Thenar motor syndrome: median mononeuropathy of the hand

To evaluate a population with presumed carpal tunnel syndrome (CTS) for isolated involvement of the median motor nerve distal latency at or near the thenar eminence.

This prospective study performed in various outpatient venues randomly selected 500 railroad workers with hand symptoms and presumed CTS entering into litigation. Electrodiagnostic studies were performed by using common techniques and distances. The dominant hand of each subject was evaluated. A prolonged median minus ulnar nerve distal latency differential (MUD) was the standard for determining CTS. Subjects with an isolated delay of motor MUD, with a normal sensory and midpalmar MUD, were classified as having thenar motor syndrome (TMS).

After eliminating all subjects who did not meet the criteria of the electrodiagnostic studies, there remained a cohort of 126 subjects. There were 38 who met the criteria for TMS, and all others were classified as true CTS. Both groups were overweight, and the body mass index of the true CTS group was significantly higher (P < 0.006).

TMS is a distinct electrodiagnostic entity. As an artifact of the study design (the exclusion of subjects with prior surgery), the relative frequency of TMS was increased. Aside from body mass index, no anthropometric or demographic difference between TMS and true CTS was demonstrated. Although no occupation source was identified, a possible pathogenesis includes local trauma superimposed on a common anatomic variant.