Prognosis

SMA encompasses a spectrum of phenotypes ranging from severe forms with early onset to milder forms with later onset. The natural history of SMA according to phenotype is summarized as follows:

●SMA type 0, with prenatal onset, is associated with early death from respiratory failure, usually within weeks after birth.

●SMA type 1, with onset between birth and age six months, leads to death from respiratory failure before the age of two years.

●SMA type 2, with onset between 3 and 15 months of age, is notable for inability to achieve independent walking or standing but is compatible with survival into adulthood. Most affected individuals live to age 25 years.

●SMA type 3, with onset between age 18 months and adulthood, is characterized by slowly progressive proximal weakness, which may lead to loss of independent ambulation, and a normal lifespan.

●SMA type 4, with adult onset, is otherwise similar to SMA type 3 and is associated with a normal lifespan.

The eventual impact of novel disease-modifying therapy (eg, Nusinersen) for ameliorating the expected course of SMA is uncertain.

Limited data suggest that survival has increased in patients with SMA type 1 born from 1995 through 2006 compared with those born from 1980 to 1994. Ventilation for >16 hours a day, use of mechanical insufflation-exsufflation device, and gastrostomy tube feeding were significantly and independently associated with prolonged survival, while year of birth was not. Thus, longer survival in the later time period appears to be related to more aggressive care.