ASDA, American Sleep Disorders Association; EMG, electromyogram; GER, gastroesophageal reflux; LES, lower esophageal sphincter; NREM, non-rapid eye movement; REM, rapid eye movement; RMMA, rhythmic masticatory muscle activity; SB, sleep bruxism; SD, standard deviation; UES, upper esophageal sphincter.

There are several reports suggesting that forward head posture is associated with temporomandibular disorders and restraint of mandibular growth, possibly due to mandibular displacement posteriorly. However, there have been few reports in which the condylar position was examined in forward head posture. The purpose of this study was to test the hypothesis that the condyle moves posteriorly in the forward head posture. The condylar position and electromyography from the masseter, temporal and digastric muscles were recorded on 15 healthy male adults at mandibular rest position in the natural head posture and deliberate forward head posture. The condylar position in the deliberate forward head posture was significantly more posterior than that in the natural head posture. The activity of the masseter and digastric muscles in the deliberate forward head posture was slightly increased. These results suggest that the condyle moves posteriorly in subjects with forward head posture.

Bruxism is a repetitive jaw-muscle activity characterized by clenching or grinding of the teeth and/or bracing or thrusting of the mandible. Recent advances have clarified the relationship between gastroesophageal reflux and sleep bruxism (SB). However, the influence of pharmacological elimination of gastric acid secretion on SB has not been confirmed. The authors aimed to assess the efficacy of a proton pump inhibitor (PPI) on SB and to examine the gastrointestinal (GI) symptoms and endoscopic findings of the upper GI tract in SB patients. The authors performed a randomized double-blind placebo-controlled crossover study at Kagoshima University Hospital. Twelve patients with polysomnography (PSG)-diagnosed SB underwent an assessment of GI symptoms using the frequency scale for the symptoms of gastroesophageal reflux disease (FSSG) and esophagogastroduodenoscopy. At baseline (i.e., before interventions), the mean frequencies of electromyography (EMG) bursts and rhythmic masticatory muscle activity (RMMA) episodes were 65.4 ± 49.0 bursts/h and 7.0 ± 4.8 episodes/h, respectively, and at least 1 RMMA episode with grinding noise was confirmed in all participants. The mean FSSG score was 8.4 ± 5.6, and 41.7% of patients were diagnosed with gastroesophageal reflux disease. Mild reflux esophagitis was confirmed in 6 patients. PSG, including EMG of the left masseter muscle and audio-video recording, was performed on days 4 and 5 of administration of 10 mg of the PPI (rabeprazole) or placebo. PPI administration yielded a significant reduction in the frequency of EMG bursts, RMMA episodes, and grinding noise. No significant differences were observed regarding the swallowing events and sleep variables. Since the clinical application of PPI for SB treatment should remain on hold at present, the results of this trial highlight the potential application of pharmacological gastroesophageal reflux disease treatment for SB patients. Larger scale studies are warranted to corroborate these findings. (UMIN Clinical Trials Registry: UMIN000004577).

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Recent studies have been revealing the relationship between the stomatognathic system and the gastrointestinal tract. However, the effect of oesophageal acid stimulation on masticatory muscle activity during wakefulness has not been fully elucidated. To examine whether intra-oesophageal acidification induces masticatory muscle activity, a randomised trial was conducted investigating the effect of oesophageal acid infusion on masseter muscle activity, autonomic nervous system (ANS) activity and subjective symptoms. Polygraphic monitoring consisting of electromyography of the masseter muscle, electrocardiography and audio-video recording was performed in 15 healthy adult men, using three different 30-min interventions: (i) no infusion, (ii) intra-oesophageal saline infusion and (iii) intra-oesophageal infusion of acidic solution (0·1 N HCl; pH 1·2). This study was registered with the UMIN Clinical Trials Registry, UMIN000005350. Oesophageal acid stimulation significantly increased masseter muscle activity during wakefulness, especially when no behaviour was performed in the oro-facial region. Chest discomfort, including heartburn, also increased significantly after oesophageal acid stimulation; however, no significant correlation was observed between increased subjective symptoms and masseter muscle activity. Oesophageal acid infusion also altered ANS activity; a significant correlation was observed between masticatory muscle changes and parasympathetic nervous system activity. These findings suggest that oesophageal-derived ANS modulation induces masseter muscle activity, irrespective of the presence or absence of subjective gastrointestinal symptoms.