A 28-year-old woman with newly diagnosed pulmonary tuberculosis is started on standard four-drug therapy (HRZE). She returns to the clinic after 2 weeks complaining of blurred vision, difficulty distinguishing red and green colors, and peripheral numbness in her feet. Visual acuity is 6/12 in both eyes. Fundoscopy shows temporal disc pallor. Serum creatinine is 0.9 mg/dL. Which drug is responsible for these adverse effects and what is the mechanism?

Explanation

## Clinical Presentation Analysis The patient presents with a triad of neurological and ophthalmological findings: - **Blurred vision** with **red-green color blindness** (dyschromatopsia) - **Temporal disc pallor** on fundoscopy (optic atrophy) - **Peripheral neuropathy** (paresthesias in feet) - Normal renal function (creatinine 0.9 mg/dL) - Early onset (2 weeks into therapy) ## Ethambutol: Optic Neuritis Profile | Feature | Details | |---------|----------| | **Incidence** | 1–5% (dose and duration dependent) | | **Onset** | 2–8 weeks after starting therapy | | **Mechanism** | Mitochondrial toxicity → retrobulbar optic neuritis | | **Visual symptoms** | Blurred vision, scotomas, loss of visual acuity | | **Color vision** | Red-green dyschromatopsia (pathognomonic) | | **Fundoscopy** | Temporal disc pallor, optic atrophy (if chronic) | | **Reversibility** | Reversible if caught early; irreversible if delayed | | **Risk factors** | High dose (>25 mg/kg/day), renal impairment, prolonged use | ## Key Point: **Ethambutol causes dose-dependent optic neuritis via mitochondrial dysfunction.** Red-green color blindness is the hallmark and most sensitive early sign of ethambutol toxicity. ## High-Yield: **Mnemonic: EMB = Eye Damage** — Ethambutol causes optic neuritis with red-green color blindness, temporal disc pallor, and potentially irreversible vision loss if not detected early. ## Mechanism of Ethambutol Toxicity ```mermaid flowchart TD A[Ethambutol absorbed]:::action A --> B[Accumulates in retrobulbar optic nerve]:::action B --> C[Inhibits mitochondrial cytochrome oxidase]:::action C --> D[↓ ATP production in retinal ganglion cells]:::action D --> E[Retrobulbar optic neuritis]:::outcome E --> F[Red-green dyschromatopsia]:::outcome E --> G[Blurred vision & scotomas]:::outcome E --> H[Temporal disc pallor]:::outcome I[Early detection & withdrawal]:::action I --> J[Visual recovery possible]:::outcome K[Delayed detection]:::urgent K --> L[Irreversible optic atrophy]:::urgent ``` ## Clinical Pearl: **Baseline and periodic color vision testing (Ishihara chart) is mandatory for all patients on ethambutol.** Any complaint of visual disturbance or color blindness mandates immediate ophthalmology review and drug withdrawal. ## Why NOT the other drugs? ### Isoniazid - Causes **peripheral neuropathy via pyridoxine (B6) depletion**, not optic neuritis - Presents with distal, symmetrical sensory neuropathy (paresthesias, numbness) - **Does NOT cause color blindness or optic neuritis** - Prevented by pyridoxine supplementation (10 mg/day) ### Rifampicin - Causes **orange discoloration of body fluids** (urine, tears, sweat), not color blindness - **Does NOT cause optic neuritis or visual symptoms** - Minimal ocular toxicity ### Pyrazinamide - Causes **hyperuricemia** → gout and arthralgia - **Does NOT cause optic neuritis or color blindness** - Hepatotoxicity is its main concern, not neurological effects ## Management of Ethambutol Toxicity 1. **Immediate withdrawal** of ethambutol 2. **Ophthalmology referral** for formal visual field and color vision testing 3. **Continue HRZ** (isoniazid, rifampicin, pyrazinamide) 4. **Extend therapy duration** to compensate for loss of ethambutol 5. **Monitor recovery**: Visual symptoms often improve within weeks if caught early [cite:KD Tripathi 8e Ch 47; Harrison 21e Ch 158]