Introduction: Patients often go to the physician with medically unexplained symptoms (MUS). MUS can be autonomic nervous system-related “unspecific” symptoms, such as palpitations, heart rhythm alterations, temperature dysregulation (hand, feet), anxiety, or depressive manifestations, fatigue, somnolence, nausea, hyperalgesia with varying pains and aches, dizziness, etc. Methods: In this real-world study, we investigated MUS in a cohort of unselected outpatients from general practitioners in Italy. It was our aim to increase the understanding of MUS by using principal component analyses to identify any subcategories of MUS and to check a role of chronic inflammatory diseases. Additionally, we studied cerebral blood oxygen (rCBO2) and associations with MUS and chronic inflammatory disease. Results: Participants included 1,597 subjects (50.6 ± 0.4 years, 65%/35% women/men). According to ICD-10 codes, 137 subjects had chronic inflammatory diseases. MUS were checked by a questionnaire with a numeric rating scale and cerebral blood flow with optical techniques. The analyses of men and women were stratified. Psychological symptom severity was higher in the inflamed compared to the non-inflamed group (fatigue, insomnia in women and men; recent mood changes, daytime sleepiness, anxiety, apathy, cold hands only in women; abnormal appetite and heart rhythm problems only in men). Principal component analysis with MUS provided new subcategories: brain symptoms, gut symptoms, and unspecific symptoms. Brain and gut symptoms were higher in inflamed women and men. Chronic inflammatory diseases and pain were tightly interrelated in men and women (p < 0.0001). In women, not in men, average frontal rCBO2 content was higher in inflamed compared to non-inflamed subjects. In men, not in women, individuals with pain demonstrated a lower average frontal rCBO2 content compared to pain-free men. MUS did not relate to rCBO2 parameters. Conclusion: This study shows close relationships between MUS and chronic inflammatory diseases but not between MUS and rCBO2 parameters.

Immune system and central nervous system (CNS) are two selfish organs, which hierarchically control self-allocation of energy in critical states [1]. In challenging situations, either the immune system (stimulus: e.g., infectious agent) or the CNS (stimulus: e.g., psychological stress) initiate a program that typically blocks the other selfish organ [1‒4]. In a situation with chronic peripheral inflammation, a complex program of CNS-related phenomena starts to inhibit brain function and related skeletal muscular activity in order to spare energy for immune function [5]. This leads to psychological sequelae that are often summarized under the heading of sickness behavior [6, 7], which is a forerunner of major depression in vulnerable people [2, 8]. The phenomenon is well known under controlled conditions such as, for example, administration to healthy people of lipopolysaccharide i.v. in the laboratory [9], but the situation in the real world – in the setting of a general practitioner – is not often studied.

In the environment of the general practitioner, patients appear with “medically unexplained symptoms (MUS)” that might belong to sickness behavior. MUS can be autonomic nervous system-related “unspecific” symptoms, such as palpitations, heart rhythm alterations, temperature dysregulation (hand, feet), anxiety or depressive manifestations, fatigue, somnolence, nausea, hyperalgesia with varying pains and aches, dizziness, etc. [10‒13]. The physician is confronted with these typical phenomena. We aimed to study these MUS under real-world conditions in the general practice. MUS can either be related to brain function or also to peripheral bodily function; thus, we aimed to find a content structure behind these MUS. A causal platform of MUS can be chronic inflammation. Thus, we were particularly interested to see how chronic inflammatory diseases are related to MUS.

Along with the collection of MUS in these patients, we were able to study cerebral blood oxygen (rCBO2) content. This part of the work was explorative because no similar studies were known that linked MUS, chronic inflammation, and rCBO2 content. rCBO2 content might be related to inflammation, which can be observed in acute mountain sickness [14, 15] or because of obstructive sleep apnea [16]. Acute brain hypoxia induces functional alterations in the brain, such as decreased cognitive performance and altered frontal/cortical connectivity [17]. Thus, links between MUS, chronic inflammatory diseases, and rCBO2 might exist.

This study aimed to investigate the following parts: (1) What are typical MUS under real-world conditions in general practice and what is the role of chronic inflammatory diseases on MUS? (2) Is there a content structure that can define different MUS subcategories? (3) Are the different MUS categories dependent on chronic inflammatory diseases? (4) Does chronic inflammatory diseases influence CBO2 parameters, and are CBO2 parameters related to MUS?

Patients

In this study, a total of 1,597 unselected participants were recruited during a visit to general medical practitioners in Italy. Since several years, participating physicians work with a health-related questionnaire and noninvasive methods like bioimpedance analysis (BIA-ACC®, BioTekna, Marcon, Italy) and optical determination of rCBO2 (HemoEncephaloGraphy Technology [HEG], BioTekna, Marcon, Italy) (techniques are given below). All patients entered this explorative study without any further selection on a consecutive basis in order to create a real-world situation beyond classical clinical studies. All data were collected during one session of investigation. However, patients with severe mental/psychiatric disorders, epilepsy, cancer, established or suspected pregnancy were excluded from the study.

The HEG device has been registered at the Italian Ministry of Health – National Classification of Medical Devices. The HEG device has been validated and CE-certified as a noninvasive medical device used for diagnostic and monitoring purposes and has been used in the EU since 2004. Before the measurements, patient received all information and gave oral consent. The retrospective use of the patient data obtained with the above devices in routine medical evaluations for anonymous analysis and publication has been approved earlier for another study by the Ethics Committee of the University Research Institute of Maternal and Child Health and Precision Medicine, National and Kapodistrian University of Athens, Athens, Greece (ethics document without registration number can be obtained through the corresponding author). The ethical review for the study was exempt, as this previous study with the same design has been approved in another EU country (Greece). The procedures were in accordance with the ethical standards of the responsible committee on human experimentation and with the Helsinki Declaration of 1975, as revised in 1983. Reporting of the study conforms to STROBE statements along with references to STROBE and the broader EQUATOR guidelines [18]. Due to the nature of the study, with electronic submission of information and due to ethical guidelines, the authors did not receive several critical patient-related data including blood material and exact diagnoses.

