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EMG / NCV

  • EMG/NCV is not helpful until 6 weeks after injury. Even complete nerve transection may not become apparent fot 2 to 6 weeks after injury.

Indication of denervation

  • Fibrillations
  • Postive sharp waves
  • Polyphasic potentials
  • typically take over 3 weeks to appear.

Demyelinating Neuropathy

  • slow conduction velocity, prolonged sensory and motor latencies, generally preserved amplitueds
  • DDX: Charcot-Marie-Tooth disease type 1A, acquired demyelinating neuropathy(conduction velocities typically vary from nerve to nerve, typically have conduction block; example = Gullian-Barre syndrome)

Axonal Neuropathy

  • reduced sensory and motor amplitudes, normal latencies and velocities, fibrillations and positive sharp waves
  • Features of reinnervation = large-amplitude, long-duration motor unit action potentials
  • DDX: idiopathic(evaluate with fasting blood glucose and 2-hour glucose tolerance test), diabetic neuropathy, toxic disturbance, alcoholism, cisplatin-induced neuropathy.

Terms

  • Conduction block: compound musle action potential amplitudes drop by more the 40% on proximal stimulation compared to distal stimulation. Highly suggestive of acquired demyelinating neuropathy.

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