How can a demyelinating peripheral neuropathy and an axonal peripheral neuropathy be differentiated by NCS and needle EMG?
Axonal loss and demyelination rarely occur in strict isolation, but some electrodiagnostic features may indicate relatively more damage to myelin versus axons. The features of demyelinating neuropathies include moderate to severe slowing of conduction velocity, temporal dispersion of evoked sensory or motor action potentials, conduction block, and prolonged distal latencies. Axonal neuropathies show milder slowing of nerve conduction, with generally low sensory and motor amplitudes on NCS. The needle EMG shows denervation abnormalities early in axonal neuropathies and only later in demyelinating neuropathies when axons are secondarily affected.