The mean age of the 1,597 participants was 50.6 ± 0.4 years, 64.8%/35.2% were female/male, weight was 69.5 ± 0.4 kg, body height was 1.67 ± 0.01 m, and mean body mass index was 24.7 ± 0.1 kg/m2. Physicians did not communicate the exact diagnosis, but they categorized their patients in the electronic submission form using ICD-10 codes necessary in Italy for administrative and insurance purposes. This ICD-10-dependent classification yielded a subgroup of patients with chronic inflammatory diseases, and, in addition, C-reactive protein serum levels supported the subgroup classification (blood values not communicated). This yielded a subgroup of 137 patients with chronic inflammatory diseases that was not different in age (50.4 ± 0.4 year) and sex (64%/36% women/men) compared to the non-inflamed group (n = 1,460; 53.6 ± 1.3 year; 74%/26%).

Questionnaire Variables

Chronic stress has been associated with autonomic nervous system-related “nonspecific” symptoms, such as palpitations, heart rhythm alterations, temperature dysregulation (hand, feet), anxiety, or depressive manifestations, etc. Similarly, chronic activation of the immune system has been linked to “nonspecific”, “sickness behavior”-type manifestations, such as fatigue, somnolence, nausea, hyperalgesia with varying pains and aches, dizziness, etc. [10‒13]. These very common clinical manifestations associated with chronic stress and inflammation bringing many patients to the general practitioner. Physicians, despite a full physical examination and many laboratory evaluations, failed to come up with a concrete diagnosis, and the term “medically unexplained symptoms” has been commonly used to describe a cluster of such manifestations [10‒13]. We use the abbreviation MUS for the rest of the text.

Similar to another study [19], we used a published questionnaire in the Italian language that included questions for MUS focusing on general signs and symptoms such as fatigue, mood alterations, sleep alterations (waking-up, insomnia), daytime sleepiness, anxiety, apathy, panic attacks, heart rhythm alterations, changes in eating behavior and appetite, nocturnal eating, abdominal symptoms of irritable bowel syndrome (bloating, nausea, flatulence, obstipation, etc.), cold hand and feet, nocturnal sweating, awakening with bad mood, feelings of unjustified guilt, feeling of anhedonia, and weight loss. In addition, we asked for general pain all over the body without naming a special location.

For every MUS item, grading of severity was estimated with a numeric rating scale with a minimum of 0 points and a maximum of 10 points provided as an electronic visual analog scale. In addition, overall pain was scored similarly using an electronic visual analog scale (minimum = 0, maximum = 10).

Measurement of Parameters of Frontal Cerebral Oxygen Content

Cerebral blood oxygenation (rCBO2) was measured using the HemoEncephaloGraphy technology (HEG; BioTekna, Marcon, Italy). HEG is a noninvasive hemodynamic medical device, which measures in real time the changes in hemoglobin concentration, cerebral blood flow (CBF), and oxygen of the prefrontal cortex (rCBO2). This HEG device is based on near-infrared spectroscopy complementary to other imaging methods using the hemodynamic response such as functional magnetic resonance imaging (fMRI) and positron emission tomography (PET). The technique of near-infrared spectrometry has been extensively evaluated elsewhere [20].

The HEG medical device, thanks to the noninvasive plethysmographic measurement technology with a sensor positioned on the patient’s forehead, allows in just 5 min to easily detect cerebral blood oxygenation of the prefrontal cortex (rCBO2) independent of skull thickness or amount of tissue between the frontal skin and cerebral vessel. For the measurements, the patient is seated in a comfortable position with the forearms resting on a table. The technique is described in Figure 1.

Fig. 1.

The noninvasive plethysmographic HemoEncephaloGraphy (HEG) technology. The unit of measure for cerebral oxygen (rCBO2) is measured in scalar units from 0 to 200. With the help of the receiver photodiode, rCBO2 is measured as the ratio between absorption of reflected red light with 660 nm wavelength (RED) and absorption of reflected infrared light (IR) at 940 nm wavelength. The red and IRs are alternately shown to the brain tissue and the absorption of reflected light is measured with a photodiode. There is a large difference in the attenuation of red light (RED) between oxygen-rich and oxygen-deficient hemoglobin, while IR is minimally modified. With the HEG device, 3 parameters of the frontal CBO2 were studied in 5 min of continuous recording: mean frontal CBO2 content (average CBO2), increase/decrease in frontal CBO2 over time (5-min CBO2 slope), and change in frontal CBO2 over time (CBO2 standard deviation [SD] over 5 min).

Fig. 1.

The noninvasive plethysmographic HemoEncephaloGraphy (HEG) technology. The unit of measure for cerebral oxygen (rCBO2) is measured in scalar units from 0 to 200. With the help of the receiver photodiode, rCBO2 is measured as the ratio between absorption of reflected red light with 660 nm wavelength (RED) and absorption of reflected infrared light (IR) at 940 nm wavelength. The red and IRs are alternately shown to the brain tissue and the absorption of reflected light is measured with a photodiode. There is a large difference in the attenuation of red light (RED) between oxygen-rich and oxygen-deficient hemoglobin, while IR is minimally modified. With the HEG device, 3 parameters of the frontal CBO2 were studied in 5 min of continuous recording: mean frontal CBO2 content (average CBO2), increase/decrease in frontal CBO2 over time (5-min CBO2 slope), and change in frontal CBO2 over time (CBO2 standard deviation [SD] over 5 min).

Close modal

Statistical Analyses

This was an explorative study and patients were included consecutively. A prior sample size was not determined due to the explorative nature of the study. All data are given as mean ± SEM. Normal distribution was checked using Kolomogorov-Smirnov test (IBM SPSS Statistics for Windows, Version 26.0. Armonk, NY: IBM Corp.). Data are presented as vertical bar graphs with error bars when data are normally distributed. Frequencies were compared with the χ2 test and group medians were compared by the non-parametric Mann-Whitney U test (both IBM SPSS Statistics). Spearman rank correlation analysis was used to study the interrelation between variables (IBM SPSS Statistics). p < 0.05 was the significance level.

