Spinal muscular atrophy type 3

Last updated Feb. 2, 2023, by Marisa Wexler, MS
Fact-checked by Ana de Barros, PhD

Type 3 is a milder form of spinal muscular atrophy (SMA), a rare genetic disease characterized by the progressive loss of motor neurons, the nerve cells that control voluntary movement. SMA results in muscle weakness and wasting among patients. One of five main forms of the disease, Type 3 is sometimes called juvenile SMA or Kugelberg-Welander syndrome.

Like the other four types, SMA type 3 is caused by mutations in both copies — one inherited from each biological parent — of the SMN1 gene. This gene provides instructions to produce SMN, a protein essential for motor neuron and muscle health.

What are the symptoms of SMA type 3?

In type 3 SMA, symptoms appear during childhood, but at later times than in the more severe disease types 2, 1, and 0. SMA type 3 symptoms may appear anywhere between 18 months of age and the end of adolescence.

Children with SMA type 3 will gain the ability to walk as toddlers, but as the disease advances, progressive muscle weakness can make walking more challenging. Some patients will lose the ability to walk independently. Other common manifestations of SMA type 3 include:

• muscle weakness, usually affecting the legs more than the arms
• poor balance and an increased risk of falls
• scoliosis, or an abnormal sideways curvature of the spine, and foot deformities.

Type 3 sometimes is broken down into two subtypes: type 3a, in which the disease manifests between 18 months and 3 years of age, and type 3b, where symptoms appear after age 3. Generally, a later age at onset is associated with a reduced decline in motor function in the long term.

Unlike children with more severe types, most people with this milder SMA type do not have swallowing or breathing problems. If these difficulties do occur, they are usually mild or only appear later in life.

How is SMA type 3 diagnosed?

When symptoms are present or if there is a family history of SMA, a diagnosis can be made through genetic testing that looks for disease-causing mutations in the SMN1 gene. This is usually done with a blood sample.

It’s becoming increasingly routine in many U.S. states and worldwide for babies to be tested in the first days of life — during newborn screening that aims to identify genetic conditions and other diseases at birth. DNA testing also can be performed on a fetus, a process known as prenatal screening.

If SMA is found, genetic testing also can be used 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 disease type.

Genetic testing also can be used to identify an SMA carrier — an individual who carries one mutated SMN1 copy but not two. These carriers will not develop SMA themselves but could pass the mutated gene on to their children.

If both parents carry one mutated copy of SMN1, they will have a 25% chance of having a biological child with any of the most common SMA types, and a 50% risk of each of their biological children being a carrier like them. There’s also a 25% chance any offspring will neither have SMA nor be a carrier.

Further tests, such as an electromyogram or muscle biopsy, may be considered if uncertainty about the diagnosis remains. However, such tests are not usually needed to confirm an SMA diagnosis.

Electromyography is a technique that measures muscles’ responses to electrical stimulations by motor neurons. It assesses the health of muscles and the motor neurons that control them. In a muscle biopsy, a small section of muscle tissue is removed, usually from the upper thigh, and examined to see whether it has SMA-associated features.

What are the treatment options for SMA type 3?

In the U.S., three medications are approved for the treatment of type 3 SMA. These therapies, all developed in the last decade, are Biogen’s Spinraza, Evrysdi by Roche and Genentech, and Zolgensma from Novartis Gene Therapies. All are also widely approved elsewhere, though specific indications may vary.

• Spinraza (nusinersen), administered via injections into the spine every four months, is approved to treat patients of all ages with all main types of SMA.
• Evrysdi (risdiplam) is a daily oral therapy that’s approved to treat all main types of SMA in patients who are at least 2 months old.
• Zolgensma (onasemnogene abeparvovec-xioi) is a one-time gene therapy for SMA that’s approved to treat children 2 and younger with all main types of SMA.

None of these is a specific SMA type 3 treatment, and there is no cure for any form of the disease. But all of these therapies have been shown to slow the progression of SMA in clinical trials.

Generally, these medicines are effective at preventing disease progression, but have limited ability to fix damage that’s already happened. Thus, clinical outcomes are generally best when treatment is initiated as early as possible.

Supportive care and assistive devices

Many people living with SMA type 3 benefit from additional forms of supportive care along with disease-modifying treatment. These may include:

• physical therapy and exercise to improve motor function
• occupational therapy, which aims to create strategies for navigating daily life with a chronic illness
• dietary counseling to ensure adequate nutrition and digestive health
• palliative care, a broad branch of therapy that works to improve life quality for people affected by chronic diseases.

Specialized equipment also may help to make living with SMA on a day-to-day basis more manageable. Some available options are:

• mobility equipment like crutches, walkers, or wheelchairs to help patients get around
• braces, standers, and other devices that can provide structural support to the body and aid in everyday activities.

What is the life expectancy for people with SMA type 3?

Compared with the general population, SMA type 3 life expectancy is generally unaffected. People with type 3 typically have a normal life span, even in the absence of treatment. This is one reason that type 3 is considered a milder form of SMA; the more severe types (0, 1, and 2) are associated with reduced life expectancy when untreated.

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.