Nerve conduction studies (NCS) provide several parameters that inform the investigator about the number of axons and the conductive properties of a nerve. Distal stimulation of motor nerves with responses recorded from muscles give information on the maximum compound motor action potential amplitudes, reflecting the number of motor axons and in pathological states the extent of reinnervation, and distal motor latencies, that reflect the integrity of the most distal part of the myelin sheath. In demyelinating neuropathies, conduction velocities often fall dramatically, as the loss of myelin sheaths between the nodes of Ranvier prevents adequate saltatory impulse conduction. In axonal neuropathies, the loss of the largest and fastest-conducting axons will eventually also lead to a decrease in NCV, but to a much lesser extent as in demyelinating disorders, and not reaching the demyelinating range of < 70% of the lower limit of normal until the CMAP becomes less than 1 mV.
In a similar fashion, responses can be recorded from sensory nerves. In a clinical setting, SNAP amplitudes reflect the number of axons in a particular nerve in a more reliable way than the CMAP amplitude does. This is because the CMAP amplitude can be maintained even when axonal degeneration occurs by regional sprouting and reinnervation of motor units. SNAP NCVs also provide an indication of the integrity and quality of the myelin sheath. Nowadays, nerve conduction studies are often combined with ultrasound information regarding the imaging aspect of the nerves studied.
Proximal conduction studies include recording of F-responses and H-reflexes. F-responses are elicited by the antidromic conduction of a supramaximal electrical stimulus in a motor nerve, that causes depolarization and backfiring of a few anterior horn cells. Another proximal conduction technique is the so-called H-reflex. H-reflexes are CMAPs elicited by afferent activation of a monosynaptic reflex arc in the spinal cord, and as such they are the electrical equivalent of tendon reflexes. H-reflexes are used routinely for assessment of the proximal segments of these nerves.
In polyneuropathies needle EMG supplements the NCS findings. It can help demonstrate a distal to proximal gradient in the extent of axonal damage, and the amount of spontaneous activity gives an impression of the speed with which the disorder progresses (i.e. if there has been time for reinnervation to occur or not). In cases were demyelination is predominant it can show a reduced recruitment pattern signifying conduction block, and also the extent of concomitant axonal damage as described above.
