Spinal muscular atrophy type 3 is a rare and less severe form of the condition. Symptoms usually start in childhood or adolescence, and life expectancy is typically unaffected. Treatment may help slow down disease progression.
Spinal muscular atrophy (SMA) is an inherited genetic condition that causes the breakdown of the nerves in your spine and brain stem that allow you to move your muscles. This leads to increasing weakness and shrinking of your muscles.
SMA type 3 is one of the four main types of SMA. It’s also called Kugelberg-Welander disease.
Symptoms of SMA type 3 develop after the age of 18 months and before adulthood. It causes milder symptoms than SMA types 1 and 2, which often lead to early death. People with SMA type 3 typically have full life spans but often lose the ability to walk in adulthood.
In this article, we examine SMA type 3 in more detail, including its symptoms, treatment options, and causes.
Spinal muscular atrophy type 3 facts
- SMA type 3 begins after the age of 18 months, with an average age of onset of about
- SMA affects about
1 in 10,000 people. SMA type 3 makes up about 13%of U.S. cases. Some expertsclassify SMA type 3 as early onset (3A) if it develops before age 3 years and as later onset (3B) if it develops at ages 3–21 years.
SMA is a progressive disease, meaning its symptoms get worse over time.
The first symptoms of SMA type 3 often include:
- trouble climbing stairs
- weakness in your thighs
- frequent falls
Other possible symptoms include:
A recessive mutation of the SMN1 gene causes all forms of SMA. “Recessive” means you must receive the mutation from both parents to develop the condition. The SMN1 gene gives your cells the code they need to create an essential protein called survival motor neuron (SMN) protein.
Your body has an almost identical backup gene to SMN1 called SMN2, but this backup gene creates a less stable and shorter protein.
The number of copies of this backup gene relates to the type of SMA that develops. People with SMA type 2 or 3 tend to have
If you have SMA, the lack of SMN leads to degeneration of neurons in your spinal cord and brain stem that allow you to move your muscles.
Genetic testing can reveal the presence of a mutated SMN1 gene. This can occur even before a baby is born or during standard newborn blood testing.
These tests can reveal up to
- age of onset
- current age
- severity of symptoms
All people over 7 years of age who did not receive treatment showed a decline in function, while all people over 7 years of age who were treated with nusinersen showed improvement.
In people under the age of 7 years, researchers observed improvement in both those who were treated and those who were not. But those treated with nusinersen showed greater improvement.
Another drug, Evrysdi, is also FDA approved to treat SMA in infants over the age of 2 months and adults.
Other treatment options for SMA aim to improve quality of life and reduce symptoms. They include:
- physical and occupational therapy
- splints or braces
- postural support devices
- emotional support
SMA type 3 is milder than type 1 and type 2. Most people with this condition learn to walk. Some people with SMA type 3 may need to use a wheelchair during childhood, while others can continue to walk into adulthood.
SMA type 3 tends to progress slowly and
Here’s a look at how SMA type 3 differs from other types of SMA:
|Age of onset
|less than 6 months
|after 18 months and before adulthood
|less severe than type 1
|less severe than type 1 and type 2
|usually causes only mild difficulties late in adulthood
|• weakness and lack of muscle tone in arms and legs
• difficulty eating, moving, breathing, and swallowing
|• trouble standing and walking
• weakness in arms and legs
• tremors in hands and fingers
|• trouble walking or getting up from a seated position
• balance problems
• difficulty climbing stairs
|• weakness in hands and feet
• difficulty walking
• shaking and twitching
Here are some questions that people often ask about SMA type 3.
What is the life expectancy of a child with SMA type 3?
Children with SMA type 3 generally have
Is SMA type 3 curable?
There is currently
Is SMA type 3 progressive?
SMA type 3 is a progressive condition. Children tend to gain motor function until about
SMA type 3 is a progressive neurological condition that can cause difficulties with movement and muscle weakness. Symptoms develop after the age of 18 months.
Most people with SMA type 3 eventually lose the ability to walk, but the condition often does not affect life expectancy. The medication nusinersen may improve the outlook for SMA. Researchers are continuing to examine other treatment options.