Significance of A-waves recorded in routine motor nerve conduction studies.

The occurrence of A-waves during routine F-wave studies was investigated in 556 consecutive patients referred to the Department of Clinical Neurophysiology at the University Hospital in Uppsala for various neuromuscular disorders. Altogether, 2367 nerves in the upper and lower extremities were studied. An A-wave, with a nearly constant latency and a uniform shape on consecutive stimulations, could be recorded in 184 nerves (7.8%) out of 124 patients (22.3%). More than 50% of patients with A-waves had various types of polyneuropathies. Of all patients with polyneuropathy, 65% had at least one nerve with A-waves. A-waves occurred somewhat less frequently in patients with radiculopathies. In other proximal local nerve lesions they were found less often and only exceptionally in patients with distal nerve lesions. A-waves were present in 6 out of 10 patients with motor neurone diseases. There was no correlation between the number of A-waves found in one nerve or the number of nerves in a given patient with A-waves and the aetiology or severity of the underlying disease. A-waves were found in 11 patients referred for various neurological symptoms in whom other neurophysiological findings were normal. This might be interpreted as an early sign of underlying disease because in 100 healthy controls no A-waves could be elicited, with the exception of 3 subjects who had A-waves in the abductor hallucis muscle when the tibial nerve was stimulated. We conclude that the appearance of A-waves should be considered a sign of either a local nerve lesion or a generalised neuropathy in all other nerves except for the tibial nerve.