Causal relationship between ADHD and frozen shoulder: Two-sample Mendelian randomization

To investigate the causal relationship between attention deficit and hyperactivity disorder (ADHD) and frozen shoulder using Mendelian randomization (MR). Data were pooled from large-scale genome wide association studies, and genetic loci that were independent of each other and associated with ADHD and frozen shoulder in people of European ancestry were selected as instrumental variables. Three MR analyses, inverse variance weighting, weighted median and MR-Egger, were used to investigate the causal relationship between ADHD and frozen shoulder. Heterogeneity and multiplicity tests were used, and sensitivity analyses were performed using the "leave-one-out" method to explore the robustness of the results. The inverse variance weighting results showed an OR (95 % CI) of 1.12 (1.00-1.25), P = .046, indicating a causal relationship between ADHD and frozen shoulder. And no heterogeneity and multiplicity were found by the test and sensitivity analysis also showed robust results. The present study used a two-sample MR analysis, and by analyzing and exploring the genetic data, the study showed that ADHD is a risk factor for developing frozen shoulder, and patients with ADHD are more likely to suffer from frozen shoulder.