Types of Spinal Muscular Atrophy

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

Spinal muscular atrophy (SMA) is a disease that causes muscles in the body to get weaker and smaller. There are five types. Which one a case falls in depends on the age when symptoms started and which development milestones were reached.

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

This is the most common type of SMA. It’s sometimes called infantile onset or Werdnig-Hoffmann disease. Babies with type 1 have symptoms that start at birth up to 6 months of age. Most show signs of the disease by the time they're 3 months old.

A baby with type 1 SMA may not be able to hold their head up without help. They may have floppy arms and legs, and a hard time eating and swallowing. Babies with this type can’t sit without support.

Type 1 SMA can progress very quickly. Weak muscles can lead to respiratory infections and trouble breathing.

Many children with type 1 SMA don’t live past 2 years old.

This type of SMA usually affects children starting between about 3 and 15 months old, before they can stand or walk by themselves. Your doctor may call it intermediate SMA or Dubowitz disease.

Type 2 SMA affects a child’s legs more than their arms. Children often can sit without support, but can’t walk. They may need help sitting by the time they are teenagers or older, as the condition progresses.

A child with this type of SMA might have breathing problems, especially when they’re asleep. Depending on how bad symptoms are, their life span might be shorter than normal.

Children with this type usually begin to have symptoms between 18 months old and their teens. It’s also called Kugelberg-Welander syndrome or juvenile SMA.

Most children with type 3 SMA can walk and stand on their own. They’ll usually fall often and have trouble getting up from a chair and climbing stairs. Muscle weakness is common in their arms and legs. Eventually, they might need a wheelchair to get around.

For children, this is the mildest form of SMA. Children with type 3 usually live about as long as someone without the disease.

Also known as adult SMA, type 4 is rare. Symptoms don’t start until you’re in your 20s or 30s. You might have weak arm and leg muscles and some breathing issues.

Symptoms are mild and progress slowly. Many people with type 4 don’t know they have it until years after symptoms start. Most are able to walk and move pretty easily throughout the rest of their lives without any problem. It usually doesn’t affect how long you’ll live.