Experimentally induced rhythmic jaw muscle activities during non‐rapid eye movement sleep in freely moving guinea pigs

Sleep architecture as a candidate for phenotyping sleep bruxism: A narrative physiological review

BACKGROUND

Sleep bruxism (SB), an oral behaviour in otherwise healthy individuals, is characterised by frequent rhythmic masticatory muscle activity (RMMA) during sleep. RMMA/SB episodes occur over various sleep stages (N1-N3 and rapid eye movement (REM)), sleep cycles (non-REM to REM), and frequently with microarousals. It currently remains unclear whether these characteristics of sleep architecture are phenotype candidates for the genesis of RMMA/SB.

OBJECTIVES

This narrative review investigated the relationship between sleep architecture and the occurrence of RMMA as a SB phenotype candidate.

METHODS

PubMed research was performed using keywords related to RMMA/SB and sleep architecture.

RESULTS

In non-SB and SB healthy individuals, RMMA episodes were most frequent in the light non-REM sleep stages N1 and N2, particularly during the ascending phase of sleep cycles. The onset of RMMA/SB episodes in healthy individuals was preceded by a physiological arousal sequence of autonomic cardiovascular to cortical activation. It was not possible to extract a consistent sleep architecture pattern in the presence of sleep comorbidities. The lack of standardisation and variability between subject complexified the search for specific sleep architecture phenotype(s).

CONCLUSION

In otherwise healthy individuals, the genesis of RMMA/SB episodes is largely affected by oscillations in the sleep stage and cycle as well as the occurrence of microarousal. Furthermore, a specific sleep architecture pattern cannot be confirmed in the presence of sleep comorbidity. Further studies are needed to delineate sleep architecture phenotype candidate(s) that contribute to the more accurate diagnosis of SB and treatment approaches using standardised and innovative methodologies.

Understanding the pathophysiology of sleep bruxism based on human and animal studies: A narrative review

BACKGROUND

Sleep bruxism (SB) is a common sleep disorder that affects approximately 20% of children and 10% of adults. It may cause orodental problems, such as tooth wear, jaw pain, and temporal headaches. However, the pathophysiological mechanisms underlying SB remain largely unknown, and a definitive treatment has not yet been established.

HIGHLIGHT

Human studies involving polysomnography have shown that rhythmic masticatory muscle activity (RMMA) is more frequent in otherwise healthy individuals with SB than in normal individuals. RMMA occurs during light non-rapid eye movement (non-REM) sleep in association with transient arousals and cyclic sleep processes. To further elucidate the neurophysiological mechanisms of SB, jaw motor activities have been investigated in naturally sleeping animals. These animals exhibit various contractions of masticatory muscles, including episodes of rhythmic and repetitive masticatory muscle bursts that occurred during non-REM sleep in association with cortical and cardiac activation, similar to those found in humans. Electrical microstimulation of corticobulbar tracts may also induce rhythmic masticatory muscle contractions during non-REM sleep, suggesting that the masticatory motor system is activated during non-REM sleep via excitatory inputs to the masticatory central pattern generator.

CONCLUSION

This review article summarizes the pathophysiology of SB and putative origin of RMMA in both human and animal studies. Physiological factors contributing to RMMA in SB have been identified in human studies and may also be present in animal models. Further research is required to integrate the findings between human and animal studies to better understand the mechanisms underlying SB.

Sleep bruxism is highly prevalent in adults with obstructive sleep apnea: a large-scale polysomnographic study

STUDY OBJECTIVES

The aim was to determine the prevalence and risk factors of sleep bruxism (SB) and to investigate the relationships between SB episodes, arousals, and respiratory events in adults with obstructive sleep apnea (OSA).

METHODS

This prospective study included 914 adults with OSA (305 females, 609 males; age = 53 years [interquartile range = 17]; apnea-hypopnea index = 13.9 events/h [interquartile range = 21]). The diagnosis of SB was made when the rhythmic masticatory muscle activity (RMMA) index was at least 2 episodes/h of sleep based on a full polysomnographic recording. Binary logistic regression was performed to identify risk factors for SB. Network analysis was performed to determine the relations between RMMA, respiratory event, sleep arousal, and other factors. Further, the percentage of RMMA time-related to arousal was calculated.

RESULTS

The prevalence of SB in adults with OSA was 49.7%. Male sex, lower body mass index, and higher percentage of N1 sleep increased the odds of having SB (odds ratios = 1.425, 0.951, and 1.032, respectively; all P < .05). Network analysis showed that there were no direct associations between RMMA and apnea-hypopnea index, nor between RMMA and arousal, although 85.7% of RMMA was time-related to arousals.

CONCLUSIONS

Nearly half of adults with OSA have comorbid SB. Male sex, lower body mass index, and a higher percentage of light sleep increase the risk of having SB. Although RMMAs do not directly correlate with respiratory events and arousals, most RMMAs are time-related to arousals in adults with OSA.

CLINICAL TRIAL REGISTRATION

Registry: Netherlands Trial Register; Name: A Large Sample Polysomnographic Study on Sleep Bruxism; URL: https://trialsearch.who.int/Trial2.aspx?TrialID=NL8516; Identifier: NL8516.

CITATION

Li D, Kuang B, Lobbezoo F, de Vries N, Hilgevoord A, Aarab G. Sleep bruxism is highly prevalent in adults with obstructive sleep apnea: a large-scale polysomnographic study. J Clin Sleep Med. 2023;19(3):443-451.

Motor representation of rhythmic jaw movements in the amygdala of guinea pigs

OBJECTIVE

The areas of the amygdala contributing to rhythmic jaw movements and the movement patterns induced remain unknown. Therefore, the present study investigated the areas of the amygdala contributing to rhythmic jaw movements using repetitive electrical microstimulation techniques.

