What is spinal muscular atrophy (SMA)?

Spinal muscular atrophy (SMA) is a group of genetic diseases that cause weakness and wasting in the voluntary muscles of infants and children and, more rarely, in adults. It is a one of the most common genetic conditions affecting children. It is estimated that one in every 6,000 to 10,000 babies worldwide is born with SMA.

In more than 95 percent of cases, SMA is caused by inadequate production of a protein called survival motor neuron (SMN) protein that is essential to motor neurons. SMN is produced by SMN1 and to a lesser extent by SMN2. These genes are on chromosome 5. Typically, people have two copies of the SMN1 gene and up to two copies of the SMN2 gene in each of their motor neuron cells. In people with spinal muscular atrophy, both copies of the SMN1 gene are altered or missing. Having additional copies (three or more) of the SMN2 gene are associated with a milder disease by partially compensating for missing SMN1. Rarely, SMA is caused by mutations in genes other than SMN (non-chromosome 5).

Motor neurons lie in the anterior horn of the spinal cord and directly control the body’s skeletal muscles. Without adequate SMN protein, spinal cord motor neurons begin to shrink and die. As this happens, the child’s brain is unable to control the body's voluntary muscles, especially those in the arms and legs and in the head and neck. The muscles begin to weaken and waste away. This affects movements such as walking, crawling, head and neck control, swallowing, and breathing.

What are the different types of spinal muscular atrophy?

There are four different types of spinal muscular atrophy. The classification is determined by the developmental milestones the child has hit by the time of the disease onset. Types I and II are the most common.

The types of spinal muscular atrophy (SMA) are:

  • Type 1 (severe) SMA: This type is also called Werdnig-Hoffmann Disease. It is the most severe and the most common type of SMA. It is usually evident at birth, or in the first few months afterwards (0-6 months). Symptoms include floppy limbs and weak trunk movement. Children with this type usually have very limited ability to move. They will also have a hard time feeding and swallowing, holding their head up, and breathing. Type 1 SMA progresses rapidly, with the weakening of muscles leading to frequent respiratory infections and usually death by the age of 2. Infants with SMA type 1 can never sit.
  • Type 2 (intermediate) SMA: Symptoms usually appear between the ages of 7 to 18 months. The rate of progression can vary greatly. The disease affects the child's legs more than his or her arms. Children with SMA type 2 can never stand. Respiratory infections are also common with this type of SMA. Life expectancy can range from early childhood to adulthood, depending on the severity of the patient's condition.
  • Type 3 (mild) SMA: This type of SMA is also called Kugelberg-Welander or Juvenile Spinal Muscular Atrophy. Symptoms can first appear during a wide range of years, from 18 months to early adulthood. Patients with Type 3 SMA can stand and walk, but may have trouble getting up from sitting position. They may also experience mild muscle weakness and are at greater risk for respiratory infections. Most patients with Type 3 SMA have a life expectancy close to normal.
  • Type 4 (adult) SMA: Symptoms for this rare type of SMA do not usually emerge until the second or third decade of life. Patients with Type 4 SMA can walk during adulthood but will usually experience slowly progressive muscle weakness and other typical SMA symptoms.

If you notice any of these symptoms in your child, or yourself, you should see your doctor immediately.

Who gets spinal muscular atrophy?

People of any race or gender can be affected by spinal muscular atrophy. Additionally,

  • One in every 40 people carries the gene that causes SMA.
  • A child of two gene carriers has a 25 percent chance of being born with spinal muscular atrophy.
  • Seventy-five million Americans are estimated to be carriers of the spinal muscular atrophy gene.
  • More than 25,000 Americans are believed to suffer from spinal muscular atrophy.

Last reviewed by a Cleveland Clinic medical professional on 11/24/2015.


  • Darras BT, Markowitz JA, Monani UR, De Vivo DC. Spinal muscular atrophies. In: Darras BT, Jones HR, et al, editors. Neuromuscular Disorders of Infancy, Childhood, and Adolescence. 2nd ed. Amsterdam: Academic Press, an imprint of Elsevier; 2014: Chap 8.
  • Amato AA, Russell JA. Spinal Muscular Atrophies. In: Amato AA, Russell JA. eds. Neuromuscular Disorders, 2e. New York, NY: McGraw-Hill; 2015.
  • Genetics Home Reference. Spinal muscular atrophy Accessed 11/24/2015.
  • National Human Genome Research Institute. Learning About Spinal Muscular Atrophy Accessed 11/24/2015.
  • Muscular Dystrophy Association (MDA). Spinal muscular atrophy Accessed 11/24/2015.

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