In the tightly crowded flexor tunnel the median nerve can be compressed, especially by long-continued swelling in the synovial sheaths. The symptoms include wasting and weakness of the thenar muscles (with loss of power of opposition of the thumb) and anaesthesia over three and a half digits on the thumb side of the hand. There is no anaesthesia over the thenar eminence itself, for this area of skin is supplied by the palmar cutaneous branch of the median nerve, or occasionally by the lateral cutaneous nerve of the forearm. Either of these nerves enters the palm superficial to the retinaculum, and so escapes compression. Surgical division of the retinaculum relieves the pressure and the symptoms.
The carpal tunnel syndrome must be distinguished from median nerve damage at a higher level. In the latter case the palmar cutaneous branch will be affected, and in addition weakness of the relevant flexor muscles in the forearm (e.g. flexor pollicis longus) is a notable feature. In the carpal tunnel syndrome the terminal phalanx of the thumb can be flexed with normal power, but with higher lesions this power is lost.