Comorbidity and sex differences in functional disorders and internalizing disorders

Objective: Functional disorders (FDs) and internalizing disorders (IDs) are highly comorbid. In the current study we estimate comorbidity rates between FDs [fibromyalgia (FM), myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS), and irritable bowel syndrome (IBS)—and IDs—major depressive disorder (MDD) and generalized anxiety disorder (GAD)] by leveraging self-reported diagnostic criteria to estimate comorbidity, sex differences, and the overrepresentation of particular combinations of diagnoses. 
Method: We analyzed data from 107,849 participants (mean age=49.3 (SD=13.0), 58.6% female) of the Lifelines Cohort Study. Lifelines is a prospective population-based cohort study in the northeast of the Netherlands. Current IDs and FDs were assessed according to diagnostic criteria between 2014-2017. We estimated tetrachoric correlations between diagnoses and tested for sex differences. Additionally, we estimated the ratio of observed-to-expected frequency for combinations of diagnoses.
Results: FDs and IDs are highly comorbid (odds ratios: 3.2-12.6) with associations stronger among male participants. All three-, four-, and five-condition diagnoses were observed more frequently than expected by chance. 
Conclusion: High rates of comorbidity between FDs and IDs suggest shared risk factors. Studies that aim to explain sex differences and the overrepresentation of specific combinations of IDs and FDs will be an important contribution to understanding the etiology of these conditions.
Keywords: Functional disorders, Internalizing Disorders, Diagnosis, Comorbidity, Lifelines