In order to define a hidden content structure for MUS of the 19-item questionnaire, principal component analysis was applied (IBM SPSS Statistics). The sampling adequacy for each variable was examined on the anti-image correlation matrix and all variables had high scores. The validity of the principal component analysis for the data was assessed by the Kaiser-Meyer-Olkin measure of sampling adequacy, which was 0.913. The hypothesis that the correlation matrix is an identity matrix was rejected by the Bartlett’s test (p < 0.0001). Factors were then extracted, and factors with an eigenvalue greater than 1 were included and a varimax rotation with Kaiser normalization was used to enhance factor loading. Data for women and men are presented in a stratified form without a direct comparison of the two groups.

Chronic Inflammation Related to Subjective Symptoms

MUS in Non-Inflamed and Chronically Inflamed Subjects

The applied questionnaire focuses on MUS associated with “nonspecific” autonomic nervous system-related symptoms, “sickness behavior” type manifestations, and others [10‒13]. In a direct comparison of non-inflamed patients and patients with chronic inflammatory diseases, many symptoms appeared more often in subjects with inflammation. For every symptom, a patient was able to judge severity on a visual analog scale. Women with chronic inflammatory diseases scored higher in 9 of 19 items (Table 1). Significant differences in women were found for fatigue, recent mood changes, insomnia, daytime sleepiness, anxiety, apathy, cold hands, bad mood during awakening, and feelings of unjustified guilt (Table 1). Men with chronic inflammatory diseases scored higher in 4 of 19 items. Significant differences in men were found for fatigue, insomnia, noticeable problems with heart rhythm, and abnormal appetite (low or binge) (Table 1).

Table 1.

Symptom comparison of women and men without versus with chronic inflammation

SymptomsNon-inflamedChronic inflammationp value*
women, n = 934 (men, n = 526)women, n = 101 (men, n = 36)women (men)
Fatigue (score from 0 to 10) 3.98±0.11 (2.91±0.15) 5.58±0.34 (5.06±0.54) 0.000005 (0.000141) 
Recent mood changes (0–10) 3.10±0.11 (2.18±0.14) 4.84±0.38 (3.33±0.56) 0.000006 (n.s.) 
Insomnia (0–10) 3.05±0.11 (2.44±0.14) 5.06±0.40 (4.67±0.64) 0.000001 (0.000127) 
Daytime sleepiness (0–10) 1.87±0.09 (1.45±0.11) 2.97±0.34 (2.47±0.60) 0.000739 (n.s.) 
Anxiety (0–10) 3.71±0.12 (2.43±0.14) 5.51±0.37 (3.58±0.59) 0.000002 (n.s.) 
Apathy (0–10) 1.09±0.08 (1.05±0.10) 2.26±0.33 (2.06±0.55) 0.000057 (n.s.) 
Panic reactions (0–10) 0.61±0.06 (0.30±0.06) 1.18±0.26 (1.28±0.45) n.s. (n.s.) 
Noticeable problems with heart rhythm (0–10) 1.75±0.09 (1.02±0.10) 2.36±0.33 (3.33±0.61) n.s. (0.000001) 
Abnormal appetite (low or binge) (0–10) 1.74±0.10 (1.03±0.10) 2.68±0.35 (2.64±0.54) n.s. (0.000059) 
Nocturnal food intake (0–10) 0.25±0.04 (0.26±0.05) 0.34±0.15 (0.58±0.31) n.s. (n.s.) 
Acidic stomach (0–10) 3.19±0.12 (2.29±0.14) 4.10±0.38 (3.67±0.60) n.s. (n.s.) 
Irritated bowel (0–10) 1.88±0.10 (1.18±0.12) 2.81±0.38 (2.50±0.61) n.s. (n.s.) 
Obstipation (0–10) 1.92±0.10 (1.03±0.10) 2.39±0.35 (2.00±0.57) n.s. (n.s.) 
Cold hands (0–10) 2.33±0.11 (1.56±0.12) 3.81±0.39 (2.42±0.59) 0.000317 (n.s.) 
Nocturnal sweating (0–10) 1.53±0.09 (0.90±0.09) 1.59±0.30 (1.28±0.46) n.s. (n.s.) 
Bad mood during awakening (0–10) 1.08±0.08 (0.95±0.10) 2.30±0.34 (1.44±0.46) 0.000060 (n.s.) 
Feelings of unjustified guilt (0–10) 1.85±0.10 (1.14±0.11) 3.13±0.38 (1.31±0.47) 0.000610 (n.s.) 
Anhedonia (0–10) 1.00±0.08 (0.85±0.10) 1.85±0.30 (2.14±0.56) n.s. (n.s.) 
Unexplained weight loss (0–10) 0.26±0.02 (0.47±0.07) 0.66±0.19 (0.69±0.33) n.s. (n.s.) 
Average of all symptoms 6.24±0.13 (4.65±0.18) 8.10±0.40 (7.50±0.70) 0.000008 (0.000024) 
SymptomsNon-inflamedChronic inflammationp value*
women, n = 934 (men, n = 526)women, n = 101 (men, n = 36)women (men)
Fatigue (score from 0 to 10) 3.98±0.11 (2.91±0.15) 5.58±0.34 (5.06±0.54) 0.000005 (0.000141) 
Recent mood changes (0–10) 3.10±0.11 (2.18±0.14) 4.84±0.38 (3.33±0.56) 0.000006 (n.s.) 
Insomnia (0–10) 3.05±0.11 (2.44±0.14) 5.06±0.40 (4.67±0.64) 0.000001 (0.000127) 
Daytime sleepiness (0–10) 1.87±0.09 (1.45±0.11) 2.97±0.34 (2.47±0.60) 0.000739 (n.s.) 
Anxiety (0–10) 3.71±0.12 (2.43±0.14) 5.51±0.37 (3.58±0.59) 0.000002 (n.s.) 
Apathy (0–10) 1.09±0.08 (1.05±0.10) 2.26±0.33 (2.06±0.55) 0.000057 (n.s.) 
Panic reactions (0–10) 0.61±0.06 (0.30±0.06) 1.18±0.26 (1.28±0.45) n.s. (n.s.) 
Noticeable problems with heart rhythm (0–10) 1.75±0.09 (1.02±0.10) 2.36±0.33 (3.33±0.61) n.s. (0.000001) 
Abnormal appetite (low or binge) (0–10) 1.74±0.10 (1.03±0.10) 2.68±0.35 (2.64±0.54) n.s. (0.000059) 
Nocturnal food intake (0–10) 0.25±0.04 (0.26±0.05) 0.34±0.15 (0.58±0.31) n.s. (n.s.) 
Acidic stomach (0–10) 3.19±0.12 (2.29±0.14) 4.10±0.38 (3.67±0.60) n.s. (n.s.) 
Irritated bowel (0–10) 1.88±0.10 (1.18±0.12) 2.81±0.38 (2.50±0.61) n.s. (n.s.) 
Obstipation (0–10) 1.92±0.10 (1.03±0.10) 2.39±0.35 (2.00±0.57) n.s. (n.s.) 
Cold hands (0–10) 2.33±0.11 (1.56±0.12) 3.81±0.39 (2.42±0.59) 0.000317 (n.s.) 
Nocturnal sweating (0–10) 1.53±0.09 (0.90±0.09) 1.59±0.30 (1.28±0.46) n.s. (n.s.) 
Bad mood during awakening (0–10) 1.08±0.08 (0.95±0.10) 2.30±0.34 (1.44±0.46) 0.000060 (n.s.) 
Feelings of unjustified guilt (0–10) 1.85±0.10 (1.14±0.11) 3.13±0.38 (1.31±0.47) 0.000610 (n.s.) 
Anhedonia (0–10) 1.00±0.08 (0.85±0.10) 1.85±0.30 (2.14±0.56) n.s. (n.s.) 
Unexplained weight loss (0–10) 0.26±0.02 (0.47±0.07) 0.66±0.19 (0.69±0.33) n.s. (n.s.) 
Average of all symptoms 6.24±0.13 (4.65±0.18) 8.10±0.40 (7.50±0.70) 0.000008 (0.000024) 

