P 143Clinical relevance of the cone isolating multifocal ERG
H. Jaegle, J. Albrecht, A. Grodon, L. T. Sharpe
Introduction: To develop procedures for recording cone-isolating mfERGs in normals and clinical patients with a loss in cone function.
Methods: The VERIS system (Ver. 3.0.1) was used in combination with "silent substitution" techniques, based on the Stockman & Sharpe (1999) cone fundamentals, to isolate responses from individual cone classes in the human mFERG. The stimuli were generated on a Sony Trinitron F500 monitor. Corneal ERG responses were recorded with a DTL fiber electrode. Michelson contrasts of 47%, 47% and 92% were achieved for the L-, M- and S-cone stimuli, respectively. 103 locations were stimulated in the visual field of view of 90° x 90° at a viewing distance of 16 cm. Subjects were normals or patients with either dominant or nondominant forms of RP with a visual acuity better than 0.6 and stable fixation.
Results: In normals, the latencies for the L- and M-cone driven responses in the central fovea were c. 30 ms with the M-cone preceding the L-cone driven response by c. 3 ms. The S-cone driven response latency in the center was c. 36 ms with a late component at c. 65 ms. All types of latency decreased with retinal eccentricity. In both forms of RP, the S-cone driven signals were more severely impaired than the L- or M-cone driven ones. In less advanced cases, no S-cone driven signals could be measured at all in the pathological retinal regions (as revealed by visual field defects); whereas, in the normal appearing regions, S-cone driven amplitudes were reduced by 30 - 40% and their latencies increased from 65 ms up to c. 80 ms. In pathological regions, the M-cone driven signals were more impaired than the L- cone driven ones, but both types had reduced amplitudes and increased latencies. In more advanced cases of RP, the amplitudes of the three cone-driven signals were severely reduced (by c. 90%) and their latencies were very prolonged across the entire retina.
Diskussion: The topography of the L-, M- and S-cone driven signals can be measured in color normals as well as patients with RP using the multifocal ERG. The method can be used to assess localized damage to all three cone classes. Typically, in the early stages of RP, the S-cone driven signals are more severely impaired than the M- and L-cone driven ones.
University Eye Clinic, Department for Pathophysiology of Vision and Neuro-Ophthalmology, Schleichstr. 12-16, D-72076 Tuebingen