General Condition Information
- Spinal Muscular Atrophy due to homozygous deletion of exon 7 in SMN1
- Werdnig-Hoffman Disease
- It is estimated that hundreds of babies are born with this condition each year in the United States.
- Visit GeneReviews to learn more about how often this condition occurs.
What is Spinal muscular atrophy
Spinal muscular atrophy (SMA) is an inherited (genetic) condition that affects the nerve cells that carry messages from the brain to the muscles of the body.
The brain uses nerves called motor neurons to control muscle movement. Motor neurons need the survival motor neuron (SMN) protein to work correctly.
In SMA, your baby’s body cannot make enough SMN protein. When a baby cannot make enough SMN protein, motor neurons die, damaging the connection between the brain and the muscles. The brain will then have trouble telling the muscles to move, which causes the muscles to break down.
This results in muscle weakness and decreased muscle size (atrophy), which can cause breathing and swallowing difficulties. Loss of motor neurons also leads to other signs and symptoms of the condition.
There are several forms of SMA, and the severity of the condition depends on how much normal SMN protein your baby makes.
Newborn Screening and Follow-Up
Newborn screening for SMA is done using a small amount of blood collected from your baby’s heel. To learn more about this process, visit the Blood Spot Screening page.
During screening, a special machine looks for a specific change in the SMN1 gene in your baby’s blood. Babies with this specific change in the SMN1 gene might have SMA.
If your baby’s blood spot screening result for SMA is out-of-range, your baby’s health care provider will contact you. Together, you will discuss next steps and follow-up plans.
An out-of-range screening result does not mean that your baby definitely has the condition. It does mean that your baby needs more follow-up testing. To learn more about screening results, visit the Blood Spot Screening Results page.
Your baby may need follow-up genetic testing after an out-of-range screening result.
You should complete any recommended follow-up testing as soon as possible. Babies with this condition can have serious health problems soon after birth if they are not diagnosed and treated quickly.
False-negative results may happen. Newborn screening only looks for one specific change in the SMN1 gene (the absence of exon 7). It is estimated that 2 to 5 percent of SMA cases are caused by a different genetic change that is not currently detected by newborn screening.
Newborn screening helps babies lead healthier lives. If your baby has an out-of-range result, follow up with your health care provider quickly. It is important to follow their instructions. Your baby may need to get treatment right away, even if they are not showing signs or symptoms. In some cases, your baby’s health care provider may decide it is best to watch (monitor) your baby to decide next steps. Careful monitoring and early treatment will help your baby stay as healthy as possible.
The timing and severity of SMA signs and symptoms depend on the form of the condition. In the most severe form, signs appear in the womb (before birth). In the most common form, signs appear soon after birth or within the first few months of life. In the least severe form, signs may not appear until early adulthood.
Signs of the condition may include the following:
- Floppy arms and legs (hypotonia)
- Poor growth
- Breathing problems
- Problems with swallowing
- Joint problems
The more SMN1 and SMN2 genes you have in your cells, the more SMN protein your body can make.
SMA stems from a change in the SMN1 gene, most often a deletion or absence of a part of the gene called exon 7. The SMN1 gene gives the body instructions for making the SMN protein that keeps motor neurons healthy and able to carry messages from the brain and spinal cord to different parts of the body.
Without a working SMN1 gene, your baby cannot make enough SMN protein. As a result, motor neurons can get sick and die. This damages the connection between the brain and the muscles, leading to muscle weakness. The SMN2 gene also provides instructions for SMN protein. The amount of SMN protein that SMN2 can make depends on the number of copies of SMN2 a baby has. The more SMN2 copies, the more SMN protein and, usually, the milder the signs and symptoms.
- SMA is an autosomal recessive condition. Babies inherit the condition when each parent passes down a nonworking SMN1 gene to their baby. Only babies with two nonworking SMN1 genes—one from the mom and one from the dad—have this condition.
- People with one working copy and one nonworking copy of the SMN1 gene are called carriers.
- Carriers do not have or develop the condition. However, they may pass down a nonworking copy of the gene to their children.
- If two parents are carriers of a nonworking copy of the SMN1 gene, they have a 1 in 4 chance of having a child with SMA.
- Carriers for SMA often do not know they are carriers before having a child with the condition. In most cases, families have no history of the condition until the birth of a child with SMA.
- Parents who already have a child with SMA still have a 1 in 4 chance of having another child with SMA. This 1 in 4 chance stays the same for all future children.
- Genetic counselors and medical geneticists can help families learn about this condition and the chance of having children with it. Visit the National Society of Genetic Counselors to find a genetic counselor and the American College of Medical Genetics and Genomics to find a medical geneticist.
Treatment and Management
Two new treatments can change how the genes work in some cases of SMA. A medicine called nusinersen can help the SMN2 gene make more SMN protein. Gene therapy can replace the nonworking SMN1 gene with a working copy. It is important to talk to your health care provider about which treatment(s) are best for your baby. The goal of these treatments is to prevent muscle weakness and resulting health problems.
Some children with SMA will require other types of support that can include:
- Physical therapy
- Feeding support for babies who cannot swallow
- Breathing support
The outcomes of these treatments vary depending on the form and severity of the condition.