Anatomy of the Carpal Tunnel and Mechanism of Compression

The carpal tunnel is a narrow canal at the level of the wrist whose floor and side walls are formed by the carpal bones and whose roof is the transverse carpal ligament. Nine finger flexor tendons and the median nerve pass through it.

Because its walls are bone and stout ligament, the canal cannot expand. Anything that raises the pressure inside it — swelling of the tendon sheaths, fluid retention, a mass within the canal, or a congenitally narrow canal — presses directly on the median nerve.

Raised pressure first impairs blood flow within the nerve itself; at this stage symptoms are intermittent and vary with hand position. If compression persists, the myelin sheath breaks down and, in advanced disease, nerve fibres are permanently lost and the thenar muscles at the base of the thumb waste away.

The median nerve supplies sensation to the thumb, index finger, middle finger and half of the ring finger, and powers the muscles that lift the thumb out of the plane of the palm and oppose it to the other fingers. The confinement of symptoms to this territory is the basis of the diagnosis.

Risk Factors and Associated Conditions

• Female sex and the 40 - 60 age range: several times more common in women
• Repetitive hand use: prolonged gripping, vibrating tools, work involving constant wrist flexion and extension
• Pregnancy: caused by fluid retention and largely resolves spontaneously after delivery
• Diabetes: increases the nerve's susceptibility to compression, and coexisting polyneuropathy complicates the picture
• Hypothyroidism, rheumatoid arthritis, gout, amyloidosis, chronic kidney failure and dialysis
• Obesity: risk rises markedly with body mass index
• Narrowing of the canal after a wrist fracture or dislocation
• Masses within the canal: ganglion cyst, tenosynovitis, anatomical variants

Symptoms

• Numbness and tingling in the thumb, index and middle fingers: sparing of the little finger is typical and useful in differential diagnosis
• Night pain and waking from sleep: the most characteristic complaint; patients obtain relief by shaking or rubbing the hand
• Symptoms provoked by driving, holding a phone or holding a newspaper
• Burning pain in the hand, which in some patients radiates to the forearm and shoulder
• Reduced grip strength and dropping objects
• Loss of fine dexterity: doing up buttons, threading a needle, picking up coins
• Thenar atrophy in advanced disease: visible wasting at the base of the thumb with permanent loss of strength
• Constant numbness in advanced disease: intermittent symptoms are replaced by persistent sensory loss

Making the Diagnosis

The diagnosis rests primarily on the history and physical examination. The distribution of symptoms, the night-time worsening and the description of relief by shaking the hand are highly indicative.

Examination includes sensory testing, measurement of thumb strength and assessment of the thenar muscle bulk. Provocation tests support the diagnosis: in the Phalen test the wrists are held flexed and numbness appears within a minute; Tinel's sign is tingling radiating into the fingers when the canal is tapped; the carpal compression test reproduces symptoms when pressure is applied over the canal.

Nerve conduction studies (EMG) confirm the diagnosis, grade the severity of compression and distinguish other causes such as cervical radiculopathy or polyneuropathy. They are the standard preoperative investigation in patients scheduled for surgery, although the result can be normal in early disease and does not exclude it.

Ultrasound demonstrates an increased cross-sectional area of the median nerve at the canal entrance and identifies causes within the canal such as a ganglion cyst or tenosynovitis. Plain radiographs are obtained where fracture or arthritis is suspected. Cervical MRI may be required when a disc herniation is in the differential.

Non-operative Treatment

In mild and moderate cases without thenar wasting, treatment begins non-operatively. Success is greatest in patients whose symptoms have lasted less than a year and who have no constant daytime numbness.

• Night splinting: a splint holding the wrist in neutral protects the nerve during sleep, when canal pressure is highest. It is the cornerstone of treatment and should be worn regularly for at least 6 - 8 weeks
• Activity modification: reducing repetitive wrist movements, adjusting work height and keyboard or mouse position, avoiding vibrating tools
• Nerve and tendon gliding exercises: aim to restore free gliding of the median nerve within the canal and are useful in addition to splinting
• Anti-inflammatory medication: provides short-term pain control but does not alter the course of the disease
• Corticosteroid injection into the canal: markedly reduces symptoms and supports the diagnosis, though the effect is temporary in most patients. Repeated injections are avoided
• Treatment of underlying disease: correction of thyroid function, glycaemic control, weight loss
• In pregnancy, splinting and observation are preferred, as the condition usually resolves after delivery

General Surgical Risks

• Wound infection: around 1 per cent and usually superficial
• Bleeding and haematoma
• Anaesthetic risks: very low, as local anaesthesia is used
• Wound healing problems and hypertrophic scar

Risks Specific to This Operation

• Injury to the median nerve or its motor branch: rare, but can cause permanent loss of thumb strength
• Incomplete division of the ligament: the commonest cause of persistent symptoms and may require revision surgery
• Pillar pain: tenderness on either side of the palm, present in a significant minority of patients and usually settling within 3 - 6 months
• Scar tenderness and temporary loss of grip strength: expected during the first 6 - 12 weeks
• Recurrence: uncommon in the long term, usually due to incomplete release or scar formation
• Complex regional pain syndrome: rare, requiring early recognition and intensive hand therapy
• Incomplete resolution of numbness in advanced cases: where nerve damage is established, surgery halts progression but may not reverse the deficit