Lumbrical sparing in carpal tunnel syndrome: anatomic, physiologic, and diagnostic implications.

Motor axons supplying lumbrical muscles are less severely affected than axons supplying thenar muscles in the carpal tunnel syndrome; sometimes lumbrical motor fibers are less affected than digit 2 sensory fibers. This pattern is consistent with compression of both the anterior and posterior aspects of the median nerve in the carpal tunnel because nerve fibers responsible for thenar, lumbrical, and digit 2 functions lie in an anterior-posterior gradient within the distal median nerve. Recognition of lumbrical sparing supports the electrodiagnosis of carpal tunnel syndrome when the distal latency to thenar muscles or the palm-to-wrist mixed median nerve conduction velocity is normal.