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
- Postive sharp waves
- Polyphasic potentials
- typically take over 3 weeks to appear.
- 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)
- 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.
- 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.