The electrophysiological characteristics of ethambutol toxic optic neuropathy

Abstract Purpose: To report the electrophysiological and ophthalmic characteristics of ethambutol optic neuropathy (EON). Methods : Retrospective case review of clinical features and electrophysiology in 31 patients (62 eyes). Ten cases received longitudinal monitoring for up to 3 years. Results: Reduced visual acuity (worse than logMAR 0.2) was present in 60/62 eyes. Colour vision was tested in 31 eyes; all showed impairment. Pattern VEPs were delayed in 33 of 45 eyes with detectable responses; they were subnormal (<5µV) in 19 eyes. Subnormal or borderline responses of normal peak time were present in 10 of 12 eyes. Seventeen eyes had no detectable pattern VEP. Peak times in 9 eyes, and amplitudes in 16 eyes, improved following drug cessation; mild PVEP worsening occurred in 2 eyes. Flash VEPs were delayed in 20 eyes of 12 patients; 5 eyes had no detectable FVEP. Peak times (6 eyes) and amplitudes (16 eyes) improved following drug cessation. PERG showed retinal ganglion cell dysfunction in 26 eyes, usually N95 component reduction. Full-field ERGs were mildly abnormal in 7 eyes of 4 patients. VA improved in 16 of 20 eyes following drug cessation. The presence of pale or swollen optic discs, or of renal disease, was associated with lack of improvement or worsening of VEPs at follow-up. Conclusions: Ethambutol may cause severe optic nerve and retinal ganglion cell dysfunction; appropriate caution is necessary. Severe VEP abnormalities can resolve following cessation of ethambutol, highlighting the need for early diagnosis and cessation of the drug. Visual acuity has poor predictive value.