69 Retinitis Pigmentosa

P degeneration, as seen through the ophthalmoscope, was first described by van Trigt in 1853 and named retinitis pigmentosa by Donders in the Netherlands.40 The term retinitis pigmentosa (RP) has traditionally included a group of hereditary retinal degenerations with characteristic features. These features include night blindness, progressive field constriction with relative preservation of macular function, and pigmentary disturbances within the posterior pole (figure 69.1). The prevalence of RP in different countries varies from one in 3000 to one in 4000 individuals.100 The number of affected individuals in the United States is estimated to be between 50,000 and 100,000. Approximately 20% of these cases are autosomal-dominant, 10% are X- linked, 20% are autosomal-recessive, and the remainder are isolated (simplex RP; no known family history).30 Most patients with RP are nonsyndromic; that is, they do not have any other associated systemic disease. The most common exception is Usher syndrome, which accounts for approximately 10–15% of RP and is associated with either profound (Type I), partial (Type II), or, extremely rarely, progressive (Type III) hearing loss. Other syndromic conditions with associated RP include Bassen-Kornzweig syndrome (abetalipoproteinemia), Refsum disease, Laurence-Moon-Bardet- Biedl syndrome, neuronal ceroid lipofuscinosis (Batten disease), Alström disease, and Kearns-Sayre syndrome.

The term retinitis pigmentosa has been retained for historical reasons even though it is merely descriptive and inappropriate, since it implies an inflammatory condition. In fact, this descriptive term for the entire category of diseases will gradually be replaced by specific mechanistic disease names reflecting the disease-causing mutation. At the present time, we have a transitional situation, in which we are still trying to force patients into descriptive categories such as retinitis pigmentosa, cone-rod dystrophy, and pattern dystrophy. In many cases, the fits do not work, and even within a single family with a RDS-peripherin mutation, for example, we may find individuals with different “diagnoses.”64,112 As knowledge evolves, it will become increasingly more informative to describe patients with names related to their mutations, such as RP1 or RDS/peripherin, than just RP. When the specific mutation is known, the patient can be counseled more accurately with respect to rate of progression. The number of known locations and specific cloned genes is constantly growing (see http://www.sph.uth.tmc.edu/retnet). In

the summary that follows, the current state of knowledge with regard to genes that cause retinitis pigmentosa will be presented. In the second part of the chapter, a summary will be given of some of the ERG protocols that are available for characterizing phenotype. It should become clear that one current challenge is to broaden the scope of the ERG and ancillary functional techniques to enrich the description of phenotype.

Genetic analysis of retinitis pigmentosa

Because the eye is readily accessible, retinal disorders have played an important role in the development of mammalian genetics.12 The first autosomal-dominant pedigree in human genetics to be fully documented involved the descendents of Jean Nougaret (1637–1719), a French butcher who had congenital stationary night blindness (CSNB). The first mammalian genetic linkage, by Haldane in 1915, was between the mouse pink-eye dilute and albino loci.12

The first mapped gene for RP was RP2, a gene for X- linked retinitis pigmentosa (XlRP) that maps to Xp11.13 Subsequently, one form of adRP was mapped to 3q.79 Shortly thereafter, the 3q form of adRP was shown to be caused by mutations in rhodopsin.41,44 Since then, additional genes causing adRP have been mapped to 6p, 7p, 8q, 17p, and 18q. Of these, the gene on 6p has been identified as the gene for peripherin; RP1 on 8q has been identified as a photoreceptor-specific protein of unknown function but with similarity over a short region to doublecortin.26 AdRP can also result from mutations in developmental regulatory genes such as NRL, a neural lucine zipper.115 Autosomaldominant cone-rod dystrophy (CORD2) results from mutations in CRX, a cone-rod otx-like photoreceptor transcription factor.46,47,102,107 Linkage analysis suggests that there are at least four additional genes for XlRP.48,53,83,88 The RP GTPase regulator (RPGR) gene maps to Xp21.180 and is responsible for RP3, the most common form of XlRP. A rare form of XlRP (RP24) maps to Xq26–27.48 Autosomal-reces- sive RP may be caused by mutations in any of a number of genes, including rhodopsin;99 PDE6B, which maps to 4p;77 the rod cGMP-gated channel (CNGA1), which also maps to 4p;76 PDE6A, which maps to 5q;42 TULP1, which maps to 6p;52 and RLBP1, cellular retinaldehyde–binding protein, which maps to 15q.81 Table 69.1, modified from a table in

F 69.1 Fundus photograph showing the posterior pole of a 42-year-old patient with XlRP. Note the “waxy disk,” the attenu-

ated retinal vessels, and the bone spicule–like pigmentary deposits throughout the midperiphery. (See also color plate 39.)

Bessant et al.,12 provides a list of genes and chromosomal loci known to cause retinitis pigmentosa.

Considerable progress has been made recently in determining the phenotype and mechanisms of functional loss for mutations that are primarily structural. Approximately 30–40% of all adRP cases are caused by rhodopsin mutations. Rhodopsin plays both a structural role as the most abundant outer segment protein and a functional role as the chromophore and initiator of the phototransduction cascade. Consequently, rhodopsin mutations produce a spectrum of clinical phenotypes, including type 1 adRP, type 2 adRP, sector adRP, and autosomal-dominant congenital stationary night blindness. In addition, rhodopsin mutations have been shown to be the cause of approximately 2% of recessive cases of RP.99 The second most common gene known to cause adRP is peripherin/RDS, the human homolog of the gene that was first identified and isolated as the cause of mouse “retinal degeneration slow” (rds).36,110 The rds gene encodes an integral membrane glycoprotein located in outer segment discs.3 Rod outer segment protein 1 (ROM1) has a similar predicted secondary structure and the same outer segment distribution as peripherin/RDS.5 Although the function of peripherin/RDS and ROM1 have not been firmly established, there is indirect evidence that they may be members of a new class of adhesion proteins that stabilize the rims of outer segment disks through homophilic and/or heterophilic interactions across the intradiscal space.5,49,109 In particular, these interactions may involve residues within their highly conserved extracellular (intradiscal) D2 loops. The clinical phenotypes include adRP, dominant retinitis punctata albescens, dominant butterfly-shaped pigment dystrophy of the fovea, and autosomal-dominant macular degeneration. That peripherin/RDS mutation can cause either RP and/or macular degeneration is consistent with the observation that the protein is expressed in both rods and cones, though its exact functional role in each photoreceptor must be different. Arg- 172-Trp mutations, for example, cause cone degeneration but appear to have no deleterious effects on rods.114 Finally, there is digenic RP66 that results from a combination of one mutation in peripherin/RDS and one in ROM1. Neither mutation alone in a heterozygote causes clinically significant degeneration.

Several approaches are available for studying mechanisms by which specific mutations lead to rod cell functional abnormalities and eventual death. Mutant opsins have been grouped on the basis of their behavior in tissue culture cells.105 Class I mutants had characteristics similar to those of wild-type rhodopsin. For example, their absorbance spectrum was normal, and transport from the endoplasmic reticulum (ER) to the plasma membrane was apparently successful. Class II mutants differed substantially from the wild-type. Their absorbance was 1–5% of wild-type levels,

their glycosylation patterns were abnormal, and transport of the proteins to the plasma membrane was not successful. Some of these proteins failed to leave the ER at all (Class IIa), while others were found equally distributed between the ER and the plasma membrane (Class IIb). However, the behavior of mutations in model systems must be compared to their action in vivo and ultimately in human patients, in whom the consequences are not always predictable. Transgenic mouse technology offers one in vivo system for studying the action of mutant genes. Among the many transgenic lines available are those expressing P23H, Q344X, K296E, and P347S rhodopsin mutations; P216L and L185P peripherin/RDS mutations; rom1 knock-outs; and rhodopsin knock-outs. Differences in vitro and in vivo are seen, for example, with the Q344X rhodopsin mutation. In vivo studies show increased retention in the cell body,106 whereas in culture, Q344X is a Class I mutant that is efficiently transported to the cell membrane. The P23H rhodopsin mutation is a Class II mutant in culture,105 but the mutant protein is present in the outer segment of the transgenic mouse.87 The mutant protein reduces the gain of transducin activation in patients22 but apparently not early in degeneration for the VPP transgenic mouse expressing the P23H mutation.50 The K296E mutant activates transducin independent of light in vitro97 but is phosphorylated and stably bound to arrestin in the transgenic mouse.73

In both human RP and mouse models of RP due to mutations in genes specific to rods, it is nevertheless the case that the loss of rod photoreceptors is accompanied by the gradual degeneration of cones. Photoreceptor loss occurs primarily through programmed cell death (apoptosis).96 It appears that degenerating photoreceptors induce apoptosis in initially healthy neighboring cells,63 and there is an accumulating body of evidence that neuroprotective factors may be capable of rescuing photoreceptors from this fate.72

M A P C

Whereas many of the negative effects of dominantly inherited mutations appear to be due to the effect on structural proteins, it is clear that the mutations that cause recessive forms of RP tend to involve either phototransduction or the visual cycle. Processes underlying activation and recovery in vertebrate rods are now understood in great detail. During the excitation phase of the rod photoresponse, light stimulates an enzymatic cascade that culminates in the hydrolysis of cyclic GMP.68,104 The interaction of excited rhodopsin (R*) with many G-protein molecules (transducin) causes each of them to release GDP and bind GTP. Transducin-GTP then activates cGMP phosphodiesterase (PDE), which hydrolyzes cGMP to 5’-GMP. The drop in cGMP caused by PDE activation causes closure of channels held open by cGMP in darkness, halting the continuous entry of Na+ and Ca2+ ions, and results in a transient hyperpolarization of the cell. Several

processes contribute to the recovery phase of the photoresponse. First, photolyzed rhodopsin is phosphorylated by rhodopsin kinase, which decreases the ability of R* to stimulate transducin. It also stimulates the binding of arrestin to the photolyzed rhodopsin, further reducing its ability to activate transducin. Activated transducin a subunits (Ta) already formed by photolyzed rhodopsin deactivate when their bound GTP is hydrolyzed to GDP. GTP hydrolysis appears to be modulated by the RGS-9.31,54 Ta-GDP then reassociates with Tbg and releases the PDEg subunit, which reinhibits PDE activity. Light-stimulated hydrolysis of cGMP within rod photoreceptors reduces the activity of cGMP-gated cation channels in the photoreceptor plasma membrane. Because these channels are the major route by which Ca2+ enters the photoreceptor, the intracellular concentration of Ca2+ falls. Lowered Ca2+ levels stimulate the activity of guanylate cyclase, which then accelerates cGMP resynthesis and the restoration of the dark conductance.

