Symptoms of Spinal Muscular Atrophy

The most common symptoms of spinal muscular atrophy (SMA) are muscle weakness and low muscle tone. Recognizing these and other symptoms early can help people get a diagnosis and start treatment as soon as possible.

Specific symptoms and their severity vary depending on the type of SMA. The risk of developing complications also varies. Even people with the same type of SMA can have very different experiences. There is no way to predict disease outcomes.

If you notice these symptoms, talk to your doctor. They can distinguish symptoms of SMA from other neuromuscular disorders and perform genetic testing to confirm a diagnosis.

What are common symptoms leading to diagnosis of spinal muscular atrophy?

Early diagnosis is important, especially because of new treatments that have been approved for SMA. Starting treatment earlier increases the chance of better health outcomes. However, children and families often experience a delay in diagnosis because symptoms are similar to other neuromuscular disorders. Universal screening of newborns for SMA would reduce delays in diagnosis and treatment.1

Usually, parents first recognize motor delays in their children. They may be a sign of SMA. The most common early symptoms of SMA are weakness and low muscle tone (hypotonia). Other signs that infants, children, or adults may have SMA include:2-4

  • Family history of motor difficulties
  • Loss of motor skills, such as poor head control or difficulty moving legs and arms
  • Weak cough or cry
  • Overall weakness, especially in muscles closer to the torso (proximal muscles)
  • Low or absent reflexes (hyporeflexia or areflexia)
  • Tongue twitches (fasciculations)
  • Difficulty achieving motor milestones, such as sitting, standing, or walking
  • Difficulty swallowing (dysphagia) and poor feeding

Doctors will perform tests to rule out other diagnoses and pinpoint SMA. If SMA is suspected, doctors can perform a blood test to see if the SMN1 gene is missing or altered. Other tests may confirm the diagnosis, such as electromyography and muscle biopsy.2,5,6

How do symptoms vary by type?

Symptoms of SMA vary greatly. Most of the variation is due to differences in how well the SMN2 back-up gene can make up for the loss of SMN1.1,7,8

SMA is classified into types depending on when symptoms begin and what motor milestones are achieved. However, even people with the same type of SMA can have very different symptoms.1,7,8

As new treatments are being developed and used, the progression of SMA is changing. It is impossible to predict how a person’s symptoms will progress.1,7,8

Type 1 (Werdnig-Hoffmann disease)

Symptoms of type 1 SMA appear during the first 6 months of life. Common symptoms of infants with type 1 SMA include:1,3,9

  • Severe muscle weakness and low muscle tone
  • Poor head control
  • No reflexes
  • Inability to sit unsupported
  • “Frog leg” posture when lying on back
  • Bell-shaped chest
  • Inhaling fluid into the lungs (aspiration)
  • Weak cry and cough
  • Tongue twitches
  • Difficulty swallowing

Despite severe weakness, infants with type 1 SMA have normal mental and emotional function. They are alert, attentive, social, and intelligent.3

Type 2 (Dubowitz disease)

Symptoms of type 2 SMA usually appear between 6 and 18 months of age. Common symptoms of type 2 SMA include:1,3

  • Delayed ability to sit unsupported
  • Inability to stand or walk independently
  • Muscle weakness, especially in legs
  • Low muscle tone and no reflexes
  • Weak cough and difficulty swallowing and breathing
  • Tongue twitches

Type 3 (Kugelberg-Welander disease)

Symptoms of type 3 SMA usually appear between 18 months old and adulthood. Symptoms of type 3 are milder than type 2 but vary greatly between people. These include:3,4,10

  • Progressive weakness, especially in legs
  • Loss of ability to stand or walk independently

Type 4

Symptoms of type 4 SMA usually appear after 30 years old. Muscle weakness or discomfort is relatively mild. People can walk, swallow, and breathe normally.1,3

What are common complications of SMA?

Much of the focus of SMA treatment is on preventing complications. Some of these complications are life-threatening, while others can permanently affect mobility. The risk of complications is much higher for the more severe types of SMA, especially when symptoms begin earlier. However, it is impossible to predict whether complications will occur.1,3

Some of the most common complications of SMA include:3,4,11

  • Scoliosis (curvature of the spine) and other spinal problems
  • Respiratory problems leading to respiratory failure
  • Low air volume capacity of the lungs (restrictive lung disease)
  • Joint shortening and stiffening (contractures)
  • Foot deformities
  • Malnutrition and low weight gain
  • Lung infection from inhaling liquids (aspiration pneumonia)

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Written by: Matt Zajac | Last reviewed: August 2021