PURPOSE: To study the functional state of the retina beneath an epimacular membrane by means of focal macular electroretinography. METHODS: Focal macular electroretinograms of 30 consecutive patients (age 25 to 75 years) with unilateral idiopathic epimacular membrane were recorded prospectively. The amplitudes and implicit times of the a and b waves and the oscillatory potentials (O1, O2, O3) recorded from the eye with idiopathic epimacular membrane were compared with those of the normal fellow eye. RESULTS: There was a statistically significant reduction in the amplitude of the a wave (P < .001), the b wave (P < .001), and the oscillatory potentials (O1 + O2 + O3; P < .001) of the affected eyes. The reduction of the b-wave amplitude was significantly greater than that of the a wave, resulting in a lower b/a wave ratio (P = .003) in the affected eyes. The reduction in the amplitude of the oscillatory potentials was significantly greater than that of the other two components (P .001). The implicit times were significantly prolonged for the a wave (P < .001), the b wave (P = .004), and O3 (P < .001). There was a significant correlation between relative b-wave amplitude (affected/normal fellow eye) and the visual acuity (r = -0.50, P = .007). These findings were similar to those we reported for eyes with aphakic cystoid macular edema. CONCLUSIONS: The focal macular electroretinograms in eyes with epimacular membrane disclosed abnormalities that were similar to those of eyes with cystoid macular edema. This suggests that the epimacular membrane probably induced damage to the neurons in the inner retinal layers. Dysfunction of these neurons is one possible cause of visual impairment in this disease.
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