It is clear that rod photoreceptors in many warm-blooded animals, including rats, rabbits, cows, monkeys,84,108 and humans,59 show the same kind of background adaptation once thought to be characteristic of lower vertebrates. Flashes superimposed on a steady background become smaller and more rapid than in the dark, with a pronounced shortening in the time to peak photoresponse.7,43,86 The flash sensitivity, defined as the change in current per photon absorbed, declines linearly with log increases in background intensity. During recovery in the dark following bright light exposure, sensitivity remains low (bleaching adaptation) as long as naked opsin is still present. Recent evidence suggests that the underlying mechanism may be similar to background adaptation.35,37 At this time, it is not clear where in the photoreceptor enzymatic cascade adaptation occurs, but there is evidence that it may occur at almost any step. It probably does not occur in the initial kinetics of transducin activation6,59,70,92 (but see Jones65 and Lagnado and Baylor69). However, adaptation could influence the duration of transducin activation, the turnoff of activated PDE or channel sensitivity. A large body of evidence suggests that the drop in cytoplasmic Ca2+ caused by channel closing activates a negative feedback loop by stimulating guanylate cyclase and cGMP recovery.104 Thus, the decrease in cytoplasmic calcium may be crucial in regulating the adaptation of the photoresponse.78,93

M A V C The first step in rod vision is the absorption of a photon by rhodopsin in the rod outer segment. This leads to the 11-cis to all-trans isomerization of the retinaldehyde chromophore. Before light sensitivity can be regained through the regeneration of rhodopsin, the all-trans-retinaldehyde must dissociate from the opsin apoprotein and reisomerize to 11-cis-retinalde- hyde. This process, known as the visual cycle, is well under-

stood in the rods.25,39,94 Following photoisomerization and reduction by all-trans-retinal dehydrogenase, the resulting all-trans-retinol is translocated across the extracellular space from the rod outer segment to the RPE. It is reisomerized to 11-cis-retinol in a two-step process involving synthesis of a fatty-acyl ester by lecithin-retinol acyltransferase and ester hydrolysis coupled energetically to trans-to-cis isomerization by isomerohydrolase. Finally, 11-cis-retinol is oxidized to 11- cis-retinal by 11-cis-retinol dehydrogenase in RPE cells. The 11-cis-retinal moves back to the rod outer segments, where it combines with opsin to form rhodopsin.

Only recently has it become apparent that mutations in genes encoding components of the visual cycle can lead to retinal disease. Mutations in the gene for RPE65, which has been proposed as the isomerohydrolase in the RPE for the trans-to-cis isomerization,95 lead to a form of Leber congenital amaurosis.82 Mutations in ABCR, which encodes a rod disk rim transporter for retinal,4,113 cause Stargardt disease,1 and recessive CRD23,38 and may be risk factors for agerelated macular degeneration.2 Mutations in RDH5, the gene that encodes NAD/NADP-dependent 11-cis-retinol dehydrogenase, are associated with fundus albipunctatus.116

ERG measures of retinal function in retinitis pigmentosa

The preceding brief review illustrates the dramatic progress that has been made in molecular biology relating to retinitis pigmentosa over the past 20 years. To adequately characterize phenotype, it is necessary to conduct tests that reflect properties of photoreceptor structure, phototransduction, the visual cycle, and adaptation. Many of the properties can be assesses by evolving ERG protocols. These protocols can be applied to patients with retinitis pigmentosa to reveal mechanisms of photoreceptor degeneration and guide the search for disease-causing mutations.

ISCEV S P Representive ERGs from a normal subject are shown in figure 69.2. The responses shown are those of the ISCEV (International Society for Clinical Electrophysiology and Vision) standard protocol.75 The ISCEV standard prescribes a standard stimulus of 1–3 cd s/m2 and recording guidelines so that ERGs can be compared and interpreted across different clinics worldwide. The standard provides a core of key responses for comparison. The standard is not intended to be a comprehensive protocol; indeed, clinics are expected to expand the protocol as appropriate for the particular disease under consideration. Examples of expansions of the protocol for RP will be shown in subsequent sections.

Each ERG clinic needs to establish upper and lower limits of normal. Generally, this is done by recruiting and testing normal subjects of different ages. Although there is no

F 69.2 ISCEV standard responses from a normal subject. Spikes superimposed on the cone flicker (31 Hz) indicate the stimulus flash.

F 69.3 Variation in log amplitude with age. Left, Rod response. Solid curve is best-fit exponential with half amplitude at age 69 years. Right, Cone response to 31-Hz flicker. Solid curve is

“right” number, 100 is probably a reasonable target number so that age trends can be identified. Generally, amplitudes are converted to log values because these more closely approximate a normal distribution.11,15 It is then possible to determine the upper and limits of normal (typically with p < .05) from the mean and standard deviation of this distribution example. These normal limits can also be adjusted for age. Figure 69.3, for example, shows the age-related variation in rod amplitude (figure 69.3A) and in cone ERG amplitude to 30-Hz flicker (figure 69.3B). Much of the decline in sensitivity with age appears to originate at the photoreceptor level.24

Examples of ISCEV protocol responses from patients with different forms of RP are shown in figure 69.4. These examples are chosen to illustrate some very broad generalizations. One is that rod ERG responses are severely attenuated at an early age. An exception to this generalization can occur in some types of adRP mutation, where individuals can retain rod responses well into adulthood. XlRP tends to be the most severe form, with severe attenuation of both rod and cone responses by the teenage years. Patients with conerod dystrophy tend to have low acuity owing to macular involvement and cone ERG loss that is equal to or greater than rod ERG loss. Leber congenital amaurosis is characterized by severe loss of retinal function at or soon after birth. Note that a consistent finding in virtually all kinds of RP and allied retinal degenerations is the delay in cone b- wave implicit time.9 The upper limit (p < .05) of cone b-wave implicit time to 30-Hz flicker is slightly less than 33.3 ms (one

best-fit exponential with half-amplitude at age 70 years. Open circles indicate female; solid circles indicate male. (From Birch DG, Anderson JL.15)

F 69.4 ISCEV standard ERG responses in selected patients with retinitis pigmentosa. All show reduced rod

cycle). The spikes superimposed on the flicker responses indicate each flash (cycle). Thus, responses that peak to the right of the spike are delayed well beyond the normal upper limit of cone b-wave implicit time. Despite considerable attention over the past 20 years, the cause of these delays is still not understood completely. A small portion of the delay originates in the cone photoreceptors, which typically have reduced sensitivity in their response to light.61 This reduction in gain will be reflected in the b-wave as an increase in implicit time, but the magnitude should be on the order of 2–3 ms rather than the 10–15 ms that is often found. Abnormal rod-cone interactions have also been proposed as the source of the delay, since rods act to speed up cones in normal subjects.21

S F -F ERG One of the key questions concerning the full-field ERG in diagnostic use involves the sensitivity of the test. How confident can we be, for example, that an individual with a normal ERG will never develop RP? Traditionally, it has been difficult to answer this question, since patients receiving ERGs tend to have prior clinical evidence of RP. In those rare reports of attempts to rule out disease in asymptomatic family members, there are few

responses and reduced and delayed cone responses. (See text for details.)

reported assessments of accuracy from following the status of the individuals later in life. ERG testing of asymptomatic individuals is usually performed in XlRP or adRP pedigrees. In the case of XlRP, the ERG seems to be abnormal in virtually all infants and children. This is particularly important, since we can readily identify carriers of the XlRP gene through either clinical exam45 or full-field ERG.10 These women are often eager to have their sons tested at an early age. We tested and followed a group of 14 at-risk males with full-field pupillometric measures and ERGs. The nine who subsequently were diagnosed with XlRP had elevated pupil thresholds as infants, and all had reduced amplitudes and delayed b-wave implicit times when first tested with the ERG at age 5. Five of the infants with normal test results did not subsequently show any evidence of XlRP. Subsequently, we have tested 106 boys with a subsequently-confirmed diagnosis of XlRP. Only two (1.8%) showed full-field ERG amplitudes within the normal range and none had normal cone implicit time.

Our ability to determine the prognostic value of the fullfield ERG has also been changed by the molecular revolution, which has provided a gold standard for evaluating the sensitivity of the ERG in families where the disease-

F 69.5 ISCEV standard ERG responses in selected patients with retinitis pigmentosa. The first two columns show responses from patients with adRP and known mutations retaining normal (#4957) or near-normal (#6659) responses. The final three columns

associated gene is known. Particularly in families with adRP, DNA samples can be obtained at the time of the ERG to determine the presence or absence of a mutation. The vast majority of affected infants show abnormal full-field ERG responses on their initial visit, but very occasionally, we encounter normal responses in a young patient with a mutation that is thought to be disease-causing. Examples of both normal and reduced ISCEV standard ERGs from select patients with RP are shown in figure 69.5. The examples of ERGs within the normal range should provide ample evidence of the caution that must be exercised in interpreting results, especially from young family members.

Extensions of the ISCEV ERG protocol

A -R I F The ISCEV standard rod response is elicited by a retinal illuminance lying at the upper end of the linear range of the b-wave amplitude-retinal illuminance function. Either a change in effective light energy (neutral density effect) or a change in the response per unit energy (response compression) can produce a reduction in amplitude. These alternatives can be distinguished by recording responses to a range of retinal

are from a patient with XlRP followed over 11 years. Soon after birth, rod and cone responses were within the normal range for this age. By 7 years, rod and cone responses were small and delayed.

illuminances and plotting the amplitude-retinal illuminance relationship.

