Antimicrobials · Antimycobacterial Agents
The facts most likely to be tested
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Ethambutol functions by inhibiting the enzyme arabinosyl transferase, which disrupts the synthesis of the mycobacterial cell wall.
The most significant and dose-dependent adverse effect of ethambutol is optic neuritis, which manifests as decreased visual acuity and red-green color blindness.
Baseline and periodic visual acuity and color vision testing are mandatory for all patients receiving ethambutol therapy.
Ethambutol is a bacteriostatic agent used in combination with other antimycobacterial drugs for Mycobacterium tuberculosis and also for Mycobacterium avium complex and M. kansasii disease.
Patients may develop hyperuricemia secondary to ethambutol use, which can precipitate an acute attack of gout.
Ethambutol is primarily excreted by the kidneys, necessitating dose adjustment in patients with significant renal impairment.
Discontinuation of ethambutol is the definitive management for patients who develop optic neuropathy to prevent permanent vision loss.
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A 42-year-old male with a history of pulmonary tuberculosis is currently on a four-drug regimen including isoniazid, rifampin, pyrazinamide, and ethambutol. He presents to the clinic complaining of difficulty reading and states that he has trouble distinguishing between certain colors, specifically noting that traffic lights look different. His physical examination reveals decreased visual acuity on Snellen chart testing and impaired red-green color discrimination on Ishihara plates. Funduscopic examination is unremarkable. His renal function is within normal limits.
Which of the patient's medications is the most likely cause of his current symptoms?
Ethambutol
The patient is exhibiting classic signs of ethambutol-induced optic neuritis, characterized by red-green color blindness and decreased visual acuity, which is a high-yield side effect tested on boards.
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High yield triage
Classification
Antimycobacterial agent; bacteriostatic inhibitor of cell wall synthesis.
Indications
First-line treatment for Tuberculosis (TB) in combination therapy.
Mechanism of Action
Inhibits arabinosyl transferase, disrupting mycobacterial cell wall synthesis.
Side Effects
Optic neuritis (red-green color blindness), hyperuricemia.
Contraindications / Monitoring
Pre-existing optic neuritis. Monitor visual acuity and color vision.
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Mechanism of Action
Ethambutol inhibits the enzyme arabinosyl transferase, which is essential for the polymerization of arabinogalactan in the mycobacterial cell wall. By blocking this step, it impairs the structural integrity of the cell wall. It is specifically active against Mycobacterium tuberculosis and is considered bacteriostatic.
Unique Properties
Unlike other RIPE drugs (Rifampin, Isoniazid, Pyrazinamide), Ethambutol is primarily bacteriostatic rather than bactericidal. It is uniquely associated with dose-dependent ocular toxicity that requires specific baseline testing.
Indications
Ethambutol is a core component of the initial four-drug regimen for active pulmonary tuberculosis. It is also utilized in the treatment of Mycobacterium avium complex (MAC) infections. It is added to prevent the emergence of drug-resistant strains.
Pharmacokinetics
The drug is primarily excreted via the kidneys; therefore, dosage adjustments are mandatory in patients with renal impairment. CSF penetration is poor and erratic even when the meninges are inflamed, so ethambutol is not a preferred agent for tuberculous meningitis. Only a minor fraction is hepatically metabolized, but liver toxicity, including fatalities, has been reported - obtain baseline and periodic hepatic function.
Side Effects & Adverse Events
The most critical adverse effect is optic neuritis, which manifests as decreased visual acuity and loss of red-green color discrimination. Patients may also experience hyperuricemia, which can precipitate gout flares. Less common effects include peripheral neuropathy and gastrointestinal distress.
Contraindications
Hypersensitivity is an absolute contraindication. Ethambutol is also contraindicated in patients unable to appreciate and report visual changes - young children, unconscious or sedated patients. It is contraindicated in known optic neuritis unless clinical judgment determines benefit outweighs risk. Use with extreme caution in severe diabetic retinopathy. Ophthalmologic testing must be repeated monthly at doses >15 mg/kg/day.
Monitoring
Baseline and periodic visual acuity and color vision testing (e.g., Ishihara plates) are mandatory. Patients should be instructed to report any changes in vision immediately. Serum uric acid levels should be monitored if the patient has a history of gout.
Clinical Pearls
On boards, always associate Ethambutol with the red-green color blindness buzzword. If a patient on RIPE therapy complains of blurry vision or difficulty distinguishing colors, the Ethambutol must be discontinued immediately to prevent permanent damage.