Diagnostic Significance of Distal Compound Muscle Action Potential (CMAP) Duration-A Case Report

Abstract

The diagnostic criteria of chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) are based on clinical findings and auxiliary investigations, of which the most important ones are neurophysiological findings. Since the European Federation of Neurological Societies/ Peripheral Nerve Society (EFNS/PNS) incorporated distal CMAP duration into the neurophysiological diagnostic criteria, their sensitivity increased to 81.3% and specificity to 96.2%. We describe a case of a 58-year-old man who was referred to our electromyography (EMG) laboratory for a second opinion regarding his diagnosis. The patient exhibited sensory-motor polyneuropathy with prominent hypotrophy of the calf muscles. Electrodiagnostic testing prior to admission had revealed a probable axonal neuropathy. In our EMG laboratory, we accomplished nerve conduction studies (NCS) of both hands and legs and we noted a prominent increase of distal CMAP duration, whereas distal motor latencies (DML) were within normal limits. As the CMAP amplitude in the tibial and peroneal nerves was less than 1 millivolt (mV), the existence of myelinopathy could not be inferred from any other parameters identified in the lower extremities. Neurophysiological investigation of both hands was necessary, even though the clinical picture of polyneuropathy was substantially more prominent in the legs. It is important to distinguish between axonal and myelin sheath lesions in order to understand the pathogenetic aspects of neuropathy. In addition, treatment of demyelinating autoimmune neuropathies can be offered and it is effective. In our patient, distal CMAP duration was the main discriminating parameter among the various neurophysiological parameters of the CIDP diagnostic criteria.