Amyotrophic lateral sclerosis (ALS), also known as motor neuron disease (MND) or Lou Gehrig's disease, is a rare, terminal neurodegenerative disease defined by the progressive loss of both upper and lower motor neurons that normally control voluntary muscle contraction. ALS is the most common of the motor neuron diseases. ALS often presents in its early stages with gradual muscle stiffness, twitches, weakness, and wasting. Motor neuron loss typically continues until the abilities to eat, speak, move, and breathe without mechanical support are lost. It is estimated that at least 50% of people with ALS experience significant changes in thinking and behavior, with 15% of individuals going on to develop frontotemporal dementia. An ALS diagnosis is made based on a person's signs and symptoms, with additional testing conducted to rule out other potential causes. Depending on which areas of the body are affected first, ALS may be classified as limb-onset (beginning with weakness in the arms or legs) or bulbar-onset (beginning with difficulty in speaking and/or swallowing). Respiratory onset occurs in approximately 1%–3% of cases.
Most cases of ALS (about 90–95%) have no known cause and are known as sporadic ALS. Both genetic and environmental factors are believed to be involved in the onset of ALS. Approximately 5–10% of ALS cases have a known genetic cause and often linked to a family history of ALS; such cases are known as familial ALS or hereditary ALS. Four disease-linked genes are responsible for approximately half of all genetic cases.
