A three generations nation-wide population study of family load estimates in bipolar disorder with different age at onset

ObjectivesThis nation-wide register-based study investigates how often bipolar disorder (BD) occurs in affected families compared to control families by estimating the family load as a random effect; this effect measures the degree of dependence among family members in relation to BD. Furthermore, the study addresses the impact of certain risk factors, namely, sex, age at onset of BD, degree of urbanization, year of birth, month of birth, and maternal and paternal age at birth.MethodA total of N=1204 children and adolescent psychiatric cases born between 1950 and 1997 and registered in the Danish Central Psychiatric Register (DPCR) developed BD before the age of 58 years. N=3553 controls without any psychiatric diagnosis were matched for age, gender, and region of residence. Psychiatric diagnoses were also obtained on the relatives, e.g. parents, siblings, and offspring as a part of the Danish Three Generation Study (3GS). A family component was obtained by using different regression models.ResultsFamilial factors accounted for 20% of the variation in disease outcome when controlling for year and month of birth, sex, and degree of urbanization. Only female sex was associated with an increased hazard ratio of BD. Also having a mother, father or a sibling with the disorder was proven to be a significant risk factor. Furthermore, case relatives did not develop BD earlier than control relatives.ConclusionThese findings based on a very large and representative dataset provide further and very solid evidence for the high family aggregation of BD.