Investigation Of Neuropathy

Sensory conduction can also be measured:

Record 2

Record 1

Stim.

Record 2

Record 1

Stim.

The index finger is stimulated and the evoked sensory potential recorded at wrist and elbow. Measurements enable calculation to be made of the latencies and conduction velocity. Note the considerable difference in amplitude between sensory and motor evoked potentials.

Distance between two recording sites Conduction velocity = Difference in latency between the two evoked responses.

GENERAL OBSERVATIONS

Amplitude of response - a function of the number of axons which respond to stimulation. Latency of response - a function of the speed at which the largest fibres in a nerve will conduct. Axonal degeneration -> reduced amplitude or absence of response to stimulation with mild slowing of conduction velocity. Demyelinative disorders -» marked slowing of conduction velocity (30% at least reduced) with progressive reduction of amplitude.

Localised compression of nerve —► slowing of conduction in region of block, e.g. over the elbow when ulnar nerve is compressed at that site. Conduction block, distant from entrapment sites may suggest multifocal motor neuropathy (see page 427).

The concentric needle has at its centre an insulated wire and potential differences between the centre and outer casing are measured

The concentric needle has at its centre an insulated wire and potential differences between the centre and outer casing are measured

2. Electromyography

A fine needle is inserted into the muscle and the recorded activity displayed on an oscilloscope. Electromyography is primarily of value in muscle disease but can also give indirect evidence of a neuropathic process. The presence of denervation in paraspinal muscles indicates proximal nerve root disease.

If chronic denervation has occurred, reinnervation may be present with long duration high amplitude motor unit potentials.

Also, with voluntary efforts, poor recruitment of motor units is seen on the oscilloscope screen.

3. Nerve biopsy

A biopsy is most likely to aid diagnosis in asymmetric multiple mononeuropathies (vasculitis, amyloidosis, sarcoidosis etc). The sural nerve is usually chosen, provided its sensory conduction is abnormal.

2mV^Normal

Reinnervation

Long duration, polyphasia

high amplitude.

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