Data are given as means ± SEM. Data of men are given in parentheses.

*Mann-Whitney U test; correction of the p value according to Bonferroni: sig. p value = 0.05/25 = 0.002 (number 25 includes also variables in Table 3).

In order to check whether one can group the 19 heterogeneous symptoms into a smaller number of subcategories, principal component analysis was performed in the entire group of women and men. With this technique, four different subcategories regrouped the 19 mixed symptoms, as demonstrated in Table 2. These new factors were used in further analyses.

Table 2.

Factor analysis of 19 diverse MUS derived from 1,597 subjects

Final statistics of the principal components analysis
factor 1factor 2factor 3factor 4
Eigenvalue 5.556 1.308 1.090 1.036 
% of variance 15.495 13.971 10.749 7.106 
Cumulative percentage of variance 15.495 29.466 40.214 47.320 
Factor matrix after varimax rotation 
Anhedonia 0.730    
Feelings of unjustified guilt 0.644    
Bad mood during awakening 0.628    
Apathy 0.592    
Panic reactions 0.417    
Insomnia  0.660   
Fatigue  0.624   
Recent mood changes  0.589   
Anxiety  0.560   
Daytime sleepiness  0.527   
Noticeable problems with heart rhythm  0.434   
Obstipation   0.603  
Unexplained weight loss   0.596  
Irritated bowel   0.562  
Acidic stomach   0.546  
Cold hands   0.434  
Nocturnal food intake    0.723 
Nocturnal sweating    0.539 
Abnormal appetite (low or binge)    0.459 
Final statistics of the principal components analysis
factor 1factor 2factor 3factor 4
Eigenvalue 5.556 1.308 1.090 1.036 
% of variance 15.495 13.971 10.749 7.106 
Cumulative percentage of variance 15.495 29.466 40.214 47.320 
Factor matrix after varimax rotation 
Anhedonia 0.730    
Feelings of unjustified guilt 0.644    
Bad mood during awakening 0.628    
Apathy 0.592    
Panic reactions 0.417    
Insomnia  0.660   
Fatigue  0.624   
Recent mood changes  0.589   
Anxiety  0.560   
Daytime sleepiness  0.527   
Noticeable problems with heart rhythm  0.434   
Obstipation   0.603  
Unexplained weight loss   0.596  
Irritated bowel   0.562  
Acidic stomach   0.546  
Cold hands   0.434  
Nocturnal food intake    0.723 
Nocturnal sweating    0.539 
Abnormal appetite (low or binge)    0.459 

The different symptoms loaded variably on four factors. Factor loadings less than 0.400 are not shown.

The first two subcategories mainly include brain symptoms (factor 1 and factor 2), and they explain together 29.466% of the variation (third row in Table 2). In further analyses, a combined factor – called “brain symptoms” – was generated as the sum of factor 1 and factor 2. Factor 3 is mainly linked to symptoms of the gastrointestinal tract (“gut symptoms”), and factor 4 to eating behavior and nighttime sweating (“unspecific symptoms”). All factors together explain 47.320% of the entire variation (Table 2, third row: “cumulative percentage of variance”). Factor analysis allows creating new variables used in further analyses.

Thus, we compared the new factors in non-inflamed compared to chronically inflamed patients (Table 3). Significant differences in women were found for the combination of factor 1 and factor 2 (“brain symptoms”), factor 1 alone, factor 2 alone, and factor 3 (Table 3). Men differed only in the combination of factor 1 and factor 2 (“brain symptoms”), factor 2 alone, and factor 3 (Table 3). For women and men, no differences were observed for factor 4, which described nocturnal food intake, nocturnal sweating, and abnormal appetite (“unspecific symptoms”).

