Clinical ERG findings in ethambutol intoxication.

The human electroretinogram (ERG), evoked by white flashes of extremely short duration (10 microseconds), shows a typical dependence on flash intensity. Increasing stimulus intensity increases the amplitude of the a-wave until saturation is reached. The amplitude of the b-wave reaches a maximum value with flashes of middle intensity, then decreases at higher stimulus strengths. The values of a-wave amplitude saturation, defined as 100%, may serve as a basis for standardizing the various amplitude-intensity relationships of the a- and b-wave. The b-wave function, calculated in this way, shows different maximum values depending on whether it was determined after light adaptation or in dark adaptation and low interindividual variability. However, the difference between bmax in dark and light adaptation is markedly decreased in the case of ethambutol intoxication. ERG changes are only detected in severe cases (total central scotoma) and are below the discrimination level in cases with moderate symptoms (relative central scotoma, visual acuity greater than 0.4-0.5).