Motor neuron diseases (muscle stimulation disorders) are characterized by progressive deterioration of the nerve cells that initiate muscle movement. As a result, the muscles stimulated by these nerves also deteriorate and can no longer function normally.
For normal muscle function, muscle tissue and nerve connections between the brain and muscle must be normal. Muscle movement is initiated by nerve cells that are located in the spinal cord and in the front part of the brain (called the motor cortex—see Using the Brain to Move a Muscle). These nerve cells connect with nerves that stimulate muscles to move (called motor nerves). In motor neuron diseases, these nerve cells progressively deteriorate. As a result, muscles weaken, waste away (atrophy), and can become completely paralyzed even though the muscles themselves are not the cause of the problem.
Motor neuron diseases have various forms, such as amyotrophic lateral sclerosis (the most common), primary lateral sclerosis, progressive pseudobulbar palsy, progressive muscular atrophy, progressive bulbar palsy, and postpolio syndrome. These disorders are more common among men and usually develop in people who are in their 50s. The cause is usually unknown. About 5 to 7% of people who have a motor neuron disease have a hereditary type and thus have family members who also have the disease.
Different parts of the nervous system may be affected first. For example, some forms of motor neuron disease affect the mouth and throat first. Others affect a hand or foot first or affect them most severely.
Long-lasting paralysis can lead to permanent shortening of muscles (contractures).
Muscle strength is affected, but people do not have pain or any changes in sensation. Depression is common.
Amyotrophic lateral sclerosis (Lou Gehrig disease):
This progressive form begins with weakness, often in the hands and less frequently in the feet or mouth and throat. Weakness may progress more on one side of the body than on the other and usually proceeds up the arm or leg. Muscles, usually those in the hands and feet, start to waste away (atrophy). Muscle cramps are also common and may occur before the weakness, but no changes in sensation occur. People may lose weight and feel unusually tired.
Over time, weakness increases.
Muscles twitch (called fasciculations). Muscle tone typically increases, and muscles tend to become stiff and tight, leading to muscle spasms (called spasticity). Movements are stiff and clumsy. In some people, muscle tone decreases, making the limbs seem loose and floppy.
Controlling facial expressions may become difficult. Weakening of muscles in the throat may lead to slurred speech and difficulty swallowing (dysphagia). Because swallowing is difficult, people sometimes drool and are more likely to choke on liquids. Food or saliva can be inhaled (aspirated) into the lungs, increasing the risk of pneumonia (see Aspiration Pneumonia). The voice usually sounds nasal but may be hoarse.
As symptoms progress, people may be unable to control emotional responses and may laugh or cry inappropriately.
Eventually, the muscles involved in breathing weaken, leading to breathing problems. Some people need a ventilator to breathe.
How rapidly amyotrophic lateral sclerosis progresses varies:
Primary lateral sclerosis and progressive pseudobulbar palsy:
These forms 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. In both disorders, muscles are weak and very stiff and tight (spastic). Muscles twitch (called fasciculations) and waste away.
Emotions may be changeable: People with progressive pseudobulbar palsy may switch from happiness to sadness quickly and without reason. Inappropriate emotional outbursts are common.
Symptoms usually progress for several years before total disability results.
Progressive muscular atrophy:
This form can develop at any age. It 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.
The hands are usually affected first, followed by the arms, shoulders, and legs. Eventually, the whole body is affected.
Many people with this form survive 25 years or longer.
Progressive bulbar palsy:
In this form, the nerves controlling the muscles of chewing, swallowing, and talking are affected, making these functions increasingly difficult. The voice may have a nasal tone. In some people, emotions are changeable. Because swallowing is difficult, food or saliva is often inhaled 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 symptoms appear.
Some people who have had polio develop this syndrome decades after they have recovered from polio. Postpolio syndrome is more likely to develop in older people and in people who had a severe initial case of polio. Muscles, usually those that were affected by polio, may become tired, painful, and weak and may waste away. However, in most people who have had polio, such symptoms are not due to postpolio syndrome but to the development of a new disorder, such as diabetes, a ruptured (herniated) disk, or osteoarthritis, or to age-related loss of certain nerve cells in areas where polio had already reduced the number of these nerve cells.
Postpolio syndrome is sometimes considered a motor neuron disease but is sometimes considered a separate disorder.
Doctors suspect motor neuron disease in adults who have progressive muscle weakness without pain or loss of sensation. Doctors ask people which parts of the body are affected, when symptoms started, which symptoms appeared first, and how the symptoms have changed over time. This information gives them clues about the cause of symptoms.
Muscle weakness can have many causes (see Causes). Diagnostic tests, such as the following, are done to help narrow the possibilities:
To test for other disorders, doctors may do blood tests to check for infections and metabolic disorders, urine tests to check for heavy metals (such as lead or mercury) if people have been exposed to them, a spinal tap (lumbar puncture) to check for inflammation, and genetic tests to check for hereditary disorders such as hereditary neuropathies.
Over time, motor neuron diseases tend to cause symptoms that are so characteristic that the diagnosis is obvious without any testing.
Motor neuron diseases have no specific treatment or cure.
Care provided by a team of several types of health care practitioners (a multidisciplinary team) helps people cope with progressive disability. Physical therapy helps people maintain muscle strength and keep joints flexible and thus helps prevent contractures. Nurses or other caregivers must feed people with swallowing difficulties carefully to prevent choking. Some people must be fed through a tube inserted through the abdominal wall into the stomach (gastrostomy tube).
Certain drugs can help relieve symptoms:
In some people with amyotrophic lateral sclerosis, riluzole, a drug that protects nerve cells, can prolong life for a few months. It is taken by mouth.
If pain develops as the disease progresses (for example, if pain occurs when a person has to sit in one position too long), opioids and benzodiazepines, which are mild sedatives, may be used.
In a few people with progressive bulbar palsy, surgery to improve swallowing helps.
Because amyotrophic lateral sclerosis and progressive bulbar palsy are progressive and incurable, people who have one of these diseases are advised to establish advanced directives that specify what kind of care they want at the end of life (see Advance Directives).
Last full review/revision December 2014 by Michael Rubin, MDCM