Spinal muscular atrophy type 3
Reviewed by Type 3 is a milder form of spinal muscular atrophy (SMA), a rare genetic disease that results in muscle weakness and wasting. One of five main forms of the disease, type 3 is sometimes called juvenile spinal muscular atrophy or Kugelberg-Welander disease.
Symptoms tend to begin after 18 months of age but before adulthood. People with SMA type 3 can typically learn to sit, stand, and walk without aid, but may require mobility support as the disease progresses.
As with the other four types of SMA, type 3 results from mutations in both copies — one inherited from each biological parent — of the SMN1 gene. This gene provides instructions to produce the SMN protein, which is essential for motor neuron and muscle health.
What is SMA type 3?
SMA type 3 is a genetic disease characterized by the progressive loss of motor neurons. These nerve cells control voluntary movement.
There are five different types of SMA, determined by when symptoms typically appear and how severe they are. As type 3 is less severe than types 0-2, it is sometimes referred to as mild SMA.
Some people may also divide type 3 SMA into the following subcategories depending on when the symptoms appear:
- Type 3A: symptoms appear between 18 months and 3 years of age
- Type 3B: symptoms appear after 3 years of age
What are the symptoms of SMA type 3?
The symptoms of SMA type 3 include muscle weakness that becomes more pronounced over time. They include:
- muscle weakness, usually affecting the legs more than the arms
- difficulty with walking
- scoliosis, or an abnormal sideways curvature of the spine
- poor balance and an increased risk of falls
With SMA type 3, children will gain the ability to walk as toddlers. However, as the disease advances, progressive muscle weakness can make walking more challenging. Some patients will eventually lose the ability to walk independently.
Generally, a later age of onset is associated with less long-term decline in motor function.
Unlike children with more severe types, adults with type 3 SMA do not have swallowing or breathing problems. If these difficulties do occur, they are usually mild or only appear later in life.
When do SMA type 3 symptoms begin?
SMA type 3 symptoms may appear anywhere between 18 months of age and the end of adolescence.
The more severe forms of SMA, types 2, 1, and 0, also appear during childhood, but at much earlier stages than type 3.
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| SMA type 3 | Characteristics |
|---|---|
| Age of onset | 18 months to end of adolescence |
| Symptoms | Muscle weakness, especially in the legs; can stand and walk but may require support as disease progresses; ability might be lost later in life; difficulty balancing; scoliosis |
| Prognosis | Life expectancy is usually unaffected |
How is SMA type 3 diagnosed?
Doctors can make a diagnosis when symptoms are present or when there is a family history of SMA, using genetic testing to look for disease-causing mutations in the SMN1 gene. This usually requires a blood sample.
In the U.S., all 50 states screen newborns for SMA in the first days of life, but screening is not routine in the U.K. and many other countries. Early diagnosis allows treatment to begin before symptoms appear, which can substantially improve outcomes.
Doctors can also perform DNA testing on a fetus, a process known as prenatal screening.
If SMA is found, doctors can also use genetic testing to measure the number of copies of the “backup” SMN2 gene that influences disease severity. Typically, a higher number of SMN2 copies is associated with less severe SMA, and most type 3 patients carry three or four copies.
Combining these genetic results with age at symptom onset and symptom severity helps to confirm the type of SMA.
Further tests, such as an electromyogram or a muscle biopsy, may be considered if diagnostic uncertainty remains. However, these tests are usually unnecessary, as genetic testing can confirm more than 99% of SMA cases when both deletion analysis and gene sequencing are used.
How is SMA type 3 treated?
In the U.S., four medications have received approval for the treatment of SMA type 3. Most are also widely approved elsewhere, though specific indications may vary.
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| Treatment | How it’s given | Who it’s approved for in the U.S. |
|---|---|---|
| Spinraza (nusinersen) | injections into the spinal fluid, with loading doses followed by maintenance doses every four months | children and adults with SMA |
| Evrysdi (risdiplam) | daily oral medication | children and adults with SMA |
| Zolgensma (onasemnogene abeparvovec-xioi) | one-time intravenous infusion | children younger than 2 with SMA and mutations in both copies of the SMN1 gene |
| Itvisma (onasemnogene abeparvovec-brve) | one-time injection into the spinal fluid | children and adults ages 2 and older with SMA and a confirmed SMN1 gene mutation |
None of these therapies is a specific SMA type 3 treatment, and there is no cure for any form of the disease. However, clinical trials have shown that all of these therapies can slow the progression of SMA type 3.
Generally, these medicines are effective at preventing SMA type 3 progression but have limited ability to fix damage that has already happened. As a result, clinical outcomes are generally best when people start treatment as early as possible.
How can SMA type 3 affect walking and mobility?
SMA is a degenerative disease. As it causes muscles to weaken and waste away, this can make it more difficult for people to walk and keep mobile.
While SMA type 3 is less severe than other forms of the condition, it can still reduce mobility.
Although most people with this type of SMA can walk, they may find that doing so becomes more difficult over time, with some potentially needing to use wheelchairs. Other people may find it hard to run, get up from a seated position, or use the stairs.
Many people living with SMA type 3 benefit from additional forms of supportive care along with disease-modifying treatment to support their mobility. These may include:
- physical therapy and exercise to improve motor function and muscle strength
- occupational therapy, which aims to create strategies for navigating daily life with a chronic illness
- palliative care, a broad branch of therapy that works to improve quality of life for people affected by chronic diseases.
Specialized equipment may also help to make living with SMA more manageable, including:
- mobility equipment like crutches, walkers, or wheelchairs to help patients get around
- braces, standing devices, and other devices that can provide structural support to the body and aid in everyday activities.
What is the outlook for people with SMA type 3?
People with SMA type 3 tend to have a similar life expectancy to the general population.
People with type 3 typically have a normal lifespan, even without receiving treatment for the condition. The more severe types (0, 1, and 2) are associated with reduced life expectancy when people do not receive treatment.
Questions to ask your SMA care team
Here are some questions that people may find helpful to ask a care team about SMA type 3:
- What type of disease-modifying therapy will be most suitable?
- What exercises are safe for me to do at home?
- Are there any signs of disease progression to look out for?
- What kinds of specialized equipment might I need in the future?
- Are there any dietary approaches that might help?
- Will I need to make any accommodations in my daily routine?
SMA News Today is strictly a news and information website about the disease. It does not provide medical advice, diagnosis, or treatment. This content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this website.
