People with type 3 spinal muscular atrophy (SMA) have milder symptoms that progress slowly. They can stand and walk independently at some point but may lose the ability later in life. Muscle weakness, especially in the legs, may impair certain motor functions.1-3
If you notice these symptoms in yourself or your child, talk to your doctor. They can perform tests to diagnose type 3 SMA. As type 3 SMA progresses, muscle weakness can get worse. However, most people with type 3 SMA do not experience life-threatening complications and have a normal life expectancy.1-3
What are the symptoms leading to a diagnosis of type 3 SMA?
Symptoms of type 3 SMA usually appear between 18 months old and adulthood. The first symptoms are usually weakness in muscles closer to the torso (proximal muscles). Legs are often more affected than arms. Overall, symptoms of type 3 SMA are milder than types 1 and 2.
Some common experiences among people with type 3 SMA include:1-3
- Problems climbing stairs and running, which worsen over time
- Difficulty standing from a seated or lying position
- Poor balance and increased risk of falls
- Slight tremor
There is wide variation among people with type 3 SMA. It is sometimes broken down into 2 subgroups:3-5
- SMA type 3a, in which symptoms begin before 3 years old
- SMA type 3b, in which symptoms begin after 3 years old
Symptoms of type 3b are milder and progress slower than symptoms of type 3a. For example, people with type 3a may not be able to climb stairs. People with type 3b are twice as likely to keep the ability to walk at age 20 and older. However, even within subgroups, people can have very different experiences.2,5,6
How are the symptoms of type 3 SMA evaluated?
Motor scales can be used to monitor symptoms of type 3 SMA. This helps doctors track the full extent of the disease as it progresses. It can also help them evaluate the success of any treatments.7
One scale often used in type 3 SMA is the Hammersmith Functional Motor Scale (HFMS). The scale includes 20 tests of different movements and posture changes. Each item is scored from 0 to 2, with a higher score reflecting more independent movements. Doctors also often use modified and expanded versions of the HFMS.8
What are some complications of this type?
As muscle weakness progresses, people with type 3 SMA can develop a few complications. The progression is usually slow. People with type 3 SMA usually do not develop more severe complications, like scoliosis or respiratory failure.2
Loss of ability to stand or walk
People with type 3 SMA can walk independently at some point in their life. However, many people lose the ability to stand or walk with time. This may cause people to become dependent on a wheelchair.1,2
The chance of losing the ability to walk is higher if symptoms start earlier. People with type 3a SMA have a 44 percent chance of being able to walk at age 20. People with type 3b SMA have an 89 percent chance of being able to walk at age 20.6,9
New treatments are beginning to change the expected outcomes of type 3 SMA. For example, some children treated with Spinraza® (nusinersen) show motor function improvements not previously seen in people with type 3 SMA.10