Introduction

Glial cell derived neurotrophic factor (GDNF) has an important role in the regulation of neuroinflammatory processes and pain syndrome of many diseases. Neuroinflammation causes GDNF expression in activated astrocytes and microglia, infiltrating macrophages, nestin-positive reactive astrocytes and neuroglial cells. Disease-related overexpression of GDNF can be compensatory and have a tissue-protective effect, as well as being harmful with negative consequences, depending on the location in the brain and the level and duration of glial cell activation [1]. Studies of GDNF have demonstrated its neuroprotective properties [2] and its ability to regulate dopamine metabolism and improve brain biodistribution [3]. The available data indicate an important role of GDNF in the regulation of the psycho-emotional sphere; the disturbance of the psycho-emotional sphere is quite often associated with chronic pain syndrome [4].

Alexithymia – difficulties with describing and verbalizing one’s own emotions and sensations – belongs to disorders of the psycho-emotional sphere. Modern studies have shown the association of alexithymia with chronic pain [5], systemic autoimmune diseases [6] and pathological conditions with somatic symptoms of uncertain origin [7]. Rheumatoid arthritis (RA) patients are characterized by significantly higher scores of alexithymic scales [6]; alexithymia in these patients is closely related to the deterioration of the psycho-emotional state and the severity of chronic pain [8].

The relationship between alexithymia and GDNF level in the blood plasma of RA patients needs more detailed research. The results of the study may provide valuable information regarding the biological role of GDNF in RA and the details of the relationship between alexithymia and GDNF in the pathogenetic mechanisms of RA.

The aim of the study was to determine the level of GDNF in the blood plasma of RA patients according to the presence of alexithymia and evaluate the relationship of GDNF level with clinical manifestations and quality of life.

Material and methods

We examined 88 patients (15 men and 73 women) who were hospitalized at the Center of Rheumatology, Osteoporosis and Biological Therapy of the communal non-profit enterprise Vinnytsya Regional Clinical Hospital named after M.I. Pirogov. Diagnosis of RA was established according to the 2010 ACR/EULAR Rheumatoid Arthritis Classification Criteria. For the measurement of plasma GDNF level, a total of 10 ml of blood was drawn from each participant. All blood samples were centrifuged and stored at –70oC immediately after collection. The GDNF in the blood plasma was determined by the ELISA method using the Human GDNF (Glial Cell Line Derived Neurotrophic Factor) ELISA Kit (Elabscience, USA, Lot CV09HB482125) according to the manufacturer’s instructions. Rheumatoid arthritis activity was assessed by the DAS28 (ESR) index [9], the Simple Disease Activity Index (SDAI) [10] and the Rheumatoid Arthritis Clinical Disease Activity Index (CDAI) [11]. Pain intensity was assessed by the Visual Analogue Scale (according to the assessment of the patient – VAS-P and the assessment of the doctor – VAS-D) [12]. General state of health and functions was assessed according to the Health Assessment Questionnaire (HAQ) [13]. Alexithymia was revealed by the Ukrainian version of the Toronto Alexithymia Scale (TAS-20) [14]; if the value of the TAS-20 was above 60 points, alexithymia was established in patients. The disability was determined by the validated and cross-cultural adapted Ukrainian version of the Disability Rating Index (DRI) questionnaire [15]. Quality of life was assessed by the SF-36 Health Survey [16].

Statistical analysis

The differences were statistically evaluated using Fisher’s and the Mann-Whitney non-parametric tests. Correlational analysis was performed by the Spearman rank order correlations method.

Bioethical standards

The study was conducted in accordance with the Declaration of Helsinki and was approved by the Bioethics Committee of National Pirogov Memorial Medical University, Vinnytsya (protocol No. 9 from 25 Oct 2021).

Results

Clinical characteristics of rheumatoid arthritis patients

According to the results from Table I we can conclude that the group of patients consisted of middle-aged people with a duration of disease more than 8 years. Approximately one third (34.1%) of patients were seropositive. Among the examined patients the second radiological stage prevailed (40.9%); the percentage of patients with the first and third stages was approximately equal (28.5% and 26.1%, respectively); and the smallest group comprised patients with the fourth stage (4.5%). The examined patients were characterized by high disease activity, low indicators of health status and functioning and low quality of life.

