Anatomy of the Carpal Tunnel

The floor and sides of the carpal tunnel are formed by the carpal bones; the roof by the flexor retinaculum. The median nerve lies between the flexor retinaculum and the finger flexors and pollicus longus tendons and their tenosynovium. The flexor retinaculum acts to prevent bowstringing of the flexor tendons. When the flexors contract, especially with the wrist in flexion, the tendons in the carpal tunnel compress the median nerve between the tenosynovium and the flexor retinaculum.

Etiology

Most cases of CTS are considered idiopathic. Nonspecific flexor tenosynovitis has been reported as the most common cause of CTS. The most common histologic findings in CTS are noninflammatory synovial fibrosis ² and vascular proliferation. Conceptually, it helps to think of factors that cause the canal to get smaller or factors that cause the contents to swell.

Causes of CTS include the following³:

Risk Factors and Frequency of Occurrence

CTS most often occurs after age 30 and women are affected three times as often as men.⁴ Repetitive strain injury (RSI) from light duty work as a cause of CTS is not well supported by the current literature.⁵ However, a study of 501 participants, 156 of whom were diagnosed with CTS, showed the following risk factors: repetitive activities with a flexed or extended wrist, hysterectomy without oophorectomy, and menopausal women who had their last menstrual period 6 to 12 months earlier.⁶ Others report increased the incidence of CTS due to pregnancy, hypothyroidism, diabetes, and recent menopause.⁷

History

CTS typically has an insidious onset of intermittent tingling or numbness, often bilaterally, of the first three fingers and the lateral one half of the fourth digit. Symptoms are frequently worse at night, often waking the patient. As the condition progresses, loss of fine motor skills and loss of grip strength with “things slipping from my fingers” are commonly reported. Relief is gained from shaking the affected hand or hands. Katz and Stirrat⁸ developed a self-administered hand diagram test that has an 80% sensitivity and a 90% specificity for CTS. Loss of finger dexterity and atrophy are late complaints.

Physical Examination

Examination may show decreased sensation over the distribution of the median nerve to sharp, light touch and vibration (128-Hz tuning fork). Surprisingly, many patients are not able to localize the symptoms to the median distribution and feel that the whole hand is involved, which may be due to autonomic nerve involvement. Tingling in the distribution of the median nerve produced by tapping on the wrist crease (positive Tinel’s sign), tingling in the distribution of the median nerve produced by holding the wrist in forced flexion for 60 to 90 seconds (Phalen’s test), or tingling produced by forcibly extending the wrist and applying pressure over the median nerve (reverse Phalen’s test) are signs of CTS. Early on, the perceived loss of strength may be due to loss of sensory feedback, not loss of strength. Manual muscle tests may elicit weakness of the LOAF muscles (muscles innervated by the median nerve in the hand; Lubricals first and second, Opponens pollicus, Abductor pollicus, Flexor pollicis brevis). As CTS advances, atrophy of the thenar eminence and weakness of the thumb abductor muscle may develop.

Other Diagnostic Tests

The carpal compression test, when the examiner exerts direct compression with the thumbs over the patient’s carpal tunnel for 30 seconds and reproduces the symptoms, was reported as having 89% sensitivity and 96% specificity when performed with the Durkan guage.⁹ Other studies have not reproduced these findings but have reported “Tinel’s, Phalen’s, Reverse Phalen’s, and carpal tunnel compression tests are more sensitive, as well as being specific tests for the diagnosis of tenosynovitis of the flexor muscles of the hand, rather than being specific tests for CTS.”^34,35 Weakness of the thumb abductor muscle is a strong indication to order neurophysiologic testing (nerve conduction studies [NCS] and/or electromyography). In CTS, NCS may show motor and sensory latencies across the wrist.

Magnetic resonance imaging (MRI) and high-definition ultrasound imaging can measure the dimensions of the carpal tunnel and size of the median nerve, but these tests are expensive and not needed in uncomplicated CTS.