Functional somatic syndrome

Functional somatic syndrome (FSS) (sometimes termed "non-specific physical disorders") is a group of symptoms or conditions in which chronic symptoms occur, but no cause is found. Fibromyalgia is the leading FSS condition, among many. Chronic fatigue syndrome has been seen by some as an FSS condition. FSS conditions are highly prevalent, but little is known about their etiology.

Functional somatic syndromes are very common, and are estimated to affect about 8-16% of the general population.

FSS refers to disturbances in bodily functioning where aetiology is unknown. The term ‘functional somatic disorders’ (FSD) was proposed in 2020.

"Medically unexplained physical symptoms" only include symptoms where no explanation is found at all, but not poorly understood syndromes like fibromyalgia or IBS. These symptoms can sometimes be worsened in the presence of mental health problems.

"Persistent physical symptoms" includes FSS situations but also situations where persistent physical symptoms are caused by a known illness, such as arthritis.

In "somatic symptom disorder" chronic physical symptoms, which may or may not be linked to a known illness, coincide with excessive and maladaptive thoughts, emotions, and behaviors connected to those symptoms. In FSS these features are not present.

This term does not appear in the ICD-11. Proposals for classifications have been made.

Functional somatic syndromes are characterized by ambiguous, non-specific symptoms that appear in otherwise-healthy people. Overlap in symptomology exists across diagnoses, including gastrointestinal issues, pain, fatigue, cognitive difficulties, and sleep difficulties. Some have proposed to group symptoms into clusters or into one general functional somatic disorder given the finding of correlations between symptoms and underlying etiologies.