Ординатура / Офтальмология / Английские материалы / Ocular Disease Mechanisms and Management_Levin, Albert_2010

First, lactate dehydrogenase activity was shown to be high in the inner segments, outer nuclear layer, and outer plexiform layer of monkey retina, and about half as great in the inner retina.87 The inner retina relies much more on glycolysis in animals such as rabbits, which have little retinal circulation87 and very low levels of oxygen and oxidative metabolism.88,89 Second, because lactate production is correlated with H+ production, pH gradients across the retina have also been used as a surrogate measure of glycolysis. In cat, the highest [H+] is in the outer nuclear layer,90,91 and mathematical modeling indicates that both the outer nuclear layer and inner segments produce H+.90 It should be noted that the H+ measurements actually reveal the layers in which H+ is extruded from cells, i.e., where the transporters are, and not necessarily the layers with the highest intracellular production of H+.

The rate of photoreceptor glycolysis, like the rate of oxidative metabolism, decreases with illumination, by less than 10% in rat,76 and about 50% in cat43,90 and pig.44 Presumably the reason for high glycolysis in darkness is that there is not enough oxygen available to produce the required ATP oxidatively, and the very low Po2 observed in the distal retina of cat and monkey supports this – a small part of the retina is normally almost anoxic. However, extremely low intraretinal Po2 has not been observed in rat retina84,85 where glycolysis is still pronounced in dark adaptation, and glycolysis is not reduced to zero during illumination, a condition in which oxygen is not limiting in any species. Consequently, the reason for the constitutively high level of aerobic glycolysis is not completely clear.

Effects of hypoxia

Decreases in blood Po2 affect the inner and outer retina differently (Figure 73.4). Inner retinal Po2 is well protected by the metabolic regulation of the retinal circulation discussed above, and Pao2 must fall to 40 mmHg or less before inner retinal Po2 is affected in cats. It should be emphasized that this result is for the normal retina, and it is expected that diseases that affect retinal vascular autoregulation may be more detrimental to inner retinal Po2. Unlike the retinal

Retinal depth (%)

Figure 73.4  Oxygen profiles in hypoxia in the dark-adapted cat retina. Arterial Po2 in the three profiles was 93.4, 63.8, and 41.1 mmHg from top to bottom. (Modified from Linsenmeier RA, Braun RD. Oxygen distribution and consumption in the cat retina during normoxia and hypoxemia. J Gen Physiol 1992;99:177–197.)

Pathophysiology

circulation, and as also shown in Figure 73.4, choroidal Po2 decreases in even relatively mild episodes of hypoxia, decreasing the flux of oxygen to the photoreceptors in the dark, and reducing their oxygen consumption.67

Compensating at least to some extent for the loss of oxidative metabolism in the outer retina in hypoxia is a strong Pasteur effect, a dramatic increase in glycolytic activity that occurs in all species studied.76,88,92,93 The changes in retinal function resulting from hypoxia are complex94,95 and will not be considered here. While the Pasteur effect is very strong in the photoreceptors, it cannot completely compensate when there is no oxygen, so ATP levels fall76 and the electroretinogram cannot be sustained during anoxia. Lactate production is low in the inner retina (Figure 73.3), and there is little information on whether there is a Pasteur effect of any consequence in the inner retina.

Processes that use ATP

Photoreceptors

It is instructive to evaluate how ATP is used by the retina. Photoreceptors appear to use the largest fraction of their ATP for the Na+/K+ ATPase that is localized to the inner segments.88,96,97 This requires more ATP than in many cells, because many cyclic nucleotide-gated light-dependent channels are open in the dark, giving photoreceptors an unusually high conductance to Na+ and Ca2+. The substantial Ca2+ influx is handled by an electrogenic transporter in the outer segment that moves one K+ and one Ca2+ out for every 4 Na+ that enter.98 The Na+ influx through the Ca2+ exchanger, plus the influx through the light-dependent channels, both have to be handled by the Na+/K+ ATPase. In the dark this process alone is estimated to require half of the ATP used by the photoreceptor.88 This ATP requirement decreases substantially during illumination, but only approaches zero if all the light-dependent channels close.

The only other component of ATP utilization that appears to be strongly light-dependent is the turnover of cGMP. In the dark, guanylate cyclase catalyzes a constant production of cGMP from guanosine triphosphate (GTP), and there is a constant breakdown of cGMP by PDE. A pulse of light causes a transient increase in PDE activity, lowering cGMP and closing channels, as is well known.99,100 A longer episode of illumination leads not only to activation of PDE but also to activation of guanylate cyclase, because channel closure reduces intracellular Ca2+, and Ca2+ negatively regulates guanylate cyclase. Thus, during illumination, the concentration of cGMP decreases, but the rates of formation and degradation of cGMP increase. This increased turnover requires more GTP, and in turn, more ATP. Thus there is expected to be a component of photoreceptor metabolic rate that increases during illumination, and such a component has been demonstrated in several studies.88,97,101 It can be isolated by eliminating the larger Na+ pumping component pharmacologically.

