Spinal Muscular Atrophy

What is Spinal Muscular Atrophy?

Spinal muscular atrophy (SMA) is a disease in which motor nerves in the brain and spinal cord die, impairing a person’s ability to move. These nerves control our ability to sit up, crawl, and walk. In severe cases, a person will not be able to sit up independently, and their breathing and swallowing may also be impaired. In the mildest cases, symptoms begin in adulthood and make independent movement more difficult, but still possible. Historically, five main subtypes of spinal muscular atrophy were described, but use of these subtypes is changing due to the impact of treatments on the health and lifespan of people with SMA. It is not always possible to predict which type of SMA a child could have based on the genetic test results. The disease is caused by a shortage in SMN protein, which helps preserve motor neuron function. Without it, the neurons cannot pass messages from the brain to the muscles of the body.


SMA Type 0

Type 0 is the most severe form of SMA. Once born, the infant will have little ability to move and may not be able to breathe and swallow independently without treatment.


Type I – Also called Werdnig-Hoffmann Disease

Type I is also severe. Infants with SMA type I often have symptoms before 6 months of age, such as trouble breathing and swallowing. They cannot sit up without support and will not develop motor skills without treatment. Their intellect, however, is normal.


Type II – Also called Dubowitz Disease

In children with type II SMA, muscle weakness becomes apparent between the ages of 6 and 12 months and progressively worsens into their mid-teens without treatment. Also, they would not be able to stand or walk without assistance and have poor muscle tone and strength. Their intelligence is typically normal.


Type III – Also called Kugelberg-Welander Disease

Type III SMA is a milder form of the disease. Its symptoms begin sometime between the age of one year and early adulthood. Without treatment, they may fall repeatedly in childhood and have trouble with stairs. Also, their muscles would be weaker than normal, though they may be able to stand and walk without aid at some point in their life. However, they might lose this ability later in life.



Type IV is the mildest form of spinal muscular atrophy. Without treatment, muscle weakness does not typically begin until their 20s or 30s. The person can still walk and move independently but may have mild to moderate tremors and/or twitching.


How common is Spinal Muscular Atrophy?

In the United States, 1 in every 6,000 to 10,000 people develop spinal muscular atrophy and about 1 in 40 is a carrier of the disease. It has been found in people of every race. Carrier rates for various populations include White (1 in 35), Ashkenazi Jewish (1 in 41), Asian (1 in 50), African American (1 in 66), and Hispanic (1 in 81).


How does Spinal Muscular Atrophy inheritance work?

SMA is an autosomal recessive genetic disorder, meaning it occurs when a child has no working copies of the SMN1 gene. Both of the child’s parents are typically carriers, having only a single working copy of the SMN1 gene instead of 2 copies. Carriers do not show symptoms of the disease. When two carriers have a child, there is a 25% chance that the child will inherit no working copies of the gene, leading to SMA. There is a 50% chance that the child will be a carrier like the parents and a 25% chance that the child will inherit two working copies of the gene.


Why do genetic testing for Spinal Muscular Atrophy?

Genetic testing plays a crucial role in diagnosing SMA and finding carriers. Early diagnosis allows for prompt intervention and access to treatments that can slow down the disease’s progression. Moreover, genetic testing is vital for family planning purposes, as identifying carriers can help prospective parents make informed decisions and seek medical advice if they plan to have children.


How is Spinal Muscular Atrophy treated?

Medical advancements have led to the development of treatments that have significantly improved the health of affected individuals. One such treatment is gene therapy, which involves the transfer of genetic material into cells to increase the production of SMN protein. Early detection and intervention are crucial for better outcomes, as treatment is more effective when started in the early stages of the disease, or before symptoms arise. Once neurological symptoms occur, there may be no way to reverse the damage caused by SMA.
In addition, supportive services can be used to address some symptoms. For children with the more severe forms of spinal muscular atrophy, mechanical breathing aids may prolong lifespan. If getting enough nutrition is an issue, some people with SMA have feeding tubes. Those with milder forms of the disease sometimes choose to have surgery to correct curvature of the spine (scoliosis) or joint problems.


How can Spinal Muscular Atrophy be prevented?

There is no cure for spinal muscular atrophy, however treatments have made a major impact on disease progression. Insurance coverage for these treatments is improving, but cost can be a factor in accessing treatment options. Research is ongoing to improve outcomes in people with SMA.


Ideally, a couple should be aware of their carrier status, so that if they have a baby with SMA, treatment can be initiated before symptoms arise. Also, if they know their carrier status before pregnancy, a couple can use options like IVF (in vitro fertilization) and PGT (preimplantation genetic testing) to ensure the child inherits at least one working copy of the SMN1gene, and never develops the condition.


What is the prognosis for a person with Spinal Muscular Atrophy?
The prognosis for a person with SMA varies greatly depending on which type of the disease they have, whether they had access to treatment, and when the treatment began.


SMA Type 0
Without treatment, the disease is typically fatal between 2 and 6 months of age. These infants do not develop any motor skills expected of infants their age.


Type I
Without treatment, this type of SMA is usually fatal within two years


Type II
Without treatment, most people with type II SMA lived to the age of 25. They were often able to sit independently when placed in a sitting position, but lost this ability by their mid-teens.


Type III
People with type III SMA may live a normal lifespan. Many learn to walk independently, though most would lose the ability to do so by their 30s or 40s without treatment.


Type IV
A normal lifespan is possible for people with type IV SMA, even without treatment. They do not develop symptoms until their 20s or 30s and usually retain the ability to walk independently.

  • Cure SMA

A group that aims to advance research on SMA, and support families affected by it.
925 Busse Road
Elk Grove Village, IL 60007
Phone: (800) 886-1762


  • SMA Foundation

The foundation supports SMA research, with a goal of accelerating treatment development.
970 W Broadway STE E
PMB 140
Jackson, WY 83001
Phone: (646) 253-7100

  • MedlinePlus

Explanations of an extensive number of genetic diseases written for everyday people by the U.S. government’s National Institutes of Health.


Other Names for Spinal Muscular Atrophy

  • SMA

Take action now to assess your risk for spinal muscular atrophy and your risk for passing it to your children. To get started with a JScreen genetic test, click here.

Source: Myriad Women’s Health.