Vol. 6, Issue 1, Part A (2024)

Introduction: Ethambutol is a first line drug against tuberculosis. It can cause side effects like optic neuropathy, hyperuricemia and allergic reactions. Optic neuritis due to ethambutol is said to be dose dependent and with varying reversibility on discontinuation of the treatment in different studies. Several studies showed incidence of TON close to 1%.
Aim: To study the incidence and clinical profile of patients developing toxic optic neuropathy after receiving standard Anti tuberculous treatment (ATT).
Methods: We evaluated the enrolled patients who met the inclusion criteria between December 2021 and December 2023 by assessing their BMI, comorbidities and conducted an ophthalmic evaluation at the start of treatment and periodically thereafter at 3 monthly interval and 4 weeks after stoppage of treatment. After the development of defective vision, visual evoked response was done, ethambutol was stopped and patients were reassessed periodically. The data was processed with SPSS software version 20.0.Descriptive data were explained in frequency. Pearson’s Chi - square test was used to compare the categorical variables and Fischer exact test when expected values were less than 5 with a p value <0.05 significance.
Results: Among 86 patients studied there was equal distribution of males and females. 7 (8.1%) presented with Toxic Optic Neuritis (TON). All TON patients were given ethambutol in the dose more than 15 mg /kg. 71.4% TON occurred after 8 weeks of ATT. Colour vision was normal in 43% of patients even when they presented with blurred vision. However Visual evoked potential (VEP) showed increased latency in all patients. Only 57.1% showed vision improvement to pre - optic neuropathy levels. 
Conclusion: TON is commonly seen with higher doses of ethambutol and after 4 months of therapy. Apart from visual acuity, visual field, colour vision and fundus examination, visual evoked response and OCT may be done to detect early optic nerve involvement.