Table 3.

Factor comparison of women and men without versus with chronic inflammation

Factors**Non-inflamedChronic inflammationp value*
women, n = 934 (men, n = 526)women, n = 101 (men, n = 36)women (men)
Factor 1 + factor 2 (brain symptoms) 23.07±0.62 (16.72±0.82) 37.04±2.27 (30.67±3.90) 8.00 × 10−10 (1.32 × 10−5
Factor 1 (factor 1 alone) 5.63±0.27 (4.29±0.34) 10.71±1.14 (8.22±1.87) 1.67 × 10−6 (n.s.) 
Factor 2 (factor 2 alone) 17.45±0.43 (12.43±0.56) 26.33±1.40 (22.44±2.30) 3.55 × 10−9 (5.28 × 10−6
Factor 3 (gut symptoms) 9.58±0.28 (6.53±0.35) 13.77±1.04 (11.28±1.68) 8.00 × 10−5 (0.001) 
Factor 4 (unspecific symptoms) 3.51±0.16 (2.19±0.18) 4.61±0.56 (4.50±1.03) n.s. (n.s.) 
Factors**Non-inflamedChronic inflammationp value*
women, n = 934 (men, n = 526)women, n = 101 (men, n = 36)women (men)
Factor 1 + factor 2 (brain symptoms) 23.07±0.62 (16.72±0.82) 37.04±2.27 (30.67±3.90) 8.00 × 10−10 (1.32 × 10−5
Factor 1 (factor 1 alone) 5.63±0.27 (4.29±0.34) 10.71±1.14 (8.22±1.87) 1.67 × 10−6 (n.s.) 
Factor 2 (factor 2 alone) 17.45±0.43 (12.43±0.56) 26.33±1.40 (22.44±2.30) 3.55 × 10−9 (5.28 × 10−6
Factor 3 (gut symptoms) 9.58±0.28 (6.53±0.35) 13.77±1.04 (11.28±1.68) 8.00 × 10−5 (0.001) 
Factor 4 (unspecific symptoms) 3.51±0.16 (2.19±0.18) 4.61±0.56 (4.50±1.03) n.s. (n.s.) 

Data are given as means ± SEM. Data of men are given in parentheses.

*Mann-Whitney U test; correction of the p value according to Bonferroni: sig. p value = 0.05/25 = 0.002 (number 25 includes also variables in Table 1).

**Factors are calculated by summing up the individual visual analog scale values for the symptoms of Table 1 allocated to the factor given in Table 2.

In summary, chronically inflamed women and men often demonstrate a higher score for different MUS. MUS can be regrouped into four reasonable categories that were higher in chronic inflammatory diseases compared to non-inflamed subjects.

Interrelation of Pain and Chronic Inflammation

Patients with chronic inflammatory diseases scored higher on the overall pain score, which was true for women and men (Fig. 2). In addition, in women and men, inflammation positively correlated with pain (women: RRank = 0.172, p < 10−6; men: RRank = 0.215, p < 10−5). Furthermore, pain score positively correlated with all four factors of Table 3 in women and men (all p value below 10−8) and, of course, with individual MUS (data not shown). This shows the tight interrelation of pain and inflammation and pain and reported MUS.

Fig. 2.

Pain and inflammation. Data are given as means ± SEM. Red bars represent subjects with chronic inflammatory diseases.

Fig. 2.

Pain and inflammation. Data are given as means ± SEM. Red bars represent subjects with chronic inflammatory diseases.

Close modal

Chronic Inflammation Related to Frontal Blood Oxygen Content

The data above clearly linked chronic inflammatory diseases and pain to subjective symptoms. We were able to define subjective factors related to brain symptoms and gut symptoms. Brain and gut symptoms were tightly linked to inflammation. Beyond subjective symptoms, we expected that chronic inflammatory diseases were also related to objectively measurable parameters of frontal cerebral oxygen content.

Frontal rCBO2 and Chronic Inflammatory Diseases

Three frontal CBO2 parameters were studied over 5 min of continuous recording: the average frontal CBO2 content (avg.CBO2), the increase/decrease of frontal CBO2 over time (CBO2 slope over 5 min), and the variation of frontal CBO2 over time (CBO2 standard deviation [SD] over 5 min). In the first analysis in women and separately in men, different results were observed for CBO2 slope (women vs. men: −1.05 ± 0.7 vs. −1.52 ± 0.12 scalar unit x time, p < 0.001) and CBO2 SD (2.46 ± 0.11 vs. 3.67 ± 0.68 scalar units), but avg.CBO2 was similar (33.53 ± 0.53 vs. 35.04 ± 0.93 scalar units). This prompted us to study women and men in stratified analyses.

In the subchapters above, we recognized that MUS and the derived factors were related to chronic inflammatory diseases in women and men. The question appeared to be whether frontal CBO2 parameters were also related to inflammation. Indeed, in women, the average frontal CBO2 was lower in non-inflamed compared to subjects with chronic inflammatory diseases that was not significant in men (Fig. 3).

Fig. 3.

Average frontal CBO2 in non-inflamed and chronically inflamed women and men. Red bars represent results of patients with chronic inflammatory diseases. Data are given as means ± SEM because data were normally distributed.

Fig. 3.

Average frontal CBO2 in non-inflamed and chronically inflamed women and men. Red bars represent results of patients with chronic inflammatory diseases. Data are given as means ± SEM because data were normally distributed.

Close modal

In women and in men, CBO2 slope over 5 min and CBO2 SD did not differ between non-inflamed and chronically inflamed subjects (data not shown). In addition, the individual symptom scores and the factors of Table 3 were not related to one of the CBO2 parameters, neither in women nor in men (data not shown).

