Clinical Uses of Nerve Conduction Studies

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Dr. Labrum Peripheral Neuropathy Program

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Motor conduction studies are helpful in indicating that weakness is due to pathology of the peripheral nerves rather than other parts of the motor unit. Sensory conduction studies may indicate that sensory symptoms are due to an impairment of peripheral nerve function or, when normal, to a lesion proximal to the dorsal root ganglia. Motor and sensory conduction studies are important in determining the presence and extent of a peripheral neuropathy, distinguishing between a polyneuropathy and mononeuropathy multiplex, recognizing the selective involvement of motor or sensory fibers, and following the course of peripheral nerve disease. The nature of the abnormalities found on motor or sensory conduction studies may suggest the type of underlying pathology and, in particular, whether this is primarily axonal loss or segmental demyelination. y Nerve conduction studies are also important as a means of recognizing subclinical polyneuropathies and in detecting and localizing a focal lesion involving individual nerves.

Focal nerve lesions may result from injury or compression

Figure 24-22 Arrangement for measuring motor conduction velocity in the forearm segment of the median nerve. The nerve is stimulated difAJ and proximally.B), and the evoked potentials are recorded from the abductor pollicis brevis muscle. (Ground electrode not shown.) Maximal motor conduction velocity (m/sec) between the proximal and distal sites of stimulation is calculated by dividing the distance between these sites by the difference in latency of the resfF/om Aminoff MJ: Electromyography in Clinical Practice, 3rd ed. New York, Churchill Livingstone, 1998.)

at sites of entrapment of the nerve by neighboring anatomical structures. Common entrapment neuropathies include compression of the median nerve at the wrist (carpal tunnel syndrome); the ulnar nerve at the elbow, cubital tunnel, or Guyon's canal; and the peroneal nerve at the head of the fibula. Electrophysiological studies are very important in localizing the lesion in such disorders and also in excluding the possibility of a subclinical polyneuropathy manifest primarily by the entrapment neuropathy. The findings depend on the duration and severity of nerve damage, the rapidity of its evolution, and the underlying disorder. y

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