a heterogeneous group of degenerative diseases of the anterior horn cells in the spinal cord and motor nuclei of the brainstem; all are characterized by weakness. Upper motor neurons remain normal. These diseases include Werdnig-Hoffmann disease, spinal muscular atrophy types I and II (SMA1, SMA2); and Kugelberg-Welander disease, spinal muscular atrophy type III (SMA3).Fazio-Londe disease.
