Monarch Disease Ontology term MONDO_0009669 (spinal muscular atrophy, type 1) is a severe infantile form of proximal spinal muscular atrophy characterized by severe and progressive muscle weakness and hypotonia resulting from the degeneration and loss of the lower motor neurons in the spinal cord and the brain stem nuclei. Also known as: HMN (hereditary motor neuropathy) proximal type I, SMA type 1, SMA type I, SMA-I, SMA1, SMNI, Werdnig Hoffmann disease, Werdnig-Hoffman disease.