Topography of Cone Electrophysiology in the Enhanced S Cone Syndrome

Abstract

purpose. To investigate the topography of cone electroretinographic (ERG) responses in the enhanced S cone syndrome (ESCS).

methods. A 19-year-old female with ESCS who was one of the original cases defining the syndrome was studied. Full-field, focal (Maculoscope) and multifocal (VERIS) ERGs were performed using white light. Multifocal ERG responses were also generated with red and blue stimuli and with a slow m-sequence to elicit off-responses. Results were analyzed by averaging data in rings at increasing eccentricity from the fovea and compared to data recorded identically from a normal subject.

results. The full-field ERG from this patient showed typical large slow photopic waveforms and was unchanged from recordings made 9 years earlier. The focal ERG showed signals of borderline low amplitude from the fovea with the multifocal ERG, the ESCS responses from the central macula had a relatively normal waveform, and those 9° to 20° from fixation showed the prolonged waveform that characterizes the full-field ERG. Responses were larger to blue light than red light in ESCS in both center and periphery. The central ESCS responses were relatively normal in timing to both red and blue light, whereas the peripheral ESCS responses were markedly delayed to both. Off-responses were seen in ESCS only near the foveal center.

conclusions. The marked differences between central and peripheral ERG responses in ESCS suggest that there are different distributions of S, L, and M cones in these regions and that S cones may feed into different neural pathways in the center and periphery. It was postulated that in ESCS, S cones may partially replace L and M cones centrally and feed into the usual S cone pathways. In the periphery, however, there is little L and M cone b-wave activity in ESCS, and S cones may usurp both the space and neural pathways of the rods.

The enhanced S cone syndrome (ESCS) is a rare inherited retinal disorder characterized by an absence of rod function and by large-amplitude S cone–mediated electrical responses that dominate the photopic electroretinogram. ^{1 2} It appears to be a relatively stationary abnormality with respect to rod and S-cone function, although cystic maculopathy may develop in some patients. Clinically the fundus shows yellowish retinal pigment epithelial lesions in the region of the retinal vascular arcades. ESCS may lie on a spectrum with patients described as having Goldmann–Favre disease who show extensive retinal pigment epithelial and retinal degeneration with low-amplitude electroretinographic (ERG) signals but otherwise similar ERG characteristics. 

Although several papers have demonstrated S cone spectral sensitivity for the conventional ERG responses of these patients, the amount of L and M cone activity has not been determined. ESCS patients have good acuity (in the absence of cystic maculopathy) and normal central color vision testing, which indicates that trichromatic cone function and circuitry must be present at least in the central macula. Evidence has been presented by Hood et al. ³ that ESCS patients have an abnormally large number of S cones—estimated at upwards of 75 times the normal number (S cones are normally only a small percentage of the cone population ⁴ ). To make room for these cones, Hood et al. speculated that S cones developing earlier than rods may in some way limit the development of rods. 

Using the new technique of multifocal electroretinography ⁵ and focal electroretinography, ⁶ we have investigated the topography of cone responses in one of the original patients from whom the ESCS was recognized and defined. The results give a 9-year follow-up on ERG abnormalities and show ERG differences between the central macula and the extramacular retina that have implications for the distribution of S, L, and M cones and their neural connections. 

Methods

This study conformed to the tenets of the Declaration of Helsinki. Both patient and control subjects gave informed consent, and the research was approved by our institutional Panel on Human Subjects in Medical Research. The subject of this study was the 19-year-old female whose unusual ERG findings led to the description of the ESCS. ^{1 7} She was reexamined clinically and Goldmann fields obtained. Color vision was tested with the Farnsworth Panel D-15 test and the Lanthony Desaturated Panel D-15 test. Conventional ERG recordings were obtained with an LKC UTAS recording system (LKC Instruments, Gaithersburg, MD), using bipolar contact lens electrodes (Hansen Laboratories, Iowa City, IA, or Doran Instruments, Littleton, MA) and stimulus conditions that conformed to the International Society for Clinical Electrophysiology of Vision Standard for Electroretinography. ⁸ The eyes were dilated with 1% tropicamide and 2.5% phenylephrine, and corneal anesthesia produced with 0.5% proparacaine. Focal ERGs were obtained with a Maculoscope (Doran Instruments), a modified direct ophthalmoscope that projects a bright annulus onto the retina, with a rapidly flickering core that elicits a focal cone ERG. ⁶  

