Abstract
Background: People with severe mental disorders have in numerous studies been shown to have an excess somatic comorbidity compared to the general population. Furthermore, individuals with somatic diseases also have an increased comorbidity of mental disorders. A possible contributing factor could be shared genetic risk factors explaining part of the excess co-occurrence of somatic diseases and mental disorders. Previous studies have mainly investigated the co-occurrence in the individual, which makes it difficult to disentangle the genetic contribution vs the influence of medication and lifestyle factors. Consequently, we aimed to utilize the extensive nationwide Danish registers to study if a family history with somatic diseases increases the risk of severe mental disorders.
Methods: We utilized the extensive nationwide Danish registers to identify any somatic diseases in the parents. The outcome was diagnosis in the offspring with severe mental disorders (schizophrenia, bipolar disorder, and unipolar disorder). We compared the incidence of getting one of the severe mental disorders if there was a somatic disorder in the parents compared to those without parents with somatic diseases. The main outcome measures were incidence rate ratios (IRR) estimated with Poisson regression analysis. All analyses were adjusted for the age, calendar time, and gender.
Results: We found a higher risk for schizophrenia with an IRR of 1.28 (95% CI: 1.23–1.32) if a parent had a somatic disorder, compared to individuals whose parents did not have a somatic disease. It was slightly attenuated when we additionally adjusted for socioeconomic status in the parents (highest education), psychiatric disorder in the parents, and somatic disorders in the proband, IRR = 1.11 (95% CI: 1.07–1.15). The risk for unipolar depression was increased with an IRR of 1.23 (95% CI: 1.21–1.25), also with a slight attenuation in the fully adjusted model. The risk for bipolar disorder was only slightly increased with an IRR of 1.11 (95% CI: 1.06 - 1.17), with no significant effect after full adjustment.
Conclusion: Severe psychiatric disorders are very heritable disorders clearly demonstrated in twin studies, genetic studies, and large scale population based studies. The results presented in this study could suggest that there is also heritability between severe somatic diseases and severe psychiatric disorders. This could add to the explanation of the excess somatic comorbidity leading to early mortality found in persons with severe mental disorder.