Mechanisms Of Change in Psychotherapy: The Effects of Cognitive Behavioral Therapy and Psychodynamic Therapy in Once - Versus Twice - Weekly Sessions on Outcomes in Depression.

Major depressive disorder (MDD) leads to significant disability, mortality, and economic strain. It ranks fourth globally in terms of disease burden and is expected to rank first in high-income countries by 2030, highlighting an urgent need for effective treatments. Evidence-based psychotherapy methods and antidepressant medications are equally effective treatments. Cognitive Behavioral Therapy (CBT) and Short-Term Psychodynamic Psychotherapy (STPP) are two evidence-based treatments for depression that are widely used. Meta-analyses indicate that CBT and STPP are one average equally effective and superior to no treatment. However, many patients do not respond sufficiently and relapse rates after acute phase treatment are high, estimated at up to 40%.

The guidelines from the UK National Institute for Health and Care Excellence (NICE) underscore the importance of personalized care. They recommend various therapies, including CBT and STPP, tailored to individual needs and preferences of patients with both mild and severe depression. However, the empirical evidence supporting personalized care is limited and insufficient.

Earlier research and theoretical insights suggest two promising strategies to enhance the effectiveness of psychotherapy. First, increasing the frequency of therapy sessions may lead to better treatment outcomes. Second, conducting experimental studies that explore "what works for whom and how" may provide knowledge that can improve efficacy, raise the proportion of responders, and reduce relapse risk. Based on such research, tailoring interventions to individual needs can further optimize treatment success.

2\. Background 2.1 Once versus twice weekly sessions of psychotherapy A meta-regression analysis by Cuijpers et al. found a strong association between session frequency and treatment effect size. Specifically, increasing the frequency from one to two sessions per week-while keeping the total number of sessions constant-led to a substantial increase in effect size (g = 0.45). These findings suggest that concentrating psychotherapy sessions within a shorter time frame may enhance treatment efficacy. However, as Cuijpers et al. (2013) concluded, more research is necessary to validate the robustness of these results. The optimal duration and intensity of psychotherapy remain unclear, and most previous studies have lacked controlled designs.

To our knowledge, only one randomized controlled trial (RCT) has directly compared the effects of once-weekly versus twice-weekly psychotherapy sessions. Bruijniks et al. conducted an RCT investigating whether patients with depression receiving twice-weekly sessions of cognitive behavioral therapy (CBT) or interpersonal therapy (IPT) showed greater symptom improvement than those receiving once-weekly sessions of the same therapies. The study found an effect size of 0.55 in favor of twice-weekly sessions. However, in a two-year follow-up study, Bruijniks et al. reported that while the differences persisted for up to nine months, no significant differences remained at the 24-month follow-up.

The long-term effects of increasing session frequency in patients with MDD remain uncertain. It is likely that different trajectories of improvement emerge between those receiving once versus twice-weekly therapy. A deeper understanding of these trajectories has the potential to refine treatment strategies for MDD, ultimately benefiting patients, clinicians, psychotherapy trainers, healthcare managers, and policymakers. Different trajectories of improvement are likely for patients receiving one session per week compared to those receiving two sessions per week. Understanding these differences has the potential to improve outcome for the treatment of MDD.

2.2 What works for whom? (Moderators/Specific markers) Patients with MDD differ in symptom severity and characteristics regarding comorbidity, personality traits, and interpersonal functioning. The present project is a pre-planned follow-up of the Mechanisms Of change in Psychotherapy - I (MOP I) study. Further knowledge into how specific markers (e.g. various sociodemographic; age, gender, education level, comorbidities, personality traits, and relational competence) impact the treatment outcomes of CBT versus STPP are needed. Further we, aim to replicate the preliminary findings from MOP I, which indicated that by evaluating patients' narratives regarding the perceived causes of their depression and the conditions necessary for recovery, a meaningful match could be established. Specifically, when the scoring of a patient's narrative aligned with the therapeutic approach they received-either psychodynamic therapy (PDT) or cognitive behavioral therapy (CBT)-the likelihood of significant improvement in their depressive symptoms was substantially higher. The results may aid in selecting the optimal treatment modality of choice for patients with a depressive disorder.

