Electrodiagnostic significance of supramaximally stimulated A-waves

A-waves are generally considered a nonspecific finding of unclear electrodiagnostic and clinical significance. We systematically identified A-waves during routine F-wave studies and defined them as supramaximally elicited reproducible intermediate to late responses that are clearly separate from the M-responses. In patients with A-waves, we noted electrophysiologic diagnoses, the nerve in which the A-wave was identified, the presence of A-waves in multiple nerves, and A-wave morphology. In 54 of 1,258 studies performed, A-waves were present in one or more nerves. Electrophysiologic diagnoses in patients with A-waves included diffuse axonal neuropathy (11.5%), demyelinating neuropathy (66.7%), motor neuron disease (6.5%), radiculopathy (3.6%), mononeuropathy (3.9%), and normal (tibial nerve only) 0.7%. A-waves were abnormal when found in any nerve except the tibial nerve. They were particularly prevalent and present in multiple nerves in acquired and hereditary demyelinating neuropathies, and they more often had a complex morphology. We postulate that demyelination is the crucial underlying pathophysiologic correlate of the supramaximally stimulated A-wave.