Recovery after severe ethambutol intoxication - psychophysical and electrophysiological correlations

Six patients with severe ocular side effects caused by therapeutical doses of the tuberculostatic drug ethambutol were investigated during the course of recovery with psychophysical and electrophysiological methods. Three patients developed an optic atrophy with permanently reduced vision as a likely consequence of additional risk factors such as diabetes, alcohol abuse, and reduced kidney function. The severity of the neuritis of the optic nerve was not related to the total intake of ethambutol. The likelihood of a permanent ocular damage increased sharply if the visual acuity had dropped below a value of 1/10. Permanently prolonged latency of the P-100 component was found in visual evoked potentials even in cases with good recovery from ethambutol-induced damage. The recovery of color vision could be monitored very well with the Farnsworth-Munsell 100-Hue Test which revealed a diffuse impairment of color discrimination with a slight prevalence of the red-green axis. In addition to the known disturbances of the red-green antagonistic neurons, it could be demonstrated by measuring transient tritanopia and spectral sensitivity functions that ethambutol also affects the blue-yellow antagonism at the retinal level.