Frontal Cerebral Blood Oxygen and Pain

In the subchapters above, we recognized that inflammation and pain were related to each other in women and men. The question appeared to be whether frontal CBO2 parameters were also related to pain. Indeed, in men, the average frontal CBO2 was lower in men with chronic pain compared to subjects without pain, which reached the level of a statistical trend, which was not observed in women (Fig. 4). No differences were found for the other CBO2 parameters in the two groups with and without pain, neither in women nor in men (data not shown).

Fig. 4.

Average frontal CBO2 in patients without and with pain in women and men. Red bars represent results of patients with chronic pain. Data are given as means ± SEM because data were normally distributed.

Fig. 4.

Average frontal CBO2 in patients without and with pain in women and men. Red bars represent results of patients with chronic pain. Data are given as means ± SEM because data were normally distributed.

Close modal

This real-world study on patients from general practitioners in Italy found a close interrelation between MUS and chronic inflammatory diseases in women and men. The results are presented in a stratified form for women and men separately; thus, no direct comparisons of women and men were carried out. Factor analysis provides four different factors by regrouping the 19 MUS. Newly described factors like “brain symptoms” and “gut symptoms” are higher in chronically inflamed than non-inflamed women and men. Chronic inflammatory diseases and pain are tightly interrelated in women and men, and pain is interrelated to brain symptoms and gut symptoms in women and men. In women, not in men, the average frontal CBO2 content was higher in chronically inflamed compared to non-inflamed subjects. In men, not in women, individuals with pain demonstrated a lower average frontal CBO2 content compared to men without pain.

Our study reports on information of typical MUS that is routinely recorded in the general practice of associated physicians in Italy. These doctors also apply the noninvasive techniques of the optical frontal CBO2 measurement. In multiple sessions, they were trained to correctly perform the different test procedures. As predefined in ethical guidelines, doctors transmit the information electronically without personal data of the patients to one of us (D.B.). This anonymous submission of data led to several publications using similar techniques [21‒26]. In the context of new developments, the optical measurement of CBO2 parameters was added to the armamentarium in this cross-sectional study.

In this real-world approach, we confirmed a clear interrelation between chronic inflammatory diseases and MUS, which was particularly obvious in women but also in men. The average symptom severity was higher in patients with chronic inflammatory diseases compared to non-inflamed women and men. This study confirms short-time experimental studies with injected lipopolysaccharide, which increases the inflammatory load for a short time [9, 27‒30]. These studies were prepared by many experiments in rodents (e.g., summarized in [31]).

In the present study, we planned to recategorize the 19 investigated MUS into meaningful factors that to be used in further analyses. Principal component analysis provided four different factors leading to different grouping of symptoms. Factor 1 and factor 2 were combined to a new category called “brain symptoms,” which is complemented by a subcategory of “gut symptoms” and a subcategory of “unspecific symptoms.” Brain symptoms and gut symptoms are higher in subjects with chronic inflammatory diseases than non-inflamed women and men. Whether or not these new brain and gut subcategories will be used in future analyses must stand the test of time.

The link between pain and inflammation goes back to the old work of neurogenic inflammation [32]. More links between inflammation and pain started to emerge in the 1990s when glial activation on the spinal level became an important element [33]. In our present study in the real-world environment, we observed that pain related to MUS in women and men.

In order to link aspects of cortical oxygen content to inflammation and pain, the optical technique of CBO2 measurement was applied. In women, not in men, average frontal CBO2 content was higher in patients with chronic inflammatory diseases compared to non-inflamed subjects. In men, not in women, patients with pain demonstrated – in the form of a trend – a lower average frontal CBO2 content compared to men without pain. To the best of our knowledge, we do not know of other studies that have examined CBO2 parameters in outpatient subjects. Thus, the meaning of these dichotomous findings in women and men is presently unclear. One can only speculate that in women, the aspect of chronic inflammatory diseases plays a stronger role for CBO2 data (higher prevalence of these diseases), whereas in men, pain has a more important role.

Nevertheless, either inflammation in women and pain in men is interrelated with CBO2 content in the frontal cortex. Similar to studies looking on acute brain hypoxia [14‒17], the inflammation-induced CBO2 change in women and the pain-induced CBO2 changes in men might be linked to symptoms. However, we were not able to see this direct interrelation because CBO2 parameters were not related to individual symptoms, brain symptoms, or gut symptoms. This fact surprised us because we expected that CBO2 changes related to subjectively measured symptom severity. A careful literature search in PubMed revealed a substantial lack of information concerning the interrelation of inflammation and CBO2. Thus, we cannot easily interpret our data, but this is different for the pain aspects.

Indeed, chronic pain was related to lower cerebral blood flow in patients with spinal cord injury [34, 35]. Short acute pain to the gingiva also reduced prefrontal cerebral hemodynamics [36]. Thus, our results with lower frontal CBO2 content in men with pain fit into the concept of lowered cortical hemodynamics. It remains unclear why this is absent in women.

In general, it is not clear why results are contrasting in women and men. However, women usually have more chronic inflammatory diseases [37]. Thus, the link between chronic inflammation and symptoms or CBO2 parameters might be more obvious in women than men.

In conclusion, we saw a clear link between MUS and inflammation in women and in men. A newly described subcategory “brain symptoms” and “gut symptoms” are higher in chronically inflamed than non-inflamed women and men. Chronic inflammatory diseases and pain are tightly interrelated in women and men, and pain is interrelated to brain and gut symptoms in women and men. In this first study, we did not see an interrelation between CBO2 parameters and MUS, i.e., brain symptoms or gut symptoms. Future studies need to address this link in women and men in larger groups with chronic inflammation.