Table I

Clinical characteristics of RA patients

IndicatorValue [n (%) or M ±SD]
Age [years]50.8 ±10.9
Sex
 Male15 (17.0)
 Female73 (83.0)
Duration of the disease [years]8.2 ±4.5
Seropositivity30 (34.1)
X-ray stage I25 (28.5)
X-ray stage II36 (40.9)
X-ray stage III23 (26.1)
X-ray stage IV4 (4.5)
ESR [mm/h]23.36 ±14.47
DAS28 (ESR) [points]5.66 ±1.15
SDAI [points]33.98 ±11.80
CDAI [points]32.93 ±11.80
VAS-P [points]6.52 ±1.51
VAS-D [points]5.94 ±1.33
HAQ [points]1.24 ±0.69
DRI [points]34.14 ±18.43
TAS-20 [points]49.31 ±14.72
Alexithymia36 (40.9)
GDNF level in blood plasma [pg/ml]3.73 ±2.59
Quality of life according to the SF-36 Health Survey [points]
 Physical functioning54.77 ±19.03
 Physical role functioning50.85 ±27.97
 Bodily pain48.38 ±22.98
 General health perceptions44.66 ±20.44
 Physical component of health49.68 ±20.58
 Mental health55.57 ±19.66
 Emotional role functioning52.65 ±24.12
 Social role functioning54.69 ±26.21
 Vitality53.47 ±18.06
 Mental component of health54.00 ±19.40

[i] CDAI – Clinical Disease Activity Index, DAS28 (ESR) – Disease Activity Score with 28-joint count with erythrocyte sedimentation rate, DRI – Disability Rating Index, ESR – erythrocyte sedimentation rate, GDNF – glial cell derived neurotrophic factor, HAQ – Health Assessment Questionnaire, SDAI – Simple Disease Activity Index, VAS-P – Visual Analogue Scale Patient, VAS-D – Visual Analogue Scale Patient Doctor, TAS-20 – Toronto Alexithymia Scale.

Characteristics of rheumatoid arthritis patients with different levels of glial cell derived neurotrophic factor in blood plasma

The average GDNF level in the examined patients was 3.73 ±2.59 pg/ml (95% confidence interval [95% CI]: 3.18–4.28 pg/ml, median: 2.96 pg/ml, interquartile range [IQR]: 1.93–4.74 pg/ml).

Based on the results of measuring the level of GDNF in blood plasma, we distinguished two groups of the examined patients. The median value of the GDNF level was 2.96 pg/ml, and it was determined as the borderline level. Accordingly, patients with the GDNF level up to and including 2.96 pg/ml (GDNF level below the median) were assigned to the first group, and patients with GDNF above 2.96 pg/ml were assigned to the second group (GDNF level above the median). The number of patients in each group was 44.

According to the results from Table II, patients with the GDNF level in blood plasma above the median had higher indicators of the TAS-20; at the same time, differences were found for the general indicator of alexithymia (p < 0.1).

Table II

Characteristics of RA patients with different levels of GDNF in blood plasma (M ±SD)

