What is SMA
Information of SMA Disease
Causes & Diagnosis
Spinal Muscular Atrophy (SMA) is an autosomal recessive disease, which means that both parents must be carriers. Both parents must have the gene responsible and these genes must be passed onto their child. When a child has received this gene from each of its parents it will then be affected by SMA. The gene codes for a protein known as the Survival Motor Neuron (SMN). The precise function of the protein is uncertain but it is important in keeping the motor nerves alive. Although both parents are carriers, the likelihood of passing the gene along to a child and having an affected child is 25%, or 1 in 4.
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.
It is important that we all understand the tests that we must endure. This is especially important when it comes to our children. After having gone through this testing, we often do not remember to ask for an explanation of all of the tests. If and when did ask, we weren't clearheaded enough to hear or understand the explanation. We, as parents, have gathered information to answer these initial questions and explain as simply as possible their diagnostic uses. Up until 1995, there were 3 major lab tests for diagnostic as well as the clinical exam.
As recently as the fall of 1995, probes were updated that detect gene deletions in Types I, II and III SMA were reported. One of these probes is for a gene called Survival Motor Neuron (SMN) and detects the absence of gene sequences in approximately 90-94% of SMA patients. These sequences are not present in normal individuals. This information makes this SMN gene test very useful for the diagnosis of SMA.
However the defect in this gene cannot be used to indicate the severity of the disease. It is believed that a blood test to screen for SMN deletion is all that is necessary to make the diagnosis. It may not be necessary to perform the EMG and muscle biopsy.
The results may show that there is no deletion of the SMN gene. If this is the case than a muscle biopsy would be necessary to confirm the diagnosis.
1. Serum Enzymes
This is a regular blood test. The enzyme most commonly studied in CPK (creatine-phosphokinase). In Type I (Werdnig-Hoffmann) this enzyme tends to be normal, but moderate elevation may occur in the milder forms.
2. EMG (Electromyography)
This test measures the electrical activity of muscle. In this procedure small needles are inserted into the patients muscles, usually the arms and thighs, while an electrical pattern is observed and recorded. The readout is similar to that of an EKG or lie detector. In addition, a nerve conduction velocity (NVC) may also be performed. In this test the response of a nerve to an electrical stimulus is measured.
When performing this test on a child, if at all possible, it should be performed by a doctor experienced in dealing with children. Also be sure to bring lots of things with which to keep your child occupied. Hold your child on your lap during the procedure, as it is a tremendous help in making an unpleasant procedure somewhat bearable.
Ask your doctor whether your child/the patient should be given a mild pain killer or sedative prior to the test.
3. Muscle Biopsy
This is a surgical procedure where an incision, approximately 3 inches long, is made and a small section of muscles is removed, usually from the thigh. The biopsy is used to check for degeneration. Although many doctors may persuade you of the necessity of a general anesthetic, this procedure can be done with a local anesthetic. It is an especially important point when dealing with children who may be suffering from a neuromuscular disorder and may have weak respiratory function.
PLEASE NOTE: THERE IS AN ALTERNATIVE TO THE BIOPSY. IT IS A PROCEDURE KNOWN AS A NEEDLE BIOPSY. INSTEAD OF A 2 TO 3 INCH INCISION, ONLY A SMALL NICK IN THE SKIN IS NECESSARY. BE SURE TO INQUIRE ABOUT THIS PROCEDURE.
No matter what, understand that you, as a parent or as a patient, have rights and that you are not alone. Most hospitals have social service departments that can give you a shoulder to lean on.
Don't be afraid to say NO if something doesn't seem right. Don't be intimidated or afraid to ask questions. If you forget to ask something, call your doctor or contact FAMILIES OF SMA for suggestions.