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Table 1 Clinical classification of spinal muscular atrophy

From: Natural history of lung function in spinal muscular atrophy

SMA type and sub-classificationAge at onsetHighest achieved motor milestones
10–6 monthsNever acquires ability to sit unsupported
0/1aPrenatal / neonatal0/1a: Symptoms in prenatal and/or neonatal period, no head control
1b (‘classic’)1–6 months1b: No head control and no ability to roll over
1c3–6 months1c: 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.
26–18 monthsAble 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 monthsAble to walk unsupported
3a18–36 months 
3b> 36 months 
4During adulthood, i.e. ≥ 18 yearsAble to walk unsupported