Neuropathic pain: a principal component of pain that should not be neglected in ankylosing spondylitis

Background Pain in AS is currently considered an inammatory pain (IP). However, it was found that some patients still had the sensation of pain even without inammation. Our study was to investigate whether ankylosing spondylitis (AS) pain has a neuropathic pain (NeP) component. Methods The study consisted of three parts. The rst included 182 AS patients to assess neuropathic pain in AS patients. The second included63 patients to evaluate pain improvement after etanercept therapy. The third included 20 AS patients and 10 healthy controls (HCs) to detect serum neurotransmitters. We found out that 55 paitents (30.21%) had possible NeP, and 14 (7.70%) had denite NeP. There were signicant differences between the groups with respect to nocturnal pain (NP), peripheral pain (PP), total back pain (TBP), BASDAI, ASDAS-CRP, HAD-A, HAD-D and BASDAI-fatigue but not CRP concentrations. Principal component analysis (PCA) of AS pain revealed that the weight of NeP was greater than PP in the rst principal component (0.703 vs 0.639). Structural equation modelling (SEM) revealed that NeP altered disease activity (β = 0.62, P < 0.001), which inuenced psychological status (β = 0.42, P < 0.001). Of 63 patients who used etanercept for 3 months, signicant improvement was found in NP, TBP and PP (all P < 0.0001) but not in PDQ (10.60 ± 6.85vs 9.98 ± 6.40, P = 0.067). Serum norepinephrine concentrations in patients with positive PDQ were greater than those in patients with negative PDQ and HC. on this nding, we are using pregabalin to treat NeP in AS patients, and related data are being collected. This study has some limitations. We only detected neurotransmitters in the peripheral blood of a limited number of AS patients. We plan to further expand the sample size in our ongoing research and use head MRI to explore the structural or functional changes in the pain-related regions of the central nervous system.


Abstract
Background Pain in AS is currently considered an in ammatory pain (IP). However, it was found that some patients still had the sensation of pain even without in ammation. Our study was to investigate whether ankylosing spondylitis (AS) pain has a neuropathic pain (NeP) component.

Methods
The study consisted of three parts. The rst included 182 AS patients to assess neuropathic pain in AS patients. The second included63 patients to evaluate pain improvement after etanercept therapy. The third included 20 AS patients and 10 healthy controls (HCs) to detect serum neurotransmitters.

Results
We found out that 55 paitents (30.21%) had possible NeP, and 14 (7.70%) had de nite NeP. There were signi cant differences between the groups with respect to nocturnal pain (NP), peripheral pain (PP), total back pain (TBP), BASDAI, ASDAS-CRP, HAD-A, HAD-D and BASDAI-fatigue but not CRP concentrations. Principal component analysis (PCA) of AS pain revealed that the weight of NeP was greater than PP in the rst principal component (0.703 vs 0.639). Structural equation modelling (SEM) revealed that NeP altered disease activity (β = 0.62, P < 0.001), which in uenced psychological status (β = 0.42, P < 0.001). Of 63 patients who used etanercept for 3 months, signi cant improvement was found in NP, TBP and PP (all P < 0.0001) but not in PDQ (10.60 ± 6.85vs 9.98 ± 6.40, P = 0.067). Serum norepinephrine concentrations in patients with positive PDQ were greater than those in patients with negative PDQ and HC.

Conclusions
We conclude that NeP contributes to pain in AS patients.

