Bitemporal visual field defects in ethambutol-induced optic neuropathy

Ethambutol-induced optic neuropathy is well documented and most frequently associated with central or cecocentral scotomas. We designed a study to characterize the subset of patients who exhibit bitemporal visual field defects.

A computer search was performed for patients evaluated in a university academic neuro-ophthalmology consultative practice to identify those with the diagnosis of ethambutol-induced optic neuropathy. Clinical features were tabulated, including dose and duration of ethambutol use, time to onset of visual loss, initial and follow-up visual acuities, automated perimetry, optic disc appearance, and MRI features. Assessments for bitemporal visual field defect with alignment on vertical midline and for visual improvement after discontinuing ethambutol were performed.

Nineteen cases of ethambutol-induced optic neuropathy were identified; All but 2 eyes demonstrated visual field defects worse in the temporal fields, most with margination along the vertical midline with superimposed central or cecocentral scotomas. Six cases (12 eyes) showed bitemporal defects with such margination without superimposed scotomas. Median time to onset of visual loss was 6.0 months. Visual improvement occurred (of 17 cases with data available) by at least 3 Snellen lines in 17 of 34 eyes (50%); mean visual acuity improvement was 3.74 lines (median, 3.0). Visual improvement by at least 3.0 decibels (dB) mean deviation (MD) on automated perimetry occurred in 27 of 34 eyes (79%); mean improvement in MD was 7.82 dB (median, 7.86). Median follow-up was 8.0 months. None had MRI abnormality in the chiasmal region.

Bitemporal visual field defects are common in ethambutol-induced optic neuropathy. The pattern may mimic chiasmal compression, and neuroimaging is required. It may reflect susceptibility to toxicity of chiasmal crossing fibers.