[An analysis of A-wave with slowed conduction in its afferent and efferent pathways below the branching point]

Motor nerve conduction study was carried out in a 59-year-old woman with 20 years' history of diabetes mellitus. When her right ulnar nerve was stimulated at the wrist, the needle electrode penetrated into the first dorsal interosseous muscle revealed an existence of A-wave. Following characteristics were observed: (1) A-wave appeared with more than 20 mA of the stimuli, in a all-or-none manner at the latency of 54 msec; (2) when stimulated more than 50 mA, A-wave jumped at 10 msec of the latency and were superimposed upon the M-wave; (3) A-wave had a stable latency; (4) when stimulated at the more proximal site, A-wave had longer latency (direct pathway) by strong stimuli, and the potential had shorter latency (indirect pathway) by weak stimuli; and (5) A-wave was not collided by paired stimuli. A-wave conduction velocity was slower than 10 m/sec in the distal part of the reflecting point both in afferent and efferent pathways. This report is the first to show strikingly slowed conduction of A-wave distal to the reflecting point compared to the proximal portion. This fact supports the thesis that A-wave is generated from the branching of immature regenerating fibers. M-wave conduction velocity also decreased around the reflecting point of the A-wave, suggesting that, in addition to moderate diabetic polyneuropathy, entrapment at the cubital tunnel may be involved in generating this A-wave.