Background
Ankylosing spondylitis (AS) is an autoin ammatory disease whose aetiology remains to be established. It mainly involves the axial skeleton and is characterized by sacroiliac arthritis and enthesitis with pain, stiffness and joint deformity as the main symptoms [1].Pain in AS is currently considered an in ammatory pain (IP). However, pain in AS is not always correlated with the in ammatory indexes of the disease, such as Creactive protein (CRP) concentrations or the erythrocyte sedimentation rate (ESR). Biologics are the most effective treatment for AS at present. It was reportedthat the serological parameters and radiological outcomes were improved in approximately60% of AS patients who received tumour necrosis factor (TNF) inhibitors [2].However, it was found in clinical practice that some patients still had the sensation of pain even though the in ammatory indexes had resolved normally after administration of the biologics. In 2017, Bidad et al.found that AS-associated pain was not merely a kind of IP but also involved neuropathic pain (NeP) [3].
Currently, there are no large-scale clinical trials distinguishing IP from NeP. Given the large population and economic condition of most AS patients in China, it is extremely important to nd an accurate and convenient tool for differentiating between IP and NeP. The painDETECT Questionnaire (PDQ) has proven to be a simple and reliable tool that screens for NeP;it consists of 9 simple self-assessment questions suitable for patient selfscoring and large-scale epidemiological investigation [4].The aims of our research were to comprehensively assess the clinical symptoms of AS pain by making use of the PDQ as part of the NeP assessment in combination with NP, TBP, PP, AS activity, anxiety, depression and fatigue. Methods 1. Part one: This part of the study included 182 patients who were con rmed in our Department of Rheumatology & Immunology to have AS. AS was diagnosedfollowing the 1984 Modi ed New York Criteria for Ankylosing Spondylitis [5].The general data collected for these patients included age, gender and the extent of AS. The Bath Ankylosing Spondylitis Disease Activity Index(BASDAI) [6],the CRP-based Ankylosing Spondylitis Disease Activity Score(ASDAS-CRP) [7],the BASDAI-Fatigue score [8],and the PDQ [4] were used, and anxiety and depression levels were determined using the Hospital Anxiety and Depression Scale (HADS) [9]. NP and BP were assessed by a visual analogue scale (VAS). PP was assessed by Q3 of BASDAI.
2. Part two: This part of the study included 63 AS patients (31 with PDQ>12 and 32 with PDQ≤ 12) who were given a subcutaneous injection of 25 mg etanercept for 12 weeks. Before drug administration, all patients underwent strict screening tests for tuberculosis, hepatitis and tumours. The main clinical parameters included age, gender, ASDAS-CRP, BASDAI-Fatigue, PDQ and HAD, which were recorded before and after drug administration.
3. Part three: This part of the study involved 20AS patients (6 with PDQ>19 and 14 with PDQ≤12). Ten age-and gender-matched healthy individuals were used as the controls. Serum neurotransmitter concentrations, including norepinephrine (NE), 5-hydroxytryptamine (5HT), glycine (Gly), glutamic acid (Glu), γ-aminobutyric acid (GABA), 5-hydroxyindoleacetic acid (5HIAA), kynurenine (KYN), and tryptophan (TRP), were measured. Differences in the concentrations of these neurotransmitters were compared among the three groups. All blood samples were sent to Anpel Laboratory Technologies (Shanghai, China) for testing. 4. PDQ calculation: The PDQ included 9 simple self-assessment questions with no need for physical examination and therefore was suitable for self-assessment and large-scale epidemiological investigation. Of the 9 questions, 7 were weighted sensory items scored by a 0-5 scoring system to indicate no manifestation to a severe degree, and the other 2 were related to radiating pain and the mode of pain onset. The total score of the nine items indicated the following: ≤12, a low possibility of NeP; 13-18, the existence of mixed pain; and ≥ 19, the possible presence of NeP.
HADS score: Anxiety and depression were evaluated usingthe HADS. Brie y, a score of 0-7 indicated no anxiety or depression, 8-10 indicated possible anxiety or depression, and 11-21 indicated a high probability of both conditions.

Statistical analysis
Statistical analyses were conducted using SPSS software (ver. 16.0). Data that had a normal distribution are presented as the mean ± SD.
Continuous variables that had a normal distribution were evaluated using a two-sample t-test. Analysis of variance (ANOVA) was employed to look for multigroup differences, and unordered categorical variables were assessed using the chi-squared test. Principal component analysis (PCA) was performed by dimension reduction processing with SPSS statistical software. SEM was analysed using Amos ver. 23.0.0, and any possible relationships between latent variables were evaluated using SEM.  The common factor (CF) of pain was calculated with a rotated component matrix, indicating that there are three major dimensions of pain in AS patients: CF 1 represents NP and TBP, CF 2 represents peripheral pain, and CF 3 represents NeP. CF analysis showed that NeP was a relatively independent type of AS pain (Table 2).

General clinical information of AS patients
3. Use of SEM: SEM was used to analyse the impact of NeP on disease activity and psychology. BASDAI and ASDAS-CRP were employed to assess the disease activity index, while HAD-A and HAD-D assessed the psychological indexes of patients. SEM was successfully established, with a probability level of 0.227, a CMIN/DF ratio of 1.413, a CFI of 0.996, an NFI of 0.987, and an RMSEA of 0.048. The SEM results showed that PDQ could alter disease activity (β = 0.62, P< 0.001), which in turn affected psychological status (β = 0.42, P< 0.001) (Fig. 1).
4. Pain improvement after biological treatment: Of the 63 AS patients who used etanercept for 12 weeks, PDQ was ≤ 12 in 32 patients, and PDQ was > 12 in the remaining 31 patients. The results suggested that the degrees of NP, TBP and PP were signi cantly decreased after drug administration, but PDQ was not signi cantly decreased (10.60 ± 6.85 vs 9.98 ± 6.40 P = 0.067). Subgroup analysis revealed that PDQ was decreased signi cantly in AS patients with PDQ > 12 (17.06 ± 2.40 vs 15.55 ± 3.69, P = 0.006) and did not show a signi cant decrease in AS patients with PDQ ≤ 12 (4.34 ± 2.47 vs 4.59 ± 2.79, P = 0.508).

