are substantially reduced and delayed, consistent with a severe attenuation of L/M cone input. It is now well-established that this unusual phenotype results from an abnormally large number of S cones which develop in these patients at the expense of rods and L/M cones.

3.1.4.4 Leber Congenital Amaurosis

Leber congenital amaurosis (LCA) is a clinically heterogeneous group of retinal degenerations with an extremely severe phenotype. Low vision is typically noticed soon after birth and is frequently accompanied by nystagmus, an oculodigital sign, and photoaversion. Often, poor vision is apparent in infants whose retinas appear normal. However, the ERG in LCA is either nondetectable or, as in Fig. 3.9, markedly reduced.

Mutations in a number of genes have been identified as causative in LCA. RPE65 is a retinal pigment epithelium-specific protein known to play a key role in the visual cycle [60]. This defect can exist at birth prior to extensive retinal degeneration, thereby raising the possibility that gene therapy may rescue photoreceptors in humans as it has done in animal models [61]. Other LCA genes include a retina-specific guanylate cyclase [62], a cone-rod homeobox protein (CRX) [63, 64], and a photoreceptor/pineal expressed protein (AIPL1) [65].

3.2 Focal and Multifocal ERG

While the full-field ERG can provide an objective, quantitative assessment of widespread retinal disease, it is of relatively little value for assessing disease limited to the macula. Although cone density is highest in the fovea, the absolute number of cones in the macula is low relative to the total number of cones in the retina [66]. Diseases limited to the macula typically do not produce full-field ERG abnormalities since the macula contains only about 7% of the total cone population.

Various focal cone ERG (fERG) techniques have evolved as objective measure of the functional integrity of the macula. An effective method of testing with direct visualization of the fundus is through a hand-held stimulator-ophthalmoscope [67]. A typical commercial instrument produces a white flickering (typically around 40 Hz) stimulus of 3 or 4° centered within a steady white annular surround. The intensity of the annulus is higher than that of the stimulus in order to mask stray light and enable direct visualization of the fundus during testing. The fERG requires a contact lens electrode on the eye and good cooperation from the patient in keeping the eye steady. Thus, it is only feasible in children older than about age 6 and is most useful as a technique for evaluating macular integrity in cases of visual loss of unknown etiology.

As shown in Fig. 3.10, representative responses from the fovea in a normal subject are approximately twice the amplitude of responses from the parafovea (centered

midway between fovea and disk). As a control, stimuli falling on the disk produce little or no response. Because the fERG is generated exclusively by macular cones, there is a correlation between fERG amplitude and Snellen visual acuity in patients with retinal disease involving the macula [68, 69], especially in the range of 20/20 to 20/80. Traditional fERG technology is an extension of full-field ERG technology in that similar recording and signal-averaging techniques are used for both. The difference is in the size of the stimulus. In principle, it would be possible to record fERGs from multiple regions across the retina to determine the topography of retinal sensitivity. In practice, however, time constraints limit recording to two or three locations and preclude screening of large retinal regions by sequential mapping. Furthermore, the sensitivity of the fERG with sequential recordings is compromised by the difficulty of separating true variation among retinal regions from variability in the responses over time.

An innovative solution to these limitations in the traditional fERG is to simultaneously record from multiple retinal areas. This powerful technology originated in the early 90s [70] and is made possible by the advent of powerful yet affordable computers. A crucial recent advance in mfERG technology is the capacity to visualize the fundus during the test session. Commercial mfERG systems deliver the stimulus display through either a modified fundus camera or a scanning laser ophthalmoscope, so that the location of the stimulus array with respect to the fovea and the disk can be monitored. It is possible to perform reliable mfERG testing in children. With extra coaxing, children who are 10 years and older can be tested in the normal manner. Children under 6 years of age (dependent on body weight) can be tested successfully while sedated. At centers like The Hospital for Sick Children (SickKids) in Toronto, Canada, the child is sedated in the ophthalmology sedation unit after he/she has been deemed medically fit for sedation. Presedation feeding instructions include no solid food within the previous 12 h, no formula within 6 h, and no clear fluids within 3 h. Chloral hydrate is administered orally (25–80 mg/kg of body weight; maximum single dose of 2 or 1 g for those weighing 15 kg or less). Before ERG assessment, both pupils are dilated pharmacologically with 1% cyclopentolate and 2.5% phenylephrine in children and with 0.5% cyclopentolate in infants less than 6 months. The child is swaddled and placed on a movable stretcher; vital signs are monitored at all times by a registered nurse. The child, on the stretcher, is moved to the Visual

Electrophysiology Unit. Before testing, one drop of proparacaine 0.5% is placed in the lower fornix. A bipolar Burian–Allen electrode with built-in infrared port (Electro-diagnostic Imaging, San Mateo, CA, USA) of the appropriate size for the individual child’s eye is placed on the corneal surface. A gold-disc electrode, filled with electro-conductive paste, is attached to the forehead to serve as the ground.