The HEG device has been registered with the Italian Ministry of Health – National Classification of Medical Devices. The HEG device has been validated and CE-certified as a noninvasive medical device used for diagnostic and monitoring purposes and has been used in the EU since 2004. Before the measurements, patient received all information and gave oral consent. The retrospective use of the patient data obtained with the abovementioned devices in routine medical evaluations for anonymous analysis and publication has been approved earlier for another study by the Ethics Committee of the University Research Institute of Maternal and Child Health and Precision Medicine, National and Kapodistrian University of Athens, Athens, Greece (ethics document without registration number can be obtained through the corresponding author). The ethical review for the study was exempt as this previous study with the same design has been approved in another EU country (Greece). The procedures were in accordance with the ethical standards of the responsible committee on human experimentation and with the Helsinki Declaration of 1975, as revised in 1983. Reporting of the study conforms to STROBE statements along with references to STROBE and the broader EQUATOR guidelines [18]. Due to the nature of the study with electronic submission of information and due to ethical guidelines, the authors did not receive several critical patient-related data including blood material and exact diagnoses.

D.B. is director of research and development at BioTekna, which sells the HemiEncephaloGraphy (HEG) device used in this study.

There was no specific funding of this work. BioTekna, Marcon, Italy, provided research data.

R.H.S. created tables and figures, and he statistically analyzed the data. R.H.S. wrote the draft version of the manuscript. D.B. discussed and corrected the tables, figures, statistical analysis, and text.

The data that support the findings of this study are not publicly available due to privacy reasons but are available from the corresponding author upon reasonable request.

