Distal Median Nerve

Anteriorly, within the carpal tunnel, the median nerve is bounded by the stiff flexor retinaculum and posteriorly by the carpal bones. In contrast, the ulnar nerve does not travel through the carpal tunnel; rather, it has a very medial position, reaching the hand through the much less restrictive Guyon canal. Thus it is relatively uncommon to have both the median and ulnar nerves affected concomitantly at the wrist, unless there is a diffuse polyneuropathy, such as found in a patient with diabetes mellitus, or amyloidosis. Once the median nerve leaves this anatomically defined tunnel, it divides into its terminal sensory and motor branches, providing motor and sensory function to 60% of the hand. These branches directly accompany tendons originating from the digital flexor muscles, including the flexor digitorum superficialis, flexor digitorum profundus, and the flexor pollicis longus. The muscular branch arises close to, or is initially united with, the common palmar digital nerve to the thumb. This curves outward over or through the flexor pollicis brevis to supply its superficial head before dividing to innervate the abductor pollicis brevis and opponens pollicis muscles, all C8, T1–innervated muscles. In addition, this muscular branch usually innervates both the first and second lumbrical muscles. Only one thenar muscle, the adductor pollicis (C8, T1) is not innervated by the median nerve; this has an ulnar nerve innervation.

There is one other rare motor variant within the hand—Riche-Cannieu anastomosis. Here the deep ulnar branch of the ulnar nerve (C8, T1) communicates with the terminal motor branch of the median nerve. Because the former nerve has its own clinical presentation, such a variation can rarely lead to diagnostic confusion. Another rare, important anatomic variation occurs if the distal median motor nerve branches off earlier than usual within the carpal tunnel, exiting it by directly piercing the flexor retinaculum, rather than leaving the carpal tunnel at its most distal extent near the thenar eminence. This anatomic variant is of potential clinical concern if an incision is made through this area when operating on a patient with CTS symptoms In the circumstance wherein this unusual branch is not recognized, the motor function of the thumb may be inadvertently and severely compromised if this unexpected branch is severed. Thus the surgeon needs to carefully inspect the site at the time of the operation to avoid damage to this branch, if present.

CARPAL TUNNEL SYNDROME

Carpal tunnel syndrome (CTS) is the most common adult mononeuropathy; it is three times more common in women, usually manifesting in middle to late life. In contrast, CTS rarely occurs in young children. The symptoms of CTS include feelings of numbness and tingling in the fingers of one or both hands. Although some patients report that their hand is simply “asleep,” at times, these paresthesias have an annoying, allodynic quality. Many individuals report that all their fingers are affected. This is conceivable when one considers that the cortical representation of the thumb, index, and middle fingers covers a much larger portion of the parietal cortex than is available for the ring and little fingers, thus allowing the misperception that all fingers are affected. Rarely, there is a component of dysautonomia with changes in temperature, color, and sweating.

Typically, these symptoms occur at night, awakening the patient from sleep. In addition, many patients also note similar symptoms on first awakening in the morning or later in the day while driving, related to holding a steering wheel. Very often, CTS patients believe that they have been sleeping on the hand, and this is thought to be supported by their observation that a simple shaking of the fingers and hand will abort symptoms. Occasionally, there is radiation of the pain into the volar forearm; rarely, this will spread into the upper arm.

Occupational activities requiring continued use of the hands, such as in carpenters, bakers, electricians, or painters, predispose to CTS at a younger age, that is, in the 30s and 40s, whereas persons without such predisposing activities present in later life. Various hobbies, including artistic painting, sewing, crocheting, or sculpturing, may also predispose the artisan to classic CTS symptoms. Athletic endeavors, including skiing, rowing, and bicycle riding, also provide the potential for CTS development. Of interest, although many of us use computer keyboards for many hours a day, there is no increased occurrence of CTS in this setting.

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