Prospective evaluation of visual function for early detection of ethambutol toxicity

Aim: The aim of the study was to evaluate various visual parameters for early detection of ethambutol toxicity. Method: This was a prospective study of 104 eyes of 52 patients being treated with ethambutol in the Directly Observed Treatment Strategy Centre (Dr R P Centre for Opthalmic Sciences, New Delhi, India). Visual acuity, visual fields, visual evoked responses (VER), stereoacuity and retinal nerve fibre layer (RNFL) thickness on optical coherence tomography (OCT) were assessed. Examinations were done before the start of therapy, after 1 and 2 months of treatment, and 1 month after stopping ethambutol. Results: No visual functional defect was noted at baseline. On follow-up, visual acuity, colour vision, contrast sensitivity, fundus and stereoacuity were not affected in any patient. Visual field defects developed in 7.69% (8/104) of the eyes. Pattern-VER showed an increased mean latency of the P-100 wave after 1 and 2 months of therapy (p<0.001 for both) with 14.42% (15/104) of eyes showing more than 10 ms increase in latency. On OCT, significant loss of mean temporal RNFL thickness was detected in 2.88% (3/104) of eyes individually. Overall, 19.23% (20/104) of the studied eyes showed sub-clinical toxicity. Reversal of this observed toxicity on pattern-VER and visual fields was seen in 80% of eyes after 1 month of stoppage of ethambutol; however, mean VER latency remained delayed (p = 0.002). Conclusion: Pattern-VER and visual field examinations are sensitive tests to detect early toxicity. Together with OCT, they may help to identify patients who are likely to develop clinical toxicity.