Table 1.

Clinical SMA Types⁴

SMA Type	Clinical Manifestations	SMN2 Copy Number
0	Reduced prenatal movement, symptoms at birth, death early	Typically 1
1	Symptoms by 6 months of age; lack of sitting; respiratory, feeding, orthopedic complications	Most commonly 2, sometimes 1 or 3
2	Symptoms between 6–18 months, able to sit but not walk, respiratory, feeding and orthopedic complications	Most commonly 3, sometimes 2 or 4
3	Symptoms after 18 months, able to sit, stand, and walk. May have respiratory, feeding, and orthopedic complications	Most commonly 3–4, but variable
4	Symptoms onset in adulthood, typically mild	Typically 4

SMA, spinal muscular atrophy; SMN, survival motor neuron