Topography of cone electrophysiology in the enhanced S cone syndrome

PURPOSE. To investigate the topography of cone electroretinographic (ERG) r esponses in the enhanced S cone syndrome (ESCS). METHODS. A 19-year-old female with ESCS who was one of the original cases d efining the syndrome was studied. Full-field, focal (Maculoscope) and multi focal (VERIS) ERGs were performed using white light. Multifocal ERG respons es 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 ide ntically from a normal subject. RESULTS. The full-field ERG from this patient showed typical large slow pho topic waveforms and was unchanged from recordings made 9 years earlier. The focal ERG showed signals of borderline low amplitude from the fovea with t he multifocal ERG, the ESCS responses from the central macula had a relativ ely normal waveform, and those 9 degrees to 20 degrees from fixation showed the prolonged waveform that characterizes the full-field ERG. Responses we re 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 a nd 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 resp onses 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 pat hways 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 con e 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.