GENERATOR SOURCES FOR THE EARLY AND LATE ULNAR HYPOTHENAR PREMOTOR POTENTIALS - SHORT SEGMENT ELECTROPHYSIOLOGIC STUDIES AND CADAVERIC DISSECTION

Objective: Determine the generator sources for the ulnar hypothenar pr emotor potentials (PMPs). Design: Observational. Setting: EMG laborato ry. Subjects: Ten asymptomatic adult volunteers, three cadaver hands. Main Outcome Measure: Far-field versus near-field characteristics of r ecorded PMPs as determined by bipolar and referential recording electr ode montages. A possible anatomic basis for any observed differences b etween ulnar PMPs and previously studied median PMPs was explored thro ugh cadaveric dissection. Results: An early PMP (E-PMP) had a latency that varied with changes in the position of GI only. A late PMP (L-PMP ) was seen only when G(1) and G(2) were on different volumes (palm vs fifth digit, or second digit vs fifth digit); its latency did not vary significantly with changes in the position of G(1) and G(2). Conclusi ons: (1) E-PMP is a near-field potential generated by the ulnar nerve passing near the G(1) electrode. (2) L-PMP represents a far-field pote ntial generated by the ulnar digital nerves as they traverse from the hand volume containing G(1) to the finger volume containing G(2). (3) Greater L-PMP-to-CMAP separation in the median than in the ulnar nerve was explained by cadaveric dissection, which revealed that the motor branch (responsible for the trailing CMAP) is longer in the median ner ve than in the ulnar nerve relative to each nerve's corresponding digi tal sensory branch (responsible for the preceding L-PMP). (4) The PMP that is typically recorded with G(1) at the hypothenar motor point and G(2) on the fifth digit most likely represents E-PMP, (5) Any propose d diagnostic use of the ulnar PMPs must take into consideration these generator sources. (C) 1996 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and rehabilita tion