Cone dysfunction in patients with ethambutol toxicity

Dear Editor, The newer guidelines by the Revised National Tuberculosis Control Program (RNTCP) recommended the use of ethambutol daily instead of three times/week and in both intensive and continuation phases. This has led to an increased incidence of ethambutol-related optic neuropathy. We describe ethambutol-induced cone dysfunction in three cases. Interestingly, they developed symptoms of intolerance to light and a decrease in vision at around 5 months while on ATT. The age group was between 40 and 72 years, and they were all females. Color vision was abnormal. Fundus examination was normal in case 1 and case 3, while case 2 had drusen-like deposits temporal to the fovea. VEP was normal in case 1 and case 3, but case 2 showed prolonged latency of P100. OCT showed EZ disruption (OU) in case 1, thinning of the ganglion cell inner plexiform layer in case 2, and was normal in case 3. Visual fields showed central (OD) and centrocecal scotomas (OS) in case 1, bitemporal defects in case 2, and central scotomas (OU) in case 3. MRI orbits and cranium were normal in all. ERG showed a delayed implicit time in flicker response, suggestive of cone dysfunction in all three cases. INH and ethambutol were stopped, and all cases received oral vitamin B12, zinc, folic acid, and pyridoxine. There was normalization of the flicker response (OU) 6 weeks after stopping ethambutol in case 1 but no change in case 2 and case 3. A composite photograph showing fundus photo(a,b), visual fields(c,d), swept source OCT(e,f) and ERG (g,f) in case 1 is highlighted as Figure 1. Improvement in vision and a decrease in light intolerance were seen with time as indicated in Table 1.Figure 1: Composite photograph of case 1 showing colour normal fundus image of right eye (a) and left eye (b); visual fields 10-2 showing central scotoma in right eye(c) and centrocecal scotoma in left eye (d); Swept-source optical coherence tomography of right eye (e) and left eye(f)showing ellipsoid zone disruption in parafoveal region(yellow oval); ERG at presentation showing delayed implicit time in flicker response in both eyes (g) and improvement after stopping ethambutol (h)Table 1: Summary of three cases with cone dysfunction due to ethambutolTo conclude, ethambutol not only causes optic nerve toxicity. Electrophysiological tests have demonstrated the involvement of various retinal layers in ethambutol toxicity.[1-4] Ethambutol-induced cone dysfunction is highlighted through this report. Newer symptoms like intolerance to light, apart from a decrease in vision while on ATT, should alert treating physicians on possible ethambutol-related ocular toxicity. Statement of ethics Verbal and written informed consent was obtained from the patient for publication of this case report and accompanying image. Financial support and sponsorship: Nil. Conflicts of interest: There are no conflicts of interest.