A 5-year-old from a rural area presents with acute onset of asymmetric flaccid paralysis of one lower limb, fever having resolved 2 days prior, and CSF showing lymphocytic pleocytosis with elevated protein and normal glucose. Which finding on EMG/nerve conduction studies would best confirm lower motor neuron involvement?
- A Reduced nerve conduction velocity with conduction block
- B Normal motor nerve conduction velocity with reduced CMAP amplitude and denervation potentials on EMG ✓
- C Sensory conduction abnormalities with normal motor conduction
- D Increased F-wave latency with normal CMAP amplitude
Explanation
Poliomyelitis causes selective destruction of anterior horn cells (lower motor neurons), sparing sensory neurons. EMG/NCS findings in polio-related flaccid paralysis: motor nerve conduction velocity is normal or near-normal (as axons of surviving neurons conduct normally), but CMAP (compound muscle action potential) amplitude is reduced (fewer motor units functioning). EMG shows denervation potentials (positive sharp waves, fibrillation potentials) and reduced motor unit recruitment in affected muscles. Sensory studies are normal (sensory neurons unaffected). Reduced NCV with conduction block suggests Guillain-Barré syndrome (demyelinating). Increased F-wave latency is non-specific.
Reference: Ghai Essential Pediatrics, 10th ed.
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