Carpal tunnel syndrome

Carpal tunnel syndrome (CTS) is a type of entrapment neuropathy - a form of peripheral neuropathy in which a peripheral nerve is compressed, stretched, or angulated by neighbouring structures. It is the most common entrapment neuropathy in the upper limb.

It occurs predominantly between the ages of 45-54 in women and 75-84 in men. In the UK, it affects around 88 men per 100,000 and around 193 women per 100,000.

🦴 Anatomy

The carpal tunnel is a narrow, rigid passageway located at the base of the palm. It is formed by the carpal bones at the base and sides while the roof is formed by the transverse carpal ligament (also known as the flexor retinaculum). The transverse carpal ligament stretches from the pisiform and hook of hamate (on the ulnar side) across to the scaphoid and trapezium (on the radial side). The tunnel serves as a conduit for a handful (literally) of important anatomical structures. It provides them with a route while providing protection too.

The structures within the carpal tunnel include:

1. Median nerve

This is the primary nerve the traverses the carpal tunnel. It is a mixed nerve responsible for both sensory and motor function. It forms from the union of the lateral cord of the brachial plexus (C5, C6, C7) and the medial cord of the brachial plexus (C8, T1). It descends lateral to the brachial artery before crossing anteromedially at the brachium. It continues its route through to the carpal tunnel where it passes superficially to the flexor tendons within tunnel.

1. Sensory function - it provides cutaneous innervation to the palmar surface of the thumb, index finger, middle finger and radial half of the ring finger (the lateral three and a half digits). It also provides cutaneous innervation to the fingertips of the lateral three and a half digits. This is achieved by the palmar digital cutaneous branches. Another branch, which is the palmar cutaneous branch innervates the lateral aspect of the palm but does not pass through the carpal tunnel and is spared in CTS.
2. Motor function - the median nerve provides motor innervation to a couple of muscles of the thenar eminence. This includes:
1. Abductor pollicis brevis - abducts the thumb and is essential for gripping and pinching.
2. Opponens pollicis - opposes the thumb which allows it to touch the tips of other fingers.
3. Flexor pollicis brevis (superficial head) - flexes the thumb at the metacarpophalangeal (MCP) joints.
4. Lumbricals (I and II) - flexes the MCP joints and extends the interphalangeal (IP) joints of the index and middle fingers.

It is the compression of the median nerve that is responsible for the symptoms of CTS.

2. Flexor tendons

There are multiple tendons of the extrinsic muscles of the hand that pass through the carpal tunnel. This includes:

1. Flexor digitorum superficialis (FDS) tendons - flexes the proximal IP joints of the fingers.
2. Flexor digitorum profundus (FDP) tendons - flexes the distal IP joints of the fingers.
3. Flexor pollicis longus (FPL) tendon - flexes the thumb.

These tendons are encased in synovial sheaths which allow for smooth, gliding movements within the confined space of the carpal tunnel. However, these sheaths may become inflamed or swollen which can increase pressure within the tunnel (as seen in tensoynovitis, for example).

Pathophysiology

CTS arises due to increased pressure within the carpal tunnel. This rise in pressure is multifactorial with both intrinsic and extrinsic factors involved:

• Anatomical variations - such as a smaller carpal tunnel.
• Tenosynovitis - inflammation of the tendon sheaths.
• Osteoarthritis of the MCP joint of the thumb - may lead to compression of the median nerve with osteophytes.
• Rheumatoid arthritis and other inflammatory joint diseases - can lead to synovitis of the carpal tunnel.
• Ganglion cysts - ganglion cyst formation in this region is fairly common and benign but may lead to compression of the median nerve.
• Repetitive hand and wrist movements - activities such as gardening, assembly line work, using vibrating hand tools and occupations that need forceful hand grip.
• Obesity - both increased surrounding tissue volume as well as metabolic alterations that lead to fluid retention and swelling exacerbate compression of the nerve.
• Pregnancy - fluid retention in pregnancy can swell the tissues in the carpal tunnel.
• Hypothyroidism - may also lead to fluid retention and tissue swelling.
• Diabetes mellitus - may result in nerve dysfunction and microvascular disease that adds to nerve ischaemia.