The multifocal records were recorded with a VERIS recording system (Tomey Instruments, Nagoya, Japan, and ElectroDiagnostic Imaging, San Mateo, CA). Stimuli were presented on a high luminance monochrome monitor with a P104 phosphor, as a 103- or 241-element array of hexagonal cells. The pupils were dilated, and contact lenses were used as for the conventional ERG. The overall stimulated field measured approximately 50° in diameter. The multifocal stimulus was binary between a bright and a dark phase. In all experiments stimulus contrast was higher than 98% measured on the screen. Amplifier filter settings encompassed a band-pass of 1 to 300 Hz. The retinal response distributions to chromatic and achromatic stimulations were compared. The bright phase of the achromatic (white) stimulus was 200 candela per meters squared (cd/m²). Red and blue stimuli were generated by viewing the same display through Wratten 29 and 47B filters, respectively. The bright phase of the red stimulus measured 15 cd/m, ² and that of the blue stimulus 5 cd/m². The identical protocols were also run on a normal control subject, a 37-year-old female with 20/20 visual acuity and no retinal disease. 

In most recordings the pseudorandom multifocal stimulus was updated at the video rate of 75 frames per second in accordance with a binary m-sequence (here considered as a sequence of + and −). When the element of binary m-sequence was +, the focal stimulus was bright and when it was −, the focal stimulus was dark. In an attempt to distinguish the on- and off-phase of the response the stimulation was slowed down and modified as follows: The pseudorandom stimulus was updated only after every 16 video frames. When the element of binary m-sequence was +, the focal stimulation was left in the bright state for 8 frames and in the dark state for the second half of the base interval. When the controlling element was −, the patch was dark for the entire base interval. Note, however, that during its bright phase a stimulus patch was not continuously bright. The short persistence screen phosphor was scanned 8 times at 13.3-msec intervals. This resulted in 8 light flashes 13.3 msec apart. Although this flicker within the central 50° of the visual field could not be resolved perceptually, it was reflected in the normal ERG response. Using a television monitor for this mode of stimulation, one can only roughly approximate the experimental condition of a continuous long flash of light for the discrimination of the on- and off-responses. 

Results

The conventional ERG of our ESCS patient showed no dark-adapted rod response to a dim flash but a large, slow, rod-like ERG waveform to a strong stimulus that was nearly identical after dark- or light-adaptation (Fig. 1) . This response has been shown to have the spectral sensitivity of S cones. ² This full-field ERG response was essentially unchanged over nearly 9 years since her initial evaluation. ⁷ There also have been no changes in visual acuity (OD 20/25, OS 20/20), color vision (normal to Panel D-15 tests), Goldmann visual fields (no scotomas or constriction), or fundus appearance (stable sparse yellow lesions in the arcade regions). 

Maculoscope recordings showed borderline normal amplitudes (Fig. 2) when the 4° stimulus was centered on the fovea. There was no response when the stimulus was centered 5° off-center in the parafovea, but responses were obtained from stimuli centered at the foveal edge (2° off-center). 

Figure 3 shows a sample multifocal ERG stimulus array and response arrays from both the ESCS patient and a normal subject. The ESCS responses consisted mostly of monophasic “negative” waveforms except near the center of the macula. The dependence of ESCS responses on eccentricity can be seen better when responses from concentric rings of stimulus cells are averaged together. Figure 4 shows such ring averages for our ESCS patient and a normal subject. The central ESCS response rings (1–4) lack a normal b-wave peak near 28 msec, but have a peak near 35 msec (↓). In the peripheral rings (6–9), however, the ESCS waveform is extremely prolonged, with a slow a-wave and with a b-wave peak near 60 msec (⇓). We use the terms“ a-wave” and “b-wave” here by convention to describe the initial negative, and subsequent positive, major waves of the multifocal ERG. The cellular origin of these waves may not be identical in multifocal and conventional ERGs, but there is evidence for a good deal of homology. ⁹  

