Table 1.

Classification and subtypes of spinal muscular atrophy.

SMA Type (Historical Name)	OMIM	Onset Age	Motor Milestones Achieved	Subclassification	Natural History	Other Features	Estimated SMN2 Copies	Estimated SMA Proportion
Type 0	-	Prenatal or at birth	Never sits, never head control	-	Death < 1 mo if untreated	Joint contractures, cardiac defect, facial diplegia, immediate respiratory failure after birth	1 SMN2 copy in ~100% of patients	Unclear, Maybe < 1%
Type 1 (Werdnig-Hoffmann disease)	253300	0–6 mo	Never sits, some achieve head control	1A: Onset < 1 mo, usually by 2 wk; head control absent 1B: Onset 1–3 mo; poor or absent head control 1C: Onset 3–6 mo; head control achieved	1A: Death < 6 mo if untreated 1B and 1C: death < 2 yr if untreated	1A: Very similar to type 0 SMA 1B and 1C: Tongue fasciculation, swallowing difficulties, early respiratory failure	1 or 2 SMN2 Copies in ~80% of patients	~60%
Type 2 (Dubowitz disease)	253550	7–18 mo	Sits but never stands	2A: Sits independently, may lose the ability to sit in later life 2B: Sits independently, maintains the ability to sit According to functional level, decimal classification ranging from 2.1 to 2.9	Usually survive >2 yr; ~70% alive at 25 yr	Proximal weakness, postural hand tremor, normal intellectual ability, kyphoscoliosis	3 SMN2 copies in >70% patients	~27%
Type 3 (Kugelberg–Welander disease)	253400	>18 mo	Stands and walks	3A: Onset between 18 and 36 mo 3B: Onset >3 yr	Survival into adulthood	May have hand tremor, resembles muscular dystrophy 3A: Scoliosis, usually early loss of ambulation	3 or 4 SMN2 copies in ~95% of patients	~12%
Type 4	271150	10–30 yr, usually >21 yr	Stands and walks	-	Survival into adulthood	Usually preserved walking ability	4 or more SMN2 copies in >90%	~1%

SMA: spinal muscular atrophy; mo: months; yr: years.