Carpal tunnel is formed posteriorly by the carpal bones and extrinsic ligaments, and anteriorly by the flexor retinaculum or transverse carpal ligament, a thin fibrous band which inserts into the scaphoid and trapezium on the radial aspect and into the pisiform and hook of hamate on the ulnar side.
The median nerve runs superficial to the FDS (flexor digitorum superficialis) tendon for the second finger and medial to the FPL (flexor pollicis longus) tendon.
USG finding in carpal tunnel syndrome:
Median nerve typically swollen at the proximal carpal tunnel and flattened as it passes beneath the flexor retinaculum and at the distal extent of the tunnel regardless of the cause of the compression.
An abrupt change in the caliber of the nerve at the proximal carpal tunnel is referred to as the ‘notch sign’.
An increased cross section area of more than 10mm2 at the proximal carpal tunnel (scaphoid-pisiform level ) is generally accepted to be diagnostic of carpal tunnel syndrome.