Brainstem reflexes in patients with sleep bruxism: Masseter inhibitory reflex responses and auditory startle reaction

Abstract

Background Sleep bruxism (SB) is a stereotyped movement disorder characterized by grinding or clenching of the teeth during sleep. Neurophysiologic methods offer approaches to study the excitability in SB by recording the motor potentials evoked by masseter muscles and masseter inhibitory reflex (MIR). Auditory startle reaction (ASR) has been found to be increased in restless leg syndrome but it has not been studied in SB. We aimed to show the hyperexcitability in the central jaw motor pathways in patients with SB by studying MIR and ASR responses. Methods 10 patients (mean age 48,4 ± 10,2 years; 8 male and 2 female) and 20 healthy subjects (mean age 34,4 ± 11,2 years; 6 male and 14 female) were enrolled in the study. The EMG activity was recorded from two masseter muscles by surface electrodes. The duration and latency of early and late silent periods and suppression ratios obtained from patient and control groups were compared. ASR was recorded from o. oculi, masseter, sternocleidomastoid, biceps brachii and tibialis anterior muscles and probability and latencies were evaluated. SPSS for Windows 11.5 package was employed. Results The differences in silent period onset latencies between patients and normal subjects were not significant but durations of early and late silent periods were shorter in SB group. Conclusions These results may indicate a reduced capacity for inhibition by the circuits responsible for the late period in SB and this increased excitability in central motor pathways could derive from an impaired modulation of subcortical structures especially brainstem inhibitory circuits and not from altered cortical mechanisms. Since ASR did not differ between the control and patient groups, it can be possible to say that there’s no abnormality in brainstem and reticulospinal circuits in SB patients.