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. Multiple studies have investigated seasonal influence on GCA incidence, yielding contradictory results. This meta-analysis aims to determine the pooled seasonal influence on GCA incidence and if it is modulated by latitude. MEDLINE and Scopus databases were searched for articles reporting on incidence and describing seasonal or monthly proportions of GCA. 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 39,829 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 [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 [1.09-1.28]. Overall, there was no significant pooled seasonal pattern; however, a warm-season pattern was driven by studies from southern locations, linking latitude and seasonal influence as environmental factors in GCA.