Figure 5 shows ring-averaged multifocal ERGs to red and blue stimuli. The ESCS responses were in general of normal amplitude to red stimuli but larger by roughly a factor of two to blue stimuli. The central ESCS responses (rings 1 and 2) to red stimuli showed relatively normal b-wave peak timing (○), although the ESCS waveforms do not show the normal descent after the b-wave. The central ESCS responses to blue also had a relatively normal waveform but a somewhat more delayed b-wave peak at approximately 45 msec (•). A small residual peak, with the same time-to-peak as the response to red, is visible on the rising waveform (○). The peripheral ESCS responses (rings 4–6) were much slower than normal to both red and blue stimuli, with a prolonged a-wave and a delayed b-wave. The ESCS b-wave time-to-peak was much slower to blue light (♦; almost 80 msec) than to red light (⋄; near 55 msec). A small hump with the timing of the red b-wave (near 55 msec) is seen with a blue stimulation (⋄). 

Figure 6 shows multifocal off-responses, produced with slow m-sequence blue-light stimulation and a long recording interval (see the Methods section) in a normal subject and in our ESCS patient. A small off-response from the central pixels was observed in our ESCS patient, which had latency similar to that of the normal subject. This response was not evident from rings beyond 7° off-center, even though the normal response grows in size. The oscillations that appear during the on-phase in the periphery of the normal subject are responses to the individual frames of the CRT display (13.33-msec intervals). The ERG of the ESCS patient did not follow this high frequency component of the stimulus with blue light stimulation. 

Discussion

The full-field ERG responses of our ESCS patient are the typical slow waveforms of this syndrome (indeed, this subject was one of the initial cases that defined this syndrome), ^{1 7} and the lack of progression in 9 years is consistent with other reports that suggest a stationary or extremely slow progressive disorder. ¹⁰  

The normal multifocal ERG to white light shows little change in ERG waveform across the posterior pole. Our results show that the multifocal ERG in ESCS is characterized by a marked difference between responses from the central macula (within 5–7° of the foveal center) and those from more peripheral regions (7–20° eccentricity). In the central area the ESCS responses had a relatively normal waveform that was only modestly delayed in a-wave and b-wave time-to-peak. In the more peripheral regions the waveforms showed a striking prolongation of the a-wave, and the b-wave time-to-peak was much slower than normal. The peripheral responses to white light match closely the appearance and timing of the full-field ERG in ESCS. 

The results to colored stimulation are more complicated. The full-field ERG in ESCS shows much greater blue sensitivity than normal. However, in the areas of posterior retina covered by the multifocal ERG, we found blue responses to be only about twice normal in amplitude, whereas red responses were of normal size. In the central 7° of eccentricity, the ESCS responses to red and blue were relatively normal in waveform (a bit delayed to blue stimuli). But in the more peripheral areas (9–20° from center), the ESCS waveforms to both red and blue were very prolonged and there was a marked difference in b-wave time-to-peak between stimulation with red (50 msec) or blue (80 msec) light. 

How can these findings be explained? In a normal eye S cones are absent in the central 100 μg of the foveola, but constitute 1% to 2% of the cones in the fovea (with a density of 1–2000/mm²), and approximately 8% of cones at 4° from the center. ⁴ If S cones increase to 75 times normal density and replace rods in ESCS, as suggested by Hood et al., ³ there would be plenty of room for S cones outside the macula where rods can reach a maximum density of approximately 140,000/mm², and S cones are normally only 600/mm². However, there would not be enough room for a 75-fold increase in S cones added to the normal population of L and M cones in the fovea (with a peak density of 140,000 mm²). ¹¹ Any large increase in S cones in the foveal region would, to some degree, have to be at the expense of L and M cones. 

