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

Disorders of Muscle Stimulation

For normal muscle function, muscle tissue and nerve connections between the brain and muscle must be normal. If the motor nerves do not stimulate muscles normally, muscles weaken, waste away (atrophy), and can become completely paralyzed even though the muscles themselves are not the cause of the problem.

Muscle stimulation disorders (also called motor neuron disorders) include amyotrophic lateral sclerosis, primary lateral sclerosis, progressive pseudobulbar palsy, progressive muscular atrophy, progressive bulbar palsy, and postpolio syndrome. Motor neuron disorders are more common among men and usually develop in people who are in their 50s. In most people who have one of these disorders, the cause is unknown. About 10% of people who have a motor neuron disorder have a hereditary type and thus have family members who also have the disorder.

In all of these disorders, motor nerves in the spinal cord or brain progressively deteriorate, causing muscle weakness that can progress to paralysis. However, in each disorder, a different part of the nervous system is affected. Consequently, each disorder primarily affects different muscles and different parts of the body.

Symptoms

Amyotrophic Lateral Sclerosis (Lou Gehrig's Disease): This disorder is progressive. It begins with weakness, often in the hands and less frequently in the feet. Weakness may progress more on one side of the body than on the other and generally proceeds up the arm or leg. Cramps are also common and may precede the weakness, but no changes in sensation occur. Over time, weakness increases, and spasticity develops. Muscles twitch and become tight, followed by muscle spasms. Tremors may appear. Weakening of muscles in the throat may lead to difficulty speaking (dysarthria) and swallowing (dysphagia). Eventually, the muscles involved in breathing may weaken, leading to breathing problems; some people may need a ventilator to breathe.

How rapidly amyotrophic lateral sclerosis progresses varies. About 50% of people with the disorder die within 3 years of the first symptoms, 10% live 10 years or more, and a few people survive as long as 30 years.

Primary Lateral Sclerosis and Progressive Pseudobulbar Palsy: These disorders are rare, slowly progressive variants of amyotrophic lateral sclerosis. Primary lateral sclerosis affects mainly the arms and legs, and progressive pseudobulbar palsy affects mainly the muscles of the face, jaw, and throat. Emotions may be changeable: People with progressive pseudobulbar palsy may switch from happiness to sadness quickly and without reason. Inappropriate emotional outbursts are common. In both disorders, severe stiffness accompanies muscle weakness. The disorders usually progress for several years before total disability results.

Progressive Muscular Atrophy: This disorder is similar to amyotrophic lateral sclerosis, but it progresses more slowly, spasticity does not occur, and muscle weakness is less severe. Involuntary contractions or twitching of muscle fibers may be the earliest symptoms. Many people with this disorder survive 25 years or longer.

Progressive Bulbar Palsy: In this disorder, the nerves controlling the muscles of chewing, swallowing, and talking are affected, making these functions increasingly difficult. Because swallowing is difficult, food or saliva is often inhaled (aspirated) into the lungs, causing choking or gagging and increasing the risk of pneumonia. Death, which is often due to pneumonia, usually occurs 1 to 3 years after the disorder begins.

Postpolio Syndrome: Some people who have had polio may develop tired, painful, and weak muscles 15 years or more after their recovery from polio. Sometimes muscle tissue also wastes away, suggesting a reactivation of the polio infection. However, in most people who have had polio, such symptoms are not due to the postpolio syndrome but to the development of a new disorder, such as diabetes, a slipped (herniated) disk, or osteoarthritis.

Diagnosis

Doctors suspect one of these disorders in adults who have progressive muscle weakness without pain or loss of sensation. Muscle weakness can have many causes (see Section 5, Chapter 59), so diagnostic procedures are performed to help narrow the possibilities. For example, electromyography (see Section 6, Chapter 77), which records electrical activity in muscles, can help determine whether the problem is in the nerves or muscles. However, such procedures cannot determine which nerve disorder is causing the problem. The diagnosis is also based on which parts of the body are affected, when the disorder started, what symptoms appeared first, and how the symptoms changed over time.

Treatment

Motor neuron disorders have no specific treatment or cure. Physical therapy helps people maintain muscle strength and helps prevent tightening of the muscles (contractures). People with swallowing difficulties must be fed carefully to prevent choking; some must be fed through a tube inserted through the abdominal wall into the stomach (gastrostomy tube). Baclofen, phenytoin, or quinine may help decrease cramps. Amitriptyline, an antidepressant, may be given, not for its antidepressant effects, but for one of its anticholinergic effects--reduction of saliva formation.

Because amyotrophic lateral sclerosis and progressive bulbar palsy are progressive and incurable, people with one of these disorders are advised to establish advanced directives, specifying what kind of care they want at the end of life (see Section 1, Chapter 9).

See the sidebar Overactive Nerves: Two Syndromes.