Types of Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a group of inherited disorders. SMA causes certain muscles to become weak and waste away (atrophy). SMA may look different in every person. Historically, doctors have considered 5 primary types of SMA. These primary types of SMA are caused by changes in the survival motor neuron 1 (SMN1) gene. Other rare types of SMA are caused by other genes.^1,2

How is spinal muscular atrophy (SMA) classified?

The 5 primary SMA classifications are based on age of onset, symptoms, and life expectancy. Classifying SMA types helps doctors make treatment decisions. However, SMA varies from person to person, even between people who have the same type. As new treatments impact the course of the disease, it may change how doctors classify SMA types.^1-3

What are the 5 types of SMA?

The 5 subtypes of SMA are:^1-4

• Type 0 – This type of SMA is very rare and severe. Symptoms begin before birth as decreased movement of the unborn baby. After birth the baby typically has difficulty breathing and eating. The life expectancy is a few months.
• Type 1 – Type 1 SMA is the most common type of SMA, making up about 60 percent of cases. It is typically diagnosed within the first 6 months of life. Babies with type 1 SMA may have muscle weakness and trouble breathing. Without treatment or breathing support, the life expectancy can be less than 2 years.
• Type 2 – In type 2 SMA, symptoms appear when the child is between 6 months and 18 months old. This type is also called Werdnig-Hoffmann disease. Symptoms of this type are less severe than types 0 or 1, but a child who has type 2 may require a wheelchair. Many people with type 2 SMA can live until their 30s or 40s.
• Type 3 – In type 3 SMA, symptoms appear after 18 months. This type is also called Kugelbert-Welander disease. It may lead to difficulty walking, but it typically does not impact breathing or life expectancy.
• Type 4 – Type 4 SMA is the mildest. Symptoms usually begin after age 21. Most people remain fully mobile. Type 4 SMA does not typically impact life expectancy.

How do new treatments change how we classify SMA?

Starting in 2016, new treatments started to be approved for SMA. These include disease modifying treatments and gene replacement therapy, both of which can improve outcomes for people with SMA. Also, improvements in screening can mean that SMA can be diagnosed earlier.^1-4

These improvements mean that people with SMA can avoid some symptoms and have better outcomes. This changes how doctors view the SMA classification. For example, in the past, a child with SMA who showed symptoms before 6 months old would have been diagnosed as having type 1 SMA. But with treatment, they may be able to sit, which is typical for type 2 SMA.^1-3

Doctors and researchers may now consider different criteria when discussing or treating SMA. These criteria can include:^1-4

• Age when symptoms start and how severe they are
• Motor skills, such as sitting or walking
• Number of copies of the gene that acts as backup for the SMN1 gene (SMN2)
• Age at the start of drug treatment

If you have questions about SMA classification, talk with a doctor. The original classification system may still apply to adults, teens, and some children with SMA. But luckily, breakthroughs in medicine have improved outcomes for many children with SMA.^1-3