Detection of neurotransmitters in AS patients:
Of the 20 AS patients in the third part of the study, PDQ was > 19 in 6 patients and PDQ ≤ 12 in 14 patients. The NE concentration in patients with PDQs > 19 was signi cantly higher than that in patients with PDQs ≤ 12 and healthy controls (0.21 ± 0.06 vs 0.13 ± 0.05 vs 0.07 ± 0.02, P< 0.001). There was no signi cant difference in other neurotransmitter concentrations.
The primary symptom of AS is pain, which is currently recognized as a type of IP. [10] For this reason, previous research and the development of new anti-AS medications have mainly focused on drugs that can block in ammatory factors [11][12] [13].Given the large capacity of outpatients and the short duration of communication between the physician and the patient in the actual clinical situation in China, both the physician and the patient usually focus their attention on laboratory and imaging ndings and pay scant attention to pain and other subjective symptoms. However, it is often found that some AS patients complain of pain even when in ammatory indexes such as ESR and CRP are within the normal ranges or after the administration of biologics. In addition, this kind of pain often has the characteristics of NeP:numbness and pricking sensations. It seems too simple and biased to explain the clinical symptoms of AS simply as IP. More studies in recent years have found that pain in AS patients is not always correlated with in ammation [14] [15]. Freynhagen et al. investigated more than 8,000 patients with chronic pain and found that approximately one-third of them had pain of a neuropathic nature [16]. Using head MRI, Wu et al. demonstrated NeP in AS patients and found that the corresponding abnormality in the grey matter was correlated with clinical symptoms [17].
In the present study, approximately one-third of 182 AS patients had possible or de nite NeP. Borman et al [18].used the Leeds Assessment of Neuropathic Symptoms and Signs (LANSS) and Douleur Neuropathique 4 (DN4) to assess pain in 58 AS patients and found that more than 50% had NeP. Choi et al.used the PDQ to assess 105 AS patients and reported that 15 (14.2%) had NeP. All these ndings suggest that NeP universally occurs in AS patients [19]. In addition, the PCA of pain in our study showed that the weight of NeP was greater than that of PP. CF analysis showed that pain in AS consists of three dimensions,that is, IP (NP and TBP), NeP and PP, which are relatively independent and represent the three dimensions of AS pain. This nding suggests that NeP is an important component of AS pain. The SEM results suggested that NeP affects the disease activity of AS, which further alters the psychological status of AS patients. Based on the above results, we believe that the incidence of NeP is relatively high in AS patients and that NeP is an important component of AS pain, which has a tremendous impact on both AS disease activity and the psychological status of AS patients.
Studies on the therapeutic effect of TNF-α on NeP have produced meaningful results. Some studies have reported that local or spinal administration of drugs that antagonize TNF-α activity attenuates pain behaviour in neuropathic animal models [20] [21].Other studiesfound that mechanical allodynia in a rat model of central neuropathic pain, produced by T13 spinal cord hemisection, was attenuated by the immediate intrathecal administration of etanercept 1-4 weeks after injury to the spinal cord [22].However, to the best of our knowledge, we are unaware of any randomized controlled clinical trials of in iximab or etanercept for the treatment of other forms of NeP. TNF inhibitors are currently the most effective drugs for AS. The results of our study demonstrated that axial pain, PP and various in ammatory indexes were improved signi cantly in AS patients who were administered etanercept for 12 weeks, but PDQ was not improved signi cantly, suggesting that TNF inhibitors did not work as effectively as expected. This may be because AS-associated NeP may have other action mechanisms other than the TNF in ammatory pathway. Given this postulation, we detected 8 neurotransmitters in the peripheral blood of 20 AS patients and found that the serum NE concentrations in AS patients with high PDQ were signi cantly elevated. Both NE and 5HT are pain-related neurotransmitters that participate in regulating both algesic and analgesic processes. Peripherally, these neurotransmitters are algesic factors that exert their regulatory effect on local neurocytes via second messengers such as calcium ions or cyclic nucleotides or through a paracrine pathway producing pain by the stimulation of sensory nerve endings. Detection of the concentration of algesic factors in peripheral blood can, to some extent, re ect the degree of pain [23].Based on this nding, we are using pregabalin to treat NeP in AS patients, and related data are being collected. This study has some limitations. We only detected neurotransmitters in the peripheral blood of a limited number of AS patients. We plan to further expand the sample size in our ongoing research and use head MRI to explore the structural or functional changes in the pain-related regions of the central nervous system.

Conclusions
In summary, NeP is a common occurrence in AS patients as well as an important component and one of the three dimensions of AS pain. TNF inhibitors do not seem to be as effective as expected in improving AS-associated NeP. The NE concentrations in the peripheral blood of AS patients with NeP complications are signi cantly elevated, suggesting that interference with neurotransmitter actions may be a promising new approach to the treatment of AS-associated NeP.

Consent for publication
Not applicable.

Availability of data and material
All relevant data are contained within the paper.

Competing interests
The authors declare that they have no competing interests.

Funding
This study was funded by Shanghai Municipal Key Clinical Specialty (shslczdzk02602).
Authors' contributions LZ, TL, and HX were responsible for study conception and design. HL, XW, JB and LL were responsible for the acquisition of data. LY, JY, and JZ performed the data analysis and interpretation. All authors were involved in drafting the article or revising it critically for important intellectual content, and all authors approved the nal version to be published. BASDAI=Bath assessment of ankylosing spondylitis disease activity index; HAQ=Health assessment questionnaire; HAD-An=Hospital Anxiety and Depression Scale-Anxiety; HAD-D=Hospital Anxiety and Depression Scale-Depression