Predictors of Somatic Symptom Severity in Adult Psychiatric Outpatients: A Cross-sectional Heart Rate Variability and Quantitative Electroencephalography Study

Objective: To examine the relative contributions of clinical, autonomic, and neurophysiological variables to somatic symptom severity among adults with major depressive disorder (MDD) or attention-deficit/hyperactivity disorder (ADHD). Methods: We analyzed data from 103 outpatients (MDD = 55; ADHD = 48) who completed quantitative electroencephalography (QEEG), heart rate variability (HRV), and questionnaires (Patient Health Questionnaire-15, Beck Depression Inventory-II, Pittsburgh Sleep Quality Index, Korean Adult ADHD Rating Scale). Hierarchical regression models sequentially entered demographics, clinical symptoms, HRV total power, and QEEG beta-band coherence. Results: = 0.124). Conclusion: Sleep disturbance showed the strongest association with somatic symptoms, followed by female sex and depressive symptoms. Limited role of categorical diagnosis supports a dimensional framework. Reduced autonomic flexibility and disrupted cortical connectivity may underlie bodily distress, warranting confirmation through longitudinal studies.