What is carpal tunnel syndrome?

Carpal tunnel syndrome is a condition in which pressure applied to the median nerve in the carpal tunnel causes symptoms. Carpal tunnel syndrome is one of a group of neurological disorders known as an entrapment neuropathy.

In the case of carpal tunnel syndrome the median nerve iscompressed where it runs through a channel in the wrist called the carpal tunnel. This tunnel has four sides; a floor and two walls formed by carpal or wrist bones and a roof made of dense unyielding fibrous tissue called the flexor retinaculum. The flexor retinaculum is on the palmar side of the carpal tunnel. The tunnel is approximately 20 mm in diameter and 25 mm in length and commences at the wrist crease.

The median nerve and nine flexor tendons run through the carpal tunnel. The median nerve transmits electrical impulses which control important thumb and finger muscles. The nerve also transmits impulses which allow feeling in the thumb, index, middle, and the thumb half of the ring finger.

The median nerve is located in the palmar aspect of the canal slightly to the radial or thumb side. The median nerve has an artery running along its palmar aspect. Branches from this artery penetrate the nerve along its course.

A rise in pressure in the carpal tunnel, for whatever reason, causes pressure on the median nerve and interrupts the blood flow to the nerve. The interruption in blood flow causes malfunction of the nerve leading to symptoms.

The flexor tendons in the carpal tunnel flex or bend the finger joints and the end joint of the thumb. The flexor tendons are surrounded by a filmy mobile tissue called tenosynovium which carries vessels to the tendons and lubricates the tendons to facilitate gliding.

What is the cause of carpal tunnel syndrome?

Carpal tunnel syndrome can be brought on in susceptible individuals by an increase in pressure in the carpal tunnel. The increase in pressure may be secondary to enlargement or hypertrophy of the synovium surrounding the flexor tendons. This can occur as a response to repetitive manual activity or from synovitis accompanying a rheumatological condition such as rheumatoid arthritis.

Fluid retention in the hand at night and during pregnancy can increase carpal tunnel pressure leading to the development of carpal tunnel syndrome symptoms. Often symptoms of the condition experienced during pregnancy resolve spontaneously following delivery.

Trauma to the bony walls of the carpal tunnel leading to fracture and distortion of the tunnel, or frank dislocation of carpal bones into the tunnel, can lead to an increase in pressure causing acute post-traumatic carpal tunnel syndrome.

Some medical conditions can predispose patients to carpal tunnel syndrome. These conditions include hypothyroidism, rheumatoid arthritis and diabetes.

Compression of the median nerve can also occur in the forearm just below the elbow. This is known as the pronator teres syndrome and has symptoms similar but not identical to carpal tunnel syndrome.

Medical conditions of the median nerve not due to compression can cause symptoms and signs similar to carpal tunnel syndrome.

Compression of nerves in the neck can cause symptoms mimicking carpal tunnel syndrome.

These latter three conditions need to be carefully excluded before a diagnosis of carpal tunnel syndrome can be confirmed.

Signs and symptoms

The median nerve collects and distributes to the brain incoming sensations such as pain, temperature and light touch from the palmar aspect of the thumb, index, and middle and thumb half of the ring finger.

The nerve transmits signals to the muscles of the thenar eminence or ball of the thumb via its motor or recurrent branch. These thenar muscles are very important in controlling the thumb's ability to oppose to the fingers and are involved in all aspects of thumb pinch grip.

Additional small motor or muscle fibre branches supply the lumbrical muscles of the index and middle fingers.

Dysfunction of the median nerve caused by carpal tunnel syndrome usually commences by causing sensory symptoms in the first instance. Usually initial symptoms include pain, which has a dull, deep and boring nature, and is often localised to the palmar aspect of the wrist and hand. Sometimes the pain radiates up along the palmar aspect of the forearm, arm and even to the shoulder.

Numbness in part or all of the territory supplied by the median nerve, pins and needles or tingling in the median nerve territory are also common symptoms.

Sensory symptoms often develop at night causing loss of sleep. Patients suffering such symptoms try and obtain relief by vigorous exercise of the fingers and hand. Some patients report that placing their hands in the fridge leads to relief. Patients commonly report that their symptoms involve ''all of their fingers'' but after being specifically instructed to observe carefully whether or not the little finger and little finger side of the ring finger is involved, will confirm that these areas supplied by the ulnar nerve are not involved in the symptoms.

Muscle symptoms of weakness are sometimes experienced during episodes of carpal tunnel syndrome, but are not usually noticed until the condition has become chronic and severe. Symptoms of pinch grip weakness, difficulty with manipulating or maneuvering fine objects, such as doing up buttons or winding a watch are then reported.

Symptoms are often brought on by specific activities such as reading a newspaper, driving a car, hanging out washing or repetitive activity at work. Usually cessation of the activity results in rapid resolution of symptoms.

If carpal tunnel syndrome symptoms resolve spontaneously, or treatment results in resolution of symptoms with a period of say 3-6 months, there are usually no long lasting sequelae.

If the median nerve is subject to increased pressure over a long period of time, irreversible damage can occur within the nerve leading to scarring or neurofibrosis.

How is a diagnosis confirmed?

As with many medical conditions an accurate history is a foundation for the diagnosis.

The patient will often spontaneously mention many of the symptoms already discussed hitherto.

The general medical status of the patient needs to be determined to see if there may be another cause for the symptoms. The examination usually need only involve the upper extremity and, in particular, the hand. The examiner observes the hand for wasting of the thenar or thumb muscles, perhaps a reduced sensibility in the median nerve distribution and in severe cases the skin may feel dry from reduced sweating in the median territory.

