Optic neuritis, ethambutol causing

Ethambutol (Myambutel) [Antitubercular Agent] Uses Pulm TB other mycobacterial Infxns, MAC Action i RNA synth Dose Adults Feds >12 y. 15-25 mg/kg/d PO single dose X in renal impair, take w/ food, avoid antacids Caution [B, +] Contra Unconscious pts, optic neuritis Disp Tabs SE HA, hyperuricemia, acute gout, abd pain, T LFTs, optic neuritis, GI upset Interactions T Neurotox W/ neurotoxic drugs X effects W/ A1 salts EMS May affect glucose (hypoglycemia) may cause vision problems OD Sxs unknown activated charcoal may be effective symptomatic and supportive... 

A. Ethambutol is associated with retrobulbar neuritis, resulting in loss of central vision and impaired red-green discrimination. Ethionamide (B) is an analogue of isonicotinic acid and is associated with GI intolerance and peripheral neuropathy, but not the optic neuritis or color vision discrimination problems. Aminosalicylic acid (C) can cause GI irritation and bleeding problems, so caution is required in peptic ulcer patients. It has no neurological side effects. Rifampin (D) is associated with red-orange discoloration of saliva, tears, and urine but not the color vision problems. Isoniazid (E) is associated with peripheral neuritis in chronic alcoholics and malnourished individuals and requires pyridoxine supplements. It is not associated with optic neuritis. 

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). 

Side effects caused by isoniazid, rifampin, pyrazinamide, and ethambutol are common and can include hepatotoxic-ity, peripheral neuropathy, optic neuritis, and Gl side effects. All four agents can potentially be hepatotoxic, but this side effect is most frequently associated with isoniazid and rifampin. Peripheral neuropathy is most commonly associated with isoniazid, whereas optic neuritis is associated with ethambutol. The metabolism of isoniazid is genetically predetermined. Patients of Scandinavian, European, and African descent metabolize isoniazid slower (slow acetylators) and are therefore more predisposed to hepatotoxicity and peripheral neuropathy due to isoniazid. Fast acetylators include people of Asian or American Indian descent and are less predisposed to these adverse effects. 

Ethambutol The (S,S)-form of ethambutol is a tuberculostatic agent the (i ,i )-form causes optical neuritis that can lead to blindness... 

The mean serum levels of a 300-mg dose of isoniazid were not signifieant-ly ehanged in 10 patients with tubereulosis when they were given a single 20-mg/kg dose of ethambutol. The possible effeets ofeoneurrent use over a period of time were not studied. However, there is some evidence that the optic neuropathy caused by ethambutol may be increased by isoniazid, and any effects resolve more slowly after the use of isoniazid. One group of authors recommends that both ethambutol and isoniazid should be stopped immediately if severe optic neuritis occurs. They further recommend that isoniazid should be stopped if less severe optic neuritis does not improve within 6 weeks after stopping ethambutol. ... 

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