Seasonality and latitude as linked environmental factors in Giant Cell Arteritis incidence: A systematic review and meta-analysis

Giant Cell Arteritis (GCA) incidence is influenced by various unexplained factors. Over the past 60 years, studies have investigated seasonal influence on GCA incidence, yielding contradictory results. This systematic review and meta-analysis aim to determine whether there is a pooled seasonal influence on GCA incidence and if it is modulated by latitude. PubMed and Scopus databases were searched for studies on seasonal influence on GCA incidence without date restriction. Articles reporting incidence and describing seasonal or monthly proportions of GCA were considered for inclusion. The primary outcome measured was the Seasonal Incidence Risk Ratio (SIRR) defined as the incidence of GCA in warm seasons (Spring and Summer) over GCA incidence in cold seasons (Autumn and Winter). Meta-analysis of GCA incidence variations with season was performed on the pooled SIRR. Nineteen articles describing GCA incidence and seasonal variations in 39829 patients were included, 10 studies reported a significant seasonal pattern in the incidence of GCA with seven studies reporting a warm seasonal pattern and three studies reporting a cold seasonal while 9 studies did not report a significant seasonal pattern. The pooled SIRR estimate in this meta-analysis was 1.08 (95% CI [0.99-1.17]). We observed a significant reverse correlation between SIRR and the studies’ location latitude r= -0.595 (p=0.015), additionally, we observed an inflexion latitude line in Lyon, France (45.8), all studies performed southern to that line reported higher proportion of warm seasons cases. The pooled SIRR of studies performed southern to the inflexion line was 1.18 (95% CI [1.09-1.28]). this meta-analysis shows a global pooled trend towards a warm seasonal pattern, mainly driven by southern locations of the included studies, linking two largely discussed environmental factors in GCA, in this case, latitude and seasonal influence which should encourage epidemiological research in southern regions of the globe in order to display globally representative data.