Amyotrophic lateral sclerosis (ALS) is the most common form of progressive motor neuron disease (Table 185-1). ALS is caused by degeneration of motor neurons at all levels of the CNS, including anterior horns of the spinal cord, brainstem motor nuclei, and motor cortex. Familial ALS (FALS) represents 5-10% of the total and is inherited usually as an autosomal dominant disorder.