Spinal muscular atrophy (SMA), the number one genetic killer of children under the age of two, is an often fatal disease that destroys the nerves controlling voluntary muscle movement, which affects crawling, walking, head and neck control, and even swallowing.
WHO IS AFFECTED
SMA is one of the most prevalent genetic disorders.
• One in every 10,000 babies is born with SMA.
• SMA can strike anyone of any age, race or gender.
• One in every 50 people carries the gene that causes SMA
• The child of two carriers has a one in four chance of developing SMA.
• 7.5 million Americans are carriers.
THE TYPES OF SMA
SMA Patients are classified into four types based on milestones achieved at onset of SMA. Type I and II are the most prevalent.
• Type I, or Werdnig-Hoffmann Disease, is the most severe form of SMA. Type I SMA strikes infants between birth and six months old. Children affected with Type I cannot sit without support.
• Type II affects infants between seven and 18 months old. Type II patients may be able to sit unaided or even stand with support. They are at increased risk for complications from respiratory infections.
• Type III, also known as Kugelberg-Welander Disease, is the least deadly form of childhood-onset SMA. It strikes children as early as the age of 18 months, but can surface as late as adolescence. Type III patients are able to walk, but weakness is prevalent. Most patients eventually need to use a wheelchair.
• Type IV is the adult form of the disease. Symptoms tend to begin after age 35.
SMA does not affect sensation and intellectual activity in patients. It commonly is observed that patients with SMA are unusually bright and sociable.
From Families of SMA
WHAT CAUSES A GENETIC DISEASE?
For most of the traits in our bodies, we have two genes, one inherited from the mother and one from the father. As the result of a change in the gene (mutation), it may not work in the proper way.
Some diseases occur when a person has a change in only one gene. Other diseases (called "recessive") occur when both genes have changed.
What is a carrier?
A carrier is a person who has a change in one of the genes for a recessive disease. Because two changes are required to produce the disease, the person remains well and unaware of his or her carrier status.
When two carriers have a child, the child has a 25% chance for being affected. When a carrier has a child with a person who is not a carrier, the child is not at risk for the disease.
HOW ARE GENETIC TESTS USED TO FIND CARRIERS?
There are two ways that carrier testing is done. One way is by direct analysis of the genes. The genes are extracted from blood cells. The genes are tested for mutations.
The second way is to test for the amount or activity of a gene product that normally prevents disease. Affected individuals have little, if any, of the gene product. Carriers have a 50% reduction in the gene product.
The choice of test depends on its ability to detect as many carriers as possible.
HOW DO CARRIERS USE GENETIC INFORMATION?
When carriers decide to have children, the disease status of the baby can be learned before birth by using prenatal diagnosis.
Prenatal diagnosis involves collecting a small sample of placenta between 10 to 12 weeks of pregnancy or a sample of the fluid that bathes the baby between 14 to 20 weeks of pregnancy. (The length of a pregnancy is usually 40 weeks.)
Genetic testing is performed on these samples to learn whether the baby has the disease, is a carrier like the parents, or is not a carrier. Sometimes carriers may decide not to have children with other carriers.
There are companies, such as Counsyl, that provide comprehensive, affordable genetic testing. If you have insurance (even if your insurance does not cover carrier screening), you can be screened for over 100 genetic diseases for around $100. Talk with your care provider for more information.