Axial spondyloarthritis (AxSpA) is an inflammatory condition of the spine and sacroiliac joints that results in back pain and stiffness. 

Radiographic AxSpA, also known as ankylosing spondylitis (AS), is the more severe form of AxSpA, where damage to the spine is visible on radiographs. 

AxSpA is an important epidemiology concern as the disease burden is expected to increase significantly in the next decade due to improved diagnoses and disease awareness. 

GlobalData’s epidemiology analysis of AS patient characteristics has found large variations in AS patient age distribution across different markets. As such, this is an important area for further research to ensure that AS can be sufficiently treated in the future.

The cause of AxSpA and AS is currently unclear. However, HLA-B27 seropositivity has been linked to the development of the disease. The only other known risk factor for the disease is sex, as men are at a higher risk of developing the disease than women. Age may be another factor to help understand disease onset and progression. In an analysis of AS disease trends in the US, South Korea, and Japan, GlobalData epidemiologists found that the age-specific proportion of diagnosed prevalent cases of AS varies widely by market. 

GlobalData epidemiologists focused on these three markets because they had country-specific epidemiology studies that reported age-specific segmentation data. Figure 1 presents the percentage distribution of diagnosed prevalent cases of AS by age in the three markets.

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In the US, there is a strong peak in the middle-age groups, where more than 50% of the diagnosed prevalent cases of AS occurred in ages 40–59 years. The proportion of diagnosed prevalent cases in the young adult population and elderly are low. In South Korea, the peak occurs in the younger adult population, where 70% of the cases occur in those under 49 years of age. In Japan, the cases are evenly distributed in each age group, with a range of 11–17%. 

It is unclear what is causing the differences in peak age groups. All three markets do share a drop in prevalence that occurs with older age. It has been proposed that this may be caused by underdiagnosis of disease in the older population, misdiagnosis due to the elderly population having been diagnosed much earlier in life but not having the disease registered in the medical records during the study years, or poor survivorship among patients with AS. 

GlobalData epidemiologists estimate that in the combined 11 major pharmaceutical markets (11MM: US, France, Germany, Italy, Spain, UK, Japan, Australia, China, India, and South Korea), there were 4.9 million diagnosed prevalent cases of AS in 2018. GlobalData expects the disease burden to increase to 6.4 million cases by 2028, corresponding to a 10-year growth rate of 30%. 

The rate of growth for AS is even higher than that of type 2 diabetes, but few studies are being done to better understand the epidemiology of this disease. For example, the most widely cited epidemiology literature for AxSpA in the US are studies from the 1970s. Therefore, there is great unmet need for more updated, nationally representative, population-based epidemiology studies of AS to help elucidate its cause and to better understand why there are differences by country.