Infants with type 1 spinal muscular atrophy (SMA) have severe symptoms compared to those with other common types of SMA. Symptoms usually begin within 6 months of age. Infants may be asymptomatic for some time before symptoms quickly develop.
Parents are often the first to notice early signs of type 1 SMA. These symptoms are similar to other neuromuscular disorders, but your doctor can perform tests to pinpoint SMA. As type 1 SMA progresses, life-threatening complications can occur. There is no way to predict which complications will occur, especially as new treatments change outcomes.
What are the symptoms leading to a diagnosis of type 1 SMA?
The hallmark symptom of type 1 SMA is severe weakness and low muscle tone (hypotonia) in most muscles of the body. Many other symptoms and complications are caused by weakness in different areas of the body. For example, infants with type 1 SMA often:1-3
- Are unable to raise their arms against gravity or move their legs
- Have poor head control
- Have a weak cry, tongue twitches, and difficulty swallowing because of weak mouth and throat muscles
- Have a bell-shaped chest because of chest muscle weakness
- Have a “frog-leg” posture when lying down because of leg muscle weakness
- Have low or absent reflexes (areflexia)
Symptoms vary between infants with type 1 SMA. For example, some infants may have difficulty moving their arms and legs but have no trouble swallowing. Type 1 SMA is more of a spectrum of symptoms spanning from the most severe type 0 SMA to the milder type 2 SMA. Symptoms are usually more severe if they appear earlier in life.4
How are the symptoms of type 1 SMA evaluated?
Muscle weakness does not give doctors a full picture of the extent of SMA. For example, muscle weakness may affect daily functioning differently depending on which muscles are affected and how severely. Doctors often use motor tests to record functional abilities and follow disease progression. This also helps evaluate treatments.5
A commonly used measure of motor skills in infants with type 1 SMA is the CHOP INTEND scale. The full name is “The Children’s Hospital of Philadelphia Infant Test of Neuromuscular Disorders.” The test includes 16 items that evaluate unprompted or prompted movements. Questions assess head, neck, arm, and leg strength. Each item is scored from 0 to 4, with a higher score reflecting better movements.6
What are the possible complications of type 1?
As type 1 SMA develops, complications occur because of progressive muscle weakness. Some of these complications are life-threatening. Others greatly impair mobility. It is impossible to know which complications will occur and how severe they will be. Also, new treatments are changing expected outcomes. For example, some infants who have been treated with Spinraza® (nusinersen) have reached motor milestones never before seen in type 1 SMA.2,7
Some complications of this type:
Inability to sit unsupported
By definition, infants with type 1 SMA are never able to sit unsupported. However, new treatments are changing this. Some infants with type 1 SMA treated with nusinersen have achieved the ability to sit unsupported. When nusinersen is started before symptoms appear, infants with type 1 SMA have a higher chance of sitting or walking unsupported.7-9
Aspiration pneumonia
Aspiration pneumonia is a lung infection caused by aspiration (inhaling material from the stomach or mouth into the lungs). Difficulty swallowing increases the risk of liquid and other particles entering the airways. Nutritionists can help suggest a diet that focuses on foods with a low risk of aspiration. Signs of aspiration include coughing, choking, and vomiting.1,2,10
Malnutrition and slow growth
Difficulty swallowing and eating often cause reduced feeding. This can lead to malnutrition and slow growth. This is often called a “failure to thrive.” Malnutrition may contribute to decreased muscle mass and weakness. Nutritionists can suggest a diet with a proper food consistency to improve food intake.2
Respiratory failure
The most common cause of death in infants with type 1 SMA is respiratory failure due to respiratory muscle weakness. This occurs when the lungs cannot get enough oxygen to your blood. Treatment for type 1 SMA usually involves respiratory support to improve quality of life.2