Comorbid Conditions Associated with Alopecia Areata: A Systematic Review and Meta-analysis.

BACKGROUND: Alopecia areata (AA) is a complex autoimmune condition resulting in nonscarring hair loss. In recent years, many studies have provided new evidence on comorbid diseases present in patients with AA. However, some studies have conflicting results, and analyses conducting a comprehensive approach are lacking. OBJECTIVE: The aim of our study was to provide an updated systematic review and meta-analysis of medical comorbidities associated with AA. METHODS: We searched PubMed, Embase, and Web of Science for case-control, cross-sectional, and cohort studies investigating medical comorbidities in AA published from inception through 1 February 2023. RESULTS: We screened 3428 abstracts and titles and reviewed 345 full text articles for eligibility. Ultimately, 102 studies were analyzed, comprising 680,823 patients with AA and 72,011,041 healthy controls. Almost all included studies (100 of 102 studies) were of satisfactory to high quality (Newcastle-Ottawa scale score ≥ 4). Among patients with AA, comorbidities with the highest odds ratios (OR) compared with healthy controls and data available from more than one study included vitamin D deficiency (OR 10.13, 95% CI 4.24-24.20), systemic lupus erythematous (OR 5.53, 95% CI 3.31-9.23), vitiligo (OR 5.30, 95% CI 1.86-15.10), metabolic syndrome (OR 5.03, 95% CI 4.18-6.06), and Hashimoto's thyroiditis (OR 4.31, 95% CI 2.51-7.40). AA may be a protective factor for certain disorders, for which the AA group had lower odds compared with healthy controls, such as irritable bowel syndrome (OR 0.38, 95% CI 0.14-0.99) and colorectal cancer (OR 0.61, 95% CI 0.42-0.89). CONCLUSION: These findings corroborate and contextualize the risks across comorbidities for patients with AA. Further work should be done to identify the underlying pathophysiology and understand appropriate screening criteria.