Diagnosing SMA

*This information is not to take the place of actual medical advice.  If you have a question, always consult with your physician.*

Physical Characteristics:

The common physical characteristics of SMA include:

  • "Frog" shaped legs (knees apart and legs bent)

  • a sunken or narrow chest

  • a big belly

  • breathing with the belly instead of the lungs

  • a weak cry and weak cough

  • poor to no head/neck control; head tilted to one side

  • weak if any movement of the legs and upper arms

  • not able to bear any weight on legs or arms

  • hands that remain clenched or turned the wrong way

  • difficulty sucking and swallowing 

  • tongue fasciculations (tongue vibrating rapidly).

These characteristics vary in severity depending on how advanced the SMA is in each child.  Some of these characteristics may not show up until later.  

Medical Testing:

There are several medical tests that can be performed to diagnose SMA.  Following are a listing of the tests along with an explanation of how they are performed and their accuracy:

EMG (Electromyography)
An EMG test measures the electrical activity of muscle. In this procedure small needles are gently  inserted into the patient's muscles (usually the arms and thighs) while an electrical pattern is observed and recorded by a specialist.

At the same time, a nerve conduction velocity (NVC) will probably also be performed. This uses the same needles and equipment.  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. If permitted, hold your child on your lap during the procedure, to make an unpleasant procedure somewhat bearable. Your doctor may allow your child to be given a mild pain killer or sedative prior to the test.

Genetic Blood Test
Within the last decade, a blood test has now become available to detect SMA.  This blood test works by detecting deletions in gene sequences that are not missing in normal, healthy individuals.  This blood test can not tell the Type of SMA that the individual has (Type I, II, or III), and approximately 5% of individuals who do have SMA do not show the gene deletions.  However, for the 95% of individuals who do show the deletions, the diagnosis is 100% accurate, and the Type of SMA can be determined by other physical factors.  With a blood test to screen for SMN deletion together with an EMG and a clinical examination it may not be necessary for a muscle biopsy to be performed.  If the results show that there is no deletion of the SMN gene, but the clinical examination and the EMG still point to SMA, than a muscle biopsy would be necessary to confirm the diagnosis.

Muscle Biopsy
This is a surgical procedure where an incision approximately 3 inches long is made, and a small section of muscle is removed.  Usually they remove the muscle from the upper thigh. The biopsy is used to check for degeneration of muscles and special tell-tale signs in the muscles of SMA.  It is important to find a doctor used to dealing with children, and experienced in dealing with SMA. Although many doctors may persuade you of the necessity of a general anesthetic, this procedure can be done with a local anesthetic. This is especially important when dealing with children who are possibly suffering from SMA which includes by nature a weak respiratory system.  General anesthesia is not recommended for children with neuromuscular diseases such as SMA as it may be difficult for them to recover.

Needle Biopsy
There is now an alternative to a muscle biopsy. Instead of a 2-3 inch incision, only a small nick in the skin is necessary.  Be sure to ask your doctor about this possibility.

Bottom Line:
You have several options when testing for SMA.  My PERSONAL opinion and recommendation, is that the blood test is by far the easiest, least painful and least invasive of all the options, and accurately diagnoses SMA in 95% of cases.  I would recommend going with the blood test first.  If the blood test comes back negative for SMA, then a muscle biopsy/EMG will be necessary. If it comes back positive, that will be 100% accurate.  Of course, at all times follow your doctor's orders!


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