Responses to an extended range of retinal illuminances are shown in figure 69.6 for a normal subject and a patient with retinitis pigmentosa. At high retinal illuminances, responses to short-wavelength flashes include a small cone component. The amplitude of this cone component can be determined from the matched cone responses to longwavelength stimuli and subtracted to obtain the actual rod amplitude. Corresponding rod peak-to-peak amplitudes are plotted as a function of retinal illuminance in figure 69.7. The solid curve is the best fit of the saturating exponential relationship attributed to Michaelis and Menton in chemistry and first used by Naka and Rushton85 to describe intracellular responses to light. The curve plots

where V = rod peak-to-peak amplitude, Vmax = maximum rod amplitude, I = retinal illuminance, k = retinal illuminance at half-amplitude, and n is an exponent describing the slope of the function. In most normal subjects and patients, n is roughly equal to 1.0.18 Therefore, an abnormality in rod

F 69.6 Full-field ERGs obtained over an extended series of retinal illuminances in a normal subject and a patient with XlRP. Upper panels show responses to short-wave stimuli; lower panels show

F 69.7 Plots of log rod-only amplitude as a function of log retinal illuminance in a normal subject (solid symbols) and a patient with XlRP (open symbols). Curves are best fit Naka-Rushton functions. Compared to mean normal, log k was elevated 0.5 log unit and log Vmax was reduced by 0.9 log unit.

b-wave function in retinitis pigmentosa can be attributed to either a decrease in log Vmax or an increase in log k. A change in log Vmax in a given patient leads to a vertical shift, while a change in log k produces a horizontal shift. The vast major-

responses to long-wave stimuli matched photometrically to the four most intense short-wave responses (darker traces in upper panels). By subtracting the cone components, rod-only amplitudes can be isolated.

ity of patients show a decrease in log Vmax.18 Vmax reflects the total activity of all rod bipolar cells. If all rod bipolars are functioning, even with reduced sensitivity, it follows that with enough stimulation it should be possible to elicit a normal maximum response. The fact that Vmax is reduced in most patients implies that a substantial number of rod bipolars are nonfunctional, that is, have complete loss of their input due to photoreceptor degeneration. Log k reflects the sensitivity of the rod bipolar cells. Since each rod bipolar cell reflects the pooled input from many rods, it follows that either the complete loss of some of the rods from the receptive field or a shortening of the outer segments of all the rods in the receptive field will have roughly the same effect on log k.62

Extended amplitude-retinal illuminance functions are useful for following patients over time, either to determine the natural history of the disease17 or to evaluate the efficacy of therapy. ISCEV standard ERGs, extended amplituderetinal illuminance functions, and rod static perimetric fields (figure 69.8) were obtained annually in 67 patients with RP.17 On the average, log Vmax decreased by 0.06 log unit per year (12%), while log k increased by 0.08 log unit per year (16%). Since the exponent of the Michaelis-Menton function was set to 1, the variation in log rod threshold is the sum of the

789 :

F 69.8 Full-field ERGs and rod visual fields in retinitis pigmentosa. A, ISCEV standard responses. B, Log rod perimetric sensitivity values (unshaded regions lie within 2.0 log units of normal). C, Rod-only ERG series. D, Rod ERG amplitude as a function of retinal illuminance. E, Dark-adapted cone ERG series. F, Cone ERG amplitude as a function of retinal illuminance. (From Birch DG, Anderson JL.16)

F 69.9 Representative a-wave responses from a 65-year-old control subject. A, Responses in dark to intensities ranging from 3.2 to 4.4 log scotopic troland-seconds (log sc td s). B, Same four intensities presented against a 3.2 log td background. C, Rod-isolated

F 69.10 Representative a-wave responses from a 23-year-old man with XlRP. A, Responses in the dark to intensities ranging from 3.2 to 4.4 log sc td s. B, Same four intensities presented against

responses. Dashed lines show fit of the computational phototransduction model.59 D, Cone responses and model fits (dashed curves). (From Birch DG et al.24)

a 3.2 log td background. C, Rod-isolated responses with best fit of model. D, Cone responses and model fits (dashed lines). (From Birch DG et al.24)

changes in log k and log Vmax. In this particular sample of 67 patients with retinitis pigmentosa tested yearly in a prospective study, the annual increase in rod ERG threshold was 0.14 log unit (28%).

A-W A The ERG generated by a brief flash includes an initial cornea-negative a-wave, the early portion of which reflects the massed transduction activity of rod and cone photoreceptors29,55,89,101 and the later portion of which reflects inner retinal negative components.98,103 Several new developments have vastly increased the value of the ERG as a research tool for studying abnormal rod function in inherited retinal degenerations. Lamb and Pugh recently provided a quantitative description of the activation stages of transduction.71 We had previously concluded that the leading edge of the human a-wave provides a measure of human rod photoreceptor activity, since it could be fitted by traditional receptor models based on n-stage exponential filters.56–58 More recently, we showed that the Lamb and Pugh model fits the leading edge in normal human subjects slightly better than does the n-stage model.59 Since the Lamb and Pugh model is based on the actual biochemical steps in the G-protein activation cascade, it can be used to evaluate defects in the activation stages of phototransduction resulting from specific gene mutations in RP.

The leading edge of the a-wave recorded from the human eye spans approximately 5–20 ms (figure 69.9). The rodmediated component of this initial segment of the ERG is essentially a linear monitor of the rod photocurrent response27,56,57 and can be quantitatively analyzed in relation to the activation steps of phototransduction.28,33,34,60 Virtually all patients with RP show a decrease in the maximum amplitude of the photoresponse, consistent with a reduced number of cyclic GMP-gated channels (figure 69.10). The leading edge reflects the gain of phototransduction and may or not be abnormal in a given patient, depending at least in part on the type of RP.111 The b-wave and other postreceptor components begin to dominate the human ERG at approximately 10–20 ms and thus obscure the subsequent response of the rods. Photocurrent data obtained from mammalian rods in vitro8,32,67,84,108 predict a time scale of several hundred milliseconds or more, depending on flash intensity, for the rod response in vivo. Thus, in the human ERG, the period of development of the leading edge of the rod a-wave represents only a tiny fraction of the duration of the rod flash response. However, techniques have recently been developed for deriving the entire rod photoresponse from the ERG in human patients. Properties of the derived response from the human ERG (time course, sensitivity, adaptation) are comparable to those of in vitro rod photocurrent responses obtained in previous studies.90 The technique employs the paired-flash method used in recent studies of the human ERG14,19,20,90,91 and in similar in vivo studies of the mouse

ERG51,74 to analyze the recovery kinetics of the rod a-wave after a saturating test flash. In this method, the extent of recovery from rod saturation (i.e., from a condition of zero circulating current in the rods) at a given time after the test flash is determined from the a-wave response to a bright probe flash that rapidly reestablishes rod saturation (figure 69.11). It is anticipated that quantitative analysis of the full time course of the photoresponse will become an important component of phenotypic assessment in patients with RP and should provide insights into the mechanism of disease.

F 69.11 Paired-flash ERG method. A, Hypothetical ERG response to a test flash and a subsequently presented bright test flash. The lower part of the panel shows hypothetical responses to a group of probe flashes presented at differing times. Peaks of these responses are aligned to reflect the presumed fixed state of the rods at photocurrent saturation. B, Protocol for paired-flash trials to determine time course of the derived response to a fixed test flash. (From Pepperberg DR, Birch DG, Hood DC.91)

Summary

The remarkable advances in molecular biology over the past 20 years have led to a wealth of information about diseasecausing mutations. Our ability to genotype patients must be matched by a comprehensive set of tools for establishing the phenotype. More specific knowledge of genotype-phenotype relationships can provide insight into mechanism, help in patient counseling, and, perhaps most important, provide the foundation for future treatment trials as appropriate interventions become available.

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T electroretinogram (ERG) is the main clinical test that will confirm a cone degeneration or dystrophy. Cone degeneration or dysfunction may be congenital or acquired, but the diagnosis is often difficult to make, since the early fundus changes can be subtle. A standardized protocol with carefully established normal values is essential for optimally recognizing a cone dysfunction pattern. The clinician may have minimal physical evidence to motivate asking for an ERG or even, if suspicious, might not realize that the ERG is the definitive diagnostic test. Traditionally, cone dystrophy refers to congenital or very early onset cases, usually called achromatopsia, and cases with family inheritance patterns. The term cone degeneration is often used in acquired cases in which there is no family history.

Depending on the stage of disease and genetic type of cone disorder, clinical signs and fundus changes provide strong diagnostic clues that a cone disorder may be present. Patients with cone dysfunction typically complain of light sensitivity and tend to see better at dusk or in the dark.3 Most have uncorrectable subnormal vision, dark-to-light adaptation problems, and loss of hues or color blindness (variable finding). Frequently, patients do not volunteer these symptoms unless questioned directly for them. Some patients who are city dwellers will have “urban night blindness,” since at night, their cones do not function well in semilighted city areas where it is not dark enough for rods to be effective.

