Nerve conduction studies and electromyography

Sensory studies

Sensory nerves are studied by stimulating the nerve at one point along it, for example the index finger, and recording at a distant site along the nerve, for example the median aspect of the wrist (Fig. 1a). By recording the time difference for the action potential to appear at two sites along the nerve and measuring the distance between them, the mean conduction velocity can be calculated across the segment of nerve between the two recording points. The amplitude of the action potential can be recorded from the oscilloscope screen. Sensory nerves can be studied orthodromically (distal to proximal) or antidromically (proximal to distal).

Fig. 1 Method of nerve conduction studies. (a) Sensory; (b) motor.

Motor studies

Motor studies involve stimulating the motor nerve at distal and proximal sites and recording the muscle action potentials (Fig. 1b). The amplitude of the muscle response (compound muscle action potential, CMAP) to supramaximal nerve stimulation can be measured. The time from the distal site of stimulation to the onset of the CMAP is the terminal or distal motor latency. This measure includes components of nerve conduction, neuromuscular transmission and muscle activation times. The difference in time to CMAP between the two sites of stimulation and the distance between is used to measure conduction velocity.

Interpretation of nerve conduction studies

There are two types of abnormality detected by nerve conduction studies:

A survey of appropriate nerves can establish the pattern of peripheral nerve involvement. This can be:

Focal: where a single nerve is affected, e.g. the median nerve at the wrist in carpal tunnel syndrome.

Multifocal: where there are multiple focal areas of abnormality. If this affects named nerves, it is mononeuritis multiplex; if not, it is a multifocal neuropathy.

Generalized neuropathy: this can be further characterized as axonal, if responses are small with normal velocity, or demyelinating, if there is prominent slowing of motor conduction (Fig. 2). The two changes can coexist. Typical patterns of abnormality are given in Table 1.

Fig. 2 Waveforms of normal, demyelinating and axonal neuropathy.

Some peripheral nerve diseases are not detectable on routine nerve conduction studies. These include:

sensory nerve root disease (any disease proximal to the dorsal root ganglion cell)

unmyelinated fibres, ‘small fibre’ neuropathies and autonomic neuropathies, for which other tests may be valuable (p. 115)

inaccessible nerves.

Some more specialist techniques are given in Table 2.

Table 2 Special techniques in peripheral neurophysiology

Spontaneous activity

The normal muscle at rest is electrically silent. The electromyogram may record abnormal spontaneous activity:

Fasciculations: spontaneous discharges of motor units, usually indicative of denervation.

Fibrillations: spontaneous muscle fibre discharges, indicative of denervation or inflammatory muscle disease.

More complex discharges, for example, myotonic discharges are heard as a ‘dive-bomber’ sound on the loudspeaker.

Motor unit analysis

The structure of the motor unit can be examined during gentle muscle contraction. Motor units are smaller and amplitude is of shorter duration in muscle disease. In lower motor neurone lesions, the amplitude is larger and discharges are more prolonged.

Recruitment of motor units and interference pattern

These are examined as the muscle is contracted with increasing force. A full interference pattern with limited force suggests a myopathy, as pathologically some fibres in each motor unit are affected, so motor units are small. A few motor units firing at high rates suggests a lower motor neurone lesion, because in denervation there is axonal sprouting of remaining axons to take over denervated muscle fibres, so the remaining motor units are fewer but larger.

Other situations

Neuromuscular junction disorder resulting from myasthenia gravis causes a failure of neuromuscular transmission with increasing use (p. 110). The electrophysiological equivalent of fatiguability is the decline in amplitude of the compound motor action potential on repetitive stimulation of the motor nerve. Single-fibre electromyography may also help in diagnosis (see Table 2).

Nerve conduction studies and electromyography

Nerve conduction studies can define the pattern and nature of peripheral nerve abnormalities.

Electromyography can detect denervation, the distribution of denervation and abnormalities in muscle disease.

Some patterns of peripheral nerve disease can only be detected indirectly using neurophysiology.