Stimulation of myofascial trigger points in the sternocleidomastoid evokes facial thermal response correlated with the referred pain

The purpose of this research is to identify and correlate the referred pain evocated by myofascial trigger points (TrPs) pressure pain threshold (PPT) in the sternocleidomastoid muscle using thermal infrared imaging (IR). Facial IR images of 46 volunteers (21 male and 25 female, average age 32 ± 6.3) undergoing PPT of five TrPs locations on the sternocleidomastoid belly were recorded. Each PPT lasted 10 s, with an interstimulus interval of 2 min. Sixteen thermal IR images were recorded for each subject: at baseline (t0), 2 s before PPT (t1), 2 s (t2) and 60 s (t3) after PPT of each TrPs location. During the interstimulus interval, subjects were asked to draw over a head– neck template displayed on a computer screen the areas of referred pain eventually evoked by the stimulation and the referred pain intensity by means of a Visual Analogue Scale (VAS). The VAS template was then superimposed with the IR records. Two temperature (T) variations were calculated: ∆T1 = T(t2) − T(t1) and ∆T2 = T(t3) − T(t1). Differences in ∆T range ≥ 0.2◦C have been considered significant. In 77% of the superimpositions, the referred pain area corresponded to a ∆T2 ≥ 0.2◦C while only the 59% corresponded to a ∆T1 ≥ 0.2◦C. In 19% of superimpositions, a ∆T2 ≥ 0.2◦C did not correspond to a referred pain area indicated by the patient, and this percentage lowers to 4% for ∆T1 ≥ 0.2◦C. None of the areas that reported a VAS of 0 or 1 showed a ∆T1 ≥ 0.2◦C or a ∆T2 ≥ 0.2◦C. Considering the limitations of this pilot study, IR could be used to identify referred pain evocated by TrPs on sternocleidomastoid muscle.