Common fundus findings include a circumscribed granularity or atrophy of the macular area and temporal optic pallor or atrophy. Congenital or early-onset cases will typically have nystagmus, which is often the symptom that brings the child to the eye doctor. Some X-linked cone dystrophy patients have confluent retinal areas of tapetal-like sheen (figure 70.1A and 70.1B), and rare patients have crystalline deposits in the macular area (figure 70.2). Krill reported a group of patients who had abnormal retinal blood vessel formation with cone dystrophy, including cases in which the retinal vessels crossed the raphe in the macula (figure 70.3).6,8

A diagnosis of cone degeneration or dysfunction is easily confirmed by a standardized ERG. The International Society for Clinical Electrophysiology and Vision (ISCEV) standardized protocol calls for the cone and rod systems to

be tested separately, as well as together in the dark-adapted bright-flash testing. Besides using a single or averaged bright flash under light-adapted conditions, another technique for isolating the cone response is to employ a flickering bright stimulus light with a frequency greater than 20–30 cycles per second (Hz), since the rod response under standard conditions will attenuate fairly severely after 8 Hz and is absent by 20 Hz.4 The flicker stimulus, which maximally stimulates the cone system, is useful in bringing out subtle dysfunction or partial cone degenerations, which may not be as apparent by single flash techniques, as the response may be disportionately worse than the single-flash photopic response.2

Cone system dysfunction should be suspected in all patients who complain of photosensitivity, problems in light adaptation, and difficulties with color saturation or discrimination (table 70.1). Patients present with subnormal or abnormal visual acuity that is noncorrectable. A number of patients will have macular atrophy or degenerative changes, some of which start as bull’s-eye macular lesions or demonstrated “cookie cutter”—shaped macular atrophy (figure 70.4A and 70.4B). Temporal optic nervehead pallor or atrophy is common in many cone dystrophies (figure 70.5). This change may be mistaken as a “tiled” disk. Abnormal color vision is not an exclusive finding in cone degeneration and may be seen in macular degeneration in macular dystrophies without panretinal cone degeneration. Unless there is a known family history of a cone disorder, an ERG is needed to confirm the diagnosis of cone dystrophy or degeneration, since this diagnosis implies a panretinal cone disorder.

An important fact to remember is the foveal centralis contributes at most only 10–15% to the photopic b-wave amplitude. This fact was confirmed many years ago by examining patients who had foveal scars but otherwise normal retinas. In the face of a macular lesion, a large reduction in the photopic ERG means that there is a cone system dysfunction.12

Traditionally, the hereditary cone degenerations and dysfunction disorders have been classified into congenital and later onset forms (table 70.2).6,8 The two congenital cone dysfunction disorders, blue monocone monochromatism, which is X-linked, and rod monochromatism, which is

B

F 70.1 Symmetric, round atrophy of fovea centralis is typically seen in a number of types of cone dystrophy or degeneration. A, In this case of X-linked cone dystrophy with tapetal sheen, the atrophy of the foveal centralis is highlighted by the surrounding sheen. This 54-year-old man had photosensitivity OU and a history of retinal detachment in his right eye; his visual acuity was 20/200 OU. B, While the sheen is seen as patches in the periphery. These patients exhibit the Mizuo-Nakamura effect on dark adaptation. (See also color plate 40.)

autosomal-recessive, typically present with congenital nystagmus, and the diagnosis may be missed, or it may be misjudged as congenital nystagmus unless an ERG is performed. The term dystrophy has been broadly used in the ophthalmologic literature, so it is appropriate to use it in con- genital-onset cone-loss cases.

Hereditary cone degenerations have been found in all three Mendelian modes of inheritance. A list of the ones currently known are listed in table 70.2. Genetic disease databases, such as RetNet or PubMed, can be used to update this information (see http://www.sph.uth.tmc.edu/retnet or http://www.ncbi.nlm.nih.gov/entrez/query.fcgi).

F 70.2 Cone dystrophy with foveal crystals. Right eye of a 58-year-old woman with urban night blindness with nonrecordable photopic ERG and normal scotopic ERGs. Visual acuity was OD 20/40, OS 20/60, and Goldmann visual fields were full. (See also color plate 41.)

F 70.3 Fluorescein angiogram of a 13-year-old girl with a cone dystrophy. The left eye had a large retinal vessel crossing the macula with telangiectatic branches giving some late leakage and edema to the macula. The retinal vessels OD were normal.

The electroretinographic pattern in all of these cone-loss disorders is generally the same pattern: The photopic ERG is severely abnormal to nonrecordable by single-flash or computer-averaged methods, while the rod ERG is normal to subnormal (figure 70.6). While the rod tracing might not have a normal amplitude, it is well formed and stable over time in cone dystrophy patients. Dark-adapted tracings frequently show a blink response near the peak of the b-wave, since most patients are photophobic (see figure 70.6). If a scotopic red flash stimulus is employed, the early cone

response will be absent, and the later rod response will be present.

Conditions that can be confused initially with cone degeneration are early cases of cone-rod retinitis pigmentosa or cases of RP inversa, in which a cone-rod ERG pattern with dense progressive central scotoma may be found and could otherwise be mistaken for a cone degeneration with mild rod involvement.5 Checking the peripheral visual field is an important adjunctive test in all these disorders, and the field is typically stable and full over time. Visual fields, often per-

T 70.1

Signs and symptoms commonly seen in cone degeneration patients

Presenting symptoms:

1.Decreased visual acuity without obvious reason

2.Complaints of photosensitivity or glare

3.Color vision (often hue) problems

4.Problems in light or dark adaptation, particularly dark to lighted conditions

5.Central scotomata

Ophthalmoscopic signs of cone degeneration:

1.Nerve fiber loss

2.Temporal optic nervehead atrophy or loss

3.Macular degeneration, early may appear granular, later occurs as symmetric or round atrophy of fovea centralis

4.X-linked later onset patients have tapetal-like retinal sheen

A

F 70.4 Fundus photographs of patients with inherited cone dystrophies; A, A 60-year-old man with blue-cone monochromatism who recently noted some mild decreases in his central vision from 20/60 to 20/200, presumably from aging. B, A 54-year-old woman

formed serially, are an important confirmatory test for distinguishing cone disorders from progressive disorders with peripheral loss and may be diagnostic on the initial test. Some cone disorders will have central scotomata whose size is consistent with the level of visual acuity.

Clinical features of cone degeneration

The diagnosis of cone degeneration or dysfunction can be extraordinarily difficult to make in the clinical setting since, in many patients, the signs and symptoms are very subtle. However, there are diagnostic indicators that ordinarily might be ignored. These are presented below to aid in determining whether an ERG should be ordered to confirm the diagnosis (see table 70.1).

The most common presenting symptoms of cone dysfunction are subnormal visual acuity and complaints of photosensitivity, loss of color saturation, or problems in adapting from a darkened environment to a lighted one. Some patients state that they see better at dusk or in the dark. An unusual symptom that a few city-dwelling cone dystrophy patients may have is urban night blindness; in a city environment, there is usually enough light at night so that rods are unable to undergo full dark adaptation, while cones do not function well. These patients will give a history of night blindness and, from the symptoms, may be mistakenly

B

with 20/400 vision OU from a large dominant pedigree with cone dystrophy from a GUCY2D gene mutation, with foveal centralis atrophy giving a “cookie cutter” appearance to macula. This pattern is characteristic of many cone dystrophies. (See also color plate 42.)

F 70.5 Temporal optic nerve head atrophy is commonly seen in many cone degenerations; illustrated here by a 9-year-old boy with rod monochromatism with temporal pallor. Sometimes the temporal edge of the nerve is flattened or missing. (See also color plate 43.)

thought to have some form of RP. Another poorly understood group of patients who have urban night blindness have rod-cone interaction dysfunction (Frumkes effect).

The most important sign to the clinician is that the patient’s vision is not correctable to normal levels, and at times, there may be no obvious reason for the visual deficit. Many patients will have obvious macular changes such as bull’s-eye lesions (see figure 70.4B), or macular atrophy, but there are other patterns to macular tissue loss that give clues that a panretinal cone degeneration is occurring. Some patients will demonstrate crystalline deposits in the fovea centralis region, sometimes associated with a geographic atrophy pattern.

One key to recognizing cone degeneration patterns of tissue loss is to note that in most patients, the atrophy is confined to the fovea centralis and usually is symmetric between eyes (see figures 70.2A through 70.2D); the tissue change itself may consist of diffuse atrophic loss, a confluent sheen

F 70.6 ERG tracings of typical cases of cone dystrophy in which the photopic (cone) signal is nonrecordable to barely discernible (left tracings). In the rod-isolated signal (middle column), the ERG is well formed and is typically normal to subnormal. The bright-flash dark-adapted tracings are subnormal to abnormal in amplitude, and if interpreted alone without the other two tracings, would be misleading and not diagnostic of any condition. The cases illustrated here are a 54-year-old woman with dominant inherited cone dystrophy DOM CD (see figure 70.6B), whose vision

with atrophy, a granular pigmentary reaction, or even crystals. Rare cone dystrophy patients will have peripheral pigmentary deposits. Concentric confluent loss is not pathognomonic of cone degeneration but is a strong indicator for performing a standardized ERG.

Patients with congenital onset and most hereditary forms of cone dystrophy will have temporal optic atrophy; since many of these patients are myopic, the atrophy may be confused or misinterpreted as a tilted disk, or the change may be a combined effect of myopic alterations in scleral canal development and atrophy of temporal disk tissue. Many of these patients, particularly those with a congenital-onset form, will demonstrate a flattened or squashed appearance to the temporal disc (see figure 70.3B). Other patients will have distinct pallor of the temporal portion of the disk

was OD 20/200, OS 20/300; a 60-year-old man with X-linked blue cone monochromatism (XL BCM), who came from a large X-linked pedigree–his visual acuity as a young man was 20/60, but by 60 years of age it was 20/200 OU (see figure 70.6A); a 58- year-old man with X-linked cone dystrophy (XL CD) with tapetal-like sheen (see figures 70.3A and 70.3B), who presented with 20/200 vision; and a 20-year-old woman with autosomalrecessive rod monochromatism (AR RM), who presented with 20/200 vision.

without obvious tissue loss (see figure 70.5), while adult-onset cone degeneration may have no disk changes.

Another pattern of disk atrophy that can be seen is a rim of white granular or sometimes crystalline-appearing material, often present in conjunction with disk pallor. Nerve fiber layer loss is a final clue that a panretinal degeneration is present but is a nonspecific finding in a number of hereditary retinal degenerations.

