Optic nerve ethambutol

EthambutoL The cause of its specific antitubercular action is unknown. Ethambutol is given orally. It is generally well tolerated, but may cause dose-dependent damage to the optic nerve with disturbances of vision (red/green blindness, visual field defects). 

The optic nerve is of interest here because some drugs are toxic to this tissue.The antibiotics chloramphenicol, ethambutol, streptomycin, and sulfonamides can cause optic neuritis. Vitamin A, especially in large doses, can result in papilledema. Digitalis can cause retrobulbar neuritis (see Chapter 35). 

Once changes have occurred in visual acuity, visual field, or color vision, these functional changes may continue to deteriorate even after ethambutol has been discontinued. More often, however, there is recovery of pretreatment visual acuity and visual field several months or years after discontinuation of the drug. The degree of recovery depends largely on the extent to which ethambutol has compromised optic nerve function. If the ocular toxicity is not recognized early, the drug can cause permanent loss of vision, especially in older patients. 

In addition to a normal eye, vision requires an intact circuit to and from the brain itself. The optic nerve (cranial nerve II), for example, carries retinal information to the occipital cortex in the posterior aspects of the brain. In addition, information concerning pupil size, direction of gaze, simultaneous movement of the eyes (conjugate gaze), and focus clarity is relayed from the brain back to the eye through multiple nerves. Ethambutol, an antituberculous medication, affects the optic nerves (optic neuropathy) whereas organic mercury compounds have been historically associated with toxic effects on the occipital cortex (cortical blindness). 

Extrinsic species Ethambutol Intrinsic species Not Known Distribution Optic nerve fibers and retinal ganglion cells Outcome Altered physiology initially ... 

The mechanism of ethambutol-induced optic neuropathy is unclear. It has been postulated that it is caused by a disturbance in mitochondrial metabolism. Ethambutol is also a strong chelator of copper, a co-factor of cytochrome c oxidase, which is required for axonal transport in the optic nerves, failure of which, secondary to mitochondrial insufficiency, results in optic neuropathy. Ethambutol is specifically toxic to retinal... 

These results suggested that visual dysfunction due to ethambutol may be entirely attributable to retinal rather than optic nerve toxicity. These are the first reports to show abnormalities in multifocal electroretinography that correspond to bitemporal visual field defects and add to the growing evidence that ethambutol damages the retina. 

TrusieWicz, D. (1975) Farnsworth 100-Huc Test in diagnosis of ethambutol-induced damage to optic nerve. Ophthalmologica (Basel), 171, 425. 

A review of certain chemicals is essential. Ethylene glycol is an antifreeze used for gasoline engines and may produce somnolence, imreactive pupils, disc swelling, and kidney failure. Systemic lead poisoning produces headaches, coma, cranial nerve palsies, and papilledema. Wood alcohol, or methanol, may produce severe toxic neuropathy and disc edema. Drugs known to produce toxic optic neuropathy include amiodarone (an antiar-rhythmic), quinine, aminoquinolines, ibuprofen, ethambutol, isoniazid, and chloramphenicol. 

In a prospective evaluation of various visual parameters for early detection of ethambutol toxicity in 52 patients with tuberculosis attending a Directly Observed Treatment Strategy Centre [67 visual acuity, visual fields, visual-evoked responses, stereoacuity, and retinal nerve fiber layer thickness on optical coherence tomography were assessed after 1 and 2 months of treatment, and 1 month after withdrawal. There was no visual functional defect at baseline. On follow-up, visual acuity, color vision. 

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