A 28-year-old woman with drug-susceptible pulmonary TB is on standard HRZE therapy. She presents with progressive bilateral foot pain, numbness, and tingling in a stocking-glove distribution after 3 months of treatment. Nerve conduction studies show slowing of conduction velocity in both lower limbs. Which investigation is most appropriate to confirm the diagnosis of isoniazid-induced peripheral neuropathy and assess severity?

Explanation

## Investigation of Choice to Confirm Isoniazid-Induced Peripheral Neuropathy and Assess Severity ### Clinical Context **Key Point:** The stem asks which investigation is most appropriate to **confirm the diagnosis** of peripheral neuropathy **and assess severity**. Although nerve conduction studies (NCS) are already mentioned as showing slowing, the question asks for the investigation that formally confirms the neuropathy diagnosis and characterizes its severity — this is **Electromyography (EMG) with detailed nerve conduction studies**. ### Why EMG with Detailed NCS is the Correct Answer **High-Yield:** EMG combined with detailed NCS is the gold-standard investigation for: 1. **Confirming** the presence and type of peripheral neuropathy (axonal vs. demyelinating) 2. **Localizing** the lesion (peripheral nerve vs. nerve root vs. anterior horn cell) 3. **Assessing severity** — amplitude of compound muscle action potential (CMAP) and sensory nerve action potential (SNAP) correlate directly with axonal loss and functional impairment 4. **Monitoring progression** — serial NCS can track worsening or recovery **INH-Induced Neuropathy Pattern on NCS/EMG:** - Predominantly **axonal sensorimotor neuropathy** (reduced SNAP/CMAP amplitudes) - Distal > proximal involvement (stocking-glove distribution) - Slowing of conduction velocity may also be seen due to secondary demyelination - EMG shows denervation changes (fibrillations, positive sharp waves) in severe cases ### Why Serum Pyridoxine (B6) Level is NOT the Best Answer Here **Clinical Pearl:** Serum pyridoxine level identifies the **underlying mechanism/cause** (B6 depletion by INH) but does NOT: - Confirm the diagnosis of peripheral neuropathy itself - Assess the **severity** of nerve damage - Differentiate axonal from demyelinating injury The question explicitly asks to "confirm the diagnosis of peripheral neuropathy **and assess severity**" — only EMG/NCS fulfills both criteria. Serum B6 is useful as a complementary test to confirm the etiology once neuropathy is established. ### Comparison of Investigations | Investigation | Confirms Neuropathy | Assesses Severity | Identifies INH Cause | |---------------|--------------------|--------------------|----------------------| | EMG + NCS | ✅ Yes (gold standard) | ✅ Yes | ❌ No | | Serum B6 | ❌ No | ❌ No | ✅ Yes | | MRI lumbar spine | ❌ No (rules out structural) | ❌ No | ❌ No | | CSF analysis | ❌ No | ❌ No | ❌ No | ### Why Other Options Are Incorrect - **Option C (MRI lumbar spine):** Rules out compressive radiculopathy; does not confirm peripheral neuropathy or assess severity - **Option D (CSF analysis):** Used for Guillain-Barré syndrome (albuminocytological dissociation); not indicated here **Reference:** Harrison's Principles of Internal Medicine, 21st edition — Chapter on Peripheral Neuropathy; KD Tripathi Essentials of Medical Pharmacology, 8th edition — Antitubercular Drugs (Isoniazid toxicity).