Transduction requires smaller amounts of ATP and GTP for rhodopsin phosphorylation, activation of the G protein transducin, and possibly other events,96 but most are early in the transduction cascade, so they precede the amplification steps that cause cGMP turnover and Na+ pumping to have large metabolic requirements.

In addition to the above events, all of which are dependent on processes in the outer segment, the photoreceptor

may have other transport functions, notably H+ extrusion in exchange for Na+, and possibly rebalancing gradients that are altered by the operation of voltage-dependent conductances in the rest of the cell. The relative importance of these processes has recently been analyzed.102 Synthesis of membranes, particularly the synthesis of new disks, and of other lipids, proteins, and RNA is also believed to require relatively little energy,96 but some of these estimates are based on parameters from other tissues and need to be revisited.

The photoreceptor synaptic terminals in the outer plexiform layer, at the border between the inner and outer retina, are far from the main metabolic engine of the photoreceptors, and have their own mitochondria.103 There is no direct information on whether the ATP they produce is all utilized in the synapse, but this seems likely. Recycling of glutamate, filling and moving vesicles, and pumping of Na+ and Ca+ require this ATP. Because the photoreceptor releases more glutamate in darkness, all of these processes are likely to require more ATP in darkness than in light, but there are no data on this point.

Inner retina

As noted above, the inner retina in animals that have a retinal circulation uses about the same amount of oxygen as the inner retina, but the processes that require this energy

have been difficult to investigate. Working in rabbit, which as noted above uses largely glycolytically derived energy for the inner retina, Ames and Li showed that pharmacologically induced changes in glutamatergic transmission produced changes in glycolysis of up to 50%.104 Neurons in the off pathway of the retina are expected to be relatively depolarized in darkness, and would therefore have high Na+ conductance, although probably not as high as rods. They would then have relatively high oxygen demand. Neurotransmitter synthesis and recycling and, for some cells, recovery of Na+ gradients after spike generation would be expected to be the other consumers of ATP.

Conclusion

A great deal is known about retinal energy metabolism under normal and experimentally altered conditions. A number of cases can also be identified where changes in metabolism or in substrate supply play some role in disease. The remaining challenges are to sort out whether changes in metabolism in diseases are primary or secondary, and whether the knowledge about retinal energy metabolism can be used to devise therapies.

2.Alder VA, Ben-Nun J, Cringle SJ. PO2 profiles and oxygen consumption in cat retina with an occluded retinal circulation. Invest Ophthalmol Vis Sci 1990;31:1029–1034.

11.Stefansson E. Ocular oxygenation and the treatment of diabetic retinopathy. Surv Ophthalmol 2006;51:364–380.

17.Arjamaa O, Nikinmaa M. Oxygendependent diseases in the retina: role of hypoxia-inducible factors. Exp Eye Res 2006;83:473–483.

27.Penn JS, Li S, Naash MI. Ambient hypoxia reverses retinal vascular attenuation in a transgenic mouse model of autosomal dominant retinitis pigmentosa. Invest Ophthalmol Vis Sci 2000;41:4007–4013.

29.Budzynski E, Smith JH, Bryar P, et al. Effects of photocoagulation on

intraretinal PO2 in cat. Invest Ophthalmol Vis Sci 2008;49:380– 389.

31.Mervin K, Valter K, Maslim J, et al. Limiting photoreceptor death and deconstruction during experimental retinal detachment: the value of oxygen supplementation. Am J Ophthalmol 1999;128:155–164.

67.Linsenmeier RA, Braun RD. Oxygen distribution and consumption in the cat retina during normoxia and hypoxemia. J Gen Physiol 1992;99:177– 197.

68.Birol G, Wang S, Budzynski E, et al. Oxygen distribution and consumption in the macaque retina. Am J Physiol 2007;293:H1696–H1704.

73.Wang L, Tornquist P, Bill A. Glucose metabolism of the inner retina in pigs in darkness and light. Acta Physiol Scand 1997;160:71–74.

74.Bill A, Sperber GO. Aspects of oxygen and glucose consumption in the retina: effects of high intraocular pressure and light. Graefes Arch Clin Exp Ophthalmol 1990;228:124–127.

