Section 6. Brain, Spinal Cord, and Nerve Disorders

Chapter 95. Peripheral Nerve Disorders

Topics: Introduction | Disorders of Muscle Stimulation | Disorders of the Neuromuscular Junction | Plexus Disorders | Thoracic Outlet Syndromes | Mononeuropathy | Mononeuritis Multiplex | Polyneuropathy | Hereditary Neuropathies | Spinal Muscular Atrophies

Plexus Disorders

See the figure Nerve Junction Boxes: The Plexuses.

A plexus resembles an electrical junction box, which distributes wires to different parts of a house. In a plexus, nerve fibers from different spinal nerves are sorted and recombined so that all fibers going to a specific body part are put together in one nerve. Damage to nerves in the major plexuses causes problems in the arms or legs that these nerves supply. The major plexuses are the brachial plexus, which is located in the neck and distributes nerves throughout the arms, and the lumbosacral plexus, which is located in the lower back and distributes nerves to the pelvis and legs.

Causes

The most common causes of damage to a plexus are physical injury and cancer. An accident that pulls the arm or severely bends the arm at the shoulder may damage the brachial plexus (located near the shoulder). Similarly, a fall can injure the lumbosacral plexus (located near the hip). A cancer growing in the upper part of the lung can invade and destroy the brachial plexus. A cancer of the intestine, bladder, or prostate can invade the lumbosacral plexus. Other masses, such as a noncancerous (benign) tumor, an abscess, or a collection of blood (hematoma), may also cause plexus disorders by putting pressure on a plexus.

A plexus may be damaged when the body produces antibodies that attack its own tissues--an autoimmune reaction. Acute brachial neuritis (a sudden malfunction of the brachial plexus) is probably caused by an autoimmune reaction. This disorder occurs primarily in men. It typically occurs in young adults but can occur at any age.

Symptoms and Diagnosis

Malfunction of the brachial plexus causes pain and weakness in an arm. The weakness may affect only a portion of the arm, such as the forearm or biceps, or the entire arm. When the cause is an autoimmune reaction, as in acute brachial neuritis, the arm loses strength within a day to a week and regains strength slowly over a few months. Recovery from an injury also tends to occur slowly, over several months, although some severe injuries cause permanent weakness.

Malfunction of the lumbosacral plexus causes pain in the lower back and leg as well as weakness in part or all of a leg. The weakness may be limited to movements of the foot or calf, or the whole leg may become paralyzed. Recovery depends on the cause. Damage to the plexus due to an autoimmune reaction may resolve slowly over several months.

Doctors can determine that a plexus is involved based on the unpredictable, patchy pattern of sensory, muscle (motor), and reflex dysfunction. The location of the symptoms indicates which plexus is affected. Electromyography and nerve conduction studies can also help locate the damage (see Section 6, Chapter 77). Computed tomography (CT) or magnetic resonance imaging (MRI) can help determine whether a cancer, another mass, or an injury is causing the plexus disorder.

Treatment

Treatment depends on the cause of the plexus disorder. Cancer near the plexus may be treated with radiation therapy or chemotherapy. Occasionally, a cancer or another mass that is damaging the plexus must be removed surgically. Doctors sometimes prescribe corticosteroids for acute brachial neuritis and other plexus disorders thought to be caused by an autoimmune reaction, but these drugs have no proven benefit. When an injury is the cause, time for healing may be all that is needed.