Bridges Between Bipolar and Borderline Personality Disorders: Clarifying Comorbidity Through the Analysis of the Complex Network of Connections Between Symptoms

Background: The reasons for the high rates of comorbidity between Bipolar (BD) and Borderline Personality (BPD) disorders remain elusive, due to the vast array of shared clinical features, which makes the differential diagnosis difficult. This constitutes an obstacle to provide quality of care services, which results in detrimental effects on individual’s mental health. The analysis of the complex network of connections between symptoms of both disorders is a promising pathway to uncover the mechanisms underlying the comorbidity structure of both disorders. Goals: In this study, we explored the comorbidity network that represents the connections between 32 DSM-5 symptoms of BD and BPD in order to (1) compare its modular structure (i.e., the constitution of cohesive subgroups of symptoms within the comorbidity network) with the nosographic proposal of the DSM-5; (2) distinguish between the different roles those symptoms have in the comorbidity network and identify the symptoms that overlap and bridge both disorders, as well as the distinctive symptoms that better discriminate them; (3) identify the most central symptoms and those with the highest impact on the strength or on the structure of the connections on the comorbidity network; and (4) analyze the association between symptoms roles and their centrality and impact. Methods: An epidemiological sample from the National Comorbidity Survey: Baseline (NCS) was analyzed. Data regarding bipolar and borderline personality symptoms were collected through the Composite Network International Diagnostic Interview (CIDI). The network of complex interactions between symptoms was estimated using the Ising model with the L1-regularization penalty (EBIC) and the nosographic structure was detailed with Moduland algorithms. Results: Data regarding an overall sample of 7556 individuals was analyzed (48.6% male, M age = 33.400 years, SD age = 10.447). Results revealed differences between the modular structure of the comorbidity network and the DSM-5 nosographic proposal, namely about unstable relationships and substance abuse, that were assigned to the module constituted by symptoms of manic episode (ME). Symptoms such as money spending and sexual indiscretions, that overlap ME and BPD in the DSM-5, were assigned to the ME module. Psychomotor agitation, which overlaps depressive episode (DE) and ME in the DSM-5, was assigned to the DE module. Additionally, emptiness and worthlessness were identified as bridge symptoms between DE and BPD; anger and substance abuse between ME and BPD; and unstable relationships and psychomotor agitation between DE and ME. Fatigue was the most distinctive symptom of the DE module, unstable relationships of the ME module, and anger of the BPD module. Strength centrality ( r = .61, 95%CI [.33, .79], p < .001) and modular bridgeness ( r = .64, 95%CI [.38, .81], p < .001) were positively correlated with the impact on the structure of the comorbidity network; and modular overlap was negatively correlated with the impact on the strength ( r = -.43, 95%CI [-.10, -.68], p = .01) of its connections. Discussion: Results suggest a similar structure of the comorbidity network to the nosographic proposal of DSM-5. Distinctive and bridge symptoms were identified for each disorder which might help with the differential diagnosis. It can also help us to unveil possible development pathways of comorbidity that might promote an improvement in psychological treatments. Keywords: Bipolar disorder, Borderline personality disorder, Network analysis, Comorbidity.