76.Winkler BS. A quantitative assessment of glucose metabolism in the isolated rat retina. In: Christen Y, Doly M, DroyLefaix M, et al. (eds) Les Séminaires ophthalmologiques d’IPSEN. Vision et adaptation. Paris: Elsevier, 1995.

81.Yu DY, Cringle SJ, Su EN. Intraretinal oxygen distribution in the monkey retina and the response to systemic hyperoxia. Invest Ophthalmol Vis Sci 2005;46:4728–4733.

83.Medrano CJ, Fox DA. Oxygen consumption in the rat outer and inner retina: lightand pharmacologically induced inhibition. Exp Eye Res 1995;61:273–284.

88.Ames A, Li YY, Heher EG, et al. Energy metabolism of rabbit retina as related to function: high cost of Na transport. J Neurosci 1992;12:840–853.

91.Yamamoto F, Borgula GA, Steinberg RH. Effects of light and darkness on pH outside rod photoreceptors in the cat retina. Exp Eye Res 1992;54:685–697.

Clinical background

The term retinitis pigmentosa (RP) is used to describe a group of inherited disorders in which vision loss is caused by degeneration of the rod and cone photoreceptor cells of the retina. RP occurs in nonsyndromic and syndromic forms. It has recently been recognized that in many of the syndromic disorders of which RP is a part, the common link between the affected tissues is cilia, as the light-sensitive outer segments of photoreceptor cells are specialized sensory cilia (Figure 74.1).

Symptoms and signs

In all types of RP, nyctalopia (night blindness) is often the first symptom noticed. This is due to dysfunction and death of rod photoreceptor cells, which mediate vision under conditions of dim illumination. This symptom is often noticed in adolescence, but the age of onset can be variable, ranging from early childhood to adulthood. Loss of peripheral visual field follows, and is progressive, resulting in constriction of visual fields. Central vision, mediated predominantly by cone photoreceptor cells, is ultimately lost in many cases as well, often in later adulthood.1

The progression of visual symptoms is associated with dysfunction and death of photoreceptor cells and consequent changes in the retinal pigment epithelium, which is visible on fundus exam. For example, in childhood, the fundus may appear relatively healthy, or small regions of depigmentation may be noted prior to the detection of the typical bone spicule pigmentation in the midperiphery (Figure 74.2A). As more photoreceptor cells die, and visual field is lost, fundus abnormalities become more notable, with more prominent bone spicule pigmentation and associated retinal atrophy (Figure 74.2B). Eventually, attenuation of retinal blood vessels and optic atrophy are evident, as further loss of photoreceptor cells and secondary loss of retinal ganglion cells occurs (Box 74.1).

Other degenerations

In addition to RP, many other inherited retinal degenerations have been described. These have been classified clini-

Eric A Pierce

cally by their age of onset, the types of photoreceptor cells affected, the region of the retina involved, and rates of progression. Leber congenital amaurosis (LCA) is a severe earlyonset form of retinal degeneration, in which poor vision associated with nystagmus is evident early in childhood. Cone and cone–rod dystrophies are characterized by earlyonset cone dysfunction, in contrast to RP, in which rods are typically affected first. Congenital stationary night blindness is characterized by early-onset night -blindness like RP, but has a more stable clinical course.2,3

Epidemiology

The prevalence of RP in the USA, Europe, and Japan is approximately 1 in 4000.3 This translates into 1.5 million individuals affected with RP worldwide. Data from the Beijing Eye Study suggest that the prevalence in China may be higher, at approximately 1 in 1000. These data predict approximately 1.3 million people affected with RP in China alone, although this estimate is based on a relatively small sample size compared to the total Chinese population.4 Data from studies in Japan and Denmark and Kuwait indicate that RP is among the leading causes of blindness or visual impairment, especially in working-aged people, accounting for 25–29% cases in that age group (21–60 years).3

Diagnostic workup

Clinical evaluation of a patient with symptoms of RP, such as nyctalopia and decreased visual fields, involves thorough ophthalmic examination, testing of visual function, consideration of systemic evaluations, and genetic testing. Visual acuity may remain normal even in later stages of classic RP, in which rod photoreceptors are affected first. Early loss of central acuity suggests the possibility of early cone photoreceptor dysfunction. Anterior-segment exam is important to rule out other causes of vision loss, and to look for posterior subcapsular cataracts, which develop in up to 50% of patients with RP.5

Visual field testing is important both for detecting field loss for diagnostic purposes, and for following disease status over time. Full-field evaluations using a Goldmann perimeter or Humphrey field analyzer are useful for detecting the midperipheral scotomas typically observed in patients with

