Purpose: To describe the implicit time topography of multifocal electroretinograms in normal subjects and to examine the change in this topography in patients affected by retinitis pigmentosa.
Methods: Thirty normal subjects and 38 patients with retinitis pigmentosa were examined with the Visual Evoked Response Imaging System using 61 hexagonal elements within a visual field of 30 degrees radius. The peak implicit times of the 61 first-order kernels (which are analogues of the photopic electroretinogram [ERG]) were measured to determine their distribution across the retina.
Results: Implicit times had a low interindividual variability in the normal group. High implicit times were found at the blind spot, the upper and lower borders of the stimulated field, and the macula. Low values were present in the area encircling the macula and were most prominent in the temporal retina. In the group with retinitis pigmentosa, implicit times were unchanged in the central region but were prolonged in the peripheral regions.
Conclusions: The spatial distribution of multifocal ERG implicit times in a normal population follows a specific topographical pattern across the retina. This pattern has to be taken into account when interpreting results in patients. Deviations in retinitis pigmentosa were found, and they show the potential for diagnostic use.