Carpal Tunnel Syndrome

Introduction

Introduction to Carpal Tunnel Syndrome

Carpal tunnel syndrome (CTS) is the most common entrapment neuropathy, resulting from compression of the median nerve as it passes through the fibro‑osseous carpal tunnel at the wrist. The carpal tunnel is bounded by the carpal bones and the flexor retinaculum, and contains the median nerve along with nine long flexor tendons. Any condition that increases the volume of the tunnel contents or reduces the cross‑sectional area of the tunnel can elevate the pressure and compress the median nerve, producing the classic symptoms of pain, paraesthesia, and numbness in the radial three and a half digits, along with thenar motor weakness in advanced cases.

A precise knowledge of the anatomy of the carpal tunnel and its surrounding structures is essential for understanding the pathophysiology, clinical presentation, diagnosis, and safe surgical management of CTS.

Study

1. Surgical Anatomy of the Carpal Tunnel

The carpal tunnel is a narrow fibro‑osseous space at the volar aspect of the wrist. Its floor is formed by the concave arch of the carpal bones: scaphoid tubercle and trapezium laterally; capitate, trapezoid, and hamate centrally; pisiform and hook of hamate medially. The roof is the flexor retinaculum (transverse carpal ligament), a strong fibrous band attaching medially to the pisiform and hook of hamate, and laterally to the scaphoid tubercle and trapezial crest. Proximally, it is continuous with the antebrachial fascia; distally it blends with the palmar aponeurosis.

The contents of the tunnel are the median nerve and nine tendons: four flexor digitorum superficialis (FDS), four flexor digitorum profundus (FDP), and the flexor pollicis longus (FPL). The FDS tendons to the middle and ring fingers lie most superficially, directly deep to the median nerve. The median nerve lies immediately beneath the flexor retinaculum, making it the most vulnerable structure to compression. All tendons are invested in synovial sheaths: the FDS and FDP share the common ulnar bursa, while the FPL has a separate radial bursa.

2. Pathophysiology of Median Nerve Compression

Normal carpal tunnel pressure ranges from 2–10 mmHg in neutral wrist position. In CTS, the pressure can rise to over 30 mmHg, and with wrist flexion or extension, it can exceed 90 mmHg. Elevated pressure impairs epineurial blood flow, causing venous congestion, oedema, and eventually ischemia of the nerve. Sustained compression leads to segmental demyelination and axonal loss. The sensory fibres, being larger and more superficial in the nerve, are affected first, producing the initial paraesthesiae. Motor fibres are involved later, explaining the progression from sensory symptoms to thenar atrophy.

Conditions that increase tunnel contents include tenosynovitis (rheumatoid arthritis), oedema (pregnancy, hypothyroidism), masses (ganglion, lipoma), or anomalous muscles. Conditions that reduce tunnel dimensions include acromegaly, malunited distal radius fractures, and congenital narrowing.

3. Clinical Features

Patients typically present with intermittent paraesthesia and numbness in the median nerve distribution: the palmar aspect of the thumb, index, middle, and radial half of the ring finger. Pain may radiate proximally into the forearm or even the shoulder. Symptoms are often worse at night and may awaken the patient. The ‘flick sign’—shaking the hand to obtain relief—is characteristic. Sensory loss over the thenar eminence is absent because the palmar cutaneous branch of the median nerve arises proximal to the carpal tunnel and passes superficial to the flexor retinaculum.

In advanced cases, thenar motor weakness develops, manifesting as difficulty with thumb opposition and fine manipulation. The abductor pollicis brevis is the most commonly atrophied thenar muscle. Loss of two‑point discrimination in the median nerve territory indicates chronic axonal loss.

4. Provocative Tests and Diagnosis

Phalen’s test: The patient flexes both wrists maximally for 60 seconds; reproduction of symptoms is considered positive. This increases carpal tunnel pressure. Tinel’s sign: Percussion over the carpal tunnel elicits tingling in the median nerve distribution. Durkan’s compression test: Direct pressure applied over the carpal tunnel for 30 seconds reproduces symptoms. The hand elevation test (holding hands above the head) also elevates tunnel pressure and may reproduce symptoms.

Electrodiagnostic studies (nerve conduction studies and electromyography) are the gold standard for confirming CTS, demonstrating prolonged distal motor and sensory latencies of the median nerve across the wrist. High‑resolution ultrasound can show increased median nerve cross‑sectional area proximal to the tunnel and bowing of the flexor retinaculum.

5. Anatomical Variations and Their Surgical Relevance

Several variations may be encountered during carpal tunnel release. A persistent median artery, present in about 10% of individuals, accompanies the median nerve and can contribute to dynamic compression. A bifid median nerve may be associated with a persistent median artery, increasing the risk of inadvertent nerve injury during surgery. The recurrent motor branch of the median nerve normally arises distal to the flexor retinaculum, but in about 20% of cases it may pierce the retinaculum (transligamentous) or arise within the carpal tunnel and cross over the retinaculum (subligamentous), placing it at risk during release. Anomalous muscles, such as a proximal lumbrical origin or an accessory FDS belly, can also crowd the tunnel.

6. Management

Mild to moderate CTS is managed conservatively with night splints (wrist in neutral), activity modification, and corticosteroid injections into the carpal tunnel. Surgical release is indicated for severe symptoms, failed conservative treatment, or thenar atrophy. The procedure involves division of the flexor retinaculum under local or general anaesthesia, either open or endoscopic. Care must be taken to protect the palmar cutaneous branch of the median nerve, the recurrent motor branch, and the superficial palmar arch. Complete division of the retinaculum is essential to prevent recurrence.

7. Complications of Carpal Tunnel Release

Incomplete release is the most common cause of persistent or recurrent symptoms. Injury to the recurrent motor branch can cause thenar paralysis. Damage to the palmar cutaneous branch leads to painful neuroma formation. Scar tenderness, pillar pain (deep palmar pain), and flexor tendon bowstringing are other recognised complications. Complex regional pain syndrome may occur, especially in patients with poor preoperative function.

Summary

Summary of Carpal Tunnel Syndrome

Carpal tunnel syndrome is a compressive neuropathy of the median nerve within the fibro‑osseous carpal tunnel, presenting with nocturnal paraesthesiae in the lateral three and a half digits, thenar weakness, and positive provocative tests. Knowledge of the tunnel’s anatomy, including its boundaries, contents, and the course of the median nerve and its branches, is essential for accurate diagnosis, safe injection, and surgical decompression. Variations such as a bifid median nerve or aberrant motor branch must be recognised to avoid iatrogenic injury.