Spinal muscular atrophy is a genetic condition that affects the nervous system and muscles. Those with the condition experience muscle weakness and wasting.

Spinal muscular atrophy (SMA) is a genetic condition that affects 1 in 10,000 people. It impairs a person’s ability to control their muscle movement. Though everyone with SMA has a gene mutation, the disease’s onset, symptoms, and progression vary considerably.

For this reason, SMA is often categorized into four types. Different gene mutations may cause other rare forms of SMA.

Read on to learn about the different types of SMA.

All four main types of SMA result from a deficiency of a protein called SMN, which stands for “survival of motor neuron.” Motor neurons are nerve cells in the spinal cord that send signals to our muscles.

When a mutation occurs in both copies of the SMN1 gene, it leads to a deficiency in the SMN protein. If little or no SMN protein is produced, it leads to motor function issues.

Genes that neighbor SMN1, called SMN2 genes, are similar in structure to SMN1 genes. They can sometimes help offset the SMN protein deficiency. But the number of SMN2 genes fluctuates from person to person. So the type of SMA depends on how many SMN2 genes a person has to help make up for their SMN1 gene mutation.

If a person with chromosome 5–related SMA has more copies of the SMN2 gene, they can produce more working SMN protein. In return, their SMA will be milder with a later onset than someone who has fewer copies of the SMN2 gene.

SMA is an autosomal recessive disorder, so in order to inherit the disorder, a person must receive two mutated copies of the gene, one from each parent. A person with only one copy of the mutated gene is called a carrier. Carriers don’t have any symptoms of SMA, but they can pass on the mutated gene to their children.

Type 0, also called prenatal SMA, is the most severe form of SMA. It’s characterized by muscle weakness and wasting that appears during fetal development and is present at birth. Congenital heart defects are also common in people with type 0 SMA.


Infants with type 0 SMA often die within the first six months of life.

Type 1 SMA is also called infantile-onset SMA or Werdnig-Hoffmann disease. It affects around 60% of those with SMA. Usually, this type results from having only one or two copies of the SMN2 gene, one on each chromosome 5. More than half of new SMA diagnoses are type 1.

When symptoms start

Babies with type 1 SMA start showing symptoms within the first six months after birth. The average onset occurs at around 2.5 months old.


The symptoms of type 1 SMA may include:

  • weak, floppy arms and legs (hypotonia)
  • inability to sit up
  • gastroesophageal reflux (GERD)
  • problems moving, swallowing, and breathing
  • trouble raising the head
  • bone loss, fractures, and spinal abnormalities such as scoliosis


With new advances in treatment and care options, the lifespan of those with type 1 SMA is increasing. However, many babies with this form of SMA don’t live past two years of age.

Type 2 SMA is also called intermediate SMA and affects around 30% of people with the condition. In general, many people with type 2 SMA have three SMN2 genes.

When symptoms start

The symptoms of type 2 SMA usually begin by 18 months old.


Symptoms of type 2 SMA tend to be less severe than type 1. They may include:

  • weak arms and legs
  • trouble walking unassisted
  • abnormal curvature of the spine
  • weak breathing muscles
  • twitching or abnormal movements


Type 2 SMA may shorten life expectancy, but most people with type 2 SMA survive into adulthood. People with type 2 SMA often use a wheelchair. They may also need equipment to help them breathe better at night.

Type 3 SMA is also referred to as juvenile SMA, mild SMA, or Kugelberg-Welander disease. It affects around 10% of people with SMA. The symptoms of this type are more variable. People with type 3 SMA generally have between three to four SMN2 genes.

When symptoms start

The symptoms begin after 18 months of age. It’s usually diagnosed by age 3, but the exact age of onset varies. Some may not begin to experience symptoms until early adulthood.


People with type 3 SMA can usually stand and walk independently, but they may lose the ability to walk as they age. Other symptoms may include:

  • difficulty getting up from seated positions
  • balance problems
  • difficulty going up steps or running
  • muscle fatigue
  • an increase in weakness over time


Type 3 SMA doesn’t generally alter a person’s life expectancy, but people with this type may have an increased risk of becoming overweight. Their bones may also become weak and break easily.

