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 39 months.
  • SMA affects about 1 in 10,000 people. SMA type 3 makes up about 13% of U.S. cases.
  • Some experts classify 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.
Was this helpful?

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

Most people with SMA type 3 lose their ability to walk in adulthood. A later onset of symptoms is linked to a milder course of the disease.

Other possible symptoms include:

  • weakness in your legs that spreads to your arms and shoulders
  • unusual walking, including a waddling gait
  • delayed development of motor skills
  • reduced or absent reflexes
  • finger tremors
  • breathing problems and scoliosis after the loss of the ability to walk (in some cases)

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 3–4 copies of SMN2.

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 95% of SMA cases. In other cases, doctors may use the following tests to help diagnose SMA:

The Food and Drug Administration (FDA) has approved nusinersen (Spinraza) for treating all forms of SMA. This drug may help improve the progression of the disease by targeting the SMN2 gene to help your body produce the full protein typically produced by the SMN1 gene.

In a 2021 study, researchers reported improvements in function in 144 people with SMA type 3 after 12 months. But the researchers recommend interpreting the results with caution since they found differences in subgroups depending on factors such as:

  • 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.

Zolgensma, a gene replacement therapy, is FDA approved to treat SMA type 1 in infants. Its potential benefit for people with SMA type 3 is currently under investigation.

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:

Learn more about treatment for SMA.

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 does not generally affect life expectancy.

Here’s a look at how SMA type 3 differs from other types of SMA:

Type 1Type 2Type 3Type 4
Alternative nameWerdnig-Hoffman diseaseDubowitz diseaseKugelberg-Welander diseaseadult SMA
Age of onsetless than 6 months6–18 monthsafter 18 months and before adulthoodin adulthood
Severitymost severeless severe than type 1less severe than type 1 and type 2usually causes only mild difficulties late in adulthood
Symptoms• 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 typical life spans. Most develop the ability to walk but lose it at some point in adulthood.

Is SMA type 3 curable?

There is currently no cure for any type of SMA. The medication nusinersen shows promise in slowing the progression of the disease. Researchers are continuing to investigate other treatments, including gene therapy.

Is SMA type 3 progressive?

SMA type 3 is a progressive condition. Children tend to gain motor function until about age 6 years. After that, they usually lose function slowly until puberty and more quickly after puberty. The decline in function tends to be slower through adulthood.

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.