Background: The relationship between DSM-IV-TR borderline personality disorder (BPD) and bipolar disorders, especially bipolar II disorder (BP-II), is still unclear. Many recent reviews on this topic have come to opposite or different conclusions.

Study aim: The aim was to test the association between hypomania symptoms and BPD traits, as hypomania is the defining feature of BP-II in DSM-IV-TR.

Methods: During follow-up visits in a private practice, consecutive 138 remitted BP-II outpatients were re-diagnosed by a mood disorder specialist psychiatrist, using the Structured Clinical Interview for DSM-IV (as modified by Benazzi and Akiskal for better probing hypomania). Soon after, patients self-assessed (blind to interviewer) the SCID-II Personality Questionnaire for BPD. Associations and confounding were tested by logistic regression, between each criteria symptom of hypomania (apart from "racing thoughts" and "distractibility", not assessed as probing focused mainly on behavioral, observable signs), and the entire set of BPD traits. Multivariate regression was also used to jointly regress the entire set of hypomanic symptoms on the entire set of BPD traits.

Results: Mean (SD) age was 39.0 (9.8) years, females were 76.3%. Frequency of BPD traits ranged between 17% and 66% (e.g. impulsivity trait 41%, affective instability trait 63%), mean (SD) number of traits was 4.2 (2.3). The most common episodic hypomanic symptoms were elevated mood (91%) and overactivity (93%); frequency of excessive risky, impulsive activities (impulsivity) was 62%. By logistic regression the only significant association was between the episodic impulsivity of hypomania and the trait impulsivity of BPD. Multivariate regression of the entire set of hypomanic symptoms jointly regressed on the entire set of BPD traits was not statistically significant.

Discussion: The core feature of BP-II, i.e. hypomania, does not seem to have a close relationship with BDP traits in the study setting, partly running against a strong association between BPD and BP-II and a bipolar spectrum nature of BPD.