Table 2. [Spectrum of SMA Phenotypes at Presentation]. - GeneReviews®

Phenotype	Age of Onset	Life Span ¹	Motor Milestones ¹	Other Findings ¹
SMA 0	Prenatal	A few weeks, <6 mos	None achieved	Severe neonatal hypotonia Severe weakness Areflexia Respiratory failure at birth Facial diplegia ↓ fetal movements Atrial septal defects Arthrogryposis
SMA I	<6 mos	Median survival 8-10 mos	Some head control, sit w/support only	Loss of head control Mild joint contractures Normal or minimal facial weakness Variable suck & swallow difficulties
SMA II	6-18 mos	70% alive at age 25 yrs	Independent sitting when placed	Developmental delay w/loss of motor skills ↓ or absent deep tendon reflexes Proximal muscle weakness Postural tremor of fingers
SMA III	>18 mos	Normal	Independent ambulation	Proximal muscle weakness (i.e., difficulty w/stairs, running) Loss of motor skills Fatigue Postural tremor of fingers Loss of patellar reflexes
SMA IV	Adulthood	Normal	Normal	Fatigue Proximal muscle weakness

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