Spinal muscular atrophy (SMA) is an inherited disease that attacks motor neurons, the nerve cells that control our muscles. SMA hits children and adults.
Children inherit the gene for SMA from both parents. About one in 50 adults are carriers of this gene, and about one in 10,000 babies are born with the condition.
Doctors have been able to uncover what causes SMA. They now classify types of the disease. Which one you have depends on when symptoms start and how critical they are. Usually, the earlier SMA symptoms appear, the more severe the disease is.
In all types of SMA, the muscles closest to the center of the body are affected first before the disease progresses to those muscles farther away.
The symptoms of all forms of SMA can vary widely.
Type 0
Babies with this type are born with severe muscle weakness and little muscle tone. In most cases, mothers notice a drop-off in their baby’s movement before birth.
Babies with type 0 don't show any reflexes. They may be born with paralyzed faces and deformed joints. Heart defects and breathing problems also are possible.
This is the rarest form of the disease. Most babies born with SMA type 0 do not survive more than 6 months.
Type 1
Also called infantile onset SMA or Werdnig-Hoffmann disease, this is the most common form of the disease.
About 50% of babies born with SMA have type 1. Symptoms start to become noticeable by 3 months of age.
These babies:
- Have a “floppy” appearance because of the poor muscle tone on both sides of their bodies
- Have trouble lifting their heads and rolling over
- Can’t sit up without support
They may have trouble breathing, and their cry may be weak. Babies with type 1 might also struggle to get the nutrients they need because they have problems swallowing and sucking.
Life expectancy for babies diagnosed with type 1 SMA is usually 2 years or less.
Type 2
An intermediate form of the disease, type 2 usually appears when a child is between 7 and 18 months of age.
Symptoms of SMA type 2 can be moderate or severe. Usually, children with this type of the disease can sit without help, but they do need help walking.
Because of the weak muscle development and structure surrounding their spines, children with type 2 may have varying degrees of a condition called scoliosis, which is a curve in their spine. These weak spinal muscles, along with ones in the chest wall, can also make it difficult to breathe. Respiratory infections might also come.
While type 2 can be quite serious, treatments can help most children with this form of the disease live into adulthood.
Type 3
Also known as Kugelberg-Welander disease, or juvenile SMA, type 3 shows up in older children or teens. They can stand and walk without help early in their lives, but they begin to have a hard time with these motor skills as they get older.
Those with SMA type 3 can get muscle contractions in their feet. They can also have scoliosis or respiratory issues.
Muscle weakness is more evident in the legs than arms with type 3, so some people with it may need to use a wheelchair.
Most people with type 3 SMA have a normal lifespan.
Type 4
This rare, mild form of the disease usually begins in adulthood. Symptoms come on slowly and may include:
- Muscle weakness, usually in the legs or arms
- Tremors
- Mild respiratory issues
Although there is no cure for SMA, two treatments have been approved by the FDA:
Talk with your doctor about which treatment is best for you or your child.