Carpal Tunnel Syndrome
Carpal tunnel syndrome (CTS) was first described by Sir James Paget in 1863, and subsequently Dr. George Phalen described the classic Phalen’s sign in 1949.1 It occurs in 10% of the general population, and up to 50% in industrial settings. There is a female predisposition (by a factor of 3:1), more often occurs in obese middle-aged individuals (58%), more likely affecting their dominant hand, but may occur bilaterally. This entrapment is rarely familial. It is also associated with pregnancy (due to edema; 10-20%), rheumatologic diseases including rheumatoid arthritis (with tenosynovitis of the wrist), systemic lupus erythematosus, ankylosing spondylitis, eosinophilic fasciitis,2 diabetes mellitus, other ischemic polyneuropathies (may occur with arteriovenous shunts or peripheral vascular disease of the upper extremity), hypothyroidism with myxedema, acromegaly with boney enlargement, hyperparathyroidism, toxic shock syndrome, amyloidosis and multiple myeloma.1 Most commonly, it occurs following repetitive stress or overuse, but this condition may occur following trauma (direct or secondary to compression from a hematoma or old wrist fracture with callous deformity), and with soft tissue compression (lipoma, flexor tendon synovitis, ganglion cyst, or gouty tophi).3
Carpal tunnel anatomy (Figure 1) includes: medially — pisiform and hook of hamate; laterally — trapezium, scaphoid; posteriorly — lunate and capitate; and anteriorly — 9 flexor tendons (one of these tendons, namely the flexor digitorum sublimus may rarely have congenital hypertrophy), and are bound by the transverse carpal ligament and flexor retinaculum. The palmaris longus tendon lies more superficial (anterior) to the carpal tunnel.1 The median mixed (sensory and motor fibers) nerve transverses this tunnel just distal to the wrist crease and innervates the abductor pollicis brevis, Opponens and superficial head of the flexor pollicis brevis. Upon rare occasion, there is a congenital or idiopathic enlargement of the median nerve at this site. The CTS lesion usually occurs at 2-4 cm distal to the palmar wrist crease with compression of sensory branch 1 cm distal to the motor compression. The smallest crossectional area is 2-2.5 cm distal to the wrist crease. With caliper measurements, the ratio of the antero-posterior thickness to the width > 0.7 (Johnson index), increases the incidence of CTS. Within the carpal tunnel, the median nerve branches to the median recurrent nerve (turns backward into transverse ligament) innervating the abductor pollicis brevis, provides sensation to digits 1 through 3 and the radial portion of digit 4, and a distal motor branch innervates the lumbrical muscles of digits 2 and 3.3
The common symptoms of CTS include hand pain, numbness, or paresthesias of median nerve distribution (more often to digits 1- 3, but not the thenar skin because it is innervated by the palmar cutaneous branch which branches proximal to carpal tunnel). It is often associated with nocturnal awakening, and may have a sclerotomal deep aching pattern into the thenar region. The symptoms are usually intensified with repetitive wrist flexion including driving, reading, keyboarding, handicrafts, and with vibration (jack hammering). There may be volar (anterior) hand and wrist pain, as well as referral into the proximal forearm, elbow, arm or shoulder. There may be a sensation of swelling, with occasional autonomic symptoms which may include vasomotor instability or Reynaud’s syndrome. An acute traumatic presentation due to sudden forceful extension of the wrist with motor vehicle accidents, industrial injuries or sports may result in a greater likelihood of thenar weakness due to conduction block (neuropraxia). Lacerations of the wrist may result in complete or partial transection of the median nerve (neurotmesis) and flexor tendons. In chronic cases, there is a sensory predominance with the insidious onset of thenar weakness with atrophy (though the deep head of flexor pollicis brevis is spared due to ulnar nerve innervation). The patient may describe clumsiness or dropping objects.3
Extremes of wrist flexion and extension in a constricted canal as the result of high force, high repetition, vibration or awkward posture (also observed with dystonia, athetosis or catatonia) and temperature extremes are the most common ergonomic risk factors of CTS. The patient may report partial alleviation of their symptoms with shaking out hands (flick sign), placing their wrist in a neutral position and with elevation. Often, patients will present asymptomatically with “distal median neuropathy,” especially in the elderly and diabetics.4
Some patients with CTS may experience a disproportionate level of pain from their condition. Often, they may have a superimposed problem including repetitive strain of the forearm with lateral epicondylitis, arthritis/arthralgias of the hand and wrist, or their symptoms may be associated with a myofascial pain syndrome.
Further, differential diagnosis may include lesions of the musculoskeletal, peripheral and central nervous system. CTS may be coexisting, superimposed with or mimicked by these conditions (see Table 1).