Symptoms and Complications of SMA By Type

Reviewed by: HU Medical Review Board | Last reviewed: August 2021

Recognizing the symptoms of spinal muscular atrophy (SMA) early can help doctors diagnose and start treatment as soon as possible. This is especially important for infants and children with more severe types of SMA.

The severity of symptoms varies greatly depending on the type of SMA. The risk of developing certain complications also varies depending on the type. But even people with the same type can have very different experiences. There is no way to predict disease outcomes.

Type 1 (Werdnig-Hoffmann disease)


Symptoms of type 1 SMA appear before 6 months of age. Infants with type 1 SMA are never able to sit unassisted. Diagnosis of infants with type 1 usually occurs after parents notice signs of motor delays and severe weakness. Symptoms are similar to many other neuromuscular disorders. Early diagnosis and treatment can improve health outcomes.

Talk to your child’s doctor if you notice any of these symptoms:1-3

  • Severe muscle weakness
  • Low muscle tone (hypotonia) causing frog-leg posture when lying down
  • Poor head control
  • Absent reflexes (areflexia)
  • Bell-shaped chest due to use of abdominal muscles for breathing
  • Risk of inhaling fluid into the lungs (aspiration)
  • Weak cry and cough
  • Tongue weakness twitching (fasciculations)
  • Difficulty swallowing (dysphagia)

Infants with type 1 SMA have normal mental abilities. They are alert, social, and intelligent.1


Infants with type 1 SMA have a high risk of developing severe complications. As SMA progresses, tongue and throat muscles weaken, and it becomes difficult to swallow and eat. This can increase the risk of lung infection due to aspiration (aspiration pneumonia). This can also lead to malnutrition, slow growth, and a failure to thrive.1,4

Respiratory muscle weakness often leads to respiratory failure. This is the most common cause of death for infants with type 1 SMA. It usually occurs before 2 years old.3

Type 2 SMA (Dubowitz disease)


Symptoms of type 2 SMA usually appear between 6 and 18 months old. Children with type 2 SMA are able to sit unassisted but can never walk independently. Common symptoms of type 2 SMA include:1,3

  • Progressive muscle weakness, especially in the legs
  • Low muscle tone and absent reflexes
  • Weak cough, difficulty swallowing, and difficulty breathing
  • Tongue weakness twitching

Children with type 2 SMA have normal mental abilities. In fact, research shows children with SMA often have higher intelligence scores than their peers once they reach adolescence.5


Respiratory muscle weakness can lead to respiratory failure in children with type 2 SMA. Muscular weakness also often leads to joint complications, such as scoliosis (curvature of the spine) and joint contractures (shortening and tightening of joints). These and other skeletal complications severely impair mobility and movement.1,3

The combination of scoliosis and respiratory muscle weakness can also cause restrictive lung disease. This happens when the lungs can only hold a low volume of air. It can cause coughing, shortness of breath, and chest pain.6

Type 3 (Kugelberg-Welander disease)


Symptoms of type 3 SMA usually appear between 18 months and 10 years old. People with type 3 SMA can walk independently but may eventually lose the ability to walk. Symptoms of type 3 are milder than type 2 but vary greatly. The most common symptom is progressive weakness that affects the legs more than the arms. People with type 3 SMA have normal mental abilities.1


People with type 3 SMA usually do not develop scoliosis or respiratory muscle weakness. However, people who maintain the ability to walk may develop foot deformities.3

Type 4


Symptoms of type 4 SMA usually appear after 30 years old. People with type 4 SMA achieve all motor milestones and maintain the ability to walk throughout life. Muscle weakness or discomfort is relatively mild.1


Few complications of type 4 SMA have been reported. Muscle weakness can progress slowly throughout life. It rarely causes skeletal or respiratory issues.3

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