Somatic symptoms of medically unknown origin are highly prevalent in the community and clinical settings; not only the primary care setting but also the secondary care setting(1-3), and are clinically important(4). Functional somatic syndrome (FSS) refers to several related syndromes that are characterized more by symptoms, suffering, and disability than by disease-specific, demonstrable abnormalities of structure or function(6).
FSSs include various diseases in many medical specialties such as irritable bowel syndrome, functional dyspepsia, fibromyalgia syndrome, and chronic fatigue syndrome.
FSSs are expected to share a common underlying pathophysiology (6). Among the regulating systems in human body, we have been focusing on autonomic nervous system (ANS) and its relationship to stress. The following two aspects should be considered in the pathophysiology of FSS: a) dysregulation of the stress response in ANS and b) psychological factors that modulate the expression of symptoms. These two aspects of pathophysiology interact with one another.