Causal relationship between endometriosis with infertility and ankylosing spondylitis

Retrospective studies have identified an increased risk of ankylosing spondylitis (AS) in endometriosis patients. The purpose of this study was to investigate the causal relationship between clinical phenotypes of endometriosis and AS using mendelian randomized analysis (MR). MR was performed using data from genome-wide association studies (GWASs). Heterogeneity, pleiotropy and sensitivity analyses were performed to evaluate the robustness of the results by MR Egger and inverse variance weighted (IVW), leave-one-out analysis. IVW, IVW-MRE (inverse variance weighted multiplicative random effects), weighted median and MR Egger were used to explore the relationship between endometriosis and AS. The IVW analysis showed a causal relationship between infertile endometriosis and AS (OR = 0.8334, P = 0.02191), and the same result was observed with IVW-MRE (OR = 0.8334, P = 0.0007933). However, further stratified analysis showed that no matter which statistical method was used, ovarian endometriosis (IVW: OR = 0.1662, P = 0.4986; IVW-MRE: OR = 0.1662, P = 0.4986; MR Egger: OR = − 0.9577, P = 0.2798; Weighted median: OR = 0.2628, P = 0.3452), pelvic peritoneum endometriosis (IVW: OR = 0.4363, P = 0.225; IVW-MRE: OR = 0.4363, P = 0.225, MR Egger: OR = 4.159, P = 0.1705; Weighted median: OR = 0.4112, P = 0.2714), rectovaginal endometriosis (IVW: OR = 0.1365, P = 0.805; IVW-MRE: OR = 0.1365, P = 0.805) there was no causal relationship between endometriosis and AS. This study suggested that patients with infertility endometriosis are at increased risk for AS. This study supports clinicians to pay more attention to the occurrence of AS in endometriosis patients with infertility.