Spinal muscular atrophy (SMA) is a disease that affects the neurons in your spinal cord. Over time the disease can interfere with muscle control in any part of the body including those connected with breathing and swallowing. There are several types of SMA. The severity and progression of the disease depends on the type and how early symptoms first develop.
Spinal muscular atrophy is a genetic disease that runs in families. Often parents are carriers of the disease but show no symptoms. Genetic counseling can identify this disease in adult carriers and counseling can help you understand the risks of passing this disease to your children. Unfortunately, the first time this disease is discovered in a family is only after a child is diagnosed with the disease.
What Causes SMA?
Spinal muscular atrophy is one of several autosomal recessive genetic diseases. It is passed along to a child when both parents are carriers of a missing or mutated gene that affects the motor neurons in the spine. The mutated or missing gene is responsible for producing a protein responsible for the health production and function of the nerve fibers that begin in the spinal cord and branch out to all the muscles in the body.
With the gene mutation associated with SMA the lack of this protein results in nerve cells weakening and atrophying. This results in muscle weakness that generally progresses over time.
Autosomal recessive disorders are only passed along to children if both parents have the genetic defect. When both parents are carriers of the disease, a child has a one in four chance of inheriting the disease and showing some degree of symptoms.
Because genes come in pairs, an individual can have one set of healthy genes and one set of mutated genes without showing symptoms. An individual with one set of defective genes is known as a carrier. This means even though they show no signs of the disease, if they have children with another carrier, they can then pass the disease along to their children.
It is important to know that even though a child has the disease they may not necessarily develop symptoms of the disease. Also there are four types of SMA and symptoms and the severity of the disease is dependent on the type of SMA involved.
Forms of Spinal Muscular Atrophy
Life expectancy and the risk of serious complications from SMA vary according to the type of SMA which depends on the age the first symptoms develop. Children with the most severe type of SMA may have live for two years or more depending on treatment, support, and individual strength. Those children with the more mild to moderate forms can live will into adulthood with ongoing medical monitoring, treatment and support.
Type I SMA is also known as Werdnig-Hoffman Disease. This is the most severe form of the disease and is usually present at birth. Due to the progressive weakening of muscles, infants are often unable to hold their heads up, feeding or swallowing. Breathing difficulties place these infants at high risk for respiratory complications and lung collapse. Life expectance is about two years. Werdnig-Hoffman disease accounts for 60% of all SMA patients.
Type II, Intermediate form of SMA: With Type II SMA the disease is usually diagnosed between the ages of six to eighteen months. Symptoms vary and the progression of the disease varies. Some infants are able to control their head and sit on their own but are unable to walk. Life expectancy also varies but a majority of children with Type II SMA do live to adulthood.
Type III SMA: Type III is the milder form of this disease. Also known as Kugelberg-Welander or Juvenile Spinal Muscular Atrophy symptoms may first appear anytime between eighteen months and early adulthood. People with Kugelberg-Welander disease often have minor muscle weakness and difficulty walking. They are also at risk for respiratory infections. With continual medical monitoring and treating symptoms of such complications such as respiratory infections as early as possible, individuals with Type II SMA have a normal life expectancy.
Type IV SMA: Adult onset Spinal Muscular atrophy is rare. First symptoms don’t appear until after age 35 and are sometimes confused for other conditions and misdiagnosed. Progression of the disease is generally much slower than the more common forms of SMA and is more likely to first affect walking.
Testing for Spinal Muscular Atrophy
Diagnosing SMA is usually not done until symptoms are present and other conditions are ruled out. Because SMA is a genetic, a blood test is done that examines the SMN1 and SMN2 genes. Most individuals with spinal muscular atrophy are lacking the SMN1 gene or have a mutated SMN1 sequence.
An EMG test which measures the electrical activity in a muscle may be used to rule out other conditions that mimic SMA.
Prognosis for Individuals with SMA
Researchers have found that even though the SMN1 gene is the gene primarily responsible for producing the SMN protein for healthy muscle growth and development. There is also a smaller level of the SMN protein produced by the SMN2 genes.
Everyone has a certain number of SMN2 genes that continue to produce some protein. The number of SMN2 copies an individual has as well as the type of spinal muscular atrophy an individual is diagnosed with will determine their overall prognosis.
The key to maintaining as much muscular strength as possible is early diagnosis. Ongoing medical monitoring can go a long way in lessening the risk of life-threatening complications. A multi-directional team approach can help support parents in caring for their children and providing a quality of life that takes into consideration their individual challenges.
There is no cure for SMA or vaccine that can be given to prevent the disease.
For families already known to be carriers of SMA, genetic counseling can give a realistic idea of the chances for passing along the disease and help in determining the choices to be made.
Daniel Silvera is a young man who understands firsthand the challenges presented by this disease. Born with SMA and a quadriplegic for most of his 33 years of life, he has a unique perspective on the quality of life. His story challenges all of us to stop and re-examine the true meaning of life. You can read his story here.
Families of SMA. Understanding SMA
Spinal Muscular Atrophy Foundation. FAQ
National Institute of Neurological Disorders and Stroke. NINDS Spinal Muscular Atrophy Information Page.