Visual acuity, reduced ethambutol

The onset of visual loss can be sudden and dramatic, with color vision defects in the red-green or blue-yellow spectra, as well as variable field defects. In acute cases, disc edema is accompanied by sphnter hemorrhages. Retrobulbar neuritis with ethambutol can be predominantly axial, presenting with reduced visual acuity and central scotoma, or periaxial, with peripheral field defects. In non-acute tjrpes the fundi and discs appear normal (13). Visual defects can be unilateral or bilateral. 

Reduced visual acuity and central or centro-cecal scotomas on visual field testing have been reported as the usual presentation of eth-ambutol-induced optic neuropathy. Bilateral temporal hemianopia has been reported in a case of ethambutol toxicity [64 ]. 

In a retrospective study of 857 Korean patients who took ethambutol, 89 had impaired vision [65 ]. Ethambutol-induced optic neuropathy was diagnosed in during a mean follow-up period of 13 months. The average dose of ethambutol was 18 mg/kg/ day and the duration of therapy was 9.4 months. Ophthalmic findings included reduced visual acuity (n = 58), abnormal visual fields (n = 58), abnormal color vision (55), optic disc pallor (34), and increased latency on VEP tests (58). Slightly less than one-third of the patients had improved visual function after discontinuing ethambutol. The mean time to recovery was 5.4 months. However, no patient with optic disc pallor at the time of diagnosis had improved visual function. Renal dysfunction and the daily dose of ethambutol, but not the duration of treatment, contributed. The authors estimated the incidence of ethambutol-induced optic neuropathy in Koreans to be under 2%. Thus, visual function after withdrawal of ethambutol is reversible in only a minority of patients and does not occur if optic disc pallor is present. 

Two patients who developed reduced visual acuity after taking ethambutol for several months for Mycobacterium avium-intracellulare infection had bitemporal visual field defects that suggested damage to the optic chiasm [66 ]. 

Ethambutol should not be given with antacids. The ethambutol dose should be reduced to three times per week in patients with renal failure. Retrobulbar neuritis is the major adverse effect. Patients may complain of a change in visual acuity, the inability to see the color green, or both. They should be monitored monthly while on the drug using Snellen wall charts for visual acuity and Ishihara red-green color discrimination cards. ... 

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