Evidence that bipolar disorder is the poor outcome fraction of a common developmental phenotype: an 8-year cohort study in young people.

Background: Reported rates of bipolar syndromes are highly variable between studies because of age differences, differences in diagnostic criteria, or restriction of sampling to clinical contacts.
Method: In 1395 adolescents aged 14-17 years, DSM-IV (hypo)manic episodes (manic and hypomanic episodes combined), use of mental health care, and five ordinal subcategories representing the underlying continuous score of (hypo)manic symptoms ('mania symptom scale') were measured at baseline and approximately 1.5, 4 and 10 years later using the Munich-Composite International Diagnostic Interview (DIA-X/M-CIDI).
Results: Incidence rates (IRs) of both (hypo)manic episodes and (hypo)manic symptoms (at least one DSM-IV core symptom) were far higher (714/105 person-years and 1720/10(5) person-years respectively) than traditional estimates. In addition, the risk of developing (hypo)manic episodes was very low after the age of 21 years [hazard ratio (HR) 0.031, 95% confidence interval (CI) 0.0050-0.19], independent of childhood disorders such as attention deficit hyperactivity disorder (ADHD). Most individuals with hypomanic and manic episodes were never in care (87% and 62% respectively) and not presenting co-morbid depressive episodes (69% and 60% respectively). The probability of mental health care increased linearly with the number of symptoms on the mania symptom scale. The incidence of the bipolar categories, in particular at the level of clinical morbidity, was strongly associated with previous childhood disorders and male sex.
Conclusions: This study showed, for the first time, that experiencing (hypo)manic symptoms is a common adolescent phenomenon that infrequently predicts mental health care use. The findings suggest that the onset of bipolar disorder can be elucidated by studying the pathway from non-pathological behavioural expression to dysfunction and need for care.