Porth's Essentials of Pathophysiology, 4e

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Disorders of Neuromuscular Function

C h a p t e r 3 6

The successful regeneration of a nerve fiber in the PNS depends on many factors. If a nerve fiber is destroyed relatively close to the neuronal cell body, the chances are that the nerve cell will die; if it does, it will not be replaced. If a crushing type of injury has occurred, par- tial or often full recovery of function occurs. Cutting- type trauma to a nerve is an entirely different matter. Connective scar tissue forms rapidly at the wound site, and when it does, only the most rapidly regenerating axonal branches are able to get through to the intact distal endoneurial tubes. A number of scar-inhibiting agents have been used in an effort to reduce this hazard, but have met with only moderate success. In another attempt to improve nerve regeneration, various types of tubular implants have been used to fill longer gaps in the endoneurial tube. Mononeuropathies Mononeuropathies usually are caused by localized con- ditions such as trauma, compression, or infections that affect a single spinal nerve, plexus, or peripheral nerve trunk. Fractured bones may lacerate or compress nerves, excessively tight tourniquets may injure nerves directly or produce ischemic injury, and infections such as her- pes zoster may affect a single segmental afferent nerve. Recovery of nerve function usually is complete after compression lesions and incomplete or faulty after nerve transection. CarpalTunnel Syndrome. Carpal tunnel syndrome is a relatively common entrapment mononeuropathy, caused by compression of the median nerve as it travels with the flexor tendons through a canal made by the carpal bones and transverse carpal ligament 18–20 (Fig. 36-8). It can be caused by a variety of conditions that produce a reduction in the capacity of the carpal tunnel (i.e., bony or ligamentous changes) or an increase in the volume of the tunnel contents (i.e., inflammation of the tendons, synovial swelling, or tumors). Carpal tunnel syndrome may be a feature of a number of systemic diseases, such as rheumatoid arthritis, hyperthyroidism, acromegaly, and diabetes mellitus. Most cases, however, are due to repetitive use of the wrist (i.e., flexion–extension move- ments and stress associated with pinching and gripping motions). Carpal tunnel syndrome is characterized by pain, par- esthesia (tingling), and numbness of the thumb and first, second, third, and half of the fourth digits of the hand; pain in the wrist and hand, which worsens at night; atrophy of the abductor pollicis muscle; and weakness in precision grip. All of these abnormalities may con- tribute to clumsiness of fine motor activity. Diagnosis usually is based on sensory disturbances confined to median nerve distribution and a positive Tinel or Phalen sign. 18,20 The Tinel sign is the develop- ment of a tingling sensation radiating into the palm of the hand that is elicited by light percussion over the median nerve at the wrist. The Phalen maneuver is per- formed by having the person hold the wrist in complete flexion for approximately a minute; if numbness and

Transverse carpal ligament

Median nerve

FIGURE 36-8. Carpal tunnel syndrome: compression of the median nerve by the transverse carpal ligament. (Courtesy of Carole Russell Hilmer, C.M.I.)

paresthesia along the median nerve are reproduced or exaggerated, the test result is considered to be positive. Electromyography and nerve conduction studies often are done to confirm the diagnosis and exclude other causes of the disorder. Treatment includes a variety of options includ- ing nonsteroidal anti-inflammatory agents, injection of corticosteroids, immobilization of the wrist with splints, rehabilitation modalities (e.g., ultrasound, stretching, and strengthening exercises), and surgery. Measures to decrease the causative repetitive move- ments should be initiated. Splints may be confined to nighttime use. When splinting is ineffective, cor- ticosteroids may be injected into the carpal tunnel to reduce inflammation and swelling. Surgical interven- tion consists of operative division of the volar car- pal ligaments as a means of relieving pressure on the median nerve. Polyneuropathies Polyneuropathies involve demyelination or axonal degeneration of multiple peripheral nerves that leads to symmetric sensory, motor, or mixed sensorimotor defi- cits. Typically, the longest axons are involved first, with symptoms beginning in the distal part of the extremi- ties. If the autonomic nervous system is involved, there may be postural hypotension, constipation, and impo- tence. Polyneuropathies can result from immune mecha- nisms (e.g., Guillain-Barré syndrome), toxic agents (e.g., arsenic polyneuropathy, lead polyneuropathy, alcoholic