The M-response amplitude and the sensory nerve action potential amplitude are reduced in relation to number degenerated axons. EMG confirms the diagnosis by revealing signs of denervation and reinnervation. F-responses which is the recurrent motor responses after peripheral nerve stimulation occures with prolonged latencies. Even more typical is the early loss of number of F-responses indicating a conduction block. In GBS, many nerves should be studied because of the variable involvement of different nerves.
Motor nerves are usually more involved than sensory.
Nerve conduction studies may be very useful in the examination of nerve entrapment . The focal lesion may cause axonal degeneration distal to the entrapment site. The amplitude of the sensory or motor response recorded distally, is reduced both when stimulation is performed diatal and proximal to the lesion. In other types of entrapment focal demyelination may occur. A distal stimulation gives normal response but proximal stimulation gives rise to a prolonged response with a prolonged latency corresponding to a focal slowing of conduction velocity.