Table 2.

Spectrum of SMA Phenotypes

PhenotypeAge of OnsetLife SpanMotor MilestonesOther Findings
SMA 0Prenatal<6 monthsNone achieved
  • Severe neonatal hypotonia
  • Severe weakness
  • Early respiratory failure
  • Facial diplegia
SMA I<6 monthsMost often ≤2 years, but may live longerSit w/support only
  • Mild joint contractures
  • Normal or minimal facial weakness
  • Variable suck & swallow difficulties
SMA II6-18 months70% alive at age 25 yearsIndependent sitting when placedPostural tremor of fingers
SMA III>18 monthsNormalIndependent ambulation
SMA IVAdulthoodNormalNormal

From: Spinal Muscular Atrophy

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Adam MP, Ardinger HH, Pagon RA, et al., editors.
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