Known hereditary forms of cone dysfunction or dystrophy

Rod monochromatism, often called achromatopsia, is inherited in the autosomal-recessive fashion and the clinical condition has turned out to be due to a number of different gene

mutations (see table 70.2). Patients may have full to partial expression of the cone loss, with visual acuity ranging from 20/60 to 20/200. There also may be varying amounts of nystagmus, which usually improves with age. Since many of these patients have blond fundi and minimal granularity of the macula, they may be thought to have ocular albinism, but an ERG will quickly distinguish the cone dysfunction. Another clinical diagnostic technique that may be helpful for distinguishing albinism from cone dystrophy is to transilluminate the iris looking for iris atrophy; electrophy-siological testing for albinism can be done by performing lateralizing visually evoked testing (see chapter 25).

Blue cone monochromatism is an X-linked recessive congenital cone dysfunction disorder that tends to be milder than rod monochromatism. An X-linked recessive pattern of inheritance in the face of a congenital absence of cone function is a reliable indicator of this disease (see table 70.2), but molecular testing is needed to confirm the diagnosis.

Blue cone monochromatism patients may have visual acuity as good as 20/30 and occasionally as poor as 20/200; the macula develops a granular atrophy, and there is frequently severe temporal disk atrophy (see figures 70.4A and 70.4B). Carriers for the disorder may show a loss of the cone portion (x-wave) of the red stimulus dark-adapted ERG; the amount of loss is dependent on the degree of lyonization.

A late-onset X-linked cone dystrophy has been reported that has a characteristic tapetal-like sheen (see figures 70.1A and 70.1B) and changes with dark adaptation (MizuoNakamura effect). These patients in later stages often will show an inverselike macular atrophy in that their sheen highlights the macular atrophy (see figure 70.1A). As was noted above, there is a pattern of symmetric anatomic foveal tissue loss, and the sheen is missing from this area in this group of patients. Several of the affected family members have had round atrophic holes leading to retinal detachment, so these patients should be checked on a regular basis for hole formation, which will need laser prophylaxis if found. The gene for this disorder is currently unknown.

An X-linked red cone degeneration has been reported in a pedigree in which the DNA analysis with a cDNA probe found a 6.5-kb deletion in the red cone pigment gene. Clinically, patients have photosensitivity in childhood and a red color deficiency.10 The 15-year-old propositus’s maternal grandfather and great uncle had 20/200 vision with macular atrophy. The ERG showed loss of red cone function, and the 30-Hz flicker appeared to be the most affected.

Partial cone degeneration or dysfunction

A number of adult and senile patients will demonstrate partial cone loss on standardized full-field electroretinographic testing. Many of these cone degeneration patients

F 70.7 Senile cone degeneration in an 80-year-old woman with failing vision over ten years, who was found to have poor photopic ERGs with both eyes. Her right eye had a 45 uV b-wave amplitude while the left eye was barely recordable with count finger vision. Rod responses were abnormal. Visual fields were full with central scotomata. Many patients with senile cone degeneration have regional atrophy with crystallike drusen deposits. (See also color plate 44.)

have no family history and will show subnormal vision, photosensitivity, or complaints of light or dark adaptation and foveal atrophy. A few patients will have golden or yellow deposits in the macular area (figure 70.7). Some may have temporal disk atrophy. The cone ERG in some will be attenuated from 30% to 60% of normal often with increased implicit times up to 40 ms. There is a wide variety of presenting visual acuities in these patients, but they typically range from 20/40 to 20/200. Ladewig identified a group of senile cone degeneration patients, whom he did not find to be otherwise distinguishable from patients with age-related macular degeneration.9

Autosomal-dominant cone dystrophy

Autosomal-dominant cone dystrophy in most families has a distinctive appearance and clinical history. Frequently, subnormal vision will begin by the teenage years, and early macular atrophy will be seen.7 At this stage, the disease is frequently thought to be Stargardt’s disease, and some patients even have a few yellow deposits similar to flecks (see figure 70.5). In some families, patients aged 10–30 may show cone ERGs, which are barely recordable to extinguished,

while in others, the cone ERGs will be only mildly affected. In all cases over time, however, the cone ERG progressively worsens and becomes nonrecordable by single-flash technique. In some cases, flicker function may be worse than single-flash cone testing. These patients universally develop round, symmetric macular atrophy (see figures 70.7A and 70.7B). To date, three genes associated with dominant cone dystrophy have been identified (see table 70.2). The number of types of dominant cone dystrophy is not known, but it is of interest that benign concentric annular dystrophy on a follow-up report was found to have a slow cone degeneration in a dominant family.11

Autosomal-recessive cone dystrophy

Autosomal-recessive forms of cone dystrophy clearly exist and have been reported in the literature, but this group of diseases is not well understood, in part because many cases appear as isolated occurrences, and family studies have not been productive.

Management of cone disorders

By the time cone dysfunction patients are examined, most have discovered that tinted lenses are beneficial to their vision, both indoors and outdoors, although a few patients will not have tried sunglasses at all. The clinician can play an important role, emphasizing that patients with cone dysfunction do better on average than patients with more progressive problems such as retinitis pigmentosa and can be given a more encouraging outlook. The ophthalmologist also can help by recommending to patients who are not doing so that they use multiple pairs of variably tinted lenses worn according to the lighting conditions. Particularly important is reassurance that wearing tinted glasses indoors is perfectly acceptable and necessary for their condition.

Once patients with a retinal dystrophy learn that they have a problem that is considered untreatable, they often fail to seek ophthalmological care, and refractive problems may be neglected. Since many cone dystrophy patients have myopia, a current refraction is always in order. Patients with central scotomas and subnormal central vision may benefit from low visual aids and eccentric viewing. Some patients report relief of glare with antioxidant vitamins such as betacarotene and lutein.

Occasional patients will be seen who appear to have foveal structure on ophthalmoscopy yet have extinguished photopic ERGs (figure 70.8). This occurrence is in contrast to patients with bull’s-eye or cookie cutter macular lesions. The ERG will give a clear answer to whether a panretinal degenerative process is occurring in the patient and should be done

F 70.8 Cone dystrophy with apparent foveal structure. This 22-year-old woman presented with a history of color blindness and photosensitivity for at least ten years. The family history was negative. Her vision was 20/200 OU, and her photopic ERG was nonrecordable, while her scotopic waveforms were within normal limits. Her Goldmann visual field was full OU. On fundus examination, she appeared to have some foveal structure; but on close inspection, the fovea centralis showed atrophy and mild granularity. (See also color plate 45.)

when there is unexplained subnormal vision or symptoms of color desaturation and glare in patients who may have minimal fundus findings.

REFERENCES

1.Bergen AAB, Pinckers AJLG: Localization of a novel X-linked progressive cone dystrophy gene to Xq27: Evidence for genetic heterogeneity. Am J Hum Genet 1997; 60:1468–1473.

2.Carr RE, Siegel I: Electrodiagnostic. Philadelphia, FA Davis, 1990.

3.Goodman G, Ripps H, Siegel IM: Cone dysfunction syndromes. Arch Ophthalmol 1963; 70:214–231.

4.Hecht S, Schlaer S: Intermittent stimulation by light: V. The relation between intensity and critical frequency for different parts of the spectrum. J Gen Physiol 1936; 19:965.

5.Heckenlively JR: RP cone-rod degeneration. Trans Am Ophthalmol Soc 1987; 85:438–470.

6.Krill AE: Cone degenerations. In Krill AE, Deutman A (eds):

Hereditary Retinal and Choroidal Dystrophies. Hagerstown, Md, Harper and Roy, 1977.

7.Krill AE, Deutman AF: Dominant macular degenerations: The cone dystrophies. Am J Ophthalmol 1972; 73:352–369.

8.Krill AE, Deutman AF, Fishman M: The cone degenerations.

Doc Ophthalmol 1973; 35:1–80.

9.Ladewig M, Kraus H, Foerster MH, Kellner U: Cone dysfunction in patients with late-onset cone dystrophy and age-related macular degeneration. Arch Ophthalmol 2003; 121(11):1557– 1561.

10.Reichel E, Bruce AM, Sandberg MA, et al: An elec-

troretinographic and molecular genetic study of x- linked cone degeneration. Am J Ophthalmol 1989; 108:540– 547.

11.van den Biesen P, Deutman A: Evolution of benign concentric annular macular dystrophy. Am J Ophthalmol 1985; 100:73–78.

12.van Lith, GHM: The macular function in the ERG. Doc Ophthalmol Proc Ser 1976; 10:405–415.

13.Yang Z, Peachey NS, Moshfeghi DM, Thirumalaichary S, Chorich L, Shugart YY, Fan K, Zhang K: Mutations in the RPGR gene cause X-linked cone dystrophy. Hum Mol Genet 2002; 11:605–611.

N to vitamin A deficiency has been recognized since ancient Egyptian times, and of the many systemic complications of vitamin A deficiency, the retinal reaction to low vitamin A levels is the best understood. A brief discussion of the metabolism of vitamin A follows to more clearly understand these problems. Vitamin A is transported across the intestinal mucosa and is bound to lipoprotein molecules and then transported and stored in the liver as vitamin A ester. As the body needs vitamin A, these esters are transported as vitamin A alcohol (retinol) to peripheral tissues, including the retina, in conjunction with retinol-binding protein, a transport protein manufactured by the endoplasmic reticulum of the liver. In the retina, vitamin A alcohol is stored in the retinal pigment epithelium and only can be utilized by the photoreceptors after conversion to the aldehyde (retinal). This conversion utilizes the zinc-dependant enzyme alcohol dehydrogenase. Retinal then combines with the protein opsin in darkness to form rhodopsin, and if the retina is bleached by light, this complex breaks down, and the aldehyde is again reduced to the alcohol.

With vitamin A deficiency, as shown in the rat, after the initial stores of vitamin A in the liver and blood have been exhausted, the level of rod visual pigment (rhodopsin) also falls, and reciprocally, the visual threshold rises, thus leading to night blindness.4

The classic fundus picture of vitamin A deficiency, first recognized in 1915, is that of scattered multiple white or gray-white spots in the retina, seen mainly in the periphery, with their diameter being that of a retinal vein (figure 71.1).11 Such fundus changes are easily separable from other “whitedot” retinal lesions by the finding of a low vitamin A level and the clearing of these lesions with normalization of vitamin A levels.

