Background: Conflicting results in genetic studies of bipolar disorders may be due to the clinical and genetic heterogeneity of the disease. Age at onset of bipolar disorders may be a key indicator for identifying more homogeneous clinical subtypes. We tested whether early onset and late onset bipolar illness represent two different forms of bipolar illness in terms of clinical features, comorbidity and familial risk.
Methods: Among a consecutively recruited sample of 210 bipolar patients, we compared early onset (n=58) and late onset (n=39) bipolar patients; the cut-off points were age at onset before 18 years and after 40 years for the two subgroups. The subgroups were compared by independent t tests and a contingency table by raw chi-square test. Morbid risk among first-degree relatives was measured by the survival analysis method.
Results: The early onset group had the most severe form of bipolar disorder with more psychotic features (P=0.03), more mixed episodes (P=0.01), greater comorbidity with panic disorder (P=0.01) and poorer prophylactic lithium response (P=0.04). First degree relatives of early onset patients also had a higher risk of affective disorders (P=0.0002), and exhibit the more severe phenotype, i.e bipolar disorder.
Conclusion: Our data suggest that early and late onset bipolar disorders differ in clinical expression and familial risk and may therefore be considered to be different subforms of manic-depressive illness.