Enhancing treatment outcomes for comorbid depression and personality disorders: Investigating the effect of psychotherapy dosage and exploring integrated treatment approaches

Depression is a major global health burden and frequently co-occurs with personality disorders (PD), resulting in more severe symptoms, poorer treatment response, and higher relapse risk. Despite the clinical relevance of this comorbidity, research focused specifically on patients with both conditions remains limited. This dissertation examines how PD influences the presentation and treatment of depression and evaluates two strategies for improving outcomes: increasing psychotherapy dosage and exploring integrated treatments, such as Short-Term Psychodynamic Supportive Psychotherapy (SPSP) and Schema Therapy (ST), that address both depressive symptoms and underlying personality patterns. Main findings Impact of PD on depression and treatment The first part examines how comorbid PD affects depressive symptoms and therapy response. Using baseline data from two large randomised trials (PsyDos and Step-D), Chapter 2 shows that patients with PD have more severe depression but unexpectedly report better health-related quality of life, likely due to limitations of the EQ-5D in capturing personality pathology. Chapter 3 shows that although psychotherapy reduces depressive symptoms in both PD and non-PD groups, patients with PD remain more symptomatic at post-treatment and follow-up, placing them at higher risk of relapse or recurrence. Psychotherapy dosage for depression with PD The second part presents findings from the PsyDos study, a large randomised trial evaluating psychotherapy dosage in patients with depression and PD. Patients were double-randomised to 25 or 50 sessions over one year and to SPSP or ST. Chapter 5 shows that the higher dosage—50 sessions starting twice weekly—led to faster improvements in depressive and personality-related outcomes. While treatment gains persisted during the year after therapy, dosage-related differences were no longer significant at 18 and 24 months, possibly due to additional non-protocol treatments. Preliminary 48-month results suggest that the 50-session format may still be more cost-effective in the long term despite higher initial resource use. Integrated treatment for depression and PD The third part explores integrated approaches that treat depression and PD as interconnected. Chapter 7 highlights psychodynamic and schema-based frameworks as promising models. In PsyDos, both SPSP and ST produced meaningful improvements in depressive and personality-related outcomes, supporting their value as integrated alternatives to depression-focused treatments. Balancing individual and system-level needs Although 50 sessions may accelerate recovery, system-level considerations are essential. Higher dosage may be cost-effective over time but can strain limited mental healthcare resources. Conversely, insufficient treatment increases the risk of chronicity and relapse, potentially raising long-term costs. Identifying which patients benefit most from each dosage is crucial to aligning clinical gains with resource availability. Recommendations for future research Future studies should: 1. disentangle the effects of session frequency versus total session number and identify patient subgroups most responsive to each dosage 2. evaluate cost-effectiveness using mental-health-specific quality-of-life measures 3. compare integrated treatments (e.g., SPSP, ST) with depression-focused approaches 4. examine the added value of antidepressants combined with integrated therapies Recommendations for clinical practice and policy Clinicians and policymakers should consider: 1. offering 50 sessions within one year to achieve faster improvement and potentially reduce long-term societal costs 2. use SPSP and ST as alternatives, particularly after insufficient response to depression-focused therapy 3. implement a structured post-treatment follow-up, including a six-month evaluation, to reduce relapse risk and support efficient use of healthcare resources. Conclusion This dissertation highlights the urgent need to improve treatment for patients with depression and PD, who commonly experience more severe and recurrent symptoms. Higher psychotherapy dosage yields faster and initially greater improvements, while integrated approaches such as SPSP and ST show promise as effective alternatives to traditional treatments. These findings contribute to optimising care for this complex patient group.