In recent years small-bowel bypass surgery has been performed for morbid obesity and Crohn’s disease, and several reports have been forthcoming that note the development of night blindness, usually several years after surgery.3,13 In all cases parenteral vitamin A has alleviated the symptoms.

Electroretinographic (ERG) findings consist of reduced rod and cone responses with normal implicit times.7 The abnormality of both photoreceptor systems is further borne out by the dark adaptation curves, which show elevation of both rod and cone segments. With psychophysical measure-

ments of dark adaptation, the more peripheral rods respond more quickly than do the perifoveal rods.

Heckenlively (personal communication) has noted in two of his patients with vitamin A deficiency secondary to malabsorption that the photopic and dark-adapted bright-flash ERG waveforms are very similar in shape and timing, quite unlike the usual situation where the dark-adapted brightflash ERG has larger a- and b-waves as compared with the photopic ERG. Perlman et al. found a similar change in his reported case (figure 71.2).7

Kemp et al.6 studied visual function and rhodopsin levels in three subjects with vitamin A deficiency secondary to primary biliary cirrhosis and Crohn’s diseases by using twocolor adaptometry and fundus reflectometry. Employing green and red targets to test rod and cone dark adaptation thresholds before and after vitamin A supplementation, the authors were able to correlate serum vitamin A levels with cone and rod sensitivity (figure 71.3). Fundus reflectometry was used in one patient with liver disease to measure rhodopsin levels before and 3 and 9 days after starting vitamin A supplementation; there was complete recovery to normal of rhodopsin, which was correlated with the dark adaptation testing (figure 71.4).

Abnormalities in liver function have also been associated with vitamin A deficiency and night blindness, possibly due to either abnormal synthesis of retinol-binding protein, lowered serum zinc levels, or simply impaired storage areas for vitamin A esters. Diseases that illustrate these processes include biliary cirrhosis,12 cystic fibrosis,8 and chronic alcoholism,9 the latter presumably with alcoholic cirrhosis. White dots are rarely seen in these conditions and, if present, tend to be more amorphous. Electrophysiological and psychophysical studies in such patients showed elevated rod and cone thresholds on dark adaptation testing as well as reduced or undetectable rod ERGs with reduced-amplitude cone ERGs and normal implicit times. Full recovery was obtained in virtually all patients following parenteral or oral vitamin

Asupplements.

The syndrome of abetalipoproteinemia (Bassen-

Kornzweig syndrome) is associated with steatorrhea, acanthocytosis, a progressive neuromuscular degeneration, and a generalized degeneration of the retina.1 In this disorder there is a low level of all fats including the fat-soluble vitamins. The associated absence or near-absence of

F 71.1 Fundus photograph of a 53-year-old woman with documented vitamin A deficiency from complications secondary to bowel resection in Crohn’s disease. Her barely recordable ERG and night vision became normal after parenteral vitamin A and E therapy. (Courtesy of John Heckenlively, M.D.) (See also color plate 46.)

F 71.2 ERGs of a normal subject (first column) and a patient with vitamin A malabsorption. The patient’s ERG responses were measured at different dates before (second column) and after (third to fifth columns) therapy. The ERG responses were evoked by single white flashes of different intensities during the light- (1st row) and dark-

lipoproteins, among them the lipoproteins responsible for the transport of vitamin A in the blood, is the metabolic abnormality responsible for the concomitant low serum levels of this vitamin. Several studies have shown that some patients given vitamin A with subsequent normalization of their vitamin A levels will show an improvement in both dark adaptation as well as the ERG.5,10 Bishara et al. suggested that vitamin E should also be administered comcomitantly.2

In summary, from a clinical perspective, vitamin A deficiency with subsequent night blindness can occur from a number of diseases affecting different metabolic sites; these include (1) reduced intake of vitamin A and/or carotenoids such as in malnutrition, (2) reduced intestinal absorption of vitamin A such as follows intestinal bypass or resection surgery,

(3) defects in the transport of vitamin A as in BassenKornzweig syndrome, and (4) liver disease that leads to abnormalities in the normal vitamin A pathway due to reduced production of retinol-binding protein, reduced amounts of zinc, or reduced storage areas for vitamin A esters.

adapted states (second to fourth rows). The intensity of the test light is given as the density of the neutral filter interposed in the light path. The upper tracing is from the left eye and the lower from the right eye. The calibration mark equals 100mV vertically and 25ms horizontally. (From Perlman I, Barsilai D, Haim T, Schramek A.7 Used by permission.)

F 71.3 Two-color dark adaptometry of a subject with vitamin A deficiency: Relative thresholds to the green (circles) and red (crosses) stimuli. Measurements were made at a retinal eccentricity of 25 degrees along the horizontal meridian in the nasal field and followed a white bleaching exposure that removed virtually all visual pigment. A, results obtained on the first test; B, data obtained when vitamin A supplementation had led to partial recovery of visual function; C, data obtained when systemic vitamin A levels were normal. (From Kemp CM, Jacobson SG, Faulkner DJ, Walt RW.6 Used by permission.)

REFERENCES

1.Bassen FA, Kornzweig AL: Malformation of erythrocytes in a case of atypical retinitis pigmentosa. Blood 1950; 5:381–387.

2.Bishara S, Merin S, Cooper M, Aziz E, Delpre G, Deckelbaum RJ: Combined vitamin A & E therapy prevents retinal electrophysiological deterioration in abetalipoproteinemia. Br J Ophthalmol 1982; 66:767–770.

3.Brown GC, Felton SM, Benson WE: Reversible nightblindness associated with intestinal bypass surgery. Am J Ophthalmol 1980; 89:776–779.

F 71.4 A, Recovery of rhodopsin in a subject with primary bilary cirrhosis and vitamin A deficiency following a full bleaching exposure on days when serum levels were normal (filled circle), mildly abnormal (diamond), and more severely abnormal (open circle). All double-density changes have been normalized to the value obtained at 30 minutes (0 to 0.12 density units) on the day when the serum vitamin A level was normal. Error bars are 1 SD. B, corresponding dark adaptometry data for a green stimulus. (From Kemp CM, Jacobson SG, Faulkner DJ, Walt RW.6 Used by permission.)

4.Dowling JE, Wald G: The biologic formation of vitamin A acid. Proc Natl Acad Sci USA 1960; 46:587–608.

5.Gouras P, Carr RE, Gunkel RD: Retinitis pigmentosa in abetalipoproteinemia: Effects of vitamin A. Invest Ophthalmol 1971; 10:784–793.

6.Kemp CM, Jacobson SG, Faulkner DJ, Walt RW: Visual function and rhodopsin levels in humans with vitamin A deficiency. Exp Eye Res 1988; 46:188.

7.Perlman I, Barsilai D, Haim T, Schramek A: Night vision in a case of vitamin A deficiency due to malabsorption. Br J Ophthalmol 1983; 67:37–42.

8.Petersen RA, Petersen VS, Robb RM: Vitamin A deficiency with xerophthalmia and night blindness in cystic fibrosis. Am J Dis Child 1968; 116:662–665.

9.Sandberg MA, Rosen JB, Berson EL: Cone and rod function in vitamin A deficiency with chronic alcoholism and retinitis pigmentosa. Am J Ophthalmol 1977; 84:658– 665.

10.Sperling MA, Hiles DA, Kennerdel JS: ERG responses following vitamin A therapy in abetalipoproteinemia. Am J Ophthalmol 1972; 73:342–351.

11.Teng KH: Further contributions to the fundus xerophthalmicus. Ophthalmologica 1965; 150:219–238.

12.Walt RW, Kemp CM, Lyness L, Bird AC, Sherlock S: Vitamin A treatment for night blindness in primary biliary cirrhosis. Br Med J 1984; 288:1030–1031.

13.Wechsler HL: Vitamin A deficiency following small-bowel bypass surgery for obesity. Arch Dermatol 1979; 115:73–75.

. .

T electroretinogram (ERG), also referred to as a negative ERG, is a very distinctive electrophysiological finding that has significant importance in not only establishing the correct diagnosis, but also localizing the source of the abnormality within the retina. An electronegative ERG has classically been defined as an ERG in which the a-wave amplitude is normal but the b-wave amplitude is severely subnormal, being smaller in amplitude than the a-wave. Within the past two decades, the term electronegative ERG has been expanded to include not only the classic definition, but also any ERG in which the b-wave is smaller than the a-wave, even when the a-wave itself is clearly subnormal. Moreover, although the term was initially applied only to the dark-adapted mixed rod-cone bright-flash ERG, the term has recently been used to describe a similar configuration for the light-adapted cone ERG.34,72

Origins of the negative ERG

The human maximal full-field electoretinogram presented to the dark-adapted eye is mainly rod-derived and has two predominant components: An initial negative a-wave followed rapidly by a supervening positive b-wave. The a-wave has been shown to be linked to the kinetics of rod phototransduction. It is now generally agreed that the b-wave arises from ON bipolar cell depolarisation,26,40,67 though it is likely that rod inner segments, the synaptic layer between rod and bipolar cells63 and third-order neurons also contribute.83

The basic underlying retinal pathology for all electronegative ERGs must thus be a disturbance at or proximal to the photoreceptor inner segments but that relatively spares photoreceptor outer segment function. This may include disturbance of neurotransmitter release from photoreceptor inner segments, defects of the postsynaptic receptors of bipolar cells where they synapse with the photoreceptors, or any disturbance of the microvasculature of the middle retinal neurons. Etiologies that should therefore be considered include inherited retinal dystrophies and acquired processes such as inflammatory, autoimmune, vascular, or neurotoxic retinopathies.

