The prevalence of ALS is 4 to 7 cases per 100,000, with an incidence of about 1.5 per 100,000. This means that in the United States, approximately 5,000 people every year will be diagnosed with ALS, and about 25,000 people currently live with the disease.
The median age of onset is fifty-five years in most studies, although the distribution is so broad that ALS can be seen in teenagers and octogenarians.
Most cases of ALS occur in the absence of a family history of the disease (sporadic ALS). Ten percent of patients inherit the disease (familial ALS). In most cases of familial ALS, the disease is passed on in an autosomal dominant pattern. In some of these cases, the cause of the disease has been traced to a specific mutation in the gene that encodes the enzyme superoxide dismutase 1 (SOD1).
dropout of motor neurons in the brain, brain stem, and spinal cord. Motor neuron death leads to increasing weakness of the limbs, facial muscles, and diaphragm, while sensory function remains intact.