While both disorders cause mood instability and affective reactivity, the phenomenologies of the mood episodes differ. In BPD, mood swings, usually of negative affect, are triggered by interpersonal stressors or perceived stressors, are transient, last from minutes to hours, and are highly dependent on the environment. In bipolar disorder, mood swings are more spontaneous and of longer duration, especially for bipolar I disorder, and there are more extended periods of elation. In addition, in bipolar disorder, acute episodes and symptom-free intervals occur, while in BPD, the affective instability is part of a characteristic pattern of emotional responding. Data suggest that these affective problems persist throughout the life course of the disorder and may be identified by parents of children with BPD as early as infancy.20,21
The mood swings of BPD and bipolar II disorder differ in emotion type as well. Individuals with BPD swing from euthymia to anger, and euthymia is infrequent, while bipolar II disorder affective shifts are from euthymia to elation.22 Shifts triggered by interpersonal stressors in BPD, which often involve rejection or perceived abandonment, are less prevalent in bipolar disorder.23 The differentiation of BPD and rapid cycling bipolar disorder remains problematic, as both disorders involve a high degree of affective instability, and the 2 entities are likely to have significant biological and possibly genetic overlap.24 Findings from these 3 studies suggest that careful detailing of the duration of mood episodes, qualitative emotional shifts, recurrent triggering events, and longitudinal patterns (episodic vs lasting) can help distinguish between BPD and bipolar disorder, although not rapid cycling forms of bipolar disorder.22-24
Impulsivity is behavior that occurs without reflection, is inconsistent with context, and is seen in both BPD and bipolar disorder.25 Differential patterns of impulsivity have been characterized for the depressive and manic phases of bipolar disorder with motor impulsivity (tendency to act on the spur of the moment) specific to mania and non-planning impulsivity (lack of sense of the future) specific to depression. Impulsivity in BPD is also characterized as non-planning.26-28 These data support the premise that BPD may have more symptomatic overlap with the depressive pole of bipolar disorder than with the manic pole.
Similarly, BPD was distinguished from bipolar II disorder by the presence of hostility and differing patterns of impulsivity. Bipolar II disorder showed attentional impulsivity characterized by distractibility and inability to focus on a task, and BPD displayed non-planning impulsiveness. The highest rate of impulsivity was found in populations with comorbid BPD and bipolar II disorder, which suggests that this group may be at the highest risk for self-damaging behaviors.28 This finding argues for the need to make both diagnoses when appropriate.
Clinically, impulsivity is believed to be more episodic in bipolar disorder than in BPD, although inter-episode impulsivity is seen in bipolar disorder when comorbid substance abuse complicates the clinical picture.29 Impulsive acts such as suicidal behavior occur in both disorders, but in bipolar disorder these are predominantly found in the depressive phase, particularly in mixed depressive presentations, and they are related to hopelessness while in BPD, they are often a function of the inability to tolerate distress.30-32
Long-term outcome studies in bipolar disorder and BPD seem to challenge the traditional Axis I/Axis II dichotomy, in which mood disorders are widely thought of as episodic and treatable, whereas personality disorders are considered life-long and treatment refractory. Many cases of bipolar disorder assume a chronic course, with long-term morbidity and substantial inter-episode symptomatology, whereas multiyear follow-up studies of patients with BPD have found that most people eventually stop meeting threshold criteria for the disorder.5,33,34 However, there appears to be a core subset of BPD symptoms, especially in the affective and interpersonal realms, that persist even after the more dramatic impulsive or demanding behaviors have subsided. There also appears to be a subset of remission-resistant BPD patients who continue to show poor judgment and high treatment utilization.35,36
Findings from prospective studies on the interactive effects of both disorders indicate that comorbid bipolar disorder had no effect on the clinical course of BPD with respect to functional outcome, remission rates, or number of hospitalizations or other treatment utilization except for mood-stabilizer medication.9