During Capsaicin-induced Central Sensitization Brush Allodynia is Associated with Baseline Warmth Sensitivity, Whereas Mechanical Hyperalgesia is Associated with Painful Mechanical Sensibility, Anxiety and Somatization

Background: Mechanical hyperalgesia and allodynia incidence varies considerably among neuropathic pain patients. This study explored whether sensory or psychological factors associate with mechanical hyperalgesia and brush allodynia in a human experimental model. Methods: Sixty-six healthy volunteers (29 male) completed psychological questionnaires and participated in two quantitative sensory testing (QST) sessions. Warmth detection threshold (WDT), heat pain threshold (HPT) and suprathreshold mechanical pain (STMP) ratings were measured before exposure to a capsaicin-heat pain model (C-HP). After C-HP exposure, brush allodynia and STMP were measured in one session, while mechanical hyperalgesia was measured in another session. Results: WDT and HPT measured in sessions separated by one month demonstrated significant, but moderate levels of reliability (WDT: ICC=0.5, 95%CI[0.28,0.77]; HPT: ICC=0.62, 95%CI[0.40,0.77]). Brush allodynia associated with lower WDT (z=−3.06, p=0.002; ϕ=0.27). Those with allodynia showed greater hyperalgesia intensity (F=7.044, p=0.010, η p}^{2}=0.107) and area (F=9.319, p=0.004, η p}^{2}=0.163) than those without allodynia. No psychological self-report measures were significantly different between allodynic and non-allodynic groups. Intensity of hyperalgesia in response to lighter mechanical stimuli was associated with lower HPT, higher STMP ratings, and higher Pain Sensitivity Questionnaire scores at baseline. Hyperalgesia to heavier probe stimuli associated with state anxiety and to a lesser extent somatic awareness. Hyperalgesic area associated with lower baseline HPT and higher STMP ratings. Hyperalgesic area was not correlated with allodynic area across individuals. Conclusions: These findings support research in neuropathic pain patients and human experimental models that peripheral sensory input and individual sensibility are related to development of mechanical allodynia and hyperalgesia during central sensitization, while psychological factors play a lesser role.