A 28-year-old male presents to the emergency department with acute onset weakness and tingling in both legs progressing to the arms over 2 days. He reports a viral illness 3 weeks prior. On examination, he has flaccid paralysis of lower limbs with absent deep tendon reflexes. Nerve conduction studies show prolonged distal latencies and conduction block. CSF analysis reveals elevated protein with normal cell count. Which of the following best explains the pathophysiology of the conduction block observed in this patient's peripheral nerves?

Explanation

## Pathophysiology of Conduction Block in Demyelinating Disease This clinical vignette describes Guillain-Barré Syndrome (GBS), an acute demyelinating polyneuropathy. The key pathophysiological mechanism underlying conduction block is **demyelination**. ### Normal Saltatory Conduction **Key Point:** In myelinated nerves, action potentials propagate via saltatory conduction—the action potential jumps from node to Ranvier to node, with regenerative currents flowing beneath the myelin sheath. ### How Demyelination Causes Conduction Block 1. **Loss of myelin insulation** → current leaks passively across the denuded axon membrane 2. **Regenerative inward currents** (Na⁺ influx) fail to depolarize the next node to threshold 3. **Action potential cannot propagate** → conduction block on NCS (reduced compound muscle action potential amplitude) 4. **Prolonged distal latencies** occur because conduction slows in partially demyelinated segments ### Why This Explains the Clinical Findings | Finding | Mechanism | |---------|----------| | Flaccid paralysis | Motor nerve conduction blocked; no action potentials reach muscle | | Absent reflexes | Sensory and motor pathways both blocked | | Elevated CSF protein | Breakdown of blood-nerve barrier; protein leakage | | Conduction block on NCS | Demyelinated segments cannot sustain regenerative currents | **Clinical Pearl:** Conduction block (>50% reduction in CMAP amplitude) is the hallmark of demyelination and distinguishes it from axonal degeneration, where latencies are normal but amplitudes are reduced. **High-Yield:** The resting membrane potential and threshold remain normal in demyelination—the problem is **current flow**, not membrane excitability.