The underlying genetic heterogeneity in Bipolar Disorder (BD) has led to the search of potential markers associated with subtypes of the disorder; as such, age at onset (AAO) could be considered as a factor that defines more genetically homogeneous subgroups.

To analyze the modal distribution of a BD population according to the AAO of the disorder, as well as the clinical characteristics related to the distribution findings.

357 patients with a BD diagnosis were included in the study. AAO was defined as the age when the patient first met DSM-IV criteria for a major mood episode. Using an admixture analysis, patients were distributed among different parameters; and parametric analyses were conducted in order to compare the demographic and clinical characteristics between groups.

The model that best fit the observed distribution was a mixture of three Gaussian distributions (mean ± SD): 17±3.7 years, 26±8.8 years, and 35.5±12.54 years. Statistically significant differences were found with respect to social status, course of illness, suicidal behavior, rapid cycling, medical co-morbidities and lithium response (p<0.05).

Our results support the existence of a tri-modal distribution in BD defined by AAO, each one with different clinical characteristics; and suggest that early-onset and late-onset BD reflect an underlying genetic heterogeneity in bipolar disorder, being early-onset BD implicitly a more serious subtype of disorder.