People with type 2 spinal muscular atrophy (SMA) have symptoms of moderate severity. Most can sit unsupported, but they may have a delay in achieving this and lose the ability later. Muscle weakness, especially in the legs, prevents them from standing or walking independently.1-3
If you notice these symptoms in your child, talk to your doctor. They can perform tests to diagnose type 2 SMA or another neuromuscular disorder. As type 2 SMA progresses, life-threatening complications can occur. However, there is wide variation between people, and it is not possible to predict which complications will occur.
What are the symptoms leading to a diagnosis of type 2 SMA?
Children usually start showing symptoms of type 2 SMA between 6 and 18 months of age. The earliest symptoms are often a delay in motor development. For example, children with type 2 SMA may have a delay in achieving unsupported sitting.1-3
The main symptom of type 2 SMA is muscle weakness, especially in muscles closer to the torso (proximal muscles). Leg muscles are usually affected more than the arm muscles.4
Symptoms are milder than in type 1. Common symptoms include:4,5
- Tongue weakness and twitching
- Absent reflexes (areflexia)
- Difficulty swallowing (dysphagia) and breathing
- Low muscle tone (hypotonia)
- Tremor of the fingers
- Bell-shaped chest due to chest muscle weakness
There is a wide variation in symptoms among children with type 2 SMA. Usually, symptoms are more severe if they appear earlier. The progression of symptoms also varies. Some children experience a more gradual decrease in motor function than others.6,7
Children with type 2 SMA have normal mental, emotional, and social functioning. Once they reach adolescence, research shows children with SMA actually have higher intelligence scores than children without SMA.8
How are the symptoms of type 2 SMA evaluated?
Doctors often use motor scales and tests to get a full picture of the extent of SMA. This helps them follow symptom progression and evaluate treatments.9
A commonly used scale in type 2 SMA is the Hammersmith Functional Motor Scale (HFMS). The test includes 20 items that assess different movements and posture changes. Each item is scored from 0 to 2, with a higher score meaning better movement. Doctors also often use modified and expanded versions of the HFMS.10
Some possible complications of this type:
Complications often occur as muscle weakness progresses. Some of these complications are life-threatening, while others limit mobility greatly. Complications of type 2 SMA greatly vary from person to person. There is also great variation in the severity of complications. New treatments are changing expected outcomes.4,11
Inability to stand or walk independently
By definition, children with type 2 SMA can sit independently but cannot stand or walk unsupported. They may also lose the ability to sit at some point in their life. However, new treatments are changing this definition. For example, many infants with type 2 SMA who have been treated with Spinraza® (nusinersen) have large improvements in motor function. When treatment is started before symptoms appear, the chance of achieving independent walking and standing is higher.4,5,12,13
Scoliosis
Muscular weakness often leads to scoliosis, which is a curvature of the spine. Scoliosis reduces mobility and causes joints to stiffen. Much of the treatment for children with type 2 SMA is focused on physical therapy and supportive braces to delay or prevent scoliosis.4
Joint contractures
Muscular weakness also can lead to joint contractures. This is a stiffening and shortening of joints, and it causes total loss of movement. It happens when flexible muscles are replaced by stiffer tissues. Physical therapy focuses on preventing joint contractures.4,14
Malnutrition and slow growth
Difficulty swallowing often reduces food intake for children with type 2 SMA. This can lead to malnutrition and slow growth. Malnutrition may worsen muscle weakness and wasting. Dietitians can set up a diet with foods that improve feeding.4
Respiratory failure
The most common cause of death in people with type 2 SMA is respiratory failure. This occurs because of weakness in the chest and respiratory muscles. Treatment for type 2 SMA usually involves monitoring respiratory function as well as ventilation support.3,4
Restrictive lung disease
Scoliosis combined with respiratory muscle weakness can lead to restrictive lung disease. This happens when the lungs can no longer expand like normal.4