Ethambutol-induced optical neuropathy: risk of overdosing in obese subjects.

BACKGROUND: Ethambutol (EMB) is one of the first-line drugs in the treatment against tuberculosis (TB). Side-effects are infrequent, but its main adverse effect, optical neuropathy, has long been recognised. The mechanisms of action and predisposing factors have not yet been fully understood. METHOD: We conducted a retrospective study (1992--2007) in an attempt to find predisposing factors for optical neuropathy. RESULTS: Visual disturbance was reported in 1.3% of the 760 patients treated with EMB; of these, 0.8% were EMB-related. We present the six cases; four were clearly overdosed, but in two obese patients dosage was correctly calculated for total body weight (TBW). CONCLUSION: Analysis of the case histories and previous reports suggest that optical neuropathy may at least partly be caused by EMB overdosing due to daily dosing based on TBW instead of dosing on lean body mass.