Spinal Muscular Atrophy (SMA) is the number one inherited cause of death in children under the age of two. SMA is a degenerative disease of the anterior horn cells. Anterior horn cells are located in the spinal cord. SMA affects the voluntary muscles for activities such as crawling, walking, head and neck control and swallowing.
It mainly affects the proximal muscles or, in other words, the muscles closest to the point of origin – in this case, those closest to the trunk of one’s body. Weakness in the legs is generally greater than weakness in the arms. Some abnormal movements of the tongue, called tongue fasciculations, may be present in patients with Type I and some patients with Type II. The senses/feelings are normal as is intellectual activity. In fact, it often is observed that patients with SMA are unusually bright and sociable.
One in every 6,000 babies is born with SMA. Of all children diagnosed before age two, 50 percent will die before their second birthday. SMA can strike anyone of any race or gender. One in every 40 people carries the gene that causes SMA – that’s 10 million people in America alone.
Carrier testing for SMA can be done through a simple blood test, but you will need to ask your doctor for it. More information on genetic testing can be found through Genzyme Genetics and Athena Diagnostics, the major labs in the U.S. who perform the tests.
What Causes Spinal Muscular Atrophy?
Spinal Muscular Atrophy is an autosomal recessive disease, which means that both parents must be carriers. Both parents must have the gene responsible for the disease, and these genes must be passed onto their child. When a child has received this gene from each of his or her parents, the child will then be affected by SMA. Although both parents are carriers the likelihood of passing this gene along to a child and having an affected child is 25 percent, or one in four.
Familial forms (affecting other family members) of Spinal Muscular Atrophy in the older age group can occur as autosomal recessive, mutants or autosomal dominant. The genetic defects underlying these diseases make it necessary to be precise regarding the inheritance pattern in a particular family.
Types of SMA
Type I Acute (Severe) Werdnig-Hoffmann Disease
This is the most lethal form of SMA. Diagnosis is usually made before six months of age, and, in the majority of cases, before three months. Symptoms may even start in the womb. Children with Type I are not able to hold up their heads, roll over, crawl, sit up without support or walk. They have difficulty with breathing, swallowing, feeding and handling secretions. Most children with Type 1 (80%) die before they reach the age of two.
Type II (Chronic)
Diagnosis is almost always made between 15 months and two years of age. Children with Type II may sit unsupported, although they may need assistance. They may be able to learn to crawl or stand – often with the aid of braces and therapy. They will usually never walk. Age of death can vary greatly and can take place as early as age three or not until adulthood.
Type III (Mild) Kugelberg-Welander/Juvenile Spinal
Diagnosis is usually made after 18 months of age, and the patient prognosis for Type III is usually very good. In the beginning, these children are able to stand and walk, but usually have difficulty doing so. They typically have a normal lifespan, but eventually may be wheelchair bound.
Type IV (Adult Onset)
Symptoms typically begin after age 35. This form of SMA is very rare. Proper diagnosis, genetic counseling and appropriate physical therapy remain common mainstays for treatment.
SMA with Respiratory Distress (SMARD1)
SMARD1 is a neuromuscular condition causing weakness of the muscles. It is the second anterior horn cell disease in infants in which the genetic defect has been defined. SMARD1 is not linked to the SMN1 gene locus on chromosome 5q13 (classic SMA), but is caused by mutations in the IGHMBP2 gene on chromosome 11q13. Despite a substantial overlap in clinical features, the phenotypes of SMA versus SMARD1 can be distinguished.
In SMARD1, the predominating symptom is a severe respiratory distress due to a paralysis of the diaphragm. Most patients present [show symptoms] at the age of one to six months with respiratory failure and progressive muscle weakness with predominantly distal lower limb muscle involvement. Sensory and autonomic nervous systems may also be involved.
Resources for Families
Newly diagnosed families also should also visit these sites for support and information:
