The carpal tunnel is a rigid fibro-osseous canal at the wrist formed by the carpal bones posteriorly and the transverse carpal ligament (flexor retinaculum) anteriorly. It transmits the median nerve and nine flexor tendons: FPL, four FDS tendons, and four FDP tendons. Normal carpal tunnel pressure is 2-10 mmHg at rest; compression of the median nerve occurs above 20-30 mmHg. The contents of the tunnel are: the median nerve (most superficially and radially), surrounded by the nine flexor tendons.
Carpal tunnel syndrome is the most common peripheral nerve entrapment, caused by elevated intracanal pressure compressing the median nerve. Risk factors include repetitive wrist flexion, pregnancy, hypothyroidism, rheumatoid arthritis, and diabetes. Diagnosis relies on Phalen test, Tinel sign, and nerve conduction studies. Surgical carpal tunnel release (open or endoscopic) divides the transverse carpal ligament, immediately reducing tunnel pressure. The recurrent thenar motor branch must be identified and protected during surgery.
Median nerve compression within the rigid carpal tunnel produces hand numbness in the thumb, index, middle, and radial ring fingers with nocturnal waking, thenar atrophy in advanced disease, and positive Phalen and Tinel tests, treated by splinting and steroid injection conservatively or by transverse carpal ligament release surgically.
Acute elevation of carpal tunnel pressure from distal radius fracture haematoma, crush injury, or burns produces rapidly progressive median nerve ischaemia requiring emergency carpal tunnel decompression as for compartment syndrome, with a 6-hour window before irreversible nerve damage.
