degenerative neurological disorder that causes muscle atrophy and paralysis. The disease usually occurs after the age of 40; it affects men more often than women. ALS is frequently called Lou Gehrig disease in memory of the famous baseball player Lou Gehrig, who died from the disease in 1941.

ALS affects the motor neurons—i.e., those neurons that control muscular movements. The muscles innervated by the degenerating neurons become weak and eventually atrophy. Symptoms include muscle weakness, frequent falls, breathing difficulty, persistent fatigue, spasticity, and twitching. Death generally results from atrophy of the respiratory muscles. There is no cure or treatment for ALS.

Progressive muscular atrophy is a variety of ALS in which the neuron degeneration is most pronounced in the spinal cord. Symptoms are similar to ALS, though spasticity is absent and muscle weakness is less severe. Individuals with progressive muscular atrophy generally survive longer than those suffering from ALS.

Progressive bulbar palsy is another variety of ALS caused by degeneration of the cranial nerves and brainstem. Chewing, talking, and swallowing are difficult, and involuntary emotional outbursts of laughing and tongue twitching and atrophy are common. The prognosis is especially grave in this form of ALS.

Approximately 10 percent of ALS cases are hereditary. A defect in the gene that produces the enzyme superoxide dismutase, which eliminates free radicals (radical) from the body's cells, is responsible for the hereditary form of ALS. Free radicals are molecular by-products of normal cell metabolism that can accumulate in and destroy cells. The genetic defect causes ineffective production of superoxide dismutase in neutralizing free radicals, which subsequently destroy motor neurons. Genetic screening can determine potential carriers of the gene in families with a history of ALS.