Table 2.

Classification of SMA.

SMA Type	Age of Onset	Maximum Age of Survival	SMN2 Copy Number	Alternate Name
0	Prenatal	<1 month	1	-
1	0–6 months	<2 years	2	Werdnig–Hoffman disease
2	<18 months	>2 years	3,4	Dubowitz disease
3a (juvenile onset)	18 months–3 years	Adult	3,4	Kugelberg–Welander disease
3b	>3 years	Adult	4	Kugelberg–Welander disease
4	> 21 years	Adult	4–8	-