American Journal of Psychiatry | A Nationwide Study on the Risk of Autoimmune Diseases in Individuals With a Personal or a Family History of Schizophrenia and Related Psychosis

Previous research has found an increased risk of schizophrenia
in individuals with autoimmune diseases and smaller but significant
associations with a family history of autoimmune diseases. This study
investigates, for the first time, the association between schizophrenia
and subsequent autoimmune diseases (the reverse temporality) and also
considers the effect of infections, a possible risk factor for both
schizophrenia and autoimmune diseases.

Danish nationwide registers were linked to establish a cohort of
3.83 million people, identifying 39,364 individuals with
schizophrenia-like psychosis and 142,328 individuals with autoimmune
disease. Data were analyzed using survival analysis and adjusted for
calendar year, age, and sex.

Individuals with schizophrenia had an elevated risk of
subsequent autoimmune diseases, with an incidence rate ratio of 1.53
(95% CI=1.46–1.62). Among persons without hospital contacts for
infections, the effect of having schizophrenia was smaller, with an
increased incidence rate ratio of 1.32 (95% CI=1.22–1.43) for autoimmune
diseases. For individuals with schizophrenia as well as hospital
contacts for infections, the combined risk of autoimmune diseases was
2.70 (95% CI=2.51–2.89). A family history of schizophrenia slightly
increased the overall risk of developing autoimmune diseases (incidence
rate ratio=1.06, 95% CI=1.02–1.09). Autoimmune diseases developed
subsequently in 3.6% of people with schizophrenia, and 3.1% of people
with autoimmune diseases had a family history of schizophrenia.

The increased risk of subsequent autoimmune diseases in
individuals with schizophrenia may involve neuropsychiatric
manifestations from the undiagnosed autoimmune disease, medical
treatment or lifestyle associated with schizophrenia, or common
etiological mechanisms, such as infections and shared genetic factors.

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