Visual outcomes of toxic optic neuropathy secondary to Ethambutol: A retrospective observational study from India, an endemic country

PURPOSE: Ethambutol (EMB) is one of the first-line drugs used for treating tuberculosis. Vision loss due to optic nerve toxicity is a well-known potential side effect of the drug. Our aim was to evaluate the clinical features and visual outcomes of patients with EMB optic neuropathy (EON). METHODS: A retrospective, observational, single-center study of all patients who were diagnosed to have EON during January 2017–December 2019 was done. All these patients were screened in the Department of Neuro-ophthalmology at a referral tertiary eye care institution in India. Clinical features, visual outcomes, and neuroimaging findings of these patients were analyzed. RESULTS: Two hundred and fifty-six eyes of 128 patients were included. Of these, 73 were male and 55 were female. Mean age was 50.55 ± 15 years. Mean visual acuity at presentation was 1.12 ± 0.45 logarithm of the minimum angle of resolution (logMAR). One hundred and forty three eyes had normal optic disk on presentation, 111 had disk pallor, and two eyes had disk edema. The most common field defect was central/paracentral scotoma (26.2%) followed by temporal defects (24.6%). Magnetic resonance imaging (MRI) brain and orbit showed optic nerve signals in 19.6% and chiasmal signals in 5.2%. At the final follow-up, a ≥2-line vision improvement was noted in 161 eyes (62.9%), which was statistically significant. CONCLUSION: Multiple prognostic factors were analyzed to predict the visual recovery of EON. We observed that patients presenting with visual acuity worse than 6/60 had poor visual outcome and long duration of follow-up showed better visual recovery, proving the possibility of a gradual recovery pattern of EON. Interestingly, we found in our study that the chances of favorable visual outcome were directly proportionate to early diagnosis and cessation of EMB.