ESCS patients have trichromatic cone function centrally because they have good acuity and color vision. However, the full-field cone ERG signal to red light or 30-Hz flicker is weak. One possibility is that ESCS patients have modestly reduced numbers of central L and M cones (perhaps 50% of normal, as suggested by the borderline maculoscope amplitudes), whereas the loss of peripheral L and M cones is more severe (perhaps to 10%–20% of normal) to account for the low full-field L and M cone ERG. An alternative possibility is that the peripheral L and M cone pathways are abnormal so that the b-wave responses are altered (as discussed further below). S-responding cones in ESCS may have a density roughly 75 times normal over most of the peripheral retina, but are perhaps only 20 to 30 times normal in the central macula to allow space for the retained L and M cones. These distributions would account in many respects for our ERG amplitude results. The central ERG in ESCS is more blue- than red-sensitive, because with blue stimuli there would be by this estimation 20 to 30 times more S cones than normally available to produce a response. However, the response to red light should represent L and M cones only, because the S cone spectral sensitivity curve does not reach significantly into the band-pass of the red Wratten 29 filter. We thus conclude that the peripheral response to red light owes its highly abnormal waveform to an abnormal L and M cone pathway in the ESCS patient. On the other hand, the spectral sensitivity of the M cones does extend into the band-pass of the blue Wratten 47B filter. This is confirmed by the relatively normal-looking L and M cone responses to blue stimulation of the normal eye. In the ESCS patient, however, blue stimulation produces additional waveforms that are not seen with the red stimulus and that we presume represent responses from the large numbers of S cones. The central b-wave has acquired a second peak at around 42 msec (•), and there is a large and broad late peripheral component peaking near 80 msec (♦), to which we found no homologous feature in the normal eye. 

The receptor potentials of L, M, and S cones have been shown to be essentially identical. ¹² Thus, the differences in b-wave waveforms between normal S cones and normal L and M cones must reflect differences in the pathway of these signals through the retinal circuitry. Although L and M cones feed onto on- and off-bipolar cells, both rods and S cones are believed to use predominantly an on-bipolar pathway. This may underlie why the normal S cone ERG is slower and more rod-like than the conventional L- and M-dominated cone ERG. Alternatively, S cones may excite different systems of horizontal cells, other integrative cells, or both, which modulate and slow the development of the inner retinal potential changes that create the b-wave. Greenstein et al. have shown that the S-sensitive cells in ESCS are cones rather than rods. ¹³ The normal trichromatic vision in ESCS suggests that L, M, and S cones centrally communicate through relatively normal pathways. Indeed, the normal S cone ERG has a time-to-peak of approximately 40 msec, ¹⁴ which is similar to the 45-msec b-wave peak to blue light in the central rings of our ESCS patient. Outside of the central macula, however, this 45-msec peak diminishes rapidly and is overshadowed by a larger and slower response that peaks closer to 80 msec (which is long even for a rod response). It is conceivable that in the central macula of ESCS, the S cones use predominantly normal S cone pathways through the retina, whereas in the periphery where the massive numbers of S cones occupy space normally occupied by rods, the S cones communicate abnormally through rod pathways and generate an unusually slow b-wave response. 

These explanations may also apply to the changes in a-wave time-to-peak between the center and periphery. However, some of this apparent shift in a-wave time-to-peak may reflect the fact that a slower b-wave process in the periphery would be slower to reverse the negative response of the a-wave. 