The sustained compression of the median nerve leads to ischaemia and mechanical injury of nerve fibres. The compression disrupts the blood-nerve barrier and causes intraneural oedema and demyelination. Over prolonged periods it may lead to axonal loss and permanent sensorimotor deficits. Advanced cases may also present with thenar atrophy and significantly impaired hand function.

⚠️ Risk factors

😷 Presentation

The typical symptoms include intermittent paraesthesia, numbness, burning sensations, or pain in the distribution of the median nerve (the thumb, index finger, middle finger and the radial half of the ring finger).

• Symptoms may be worse at night and can wake the patient from sleep.
• Pain may radiate to the forearm (along the course of the median nerve).
• It may affect the motor function of the median nerve, resulting in loss of grip strength, hand weakness, reduced dexterity when doing daily activities.

Severe cases of CTS may result in:

• Thenar atrophy
• Trophic ulcer formation - these are chronic wounds that heal slowly. It indicates the loss of protective sensations in the fingertips that is usually provided by the median nerve.
• Absent pain - due to permanent sensory deficits.

🔍 Investigations

⭐️ NICE suggests that the diagnosis of CTS is a clinical diagnosis based on the history and clinical examination using hand provocation manoeuvres which can support the diagnosis.

The hand provocation manoeuvres include:

1. Phalen’s test - this involves flexing the wrist for 60 seconds by pushing the dorsum of both hands against each other. It is positive if pain or paraesthesia are felt along the median nerve distribution.
2. Tinel’s test - this involves tapping lightly over the median nerve at the volar surface of the wrist. It is positive if pain or paraesthesia are felt along the median nerve distribution.
3. Durkan’s test (carpal tunnel compression test) - this involves applying direct pressure over the proximal edge of the transverse carpal ligament (or the proximal wrist crease) with the thumbs. It is positive if the pressure produces/worsens the paraesthesia along the median nerve distribution.

If there are atypical symptoms, or the diagnosis is not certain, or the patient is being considered for surgery, then we may need to refer for nerve conduction studies or electromyography (EMG).

It may be useful to consider alternative diagnoses such as:

• Pronator teres syndrome - compression of the median nerve by pronator teres. It presents with the symptoms extending up to the proximal forearm.
• Flexor carpii radialis tenosynovitis - this will present with tenderness at the base of the thumb.
• Cervical radiculopathy - involvement of the C6/C7 nerve root may produce pain and paraesthesia along a similar distribution, however, there is likely to be an additional element of neck pain.
• Cubital tunnel syndrome - ulnar nerve compression at the elbow causes paraesthesia of the pinky and ulnar half of the ring finger. There may be weakness of the hand with sparing of the thumb.
• De Quervain’s tenosynovitis - this presents with pain over the distal radial styloid process. It occurs as a result of irritation to the tendons travelling to the thumb.
• Thoracic outlet syndrome - there may be neurovascular compression in the thoracic outlet which can cause shoulder and neck pain associated with finger numbness.

🧰 Management

🧰

Management of carpal tunnel syndrome:

• Patient education - such as avoiding repetitive hand/wrist movements. Encourage Occupational Health to make an assessment if there are work-place risk factors.
• Treat underlying condition (if applicable)

🥇 Patients with mild-moderate symptoms should have a 6-week trial of conservative options which include:

• Wrist splinting in a neutral position at night
• Corticosteroid injection into the carpal tunnel
• Hand exercises and median nerve mobilisation techniques

🏆 If symptoms persist after 6 weeks then refer to orthopaedic surgery for carpal tunnel decompression surgery.

If the patient has severe symptoms that impact on daily functioning then a referral for surgery may be made without a trial of conservative option.