The association and aetiological overlap between subsyndromal hypomania, psychiatric and neurodevelopmental conditions and symptoms: a twin study

Background: Subsyndromal hypomanic symptoms are fairly common in the general population but are linked to psychiatric and neurodevelopmental conditions. However, the genetic and environmental origins of these associations are unclear. This twin study examined the phenotypic and aetiological associations between subsyndromal hypomania with psychiatric and neurodevelopmental conditions and related symptoms (e.g., self-harm).Methods: Participants were 4,932 twin pairs from the Child and Adolescent Twin Study in Sweden. Hypomanic symptoms were assessed using the parent-rated Mood Disorders Questionnaire when the twins were aged 18. Specialist diagnoses of 14 conditions and symptoms were ascertained from Swedish population registries. Phenotypic associations between hypomania and these conditions/symptoms were investigated and their aetiological overlap examined using the twin method.Outcomes/findings: Subsyndromal hypomanic symptoms were significantly associated with the diagnosis of all 14 conditions/symptoms. The highest odds was for psychotic disorders (odds ratio [OR]=1·48, 95% confidence intervals [CI]=1·33-1·64, p<0·001) and the lowest was for eating disorders (other than anorexia) (OR=1·15, 95% CI 1·08-1·22, p<0·001). The genetic correlations between subsyndromal hypomania and these diagnoses ranged from 0·12 (95% CI 0·04-0·33) for eating disorders (other than anorexia) to 0·58 (95% CI 0·28-1·00) for drug misuse. The highest non-shared environmental correlation was for psychotic disorders (0·52, 95% CI -0·02-0·92) and lowest was for body dissatisfaction (0·04, 95% CI -0·01-0·08). For bipolar disorder, psychotic disorders and Attention Deficit Hyperactivity/Disorder, genetic and non-shared environmental correlations with subsyndromal hypomania were of a similar magnitude.Interpretation: The association between subsyndromal hypomania and the diagnosis of multiple psychiatric phenotypes highlights its important role in the developmental pathway to clinical disorders, its complex origins and that it may represent a quantitative trait for bipolar disorder, and various psychiatric phenotypes.