10/26/2021
The incidence of ankylosing spondylitis (AS) did not differ significantly between women and men in a large SAA-funded study of military service members, according to a report in Arthritis Care & Research co-authored by Michael H. Weisman, MD, FACP, a member of SAA’s Medical and Scientific Advisory Board.
The findings challenge the medical community’s long-held understanding that AS is more prevalent in men – a belief that has led some to mischaracterize the condition as a “man’s disease.”
Weisman and colleagues conducted a retrospective study of data from the Stanford Military Data Repository, tracking 728,556 active-duty personnel who underwent clinical screening for chronic back pain between March 2014 and June 2017. Cases of AS were identified using diagnostic codes from medical records.1
Among the study population, 438 individuals – or 0.06% of the population – were diagnosed with AS during the study period, indicating an overall incidence of 27.22 per 100,000. The incidence rate for females was actually slightly higher (31.36) than for males (26.53).1
The biggest gender difference was seen in individuals aged 24 and younger, where the incidence rate for females was nearly double the rate for males (21.43 vs. 11.11 per 100,000).2 Among older age groups, males had a slightly higher incidence rate than females.
While previous research pegged the incidence ratio of men to women with AS at 3:1, the new study suggests the ratio may be closer to 1:1.
“The significance of this study is that women appear to have this disease as often as men,” said Dr. Weisman, Professor of Medicine Emeritus at Cedars-Sinai Medical Center in Los Angeles. “The gender differences are not as strong as previously noted.”
Weisman added that the results were not too surprising to him. “On a daily basis, we see just as many new female AS patients as new male patients,” he said.
The last population-based study of the incidence of AS in the U.S. suggested AS predominantly affects men. This prior study, conducted in Olmsted County, Minnesota between 1980 and 2009, found males were three times more likely to have AS than females.3
What could account for these vastly different findings? “Every study depends on the population you’re studying and the tools you use to find those patients,” Weisman told SAA. While the prior study’s population was largely male and Caucasian, the new study draws from a broader demographic sample. “This is the first time there’s been a study of incidence in a more nationally representative sample. Individuals who join the military are generally young and they better reflect the gender and racial makeup of the U.S. They’re more representative of the population in general.”
The two studies also used different criteria to determine the incidence of AS in their sample populations. The Minnesota study used the stricter New York Criteria for the classification of AS3, which rely more heavily on moderate to severe sacroiliitis seen on x-rays (and various studies have demonstrated that men with AS tend to have more bone changes visible on x-rays than women do). Meanwhile, the new study determined incidence by the number of patients who received a diagnosis from their doctor.
Neither study is perfect, Weisman noted: “There was more possibility of misclassification in our study,” due to the somewhat subjective process clinicians use to make a diagnosis, “but there was more possibility of missing undiagnosed cases in the previous study, where only patients who fulfill classification criteria were included.”
Weisman hopes additional research will shed even more light on the incidence of AS among men and women. “We need much greater understanding about gender differences in axSpA. This study at the very least should open that door to further studies on gender differences.”
SAA proudly funded this research. SAA is a leader in spondyloarthritis (SpA) research, and sponsors a number of studies, including this one, that aim to provide greater understanding of SpA and help find a cure.4
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