Spinal muscular atrophy (SMA) type 1 is the most common and most severe form of SMA. Symptoms typically begin in the first 6 months of life. Life expectancy is less than 2 years without treatment, but new therapies can extend it.

Spinal muscular atrophy (SMA) is an inherited neuromuscular disorder that causes progressive muscle weakness. Depending on the genes involved, people can develop different types of SMA, which vary in severity.

SMA type 1 (also called Werdnig-Hoffman disease) is the most common and most severe form of SMA. Symptoms typically begin in early infancy and progress rapidly.

There is no cure for SMA. But in recent years, early diagnosis and new targeted therapies have shown great promise in improving the quality of life for the many children with SMA and their families.

How common is spinal muscular atrophy?

SMA is one of the most common recessive genetic disorders, occurring in about 1 in 10,000 live births. About 1 in 40 people carry one copy of the genetic mutation that causes the condition.

About 60% of all SMA cases are type 1.

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Children born with SMA type 1 may not show symptoms at birth, but muscle weakness will begin to appear within the first 6 months of life.

Early symptoms include:

  • difficulty with head movement
  • low muscle tone
  • inability to roll or sit
  • difficulties with feeding and swallowing
  • a bell-shaped chest
  • slow growth
  • loss of previous motor milestones

In many cases, doctors can use prenatal genetic testing to diagnose SMA before symptoms begin. In other cases, state newborn screening blood tests performed shortly after birth can identify the condition.

SMA type 1 is an inherited genetic disorder.

About 1 in 40 people in the United States carry a single mutated copy of the survival motor neuron (SMN1) gene. These people have no symptoms but can pass the mutated gene on to their children.

When two people who carry the SMN1 mutation have a child, there is a 1 in 4 chance that the child will have SMA. Children with SMA type 1 have usually inherited two mutated copies of the SMN1 gene — one from each biological parent. This pattern is called autosomal recessive inheritance.

The SMN1 gene encodes the SMN protein, which is crucial for motor neuron survival and function. When the SMN1 gene has this mutation, the body can’t properly make the protein. This results in spinal cord motor neuron damage, leading to progressive loss of muscle strength, control, and function.

Another gene, SMN2, produces smaller amounts of SMN protein. It can’t compensate fully for a mutated SMN1 gene, but the presence and number of copies of the SMN2 gene can influence a child’s symptoms. Children with SMA type 1 usually have the fewest copies of SMN2.

There is no cure for SMA. But promising treatments, such as the following, can help improve motor function and quality of life for children with SMA type 1:

  • Nusinersen (Spinraza) stimulates the production of functional SMN protein in the brain and spinal cord (the central nervous system, or CNS). A healthcare professional injects this medication at regular intervals into the fluid surrounding the spinal cord.
  • Risdiplam (Evrysdi) is an oral medication that works to increase functional SMN protein in the CNS and peripheral tissues.
  • Children less than 2 years old may also benefit from a one-time intravenous (IV) infusion of a gene replacement therapy called onasemnogene abeparvovec (Zolgensma).

These therapies may help children with SMA reach or maintain motor milestones such as rolling, sitting, and greater control of head movement. Talk with your doctor to see whether and when these therapies may be right for your child.

Your child’s healthcare team can also help guide decisions about additional supportive care, such as:

Life expectancy is limited in SMA type 1. Without breathing support, most children with SMA type 1 have historically had a life expectancy of less than 2 years.

However, researchers continue to study promising therapies that may improve both quality of life and life expectancy.

Resources for support

Receiving a diagnosis like SMA type 1 can be overwhelming, stressful, and isolating for families. But SMA is one of the most common autosomal recessive diseases, and help is available for your family.

Talk with your child’s doctor about community support and expert SMA resources in your area. Discuss and research your nearest multispecialty SMA center or clinic.

You may also find these organizations and web resources helpful as you learn more and connect with others in the SMA community:

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People with a family history of SMA are at highest risk. Talk with your doctor if you have a family history of SMA, especially if you’re planning a pregnancy. People who carry the genetic mutation have no symptoms, so it’s possible to be a carrier and not know it.

SMA type 1 is significantly more common in white people than in Black people and people of multiple ethnicities.

Because SMA is an autosomal disorder, it affects all sexes equally.

There are four primary types of SMA, with lower numbers indicating greater severity.

SMA type 1 is both the most common and the most severe type. Progressive weakness begins in early infancy. Many babies with SMA type 1 will not be able to sit on their own and will eventually need mechanical ventilation. Life expectancy is limited.

People with SMA type 2 typically do not develop symptoms until 6–18 months of age but then experience progressively severe weakness and respiratory problems. Most people with SMA type 2 survive into their 20s.

Symptoms of SMA type 3 begin after age 18 months but are usually detected by 3 years of age. Most children with SMA type 3 can walk on their own, but they may eventually need assistive devices such as walkers or wheelchairs. Lung involvement is uncommon, and life expectancy is typical.

SMA type 4 is the mildest form of SMA. Symptoms don’t appear until adulthood and usually begin with mild leg weakness. SMA type 4 doesn’t usually affect life expectancy.

SMA type 1 is the most common and most severe form of SMA, an inherited motor neuron disease that results from a mutation in the SMN1 gene.

Children born with SMA type 1 will develop progressive weakness by 6 months of age. Babies may lose milestones they had previously reached, such as the ability to control head movement and to roll. Eventually, the condition affects functions such as feeding and respiration.

A multidisciplinary team of expert SMA professionals can help your family navigate the condition. While there’s no cure, promising new therapies have emerged in recent years. Earlier diagnosis and disease-modifying treatments can improve quality of life, motor function, and life expectancy in babies born with SMA type 1.