IndicatorPatients with GDNF level below the median (n = 44)Patients with GDNF level above the median (n = 44)p-value
Results of assessment of alexithymia according to the TAS-20 [points]
 Difficulty identifying feelings14.98 ±5.8317.09 ±6.120.121
 Difficulty describing feelings11.34 ±3.8112.43 ±3.640.142
 Externally oriented thinking20.57 ±5.8622.20 ±6.200.224
 Indicator according to TAS-2046.89 ±14.4151.73 ±14.790.068
Clinical indicators
 ESR [mm/h]21.11 ±13.7125.61 ±15.020.134
 DAS28 (ESR) [points]5.56 ±0.945.76 ±1.330.096
 SDAI [points]33.11 ±10.1034.85 ±13.340.502
 CDAI – Clinical Disease Activity Index [points]31.93 ±9.7833.93 ±13.570.433
 VAS-P [points]6.34 ±1.336.70 ±1.660.205
 VAS-D [points]5.80 ±1.176.09 ±1.480.207
 HAQ [points]1.15 ±0.641.32 ±0.730.170
 DRI [points]32.70 ±17.4735.59 ±19.430.545
Quality of life according to the SF-36 Health Survey [points]
 Physical functioning58.75 ±17.3950.80 ±19.940.076
 Physical role functioning55.68 ±26.3446.02 ±29.010.087
 Bodily pain50.61 ±21.7246.14 ±24.220.354
 General health perceptions48.64 ±20.7240.68 ±19.580.062
 Physical component of health53.43 ±19.4545.93 ±21.220.063
 Mental health58.05 ±19.8453.09 ±19.390.212
 Emotional role functioning56.82 ±25.5248.48 ±22.130.080
 Social role functioning58.24 ±24.4051.14 ±27.730.194
 Vitality56.82 ±17.4950.11 ±18.190.081
 Mental component of health57.50 ±18.7550.51 ±19.620.076

[i] CDAI – Clinical Disease Activity Index, DAS28 (ESR) – Disease Activity Score with 28-joint count with erythrocyte sedimentation rate, DRI – Disability Rating Index, ESR – erythrocyte sedimentation rate, GDNF – glial cell derived neurotrophic factor, HAQ – Health Assessment Questionnaire, SDAI – Simple Disease Activity Index, VAS-P – Visual Analogue Scale Patient, VAS-D – Visual Analogue Scale Patient Doctor, TAS-20 – Toronto Alexithymia Scale.

Patients with GDNF level above the median had higher RA activity rates, pain intensity, and worse functional activity and health status, although differences between groups were not statistically significant (p > 0.05). The most significant were the differences in RA activity according to the DAS28 (p < 0.1).

Characteristics of rheumatoid arthritis patients with and without alexithymia and different levels of glial cell derived neurotrophic factor in blood plasma

To evaluate the features of the relationship between GDNF and alexithymia, patients were divided into two groups based on the results of the TAS-20. The first group included 52 patients without alexithymia (values of the TAS-20 up to and including 60 points). The second group included 36 patients with alexithymia (TAS-20 score – 61 points and above) and each of these groups was divided according the GDNF level. The average age of patients without alexithymia was 49.1 ±11.7 years, of patients with alexithymia 53.4 ±9.2 years, duration of RA was 7.0 ±3.9 years and 9.9 ±4.8 years, respectively.

Glial cell derived neurotrophic factor level in RA patients with alexithymia was higher than in patients without alexithymia: 4.08 ±2.87 pg/ml (95% CI: 3.11–5.06 pg/ml, median 3.11 pg/ml, IQR: 2.40–4.74 pg/ml) vs. 3.48 ±2.37 pg/ml (95% CI: 2.82–4.14 pg/ml, median 2.71 pg/ml, IQR: 1.87–4.66 pg/ml). The differences between groups with and without alexithymia were not statistically significant (p = 0.295).

According to the results in Table III, alexithymia has a significant effect on the differences in the indicators of patients with different levels of GDNF.

Table III

Characteristic of RA patients with and without alexithymia and different level of GDNF in blood plasma (M ±SD)

