Table 2.

Spectrum of SMA Phenotypes at Presentation

PhenotypeAge of OnsetLife Span 1Motor Milestones 1Other Findings 1
SMA 0 PrenatalA few weeks, <6 mosNone achieved
  • Severe neonatal hypotonia
  • Severe weakness
  • Areflexia
  • Respiratory failure at birth
  • Facial diplegia
  • ↓ fetal movements
  • Atrial septal defects
  • Arthrogryposis
SMA I <6 mosMedian survival 8-10 mosSome 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 mos70% alive at age 25 yrsIndependent 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 mosNormalIndependent 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 AdulthoodNormalNormal
  • Fatigue
  • Proximal muscle weakness

With supportive care only

From: Spinal Muscular Atrophy

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