A 32-year-old man from rural Maharashtra presents with acute ascending paralysis and areflexia over 3 days. On examination, he has bilateral lower limb weakness (MRC grade 2/5) with absent deep tendon reflexes. Cranial nerves are intact. CSF analysis shows protein 120 mg/dL with normal cell count. Which investigation is most specific for confirming the diagnosis of Guillain-Barré Syndrome?

Explanation

## Diagnostic Approach to Guillain-Barré Syndrome ### Role of Investigations in GBS **Key Point:** Nerve conduction studies (NCS) and electromyography (EMG) are the gold standard investigations for confirming GBS and determining its electrophysiological variant. ### Electrophysiological Findings in GBS NCS/EMG demonstrates characteristic features depending on the GBS variant: | Finding | Acute Inflammatory Demyelinating Polyneuropathy (AIDP) | Acute Motor Axonal Neuropathy (AMAN) | |---------|-------|-------| | **Conduction velocity** | Slowed (< 80% of lower limit normal) | Normal or near-normal | | **Conduction blocks** | Present (demyelinating) | Absent | | **Distal latencies** | Prolonged | Normal | | **F-wave latencies** | Prolonged or absent | Prolonged or absent | | **Needle EMG** | Fibrillations (later) | Fibrillations (early) | **High-Yield:** AIDP (80% of GBS in Western countries) shows demyelinating pattern; AMAN (more common in Asia) shows axonal pattern. ### Why NCS/EMG is Superior 1. **Specificity:** Demonstrates demyelination or axonal degeneration patterns pathognomonic for GBS 2. **Timing:** Can be abnormal within 3–5 days of symptom onset (earlier than CSF changes) 3. **Prognostication:** Axonal involvement predicts worse prognosis 4. **Variant classification:** Distinguishes AIDP from AMAN, which has different geographical distribution and prognosis **Clinical Pearl:** Early in GBS (first 3 days), NCS may be normal; repeat studies at day 7–10 increase diagnostic yield to >90%. ### Diagnostic Criteria Integration GBS diagnosis relies on: - **Clinical:** Progressive weakness + areflexia (mandatory) - **CSF:** Albuminocytologic dissociation (high protein, normal/low cells) — present in ~80% by week 1 - **NCS/EMG:** Demyelinating or axonal features — most specific confirmatory test **Mnemonic:** **AIDP-D** = Ascending, Inflammatory, Demyelinating, Polyneuropathy, Distal-latency-prolonged.