Box 74.1  Retinitis pigmentosa – clinical features

Symptoms

•Nyctalopia

•Peripheral field loss

•Progressive loss of central vision later in disease

Exam findings

•Diminished electroretinograms, rod greater than cone

•Bone spicule pigmentation

•Vascular attenuation

•Optic atrophy

RP. Progression of field loss is associated with loss of rod and cone photoreceptor function, resulting in small residual islands of vision.6

Electroretinography

Electroretinogram (ERG) testing is an important diagnostic tool for patients with RP. The ERG measures the function of rod and cone photoreceptors. It is a measure of the field potential generated by the circulating ion currents in photoreceptor cells.7 Standards for the performance of ERGs have been established by the International Society for Clinical Electrophysiology of Vision (ISCEV).8 In the ISCEV standard ERG evaluation, five steps are used to evaluate rod and then cone function: the rod ERG, the combined rod– cone ERG, oscillatory potentials, single flash cone ERG, and

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30-Hz flicker ERG.8 Responses to low levels of white or blue light in dark-adapted subjects are used to evaluate rod photoreceptor function. A brighter standard flash of white light is then used to elicit a maximal or combined response from both the rod and cone photoreceptors. A typical response includes a negatively deflected a-wave, followed by a positive b-wave (Figure 74.3). The a-wave is a measure of the photoreceptor response; the b-wave is thought to be generated by cells in the inner retina.9

Following light adaptation, cone responses to a single white flash are recorded in the presence of background illumination. Finally, flicker ERGs recorded at approximately 30 flashes per second (30 Hz) are used to measure responses from cones; rod photoreceptors cannot recover rapidly enough to respond to the rapid flashes8 (Figure 74.4).

In patients with RP, ERG responses are decreased. Indeed, decreased photoreceptor function can be detected by ERG in children who remain asymptomatic until young adulthood.1 In RP, decreased rod photoreceptor responses are typically noted first, followed by decreases in cone responses. A typical young adult with RP will have reduced amplitudes of both rod and cone responses, and delays in the response times (Figures 74.3 and 74.4). Patients with more severe retinal degeneration, such as early-onset forms of RP or LCA, may have nondetectable ERG responses (Figures 74.3 and 74.4).

ERG amplitudes can provide objective measures of retinal function, and thus are useful for accurate diagnosis and for tracking the course of disease.1,3 It has also been suggested that the amplitudes of the 30-Hz cone response can be used to provide information about visual prognosis.10 The dark

adaptation threshold can also be a useful assessment of rod photoreceptor function. This test measures the lowest intensity of white light that can be perceived in a darkadapted state.11 Optical coherence tomography (OCT) can be useful for monitoring the thickness of the retina in patients with RP.12

Genetic testing

Genetic testing to identify the mutations which cause an individual patient’s disease has become an important part of clinical care of patients with RP and related disorders. This is important because it can help confirm the diagnosis, assist

with family planning, and provide more detailed information about the prognosis of the specific form of RP identified. Genetic diagnoses will also be increasingly important as genetic treatments for RP and related disorders are developed.

Clinical genetic testing for RP is improving, and several clinical labs now provide relevant testing. An up-to-date list can be found at www.genetests.org. Identification of pathogenic mutations in patients with RP can be challenging, due in part to the polygenic nature of these disorders (see below).13 New developments with high-throughput mutation detection and sequencing will hopefully simplify this process in the relatively near future.14,15

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Systemic evaluation

As discussed below, RP and related retinal degenerations are often associated with systemic disorders. It is therefore important to consider potential disease associations when evaluating patients with RP. This is especially true for

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children. Common systemic associations include defects in other sensory or primary cilia, such as hearing loss in Usher syndrome, cystic renal disease in Alstrom, Bardet–Biedl, Joubert, and Senior Loken syndromes. Retinal degeneration can also occur in the setting of mitochondrial disease, and metabolic disorders. Three specific forms of RP that are

important to consider in children are Refsum’s disease (phytanic acid oxidase deficiency), Bassen–Kornzweig syndrome (abetalipoproteinemia), and RP with ataxia caused by α-tocopherol transport protein deficiency, as early intervention can be beneficial in these disorders.16

Differential diagnosis

Other retinal degenerative disorders can present with visual symptoms like those caused by RP. The fundus appearance in gyrate atrophy is distinct from that of RP, with patches of choroidal and retinal atrophy in the midperiphery. Plasma ornithine levels are elevated in this disease, which is caused by deficiency of the ornithine aminotransferase (OAT) gene. Fundus appearance is also helpful for distinguishing choroid­ eremia from RP. In this X-linked condition, RPE and choroidal atrophy are evident on fundus exam.17

