Ethambutol retinal toxicity: An electrophysiologic study

Background and purpose: In animal studies, ethambutol (EMB) has been shown to be toxic to cone pedicles and to cause their degeneration in the retinas of fish. The purpose of this study was to determine whether EMB is toxic t o retinas in humans. Methods: Twenty-seven patients with EMB-induced optic neuropathy and 20 nor mal control subjects were included in this study. The following details wer e recorded: age, sex, and systemic condition of the patients, daily dosage of EMB, duration of EMD treatment, visual function at the time of electroph ysiologic investigation, time from the onset of blurred vision to the disco ntinuation of EMB treatment (symptom duration), and time from termination o f EMB treatment until electrophysiologic investigation. Results: The electroretinograms were normal in 25 patients. Twelve patients had normal electro-oculogram (EOG) findings in both eyes and the remaining 15 patients had abnormal EOG findings in at least one eye. Ten eyes showed supranormal EOG (light/dark (L/D) ratios of more than 2.33, and 13 eyes ha d decreased L/D ratios (< 1.65). The symptom duration was shorter in the su pranormal EOG group. Conclusions: The results suggest that a supranormal EOG may be indicative o f an early toxic state during EMB therapy and that EMB may cause dysfunctio n of the retinal pigment epithelium.