The effect of light scattering on multifocal electroretinography

Purpose: Unclear ocular media is a very common condition of older eyes characterized by significant light scattering and image degradation. The multifocal electroretinography (MERG) is a useful objective technique to measure retinal activity but its validity in the presence of cloudy ocular media remains unclear. We tested the MERG under controlled light scattering conditions using a liquid crystal diffuser (LCD) that simulated different degrees of image degradation. Methods: The MERG were taken from 13 normal young subjects seated behind a LCD set under two conditions: scatter (visual acuity -6/18) and non-scatter (visual acuity -6/6). The pupils had been dilated and the eyes were optically corrected for the working distance. The first-order kernel MERG response was analysed. Three subjects underwent MERG measurement with two additional intermediate light scattering levels (i.e. visual acuity ∼6/9 and 6/12). Results: The macular MERG response density was reduced (p < 0.001), but the peripheral MERG response densities were increased (p < 0.001) under the scattering condition. A similar trend was also observed with intermediate degrees of light scattering. Comparing the MERG waveforms without light scattering, a new retinal response was identified with a characteristic latency of about 60 ms (P60), but it was diminished in size under the scattering conditions. Conclusions: The veiling luminance might have caused the reduction in the macular MERG response and an elevation in the peripheral retina with light scatter. The functional suppression of P60 observed under the influence of light scatter may be related to retinal adaptation. Unclear optical media will affect the interpretation of MERG results.