Effect of jaw muscle pain and soreness evoked by capsaicin before sleep onorofacial motor activity during sleep

Aims: Sleep bruxism, which is a form of orofacial motor activity (OMA), and jaw muscle pain and soreness have for a long time been thought to be mutua lly linked. The aim of this study was to investigate the effect of clinical and experimental jaw muscle pain and soreness on sleep OMA. Methods: Twelv e healthy subjects aged 21 to 31 years old participated in this study. All of them were aware of signs or symptoms of sleep OMA and were subdivided in to a group with clinical pain complaints (n = 5) and a group without pain ( n = 7). All subjects slept in the laboratory for 3 consecutive nights, incl uding a habituation night, a baseline night, and an experimental night. Ele ctroencephalographic (EEG) activity and electromyographic (EMG) activity fr om the masseter muscles were recorded during sleep. On the experimental nig ht, before sleep, all subjects received an injection of capsaicin (0.1 mL, 100 mug/mL) into the masseter muscle that had demonstrated the most EMG act ivity during the previous recordings. The OMA events and episodes were quan tified and were compared between the baseline night and the experimental ni ght. Every evening and morning during the study period, pain intensity, unp leasantness, and soreness were scored by the subjects on a visual analog sc ale (VAS), and pain detection thresholds (PDTs) in the masseter muscles and maximal voluntary occlusal force (MVOF) were also measured. Results: Pre-s leep injection of capsaicin did not cause significant differences between g roups in peak pain intensity on the VAS. The PDT and MVOF did not show any significant differences between groups, injection and non-injection sides, or baseline and experimental nights and mornings. The number of EMG episode s/hour sleep, the number of bursts/hour sleep, and total area of all bursts and episodes during the baseline night were significantly higher in the su bjects without pain than in the subjects with pain. However, the capsaicin injection did not cause any significant changes in these parameters. Conclu sion: This study suggests that an acute Pre-sleep painful stimulus does not have any effect on OMA during sleep, but the study extends previous findin gs that clinical jaw muscle pain and soreness are associated with less EMG activity in the masticatory muscles.