Early diagnosis of ethambutol side effects on the optic nerve using a special color test

Abstract

Chart ‘65’ in Stilling's pseudoisochromatic plates is the most sensitive test for ethambutol-induced side effects on the optic nerve. Since ethambutol intoxication accounts for a color-contrast defect at postreceptoral level, the size of the retinal image of the test figure plays an important role. Therefore, by measuring the distance in which the test figure can be recognized, very accurate information about the color vision defect caused by ethambutol is available. The plate is shown in a viewing box illuminated by two standardized lamps. Variations of the optotype were introduced by cutting the plate vertically. In this way, not only 65, but also 56, 59, and 95 could be shown. Thus, various colored optotypes with identical demands on color vision are at our disposal. Preliminary results from our study of more than 100 patients at the Heidelberg Tuberculosis Clinic show that about 95% of all patients tolerate the therapeutically necessary ethambutol dosage. The remaining 5% are efhambutol-at-risk patients who can be identified by our new color test.

Key Words:

• Ethambutol
• optic neuropathy
• color test