Entrapment, Median Nerve, Carpal Tunnel Syndrome
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Description
Of the peripheral nerve entrapment syndromes, this is the most common. The region of entrapment is along the median nerve as it courses through the carpal tunnel at the wrist.
Anatomy
The carpal tunnel is a narrow conduit at the wrist bordered by the carpal bones deep and the transverse carpal ligament superficially. It contains the main trunk median nerve, the flexor tendons of the hand, and tenosynovium. Thickening of the transverse carpal ligament narrows the tunnel, and may result in compression of the median nerve.
Predisposing Factors
Carpal tunnel syndrome (CTS) may occur in anyone, but may be a bit more common in certain populations. Repetitive hand motions, as can occur in work involving assembly lines, typing, jackhammering, etc., may increase the risk of developing carpal tunnel syndrome, but this is somewhat controversial. Individuals with diabetes, hypothyroidism, acromegaly, pregnancy, and multiple myeloma are at increased risk of developing CTS.
Clinical Presentation
Patients typically describe pain at the wrist and first three fingers of the hand, often worse at night. Nocturnal exacerbation is thought to occur from nerve ischemia resulting from a position of wrist flexion that commonly occurs during sleep. These symptoms are often relieved by shaking or massaging the affected hand and wrist. Other symptoms may include numbness along the palmar first three digits, and weakness of thumb abduction. Other patients complain of hand weakness, clumsiness, or a loss of fine motor control with the fingers. Other patients describe pain that radiates up the arm, and some even describe neck or shoulder pain.
Physical Examination
A careful physical examination may reveal any combination of atrophy of the thenar muscles, weakness of the first lumbrical and abductor pollicis brevis muscles, sensory loss in the first three digits and splitting the fourth digit, a Tinel sign along the median nerve at the wrist, and hypersensitivity to touch in a median nerve distribution. Typically, the palmar cutaneous sensation is spared. Sometimes patients have little or no findings on physical examination.
Electrodiagnostic Studies
Careful electrodiagnostic examination of the median nerve across the wrist may reveal slowing of conduction velocities across the carpal tunnel. Inching studies, in which conduction velocities across sequential short segments of nerve, may be quite helpful here. There may be reduced amplitudes and increased distal latencies in the median nerve at the level of the wrist as well. Electromyography of the hand muscles may reveal denervational changes, such as fibrillations or positive sharp waves, in the abductor pollicis brevis or opponens pollicis muscles.
Treatment
Treatment generally begins with avoiding aggravating or causative activities, such as using repetitive motions, use of keyboards, use of jackhammers, etc. Wrist splinting is typically the next step. Steroid injections into the carpal tunnel may also provide symptomatic relief, although this is often temporary. Patients who still do not improve are offered decompressive surgery. Some physicians advocate the treatment of advanced carpal tunnel syndrome, in which patients already have evidence of notable muscle injury, with surgery without a prolonged trial of nonoperative management.
There are a variety of techniques in use to perform median nerve decompression at the wrist in patients who have carpal tunnel syndrome. One of the most popular is a mini-open technique in which the nerve is decompressed under direct vision through an incision that is an inch or less in length. Advantages of this procedure are its relative ease to perform, good visibility of the relevant anatomy to avoid damaging nerve branches, and low complication rate. The incision does require a few weeks to heal, however.
Endoscopic carpal tunnel release uses either one or two tiny incisions through which a camera and a cutting tool are placed to perform the carpal tunnel surgery. Patients generally have less postoperative discomfort and are able to return to work more quickly compared to patients who undergo a mini-open procedure. The risk of injury to the median nerve and its branches is slightly higher, however, especially in diabetic patients.
Overall, outcomes are quite similar regardless of the method used to decompress the nerve. The most important goal of the surgery is to completely decompress the nerve across the wrist segment. Additional surgical steps such as internal neurolysis, tenosynovectomy, and transverse carpal ligament repair, do not seem to add any benefits to straightforward decompression, and probably should not be performed.
Complications
Care should be taken in the surgery to avoid injury to the recurrent motor branch, found 3 cm distal to the wrist crease on the radial side of the median nerve, and the palmar cutaneous branch, which exits the radial side of the median nerve 9 cm proximal to the wrist crease. Injury to the recurrent motor branch may result in weakness of the opponens pollicis and abductor pollicis brevis muscles. Injury to the palmar cutaneous branch may result in numbness along the central palm of the hand and a painful sensory neuroma.
Outcomes
Approximately 50% of patients will obtain relief of their symptoms with nonoperative management, whereas 90% of patients will improve with surgery.