One puzzling aspect of our data is the difference in the timing of ESCS responses to red and blue light. In the central area, red light induces a rapid b-wave that is delayed only slightly relative to normal. This probably represents primarily a response of the L and M cones that are present centrally in ESCS but may include a contribution from the normal S cone ERG. However, it is harder to explain why the prominent ESCS response to red light in the periphery peaks near 55 msec, and thus is too slow for a normal S cone response but is more rapid than the peripheral (possibly rod-pathway) S cone response in ESCS. This difference in timing between red and blue peripheral responses in ESCS is unlikely to be a result of differences in the effectiveness of our red and blue stimuli on the S cone system alone, because the latter should not respond at all to the red light. It seems more likely that the peripheral L and M cones are in some way using or interacting with the peripheral S cone pathways, rod on-pathways, or both. Neural connections between different bipolar pathways are known at the amacrine cell level, ¹⁵ although it is not known how they affect ERG recordings. The timing differences between red and blue peripheral responses may also reflect some aspect of the interactive mechanism that leads to the differences in b-wave timing between normal S and normal L and M cones, or reflect other differences in inner retinal pathways that are unknown at present. The white light waveforms in ESCS (↓ and ⇓ in Fig. 4 ) have times-to-peak that are faster than the blue-stimulus times-to-peak. These may in part represent summations of the red and blue waveforms but may also reflect the tendency for b-waves to show faster times-to-peak with brighter stimuli. The responses from the normal eye were also faster to white stimuli, which in these experiments were 100-fold brighter than red and blue. 

Some further support for these general speculations on cone distribution comes from our data on the on–off responses. Our data from the normal subject are consistent with the finding of Kondo et al. ¹⁶ that the amplitude of the off-response, relative to the b-wave, increases with eccentricity. Our ESCS patient showed off-responses in the central retina that were close in latency to those from normal cones, but those in the peripheral retina seemed to be absent. This can be explained if off-responses from the center are dominated by the L and M cone circuitry, which appears to be functional in this region of the ESCS eye. Off-responses have been observed in the full-field ERG of some ESCS eyes, even though S cone pathways seem primarily to use on-bipolar cells. ¹⁷ Our results suggest that regional differences may be relevant to understanding these observations. 

 Reprint requests: Michael F. Marmor, Department of Ophthalmology, Room A-157, Stanford University Medical Center, Stanford, CA 94305-5308.

 Submitted for publication September 15, 1998; revised January 22, 1999; accepted March 3, 1999.

 Proprietary Interest: N (Marmor and Tan), P (Sutter), E (Bearse).

 

Figure 1.

 

Full-field ERGs from our ESCS patient at almost 11 and at 19 years old.

Figure 2.

 

Focal ERG (Maculoscope) data in our ESCS patient. The stimulus is a flickering 4° circle within a bright annulus of light that spans 10°, as shown in the figure. Recordings were made with the subject fixating centrally, at the edge of the stimulus circle, or at the outer edge of the annulus to stimulate the foveal center, foveal edge, and parafovea, respectively. The range of normal values is shown for comparison.

Figure 3.

 

Multifocal ERG arrays. Top: Sample 241-stimulus cell array. During testing, each hexagon flickers on and off according to the m-sequence. The location of 20° eccentricity is shown. Middle: Topographic array of ERG responses in our ESCS patient. Note the slow negative waveform except in the center of the macula. Bottom: ERG array from a normal subject for comparison.

Figure 4.

 

Ring averages of multifocal ERG responses to white light in our ESCS patient. Ring 1 is the central response; ring 2 is the average of responses in the first ring about the center; ring 3 is the average of the next ring of responses; and so forth. The symbols ↓ and ⇓ show the timing of b-wave peaks in the central and peripheral regions, respectively. Normal responses are superimposed for comparison, with a dashed line drawn through the b-wave peaks.

Figure 5.

 

Ring responses in ESCS to red (left) and blue (right) stimuli. The data are presented as in Figure 4 , with ring 1 the most central, and ring 6 the most peripheral (near 20° from fixation). The number of rings is smaller than in Figure 4 because these responses were recorded from an array of 103 rather than 241 stimulus cells to improve signal-to-noise ratio. The open symbols (○ and ⋄) show the timing of b-wave peaks to red stimuli in the center and periphery, respectively. The filled symbols (• and ♦) show similarly the timing of b-wave peaks to blue stimuli. Normal responses, recorded under the same conditions, are superimposed as in Figure 4 .

Figure 6.

 

Ring averages of blue-light multifocal ERG off-responses in our ESCS patient and a normal subject. Off-responses (indicated by the dashed lines) are seen clearly only in rings 1 and 2 of the ESCS patient, whereas they are prominent in all rings of the normal eye and increase in size with eccentricity.