The stimulus for the multifocal ERG (mfERG) consists of one of a large number of available arrays. In general, the elements in an array are scaled with eccentricity so that focal retinal responses with approximately equal signal/noise ratios are produced across the field in normal subjects. At any given moment, about 50% of the locations in the array are at a high luminance (white) and the other locations are at a low luminance (black). The stimulation rate is the number of times per second that display is changed and is a multiple of the frame rate of the video monitor (typically 75 Hz). With every change in stimulation, each element in the array has a probability of 0.5 of being light or dark. Each element in the array is stimulated with the same sequence (an m-sequence) of light and dark, but this sequence is lagged by different amounts for each location. The responses associated with these elements are effectively uncorrelated when the lags are much greater than the duration of the response. The local response for each element in the array is computed as the cross correlation between the m-sequence and the response cycle. This response, the first order component, can be thought of as the average response from a particular retinal area unaffected by stimulation at any other region in the array, i.e., a linear approximation of the response of the small retinal region. A nonlinear response, the second order component, represents temporal interactions between flashes when the lags are short relative to the duration of the response.

For testing infants, the classical m-sequence stimulation mode is used to stimulate an array of either 61 hexagons or 103 hexagons covering the central radius of 15–20° of the retina. This stimulus is presented using a cathode ray tube mounted on an extendable arm. To ensure optimal recording, the stimulated area of the retina must be monitored continuously. This is achieved using an infrared fundus camera mounted parallel to the stimulator. Data are collected in eight 30 s intervals. After each 30 s testing period, the reliability of the signal and the patients’ fixation is assessed and the segment is repeated if necessary. Test duration is approximately 10 min for each eye tested (Fig. 3.11).

Fig. 3.11  Sedated child lying supine on stretcher. Appropriately sized corneal contact lens electrode with built-in infrared illuminator can be seen. Stimulus is presented via cathode ray tube on extended arm of the mfERG system. Stimulus array (61 hexagons) can be seen superimposed on the retina during monocular testing. (Photo: Carmelina Trimboli; Courtesy of CA Westall, PhD, Department of Ophthalmology and Visual Sciences, The Hospital for Sick Children, Toronto, Canada)

The traditional output from the mfERG is a topographicdisplayofminiaturewaveforms.Theseresponses bear a superficial resemblance to the full-field ERG in showing both a negative and positive peak, but they do not match the full-field waveform in complexity or implicit time due primarily to differences in stimuli [71]. It is often useful to average concentric rings of amplitude and to generate three-dimensional representations of the response pattern as shown for the normal infant in Fig. 3.12. With this kind of ring analysis, a paracentral scotoma is readily evident (Fig. 3.13) in a patient with Stargardt macular dystrophy (STGD) sparing the fovea. Alternately, young patients with STGD may lose amplitude in the fovea (Fig. 3.14). At more peripheral loci, amplitudes from patients with STGD may approach those of normal subjects and implicit times are not markedly delayed at any eccentricity. In the future, the mfERG may be of considerable use in treatment trials for young patients with diseases such as STGD, for which the full-field ERG has been of little value.

Fig. 3.13  MfERG from patient with STGD and macular sparing. (a) fundus appearance, (b) raw waveforms, (c) six annular rings used for annular averages, (d) 3D representation showing

parafoveal scotoma with foveal sparing, and (e) responses and parameters from each annular ring

The mfERG is becoming increasingly popular as a technique for exploring local ERG responses from the macula. Although the requirements of steady fixation present a challenge in the pediatric patient, an increasing number of clinics are attempting to make this technology available. As the amplitude of the foveal ERG response is so closely related to visual acuity in patients with macular disease [68], the mfERG is particularly useful in low vision children. A normal mfERG test result is compelling evidence for the integrity of the macula and suggests a postretinal cause for reduced visual acuity. Alternately, a normal mfERG test may suggest a psychological component to reduced acuity. The mfERG is also proving to be of value for monitoring retinal health in patients taking medications with suspected or known retinal toxicity. Vigabatrin, for example, is effective in patients with drug-resistant

epilepsy, but is known to cause visual field constriction. Several authors have identified electrodiagnostic abnormalities in the mfERG in patients on vigabatrin therapy [72–74], suggesting that this test may be sensitive for detecting an early effect on outer retinal function that may be reversible after cessation of vigabatrin treatment.

In summary, electroretinographic testing provides a comprehensive assessment of retinal function in children. Traditional full-field ERG techniques are appropriate for children from families with a history of hereditary retinal degeneration, for infants with unexplained visual loss, for children with subjective loss of night vision, and for infants with certain inborn errors of metabolism involving the eye. Focal and mfERG techniques are finding increasing acceptance in pediatric ophthalmology for regional or localized disorders possibly affecting the macula.