Cone and rod ERGs in degenerations of central retina.

Sixty-four patients with retinal degenerations at the posterior pole were reviewed and their ERGs analysed. On the basis of symptoms, visual acuity, ophthalmoscopy and cone/rod ERG the patients were divided into five diagnostic groups: Stargardt's disease, fundus flavimaculatus, cone degeneration, dominant drusen and central retinitis pigmentosa (RP). Stargardt's disease and fundus flavimaculatus show low-normal or subnormal cone b-wave amplitudes, often with prolonged peak times; the rod ERG is rarely abnormal. Cone degeneration presents with reduced visual acuity, photophobia, nystagmus and minimal fundus changes. The ERG shows severely reduced cone b-waves and increased peak times; the rod b-waves are in the low-normal or subnormal range. Dominantly inherited drusen, included for comparison, revealed little change in the ERG in spite of widespread ophthalmoscopic changes: b-wave amplitudes fall mostly in the low-normal range, and their peak times may be prolonged. RP of the central type reveals considerable variability in all clinical aspects, but the cone and rod ERGs are consistently greatly reduced, showing markedly increased peak times of the cone b-waves. Recording of the Ganzfeld ERG with cone/rod separation thus proves useful in differentiating degenerations of the central retina.