A population-based register study of the association between schizophreniaand rheumatoid arthritis

The authors investigated the association between schizophrenia and rheumato id arthritis. The design is a population-based case-control and follow-up study. The case s were 20495 patients admitted for schizophrenia and registered in the Dani sh Psychiatric Case Register. A total of 204912 persons matched on age and gender and chosen from the general population served as controls. Admission s for rheumatoid arthritis and other non-autoimmune, musculoskeletal disord ers were checked in the Danish National Patient Register. Odds ratios and r elative risks were estimated by the Mantel-Haenszel estimator and Poisson r egression. The same analyses were carried out for 10242 patients with bipol ar affective disorder and 102420 controls for comparison. Individuals with schizophrenia had a reduced risk for being admitted with rheumatoid arthrit is [odds ratio 0.44 (CI 0.24-0.81)] in the case-control study. A similar re sult was found in the follow-up study, but the incidence of the degenerativ e disorders in the musculoskeletal system was equally significantly lower i n both studies. The incidence of rheumatoid arthritis among the bipolar pat ients was the same as in the control population. The negative association b etween schizophrenia and rheumatoid arthritis may thus be the result of asc ertainment bias and selection due to under reporting and treatment of the m edical illness. Clinicians are reminded of the difficulties in detecting me dical illness among individuals with schizophrenia. (C) 1999 Elsevier Scien ce B.V. All rights reserved.