Effect of the tuberculostaticum ethambutol and stimulus intensity on chromatic discrimination in man

In goldfish it has been shown that ethambutol shifts the threshold for wave length discrimination without affecting the absolute sensitivity of the con es. In this study we demonstrate that a similar colour vision disturbance o ccurs in tuberculosis patients treated with ethambutol. After 2 months of e thambutol treatment, chromatic discrimination was measured with a computeri zed forced two choice (CD) test with isoluminant coloured stimuli and with three other colour vision tests: the Ishihara, the Oscar and the Lanthony D esaturated 15 Hue tests. The scores of the patient group (n = 19) on these four colour vision tests were compared with the scores of a group of contro l subjects (n = 33) and a group of congenital red/green colour-blind subjec ts (n = 5). A reduction of the stimulus intensity of 1 log unit caused a si gnificant reduction in red/green chromatic discrimination, measured with th e CD test in both, control subjects and patients. This intensity dependent reduction was significantly greater for patients than for controls. In this respect, man and goldfish behave similarly. Furthermore, the CD test showe d the same ethambutol-induced reduction in chromatic discrimination at low intensity for the blue/green part of the spectrum. This has not been measur ed in goldfish. The origin of this ethambutol-induced colour vision disturb ance must be at a post-photoreceptor site, because the Ishihara and Oscar t ests, both designed to screen for photoreceptor-based, or primary red/green colour vision disturbances, did not discriminate between patients and cont rol subjects. Thus, as in goldfish, we find that in patients ethambutol shi fts the threshold for chromatic discrimination without changing the absolut e sensitivity. (C) 1999 Elsevier Science Ltd. All rights reserved.