The Prevalence of SM in the U.S.

 

by Marcy Speer Phd, Duke University

Syringomyelia is considered a rare condition, and early estimates suggested that approximately 21,000 individuals were affected with this condition. Given the current estimates on U.S. population size from the recent census, this figure would suggest that about 1/18,000 Americans have syringomyelia. However, this number probably underestimates the true frequency of syringomyelia. The “perfect” study to estimate the frequency of syringomyelia would involve testing a large group of people (probably more than 10,000) selected at random to see how many have syringomyelia. This type of study has never been done, and so we have tried to estimate the prevalence of syringomyelia by pulling information from the scientific and medical literature.

We started by listing the common causes of syringomyelia. Common causes include syringomyelia associated with spina bifida, post-traumatic syringomyelia, spinal cord tumors, arachnoiditis, and idiopathic syringomyelia. By searching the literature, we found estimates for how frequently each of these conditions occurs in the population and how frequently syringomyelia is associated with them (Table 1a). We looked for upper and lower estimates of how frequently these conditions occurred to get a range of numbers. The most common cause of syringomyelia, however, is related to the hindbrain (for example, those associated with Chiari type 1 malformation, basilar invagination, etc.) and there is no available estimate of the prevalence of Chiari type 1 malformation or other hindbrain-related abnormalities available. Therefore, we utilized data published by Dr. B. Williams in 1997 and determined that about 70% of syringomyelia is due to hindbrain-related abnormality, and then we used algebra to calculate backwards (Table 1b). By combining these numbers, we estimate that the prevalence of syringomyelia in the U.S. population is between 141,420 – 209,187 people; in other words, about 1 in 1,300 to 1 in 1,900 Americans have syringomyelia.

This estimate is about 10 times higher than the earlier estimate. Could this be too high? Yes, there are several potential problems with these calculations that may cause our estimates to be too high. For instance, any of the estimates of the frequency of the causes of syringomyelia that we used to generate our overall number could be incorrect. And, some physicians may argue that syringomyelia associated with spina bifida or spinal cord injury or other causes should not be included in these numbers. The most problematic contribution to these numbers comes from our estimate of the frequency of hindbrain related abnormalities, likely the most common cause of syringomyelia. Since no good estimate of the prevalence of Chiari type 1 malformation or other types of hindbrain-related abnormality are available, the numbers utilized here could be inaccurate. On the other hand, if the relationship between fibromyalgia and/or chronic fatigue syndrome with Chiari type 1 malformation proves to be true, these numbers could be higher.

If Regis Philbin were to ask, “Is this your final answer?”, we would have to say emphatically ‘no!.’ We can say with certainty that more than 18,000 individuals in the U.S. have syringomyelia, but how much higher is not well established. Time and added insight from research will continue to refine our estimates of the prevalence of syringomyelia, but we feel this is a good starting point based on available information.

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