One of the most powerful tests is the provocation test. A provocation test is positive if it produces symptoms identical with those of which the patient is complaining. The Phalen's test is well known and involves placing the patient's wrist in a hyper-flexed position for 60 seconds to see if symptoms develop. Alternatively the reversed Phalen's test involves hyper-extending the wrist for 60 seconds.

Applying direct digital pressure to the palmar aspect of the carpal tunnel can often reproduce symptoms. This is known as the carpal tunnel compression test.

Another technique used involves applying a pneumatic tourniquet, which is blown up to occlude venous but not arterial circulation and then determining whether or not symptoms follow.

In severe cases of carpal tunnel syndrome, the Tinel's test is positive over the median nerve. This involves tapping the nerve. A positive test is one in which tingling is felt in the distribution of the median nerve. The test may on occasion be uncomfortable for the patient so caution is required in performing this test.

If a patient's history and the physical signs described above are present, further tests are usually unnecessary to establish the diagnosis.

Where there is any doubt about the diagnosis, further tests are indicated.

If a medical cause for the condition is suspected, then appropriate screening tests are indicated.

The most sensitive and specific tests for carpal tunnel syndrome are that of nerve conduction study and electromyography. These are electrical tests performed on the median nerve and the muscles it supplies. The tests require sophisticated electrophysiological equipment and the interpretation of results by a skilled neurologist or neurophysiologist.

Treatment

The simplest treatment involves eliminating physical causes for the condition such as modifying if possible the way manual tasks are performed.

Unfortunately many patients with carpal tunnel syndrome have symptoms at night which are unresponsive to behavioural modifications. Temporary relief may be afforded by splinting the wrist in a neutral position prior to the commencement of sleep.

Anti-inflammatory medication may be helpful in alleviating symptoms due to synovitis and may help modify pain.

Diuretic medication has been popular in the past, but seldom leads to lasting relief and is not a satisfactory long-term solution. Temporary relief can usually be obtained by an injection of a corticosteroid preparation into the flexor tenosynovium of the wrist. This form of treatment is particularly useful in controlling symptoms during pregnancy.

If symptoms are not short lived or controlled by the above measures, then surgical decompression of the nerve is indicated.

The surgical objective is to completely release the flexor retinaculum by dividing or cutting it. Partial release of the ligament is often accompanied by complete failure to relieve symptoms. The divided flexor retinaculum allows the carpal tunnel contents to expand slightly thus reducing pressure. The flexor retinaculum then heals with no long-term problems.

Anaesthesia

Surgery for carpal tunnel release requires blood free conditions which is best achieved by use of a tourniquet on the arm. The preferred anaesthetic technique is that of regional brachial plexus block for unilateral or one-sided release and general anaesthesia for bilateral or right and left release where bilateral axillary plexus block is contraindicated on account of drug dosage toxicity.

There are broadly speaking three surgical techniques used to perform carpal tunnel release.

The blind technique involves a skin incision at the level of the wrist crease, and then an attempt to release the flexor retinaculum is made by forcing a cutting device such as a pair of semi-open scissors distally in a blind fashion.

Complications including division of the median nerve, its branches, distal vessels and failure to decompress the ligament have been noted with such frequency as to cause this surgery to fall into disrepute.

The most common way that the carpal tunnel is safety surgically decompressed is by an open longitudinal incision commencing at the wrist, traveling some 5 cm distally in line with the 4th metacarpal. The skin, subcutaneous fat, palmar aponeurosis and transverse retinacular ligaments are sectioned serially under direct vision and the median nerve can be identified and the contents of the carpal canal inspected.

Bleeding is stopped after the tourniquet is released and the skin alone is closed with stitches. No attempt to repair the cut retinaculum is made.

A light postoperative dressing and an optional splint are applied. Stitches are removed at between 7-10 days and gentle progressive hand therapy is undertaken.

Endoscopic surgical techniques have become popular in many fields of surgery and in general offer reduced pain and faster recovery for the patient.

Endoscopic carpal tunnel release has been developed over the last decade and as with other branches of surgery, reduced pain and quicker recovery are achieved through this method.

The surgery requires sophisticated high-tech imaging and appropriate technical training on the part of the surgeon. Inappropriate technique can result in damage to the median nerve, its branches, adjacent arteries and nerves, the flexor tendons or incomplete release of the transverse retinacular ligament.

The endoscopic technique can either employ a uni-portal or double portal technique.

The most commonly employed technique involves passing a small cannula through the palmar aspect of the carpal tunnel beside the median nerve via two small skin portals above and below the carpal tunnel.

The entire length of the flexor retinaculum to be cut is under direct vision while it is cut with a specially designed blade called a hook knife introduced through the cannula.

The skin portals can be closed with steristrips, a padded dressing applied, and the patient is encouraged to mobilise as symptoms allow, taking care to prevent the wounds from becoming wet. Dressings can be removed at 7 days post-operatively and a progressive return to full function is usually achieved over a 2-3 week period.

No matter which technique is employed, despite complete relief of carpal tunnel syndrome symptoms, a few patients develop mild to moderate discomfort in the region of the wound. Fortunately this resolves spontaneously over a period of two to three months.

Surgery on patients who have severe chronic carpal tunnel syndrome where there is irreversible nerve damage, will often alleviate pain and result in reduced tingling and improved sensation, but muscle wasting and weakness may not improve significantly.

Patients who suffer severe thumb muscle wasting can be offered tendon transfers to supplement thumb function at the same time as the carpal tunnel is released.

People will often ask. "If I have surgery, can my symptoms come back?" Fortunately the answer is that recurrence of carpal tunnel syndrome following successful surgical release is extraordinarily rare.

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