Spinal muscular atrophy is the term for a group of disorders involving just anterior horn cells. Many are hereditary in nature. The clinical findings consist of the lower motor neuron signs of weakness, atrophy, fasciculations, and loss of reflexes. Werdnig-Hoffman disease is an infantile onset spinal muscular atrophy with fatal outcome due to respiratory weakness. Less common types of spinal muscular atrophy become symptomatic in childhood or adult life, creating nonfatal disabilities. Benign focal amyotrophy manifests in adults as a slowly progressive atrophy of one limb or restricted segments of limbs, with a normal life span.