Second lumbrical muscle recordings improve localization in severe carpal tunnel syndrome.

OBJECTIVE

To determine how often the second lumbrical motor potential is present when the abductor pollicis brevis (APB) motor potential is absent in severe carpal tunnel syndrome (CTS).

DESIGN

Prospective study of consecutive patients with severe CTS and an absent motor potential from the APB.

SETTING

Single-center public hospital-based electromyography lab.

PARTICIPANTS

Patients with a clinical diagnosis of CTS who had an absent median sensory response and an absent median motor response to APB on routine nerve conduction testing. Twenty-two hands of 19 patients were examined.

INTERVENTIONS: Not applicable.

MAIN OUTCOME MEASURE: Presence and distal latency of motor potential to the second lumbrical.

RESULTS

The second lumbrical potential was present in 17 hands (77%). The distal motor latency to the second lumbrical was prolonged in all (mean, 9.1ms; normative value, <4.1ms).

CONCLUSIONS

Second lumbrical recordings improve localization in many patients with severe CTS when routine median sensory and motor conduction studies produce no potentials.