OBJECTIVE:

The boundary between borderline personality disorder (BPD) and bipolar disorder (BD) is a controversial subject. Clinically, it can be difficult to diagnose patients who present with both affective instability and impulsivity. This paper reviews concepts and challenges related to the overlap of these disorders.

METHODS:

A Medline search was conducted, using the key words borderline personality disorder, bipolar disorder, affective disorder, and personality disorder. Reference lists from articles generated were also used. Publications from the last 20 years were considered.

RESULTS:

Studies demonstrate a greater cooccurrence between these 2 disorders than between BPD and other Axis I disorders or between BD and other Axis II disorders. Some authors suggest that many patients diagnosed with BPD are better described as having BD, that the bipolar classification is too narrow, or that BPD should be considered a variant of affective disorders. Others present evidence supporting BPD as a valid construct. Hypotheses about the relation between the 2 disorders and suggestions for clinical practice are offered.

CONCLUSIONS:

There appears to be sufficient evidence to consider BPD to be a valid diagnosis. Both disorders apply to heterogeneous populations, and their characteristics require further clarification. In diagnostically challenging situations, careful consideration of a patient's longitudinal history is essential. Future research will be important to ensure that our diagnostic classifications reflect clinically useful entities.