Spinal muscular atrophy types

Editors-in-Chief: C. Michael Gibson, M.S., M.D.; Priyamvada Singh, MBBS

Overview
The autosomal recessive SMA caused by mutation of SMN gene present on chromosome 5q13 comprises 95% of all SMA. SMA is mainly classified into three broad types (Type I – Type III) based on the clinical severity and prognosis. Two more type of SMA, Type 0 and Type IV have been recently defined. The different types of SMA have a clinical continuum. ==Types of SMA, , ==

Type I SMA, Werdnig-Hoffman disease


 * 1) 	Present from birth to 6 months of age
 * 2) 	Survival mostly up to 2 years of age.
 * 3) 	Never able to sit
 * 4) 	Hypotonic (predominately proximal weakness, lower limb more than upper)
 * 5) 	Poor head control
 * 6) 	Risk of aspiration, failure to thrive
 * 7) 	Normal cognition,
 * 8)      Respiratory failure common.

Type II SMA, Intermediate SMA


 * 1) 	Presents mostly at 6-12 months of age
 * 2) 	70% survive beyond 25 years of age
 * 3) 	Sitter but never able to walk
 * 4) 	Clinical features similar to type I (hypotonic with predominance proximal weakness) but of less severity than Type I SMA.
 * 5) 	Cognition normal or even more than normal.

Type III SMA, Kugelberg-Welander disease
 * 1) 	Walkers
 * 2) 	Onset of disease more than 12 months of age
 * 3) 	Life span normal
 * 4) 	Milder symptoms (less hypotonic, little or no respiratory failure) compared to type I and II.

Outliers There are two more types of SMA recently defined as Type ‘0’ and Type ‘IV’. Type 0 is the most severe of all the types of SMA with onset as early as prenatal period. Life expectancy is reduced and mostly the child is unable to survive beyond 6 months of age. Mostly they don’t attain any motor milestones can present with facial diplegia, opthalmoplegia and arthrogryposis and respiratory failure. Type IV SMA is the mildest variety.

Other: Non-5q13 associated SMA -

SMA in this category include X linked, autosomal dominant spinal muscular atrophies, distal spinal muscular atrophies, distal hereditary motor neuropathies, SMA with respiratory distress and pontocerebellar hypoplasia with infantile SMA. Infantile X-Linked SMA is similar to but distinguishable from Werdnig Hoffmann disease, manifested at or before birth in boys. Boys who inherit the gene usually die before age 2. Girls who inherit the gene are carriers, but are otherwise unaffected.It is caused by mutation of the UBE1 gene on X Chromosome