SMA is characterized by symmetric proximal muscle weakness. Sensory neurons may have also been affected in SMA type I (see below) patients. SMA is classically subdivided into three types according to age of onset and clinical severity, based on the diagnostic criteria. Type I SMA (Werdnig-Hoffmann disease; OMIM#253300) is characterized by the onset of severe muscle weakness and hypotonia in the first few months of life, and the inability to sit or walk. Fatal respiratory failure usually occurs before the age of 2 years. Type II SMA (intermediate type; OMIM#253550) is characterized by the onset of proximal muscle weakness before 18 months of age, the ability to sit but not to walk unaided, and survival beyond 4 years of age. Type III SMA (Kugelberg-Welander disease; OMIM#253400) is characterized by the onset of proximal muscle weakness after the age of 2 years, the ability to walk independently until the disease progresses, and survival into adulthood.
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