A 28-year-old woman with drug-susceptible pulmonary TB completed 2 months of intensive phase (HRZE) and is now 1 month into the continuation phase (HR). She reports progressive tingling and numbness in both feet, with difficulty walking. Neurological examination confirms distal, symmetrical, lower-limb sensorimotor polyneuropathy. Serum vitamin B12 and folate levels are normal. What is the most appropriate next step in management?

Explanation

## Clinical Context This patient has developed **isoniazid-induced peripheral neuropathy (IIPN)**, a well-recognized adverse effect occurring in 0.2–2% of patients on isoniazid. The distal, symmetrical, sensorimotor pattern and normal B12/folate exclude nutritional causes. ## Pathophysiology of IIPN **Key Point:** Isoniazid is a **pyridoxine (vitamin B6) antagonist**. It inhibits pyridoxine phosphokinase, depleting the active form (pyridoxal-5-phosphate), which is essential for: - Myelin formation - Neurotransmitter synthesis - Homocysteine metabolism This leads to demyelinating peripheral neuropathy. **High-Yield:** IIPN is **preventable and reversible** with adequate pyridoxine supplementation. The standard approach is: 1. **Add pyridoxine immediately** (do not stop isoniazid) 2. **Continue HR therapy** (isoniazid is irreplaceable in TB treatment) 3. **Monitor neurological recovery** over 2–4 weeks ## Pyridoxine Dosing for IIPN Prevention & Treatment | Clinical Scenario | Dose | Frequency | |---|---|---| | Prevention (high-risk patients) | 10 mg/kg/day (max 200 mg) | Daily | | Treatment of established IIPN | 50 mg daily (or 10 mg/kg if >60 kg) | Daily | | Duration | Continue throughout TB therapy | — | **Clinical Pearl:** Patients at **higher risk** for IIPN include those with: - Malnutrition - Diabetes mellitus - Alcoholism - Slow acetylator phenotype (genetic) - Renal failure - HIV/AIDS Preventive pyridoxine (10 mg/kg daily) should be given to all high-risk patients from the start of TB therapy. ## Management Algorithm ```mermaid flowchart TD A[Patient on INH develops<br/>peripheral neuropathy]:::outcome --> B{Confirm IIPN<br/>distal, symmetrical,<br/>sensorimotor?}:::decision B -->|Yes| C[Check B12, folate,<br/>glucose]:::action C --> D{Nutritional or<br/>metabolic cause?}:::decision D -->|Yes| E[Treat underlying<br/>cause]:::action D -->|No| F[IIPN confirmed]:::outcome F --> G[Add pyridoxine<br/>50 mg daily]:::action G --> H[Continue HR therapy]:::action H --> I[Recheck neuro status<br/>in 2 weeks]:::action I --> J{Improvement?}:::decision J -->|Yes| K[Continue both drugs<br/>until TB cure]:::action J -->|No| L[Consider INH<br/>discontinuation]:::urgent ``` ## Why Isoniazid Is NOT Discontinued Isoniazid is the **most potent anti-TB drug** and is irreplaceable in the standard regimen. Discontinuing it: - Reduces treatment efficacy - Increases risk of treatment failure and relapse - May lead to drug resistance if other drugs are inadequate **Mnemonic:** **RIPE** = Rifampicin, Isoniazid, Pyrazinamide, Ethambutol. All four are essential in the intensive phase. Isoniazid continues into the continuation phase and cannot be omitted without compromising cure rates. ## Expected Recovery Timeline **High-Yield:** With adequate pyridoxine supplementation: - Symptoms begin to improve within **2–4 weeks** - Complete resolution occurs in **2–3 months** in most cases - Nerve conduction studies normalize within 6 months - Continued pyridoxine throughout TB therapy prevents recurrence [cite:Harrison 21e Ch 158; KD Tripathi 8e Ch 48] ## Why Each Distractor Is Wrong **Option A (Pyridoxine 25 mg daily):** While the principle of adding pyridoxine is correct, the dose is **subtherapeutic**. For treatment of established IIPN, the standard dose is **50 mg daily** (or 10 mg/kg if weight >60 kg), not 25 mg. A dose of 25 mg is used only for prevention in low-risk patients. Additionally, the statement that IIPN is "self-limiting" is misleading—without pyridoxine, symptoms worsen and may become irreversible. **Option B (Discontinue INH, add ETH + FQ):** This approach sacrifices the most effective anti-TB drug (isoniazid) unnecessarily. IIPN is a **manageable side effect** with pyridoxine supplementation and is **not an indication to stop isoniazid**. Ethambutol and fluoroquinolones are less potent than isoniazid, and this regimen would reduce TB cure rates. Isoniazid should only be discontinued if IIPN is severe, progressive despite pyridoxine, or the patient is unable to tolerate it. **Option D (Switch to bedaquiline + linezolid):** This assumes drug-resistant TB, but the patient has drug-susceptible disease and is responding to standard therapy. There is no evidence of resistance. Second-line drugs are reserved for MDR-TB or XDR-TB and carry greater toxicity. This is an inappropriate escalation.