Life Expectancy and Prognosis for SMA

The life expectancy and prognosis of spinal muscular atrophy (SMA) vary widely between types. People with the most severe types may only live a few months. People with the mildest types have a normal lifespan. This variation depends mostly on how well the SMN2 back-up gene can make up for the loss of the SMN1 gene.1,2

What is prognosis?

Prognosis is the medical term for what a doctor expects your outcome to be. Think of prognosis as a forecast.

As people receive new treatments for SMA, life expectancy and prognosis may change. For example, people who may show symptoms before 6 months old (type 1) may achieve the motor milestones of someone with type 2. However, we do not yet know enough about how treatments affect life expectancy and prognosis.

Type 1

More than half of new cases of SMA are type 1. People with type 1 SMA usually can never sit independently. Without treatment, it is the leading genetic cause of death in early infancy. The life expectancy is under 2 years. The cause of death is usually respiratory failure. An early onset of symptoms is associated with a shorter life expectancy.1,2

Another way of thinking about life expectancy is a “survival probability.” This is the chance that someone with SMA lives past a certain age. For type 1 SMA, the survival probability is:3

  • 40 percent at 1 year old
  • 25 percent at 2 years old
  • 6 percent at 4 years old
  • 0 percent at 10 years old and older

Long-term survivors of type 1 SMA have been reported. This may be due to more proactive respiratory support and other care.4,5

Type 2

About 1 in 4 new cases of SMA are type 2. People with type 2 SMA usually can sit on their own but cannot walk. Life expectancy varies but is usually between 20 and 40 years. The cause of death is usually respiratory failure. For type 2 SMA, the survival probability is:2,3

  • 100 percent at 2 years old
  • 97 percent at 4 years old
  • 93 percent at 20 years old
  • 52 percent at 40 years old

Type 3

People with type 3 SMA usually can walk on their own, and they have a normal life expectancy. The survival probability is 100 percent through age 40. People with type 3 SMA may gradually lose motor function over time. About 42 percent of people with type 3 SMA lose the ability to walk. This is more likely if symptoms start earlier in life.1,3,6

Type 4

People with type 4 SMA can usually walk throughout life and have a normal life expectancy. However, they may develop gradual muscle weakness and other typical symptoms of SMA.2,7

Other types

Other rare forms of SMA vary in life expectancy and prognosis.

Type 0

Type 0 is the rarest and most severe form of SMA caused by changes in the SMN1 gene. Infants do not achieve any motor milestones. Life expectancy is 1 month. The cause of death is usually respiratory failure.2,4

X-linked infantile spinal muscular atrophy (XL-SMA)

XL-SMA is a rare form of SMA that affects boys more often than girls. It is caused by changes in the UBA1 gene. The disease course and life expectancy are similar to type 0 or 1 SMA.4

X-linked spinal and bulbar muscular atrophy (SBMA)

SBMA, also called Kennedy disease, is a rare form of SMA that affects men more often than women. It is caused by alterations in the AR gene. The disease course is similar to type 4 SMA. Symptoms appear during adulthood and muscle weakness gradually develops.4,8

Spinal muscular atrophy with respiratory distress type 1 (SMARD1)

SMARD1 is a rare form of SMA caused by changes in the IGHMBP2 gene. The progression of SMARD is different from other types of SMA. Respiratory symptoms and muscle weakness usually appear during the first few months of life and worsen until age 2. Then symptoms stabilize and may improve. Children depend on ventilation support and may not be able to sit but can participate in daily school activities.4,9

Scapuloperoneal spinal muscular atrophy (SPSMA)

SPSMA is a rare form of SMA caused by changes in the TRPV4 gene. The disease course of SPSMA is similar to type 3 or 4 SMA. Muscle weakness begins in childhood or adulthood and progresses slowly. People maintain the ability to walk until after age 50.8

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Written by: Matt Zajac | Last reviewed: August 2021