NCS Abnormalities

NCS Abnormalities

The wide variety of PNS dysfunction produces a limited number of electrophysiologic nerve conduction study abnormalities. They may be summarized as prolonged latency values and slowing of calculated NCV; evoked responses (CMAP or SNAP) that are attenuated, absent, or temporally dispersed. These changes can be focal or diffuse in nature. In general, changes along the X-axis (latency, NCV) are associated with nerve demyelination, whereas Y-axis changes (evoked response amplitude) are associated with axonal loss. However, axonal shrinkage can result in NCV slowing, and segmental demyelination of a significant number of axons can result in their inability to conduct a nerve action potential, resulting in a smaller evoked response.


X axis changes (latency, NCV)
Latency changes. Prolonged distal latency values are useful in identifying distal nerve pathology. The pattern of changes noted may be focal or diffuse in nature. For example, carpal tunnel syndrome is identified by prolonged distal median latency values. Sensory latency values are usually more sensitive indicators of early or mild pathology. Prolonged distal latency values of multiple nerves may be consistent with diffuse peripheral neuropathy or mononeuritis multiplex.
NCV slowing. Again, these changes may be focal or diffuse in nature. Peripheral polyneuropathy may be identified by NCV slowing of multiple nerves. Focal slowing of the ulnar nerve segment through the elbow compared to below elbow to wrist and axilla to above elbow segment may infer focal elbow pathology.
NCV slowing may be observed in cases of selective loss of large axons, owing to the fact that latency values and the resultant data are a function of these fibers. For example, in ALS, selective loss of large motor neurons may result in the calculation of NCV slowing of nerve segments of the limb without demyelination of these segments. The normal NCV of smaller diameter fibers is slower than the normative data.
Y-axis changes (amplitude of evoked responses)
In general, the amplitude of the evoked compound muscle action potential (CMAP) is a function of how many axons are contributing to making muscle fibers twitch. The sensory nerve action potential (SNAP), is the summated response of all sensory axons contributing to the evoked response.
CMAP or SNAP attenuation can be seen in cases of axonal loss. Attenuated or absent responses of multiple nerves may be seen in peripheral polyneuropathy with primary axonal degeneration. Severe focal nerve pathology with complete denervation ( e.g. carpal tunnel syndrome or ulnar neuropathy at the elbow) can result in absent evoked median or ulnar responses.
A focal conduction block can at times be demonstrated by an attenuated response with stimulation proximal to the compression site compared to stimulation immediately distal to the compression site. For example, ulnar neuropathy at the level of the medial epicondyle may be identified by a smaller CMAP with stimulation proximal to the medial epicondyle compared to stimulation immediately distal to the medial epicondyle.



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