Spinal muscular atrophy (SMA) type 4 is the rarest and mildest form of the condition. Symptoms don’t start until adulthood and progress slowly, but experts still recommend early treatment. It doesn’t affect your life expectancy.
SMA is a progressive genetic condition that affects your muscles and nerves. It leads to the destruction of nerve cells in your spinal cord and brainstem that control voluntary movement. Destruction of these cells leads to muscular weakness and muscle shrinkage.
Experts divide SMA into four main types. SMA type 4, or Kugelberg-Welander disease, is the rarest and mildest form of SMA. Symptoms don’t usually appear until adulthood, and the condition doesn’t usually affect life expectancy.
Read on to learn more about SMA type 4, including its symptoms, its treatment options, and how it differs from other types of SMA.
Symptoms of SMA type 4
- muscle weakness, which usually develops after age 35 years
- breathing problems
- walking problems, including a waddling gait
- muscle twitches
- enlarged calves
Usually, muscle weakness primarily affects the legs and hips first and then progresses to the shoulders and arms.
SMA is typically caused by a mutation in the SMN1 gene, which contains the instructions your body needs to build a protein called survival motor neuron (SMN) protein. The bodies of people with SMA can’t produce this protein properly. Instead, a backup gene called SMN2 produces a similar but shorter protein that doesn’t fully compensate for the loss of SMN protein.
People with SMA are entirely dependent on their SMN2 gene. The severity of their condition correlates with the number of copies of the SMN2 gene they have. People with SMA type 4 tend to have
According to a 2021 study,
A genetic blood test can detect a mutation in the SMN1 gene.
In cases where the SMN1 gene isn’t responsible for SMA, a doctor
Nusinersen, sold in the United States as Spinraza, is the first
There’s currently no data on the effectiveness of nusinersen for SMA type 4, but experts think it’s most effective to start treatment soon after diagnosis. Researchers in Japan found evidence that it helped improve the symptoms of a 71-year-old woman in a 2020 case study. However, larger studies with many people are needed to draw definitive conclusions.
Another medication called risdiplam (Evrysdi) is also FDA-approved to treat the condition in adults and children over the age of 2 months. It also targets the SMN2 gene, but most of the research on Evrysdi has been on SMA types 1, 2, and 3.
Other treatment options for SMA type 4 aim to reduce symptoms and improve quality of life. They include:
The outlook for people with SMA type 4 is better than with other types of SMA. Most people have a
SMA type 4 is the mildest form of SMA. Here’s a look at how it compares with other types.
Here are some frequently asked questions that people have about SMA type 4.
What’s the life expectancy of someone with SMA type 4?
SMA type 4 is the mildest form of SMA. It
Is SMA type 4 curable?
No type of SMA is curable, but researchers have identified the genetic cause for most people and are continuing to develop medications that can help improve the outlook of SMA.
The medication nusinersen may help improve its progression, although research looking specifically at its effect on SMA type 4 is lacking.
Is SMA type 4 progressive?
SMA type 4 is a progressive condition. However, its progression tends to be very slow compared with other types of SMA.
How common is SMA type 4?
About 1 in 40 people in the larger population carry a gene linked to SMA.
SMA type 4 is the mildest form of SMA and doesn’t cause symptoms until adulthood. At that point, it can cause progressive weakness, especially in your legs and hips.
SMA type 4 doesn’t typically affect your life span, but some people lose the ability to walk.
The medication nusinersen may help improve its progression, but its use for SMA type 4 remains under investigation. Researchers are also continuing to examine new potential treatments for SMA.