"Vulnerability can breed strength": The role of borderline personality disorder severity in movement synchrony among patients with major depressive disorder.

Objective: Patients with Major depressive disorder (MDD) and a comorbid Borderline personality disorder (BPD) show a poorer prognosis for MDD compared to patients without BPD. Little is known about the therapeutic processes underlying this prognosis. The goal of the present study was to investigate whether patients with more severe BPD symptoms experience less strengthening in patient-therapist movement synchrony (MS) throughout treatment, and whether less strengthening in MS is associated with less effective sessions. Method: Ninety-five patients participating in a randomized control trial were assessed for BPD (N = 9) using the Structured Interview for DSM-IV Personality. 1367 video-taped sessions were analyzed for MS using Motion Energy Analysis. Session effectiveness was evaluated following each session using a one-item scale taken from the Helping Skills Measure. Multilevel models were used to assess whether BPD severity predicted MS throughout treatment, and whether MS predicted session effectiveness. Results: Patients with more severe BPD symptoms experienced greater strengthening in MS throughout treatment. Less MS strengthening was associated with less session effectiveness. Conclusion: These findings indicate unique therapeutic processes that characterize individuals with more severe BPD symptoms. Shedding light on these unique processes has the potential to contribute to the personalization of MDD treatment for patients with BPD.