Pain, weakness, and loss of sensation occur in all or part of an arm or a leg.
Electromyography and nerve conduction studies help doctors locate the damage, and magnetic resonance imaging or computed tomography helps identify the cause.
Sometimes treating the disorder causing the problem improves nerve function.
(See also Overview of the Peripheral Nervous System Overview of the Peripheral Nervous System The peripheral nervous system refers to the parts of the nervous system that are outside the central nervous system, that is, those outside the brain and spinal cord. Thus, the peripheral nervous... read more .)
A plexus is like an electrical junction box, which distributes wires to different parts of a house. In a plexus, nerve fibers from different spinal nerves (which connect the spinal cord to the rest of the body) are sorted. The fibers are 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 shoulders and which distributes nerves throughout the arms
Lumbosacral plexus, which distributes nerves to the pelvis and legs
The lumbosacral plexus includes the
Lumbar plexus, located in the lower back
Sacral plexus, located in the pelvis
Nerve Junction Boxes: The Plexuses
A plexus is like an electrical junction box in 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. Four nerve plexuses are located in the trunk of the body:
Because the lumbar and sacral plexuses are interconnected, they are sometimes referred to as the lumbosacral plexus. The spinal nerves in the chest do not join a plexus. They are the intercostal nerves, which are located between the ribs. |  |
Causes of Plexus Disorders
The most common causes of damage to a plexus are
Physical injury
Cancer
An accident that pulls the arm or severely bends the arm at the shoulder may damage the brachial plexus (located near the shoulder). In newborns, the brachial plexus can be damaged during birth if the delivery requires pulling or other maneuvers. A fall can injure the lumbosacral plexus (located near the hip).
A cancer growing in the breast or upper part of the lung can invade and destroy the brachial plexus. Cancer of the intestine, bladder, or prostate can invade the lumbosacral plexus. Other masses, such as a noncancerous (benign) tumor, an abscess, or a pocket of blood (hematoma), may cause plexus disorders by putting pressure on a plexus.
Diabetes or radiation therapy for breast cancer, which can damage nerves throughout the body, may also damage nerves in a plexus.
Acute brachial neuritis (Parsonage-Turner syndrome, or neuralgic amyotrophy) is sudden malfunction of the brachial plexus due to inflammation rather than injury or cancer. It is probably caused by an autoimmune reaction Autoimmune Disorders An autoimmune disorder is a malfunction of the body's immune system that causes the body to attack its own tissues. What triggers an autoimmune disorder is not known. Symptoms vary depending... read more —when the body produces antibodies that attack its own tissues—or a virus. This disorder occurs primarily in men. It typically occurs in young adults but can occur at any age.
Symptoms of Plexus Disorders
Malfunction of the brachial plexus causes pain, weakness, and loss of sensation in an arm. All or part of the arm (such as the forearm or biceps) may be affected. If the cause is an injury, recovery tends to occur slowly, over several months. Some severe injuries cause permanent weakness.
Acute brachial neuritis causes severe pain in the upper arms and shoulders. Usually, the arm becomes weak and reflexes are impaired as the pain resolves. People may not be able to make an ο with their thumb and index finger. Weakness develops within 3 to 10 days. Then people typically regain their strength over the next few months.
Malfunction of the lumbosacral plexus causes pain in the lower back and leg as well as weakness and loss of sensation in all or part of a leg (such as the foot or calf). Recovery depends on the cause.
Diagnosis of Plexus Disorders
Electromyography and nerve conduction studies
Magnetic resonance imaging or computed tomography
Doctors suspect that a plexus is involved when symptoms are located in a part of the body supplied by a specific plexus. The location of the symptoms indicates which plexus is affected.
Electromyography and nerve conduction studies Electromyography and Nerve Conduction Studies Diagnostic procedures may be needed to confirm a diagnosis suggested by the medical history and neurologic examination. Imaging tests commonly used to diagnose nervous system (neurologic) disorders... read more 
can also help locate the damage.
Magnetic resonance imaging (MRI) or computed tomography (CT) can help determine whether a cancer, another mass, or an injury is causing the plexus disorder.
Treatment of Plexus Disorders
Treatment of the cause
When cancer or injury is the cause, sometimes surgery
Treatment of a plexus disorder depends on the cause. Cancer near the plexus may be treated with radiation therapy, chemotherapy, or both. Occasionally, a cancer or another mass that is damaging the plexus must be removed surgically.
If diabetes Treatment Diabetes mellitus is a disorder in which the body does not produce enough or respond normally to insulin, causing blood sugar (glucose) levels to be abnormally high. Urination and thirst are... read more is the cause, controlling blood sugar levels can help.
Doctors sometimes prescribe corticosteroids Corticosteroids Rheumatoid arthritis is an inflammatory arthritis in which joints, usually including those of the hands and feet, are inflamed, resulting in swelling, pain, and often destruction of joints.... read more 
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, but sometimes surgery is required.
