What is the age of onset of spinal muscular atrophy (SMA)?

Updated: May 29, 2019
  • Author: Jeffrey Rosenfeld, MD, PhD, FAAN; Chief Editor: Stephen L Nelson, Jr, MD, PhD, FAACPDM, FAAN, FAAP  more...
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The ISMAC classification system is based on the age of onset. [8] See Background, History, and Physical for a review of the existing classification systems and a brief discussion of their relevancy to the role of age in spinal muscular atrophies.

According to the ISMAC system, the age of onset for spinal muscular atrophies is as follows:

  • SMA type I (acute infantile or Werdnig Hoffman): Onset is from birth to 6 months.

  • SMA type II (chronic infantile): Onset is between 6 and 18 months.

  • SMA type III (chronic juvenile): Onset is after 18 months.

  • SMA type IV (adult onset): Onset is in adulthood (mean onset, mid 30s).

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