Type 4 is on the mild end of the spectrum of spinal muscular atrophy (SMA). People with type 4 SMA usually experience leg muscle weakness after age 30. Other mild symptoms are possible, but people with type 4 usually do not experience severe symptoms or complications.1-3
Type 4 SMA is the rarest of the major types of SMA. Fewer than 5 percent of all SMA cases are type 4. Because it is rare, there is a much smaller focus on type 4 SMA in research and clinical settings than the more common and severe types. We do not yet know as much about diagnosing, managing, and treating type 4 SMA.1-3
What are the symptoms leading to a diagnosis of type 4 SMA?
People with type 4 SMA experience symptoms after they turn 30 years old. Symptoms are similar to type 3 SMA but milder. The most common symptom is mild muscle weakness or discomfort in the legs and hips. Other symptoms reported in people with type 4 SMA include mild tremors and twitches, low reflexes, and fatigue. People with type 4 SMA usually do not experience weakness of the mouth, throat, or breathing muscles.1-3
Genetic testing is almost always used to diagnose type 4 SMA. That said, diagnosis may take some time. The milder symptoms of type 4 SMA can also be signs of many other conditions. Your doctor may rule these out with a muscle biopsy or physical exam. They may also measure your muscles’ electrical activity or order an MRI or CT scan.4
How are the symptoms of type 4 SMA evaluated?
People with type 4 SMA are usually not monitored for functional ability as closely as people with other types of SMA. This is because the disease does not place as much of a burden on daily activities as more severe types. When symptoms are evaluated for clinical or research purposes, doctors will use similar motor scales that they use for type 3 SMA or other motor neuron diseases.5
One scale often used is the Hammersmith Functional Motor Scale (HFMS), but expanded for people who have the ability to walk. 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.6
What are the possible complications of type 4 SMA?
People with type 4 SMA usually do not develop any complications. Muscle weakness progresses slowly throughout life. Problems with breathing and eating rarely occur. Life expectancy is normal.6,7
Most people with type 4 SMA maintain the ability to walk throughout life. However, some people do develop walking difficulties and need to use a wheelchair. The likelihood of a person with type 4 SMA losing the ability to walk is hard to calculate. This is because studies of people with type 4 SMA are too small.6,7