We investigated the indication of endoscopic carpal tunnel release (ECTR) for carpal tunnel syndrome (CTS) based on the results of a nerve conduction study (NCS). 38 hands of 35 patients with CTS were divided into 4 types preoperatively based on the results of the NCS. The compounds muscle action potentials (CMAPs) were recorded at the abductor pollicis brevis (APB) and the second lumbrical muscle (L2) following median nerve stimulation at the wrist. CTS with similar prolonged distal motor latency of the APB and L2 was included in type 1 (25 hands). CTS with prolonged distal motor latency of the APB longer than that of L2 (>0.7ms) was included in type 2 (10 hands). CTS with the CMAPs at the APB alone recordable was included in type 3 (3 hands). CTS in which the CMAPs at the APB and L2 were not elicited was included in type 4 (2 hands). After ECTR, all the CTS of type 1 and 2 improved well and the latency difference between APB and L2 was reduced for type 2. In 1 type 3 hand, the CMAPs at the APB were not elicited, although the CMAPs at L2 improved. In 1 of 2 type 4 hands, CMAPs at L2 alone were elicited.
In conclusion, the ECTR was indicated for CTS of type 1 and 2 abnormality, but a few patients with type 3 and 4 abnormality had poor results with the ECTR probably due to the anatomical variation of the recurrent motor branch to the APB.
