Spinal muscular atrophy (SMA) is a disease in which certain nerves in the brain and spinal cord die, impairing the person’s ability to move. Called motor neurons, these nerves control our ability to sit up, crawl, and walk. In severe cases, a person will not be able to sit up independently and their breathing and swallowing may also be impaired. In the mildest cases, symptoms begin in adulthood and make independent movement such as walking more difficult, but still possible. There are five main subtypes of spinal muscular atrophy, each described below. It is not always possible to predict which type of SMA a child could have based on the genetic mutation he or she inherits. This is true of the mutation (exon 7 deletion) for which Counsyl tests.
Type 0 is the most severe form of SMA. Symptoms can often be seen in the later stages of pregnancy as the fetus is less active than expected. Once born, the infant will have little ability to move and may not be able to breathe and swallow independently. Infants with type 0 SMA often die before the age of 6 months.
Type I is another severe form of the disease. Symptoms develop within the first six months of life. Infants with SMA type I often have trouble breathing and swallowing. Their muscle tone and strength are extremely poor; they cannot sit up without support and will not meet any motor skills milestones. Their intellect, however, is normal. Most children with type I SMA will die before the age of two.
In children with type II SMA, muscle weakness becomes apparent between the ages of 6 and 12 months. When placed in a sitting position, people with type II SMA can usually maintain the position without support, however they often lose this ability by their mid-teens. People with SMA type II cannot stand or walk without assistance. They have poor muscle tone and strength and their fingers usually tremble uncontrollably. Their intelligence is typically normal or above average.
Type III SMA is a milder form of the disease. Its symptoms begin sometime between the age of one year and early adulthood. As young children, they may fall repeatedly and have trouble walking down stairs. While their muscles are weaker than normal, people with type III SMA can usually stand and walk without assistance, although they may lose this ability later in life. The legs are often more severely affected than the arms.
Type IV is the mildest form of spinal muscular atrophy. With this form of the disease, muscle weakness does not begin until one’s 20s or 30s, or even later. This weakness is often mild to moderate, and the person can still walk and move independently. These individuals may experience mild to moderate tremors and/or twitching. The disease typically does not diminish lifespan. With all types of SMA, there can be difficulties in sleeping and gaining weight. Frequent pneumonia is common. A curvature of the spine and stiff joints are also common. Women with milder forms the disease have been known to give birth to healthy children, although many of the pregnancies had complications.The disease is caused by a shortage in SMN protein, which helps preserve motor neurons. Without it, the neurons cannot pass messages from the brain to the muscles of the body.
In the United States, 1 in every 6,000 to 10,000 people develop spinal muscular atrophy and 1 in 50 is a carrier of the disease. It has been found in people of every race, but is most common in Caucasians, of whom 1 in 35 is a carrier. Carrier rates for other populations include: Ashkenazi Jews (1 in 41 to 62), Asians (1 in 53), African Americans (1 in 66), and Hispanics (1 in 117). Studies done in specific populations have found carrier rates of 1 in 50 in Germany, 1 in 57 in Italy, and 1 in 62 in China.
There is no cure for spinal muscular atrophy, however some of its symptoms can be addressed.
For children with the more severe forms of spinal muscular atrophy, mechanical breathing aids may prolong lifespan. In some cases, breathing is more difficult at night, leading to a lack of sleep. In those cases, certain types of respiratory assistance may be helpful. If getting enough nutrition is an issue, some people with SMA have turned to feeding tubes.
Those with milder forms of the disease sometimes choose to have surgery to correct curvature of the spine (scoliosis) or joint problems. In forms of the diseases that are fatal in early childhood, these surgeries are often not done.
The prognosis for a person with SMA varies greatly depending on which type of the disease he or she has.
The disease is typically fatal between 2 and 6 months of age. These infants do not develop any motor skills expected of infants their age.
This type of SMA is usually fatal within two years. With mechanical breathing aids, children with type I SMA may live longer. There are a few known cases of SMA type I in which the child survived to adolescence or adulthood.
With type II SMA, 75% of those affected live to the age of 25. They are often able to sit independently when placed in a sitting position, but lose this ability by their mid-teens.
People with type III SMA may live a normal lifespan. Many learn to walk independently, though most lose the ability to do so by their 30s or 40s.
A normal lifespan is possible for people with type IV SMA. They do not develop symptoms until their 20s or 30s and usually retain the ability to walk independently.
While other organizations are working hard to find a treatment or cure, the Claire Altman Heine Foundation uses its funding to identify carriers of SMA, support population based SMA carrier screening, raise awareness of SMA and educate the public and medical communities about SMA.
1112 Montana Avenue # 372
Santa Monica, CA 90403
Phone: (310) 260-3262
A group of 55,000 members that aims to advance research on SMA and support families affected by it.
925 Busse Road
Elk Grove Village, IL 60007
Phone: (800) 886-1762
Explanations of an extensive number of genetic diseases written for everyday people by the U.S. government's National Institutes of Health.