[Analysis of unusual late responses of peripheral nerves]

F-waves are routinely evaluated in nerve conduction studies on proximal peripheral nerves. However, unusual late responses other than F-waves that show variation in latency and configuration sometimes confound evaluation of the F-waves. The unusual late responses were analyzed in this study. In two patients with cervical radiculopathy, two patients with brachial plexus injury and 9 patients with polyneuropathy, A-waves showing constant latency and configuration were recorded in addition to temporally dispersed F-waves. The A-waves in these patients persisted after supramaximal stimulation of the peripheral nerves, although the A-waves should normally disappear in response to supramaximal stimulation. The supramaximally stimulated A-waves were prominent in polyneuropathy and nerve lesions with demyelinating pathology. A patient with Guillain-Barré syndrome showed unusual repetitive discharges that might be important electrodiagnostic findings in the early stage of illness. In patients with myotonic dystrophy, electrical stimulation elicited repetitive discharges that continued from the M-waves. Recordings with a concentric needle electrode showed characteristics similar to those of myotonic discharges recorded during needle electrode insertion. When the repetitive discharges were mild, the F-waves were recorded. Absence of the F-waves in myotonic dystrophy might be related to the myotonic discharges elicited by electrical stimulation, and may not imply impaired conduction of the peripheral nerves. A patient with myokymia showed repetitive discharges with constant configuration but variable latency, although the F-waves were normal. These repetitive discharges may be evoked by ephaptic transmission resulting from abnormal excitability of the peripheral nerves or muscles of different etiologies.