Homotopic and heterotopic variation in skin blood flow and temperature following experimental muscle pain in humans

The aim of the present study was to explore variation in skin blood flow and temperature following experimental muscle pain. In 14 male human subjects, 2 ml and 4.8 ml of hypertonic (5.8%) saline were injected into the left tibialis anterior (TA) muscle to induce muscle pain. The subjects rated the pain intensity on a 10 cm visual analogue scale (VAS). Using laser-Doppler flowmetry and thermography, the skin blood flow and temperature were assessed at four different skin areas: ipsilateral muscle pain area (5 × 5 cm), ipsilateral referred pain area (5 × 10 cm), and two corresponding mirror areas on the contralateral non-injected leg. Compared with 2 ml hypertonic saline injection, significantly longer pain duration (1599 ± 119 s) and stronger pain intensity (VAS peak: 6.9 ± 0. 6 cm) were found after the injection of 4.8 ml hypertonic saline (P < 0.001, respectively). In addition, 4.8 ml hypertonic saline evoked a significant increase in skin blood flow and higher skin temperature around the injection site, the referred pain area, and the contralateral area to the injection site (P < 0.05). By contrast, 2 ml hypertonic saline injection only elicited an increase in skin blood flow, but not temperature, at the injection area and the contralateral mirror area to the injection site (P < 0.05). These results suggested that the vasodilation in different skin areas following intramuscular injection of hypertonic saline was dose-dependent. Injection of 4.8 ml hypertonic saline after local intramuscular anesthesia (2% lidocaine) did not evoke any significant changes in skin blood flow or skin temperature in any of the four observation areas. This suggested that both homotopic and heterotopic vascular reactions triggered by hypertonic saline stimulation of thin muscle afferent fibers were a neurogenically associated reaction.