Spinal muscular atrophy (SMA) is the leading genetic cause of infant mortality. Historically, the average life expectancy for infants diagnosed before 6 months was under 2 years, but new treatments can prolong their lifespan.

The above information comes from a 2018 study in the Journal of Pediatrics and Child Health.

It is important to note that different types of SMA have different outlooks. Types 0 and 1, the most severe forms, have a very short life expectancy without treatment.

However, newer treatments can prolong the life of someone with type 0 and type 1. Type 2 can cause significant disability, which can increase the risk of a shortened lifespan.

Types 3 and 4 do not substantially affect life expectancy, but complications can sometimes be life threatening.

Read on to learn more about SMA life expectancy, factors that affect it, how to improve it, and more.

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SMA is a group of diseases that harm motor neurons in the brainstem and spinal cord. It causes muscle weakness and dysfunction that can affect a person’s ability to move, sit, breathe, eat, walk, and talk.

There are five types ranging from 0 to 4, and a person’s life expectancy mainly depends on the type of disease they have.

Types 0, 1, and 2 cause weakness in the muscles that control breathing, shortening a person’s lifespan. Type 2 has less of an effect on respiration than types 0 and 1.

Conversely, types 3 and 4 can cause various disabilities, including mobility impairments, but they do not significantly affect lifespan.

While historical data point to a very shortened lifespan for infants with types 0, 1, and 2, new gene therapies, such as Zolgensma (onasemnogene abeparvovec-xioi), can prolong life expectancy. They prevent further lung damage by slowing the progression of the disease.

Gene therapy is an approved treatment for children with an SMA diagnosis who are younger than 2 years old and have mutations in the survival 6516 motor neuron 1 (SMN1) gene. Researchers have not proven it to be beneficial for people with SMA who are older than 2 years.

The injectable medications Spinraza (nusinerson) and Evrysdi (risdiplan) work differently than gene therapy and have approval for use in children and adults. These two medications restore the missing SMA protein. They work similarly, so doctors do not need to combine them, but they can use the medications with gene therapy.

There are five types of SMA, and their outlook and treatment options vary by type.

SMA type 0

Type 0 causes symptoms immediately at birth. Before birth, it can also cause symptoms, such as very limited movements of the growing fetus. Most notably, it can lead to respiratory failure.

Although type 0 is the most severe form of SMA, it is also the rarest. Infants with this type do not typically live past their first few months of life, and some do not survive past birth.

Emerging treatments, including gene therapy, may prolong life expectancy. However, these treatments are new and work best when they begin before symptoms become severe. It is not clear how they may help infants with type 0 SMA.

SMA type 1

Type 1 is the most common form of the disease. According to existing data, an infant with this type has an average lifespan of 2 years or less.

Symptoms usually begin between 3 and 6 months after birth. Breathing difficulties are common and, in many cases, eventually fatal.

As mentioned earlier, new treatments may prolong a person’s life. However, they will not reverse the symptoms a person already has. Treatment must begin early in life and before the onset of severe symptoms.

SMA type 2

Type 2 causes symptoms beginning at 6–18 months of age.

Infants and children with this type usually have more mobility, including the ability to sit, and do not experience the same level of disability as those with types 0 and 1.

According to current data, 70% live to the age of 25 years, and some live even longer.

As with the more severe forms of SMA, emerging treatments may prolong the life expectancy of someone with SMA type 2. However, these treatments are new, so it is unclear how much they can affect lifespan.

SMA type 3

SMA type 3 causes fewer mobility impairments. People with this form often can walk and do not typically have breathing difficulties.

Some individuals with type 3 have other disabilities or need assistive devices, such as wheelchairs, but the condition does not affect their life expectancy.

SMA type 4

SMA type 4 is the least severe form of the disease, and people with this type may only have minimal mobility impairments. The life expectancy of someone with type 4 is similar to that of individuals without SMA.

With early treatment, the prognosis of SMA may improve, and people can survive for longer than those who do not receive treatment.

Some factors affecting survival in the most severe types, which were once fatal, include:

  • starting treatment before the age of 2 years
  • not having severe symptoms at the time therapy begins
  • responding well to treatment
  • managing health complications, such as infections

Gene therapy may improve the life expectancy of someone with SMA. However, this technique is very new, and clinical trials have not lasted long enough to determine the average life expectancy following gene therapy.

Rispidlam and nusinersen, two other new drugs, can also extend the lifespan and improve the quality of life for people with SMA. However, like gene therapy, these treatments are new, and it is unclear exactly how long they might prolong a person’s life.

In the most severe forms of SMA, the outlook is better if someone receives treatment before developing symptoms.

Learn more about gene therapy for SMA.

There are five types of SMA, ranging from most severe to least severe. Most forms cause significant disability and mobility impairments, though people with type 4 sometimes have only mild symptoms.

Life expectancy depends on the SMA type a person has, which treatments they are eligible for, and how early they begin them.

Parents or caregivers of children with SMA should seek specialist care from an expert and advocate for early intervention, especially if the child is eligible for gene therapy. Adults with SMA should continue to seek care from an expert healthcare team and ask about emerging and experimental therapies.