Spinal Muscular Atrophy Type 3

Spinal muscular atrophy (SMA) is a severe inherited disease characterized by the progressive loss of motor neurons. Motor neurons are nerve cells that send signals to control voluntary muscles, and as they are lost the patient’s ability to move, swallow, and breathe typically worsens. There are many different types of SMA, based on the gene that is affected, as well as disease severity and age at onset of symptoms.

The more common types of SMA are known as SMA types 0 to 4, and are all caused by mutations in the survival motor neuron 1 (SMN1) gene that is located on a section of chromosome number 5. SMN1 contains the instructions to produce the SMN protein, which is essential for the survival of the motor neurons.

Variations in the onset and severity of SMA is thought to be due to the number of copies of the SMN2 gene a patient has. SMN2 is can also produce the SMN protein, but in very low amounts. 

What is SMA type 3?

SMA type 3 has a later onset and is generally milder than SMA types 0–2. It is also sometimes referred to as Kugelberg Welander syndrome. It is inherited in an autosomal recessive manner, which means that an individual must inherit a mutated copy of the SMN1 gene from both their mother and their father to develop the disease.

The symptoms of SMA type 3 usually develop after the child has learned to walk unaided, but they can begin to be evident at 12 months of age. SMA type 3 can be further broken down into two subgroups based on age of onset: SMA type 3a, where symptoms begin before age 3; and SMA type 3b, where symptoms begin after age 3 and usually progress more slowly.


Patients with SMA type 3 have muscle wasting, which leads to muscular weakness. As a result, they experience:

  • Problems walking unaided, climbing stairs, and running, all of which can worsen with time
  • Difficulty standing from a seated or lying position
  • Poorer balance, and an increased risk of falls
  • A slight tremor

Scoliosis, where the spine develops an abnormal curvature, is reported to develop in around 50 percent of SMA type 3 patients.

The majority of SMA type 3 patients do not have breathing problems. If they occur, they are usually mild or only appear much later in life. In some cases, however, spine deformities can cause complications that affect breathing.


If SMA type 3 is suspected, based on the symptoms or the patient’s family history, then a genetic test will be carried out. This involves a blood sample being taken and sent to a laboratory to be tested for mutations in the SMN1 gene.

As the availability and accuracy of genetic testing have improved, a genetic test is generally the only diagnostic test needed. Occasionally, however, muscle biopsies or neurophysiological studies might also be carried out. 


While no cure exists for SMA type 3 or any other type of SMA, several treatments can help patients manage the condition.

Spinraza (nusinersen) is currently the only approved medication that specifically treats the underlying cause of SMA. By boosting the amount of SMN protein produced by the SMN2 gene, Spinraza can increase motor function in patients with SMA type 3.

Physical and occupational therapy can help to maintain independence and quality of life.

If the patient has severe scoliosis, specialized back braces or surgery may be required to correct this.


SMA type 3 does not usually reduce life expectancy, as the disease tends to progress slowly. Patients may require help with mobility at later disease stages, such as a wheelchair. The timing of this can depend on the severity of the symptoms; some SMA type 3b patients can retain the ability to walk into adulthood, while SMA type 3a patients usually require a wheelchair from childhood.


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.

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