Pathogenesis

Intermittent or sustained high pressure within the tunnel (due to reduced dimensions of the tunnel or increased volume of its contents) produces ischaemia of the median nerve, resulting in impaired nerve conduction causing paraesthesia and pain. Severe symptoms are occasionally associated with weakness and wasting. If axonal injury occurs secondary to prolonged ischaemia, the nerve dysfunction may become irreversible.
Rarely, compression of more proximal parts of the nerve can occur at the forearm or elbow. The two significant conditions are pronator teres syndrome and anterior interosseous syndrome.

Epidemiology

A recent study by the UK General Practice Research Database (253 practices) revealed 87.8 men and 192.8 women with new presentations per 100,000 population.^[1] The most common patients seen were women aged 45-54. Other compression conditions, eg pronator syndrome, are much less common and tend to be seen in specific groups of workers.^[2]

Risk factors

Carpal tunnel syndrome (CTS)

Vocational activities are often involved, but many other factors play a part.^[3] There is also a theory that patients who use their hands and wrists a lot are simply more aware of the symptoms.

Risk factors include:

• Genetic - square-shaped wrist, short stature, family history, hereditary neuropathy
• Secondary causes
  • Post-Colles' fracture
  • Flexion/extension injury of the wrist
  • Conditions encroaching on the space within the carpal tunnel (eg aneurysm, neurofibroma, haemangioma, lipoma, ganglion, xanthoma and gouty tophi)
  • Diabetes
  • Thyroid disorders (mainly myxoedema)
  • Menopause (including surgically-induced)
  • Inflammatory arthritides of the wrist
  • Acromegaly
  • Renal dialysis
  • Amyloidosis
• Vocational or leisure activities involving prolonged extreme posture of the wrist, high amounts of repetitive movement or exposure to vibration and/or cold are all associated with CTS
• Other factors - pregnancy, lactation, lack of aerobic exercise, use of walking aids

Other median nerve lesions

The main risk factor is occupational, eg dentists and shipyard workers using high-powered vibrating tools^[4][5]

Presentation

Carpal tunnel syndrome (CTS)

CTS is characterised by tingling, numbness, or pain in the distribution of the median nerve (the thumb, index, and middle fingers, and half the ring finger) that is often worse at night and causes wakening.^[3][6] The offending hand may be hung out of the bed at odd angles to try to revive it. Pain may become more persistent, and may radiate to the forearm, elbow, arm and shoulder.

Weakness may be noted in hand grip and opposition of the thumb.

• Positive Phalen's test: flexing the wrist for 60 seconds causes pain or paraesthesia in the median nerve distribution.
• Positive Tinel's sign: tapping lightly over the median nerve at the wrist causes a distal paraesthesia in the median nerve distribution.
• Positive carpal tunnel compression test: pressure over the proximal edge of the carpal ligament (proximal wrist crease) with thumbs causes paraesthesia to develop or increase in the median nerve distribution.

Other median nerve lesions

Nerve damage at the elbow or forearm causes inability to flex the index finger and distal phalanx of the thumb with weak flexion of the middle finger and defective opposition of the thumb.^[7] This has been described as 'simian'. There may be significant sensory loss over the palm and some fingers with skin becoming dry, reddened and atrophic. With partial lesions, causalgia may develop hours or days later with dry scaly skin.^[8]

Differential diagnosis

Carpal tunnel syndrome (CTS)

Other conditions which should be considered include:

• Other median nerve compression syndromes
• Cervical radiculopathy
• Shoulder bursitis
• Thoracic outlet syndrome
• Transient ischaemic attack
• Myocardial ischaemia
• Tendonitis
• Fibrositis
• Lateral epicondylitis^[3][7]
• Pronator syndrome - this is compression of the median nerve where it passes between the two heads of the pronator teres, causing pain in the wrist and forearm and weakness of the thenar muscles
• Anterior interosseous syndrome - compression mainly of the motor nerve, most commonly caused by tendinous origin of the deep head of the pronator teres, causing difficulty moving the index and middle fingers

Investigations

These are useful in patients whose clinical features yield a high index of suspicion for carpal tunnel syndrome (CTS) but who fail to respond to first-line treatment.^[9]

• Electroneurography (ENG) - nerve conduction studies have an 85% sensitivity and specificity greater than 95% for diagnosing CTS.^[3] The median nerve is stimulated proximal to the carpal ligament and compound muscle action potential is picked up over the thenar eminence.
• Electromyography (EMG) - this is useful in some cases but is not as sensitive as ENG.
• Ultrasonography - this is being used increasingly as a confirmatory test. Enthusiasts cite its wide availability, lower cost, noninvasiveness, and shorter examination time than electrophysiological studies. Ultrasound views of the median nerve show widening at the inlet of the carpal tunnel or flattening along the length of the tunnel.^[10]
• MRI scan- this can be used as an alternative to ultrasonography and when electrophysiological studies are ambiguous.^[7]

Associated diseases

Carpal tunnel syndrome (CTS)

Apart from the risk factors outlined above, CTS may also be associated with pyridoxine deficiency, obesity and chondromalacia.^[11][12][13]

Management

Decision Aids

Doctors and patients can use Decision Aids together to help choose the best course of action to take.

Compare the options for Median Nerve Lesions and Carpal Tunnel Syndrome.

General measures

• Explain that the symptoms may resolve within 6 months. This is most likely to occur in young people (less than 30 years of age), if the symptoms are unilateral and of short duration, and in women in whom fluid retention due to pregnancy is the precipitating factor.^[6]
• Advise wearing a wrist splint at night that maintains the wrist at a neutral angle without applying direct compression. Any improvement should be apparent within 8 weeks of use.
• Advise minimisation of activities that exacerbate symptoms. Explain to people who work with computer keyboards that there is little evidence to suggest that modifications at their work place are likely to be of any help in relieving symptoms.
• Do not recommend the use of non-steroidal anti-inflammatory drugs or diuretic medication for carpal tunnel syndrome (CTS).^[6][14] However, rest and anti-inflammatories may be helpful in other median nerve lesions. Decompressive surgery is also sometimes required.^[7]

Corticosteroids

Orally administered corticosteroids can be effective for short-term management (two to four weeks), but adverse effects preclude their use.^[6]

Local corticosteroid injections provide greater clinical improvement in symptoms one month after injection compared with placebo but significant symptom relief beyond one month has not been demonstrated.^[15]

Surgical

• Carpal tunnel release may be performed by the open method or endoscopically. Trials suggest the effectiveness is similar in both methods, but there is a shorter recovery time after endoscopy.^[16]
• There is as yet insufficient evidence to state definitively whether surgical treatment is better than conservative therapy.
• Good results have recently been achieved using a specially developed tool (the KnifeLight®) and minimally invasive techniques.^[17]

Prognosis

Definitive statements about prognosis are difficult in carpal tunnel syndrome (CTS) because 'treatment failures' may sometimes be due to misdiagnosis.^[18] Studies suggest that some patients do get better over time without treatment.^[19] Severe symptoms seem to predict a more prolonged course.^[19] Spontaneous improvement occurs rarely with other median nerve lesions. There is a good response to surgical treatment, particularly where patients are selected on the basis of detailed investigation.^[20]