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Classic chromosome 5q spinal muscular atrophy (SMA) is one of the most common neuromuscular conditions in childhood and the most common fatal genetic disease in infants. Byers and Banker1 first described the variability of this disease in terms of clinical onset and phenotypic severity of SMA and introduced us to the clinical spectrum of “infantile SMA.” They suggested that SMA was a progressive disease. Others, however, favored a static clinical course after an initial period of deterioration. This debate has continued to the present. Most accept that a continual gradient in phenotypic severity exists, meaning every patient with SMA is truly unique. The modern classification of SMA, established in 1992, is therefore based on age at symptom onset and best motor performance achieved.2 SMA type I, also known as Werdnig-Hoffmann disease, exhibits subtypes of differing severity and, therefore, has been further divided into IA, IB, and IC, based on age at clinical onset. Dubowitz3 proposed a decimal classification system based on a continual rather than a discrete variable (e.g., SMA type 1.1, 1.5, and 1.9) to better capture the graded severity of the clinical phenotypes.
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