Spinal Muscular Atrophy Progression: What to Know

Medically Reviewed by Christopher Melinosky, MD on May 12, 2023
3 min read

How your spinal muscular atrophy (SMA) changes over time depends on many things, including which of the four types of SMA you have. They’re based on when symptoms first show and the highest physical development milestone you achieved when they started.

The most severe form, known as type 0, is rare and usually fatal, either before the child is born or shortly after.

In the other four types, a person with SMA has muscle weakness, and most start to have trouble breathing. Symptoms can range from very severe to mild.

The muscles closest to the center of the body will be affected first. They’ll also feel more impact than muscles farther away.

Think of it this way: The muscles controlling your thighs, which can help you stand, will be weaker than the muscles in your feet. The legs will also be weaker before your arms.

For some, the hands may become less strong. But, generally, the hands keep their strength the longest. Even if they weaken, you could still be able use a computer and do day-to-day things.

Another issue for those with SMA is scoliosis. That’s a side-to-side curve in the spine, and many children with SMA get it early in life. Scoliosis happens because SMA weakens the muscles that support the spine.

People with scoliosis may have:

  • Uneven shoulders and hips
  • One shoulder blade bigger than the other
  • One hip higher than the other

Scoliosis can be uncomfortable. If the curve is severe, it could affect breathing.

Although doctors prefer to treat scoliosis with a brace until growth is complete and surgery can be done, scoliosis does progress, meaning that the curve can get worse. So your doctor will pay close attention to any breathing and movement problems that come.

What may happen next depends on which type of SMA you have.

Also called Werdnig-Hoffmann disease, this is a severe form of SMA. It’s diagnosed right at birth or up to 6 months of age.

Muscle weakness becomes worse very quickly. Your child won’t be able to sit or stand without treatment.

They may have a hard time sucking and swallowing as well. That can lead to malnutrition. Some infants may look as if they are breathing with their bellies. That’s because the muscles that control breathing stop working properly. This can bring frequent breathing problems. Some of these can be serious and life-threatening.

Type 2 is diagnosed from 6 to 18 months of age. How quickly this form of SMA progresses is very different for each child.

Children with type 2 SMA can sit without assistance early in their development, but by their teens, they may not be able to do this without help.

Those with type 2 also won’t be able to walk for more than a few feet without help. In most cases, their fingers will start trembling as well. In about 2 out of 3 cases, tendon reflexes (think the reflex test you have at the doctor) will also be affected.

By their mid-teens or later, most people with type 2 SMA may not be able to sit without help.

As their muscles weaken, respiratory infections can become more likely.

This type is also called Kugelberg-Welander syndrome. It’s usually diagnosed around 18 months of age, but symptoms can start in young adulthood.

How much type 3 progresses in children will depend a lot on how much they learn to move. Children with this can learn to walk. But in time, some will start falling down a lot and having a tough time with stairs. Getting up after lying on their back will also be tough.

Like all kids with SMA, they have a greater chance of respiratory infections as their muscles get weaker.

This is the rarest form of SMA. It is first diagnosed when people are in their 20s or 30s. Disease progression is slow. People with this type of SMA can walk as adults, but as they age, their muscles will get weaker.