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Motor neuron diseases  




Definition

Motor neuron diseases are a group of progressive disorders involving the nerve cells responsible for carrying impulses that instruct the muscles in the upper and lower body to move. Motor neuron diseases are varied and destructive in their effect. They commonly have distinctive differences in their origin and causation, but a similar result in their outcome for the patient: severe muscle weakness. Amyotrophic lateral sclerosis (ALS), spinal muscular atrophy, poliomyelitis, and primary lateral sclerosis are all examples of motor neuron diseases.

Description

A motor neuron is one of the largest cells in the body. It has a large cell body with many extensions reaching out in 360° from the cell body (soma). These extensions are called dendrites and are chemically able to receive instructions from adjacent neurons. These instructions are received in the form of an impulse stimulation of a particular protein channel on the dendrite by a neurotransmitter termed acetycholine (ACh). Extending from the soma of the motor neuron is a long portion of the cell called the axon. When conditions are favorable, an electrical signal passes down the axon to a region of the cell identified as the axon terminals. These terminals also branch in many directions and have, at their tips, a region called the synaptic end bulb. This region releases ACh that crosses a small gap until it reaches a protein on another dendrite.

When motor neurons line up in a tract, they allow an electrical signal to spread from the brain to the intended muscle. There are a tremendous number of nerve tracts that extend to all the muscles of the body that are responsible for contraction and relaxation of all types of muscles, including smooth and cardiac, as well as skeletal muscle. When the motor neuron is affected or damaged and it cannot perform at peak performance, the muscles of the body are affected. Often, a disorder of the motor neurons results in progressive muscle atrophy (shrinking and wasting) of some, if not all, the muscles of the body. Muscle twitching (fasciculation) is common among these disorders. Motor neuron diseases are difficult to treat, debilitating to movement and, in some cases, fatal.

Amyotrophic lateral sclerosis (ALS) is a disorder that generally involves either the lower or upper motor systems of the body. In advanced stages, both regions of the body are affected. This disease is commonly known as Lou Gehrig's disease after the famous baseball player who died from the condition. It is caused by sclerosis (a hardening of the surrounding fibrous tissues) in the corticospinal tracts. Associated with the sclerosis is a loss of the tissue of the anterior horns (gray matter) in the spinal cord, including the brainstem. Lou Gehrig's disease is characterized by a wasting of the muscles that, in turn, produces weakness. The bulbar, or facial/mouth muscles can initially become involved, which may lead to slurring of speech and drooling. The significance of this involvement is that, with rapid progression, the patient may not be able to swallow properly. This may lead to the risk of choking and other difficulties with obtaining nutrition and proper respiration. Death from complications of ALS is common within five years.

Spinal muscular atrophies (SMAs) are a wide group of genetic disorders characterized by primary degeneration of the anterior horn cells of the spinal cord, resulting in progressive muscle weakness. Spinal muscular atrophies affect only lower motor neurons. In babies and children, many SMAs are rapidly progressive with paralysis of the legs, trunk, and eventually, the respiratory muscles. In teenagers and adults, SMAs are usually slowly progressive. Kennedy's disease, an X-linked (carried by women and passed on to male offspring) SMA, features similar wasting of facial muscles as seen in ALS, with characteristic difficulty speaking and swallowing.

Primary lateral sclerosis (PLS) is a rare motor neuron disease that resembles ALS. Primary lateral sclerosis often begins after age 50, and results in slowly progressive weakness and stiffness in the leg muscles, clumsiness, and difficulty maintaining balance. Symptoms worsen over a period of years. Muscle spasms in the legs may also occur, but in PLS, there is no evidence of the degeneration of spinal motor neurons or muscle wasting (amyotrophy) that occurs in ALS.

Unlike most motor neuron diseases, poliomyelitis results from infection with a virus. Contamination occurs through fecal or oral exposure. Once inside the body, the virus uses the cells of the gastrointestinal tract to enter the bloodstream and move throughout the body. Eventually, the poliovirus invades the nerve cells of the spinal cord and kills the motor neurons. When the motor neurons are destroyed, the muscles they connect to become damaged and weaken. The result is varying degrees of paralysis, including difficulty swallowing, walking, breathing, and control of speech.

Demographics

Motor neuron diseases are uncommon, as about one person in 50,000 is diagnosed with a motor neuron disease in the United States each year. In total, about 5,500 people in the United States each year receive a diagnosis of a motor neuron disease.

About 20,000 Americans are living with ALS and nearly 4,500 new cases are reported annually. The peak age for onset is around 55 years of age, but younger patients have been observed. Spinal muscular atrophies and primary lateral sclerosis are rare diseases.

The occurrence of poliomyelitis is seen in records of epidemics that were intricately documented in the last 100 years. A description of an epidemic in recent times in the United States discussed a low of 4,197 cases in the early 1940s to a high of 42,033 in 1949. By 1952, the number of case had reached over 58,000. In 1955, a vaccine was developed that used weakened forms of the virus. This vaccine and the subsequent Sabin vaccine nearly wiped out polio in the world. The Americas were declared free of polio in the 1990s. In 2002, there were less than 500 cases worldwide, and in 2003, that number decreased to less than 100 cases. It is expected that by the end of the year 2005, the disease will be eradicated. Although new cases have begun to appear in regions of Africa and India, the World Health Organization (WHO) is keeping track of the outbreaks, and scientists are hopeful that poliomyelitis will soon disappear from the list of motor neuron diseases.

Causes and symptoms

Causes of many motor neuron diseases are unknown, and others have varying causes according to the specific motor neuron disease. Most cases of ALS occur sporadically for an unknown reason, however, up to 10% of ALS cases are inherited. Most spinal muscular atrophies are inherited. A virus causes poliomyelitis. Additionally, environmental factors and toxins are under study as causes or triggers for motor neuron diseases.