2.3 How does psychotherapy work? (Mediators) A mediator of treatment outcome is a specific mechanism of change for a particular form of psychotherapy suggesting how or why symptom change occurs. Psychotherapy is likely to involve multiple mechanisms of change.

The theoretical framework for CBT assumes that changes in cognitive processes and underlying schemas are followed by reduced symptoms and improved functioning and quality of life. Thus, negative automatic thoughts, dysfunctional attitudes, different attributional styles, and cognitive schemas, are all potential mediators of change in CBT. Evidence from research into mechanisms of change specific to CBT suggests that rumination, worry, dysfunctional attitudes and cognitive schemas could be specific mediators (Lemmens et al., 2016).

In STPP, theoretically assumed mediators of change are improved self-understanding, improved emotional awareness, more mature defence mechanisms, and reflective functioning.

However, there is limited empirical evidence supporting the role of these theoretical constructs as mediators in psychotherapy. More research is needed to determine whether these variables mediate the effects of psychotherapy and to what extent they are specific to CBT or STPP.

3\. Aims The primary aim of this project is to investigate whether patients receiving twice-weekly sessions of CBT and STPP experience faster, greater, and sustained improvement in depressive symptoms compared to those receiving weekly sessions. Secondly, we want to examine potential moderators and mediators of change in CBT and STPP.

Session frequency We hypothesize that patients receiving therapy twice weekly, regardless of the therapeutic approach, will show greater improvement in depressive symptoms compared to those receiving therapy once weekly.

Further, we also want to examine if there are differences in each of the two treatment approaches.

With regards to once versus twice-weekly sessions we hypothesize that:

1. Patients receiving twice weekly sessions in CBT will improve more in depressive symptoms than patients receiving one weekly session. 2. Patients receiving twice weekly sessions in STPP will improve more in depressive symptoms than patients receiving one weekly session.

The second set of hypotheses pertains to the analysis of potential moderators and mediators of treatment. More specifically we want to examine the following research question:

Moderators of outcome 3. Can the findings from MOP I be replicated with respect to a questionnaire that demonstrated the ability to match patients to CBT and PDT, resulting in significantly improved treatment outcomes? 4. Are there certain patient characteristics that moderate the outcome of CBT and/or STPP respectively? 5. Are there certain patient characteristics that moderate the outcome of once vs twice weekly session of CBT and/or STPP respectively? 6. If so, which patient characteristic differentially influence outcome in the four treatment conditions? Mediators of change 6. Does improvement occur through different or similar change processes in the two treatment modalities and in once vs twice weekly sessions?

We will test the following theoretically based hypotheses:

In CBT, symptom and functional improvement are mediated by changes in negative automatic thoughts, rumination, dysfunctional attitudes, and cognitive schemas.

In STPP, symptom and functional improvement are mediated by improved self-understanding/insight, emotional awareness, tolerance for emotional distress, and more mature defence mechanisms.

4\. Study design Randomized Controlled study. This is a randomized clinical study. Patients will be randomized to either CBT (once or twice weekly) or STPP (once or twice weekly). Clinical assessments will be conducted at baseline, during therapy, at the end of therapy, and at follow-up investigations 1 and 3 years after treatment termination. The design is single blind, i.e. outcome assessors at treatment termination and further follow-up evaluations will not be aware of assigned study condition.

5\. Materials and methods 5.1. Participants Patients referred to outpatient psychiatric clinic at Nydalen Psychiatric Outpatient Clinic, Oslo University Hospital (OUS) and Vinderen Psychiatric Outpatient Clinic, Diakonhjemmet Hospital due to symptoms of depression are candidates for the MOP-study. A total of 200 patients will be included.

5.2. Inclusion criteria Patients aged between 18-65 years, with MDD according to clinical assessment and a Hamilton Depression Rating Scale (HDRS) \> 14.

