Phenotypic Classification of Spinal Muscular Atrophy

Spinal muscular atrophy results in a spectrum of disease based on time of onset and clinical severity.

Parameters:

(1) age at onset

(2) motor function

(3) lifespan

Prenatal Phenotypes (weakness present at birth)

Phenotype	Lifespan	Motor Function	Other Findings
SMA 0	short (a few months)	none	not meeting criteria for the other 2 types
congenital axonal neuropathy (CAN)	very short (a few days)	severe weakness	joint contractures, respiratory failure at birth
arthrogryposis multiplex congenital (AMC)	variable	may be able to stand	congenital joint contractures affecting 2 or more body regions

Postnatal Phenotypes

Phenotype	Synonym	Age at Onset and Death	Motor Function
SMA I (acute)	Werdnig-Hoffman	onset < 6 months; death in early childhood	sitting requires support
SMA II (chronic)	Dubowitz	6 - 18 months; many survive into adulthood	independent sitting once in place
SMA III (juvenile)	Kugelberg-Welander	onset > 12 months; normal lifespan	able to walk
SMA IV (adult)		adult onset; normal lifespan	normal motor function

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