Spinal Muscular atrophy causes

The most common form of SMA is caused by mutation of the SMN gene, and manifests over a wide range of severity affecting infants through adults. This spectrum has been divided into three groups, depending on the age of onset, and are as followed:

• Infantile SMA - Type I or Werdnig-Hoffmann disease (generally 0-6 months). SMA type I, also known as severe infantile SMA or Werdnig Hoffmann disease, is the most severe, and manifests in the first year of life. This type generally onsets quickly and unexpectedly after birth; babies diagnosed with Type I SMA do not generally live past one year of age. Pneumonia is considered the ultimate cause of death due to deterioration of survival motor neurons; these neurons trigger improper functioning of the major bodily organ systems, particularly respiratory (e.g. breathing, ridding of pooled secretions inside lungs).
• Intermediate SMA - Type II (generally 7-18 months). Type II SMA, or intermediate SMA, describes those children who are never able to stand and walk, but who are able to maintain a sitting position at least some time in their life. The onset of weakness is usually recognized some time between 6 and 18 months.
• Adult SMA - Type III or Kugelberg-Welander disease (generally >18 months). SMA type 3 describes those who are able to walk at some time.