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Spinal Muscular Atrophy Type 3

Updated on May 26, 2021
Medically reviewed by
Evelyn O. Berman, M.D.
Article written by
Victoria Menard

Spinal muscular atrophy (SMA) type 3 is a rare neuromuscular disorder characterized by hypotonia, the progressive weakening of muscles. About 1 in 8,000 to 10,000 people have some form of SMA. Symptoms range in severity depending on which type of SMA you have. People with SMA type 3 experience muscle weakness predominantly in the legs and lower body, as muscles gradually lose strength over the years.

Also known as Kugelberg-Welander Syndrome or Kugelberg-Welander disease, SMA type 3 is one of the milder forms of SMA. There are multiple treatments available for SMA type 3, including physical therapy and medication.

How Is SMA Type 3 Different From Other Types of SMA?

Five types of SMA (0, 1, 2, 3, and 4) make up 95 percent of cases. While SMA type 1 accounts for nearly half of all cases, SMA type 3 is the second most common type, comprising about 30 percent of cases. SMA type 3 is equally common in males and females.

People with SMA type 3 tend to experience less severe symptoms. Usually, they don’t experience respiratory problems, and their life expectancy is on par with that of the general population. If they do have breathing problems, it’s usually in the form of hypoventilation or obstructive sleep apnea. Many children with SMA type 3 can stand and walk, although that can become more difficult as they grow older.

While SMA type 3 affects multiple muscle groups, the legs and lower body muscles tend to be the most affected. Different people experience different degrees of mobility. For some, walking is possible, although they may have difficulty with their gait due to foot deformities. Others may use mobility devices or require assistance with daily tasks.

SMA Type 3a and Type 3b

There are two subgroups of SMA type 3, type 3a and type 3b, which are determined by age of disease onset. SMA type 3a develops in children ages 18 months to 3 years old, whereas 3b can develop in anyone ages 3 to 30 years old.

Because type 3b develops later in life, people with type 3b generally have less trouble walking than those with type 3a. SMA type 3b is sometimes grouped with SMA type 4 as “late onset SMA” because the symptoms tend to be milder than type 3a.

What Causes SMA Type 3?

Spinal muscular atrophy is inherited from parents who both possess a gene mutation for the condition, but usually do not have SMA themselves. Similar to other types of SMA, SMA type 3 develops due to mutations in the survival motor neuron 1 (SMN1) genes.

The severity of SMA is usually correlated to the number of SMN2 genes a person has. Those who have SMA type 3 (Kugelberg-Welander disease) might have three or four copies of the SMN2 gene, while those with SMA type 4 may have four or more copies of the SMN2 gene.

Symptoms of SMA Type 3

For some, symptoms of SMA type 3 can be apparent due to physicality and behavior early in life. For others who develop SMA type 3b later on, genetic testing may be required to diagnose the condition.

There are several recognizable symptoms of SMA type 3:

  • Difficulty walking or climbing stairs without help
  • Trouble balancing
  • Struggling to get up from sitting
  • Tremors (especially in the hands)

Additionally, roughly half of people with SMA type 3 develop scoliosis, a curvature of the spine.

SMA Type 3 Diagnosis

Before genetic testing was available, muscle biopsy and neurophysiological exams were used to assess whether someone had SMA type 3. These days, genetic testing is used to diagnose spinal muscular atrophy. Blood samples are evaluated for mutations of the SMN1 gene. Your doctor may refer you to a genetic counselor who can help you understand the testing process and your results.

Recent changes to the U.S. Department of Health and Human Services (HHS) Recommended Uniform Screening Panel (RUSP) for newborns may change the diagnosis process for people with SMA type 3. As of August 2020, 23 states and U.S. territories routinely screen infants for the genetic mutation that causes SMA types 0 through 4. Instead of being diagnosed when symptoms arise in childhood or early adulthood, some people may be diagnosed as newborns.

Treatments for SMA Type 3

There are multiple treatments available for SMA type 3, including medications and several forms of physical therapy to improve strength and preserve motor function.

Medication for SMA Type 3

As of August 2020, Spinraza (nusinersen) is the only medicine approved by the U.S. Food and Drug Administration (FDA) to treat multiple types of SMA in both adults and children. Spinraza is believed to increase the amount of survival motor neuron protein for people with SMA. In one clinical trial, 7 out of 7 people with SMA type 3 “achieved meaningful improvements in their walking ability.”

Physical Therapy for SMA Type 3

There are several forms of physical therapy available for SMA type 3. The best option for each person depends on a variety of factors, including their current age, the age of disease onset, and the severity of symptoms. Always consult your doctor before starting any new exercise or physical therapy regimen.

Physical therapy allows people with SMA type 3 to improve their strength and preserve the abilities they currently have. It can also work to effectively prolong the onset of some symptoms. Physical therapy for SMA type 3 can help address hypotonia, contractures, bone weakness, scoliosis, and walking and balance issues.

Some physical therapy regimens are designed to build strength. Aquatic therapy and the use of braces and mobility devices, such as walkers, can also help those who are having trouble walking regain flexibility and strength. Gait training can help people walk in a way that puts less strain on their legs. This may involve altering their stride or changing the positioning of their feet or hips.

There are also some physical therapy exercises designed to help people achieve a higher degree of functionality. Occupational therapy, for example, involves learning to efficiently do everyday activities, such as getting dressed or using the bathroom.

The end goal of physical therapy is to remove barriers and allow people with SMA type 3 to manage symptoms and maintain or improve quality of life.

Evelyn O. Berman, M.D. is a neurology and pediatric specialist and treats disorders of the brain in children. Review provided by VeriMed Healthcare Network. Learn more about her here.
Victoria Menard is a writer at MyHealthTeam. Learn more about her here.

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