The incidence of giant cell arteritis (GCA) varies by almost three-fold between various geographic regions, according to a new meta-analysis. There are several possibilities for the mechanism behind this variation, but it is not fully understood.
The epidemiology of GCA has been extensively studied, with changes in age at diagnosis and other characteristics changing over time, but the last major meta-analysis on the topic was conducted in 2008. Researchers at the University of Western Ontario in Canada performed a new review and meta-analysis, including a total of 107 studies with at least 50 patients with GCA.
The overall pooled incidence of GCA was 10.00 cases per 100,000 people over the age of 50. This incidence varied widely based on region, with the highest incidence seen in Scandinavia, at 21.57 cases per 100,000 people over 50.
This was followed by North and South America (10.89), Oceania (7.85), Europe (7.26), the Middle East (5.73), Africa (4.62), and East Asia (0.34). The highest incidence reported in any individual study was seen in Denmark, at 76.6 cases per 100,000 people over 50. The lowest was seen in a study in Hong Kong (0.34).
Scandinavia’s incidence decreased over time more than any other region, by 0.80 cases per 100,000 people per year. This corresponded to a two-thirds reduction in incidence between 1981 and 2017. The global pooled incidence reduced by a rate of 0.41 per 100,000 per year.
The mortality rate associated with GCA appears to be coming down as well. Across the years of publication, there was an overall decrease in mortality of 0.14 per 1,000 people per year (p = .00076).
A regression model showed a significant correlation between latitude and incidence of GCA (p = .0011), though not between latitude and prevalence or mortality. Senior author Dr Janet Pope told the limbic that there was likely not enough statistical power to reveal a correlation between latitude and prevalence; only nine of the studies reported prevalence.
The mechanism connecting latitude and GCA remains unclear. “We do wonder if there is an issue of both genetics and also maybe latitude (environment),” Dr Pope said. Though the connection is not yet proven for GCA, some diseases are more common in colder climates with less sunlight. Dr Pope gave the example of Canada’s high prevalence of multiple sclerosis.
Genetic susceptibility may also play a role. Polymorphisms in genes that express inflammatory mediators such as TNF, adhesion molecules, and IL-18 have been implicated in GCA. Genetic patterns might explain the higher rates seen in North America, where some specific counties with high GCA incidence had large communities with Scandinavian ancestry.
There is also the possibility that the higher rates are simply due to the existence of more advanced healthcare tracking systems in Scandinavian countries. The authors noted that they expect total numbers of GCA patients to begin increasing as the population ages.