Treatment

Several studies have been performed to assess the value of nutritional supplements for patients with RP. A randomized clinical trial demonstrated that vitamin A supplementation slowed the decline of photoreceptor cell loss as measured by 30-Hz cone ERG amplitude in patients with RP.18 Vitamin A supplementation was also associated with slower loss of visual field in the subset of trial patients who performed the visual field tests with the greatest precision.19 Studies of dietary supplementation with the omega-3 fatty acid docosahexaenoic acid (DHA), which is present at relatively high levels in photoreceptor outer-segment membranes, did not show a clear benefit.20,21

Several promising treatments for RP and related disorders are nearing or in clinical trials. The value of sustained intraocular release of the neurotrophic factor ciliary neurotrophic factor (CNTF) is being evaluated in a phase II trial, after being found to be safe in a small phase I study.22 Gene augmentation therapy for specific forms of RP, LCA, and related disorders has shown promise in preclinical and early phase I research studies.23–26 Based on these results, further clinical trials of gene therapy for LCA2, caused by mutations in the RPE65 gene, are currently in progress in England and the USA. RNA interference-mediated knockdown of mutant alleles that cause disease via dominant negative mechanisms is also being evaluated.27 Several approaches to using stem cells for the treatment of inherited retinal degenerations have also shown promise, including the use of RPE-like cells derived from human embryonic stem cells.28

Pathology

Several clinicopathologic studies of the pathology of RP have been reported. These studies show that loss of vision in endstage RP correlates with widespread loss of photoreceptors. In samples from patients with retained vision, a single layer of cones with shortened outer segments was observed in the macula, reflecting the slower loss in cones in most types of RP. Bone spicule pigmentation is caused by migration of RPE cells into the retina, where they cluster around blood vessels.29 Pathologic studies also show that the neural retina undergoes significant remodeling in human RP.30,31 Loss of

Etiology

photoreceptors, especially cones, appears to trigger retinal remodeling, with changes in cell organization and connections. These include neuronal and glial migration, rewiring of retinal circuits with elaboration of new neurites and synapses, and glial hypertrophy.29,32 These remodeling changes may affect the success of potential therapeutic strategies for RP and related disorders.

Etiology

Genetics of RP and related disorders

Nonsyndromic RP

RP and related disorders are caused by mutations in genes which encode proteins that are required for photoreceptor cell function. These disorders are genetically heterogeneous, with over 80 disease genes identified to date (Tables 74.1– 74.3). Indeed, it has recently been pointed out that photo­ receptor cells are subject to more genetic diseases than any other cell type.33 The website RetNet provides a curated listing of disease genes and loci for retinal degenerative disorders.34

It is has been estimated that 65% of RP cases are non­ syndromic.35 Of these, autosomal-recessive RP is the most common form, accounting for 50–60% of cases. This includes simplex cases of RP, which are assumed to be primarily recessive, although dominant mutations have been detected in patients with simplex disease.36,37 Autosomaldominant forms of RP are estimated to account for 30–40% of cases, and X-linked RP for 5–15% of cases.3,35 Other less common modes of inheritance have also been reported, including digenic RP requiring mutations in two genes (PRPH2 and ROM1) and RP due to mutations in mitochondrial genes.38–40

In aggregate, the mutations in the identified genes account for approximately 56% of patients with autosomal RP, 30% of patients with autosomal-recessive RP, and nearly 90% of patients with X-linked RP.3,35 This suggests that many additional disease genes remain to be identified, especially for recessive forms of RP. Consistent with this suggestion, new disease loci continue to be identified; an additional 19 genetic loci for RP and related disorders are listed in RetNet.

Syndromic RP

Approximately 25–30% of individuals with RP have associated nonocular disease. As described in more detail below, the majority of these syndromic forms of RP are cilia-related disorders, including Alstrom, Bardet–Biedl, Joubert, Senior Loken/nephronophthisis and Usher syndromes.3,35,41,42 The most common syndromic form of RP is Usher syndrome, accounting for approximately 10% of RP cases. Mutations in the USH2A gene also cause a significant proportion of recessive RP without hearing loss (Table 74.3 and Box 74.2).3

In Bardet–Biedl syndrome (BBS), RP is found in association with multiple cilia-related disorders, including cystic renal disease, polydactyly, mental retardation, obesity and gonadal malformations, diabetes, and situs inversus.41 To date, mutations in 12 genes have been identified to cause BBS, which is estimated to account for 5% of cases of RP.3,35 In addition to autosomal-recessive inheritance, oligogenic