Compound muscle action potential amplitude and area changes in normal subjects and patients with carpal tunnel syndrome.

Lesions of mixed peripheral nerves are often diagnosed by means of electrodiagnostic tests, especially by motor and sensory conduction studies. In order to clarify the varying degrees of motor nerve involvement in patients with carpal tunnel syndrome (CTS), we designed this study for comparing the results of compound muscle action potential (CMAP) amplitude and area under the negative phase between normal persons (n = 662) and patients having CTS (n = 250). The CTS cases were categorized into two distinct groups, i.e., CTS-D (n = 120) having prolonged median sensory latency and normal needle examination, CTS-AD having prolonged median sensory latency and abnormal needle findings (n = 130). Subjects in the CTS-AD group showed significant reduction, both in CMAP amplitude and area from 3rd to 7th decades; however, in the CTS-D group there were significantly smaller CMAP amplitude from 5th to 7th decades but CMAP area demonstrated wide variations. These data indicate that CMAP amplitude and area are helpful in documenting motor nerve axonal loss, but care must be exercised when attempt is made to define axonal loss based either on area or amplitude because of the temporal dispersive effects.