Carpal tunnel syndrome: the tunnel, the median nerve and the release

1. A gutter of bones

Seen from the palm, the eight carpal bones form an arch, hollow towards the palm. Four bony pillars stand up at its corners: the pisiform and the hook of the hamate on the ulnar (little finger) side, the tubercle of the scaphoid and the tubercle of the trapezium on the radial (thumb) side. The floor of the gutter is the palmar surface of the carpus and its ligaments.

Surgeon’s corner: You can feel all four pillars: the pisiform at the ulnar end of the distal wrist crease, the hook of the hamate 2 cm distal and radial to it (deep pressure), the scaphoid tubercle at the radial end of the crease. A fracture of the hook of the hamate (golf, baseball, tennis: the handle hits it) can irritate the nearby ulnar nerve.

2. The roof: the flexor retinaculum

The flexor retinaculum (transverse carpal ligament) is a tough band a few millimetres thick that bridges the gutter, from the pisiform and the hook of the hamate to the scaphoid and trapezium tubercles. Gutter plus roof make the carpal tunnel: rigid on every side, so it cannot stretch when its contents swell. Its distal edge lies about at the level of the outstretched thumb’s web, well into the palm.

Surgeon’s corner: The tunnel is narrowest at the level of the hook of the hamate. Pressure inside it is lowest with the wrist straight and rises several times in full flexion or extension: this is why a night splint in neutral relieves the symptoms and why Phalen’s test works.

3. Through the tunnel: nine tendons and one nerve

Through the tunnel run the median nerve and nine flexor tendons: four of flexor digitorum superficialis, four of flexor digitorum profundus (deeper) and flexor pollicis longus on the radial side. The nerve is the most superficial structure, just under the roof, on the radial side of the superficialis tendons. Flexor carpi radialis does not enter the tunnel: it runs in its own split of the retinaculum.

Surgeon’s corner: Anything that adds volume squeezes the nerve: tenosynovitis (overuse, rheumatoid arthritis), fluid in pregnancy, hypothyroidism, diabetes, a ganglion, a displaced distal radius fracture or a lunate dislocation. Acute carpal tunnel syndrome after such an injury (rapidly rising numbness) needs an urgent release.

4. Outside the tunnel: what is spared

Three things cross the wrist over the roof, not through it. The palmaris longus tendon fans out into the palmar aponeurosis (it is missing in about one person in seven). The palmar branch of the median nerve leaves the main nerve about 5 cm above the wrist crease and supplies the skin over the thenar eminence. The ulnar nerve and artery pass beside the pisiform in Guyon’s canal.

Surgeon’s corner: Two clues in the history: in carpal tunnel syndrome the skin of the thenar eminence feels normal (its branch is outside the tunnel), and the little finger is never involved. Numbness of the thenar skin points to a lesion higher up (pronator syndrome); little-finger numbness to the ulnar nerve or a C8 root.

5. The median nerve in the hand

Just beyond the retinaculum the nerve gives its recurrent (thenar) motor branch, which hooks back into three thenar muscles: abductor pollicis brevis, opponens pollicis and the superficial head of flexor pollicis brevis; it also supplies the first two lumbricals ("LOAF"). The rest fans out into digital nerves for the palmar skin of the thumb, index, middle and the radial half of the ring finger. The thin recurrent branch is not drawn separately in this model.

Surgeon’s corner: Test abductor pollicis brevis: the hand flat on the table, palm up, the patient lifts the thumb towards the ceiling against your finger. Weakness and a flattened thenar eminence mean long-standing, severe compression: do not wait, operate.

6. Making the diagnosis

Typical story: tingling and numbness in the thumb, index and middle finger, worst at night or when holding a phone, a book or the steering wheel, relieved by shaking the hand. Later, clumsiness and dropping things. It is common in women aged 40–60, in pregnancy, diabetes, hypothyroidism and with vibrating tools, and is often on both sides. Use the slider to flex the wrist: Phalen’s test (full flexion for 60 seconds) narrows the tunnel and brings the symptoms on.

Surgeon’s corner: Bedside: Tinel’s sign (tapping over the tunnel sends tingling into the fingers), Phalen’s test and Durkan’s compression test (thumb pressure over the tunnel for 30 seconds, the most sensitive). Confirm with nerve conduction studies (slowed sensory conduction, prolonged distal motor latency); ultrasound shows a swollen nerve at the tunnel inlet. Rule out a C6/C7 radiculopathy (neck pain, reflexes) and polyneuropathy (feet too).

7. Treatment and the carpal tunnel release

Mild or intermittent symptoms: a night splint with the wrist straight, fewer provoking activities, a steroid injection into the tunnel (often good for months). Persistent, severe or motor symptoms: surgery. In the open release a short incision runs in the palm in line with the radial border of the ring finger; the palmar aponeurosis and then the whole retinaculum are divided along its ulnar side, from the forearm fascia to its distal edge. Endoscopic release gives the same result with a slightly quicker return to work.

Surgeon’s corner: Stay on the ulnar side of the retinaculum: the recurrent motor branch leaves the radial side and sometimes pierces the ligament itself. Keep the incision ulnar to the thenar crease to spare the palmar cutaneous branch, stop where the fat around the superficial palmar arch appears distally, and do not stray into Guyon’s canal. The commonest cause of persistent symptoms is an incomplete release of the proximal or distal edge. Tenderness beside the scar ("pillar pain") is usual for a few months.