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

Abstract Background Pain in AS is currently considered an inflammatory pain (IP). However, it was found that some patients still had the sensation of pain even without inflammation. Our study was to investigate whether ankylosing spondylitis (AS) pain has a neuropathic pain (NeP) component. Methods The study consisted of three parts. The first 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 definite NeP. There were significant 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 first principal component (0.703 vs 0.639). Structural equation modelling (SEM) revealed that NeP altered disease activity (β = 0.62, P < 0.001), which influenced psychological status (β = 0.42, P < 0.001). Of 63 patients who used etanercept for 3 months, significant improvement was found in NP, TBP and PP (all P < 0.0001) but not in PDQ (10.60 ± 6.85 vs 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.