TEACHING FILES - JOINT - CASE 1
WRIST - DISCUSSION
The median nerve when irritated or compressed is responsible for the Carpal Tunnel Syndrome. Usually the cause is idiopathic . On MSUS the finding is an enlarged median nerve. Occasionally, compression of the median nerve by a bone fracture, carpal bone instability (VISI or DISI), ganglion cyst, tenosynovitis of the flexor tendons, congenital elongation of the flexor muscles into the carpal tunnel (in shorter persons with short musculotendinous junctions), thickening of the flexor retinaculum/volar ligamentous complex can be the cause - which can be seen on MSUS. The median nerve, if isolated, would become more rounded when inflamed, however, the median nerve as it transverses the carpal tunnel is rigidly held in position by the overlying restricting flexor retinaculum/volar ligamentous complex. Thus, when the median nerve swells, it cannot do so in the top to bottom direction, only in the side-to-side direction - thus, it expands. Our observations are that most adult median nerves range from 0.6 to 0.9 cm in width in the transverse view. Anything over 1.0 cm is suspect for and any measurement over 1.5 cm is consistent with Carpal Tunnel Syndrome, if suspected clinically. Always compare to the contralateral side, especially if unilateral involvement is suspected or one side is more symptomatic than the other. If one nerve is significantly larger than the other, regardless of measurements, this suggests inflammation (assuming the smaller nerve is not abnormally small/atrophied)