Disorders associated with an electronegative ERG

Many disorders have been described with electronegative ERGs. A complete listing is shown in table 72.1 together with references to chapters in this text in which more detailed descriptions may be found. Some show selective reduction in the b-wave, for example, congenital stationary night blindness (CSNB) and melanoma-associated retinopathy (MAR). Disorders in which the a-wave amplitude is typically also abnormal include diseases that affect multiple retinal cell types, such as ocular siderosis, quinine toxicity, methanol toxicity, and certain forms of retinitis pigmentosa, and vascular diseases that compromise or disrupt both choroidal and retinal circulation, such as birdshot choroidopathy and carotid insufficiency.

C S N B Probably the most frequent and best-recognized cause of a negative ERG is X-linked CSNB (Schubert-Bornschein types; see also chapter 74). CSNB is a recessive, nonprogressive retinal disorder characterized by night blindness, decreased visual acuity, myopia, nystagmus, and strabismus. In 1986, Miyake et al. proposed the existence of two distinct subtypes of CSNB, termed complete and incomplete.50 Patients with complete CSNB show moderate to severe myopia, profoundly subnormal rod function, abnormal scotopic and photopic oscillatory potentials, and a normal or near-normal photopic cone amplitude (figure 72.1). Patients with incomplete CSNB show moderate myopia to hyperopia, subnormal but measurable rod responses, subnormal but more intact oscillatory potentials, and subnormal cone function (see figure 72.1). Weleber and Tongue reported siblings with visual developmental delay and presumed autosomal-recessive CSNB with markedly subnormal ERG amplitudes during early infancy that increased at age 1 year to became consistent with the complete form of CSNB.80 Typically, with both autosomal and X-linked CSNB, the maximal scotopic ERG b-wave amplitude is markedly reduced, but the a-wave is normal or only minimally subnormal.

Two genes have been discovered for the X-linked forms of CSNB (reviewed in Weleber76). Incomplete CSNB results from mutation of the gene CACNA1F, which encodes the

a-subunit of the L-type voltage-gated calcium channel present within retinal synapses.3,71 Presumably, the mutations cause a decrease in neurotransmitter release from photoreceptor presynaptic terminals.71 Complete CSNB results from a mutation of the gene NYX, which encodes nyctalopin, a small leucine-rich proteoglycan that is thought to be essential for the development of functional ON pathway retinal interconnections, including ON bipolar cells.4,61

Controversy exists as to whether Åland Island eye disease (AIED) (also called Forsius-Eriksson ocular albinism)19 is a separate disorder or should be considered a subset of X- linked incomplete CSNB.13,83 Family studies suggest that the two disorders have the same genetic interval,1,23,66 and although mutations in CACNA1F have been found in rare AIED-like patients, no mutations have been identified in the CACNA1F gene in the original AIED patients.84 Nonetheless, AIED and incomplete CSNB are electrophysiologically similar, if not indistinguishable.78

X-L J R X-linked juvenile retinoschisis (XLRS) is probably the next most frequent and recognized genetic cause of an electronegative ERG (figure 72.2) (see chapter 73).25,36,54,68 This progressive retinal dystrophy is the most common cause of juvenile macular degeneration in males.52 The condition exhibits considerable variability at presentation. Classically, multiple peripheral retinoschises and vitreoretinal degeneration can be seen associated with a cystic, spoke-wheel maculopathy.21

Electrophysiological and psychophysiological studies of XLRS have been interpreted to suggest that oscillatory potentials (OPs) may be generated, at least in part, by interplexiform cells rather than entirely from amacrines or horizontal cells.54 Furthermore, the negative ERG in association with normal psychophysical function is strongly supportive of Müller cell dysfunction. Müller cells are not the direct generators of the OPs but reflect the signal generated in other cells.54 Abnormal a-wave responses are also seen and

F 72.1 International Society for Clinical Electrophysiology of Vision (ISCEV) standard ERG for patient with incomplete CSNB, complete CSNB, and an age-similar normal for comparison. The right and left eyes are superimposed. Note that sizable rod responses, detectable oscillatory potentials, and severely subnormal

photopic b-waves distinguish the incomplete from the complete form of CSNB. (Reproduced with permission from Weleber RG. Infantile and childhood retinal blindness: A molecular perspective (The Franceschetti Lecture). Ophthalmic Genet 2002; 23:71– 97.)

F 72.2 ISCEV standard ERG for a 36-year-old patient with X-linked retinoschisis, compared to a normal. The right and left

eyes are superimposed. Typical foveal schisis was not evident, but the foveal umbo was absent.

indicate that photoreceptor as well as inner retinal layer function may be affected in XLRS, at least in some patients.8 XLRS results from mutations within the XLRS1 gene, which encodes retinoschisin. This protein acts as a cell adhesion protein to maintain cellular organization and the synaptic structure of the retina.81 Disruption of this gene function presumably leads to the disruption of Müller cells, which results in the splitting of the inner plexiform layer. It is of note that there appears to be no good genotype-phenotype correlation. The severity of ERG abnormalities does not appear to correlate with clinical findings, age, or the type of mutation. Responses may even differ between affected males

within the same family.8

Although the electroretinogram is a key diagnostic test for X-linked retinoschisis, Sieving et al.68 have documented a normal electroretinogram scotopic b-wave in a male with molecularly confirmed X-linked retinoschisis. Caution is therefore advised in placing too much reliance on the electroretinogram to exclude the diagnosis.

R P Most patients with advanced RP of many subtypes will have an electronegative ERG with greater relative loss of b-wave than a-wave (figure 72.3). The selective loss of the b-wave probably occurs from the secondary remodeling effects of the retinal degeneration on middle and inner retinal neurons and Müller cells. These stress-induced reorganizational responses to the degeneration and death of photoreceptors lead to neuronal cell death, neuronal and glial migration, elaboration of new neurites and synapses, rewiring of retinal circuits, glial hypertrophy, and the evolution of a fibrotic glial seal that isolates the remnant neural retina from the surviving RPE and choroid.30,41 Other patients with early retinitis pigmentosa (RP) with otherwise typical clinical features have been found to have the unusual electroretinographic finding of a negative waveform to a bright flash in the dark-adapted state (figure 72.4). The ERG findings in this subset of RP patients indicate there is relatively early dysfunction not only at the level of the photoreceptor outer segment but also at or prox-

F 72.3 ISCEV standard ERG for the right and left eyes of a 35-year-old patient with autosomal-dominant RP, compared to a

normal. The fundus appearance was typical for moderately advanced RP.

F 72.4 ISCEV standard ERG for a patient with autosomal-dominant RP, compared to a normal. The fundus

imal to the photoreceptor terminal region.16 Even patients with molecular defects limited to rods, for example, RP from the P23H mutation of rhodopsin, may show an electronegative ERG (figure 72.5). The possibility of autoimmune retinopathy should be considered in patients with RP and electronegative ERG (see chapter 58).

C , C -R M D

E ERG A progressive cone dystrophy has been described in which negative scotopic and photopic fullfield ERGs were recorded. The authors comment that this is most unusual and raise the possibility that the retinal and electrophysiological defects may be inherited separately.34 More likely, the defective gene product is required for maintenance of health and function of photoreceptor (cone) inner segments and/or middle retinal neurons (bipolar cells). Electronegative ERGs have also been noted in three families with dominantly inherited cone-rod dystrophies.20,24,72

Miyake et al.48 studied four patients with a bull’s-eye maculopathy and otherwise normal fundus. Acuity and color

showed a bull’s-eye maculopathy with minimal pigment in the periphery.

vision losses were progressive, though visual fields were unaffected. A dark-adapted single-flash ERG with an intense white light stimulus was electronegative. Cone responses were relatively well preserved. The 30-Hz flicker ERG and EOG were normal.

F A Fundus albipunctatus is a rare autosomal-recessive condition characterized by numerous yellow-white punctate lesions at the level of the retinal pigment epithelium.42,49 Although originally considered a stationary disorder, Miyake et al. have reported that patients of all ages may develop atrophic macular lesions.49 Scotopic and photopic ERG responses are subnormal following conventional dark adaptation but reach normal or near-normal amplitudes following extended dark adaptation.14,42,47 However, what is not addressed in any of these studies is the negative waveform of the scotopic maximal responses with conventional dark adaptation.49

M C S R D Kellner and colleagues have described a negative ERG in a family with

F 72.5 ISCEV standard ERG for a patient with autosomaldominant RP from the P23H mutation of rhodopsin, compared to a

Müller cell sheen dystrophy. Light-adapted responses showed an unusually delayed b-wave, broad and delayed ON and OFF responses, and a missing flicker response, suggesting Müller cell dysfunction.35

F O A Dominant optic atrophy has been reported with an electronegative ERG, presumably representing a newly appreciated genetic disorder.77 Such electrophysiological abnormalities are not seen with other familial optic atrophies.

A -D I N ERG P Electrophysiologic studies were performed in an infant who presented with moderate myopia, nystagmus, visual developmental delay, and an electronegative ERG.18 These findings prompted investigation of other family members who showed similar electrophysiological abnormalities, apparently inherited as a dominant trait. Rod thresholds were normal, as were acuities and visual fields. The infant’s nystagmus resolved by age 5 years, at which point the fundi remained normal and there was no evidence

normal. An early regional pigmentary retinopathy was evident on fundus examination. The visual fields showed a ring scotoma in each eye.

of systemic disease. The authors speculated that a mutation within the gene encoding metabotropic glutamate receptor subtype 6 might be causative but found no sequence changes.

B C D Jurklies et al.31 have reported a single case of an individual with Bietti crystalline dystrophy, whom they followed over a 30-year course. Serial ERG recordings progressed from low normal amplitudes to extinction. However, during his third decade, electronegative scotopic ERG waveforms were noted.

A -D N I

V Autosomal-dominant neovascular inflammatory vitreoretinopathy is a rare genetic eye disorder first described in 1990 and linked in 1992 in a large family to the long arm of chromosome 11 (11q13).5,70 Affected patients may be asymptomatic in early adulthood but eventually acquire vitreous cells and selective loss of the b-wave of the scotopic ERG. They eventually develop cystoid macular edema, cataracts, and glaucoma as well as periph-

eral arteriolar closure, peripheral neovascularization, and peripheral retinal pigmentary retinopathy. Retinal detachments can ensue, and these patients react to surgery with a marked inflammatory response.

