Neurology -- Correspondence for Hirtz et al., 68 (5) 326-337

Correspondence published:

How common are the "common" neurologic disorders?: Adam I. Kaplin, Montel Williams, Montel Williams MS Foundation, New York, NY (1 April 2007)

Reply from the authors: Deborah G. Hirtz, David J Thurman, Katrina Gwinn-Hardy, Robert Zalutsky (1 April 2007)

How common are the "common" neurologic disorders? 1 April 2007

Adam I. Kaplin,
Johns Hopkins University School of Medicine
733 N. Broadway, Suite G49/Baltimore, MD 21205-2196,
Montel Williams, Montel Williams MS Foundation, New York, NY
Send Correspondence to journal:
Re: How common are the "common" neurologic disorders?

srubin{at}ccapr.com Adam I. Kaplin, et al.

In their article on the prevalence of "common" neurologic disorders, Hirtz et al estimate the incidence of multiple sclerosis (MS) from a meta-analysis of American and European epidemiological studies. [1] The authors highlight the urgent need for accurate figures on several neurologic disorders such as MS.
Because of a lack of adequate and current data, the authors had to use studies published between 1990 and 2005, with some of the data dating back almost a quarter of a century. From this, they estimated the number of U.S. cases of MS to be 266,000. While we appreciate the efforts of the authors from National Institutes of Health and Centers for Disease Control, we believe their methodology and their assumptions are overly conservative.
Epidemiological meta-analyses of MS are rare because of the difficulties in making comparisons of prevalence rates and the variability of surveyed populations from country to country. There is tremendous variability between countries in the prevalence of MS, as well as significant differences in access to medical care, local neurological expertise, availability of new diagnostic procedures and much more. As a result, previous studies have reported prevalence rates that vary by up to 250-fold. [2]
This latest study follows another sponsored by the NIH on the incidence and prevalence of MS in the United States that likely significantly underestimated the prevalence of MS [2] and presented scant information on key subgroups, such as African-Americans, Latinos and children.
There is considerable anecdotal evidence that the prevalence of MS in the United States is much higher. Autopsy studies show that many cases of MS go undiagnosed. The National MS Society has a database of more than 300,000 Americans who have self-identified as having MS. Based on the reviewers� own analyses of the North American studies. [1] The prevalence rate appears to be closer to 600,000. We believe the true figure could be well beyond that.
Meanwhile, mounting evidence that MS has become more common over the past 50 years [3] makes it imperative that more attention and resources be devoted to understanding and stemming this disease that usually starts in young adults.
The longstanding uncertainty over the incidence and prevalence of MS has wide-ranging implications, from the allocation of clinical research resources to prioritizing efforts to better understand the epidemiological trends and risk factors for MS. To accurately map, treat and improve the lives of people with MS requires that we make this devastating disease a national priority, one that could best be championed by the NIH.

References
1. Hirtz D, Thurman DJ, Gwinn-Hardy K, Mohamed M, Chaudhuri AR, Zalutsky R. How common are the "common" neurologic disorders? Neurology 2007;68:326-337.
2. Pugliatti M, Sotgiu S, Rosati G. The worldwide prevalence of multiple sclerosis. Clin Neurol Neurosurg 2002;104:182-191.
3. Orton SM, Herrera BM, Yee IM, et al. Sex ratio of multiple sclerosis in Canada: a longitudinal study. Lancet Neurol 2006;5:932-936. P>Disclosure: The authors report no conflicts of interest.

Reply from the authors 1 April 2007

Deborah G. Hirtz,
NINDS, NIH
6001 Executive Blvd., Rockville, Md. 20892,
David J Thurman, Katrina Gwinn-Hardy, Robert Zalutsky
Send Correspondence to journal:
Re: Reply from the authors

hirtzd{at}ninds.nih.gov Deborah G. Hirtz, et al.

We appreciate the comments of Dr. Kaplin and Mr. Williams regarding the need for accurate estimates of prevalence of neurological disorders. We would like to clarify a few points they raise regarding our estimate in relation to others.
First, our review was not a true metanalysis; we did not pool the original data from the studies we reviewed because of their heterogeneity, instead we simply described the median and range of estimates the studies yielded.
While not ideal, the inherent limitations of extrapolating findings from other countries to the United States were reduced by restricting our review to studies in developed countries where advanced health care resources were generally available. The few recently published North American studies yielded some very high estimates of MS occurrence, perhaps related to the mainly northern European origin of the communities studied, a group associated with higher MS risk not representative of the entire U.S. population.
Data reported directly from voluntary registries are not a reliable basis for estimating either incidence rates or prevalence in a population. Lack of motivation or knowledge among persons eligible to submit their names may lead to under-reporting. Conversely, over-reporting may easily occur without adequate methods to adjust for duplicate reports, eliminate unverifiable or false reports, and account in a timely manner for deaths and out-migration. The possibility of major error--either overestimation or underestimation--is large.
Among all studies we reviewed, the median estimated prevalence of MS was 0.93 per 1000. In comparison, the National Health Interview Survey of 1989-1994 yielded an MS prevalence estimate of 0.85/1,000 population [4] and a study of two counties in Colorado yielded 0.84/1000. [5] We did not include these two population-based studies in our review because Noonan et al [4] relied on self-reported diagnoses and Nelson et al [5] was published before 1990.
Finally, our estimate was limited to definite or probable cases of MS. The earlier NIH estimate of MS prevalence of 1.2 /1,000 [6] also included possible cases, which may account for much of the modest difference between the two estimates.
We strongly support relying on the best possible studies--and not anecdotal evidence or non-population-based data--conducted in the same way over time, to inform us about the true frequency and time trends of diseases with major burden to the US population.
References
4. Noonan CW, Kathman SJ, White MC. Prevalence estimates for MS in the United States and evidence for an increasing trend for women. Neurology 2002;58:136-138.
5. Nelson LM, Hamman RF, Thompson DS, et al., Higher than expected prevalence of multiple sclerosis in northern Colorado. Neuroepidemiology 1986;5:17-28.
6. Anderson DW, Ellenberg JH, Leventhal CM, et al. Revised estimate of theprevalence of multiple sclerosis in the United States. Ann. Neurol 1992;31:333-336.
Disclosure: The authors report no conflicts of interest.