Nerve conduction velocity in degenerative disease depends on which nerve fibres are affected. If the largest diameter ones are spared, velocity will be normal. Otherwise, there will be a decrease of up to 30 per cent. The localising value of nerve conduction studies in degenerative disease is limited though hardly required after focal trauma. Electromyography will usually also be needed. An abnormal finding will always be found if either the stimulating or recording electrode lies over a region of degeneration. Preganglionic pathology occurring in radiculopathies is associated with normal sensory conduction. The degree to which sensory nerve action potentials (SNAPs) are reduced in amplitude is a useful guide to the severity of the pathology within postganglionic fibres. These concepts are illustrated with simple but helpful diagrams.

Slowed conduction in a demyelinated nerve is proportional to the degree of pathology. More severe pathology leads to conduction block and ultimately to degeneration of the nerve. Degeneration causes the amplitudes of the action potentials, sensory nerve action potentials (SNAPs) or compound muscle action potentials (CMAPs), to be reduced. Unfortunately, the positively skewed distribution of these severely limits their clinical usefulness in assessing the degree of pathology. The demonstration of unstable nascent potentials can be a useful sign that motor nerve regeneration is occurring but, in general, the contribution of electrophysiological techniques to the monitoring of peripheral nerve degeneration is disappointing.

Most peripheral nerve contain both sensory and motor fibres and are called mixed nerves. It is often desirable to test these two components separately. The simple principles involved are presented. Sensory testing yields sensory nerve action potentials (SNAPs). Likewise, with motor testing we get compound muscle action potentials (CMAPs). The concept of supramaximal stimulation is discussed and important factors affecting nerve conduction are stated.