Written consent will be obtained from all patients. The participants must be able to speak and understand a Scandinavian language and have the willingness and ability to give informed consent.

5.3. Exclusion criteria Exclusion criteria are a current or past neurological illness, traumatic brain injury, current alcohol and/or substance dependency disorders, psychotic disorders, bipolar disorders, developmental disorders, and IQ \<70.

6.0. Treatment conditions 6.1 Cognitive behavioral therapy In condition 1 the treatment consists of 16 weekly CBT sessions followed by three booster sessions at monthly intervals. The treatment is thus offered within a time frame of 28 weeks. In condition 2, the treatment consists of 8 biweekly CBT sessions followed by three booster sessions at 2-week intervals. The treatment is thus offered within a time frame of 14 weeks. Treatment principles are based on "Cognitive Therapy of Depression" by Aaron Beck (Beck et al., 2024) and "Cognitive Behavior Therapy. Basic and Beyond" by Judith S. Beck (Beck, 2020).

Sessions are structured yet flexible, with active therapist involvement. Patients' complete homework and behavioral experiments. Each session starts with a mood score, reviews previous assignments, sets an agenda, and ends with a summary and new homework.

Therapists use interventions like Socratic questioning, the ABC and Diamond models, challenging automatic thoughts, behavioral activation, and identifying thinking traps. These techniques help patients examine beliefs, modify unhelpful thoughts, and engage in positive activities to break cycles of avoidance and negativity.

Early sessions focus on goal setting, case formulation, and building a therapeutic alliance. Later sessions target symptom reduction, while booster sessions reinforce progress and prevent relapse.

6.2 Short-Term Psychodynamic Psychotherapy (STPP) For patients randomized to STPP, condition 3 consists of 28 weekly sessions (Cregeen, 2018). The treatment is thus offered within a time frame of 28 weeks. Condition 4 consists of 28 biweekly sessions. The treatment is thus offered within a time frame of 14 weeks. The treatment principles are based on "Long-term psychodynamic psychotherapy" by Glen O. Gabbard (Gabbard, 2017), which according to the author also can be applied to shorter and / or time-limited therapies. This basic text outlines central principles of psychodynamic psychotherapy such as the significance of unconscious me

• General Anxiety Disorder [Срок оценки: From baseline to end of treatment at 28 weeks]
• The Work And Social Adjustment Scale (WSAS) [Срок оценки: From enrollment to end of treatment at 28 weeks]
• The Personality Inventory for DSM-5-Brief Form (PID-5-BF)-Adult [Срок оценки: From enrollment to end of treatment at 28 weeks]
• The Level of Personality Functioning Scale-Brief Form (LPFS-BF) [Срок оценки: From enrollment to end of treatment at 28 weeks]
• The Self- Reflection and Insight Scale (SRIS) [Срок оценки: From enrollment to end of treatment at 28 weeks]
• Defense Mechanisms Rating Scales-Self-Report-30 (DMRS-SR-30). [Срок оценки: From enrollment to end of treatment at 28 weeks]
• Tolerance for Emotional Distress and Emotional Clarity (TED and EC) comprise 7 items that assess the patient's tolerance for distress and emotional clarity [Срок оценки: From enrollment to end of therapy at 28 weeks]
• The Meta Cognitive Questionnaire (MCQ-30) [Срок оценки: From enrollment to end of therapy at 28 weeks]
• Positive Beliefs about Rumination(PBRS) [Срок оценки: From enrollment to end of therapy at 28 weeks]
• Dysfunctional Attitude Scale (DAS) [Срок оценки: From enrollment to end of treatment at 28 weeks]
• Ruminative Response Scale (RRS) [Срок оценки: From enrollment to end of therapy at 28 weeks]
• The Working Alliance Inventory (WAI-12-P) [Срок оценки: From two weeks after enrollment and assessed at 8 weeks, 14 weeks and end of treatment 28 weeks]