The carpal tunnel syndrome

Abstract

ERIPHERAL NERVES may be affected by a variety of injuries. If a peripheral nerve is injured in a specific location as the result of a mechanical irritation by some anatomical phenomenon, the condition is called an entrapment neuropathy. This type of nerve injury has been described by the term neuropraxia’ to characterize absence of action in an otherwise intact nerve. Neuropraxia may be caused by contusion, concussion, traction, or compression. Most peripheral nerves in their anatomic course pass through tough inelastic fibro-osseous tunnels, and are accompanied by tendons and/or blood vessels. These areas are well known and are frequently the landmarks used for local anesthetic nerve blocks. The best known of all these potential entrapment points is the carpal tunnel. Located on the volar aspect of the wrist joint it contains the median nerve. It is at this point in its peripheral course that the median nerve is most frequently injured so that it exhibits the physical symptoms and signs of neuropraxia. Although Paget in 1854 described median nerve compression at the wrist secondary to fracture callus, only in thhe past two decades has compression of the median nerve been appreciated as a distinct clinical entity and aggressively treated.*13 FUNCTIONAL ANATOMY The usual muscular distribution of the median nerve in the hand is to four and one-half muscles. They are: lumbricals I and II (the two most radial lumbricals), the abductor pollicis brevis, the opponens pollicis, and the superficial (main head) of the flexor pollicis brevis. These muscles act in conjunction with the long thumb flexor, thumb extensors, and abductor to produce almost all motions at the thumb metacarpophalangeal and carpometacarpal joints.4 The usual sensory supply of the median nerve is to the volar (palmar) side of the hand. This includes the radial half of the palm, most of the volar skin of the thumb, index, middle fingers, and radial half of the ring finger. Recently considerable attention has been given to the anatomical course of the motor branch of the median nerve which supplies the bulk of the thenar musculature. Its course may be variable both within the carpal tunnel and as it passes terminally to innervate the thenar muscles .5-7 The carpal canal also contains all the long flexor tendons to the fingers and thumb. Occasionally aberrant blood vessels are also present in the canal. All of these structures are covered by the strong transverse carpal ligament (flexor retinaculum) which extends across the concavity of the carpal arch. This ligament is attached medially to the pisiform bone and to the hamulus of the hamate bone; laterally it is attached to the tubercle of the navicular and to the greater multangular. The floor of the carpal tunnel is formed by the central carpal bones.