Applying the functional somatic disorder classification to somatic symptom disorder: Findings from the SOMA.SSD study

Functional somatic disorders (FSD), characterised by persistent physical symptoms and associated impairment, remain diagnostically fragmented across medical specialties, contributing to inconsistent patient care. The FSD classification addresses this by classifying symptom presentations into multi-system, single-system, and single-symptom subcategories, yet its utility in clinical populations is largely unexamined. This study evaluated the applicability and clinical utility of FSD classification in patients with Somatic Symptom Disorder (SSD).

Observational, cross-sectional analysis of baseline data from the SOMA.SSD cohort. Patients with SSD (N = 239), diagnosed using the Structured Clinical Interview for DSM-5-TR (SCID-5), were recruited from a German psychosomatic outpatient clinic. FSD subcategories were operationalised using the Bodily Distress Syndrome Checklist. Group differences were examined using chi-square and Mann-Whitney U tests, and binary logistic regression identified factors associated with multi-system FSD.

Most patients met multi-system FSD criteria (n = 207, 86.6%). Multi-system FSD was associated with greater psychological and behavioural burden, including higher somatosensory amplification, perceived stress, health anxiety, and negative affect, compared with other subgroups. Single-system FSD showed limited correspondence with established syndrome-based diagnoses (e.g., fibromyalgia or irritable bowel syndrome). Comorbid musculoskeletal, vascular, and neurological conditions were common but often not aligned with the presenting symptom system.

The FSD classification captured variation within SSD, supporting its value as a complementary tool for characterisation and differentiation. Transdiagnostic factors such as health anxiety, somatosensory amplification, and negative affectivity may play a key role in multi-system presentations. Future research should examine the FSD classification's utility in longitudinal and treatment settings.