Most ALS cases are sporadic (the disease appears for no apparent reason and is not passed on to children). However, in around 10% of cases, ALS is directly caused by a mistake in a gene which can be passed from parent to child. In this case, the disease is known as familial ALS. First signs of familial ALS usually occur slightly earlier (46 years old) than sporadic cases (56 years old).
Disease progression can vary from a few months to several years and is characterized by progressive muscle paralysis and amyotrophic.
Impaired respiratory function is a feature of advanced ALS and is a poor prognostic sign.