Neurodegenerative disorders

N C L The neuronal ceroid lipofuscinoses (NCL, Batten’s disease) are neurodegenerative disorders with psychomotor deterioration, seizures, visual failure, and premature death, all associated with abnormal storage of lipoproteins within lysosomes (see chapter 80). The most common forms of NCL are an infantile form (INCL, CLN1), a late infantile form (LINCL, CLN2) and a juvenile-onset form (JNCL, CLN3). The ERG is abnormal early in all three of these forms, and may take on an electronegative configuration, and eventually is totally ablated. Patients with JNCL invariably showed severe to profound ERG abnormalities when first tested, usually with no discernible rod-mediated activity and marked loss of a-wave

amplitudes with even greater loss of b-wave amplitudes, creating electronegative configuration waveforms (figure 72.6). Differences in the ERG responses were thus found that provide further clues to the earliest site of pathology within the retina.74

M IV The finding of electronegative ERGs in two cases of mucolipidosis IV suggests that the primary retinal disturbance in mucolipidosis IV may occur at or proximal to the photoreceptor terminals.59 For children with corneal cloudiness and developmental delays, the finding of an electronegative ERG (figure 72.7) should trigger the consideration of this disorder.

I R ’ D Infantile Refsum’s disease (IRD) represents a disorder of peroxisomal biogenesis and is distinct from the classical later-onset or classic form of Refsum’s disease (reviewed in Weleber79). The biochemical abnormalities in IRD are more extensive and reflect the expected multiple biochemical defects and deficiencies

F 72.6 ISCEV standard ERG for a 7-year-old patient with juvenile-onset neuronal ceroid lipofuscinosis, compared to an age-similar normal. Tracings from the right and left

eyes are superimposed. Note the differences in the vertical scale. The patient was heterozygous for the 1.02-kb deletion of

CLN3.

Rod Suppressing

Background

No Background

Photopic Single Flash

Bright White

Scotopic OPs (100-300 Hz)

Digitally Filtered

Bright White

Scotopic Single Flash

Bright White

Blue (rod response)

Red (cone & rod response)

100 V

50 ms

100 V

20 ms

50 V

50 ms

200 V

50 ms

F 72.7 ISCEV standard ERG for a 4-year-old patient with type IV mucolipidosis, compared to an age-similar

resulting from the near total absence of functional peroxisomes. Abnormal laboratory findings in IRD include elevated plasma levels of very long chain fatty acids, phytanic acid, and pipecolic acid. Patients are deficient in docosahexaenoic acid, precursors of plasmalogens, and the biliary dihydroxycholestanoic and trihydroxycholestanoic acids. Features include early-onset mental retardation, facial dysmorphism, RP, sensorineural hearing loss, hepatomegaly, osteoporosis, failure-to-thrive, and hypocholesteremia. The ERG is severely abnormal early in the course of the disease and shows an electronegative configuration (figure 72.8). The reason why an electronegative ERG is observed in some patients remains to be elucidated but may involve a function of peroxisomes that is critical for maintenance and survival of middle retinal neuronal as well as photoreceptors.75,79

D /B M D Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) patients have mutations in the dystrophin gene that

normal. Tracings from the right and left eyes are superimposed.

result in progressive muscle degeneration. The ERG often will be electronegative for these patients as well as patients with the contiguous gene deletion involving dystrophin known as Oregon eye disease, even in the absence of defective dark adaptation.27,55,56,69,78 At least four isoforms of dystrophin have been shown to be present in the outer plexiform layer of the human retina.55 Although most patients have no ocular symptoms, a reduced b-wave amplitude is typically seen in the dark-adapted ERG in individuals with mutations that result in the loss of function of these isoforms.57 Unfortunately, it appears that although ERG findings in DMD and BMD patients may correlate with molecular analysis, such testing is not discriminatory in DMD and BMD carriers.73

Acquired diseases of the eye

R D An electronegative ERG can be seen in situations in which the retinal capillary circula-

30 Hz (34 cd/m2)

+0.3 log cd-s/m2

Photopic White Flash

+0.6 log cd-s/m2

Scotopic OPs (100-300 Hz)

+0.6 log cd-s/m2

Scotopic White Flash

+0.6 log cd-s/m2

+0.0 log cd-s/m2

-0.6 log cd-s/m2

Blue (rod response)

Red (cone & rod response)

tion is extensively disrupted, such as occurs in central retinal artery occlusion32 or hemorrhagic (or ischemic) central retinal vein occlusion (CRVO) (figure 72.9).32,64 Central retinal vein occlusion can be associated with good capillary perfusion, termed nonischemic CRVO, and poorly perfused eyes, termed ischemic CRVO. Ischemic CRVO conveys a great risk of iris neovascularization (with subsequent neovascular glaucoma). Panretinal photocoagulation can prevent or ameliorate neovascular glaucoma, but questions persist as to when to offer this treatment. Various techniques (fluorescein angiography, quantitative measures of the afferent pupillary defect, and the ERG) have been evaluated to assess the extent of capillary nonperfusion in eyes with CRVO in an attempt to predict which patients with CRVO are at great-

similar normal. Tracings from the right and left eyes are superimposed.

est risk for iris neovascularization (for a review, see Bresnick9). In a retropective study of 45 patients with CRVO, Sabates, Hirose, and McMeel64 found that six patients with hemorrhagic (ischemic) disease, who had an ERG that was initially electronegative, with on average a b/a ratio of 0.84, developed neovascular glaucoma; conversely, the 27 venous stasis retinopathy (nonischemic) CRVO patients (with an average b/a ration of 1.67) did not develop neovascular glaucoma. Five of the 12 patients with undetermined retinopathy had electronegative ERGs, and two of these developed neovascular glaucoma; three had normal or near-normal ERGs, none of whom developed glaucoma. Overall, no patient with a b/a ratio greater than 1.0 developed neovascular glaucoma. Several investigators have studied the ERG in

F 72.9 ISCEV standard ERG for the right and left eyes of an 84-year-old male with an ischemic central retinal vein occlusion of the left eye. The b/a ratios were 1.4 OD and 0.9 OS. Rod and

CRVO to determine which components are most sensitive and specific for development of iris neovascularization and hence which patients would need consideration for early panretinal photocoagulation.10–12,28,29,33,37,44,45,82 Debate persists with regard to which protocol is most helpful in the clinical setting; however, the finding of an electronegative ERG in an eye with CRVO is accepted as being highly predictive of iris neovascularization.

M -A R Melanoma-associated retinopathy (MAR) is a paraneoplastic retinopathy that commonly presents after the diagnosis of melanoma has been made, often at the stage of metastases. Symptoms include shimmering, photopsias, night blindness, and mild peripheral visual field loss. The fundus may appear normal.7,15 Histologically, there is evidence of ganglion cell transsynaptic atrophy, a marked decrease in bipolar neurons in the inner nuclear layer, and relative preservation of the photoreceptors themselves.22 It appears that there is an antibody crossreactivity between an antigen on melanoma cells and a pro-

cone b-wave implicit times for the left eye were prolonged for single flash and 30 Hz flicker responses. The left eye developed neovascular glaucoma.

tein or lipid on ON bipolar cells.38 It has recently been suggested that the antigen may be transducin.58 Such a pattern of retinal degeneration explains intuitively the preferential amplitude reduction in the b-wave of the scotopic full-field ERG that results in the electronegative waveform, characteristic of patients with MAR.

Rarely, individuals present with symptoms, signs, and electrophysiologic abnormalities consistent with a diagnosis of MAR but with no evidence of malignancy. In these instances, similar autoimmune phenomena are hypothesized.17,51

B C Birdshot chorioretinopathy (BSCR) is a bilateral posterior uveitis characterized by the development of cream-colored depigmented chorioretinal lesions that, untreated, result in progressive visual loss from optic atrophy and chronic cystoid macula edema.53 Electrophysiology typically may show an initially electronegative ERG (figure 72.10) and may be associated with a diminished a-wave; however, both components of the ERG can eventually become extinguished.39,60

F 72.10 ISCEV standard ERG for a 5-year-old female with birdshot choroidopathy, compared to an age-similar normal. Tracings

T R The clinician should be aware that several pharmacological agents can be associated with the generation of negative ERGs and that, in some cases, prompt cessation of treatment may prevent further damage. These include quinine toxicity (figure 72.11),2 vincristine,62 methanol,46 and MS222 (fish anesthetic).6 A negative ERG has also been documented in ocular siderosis resulting from intraocular ferric foreign bodies.65

Concluding comments: The electronegative ERG in clinical practice

Electrophysiological testing is often performed to rule out or add weight to a suspected clinical diagnosis. In instances in which the clinical diagnosis is suspected, the appearance of a negative ERG is useful in providing confirmation. An

from the right and left eyes are superimposed. The disease responded slowly to immunosuppression therapy with modest ERG improvement.

example is X-linked juvenile retinoschisis. The diagnosis of XLRS may be difficult in instances in which the foveal abnormalities are subtle or late in the disease, when nonspecific macular atrophy may supervene. In these instances, an ERG will prove incisive.

However, clinicians are not infrequently confronted with a patient with unexplained subnormal acuity or symptoms, and the ERG is unexpectedly found to be electronegative, providing valuable information in narrowing the possible diagnoses. In some cases, the finding of an electronegative ERG in the face of normal or nonspecific clinical findings warrants prompt further evaluation of the patient for such conditions as melanoma or pharmacological toxicity.

Supported by the Foundation Fighting Blindness, Research to Prevent Blindness, and the Frost Charitable Trust.

F 72.11 ISCEV standard ERG for a 15-year-old patient with severe quinine toxicity at day 10 and day 25, compared to an age-similar normal. Tracings from the right

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