Birth order and its association with the onset of chronic fatigue syndrome

Human Biology, Aug 2002 by Brimacombe, Michael, Helmer, Drew A, Natelson, Benjamin H

Abstract Chronic fatigue syndrome (CFS) is a medically unexplained illness that is diagnosed on the basis of a clinical case definition; so it probably is an illness with multiple causes producing the same clinical picture. One way of dealing with this heterogeneity is to stratify patients based on illness onset. We hypothesized that either the whole group of CFS patients or that group which developed CFS gradually would show a relation with birth order, while patients who developed CFS suddenly, probably due to a viral illness, would not show such a relation. We hypothesized the birth order effect in the gradual onset group because those patients have more psychological problems, and birth order effects have been shown for psychological characteristics. We compared birth order in our CFS patients to that in a comparison group derived from U.S. demographic data. We found a tendency that did not reach formal statistical significance for a birth order effect in the gradual onset group, but not in either the sudden onset or combined total group. However, the birth order effect we found was due to relatively increased rates of CFS in second-born children; prior birth order studies of personality characteristics have found such effects to be skewed toward first-born children. Thus, our data do support a birth order effect in a subset of patients with CFS. The results of this study should encourage a larger multicenter study to further explore and understand this relation.

KEY WORDS: CHRONIC FATIGUE SYNDROME, ILLNESS ONSET, BIRTH ORDER

Chronic fatigue syndrome (CFS) is a medically unexplained illness characterized by new onset of fatigue that produces some degree of disability as well as a wide variety of accompanying symptoms. Although many hypotheses have been posited regarding the cause of CFS (Natelson 2001), none has proven to be definitive.

These hypotheses range from purely medical (i.e., a viral cause) to purely psychological (i.e., the problem is one of amplification of commonly occurring symptoms).

Some years ago, a CFS patient Internet bulletin board was filled with electronic mail communications concerning the idea that there was a potential birth order effect in CFS. To our knowledge, no one has actually tested to see if such an order effect does exist. We can examine the existence of a birth order effect using our population of referred CFS patients, subject to the restrictions of the existing data and in relation to birth order in the general U.S. population.

Birth order studies have been frequently used to look at psychological or psychosocial variables. Thus, the issue of whether CFS is a medical illness or a psychological state is critical in designing a birth order study. If CFS is simply an overreaction to symptoms commonly occurring in some people, then we might expect to find a birth order effect for the whole group of CFS patients. But because CFS is diagnosed based on a clinical case definition, it probably is not a single disorder but instead may be comprised of a number of disorders with different causes but a similar clinical picture. One technique recommended to reduce such heterogeneity is stratification into groups (Fukuda et al. 1994).

One source of variability among patients with CFS has to do with illness onset. Many patients report that their illness develops to its full degree in 1-2 days with an acute influenza-like onset peaking in early winter (Zhang et al. 2000). In contrast, other patients report that it takes weeks to months for their illness to develop completely; this gradual onset group has significantly more comorbid psychiatric illness than the sudden onset group (DeLuca et al. 1997).

We would not expect to find a birth order effect in individuals whose CFS follows an apparent viral infection. Alternatively, if psychological factors play a role in the gradual onset of CFS, one might see a birth order effect in this subgroup of patients. The purpose of this paper is to communicate evidence supporting this possibility.

Materials and Methods

Our plan was to compare birth order data drawn from our own patient sample with that from a comparison population taken from the 1998 U.S. National Vital Statistics database. One of the reviewers of the original version of this manuscript had a concern that the birth order distribution using the 1998 database might differ from that in previous years. One suggestion to bypass this potential problem was to use an alternative reference population of birth order-that is, one based on the birth years of the sample CFS patients. Our sample varies in age from 27 to 52 with an interquartile range of 30 to 44. The birth years associated with that sample run from the early 1950s to the early 1970s. National Vital Statistics for that time period are far less accurate than current values. Birth order distributions for Caucasian women do exist for the years 1950, 1955, and 1959 onwards. This time frame, unfortunately, leaves out several sample birth years of interest. In addition, the existing recorded values for all the sample birth years are only estimates based on a 50% sample of births (the 1967 data was based on a 20%-50% sample). As such, they display random fluctuation over time due to sampling error and not necessarily due to actual changes in the underlying birth order distribution itself. Because of these shortcomings, we did not feel that use of these numbers would address the issue of potential temporal fluctuation in the actual birth order distribution for our sample.