MOTOR NEURON DISEASES
What are motor neuron diseases?
The motor neuron diseases (MNDs) are a group of progressive neurological disorders that destroy motor neurons, the cells that control essential voluntary muscle activity such as speaking, walking, breathing, and swallowing. Normally, messages from nerve cells in the brain (called upper motor neurons) are transmitted to nerve cells in the brain stem and spinal cord (called lower motor neurons) and from them to particular muscles. Upper motor neurons direct the lower motor neurons to produce movements such as walking or chewing. Lower motor neurons control movement in the arms, legs, chest, face, throat, and tongue. When there are disruptions in these signals, the muscles do not work properly; the result can be gradual weakening, wasting away, and uncontrollable twitching (called fasciculations). When upper motor neurons are affected, the manifestations include spasticity or stiffness of limb muscles and over-activity of tendon reflexes such as knee and ankle jerks. Eventually, the ability to control voluntary movement can be lost. MNDs may be inherited or acquired.
Who is at risk?
MNDs occur in adults and children. The diseases are more common in men than in women. In adults, symptoms may appear after age 40. In children, particularly in inherited or familial forms of the disease, symptoms can be present at birth or appear before the child learns to walk.
What causes motor neuron diseases?
The causes of sporadic, or noninherited, MNDs are not known, but environmental, toxic, viral, or genetic factors may be implicated. Sporadic cases may be triggered by cancers or prolonged exposure to toxic drugs or environmental toxins. Scientists are investigating whether the body’s autoimmune reaction to viruses such as the human immunodeficiency virus can trigger MNDs.
What are the symptoms of motor neuron diseases?
A brief description of the symptoms of some of the more common MNDs follows.
Amyotrophic lateral sclerosis (ALS), also called Lou Gehrig’s disease or classical motor neuron disease, is a progressive, ultimately fatal disorder that eventually disrupts signals to all voluntary muscles. In the United States, doctors use the terms motor neuron disease and ALS interchangeably. Both upper and lower motor neurons are affected. Approximately 75 percent of people with classic ALS will also develop weakness and wasting of the bulbar muscles (muscles that control speech, swallowing, and chewing). Symptoms are usually noticed first in the arms and hands, legs, or swallowing muscles. Muscle weakness and atrophy occur disproportionately on both sides of the body. Affected individuals lose strength and the ability to move their arms, legs, and body. Other symptoms include spasticity, exaggerated reflexes, muscle cramps, fasciculations, and increased problems with swallowing and forming words. Speech can become slurred or nasal. When muscles of the diaphragm and chest wall fail to function properly, individuals lose the ability to breathe without mechanical support. Although the disease does not usually impair a person’s mind or personality, several recent studies suggest that some people with ALS may have alterations in cognitive functions such as problems with decision-making and memory.
Progressive bulbar palsy, also called progressive bulbar atrophy, involves the bulb-shaped brain stem —the region that controls lower motor neurons needed for swallowing, speaking, chewing, and other functions. Symptoms include pharyngeal muscle weakness (involved with swallowing), weak jaw and facial muscles, progressive loss of speech, and tongue muscle atrophy. Limb weakness with both lower and upper motor neuron signs is almost always evident but less prominent. Affected persons have outbursts of laughing or crying (called emotional lability). Individuals eventually become unable to eat or speak and are at increased risk of choking and aspiration pneumonia, which is caused by the passage of liquids and food through the vocal folds and into the lower airways and lungs.
Primary lateral sclerosis (PLS) affects only upper motor neurons and is nearly twice as common in men as in women. Onset generally occurs after age 50. The cause of PLS is unknown. It occurs when specific nerve cells in the cerebral cortex (the thin layer of cells covering the brain which is responsible for most higher level mental functions) that control voluntary movement gradually degenerate, causing the muscles under their control to weaken. The syndrome—which scientists believe is only rarely hereditary—progresses gradually over years or decades, leading to stiffness and clumsiness of the affected muscles.
Progressive muscular atrophy is marked by slow but progressive degeneration of only the lower motor neurons. It largely affects men, with onset earlier than in other MNDs. Weakness is typically seen first in the hands and then spreads into the lower body, where it can be severe. Other symptoms may include muscle wasting, clumsy hand movements, fasciculations, and muscle cramps. The trunk muscles and respiration may become affected. Exposure to cold can worsen symptoms. The disease develops into ALS in many instances.