Muscle weakness is the symptom common to all motor neuron diseases. Muscles of the legs are most often affected, leading to clumsiness, unstable gait, or lower limb paralysis. Muscle cramps and fasciculations (twitching) occur with most motor neuron diseases. Facial muscles may also be affected, leading to difficulty with speech (dysarthria). Later in the course of some motor neuron diseases, the muscles involved with swallowing and breathing may be impaired (dysphagia).

Diagnosis

Diagnosis of motor neuron disease is often based upon symptoms and exclusion of other neurological diseases. Nerve conduction studies can help distinguish some forms of peripheral neuropathy from motor neuron disease. Electromyelogram (EMG), a test measuring the electrical activity in muscles, can support the diagnosis of ALS and some other motor neuron diseases. Although computed tomography (CT) scans and magnetic resonance imaging (MRI) scans are often normal in persons with motor neuron disease, they may help exclude spinal malformations or tumors that could be responsible for similar symptoms. A muscle biopsy can exclude myopathies. Diagnosis of primary lateral sclerosis is especially difficult and often delayed, as it is frequently misdiagnosed as ALS. Polio may be diagnosed by recovering the virus from a stool or throat culture, examining antibodies in the blood or, rarely, by spinal fluid analysis. Finally, molecular genetic studies can aid in the diagnosis of spinal muscular trophies and the small percentage of inherited ALS cases.

Treatment team

Caring for a person with a motor neuron disease requires a network of health professionals, community resources, and friends or family members. A neurologist usually makes the diagnosis, and the neurologist and primary physician coordinate ongoing treatment and symptom relief. Physical, occupational, and respiratory therapists provide specialized care, as do nurses. Social service and mental health consultants organize support services.

Treatment

There are few specific treatments for motor neuron diseases, and efforts focus on reducing the symptoms of muscle spasm and pain while maintaining the highest practical level of overall health. Riluzole, the first drug approved by the U.S. Food and Drug Administration for the treatment of ALS, has extended the life of ALS patients by several months and also extended the time a person with ALS can effectively use his or her own muscles to breathe.

Other medications used to treat persons with motor neuron disease are designed to relieve symptoms and improve the quality of life for patients. These include medicines to help with depression, excess saliva production, sleep disturbances, and constipation.

Recovery and rehabilitation

Recovery from motor neuron diseases depends on the type of disease and the amount of muscle degeneration present. In diseases such as ALS, the emphasis is placed upon maintaining mobility and function for as long as possible, rather than recovery. With all motor neuron diseases, physical therapy can teach exercises to help with range of motion and prevent contractures (stiff muscles at the joints). Occupational therapy provides assistive devices for mobility such as wheelchairs, positioning devices, braces, and other orthotics for performing daily activities such as reaching and dressing. Respiratory therapists and speech therapists help prevent pneumonia by maintaining lung function and promoting safe eating strategies. Speech therapists also help with alternate forms of communication if facial muscles are involved.

Recovery from polio may be complete or only partial, depending on the degree of lower motor neuron damage. Years or decades after recovering from polio, persons may again experience muscle weakness and pain. This is known as postpolio syndrome. Vigorous exercise has been shown to cause additional weakness in postpolio syndrome, and physicians recommend energy conservation and lifestyle changes for these patients.

Clinical trials

The National Institutes of Health (NIH) has more than 20 clinical trials scheduled for 2004–05 for the study of motor neuron diseases, including one trial designed to evaluate a new drug, Minocycline, in the treatment of ALS. Details and up-to-date information about patient recruiting can be found at the NIH Website for clinical trails at .

Prognosis

The prognosis of persons with motor neuron diseases depends on the type of the disease and the amount and progression of muscle degeneration. Most persons with ALS die from complications of respiratory failure within five years of developing symptoms. About one out of 10 persons with ALS live a decade or longer with the disease. The prognosis for a person with spinal muscular atrophy varies greatly, according to the severity of the disease. Some forms result in immobility and death within a few years, while others impede movement, but do not affect a normal lifespan.

Special concerns

It is important to remember that even in the most severe motor neuron diseases, a person's personality, intelligence, reasoning ability, or memory are not impaired. The person with motor neuron disease also retains the senses of sight, smell, hearing, taste, and in the unaffected areas, touch.

Resources

BOOKS

Kunci, Ralph W. Motor Neuron Disease. Philadelphia: W.B. Saunders, 2002.

Oliver, David. Motor Neuron Disease: A Family Affair. London: Sheldon Press, 1995.

Silver, Julie. Postpolio Syndrome. Philadelphia: Hanley & Belfus, 2003.

Wade, Mary Dodson. ALS—Lou Gehrig's Disease. Berkeley Heights, NJ: Enslow Publishers, 2001.

OTHER

"NINDS Motor Neuron Diseases Information Page." National Institute of Neurological Disorders and Stroke. May 15, 2004 (June 1, 2004). http://www.ninds.nih.gov/health_and_medical/disorders/motor_neuron_diseases.htm.

ORGANIZATIONS

ALS Association (ALSA). 27001 Agoura Road, Suite 150, Calabasas Hills, CA 91301-5104. (818) 880-9007 or (800) 782-4747; Fax: (818) 880-9006. info@alsanational.org. http://www.alsa.org.

Families of SMA. PO Box 196, Libertyville, IL 60048-0196. (800) 886-1762; Fax: (847) 367-7623. sma@fsma.org. http://www.fsma.org.

Primary Lateral Sclerosis Newsletter. 101 Pinta Court, Los Gatos, CA 95032. (408) 356-8227; Fax: (408) 356-8227. 73112.611@compuserve.com.


Brook Ellen Hall, PhD


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