Not just sensitization: sympathetic mechanisms contribute to expand experimental referred pain

Abstract

Background: Widespread pain partially depends upon sensitization of central pain mechanisms. However, mechanisms controlling pain distribution are not completely known. The present study sought to assess skin temperature variations in the area of experimentally-induced pain and potential sex differences.

Methods: Pressure-pain thresholds (PPTs) were measured on the right infraspinatus muscle. At the end of Day 0, all participants performed an eccentric exercise of the shoulder external rotators to induce muscle soreness 24 hours after. On Day 1, participants indicated on a body chart the area of pain induced by 60 seconds of suprathreshold pressure stimulation (STPS; PPT + 20%) on the right infraspinatus muscle. Skin temperature variations in the area of referred pain were recorded with an infrared thermography camera, immediately before and after the STPS.

Results: Twenty healthy, pain-free individuals (10 females) participated. On Day 0, the pre-STPS temperature was higher than the post-STPS temperature on the arm (P = 0.001) and forearm (P = 0.003). On Day 1, the pre-STPS temperature was higher than the post-STPS temperature on the shoulder (P = 0.015), arm (P = 0.001), and forearm (P = 0.010). On Day 0, the temperature decrease after STPS in females was greater than in males on the forearm (P = 0.039). On Day 1, a greater temperature decrease was found amongst females compared with males at the shoulder (P = 0.018), arm (P = 0.046), and forearm (P = 0.005).

Conclusions: These findings indicate that sympathetic vasomotor responses contribute to expand pressure-induced referred pain, especially among females.

Keywords: Female; Pain; Pain Measurement; Pain Perception; Pain Threshold; Referred; Sex Characteristics; Shoulder; Sympathetic Nervous System; Temperature; Thermography..

Fig. 1

Two-day study protocol. PPTs: pressure pain thresholds, STPS: suprathreshold pressure stimulation, DOMS: delayed-onset muscular soreness.

Fig. 2

Body divisions show the areas of interest to quantify temperature variations in thermography images following the infraspinatus suprathreshold pressure stimulation: shoulder (a), arm (b), and forearm (c).

Fig. 3

Mean temperature variations before and after suprathreshold pressure stimulation on Day 0 and Day 1 for the shoulder, arm, and forearm regions. Error bars indicate standard error of the mean. *Significantly different compared to arm and forearm regions after Bonferroni post hoc test, P < 0.05.

Fig. 4

Mean temperature variations before and after suprathreshold pressure stimulation on Day 0 and Day 1. Error bars indicate standard error of the mean. *Significant sex differences after Bonferroni post hoc test, P < 0.05. ^#Significantly different compared to Day 0 after Bonferroni post hoc test, P < 0.05.

Fig. 5

Proportion of participants experiencing pressure-induced referred pain at the shoulder, arm, and forearm regions following suprathreshold pressure stimulation on Day 0 and Day 1.