A 28-year-old woman from Chennai presents with a 10-day history of progressive bilateral leg weakness and difficulty climbing stairs. She recalls a watery diarrheal illness 3 weeks ago. On neurological examination, she has symmetrical flaccid paralysis of lower limbs with absent knee and ankle reflexes. Cranial nerves are intact. Respiratory function is preserved (FVC 85% predicted). CSF shows protein 120 mg/dL with 2 lymphocytes/μL. Serum anti-ganglioside antibodies (anti-GQ1b) are negative. Electromyography shows reduced motor amplitudes with preserved conduction velocities. What is the most likely electrophysiological variant of GBS in this patient?

Explanation

## Electrophysiological Classification of GBS **Key Point:** GBS has distinct electrophysiological variants that correlate with antecedent infection, geographic region, and prognosis. This patient's NCS findings (reduced amplitudes, preserved velocities) indicate an **axonal** pattern, not demyelinating. ## Variant Comparison | Variant | NCS Pattern | Antecedent | Geography | Anti-Ganglioside | Prognosis | |---|---|---|---|---|---| | **AIDP** | Demyelination (↑ latency, conduction block) | Viral (URI, GI) | Temperate | Negative | Good | | **AMAN** | Axonal (↓ amplitude, normal velocity) | Campylobacter jejuni | Tropical/subtropical | Anti-GM1 (50%) | Slower recovery | | **AMSAN** | Axonal (sensory + motor) | C. jejuni | Asia, Latin America | Anti-GM1/GD1a | Poorest | | **MFS** | Variable | Viral/C. jejuni | Variable | Anti-GQ1b (90%) | Good | | **PCBV** | Axonal/demyelinating | Variable | Rare | Variable | Variable | ## Why AMAN in This Case? 1. **NCS findings:** Reduced motor amplitudes with **preserved conduction velocities** = axonal degeneration, not demyelination 2. **Antecedent:** Diarrheal illness 3 weeks prior (classic for Campylobacter, which triggers AMAN) 3. **Geography:** Chennai (tropical India) — AMAN is more common in tropical/subtropical regions 4. **Negative anti-GQ1b:** Rules out Miller Fisher syndrome 5. **Pure motor pattern:** No sensory symptoms documented → AMAN rather than AMSAN **High-Yield:** In tropical countries, AMAN accounts for 30–50% of GBS cases (vs. 5–10% in temperate regions). Campylobacter jejuni infection is the key trigger. **Clinical Pearl:** AMAN patients have slower recovery than AIDP (weeks to months vs. days to weeks) and higher residual disability rates. However, immunomodulatory therapy (IVIG/plasma exchange) is still indicated. **Mnemonic:** **AMAN = Axonal Motor Amplitude Negative** (reduced amplitude on NCS). ## Distinguishing AMAN from AIDP ```mermaid flowchart TD A[GBS with ascending paralysis]:::outcome --> B{NCS pattern?}:::decision B -->|Demyelination: ↑ latency, conduction block| C[AIDP]:::outcome B -->|Axonal: ↓ amplitude, normal velocity| D[AMAN vs AMSAN]:::outcome D --> E{Sensory involvement?}:::decision E -->|Motor only| F[AMAN]:::outcome E -->|Motor + sensory| G[AMSAN]:::outcome F --> H{Anti-ganglioside?}:::decision H -->|Anti-GM1 positive| I[Campylobacter trigger likely]:::action H -->|Negative| J[Other trigger]:::action ``` [cite:Harrison 21e Ch 385; Robbins 10e Ch 27]