## Diagnostic Confirmation in GBS **Key Point:** Nerve conduction studies (NCS) and electromyography (EMG) are the gold standard investigations for confirming Guillain-Barré Syndrome and determining its electrophysiological subtype. ### Why NCS/EMG is the Investigation of Choice NCS/EMG demonstrates characteristic findings that confirm demyelination or axonal degeneration: - **Demyelinating pattern** (AIDP — Acute Inflammatory Demyelinating Polyneuropathy, ~90% of cases in India): - Slowed conduction velocity (<80% of lower limit of normal) - Prolonged distal latencies - Conduction blocks (most specific) - Preserved compound muscle action potentials (CMAP) amplitude - **Axonal pattern** (AMAN/AMSAN — seen more in Asia): - Reduced CMAP amplitudes - Relatively preserved conduction velocities - Absent H-reflexes and F-waves **Clinical Pearl:** NCS/EMG findings evolve with disease progression. Early in the illness (first 3–5 days), NCS may appear normal; repeat testing after 1 week increases diagnostic yield. **High-Yield:** The **conduction block** on motor NCS is the most specific finding for demyelination and is pathognomonic when present. ### Role of Other Investigations | Investigation | Role in GBS | Limitations | |---|---|---| | **CSF analysis** | Albuminocytologic dissociation (↑ protein, normal WBC) supports diagnosis | Not specific; seen in other conditions; may be normal in first week | | **MRI spine** | Rules out spinal cord compression; may show nerve root enhancement | Not diagnostic for GBS; used to exclude mimics | | **Anti-ganglioside antibodies** | Prognostic value; associated with axonal subtypes | Not required for diagnosis; present in only 50–60% of cases; time-consuming | **Mnemonic:** **NCS-EMG = GOLD** — **G**old standard, **O**bjective findings, **L**ocalization of lesion, **D**ifferentiates subtypes. [cite:Harrison 21e Ch 446] 
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