Purpose: To evaluate various visual parameters for early detection of ethambutol toxicity.
Method: Prospective study of 104 eyes of 52 patients on ethambutol under Directly Observed Treatment Strategy. Parameters evaluated were visual acuity, fields, Visual Evoked Responses(VER), stereoacuity and RNFL thickness on OCT. Examinations were done before 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, color 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 P100 wave after 1and 2 months of therapy (p<0.001 for both) with 14.42% (15/104) of eyes showing more than 10 msec 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 ethambutol; however mean VER latency remained delayed(p=0.002).
Conclusion: Pattern-VER and visual field examinations are sensitive tests to detect early toxicity and along with OCT may help to identify cases likely to develop clinical toxicity.