1.
Straub
RH
,
Cutolo
M
,
Buttgereit
F
,
Pongratz
G
.
Energy regulation and neuroendocrine-immune control in chronic inflammatory diseases
.
J Intern Med
.
2010
;
267
(
6
):
543
60
. .
2.
Dantzer
R
,
O’Connor
JC
,
Freund
GG
,
Johnson
RW
,
Kelley
KW
.
From inflammation to sickness and depression: when the immune system subjugates the brain
.
Nat Rev Neurosci
.
2008
;
9
(
1
):
46
56
. .
3.
Furman
D
,
Campisi
J
,
Verdin
E
,
Carrera-Bastos
P
,
Targ
S
,
Franceschi
C
, et al
.
Chronic inflammation in the etiology of disease across the life span
.
Nat Med
.
2019
;
25
(
12
):
1822
32
. .
4.
Schiller
M
,
Ben-Shaanan
TL
,
Rolls
A
.
Neuronal regulation of immunity: why, how and where
.
Nat Rev Immunol
.
2021
;
21
(
1
):
20
36
. .
5.
Straub
RH
.
The brain and immune system prompt energy shortage in chronic inflammation and ageing
.
Nat Rev Rheumatol
.
2017
;
13
(
12
):
743
51
. .
6.
Lasselin
J
,
Schedlowski
M
,
Karshikoff
B
,
Engler
H
,
Lekander
M
,
Konsman
JP
.
Comparison of bacterial lipopolysaccharide-induced sickness behavior in rodents and humans: relevance for symptoms of anxiety and depression
.
Neurosci Biobehav Rev
.
2020
;
115
:
15
24
. .
7.
Dantzer
R
.
Evolutionary aspects of infections: inflammation and sickness behaviors
.
Curr Top Behav Neurosci
.
2023
;
61
:
1
14
. .
8.
Dean
J
,
Keshavan
M
.
The neurobiology of depression: an integrated view
.
Asian J Psychiatr
.
2017
;
27
:
101
11
. .
9.
Reichenberg
A
,
Yirmiya
R
,
Schuld
A
,
Kraus
T
,
Haack
M
,
Morag
A
, et al
.
Cytokine-associated emotional and cognitive disturbances in humans
.
Arch Gen Psychiatry
.
2001
;
58
(
5
):
445
52
. .
10.
Olde Hartman
TC
,
Hassink-Franke
LJ
,
Lucassen
PL
,
van Spaendonck
KP
,
van Weel
C
.
Explanation and relations. How do general practitioners deal with patients with persistent medically unexplained symptoms: a focus group study
.
BMC Fam Pract
.
2009
;
10
:
68
. .
11.
Gormley
KJ
.
Medically unexplained symptoms: the need for effective communication and an integrated care strategy
.
Br J Community Nurs
.
2014
;
19
(
2
):
86
90
. .
12.
Isaac
ML
,
Paauw
DS
.
Medically unexplained symptoms
.
Med Clin North Am
.
2014
;
98
(
3
):
663
72
. .
13.
Rosendal
M
,
Blankenstein
AH
,
Morriss
R
,
Fink
P
,
Sharpe
M
,
Burton
C
.
Enhanced care by generalists for functional somatic symptoms and disorders in primary care
.
Cochrane Database Syst Rev
.
2013
(
10
):
Cd008142
. .
14.
Lundeberg
J
,
Feiner
JR
,
Schober
A
,
Sall
JW
,
Eilers
H
,
Bickler
PE
.
Increased cytokines at high altitude: lack of effect of ibuprofen on acute mountain sickness, physiological variables, or cytokine levels
.
High Alt Med Biol
.
2018
;
19
(
3
):
249
58
. .
15.
Limper
U
,
Fiala
V
,
Tank
J
,
Elmenhorst
EM
,
Schaelte
G
,
Hew
YM
, et al
.
Sleeping with elevated upper body Does not attenuate acute mountain sickness: pragmatic randomized clinical trial
.
Am J Med
.
2020
;
133
(
10
):
e584
e588
. .
16.
Yang
C
,
Zhou
Y
,
Liu
H
,
Xu
P
.
The role of inflammation in cognitive impairment of obstructive sleep apnea syndrome
.
Brain Sci
.
2022
;
12
(
10
):
1303
. .
17.
Bickler
PE
,
Feiner
JR
,
Lipnick
MS
,
Batchelder
P
,
MacLeod
DB
,
Severinghaus
JW
.
Effects of acute, profound hypoxia on healthy humans: implications for safety of tests evaluating pulse oximetry or tissue oximetry performance
.
Anesth Analg
.
2017
;
124
(
1
):
146
53
. .
18.
Simera
I
,
Moher
D
,
Hoey
J
,
Schulz
KF
,
Altman
DG
.
A catalogue of reporting guidelines for health research
.
Eur J Clin Invest
.
2010
;
40
(
1
):
35
53
. .
19.
Tsigos
C
,
Stefanaki
C
,
Lambrou
GI
,
Boschiero
D
,
Chrousos
GP
.
Stress and inflammatory biomarkers and symptoms are associated with bio-impedance measures
.
Eur J Clin Invest
.
2015
;
45
(
2
):
126
34
. .
20.
Fantini
S
,
Sassaroli
A
,
Tgavalekos
KT
,
Kornbluth
J
.
Cerebral blood flow and autoregulation: current measurement techniques and prospects for noninvasive optical methods
.
Neurophotonics
.
2016
;
3
:
031411
. .
21.
Christaki
EV
,
Pervanidou
P
,
Papassotiriou
I
,
Bastaki
D
,
Valavani
E
,
Mantzou
A
, et al
.
Stress, inflammation and metabolic biomarkers are associated with body composition measures in lean, overweight, and obese children and adolescents
.
Children
.
2022
.
9
(
2
):
291
.
22.
Chrousos
GP
,
Papadopoulou-Marketou
N
,
Bacopoulou
F
,
Lucafò
M
,
Gallotta
A
,
Boschiero
D
.
Photoplethysmography (PPG)-determined heart rate variability (HRV) and extracellular water (ECW) in the evaluation of chronic stress and inflammation
.
Hormones
.
2022
;
21
(
3
):
383
90
. .
23.
Cvijetic
S
,
Macan
J
,
Boschiero
D
,
Ilich
JZ
.
Body fat and muscle in relation to heart rate variability in young-to-middle age men: a cross sectional study
.
Ann Hum Biol
.
2023
;
50
(
1
):
108
16
. .
24.
Geronikolou
S
,
Cokkinos
D
,
Boschiero
D
,
Chrousos
GP
,
Albanopoulos
K
.
Chronic systemic inflammation measured by bioimpedance technology before and after sleeve gastrectomy: a feasibility study
.
Adv Exp Med Biol
.
2021
;
1339
:
169
77
. .
25.
Günther
F
,
Ehrenstein
B
,
Hartung
W
,
Boschiero
D
,
Fleck
M
,
Straub
RH
.
Increased extracellular water measured by bioimpedance analysis and increased serum levels of atrial natriuretic peptide in polymyalgia rheumatica patients : signs of volume overload
.
Z Rheumatol
.
2021
;
80
(
2
):
140
8
. .
26.
Straub
RH
,
Ehrenstein
B
,
Gunther
F
,
Rauch
L
,
Trendafilova
N
,
Boschiero
D
, et al
.
Increased extracellular water measured by bioimpedance and by increased serum levels of atrial natriuretic peptide in RA patients-signs of volume overload
.
Clin Rheumatol
.
2017
;
36
(
5
):
1041
51
. .
27.
Chen
P
,
Yang
L
,
Tong
Y
,
Meng
L
,
Zhou
R
.
The intracerebroventricular injection of lipopolysaccharide may induce neurogenic detrusor overactivity symptoms in mice
.
Neurourol Urodyn
.
2022
;
41
(
4
):
894
904
. .
28.
Lasselin
J
,
Benson
S
,
Hebebrand
J
,
Boy
K
,
Weskamp
V
,
Handke
A
, et al
.
Immunological and behavioral responses to in vivo lipopolysaccharide administration in young and healthy obese and normal-weight humans
.
Brain Behav Immun
.
2020
;
88
:
283
93
. .
29.
Cohen
O
,
Reichenberg
A
,
Perry
C
,
Ginzberg
D
,
Pollmacher
T
,
Soreq
H
, et al
.
Endotoxin-induced changes in human working and declarative memory associate with cleavage of plasma “readthrough” acetylcholinesterase
.
J Mol Neurosci
.
2003
;
21
(
3
):
199
212
. .
30.
Benson
S
,
Labrenz
F
,
Kotulla
S
,
Brotte
L
,
Rödder
P
,
Tebbe
B
, et al
.
Amplified gut feelings under inflammation and depressed mood: a randomized fMRI trial on interoceptive pain in healthy volunteers
.
Brain Behav Immun
.
2023
;
112
:
132
7
. .
31.
Dantzer
R
,
Heijnen
CJ
,
Kavelaars
A
,
Laye
S
,
Capuron
L
.
The neuroimmune basis of fatigue
.
Trends Neurosci
.
2014
;
37
(
1
):
39
46
. .
32.
Jancso
N
,
Jancso-Gabor
A
,
Szolcsanyi
J
.
Direct evidence for neurogenic inflammation and its prevention by denervation and by pretreatment with capsaicin
.
Br J Pharmacol Chemother
.
1967
;
31
(
1
):
138
51
. .
33.
Watkins
LR
,
Maier
SF
.
Implications of immune-to-brain communication for sickness and pain
.
Proc Natl Acad Sci U S A
.
1999
;
96
(
14
):
7710
3
. .
34.
Iwabuchi
SJ
,
Xing
Y
,
Cottam
WJ
,
Drabek
MM
,
Tadjibaev
A
,
Fernandes
GS
, et al
.
Brain perfusion patterns are altered in chronic knee pain: a spatial covariance analysis of arterial spin labelling MRI
.
Pain
.
2020
;
161
(
6
):
1255
63
. .
35.
Richardson
EJ
,
Deutsch
G
,
Deshpande
HD
,
Richards
JS
.
Differences in resting cerebellar and prefrontal cortical blood flow in spinal cord injury-related neuropathic pain: a brief report
.
J Spinal Cord Med
.
2021
;
44
(
5
):
794
9
. .
36.
Sakuma
S
,
Inamoto
K
,
Yamaguchi
Y
,
Takagi
S
,
Higuchi
N
.
Changes in prefrontal cerebral hemodynamics during intermittent pain stimulation to gingiva: preliminary study using functional near infrared spectroscopy
.
J Dent Sci
.
2021
;
16
(
3
):
980
6
. .
37.
Whitacre
CC
.
Sex differences in autoimmune disease
.
Nat Immunol
.
2001
;
2
(
9
):
777
80
. .