Type 4 SMA is also called late-onset SMA. It affects around 1% of those with the condition. People with type 4 SMA have between three to five SMN2 genes and can produce a reasonable amount of SMN protein. Type 4 is the least common of the four types.

When symptoms start

Symptoms of type 4 SMA usually begin in early adulthood, typically after age 35.


Type 4 SMA may gradually worsen over time. People with this type of SMA can typically walk independently. Symptoms may include:

  • weakness in the hands and feet
  • difficulty walking
  • shaking and twitching muscles


Type 4 SMA doesn’t alter a person’s life expectancy, and the muscles used for breathing and swallowing usually aren’t affected. Most people with type 4 SMA can live and work independently and do not typically need assistive devices for mobility.

These types of SMA are rare and caused by different gene mutations than those affecting the SMN protein.

  • Spinal muscular atrophy with respiratory distress (SMARD) is a very rare form of SMA caused by a mutation of the gene IGHMBP2. SMARD is diagnosed in infants and causes severe breathing problems.
  • Kennedy’s disease, or spinal-bulbar muscular atrophy (SBMA), is a rare kind of SMA that usually only affects males. It often starts between the ages of 20 and 40. Symptoms include weakness of the facial muscles, swallowing difficulties, tremors of the hands, muscle cramps, limb weakness, and twitching. While it can also cause difficulty walking later in life, this type of SMA doesn’t usually alter life expectancy.
  • Distal SMA is a rare form caused by mutations in one of many genes, including UBA1, DYNC1H1, and GARS. It affects nerve cells in the spinal cord. Symptoms usually start during adolescence and include cramps or weakness and wasting of the muscles. It doesn’t affect life expectancy.

Below are some of the most frequently asked questions about SMA.

How common is spinal muscular dystrophy?

It’s estimated that 1 in every 10,000 babies born has SMA.

How is spinal muscular dystrophy diagnosed?

Doctors may order several tests to diagnose SMA. The most common test is a genetic test to look for the mutation that causes SMA. Other tests may include an electromyography (EMG) to measure the electrical activity of muscles, and a nerve conduction study to measure how well nerves are able to send signals. A blood test for creatine kinase (CK) may also be done to look for high levels of CK, which can be a sign of muscle damage. Rarely, a muscle biopsy may be done to look for changes in the structure of muscle fibers.

What treatment options are available for spinal muscular atrophy?

There is no cure for SMA, but there are treatments that can help manage the symptoms and improve quality of life. Disease-modifying medications, such as nusinersen (Spinraza), risdiplam (Evrysdi), and onsemnogene abeparvovec-xioli (Zolgensma) are available. These treatments can help improve muscle strength, function, and respiratory quality.

Physical therapy, occupational therapy, and braces can help maintain muscle strength and function. There are also drugs that can help reduce inflammation and slow the progression of the disease. Respiratory support may also be necessary for some people. Clinical trials are ongoing to test new treatments for SMA.

What is the prognosis for someone with spinal muscular atrophy?

The prognosis for someone with SMA depends on their SMA type. People with types 1 and 2 SMA usually have a shorter life expectancy, while people with types 3 and 4 SMA usually have an average life expectancy. However, SMA can cause complications such as respiratory problems and weak bones, which can reduce life expectancy.

There are four different types of chromosome 5–related SMA, roughly correlating with the age at which symptoms start. The type depends on the number of SMN2 genes a person has to help offset a mutation in the SMN1 gene.

In general, an earlier age of onset means fewer copies of SMN2 and a greater impact on motor function. Children with type 1 SMA typically have the lowest level of functioning, while types 2 through 4 cause less severe symptoms. It’s important to note that SMA doesn’t affect a person’s brain or ability to learn.

Other rare forms of SMA, including SMARD, SBMA, and distal SMA, are caused by different mutations with an entirely different pattern of inheritance.

If you’re concerned that you or your child may have SMA, talk with a doctor. While there’s no cure for SMA, early diagnosis and treatment can help improve quality of life.