Table 2.

Classification of clinical variant of SMA.

Type	SNM2 Copy Number	Onset	Life Survival	Motor Milestones	Peculiar Findings	Bone Involvement
SMA 0	1	Prenatal	Weeks	None achieved	Severe hypotonia Respiratory distress at birth	Generalized osteopenia Bone deformities
SMA I (Werdnig–Hoffmann disease)45% of cases	2–3	0–6 months	Death prior to age 2	Sit with support	Bell-shaped deformity of the chest “Frog-leg” lower limb posture Feeding problems (suck and/or swallowing)	Lowest aBMD Z-scores at all skeletal sites Higher fracture risk Osteoporosis*
SMA II 20% of cases	3	<18 months	70% alive at age 25 years	Independent sitting achieved by 9 months or delayed Never stand or walk independently	Postural tremor of fingers (minipolymyoclonus)	Increased bone resorption markers Low aBMD Z-scores at the LS and the LDF Fragility vertebral fractures
SMA III (Kugelberg–Welander disease)30% of cases	3–4	IIIa 18 months−3 yearsIIIb 3–30 years	Normal	Independent ambulation	No available data	Increased bone resorption markers Low aBMD Z-scores at the LDF Asymptomatic vertebral fractures
SMA IV Less than 5% of cases	4 or more	Adulthood	Normal	Normal	No available data	No available data

^*According to 2015 ISCD pediatric criteria; aBMD, areal bone mineral density; LS, lumbar spine; LDF, lateral distal femur.