Table 1.

Clinical classification of spinal muscular atrophy

SMA type and sub-classification	Age at onset	Highest achieved motor milestones
1	0–6 months	Never acquires ability to sit unsupported
0/1a	Prenatal / neonatal	0/1a: Symptoms in prenatal and/or neonatal period, no head control
1b (‘classic’)	1–6 months	1b: No head control and no ability to roll over
1c	3–6 months	1c: Will usually acquire additional motor skills, such as head control or rolling from supine to prone, or at least to one side at any stage in life.
2	6–18 months	Able to sit unsupported, not able to walk
2a		2a: unsupported sitting but not able to stand or walk even with assistance
2b		2b: in addition to unsupported sitting also able only with assistance to stand or even walk a few steps
3	> 18 months	Able to walk unsupported
3a	18–36 months
3b	> 36 months
4	During adulthood, i.e. ≥ 18 years	Able to walk unsupported