Table 2 Non-genetic causes of negative ERGs.
Aetiology | Disease | Unilateral or bilateral |
|---|---|---|
Vascular | CRAO: negative DA ERG and reduction in LA b-wave | Usually unilateral |
Ischaemic CRVO: negative DA ERG or reduced b:a ratio | Usually unilateral | |
Systemic or drug toxicity | Ingestion/overdose of agents including quinine, vigabatrin, methanol | Bilateral |
Direct ocular toxicity | Siderosis from an intraocular iron foreign body causes progressive ERG decline, affecting b-wave earlier than a-wave. | Usually unilateral |
Autoimmune paraneoplastic | MAR gives an ERG phenotype similar to complete CSNB (selective ON pathway impairment) due to antiTRPM1 antibodies. CAR can cause both a-wave and b-wave reduction, but can also give rise to a negative ERG | Usually bilateral, but can be asymmetric |
Autoimmune non-paraneoplastic | Non-paraneoplastic autoimmune retinopathy can cause both a-wave and b-wave reduction, but can also give rise to a negative ERG | Usually bilateral, but can be asymmetric |
Other inflammatory | Birdshot uveitis can cause both a-wave and b-wave reduction, as well as 30 Hz flicker amplitude reduction and peak time delay. Negative ERGs occur commonly (sometimes with increased a-wave amplitude) | Unilateral or bilateral, often asymmetric |
Inflammatory or infective occlusive vasculitis can lead to negative ERGs (due to inner retinal ischaemia) | Unilateral or bilateral, often asymmetric | |
Nutritional | Vitamin A deficiency selectively reduces rod responses, and a negative DA ERG (with subnormal a-wave) can result, representing the dark-adapted cone system response | Bilateral |