IndicatorPatients without alexithymiaPatients with alexithymiap**p***
Patients with GDNF level below the median (n = 29)Patients with GDNF level above the median (n = 23)p*Patients with GDNF level below the median, (n = 15)Patients with GDNF level above the median, (n = 21)p*
Clinical indicators
 ESR [mm/h]20.28 ±14.6317.70 ±10.930.74722.73 ±12.0334.29 ±14.220.0170.3210.000
 DAS28 (ESR) [points]5.29 ±0.995.06 ±1.400.6726.09 ±0.556.53 ±0.660.0170.0040.000
 SDAI [points]30.59 ±9.5130.17 ±15.690.48937.98 ±9.6839.98 ±7.700.3860.0440.000
 CDAI [points]29.81 ±8.9329.67 ±16.400.42336.04 ±10.3538.60 ±7.500.2680.0920.000
 VAS-P [points]5.86 ±1.365.61 ±1.370.7347.27 ±0.597.90 ±0.980.0510.0000.000
 VAS-D [points]5.41 ±1.155.09 ±1.200.3726.53 ±0.837.19 ±0.810.0200.0020.000
 HAQ [points]0.97 ±0.620.90 ±0.600.7461.51 ±0.541.78 ±0.580.0400.0060.000
 DRI [points]28.76 ±16.9824.73 ±14.780.44440.31 ±16.3447.49 ±16.950.2750.0210.000
Quality of life according to the SF-36 Health Survey [points]
 Physical functioning62.76 ±17.4561.09 ±19.300.94051.00 ±14.9039.52 ±13.780.0190.0190.000
 Physical role functioning60.34 ±26.3259.78 ±28.940.98546.67 ±24.7630.95 ±20.770.0410.1150.001
 Bodily pain58.28 ±19.2962.50 ±18.860.37835.80 ±18.6528.21 ±14.980.1600.0010.000
 General health perceptions56.72 ±17.4453.04 ±17.430.38533.00 ±17.7127.14 ±11.020.2860.0000.000
 Physical component of health59.54 ±18.4759.13 ±19.470.99341.61 ±15.8831.47 ±11.440.0280.0020.000
 Mental health68.76 ±11.7667.83 ±12.790.76637.33 ±15.3236.95 ±10.270.6720.0000.000
 Emotional role functioning66.68 ±19.9362.33 ±18.290.43037.75 ±24.7933.30 ±14.910.6200.0000.000
 Social role functioning70.26 ±19.0273.37 ±16.980.70535.00 ±15.0926.79 ±12.050.0900.0000.000
 Vitality64.66 ±14.0164.13 ±12.220.85241.67 ±13.1834.76 ±8.580.0810.0000.000
 Mental component of health67.61 ±12.7966.93 ±11.590.77537.95 ±11.5032.52 ±5.460.0520.0000.000

* Statistical significance of differences between groups of patients with GDNF level below the median and above the median.

** Statistical significance of differences between groups of patients with and without alexithymia with GDNF level below the median.

*** Statistical significance of differences between groups of patients with and without alexithymia with GDNF level above the median.

CDAI – Clinical Disease Activity Index, DAS28 (ESR) – Disease Activity Score with 28-joint count with erythrocyte sedimentation rate, DRI – Disability Rating Index, ESR – erythrocyte sedimentation rate, GDNF – glial cell derived neurotrophic factor, HAQ – Health Assessment Questionnaire, SDAI – Simple Disease Activity Index, VAS-P – Visual Analogue Scale Patient, VAS-D – Visual Analogue Scale Patient Doctor, TAS-20 – Toronto Alexithymia Scale.

Slightly higher disease activity, pain intensity and worse functional activity were found in patients without alexithymia with the GDNF level below the median. The differences between the groups with the GDNF level below and above the median were not statistically significant.

Significantly higher ESR (p < 0.05), RA activity index according to the DAS28 (p < 0.05), pain intensity according to the VAS-P (p < 0.1) and VAS-D (p < 0.05), and worse general state of health and functions according to the HAQ index (p < 0.05) were found in the group of patients with alexithymia and with the GDNF level above the median.

Patients with alexithymia and the GDNF level above the median had worse quality of life detected by the SF-36 questionnaire: physical functioning (p < 0.05), physical role functioning (p < 0.05), physical component of health (p < 0.05), social role functioning (p < 0.1), vitality (p < 0.1) and mental component of health (p < 0.1).

