Somatosensory profiling of intra-oral capsaicin and menthol in healthy subjects

Abstract

This study was designed to investigate the effect of surrogate orofacial pain models on the quantitative sensory testing (QST) profile in healthy participants. Capsaicin, menthol, or saline (control) were applied topically onto the gingiva of 15 healthy subjects for 15 min. During application, the subjects rated pain intensity on a score of 0-10, on an electronic visual analog scale (VAS). A standardized intra-oral QST protocol was performed before and immediately after application. Data obtained before and after application were compared using rank-sum tests, and QST profiles were made after Z-transformation. Application of capsaicin caused moderate levels of pain (VAS(peak) = 6.0 ± 0.7), and application of menthol produced mild levels of pain (VAS(peak) = 1.8 ± 0.6). Capsaicin induced hypersensitivity to warmth, heat pain and cold pain and hyposensitivity to mechanical stimuli. Menthol induced hypersensitivity to cold and warmth. Saline caused hypersensitivity to heat pain and hyposensitivity to mechanical stimuli. However, somatosensory profiles from Z-scores demonstrated sensory gains regarding warmth detection and heat pain only after application of capsaicin. In conclusion, a standardized battery of QST showed somatosensory changes after application of capsaicin, menthol and saline to the gingiva. However, the Z-score-based profiles may only reflect the most prominent somatosensory changes and thus represent a conservative approach for evaluation of data.

Fig. 1

Subject-reported visual analog scale (VAS) pain scores (mean values ± SEM) from topical application of capsaicin (black icons), menthol (grey icons) or saline (white icons) on the attached gingiva. Mean values (n=15) during the 900-s (15 min) recording period.

Fig. 2

Individual Z-score based quantitative sensory testing (QST) profiles from the attached gingiva after application of capsaicin (A), menthol (B) or saline (C) (n = 15). Part D shows the averaged Z scores. CDT, cold detection threshold (°C); WDT, warmth detection threshold (°C); TSL, thermal sensory limen (°C); CPT, cold pain threshold (°C); HPT, heat pain threshold (°C); PPT, pressure pain threshold (kPa); MPT, mechanical pain threshold (mN); MPS, mechanical pain sensitivity (mean pain rating, 0–100); WUR, wind-up ratio; MDT, mechanical detection threshold (mN); VDT, vibration detection threshold (/8); PHS, paradoxical heat sensation (/3); ALL, dynamic mechanical allodynia. NRS, numerical rating scale (pain intensity from 0–100). The grey zone (Z score between −1.96 and 1.96) represents the 95 % confidence interval of baseline values.