A 42-year-old woman from Bangalore presents with acute-onset weakness of the lower limbs and autonomic dysfunction 2 weeks after a respiratory tract infection. On examination, she has ascending paralysis, absent deep tendon reflexes, and elevated protein in cerebrospinal fluid (CSF) with normal cell count. Electromyography shows prolonged distal latencies and conduction blocks. A clinical diagnosis of Guillain-Barré syndrome (GBS) is made. Which of the following best explains the mechanism of motor nerve conduction failure in this patient?

Explanation

## Guillain-Barré Syndrome: Demyelinating Pathophysiology ### Clinical Presentation and Diagnosis **Key Point:** Guillain-Barré syndrome (GBS) is an acute, post-infectious, autoimmune disorder of the peripheral nervous system characterized by ascending paralysis, areflexia, and demyelination of motor nerves. ### Pathophysiology: Demyelination and Conduction Block #### 1. Autoimmune Attack on Myelin - **Molecular mimicry:** Antibodies generated against Campylobacter jejuni (or other pathogens) cross-react with gangliosides on myelin and axonal membranes - **Complement activation:** IgG and IgM antibodies bind to myelin epitopes, activating the classical complement cascade (C1q → C3 → MAC) - **Myelin destruction:** Complement-mediated lysis and macrophage infiltration strip myelin from motor nerves - **Demyelination pattern:** Typically involves motor nerves > sensory nerves; proximal > distal #### 2. Conduction Block Mechanism When myelin is lost: 1. **Exposed axon membrane** loses saltatory conduction capability 2. **Voltage-gated sodium channels** become sparse in demyelinated segments 3. **Action potential propagation fails** across the demyelinated region → **conduction block** 4. **Proximal stimulation** may elicit a response, but **distal stimulation** does not (or shows markedly reduced amplitude) ```mermaid flowchart TD A[Post-infectious state]:::outcome --> B[Molecular mimicry]:::action B --> C[Anti-ganglioside antibodies]:::outcome C --> D[Complement activation]:::action D --> E[Myelin destruction]:::outcome E --> F{Conduction block?}:::decision F -->|Demyelination| G[Slowed CV, prolonged latency]:::outcome F -->|Severe demyelination| H[Conduction block]:::urgent G --> I[Ascending paralysis]:::outcome H --> I ``` #### 3. Electrophysiological Findings in GBS | Finding | Mechanism | Timing | |---------|-----------|--------| | **Prolonged distal latencies** | Slowed conduction in demyelinated segments | Early (1–2 weeks) | | **Conduction blocks** | Complete failure of action potential propagation across demyelinated zone | Early to mid-course | | **Slowed conduction velocity** | Reduced saltatory conduction; action potential must propagate sequentially along exposed axon | Throughout | | **Prolonged F-wave latency** | Demyelination of proximal motor nerves | Early | | **Absent H-reflex** | Demyelination of sensory nerve roots | Early | **High-Yield:** **Conduction blocks** in motor nerves (not seen in primary axonal degeneration) are the electrophysiological hallmark of demyelinating GBS and distinguish it from axonal variants (AMAN, AMSAN). ### Comparison: Demyelinating vs. Axonal GBS | Feature | Demyelinating (AIDP) | Axonal (AMAN/AMSAN) | |---------|----------------------|---------------------| | **Pathology** | Myelin destruction | Axonal degeneration | | **Conduction block** | Present (early) | Absent | | **Distal latencies** | Prolonged | Normal | | **Conduction velocity** | Slowed | Normal or mildly reduced | | **EMG (late)** | Demyelination pattern | Denervation (fibrillations) | | **CSF protein** | Elevated | Elevated | | **Geography** | Worldwide (most common) | Asia, Latin America | | **Prognosis** | Generally better | Worse; more residual weakness | ### Why Demyelination Causes Weakness 1. **Conduction block** prevents action potential from reaching the motor end plate 2. **No acetylcholine release** occurs (presynaptic terminal never depolarizes) 3. **No postsynaptic depolarization** → **no muscle contraction** 4. **Clinical result:** Flaccid paralysis, absent reflexes **Clinical Pearl:** The **albuminocytologic dissociation** (elevated CSF protein with <5 WBC/μL) is pathognomonic for GBS and reflects demyelination of nerve roots with minimal inflammatory cell infiltration at the time of lumbar puncture. **Mnemonic:** **DAMP** = **D**emyelination → **A**ction potential block → **M**uscle **P**aralysis. ### Distinction from Other Neuromuscular Disorders - **Myasthenia Gravis:** Postsynaptic AChR destruction; decremental response on RNS; normal motor conduction velocity - **Lambert-Eaton:** Presynaptic VGCC blockade; incremental response on high-frequency stimulation; normal conduction velocity - **Botulism:** Presynaptic ACh release failure; normal conduction velocity; brief, small, abundant motor action potentials (BSAP) [cite:Harrison 21e Ch 380; Guyton & Hall 14e Ch 8; Robbins 10e Ch 27]