What Is Carpal Tunnel Syndrome?

The Carpal Tunnel: Anatomy

The carpal tunnel is a rigid, non-expandable fibro-osseous channel at the base of the wrist formed by the carpal bones on three sides (forming a concave arch) and the transverse carpal ligament (flexor retinaculum) as its palmar roof. Through this tunnel pass nine flexor tendons (four flexor digitorum superficialis, four flexor digitorum profundus, and one flexor pollicis longus) and the median nerve. The median nerve provides sensory innervation to the thumb, index finger, middle finger, and the radial half of the ring finger, and motor innervation to the thenar muscles (the intrinsic thumb muscles responsible for opposition and pinch grip). The tunnel's rigid walls mean that any process increasing its internal volume — swelling, tendon thickening, or anatomical variation — necessarily increases pressure on the median nerve, which has the least mechanical tolerance of the tunnel's contents.

Mechanism of Nerve Compression

Elevated carpal tunnel pressure compresses the median nerve, initially impairing microvascular flow and venous drainage within the nerve (the epineural microvasculature is the most pressure-sensitive component). Sustained or repetitive elevated pressure produces segmental demyelination of the nerve fibres at the compression site, slowing nerve conduction velocity — the finding that nerve conduction studies detect. With prolonged compression, axonal changes develop, producing the motor weakness and thenar atrophy of advanced carpal tunnel syndrome. The pressure dynamics within the tunnel are substantially influenced by wrist position: carpal tunnel pressure is lowest in neutral wrist position and rises dramatically with wrist flexion and extension, explaining why symptoms characteristically worsen at night (when the wrist tends to fall into flexion during sleep) and with sustained grip or wrist-loaded activities.

Why Does It Develop?

Most cases of carpal tunnel syndrome are idiopathic — no single causative factor can be identified. However, several conditions substantially increase risk by reducing tunnel volume or increasing median nerve vulnerability: pregnancy (fluid retention increases tunnel pressure — affects up to 20% of pregnant women, typically resolving postpartum); hypothyroidism (myxoedematous tissue infiltration); rheumatoid arthritis (tenosynovial inflammation); diabetes mellitus (peripheral nerve vulnerability through metabolic axonopathy); obesity; wrist fracture (altered tunnel anatomy); and repetitive occupational wrist loading (forceful grip combined with repetitive wrist flexion-extension). Women are affected two to three times more commonly than men, partly attributable to anatomically smaller carpal tunnel dimensions.

Symptoms and Diagnosis

The cardinal symptoms are nocturnal paraesthesia (numbness, tingling, or burning in the thumb, index, and middle fingers, waking the patient and relieved by shaking or "flicking" the hand — the "flick sign"), and hand symptoms provoked by sustained grip or wrist-loaded activity. Thenar weakness (reduced thumb opposition and pinch grip) indicates more advanced motor involvement. The Phalen test (sustained wrist flexion for 60 seconds reproducing symptoms) and Tinel sign (percussion over the carpal tunnel producing distal paraesthesia) are clinically useful, though their sensitivity and specificity are moderate. Nerve conduction studies confirm the diagnosis and quantify severity, informing the decision between conservative management and surgical referral.

Conservative Management

Conservative management is appropriate for mild to moderate carpal tunnel syndrome. Neutral-position wrist splinting — particularly nocturnal splinting — is the most evidence-supported non-surgical intervention, reducing carpal tunnel pressure to its minimum and preventing the nocturnal flexion that drives nightly symptoms. Median nerve mobilisation exercises (nerve gliding) reduce adhesions around the nerve and improve its excursion within the tunnel. Soft tissue therapy to the forearm flexors and thenar muscles reduces muscular contribution to tunnel crowding. Activity modification to reduce sustained grip and wrist-loading positions addresses occupational drivers. Corticosteroid injection into the carpal tunnel provides significant temporary symptom relief and may delay or negate the need for surgery in a proportion of cases. A comprehensive conservative programme should be trialled for eight to twelve weeks before surgical referral in mild to moderate presentations.

When Is Surgery Indicated?

Surgical carpal tunnel release — division of the transverse carpal ligament, permanently enlarging the tunnel — is indicated when conservative management fails, when moderate to severe nerve conduction changes are documented, when thenar atrophy is present, or when the patient cannot tolerate the functional limitation. Carpal tunnel release is one of the most successful and commonly performed peripheral nerve surgeries, with high rates of symptom resolution when performed for appropriate indications. Symptom recurrence following surgery is uncommon (5–10%) and is usually attributable to incomplete ligament division, scar formation, or unaddressed contributing factors such as cervical radiculopathy.