Familial coaggregation and shared familiality of functional and internalizing disorders in the Lifelines cohort.

Background: Functional disorders (FDs) are characterized by persistent somatic symptoms and are highly comorbid with internalizing disorders (IDs). To provide much-needed insight into FD etiology, we evaluated FD and ID familial coaggregation and shared familiality.

Methods: Lifelines is a three-generation cohort study, which assessed three FDs (myalgic encephalomyelitis/chronic fatigue syndrome [ME/CFS], irritable bowel syndrome [IBS], and fibromyalgia [FM]) and six IDs (major depressive disorder [MDD], dysthymia [DYS], generalized anxiety disorder [GAD], agoraphobia [AGPH], social phobia [SPH], and panic disorder [PD]) according to diagnostic criteria. Based on 153,803 individuals, including 90,397 with a first-degree relative in Lifelines, we calculated recurrence risk ratios (λs) and tetrachoric correlations to evaluate familial aggregation and coaggregation of these disorders in first-degree relatives. We then estimated their familiality and familial correlations.

Results: Familial aggregation was observed across disorders, with λ ranging from 1.45 to 2.23 within disorders and from 1.17 to 1.94 across disorders. Familiality estimates ranged from 22% (95% confidence interval [CI]: 16-29) for IBS to 42% (95% CI: 33-50) for ME/CFS. Familial correlations ranged from +0.37 (95% CI: 0.24-0.51) between FM and AGPH to +0.97 (95% CI: 0.80-1) between ME/CFS and FM. The highest familial correlation between an ID and FD was +0.83 (95% CI: 0.66-0.99) for MDD and ME/CFS.

Conclusions: There is a clear familial component to FDs, which is partially shared with IDs. This suggests that IDs and FDs share both genetic and family-environmental risk factors. Of the FDs, ME/CFS is most closely related to IDs.