Comparison of patients with the same GDNF level depending on the presence of alexithymia revealed even more significant differences which proved the significant influence of alexithymia on the clinical characteristics of patients and the relationship with GDNF. Patients with alexithymia had significantly worse results for all evaluated clinical indicators and indicators of quality of life. In patients with the GDNF level below the median, significant differences were found between patients with and without alexithymia according to the DAS28 (p < 0.01), SDAI (p < 0.05), VAS-P (p < 0.001) and VAS-D (p < 0.01), HAQ (p < 0.01), DRI (p < 0.05), scales of physical functioning (p < 0.05), bodily pain (p < 0.01), general health perceptions (p < 0.001), physical component of health (p < 0.01), mental health (p < 0.001), emotional role functioning (p < 0.001), social role functioning (p < 0.001), vitality (p < 0.001) and mental component of health (p < 0.001). In the group of patients with the GDNF level above the median, the differences were even more significant: statistically significant differences were found for all indicators, and the level of statistical significance of the differences was very high (p < 0.001).

The assessment of correlations between alexithymia and GDNF level proved their complex nature. Analyzing correlations in all patients (without differentiation into groups with and without alexithymia) and in patients without alexithymia significant relationships were not found. Patients with alexithymia had significant correlations of moderate strength between the level of GDNF and pain severity: VAS-P – rS = 0.338, p = 0.044; VAS-D – rS = 0.446, p = 0.006.

Discussion

Comparing the results of our study with previous studies, which found in patients with RA an increase in the level of brain-derived neurotrophic factor (BDNF) and a decrease in the level of GDNF compared to the control group [17], there can be noted some consistency of the data we obtained with the results of previous studies. Experimental models of GDNF influence proved the dependence of GDNF effects on the duration of its action: in an acute process GDNF showed neuroregenerative properties [1820], while long-term expression and high concentration of GDNF caused a negative effect on the homeostasis of neurotransmitters [2124]. The patients in our study represented persons with a chronic disease (average duration more than 8 years); the association of GDNF level with disease activity and worse quality of life could be due to this factor. Interpreting the data of our study, one should take into account the report of Lundborg et al. that the level of GDNF in chronic pain increased intrathecally but decreased in blood plasma [25], as well as the data of Hulander et al., indicating that a reduced serum level of GDNF may lead to reduced activation of inflammatory pathways and that GDNF expression is significantly modulated by external influences [26].

Higher levels of RA activity, pain intensity and worse functional capacity and health status of patients were associated with a higher level of GDNF – generally in line with data from research by Elfving et al.; the authors found that the level of GDNF in the plasma of RA patients was higher than in the control group and correlated with the severity of the pain syndrome [27]. At the same time the available studies testify to the complex and ambiguous nature of the physiological function of GDNF [28], and in evaluating the regularities, one should take into account as many relevant factors as possible that can influence the effects of GDNF.

In our opinion, alexithymia can be one of these factors. The data of our study proved that the RA patients had a higher level of alexithymia. This data are consistent with the results of Vadacca et al. [6] and Chimenti et al. [29]; they found a significant prevalence of alexithymia in RA patients, as well as the relationship of alexithymia in RA with the psychoemotional state of patients [8] and pain perception [30].

As the relationship between alexithymia and the level of GDNF in RA patients has not been studied yet, based on the data of our study we can assume that the presence of alexithymia not only modifies the clinical manifestation of RA, but also significantly changes the physiological effect of GDNF.

In the group of patients with alexithymia the level of GDNF was directly correlated with disease activity and pain intensity, while in the group of patients without alexithymia, significant correlations were not found. At the same time, in the group of patients with alexithymia, significant differences in disease activity, severity of pain, functional activity and health status of patients were found between patients with different levels of GDNF, while in the group of patients without alexithymia, such differences were not found.

In our opinion the features of the quality of life are a natural reflection of the trends revealed regarding disease activity, pain intensity, functional activity, general state of health and the state of the psycho-emotional sphere. As patients with a higher level of GDNF were characterized by a worse state of these functions, it seems logical that the quality of life in the related areas would decrease.

Conclusions

Alexithymia was found in 40% of RA patients.

Rheumatoid arthritis patients with alexithymia had a nonsignificantly higher GDNF level compared to patients without alexithymia.

In RA patients with alexithymia, an association of GDNF level with rheumatoid arthritis activity, loss of functional capacity and reduced quality of life was established.

Alexithymia in RA patients is an important factor in the clinical manifestation of RA and modification of the pathophysiological role of GDNF.