DESIGN

Experiments were performed on head-restrained guinea pigs under ketamine-xylazine anesthesia. EMG activities in the masseter and digastric muscles and jaw movements were recorded. Short- and long-train electrical microstimulations of the amygdala were performed and the patterns of jaw movements induced were analyzed quantitatively.

RESULT

The short-train stimulation induced short-latency EMG responses in the masseter and/or digastric muscles. The stimulation sites inducing short-latency EMG responses were distributed within the ventral part of the amygdala, which covered the medial, basal, and cortical nuclei. The long-train stimulation induced tonic jaw opening and two types of rhythmic jaw movements: those with or without lateral jaw shifts, which were characterized by a larger jaw gape and ipsilateral jaw excursion, respectively. Rhythmic jaw movements with lateral jaw shifts were characterized by overlapping masseter and digastric EMG activities. However, rhythmic patterns did not differ between the two types of rhythmic jaw movements. The stimulation sites that induced rhythmic jaw movements were more localized to the cortical nucleus.

CONCLUSIONS

The present results suggest that the ventral part of the amygdala is involved in the induction of rhythmic jaw movements in guinea pigs. The putative roles of the limbic system in the genesis of functional (e.g., chewing) and non-functional (e.g., bruxism) rhythmic oromotor movements warrant further study.

After-effects of acute footshock stress on sleep states and rhythmic masticatory muscle activity during sleep in guinea pigs

This study investigated the effects of acute footshock stress (FS) on the occurrence of rhythmic masticatory muscle activity (RMMA) during sleep in guinea pigs. Animals were prepared for chronic recordings from electroencephalogram, electrooculogram and electromyograms of neck and masseter muscles. The signals were recorded for six hours on the two successive days: the first day with stress-free condition (non-FS condition) and the second day with acute FS (FS condition). Sleep/wake states and RMMA were scored visually. Sleep variables and the frequency of RMMA occurring during non-rapid eye movement (NREM) sleep were compared during 6-h periods between the two conditions. Compared to non-FS condition, the amount of total sleep and NREM sleep significantly reduced during 2 h following the acute FS in the FS condition. Similarly, the frequency of RMMA significantly increased during 2 h following the acute FS for the FS condition compared to non-FS condition. During 2-6 h after FS in the FS condition, sleep variables and the frequency of RMMA did not differ from those without FS in the non-FS condition. These results suggest that acute experimental stress can induce transient changes in sleep-wake states and the occurrence of RMMA in experimental animals.

A lack of specific motor patterns between rhythmic/non-rhythmic masticatory muscle activity and bodily movements in sleep bruxism

Purpose The aims of the present study were to investigate the temporal relationships between jaw and bodily movements and clarify motor processes in the genesis of rhythmic masticatory muscle activity (RMMA) in sleep bruxism (SB).Methods Video-polysomnography recordings were obtained from ten subjects with SB (mean age: 23.4 ± 1.6 years) and ten matched normal controls (CTL) (mean age: 24.4 ± 3.2 years). RMMA and nonspecific masseter activity (NSMA) were scored in association with bodily movements in the leg, arm, head, and trunk using electromyography and video recordings. The relationship between oromotor episodes and bodily movements was assessed in terms of sleep stage distributions and temporal relationships. Cardiac changes preceding oromotor episodes in stage N2 were assessed.Results Approximately 80% of RMMA and NSMA were associated with movements in one or more body sites. RMMA and NSMA were more frequently associated with movements of the leg (70-75%) and arm (40-55%) than movements of the head (17-22%) and trunk (5-25%). The relationship between oromotor episodes and bodily movements did not significantly differ among sleep stages. Oromotor episodes and bodily movements did not show a consistent temporal pattern in the SB and CTL groups. Regardless of the temporal relationship between oromotor episodes and bodily movements, the mean heart rate significantly increased by 5 beats before the onset of oromotor episodes.Conclusions No specific temporal motor patterns were found between RMMA and bodily movements. RMMA and NSMA represent a repertoire of arousal-related autonomic motor responses during sleep.

Sleep stage-dependent changes in tonic masseter and cortical activities in young subjects with primary sleep bruxism

STUDY OBJECTIVES

The present study investigated the hypothesis that subjects with primary sleep bruxism (SB) exhibit masseter and cortical hyperactivities during quiet sleep periods that are associated with a high frequency of rhythmic masticatory muscle activity (RMMA).

METHODS

Fifteen SB and ten control participants underwent polysomnographic recordings. The frequencies of oromotor events and arousals and the percentage of arousals with oromotor events were assessed. Masseter muscle tone during sleep was quantified using a cluster analysis. Electroencephalography power and heart rate variability were quantified and then compared between the two groups and among sleep stages.

RESULTS

The frequency of RMMA and percentage of arousals with RMMA were significantly higher in SB subjects than in controls in all stages, while these variables for non-rhythmic oromotor events did not significantly differ between the groups. In SB subjects, the frequency of RMMA was the highest in stage N1 and the lowest in stages N3 and R, while the percentage of arousals with RMMA was higher in stage N3 than stages N1 and R. The cluster analysis classified masseter activity during sleep into two clusters for masseter tone and contractions. Masseter muscle tone showed typical stage-dependent changes in both groups, but did not significantly differ between the groups. Furthermore, no significant differences were observed in electroencephalography power or heart rate variability between the groups.

CONCLUSION

Young SB subjects exhibited sleep stage-dependent increases in the responsiveness of RMMA to transient arousals, but did not show masseter or cortical hyperactivity during sleep.