2. Photophobia

3. Retinal or macular

a.Retinitis

b.Vascular disorder

c.Edema

d.Pigmentary changes

Possible

1. Hemorrhages

2. Paresis of extraocular muscles

3. Eyelids or conjunctiva

a.Lyell’s syndrome

b.Erythema multiforme

Unconditional/Unclassified

1. Mydriasis

2. Spasms

3. Visual hallucinations

Clinical significance

Ethambutol is still a first-line agent used in the multidrug treatment of tuberculosis. Its use is on the increase since there are more resistant forms of tuberculosis occurring. The most significant adverse effect of ethambutol is optic neuritis, which is usually bilateral and can be asymmetric. Ethambutol toxicity may affect only the small caliber papillo-macular bundle axons and

optic atrophy will not develop until months after the fibers are lost. This means objective findings on the fundus exam are frequently absent. Optic neuropathy may occur, on average, at 2–5 months after starting therapy. The earliest ophthalmologic findings in toxic optic neuropathy from ethambutol may be loss of visual acuity, color vision loss or central scotomas. Ethambutol also has an affinity for the optic chiasm with bitemporal visual field defects manifesting toxicity. Some authors have proposed electrophysiologic tests to screen for toxicity due to ethambutol. A multifocal ERG may be of value to diagnose and monitor patients taking ethambutol, and full field ERGs and EOGs also have demonstrated abnormalities. Contrast sensitivity measurement may also be useful in detecting subclinical ethambutol toxic optic neuropathy. Optical coherence tomography shows promise as a screening tool to detect subclinical optic neuropathy.

Isoniazid is frequently prescribed concomitantly with ethambutol for tuberculosis due to multiple cases of drug resistance to single agent therapy. Isoniazid has also been associated with optic neuropathy, and differentiating toxicity due to ethambutol versus isoniazid can be challenging. In general, the toxicity from isoniazid is less frequent, less severe and is usually reversible. When in doubt, a dechallenge with isoniazid and/or ethambutol may need to be undertaken after consultation with the primary care physician.

Once a defect is found and the ethambutol is discontinued, occasionally the abnormality may continue to progress