Central Processing of Odour Stimuli in Patients With Multi-systemic Functional Somatic Disorder, Multiple Chemical Sensitivityor Post Covid Compared to Healthy Controls - the Pilot Study

Background:

Functional somatic disorders (FSDs) are frequent in all medical settings and characterized by persistent physical symptoms that cannot be explained by other somatic or psychiatric conditions. Numerous terms have been used for the phenomenon, such as medically unexplained symptoms, somatoform disorders, and functional somatic syndromes (FSS). Various types of FSS exist, including chronic fatigue syndrome, myalgic encephalopathy, fibromyalgia, irritable bowel syndrome, and multiple chemical sensitivity (MCS), however previous studies suggest significant overlaps among most of them.

Based on empirical research, a phenotype of multi-systemic FSD or multi-organ bodily distress syndrome (multi-organ BDS) has been identified in the most severely affected patients who have symptoms from multiple organ systems, thus fulfilling the criteria for multiple FSS. Multi-organ BDS is a research diagnosis, and the terms FSD and BDS are used as synonyms. This diagnosis is defined by an identifiable physical symptom pattern with symptoms from four groups (a cardiopulmonary, a gastrointestinal, a musculoskeletal, and a general symptom group). The diagnosis has been included in the draft of WHO's ICD-11 for Primary Health Care under the name Bodily Stress Disorder. Multiple chemical sensitivity (MCS) is non-allergic and falls within the following two diagnostic categories: FSS and single-organ BDS. MCS is characterized by odour intolerance and various somatic symptoms attributed to the influence of toxic environmental chemicals in low usually harmless doses. Post-covid is charcterised by WHO by as continuation or development of new symptoms 3 months after the initial SARS-CoV-2 infection, with these symptoms lasting for at least 2 months with no other explanation. the symptoms includefatigue, shortness of breath and cognitive dysfunction over 200 different symptoms have been reported that can have an impact on everyday functioning.The studies conducted by Per Fink and colleagues in recent years imply a high prevalence of FSD (about 10%) in the Danish population, and that FSD is associated with severe individual and social consequences.

There is no clear explanation for the pathophysiology of FSD. Numerous hypotheses for the aetiology of FSD and MCS exist, including inflammation, immune system involvement, central sensitization, mental or physical lifelong stress, and hyperactivity of the autonomic nervous system, but recent studies raise a neurobiological basis in this issue. However, most neuroscientific studies lack generalizability due to a gap in the delimitation of FSD, which makes it difficult to treat FSDs adequately.

Hypotheses concerning odour intolerance include stressors of the olfactory system and immunological sensitization. Clinical observations indicate that hypersensitivity to olfactory stimuli is also at play in patients with BDS, and a study indicates that this also applies to patients with functional motor disorders.