The purpose of this study is to evaluate whether cognitive-behavioral therapy enriched with strategies from emotion regulation training leads to better improvement in somatic symptoms and comorbid problems than cognitive-behavioral therapy alone.

Somatic symptoms not caused by a known biomedical condition ("somatoform disorders") are highly prevalent, involve a high risk of chronicity, are one of the major reasons for doctor visits, and are a tremendous burden for health care systems. Despite the economic relevance, research activities are disproportionately low. The only intervention with an evidence grade I is cognitive-behavioral therapy (CBT). However, average effect sizes for CBT in somatization syndromes are only moderate (Cohen's d < 0.5), and more powerful interventions are needed.

There is convincing evidence that patients with somatoform disorders have emotion regulation deficits, which are not addressed by current CBT approaches. Based on positive results of a small pilot study it is expected that enriching CBT programs with emotion regulation techniques improves treatment outcome. The primary goal of the proposed project is therefore to evaluate this hypothesis in a randomized design.

Patients will be primarily recruited via referrals by primary care doctors. After a screening phase a baseline assessments with different self- and clinician-rating scales (see Outcome Measures) follows. If participants fulfill the eligibility criteria they will be randomized to one of the two study arms: cognitive-behavioral therapy vs. cognitive-behavioral therapy enriched with emotion regulation strategies. After every therapy session patients will be screened in regard to aspects of therapeutic alliance, adverse events, and symptom intensity/annoyance. The post assessment takes place after the 20th session and a follow-up is planned six months after post treatment.

Different methods will be applied to prevent bias and to assure a high quality level of the current study. Data handling, data monitoring and statistical analyses will be supervised by the Coordinating Center for Clinical Trials (KKS) of Philipps-University of Marburg; data quality and safety principles will be applied. Additionally an independent Data Safety Monitoring Board will be nominated. The study center will visit all study sites regularly to verify correct procedures, data sampling, and data management. Randomisation occurs and is controlled centrally through the randomisation's central office in KKS Marburg. Furthermore the current study constitutes a single-blinded trial. Assessment interviews are conducted and analyzed by people blinded to the treatment condition. Additionally, treatments are manualized, and therapists receive an intense training. Treatment fidelity/integrity is analyzed with rating schemes for 5% randomly selected videotaped treatment sessions. Allowed additional treatments during study inclusion are thoroughly monitored and analyzed.

The sample size calculation is based on the primary outcome variable "somatization severity index" of the Screening of Somatoform Disorders (SOMS-7T). Based on results of the main validation study of SOMS-7T, meta-analytic estimations, and results of a pilot study of the efficacy of ENCERT, the power calculations yield a necessary total sample size of N=194 to detect a clinical relevant difference of 4 points symptom reduction on SOMS-7T between the two treatments with a power of 0.80 and an alpha=.05. With regard to an estimated drop-out rate of 20%, N=244 patients are to be recruited.

As main efficacy analysis the primary outcome shall be analyzed with linear mixed-effect models. It will be done according to the Intention-to-Treat (ITT) principle: to consider missing values as a potential source of bias, they will be handled according to the framework by Rubin. Secondary statistical analyses focus on establishing longer term treatment efficacy and describing the pattern of change. For this purpose the mean response will be modeled as a function of time with a separate mixed effects linear model over all assessments. Furthermore, interindividual differences in intraindividual change will be modeled using multilevel analysis for longitudinal data. Multilevel longitudinal mediation analyses will be conducted in order to test whether the effect of treatment condition on intraindividual changes in somatic symptom severity can be explained by intraindividual changes in emotion regulation skills.

Inclusion Criteria (are based on DSM-V diagnosis "somatic symptom disorder [SSD] 300.82"):

• Multiple distressing somatic symptoms (≥ 3 symptoms) not fully explained by a medical condition

• PDI ≥ 4

• Patient Health Questionnaire-15 (PHQ-15) ≥ 5

• Requested psychological criteria for SSD (at least 1 of 3):

  1. Disproportionate and persistent thoughts about the seriousness of one's symptoms
  2. Persistently high level of anxiety about health or symptoms
  3. Excessive time and energy devoted to these symptoms or health concerns

• Symptom duration ≥ 6 months

• Age: 18-69 years

• Comorbidity (depression, other mental disorders) allowed, as long as somatic symptoms are considered to be the major problem by therapist and patient

• Thorough medical check for medical disease that might fully explain the somatic symptoms

• Documented medical evaluation

Exclusion Criteria:

• Severe alcohol/drug addiction
• Acquired brain injuries
• Psychoses (history of schizophrenia spectrum disorders; bipolar disorders)
• Primary disorder requesting other treatments (e.g., suicidality)
• Biomedical etiology of major symptoms (also if detected during treatment course
• Ongoing psychotherapy
• Continuous or intermittent, high-dosage (on average more than once per 2 weeks) benzodiazepine treatment
• Continuous antipsychotic treatment
• Continuous opioid treatment
• For patients on medication with antidepressants: treatment regime changes during the time between 4 weeks prior to treatment until follow-up