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J Dent Res 86(9):837-842, 2007
© 2007 International and American Associations for Dental Research

RESEARCH REPORT
Clinical

Identification of a Sleep Bruxism Subgroup with a Higher Risk of Pain

P.H. Rompré1, D. Daigle-Landry1, F. Guitard1, J.Y. Montplaisir2, and G.J. Lavigne1,2,*

1 Faculty of Dental Medicine, Université de Montréal, C.P. 6128, succ. Centre-Ville, Montréal, Canada, H3C 3J7; and
2 Centre d’étude du sommeil et des rythmes biologiques, Hôpital du Sacré-Coeur, Canada

* corresponding author, gilles.lavigne{at}umontreal.ca


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Sleep bruxism research diagnostic criteria (SB-RDC) have been applied since 1996. This study was performed to validate these criteria and to challenge the hypothesis that pain is associated with lower frequencies of orofacial activities. Polygraphic recordings were made of 100 individuals presenting with a clinical diagnosis of sleep bruxism and 43 control individuals. TwoStep Cluster analyses (SPSS) were performed with sleep bruxism variables to reveal groupings among sleep bruxers and control individuals. Participants completed questionnaires during screening, diagnosis, and recording sessions. Cluster analysis identified three subgroups of sleep bruxers. Interestingly, 45 of the 46 sleep bruxers with values below SB-RDC were classified in the low-frequency cluster. These individuals were more likely to complain of pain and fatigue of masticatory muscles than were the higher-frequency sleep bruxers (odds ratios > 3.9, p < 0.01). Sleep bruxers were distributed among three heterogeneous groups. Sleep bruxers with low frequencies of orofacial activities were more at risk of reporting pain.

KEY WORDS: sleep bruxism • polygraphy • cluster analysis • diagnostic criteria • tooth grinding • pain


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Sleep bruxism was recently classified as a movement disorder during sleep (AASM, 2005). In 1996, we proposed sleep bruxism research diagnostic criteria (SB-RDC) for polygraphic recording of sleep bruxism motor activity (Lavigne et al., 1996). These criteria were based on a small sample size (18 sleep bruxers and 18 control individuals). They were derived from electromyographic and audio-video recognition of jaw muscle activity in relation to: (1) a positive report of tooth grinding during sleep; and (2) the presence of sleep bruxism in a sleep laboratory. The criteria were: > 4 sleep bruxism episodes/hr of sleep, > 25 sleep bruxism bursts/hr of sleep, and > 1 sleep bruxism episode with tooth-grinding sounds.

Over the last 15 years, we have made sleep laboratory recordings of 100 individuals with a positive home history of tooth grinding. Interestingly, half of the persons presented a low frequency of sleep bruxism episodes per hour of sleep (lower than 4 episodes/hr) and few tooth-grinding episodes, in spite of their sleep partners’ complaints of frequent grinding noise. One explanation may be that these persons experienced pain, which caused them to have fewer orofacial activities, in accordance with the pain adaptation model (Lund, 1995; Lavigne et al., 1997). Therefore, the objectives of this paper were to validate the 1996 SB-RDC in a large number of control individuals and sleep bruxers recorded in the sleep laboratory, and to challenge the hypothesis that pain is associated with lower frequencies of orofacial activities.


   MATERIALS & METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Participants
Data from 100 sleep bruxers and 43 control individuals, recorded in the sleep laboratory since 1990, were used for the analysis. Sleep bruxers were chosen based on: (1) a history of frequent tooth grinding occurring at least 3 nights per week for the preceding 6 mos, as confirmed by a sleep partner; (2) clinical presence of tooth wear; (3) masseter muscle hypertrophy; and (4) report of jaw muscle fatigue or tenderness in the morning (American Sleep Disorders Association, 1997; AASM, 2005; Lavigne et al., 2005). Control individuals were selected on the basis of the absence of a history of tooth grinding during sleep and of other clinical evidence of sleep bruxism (American Sleep Disorders Association, 1997; AASM, 2005; Lavigne et al., 2005). Exclusion criteria for both sleep bruxers and control individuals were TMD as a primary complaint, a medical history of neurological disorders, mental disorders, or sleep disorders (e.g., apnea, periodic leg movements, insomnia). At the time of recordings, none of the participants was taking medication, or was under the influence of alcohol, nicotine, or caffeine. All participants provided informed consent according to the institutional rules (Hôpital du Sacré-Coeur).

Polygraphic Recordings
Individuals were studied in the sleep laboratory for two consecutive nights. The first night allowed them to adapt to the laboratory setting and permitted researchers to rule out sleep disorders. On the second night, sleep bruxism and sleep were analyzed. Polygraphic recordings, with surface electrodes, included two electroencephalograms (EEG; C3A2, O2A1), bilateral electro-oculograms, an electrocardiogram, and electromyograms (EMG) from chin/suprahyoid, bilateral masseter, temporalis, and tibialis muscles. Respiration was monitored with a nasal flow sensor. All signals were amplified and recorded at a sampling rate of 128 Hz and stored for off-line analysis by Harmonie Software (Stellate Systems, Montréal, Canada). Audio and video recordings were carried out simultaneously to distinguish sleep bruxism episodes from non-specific orofacial activities (Velly-Miguel et al., 1992; Kato et al., 1999).

Sleep and Sleep Bruxism Scoring
Sleep was scored according to standard criteria (Rechtschaffen and Kales, 1968). A micro-arousal was defined as an abrupt EEG frequency shift (> 3 sec) without complete awakening (American Sleep Disorders Association, 1991). Sleep bruxism episodes were scored into phasic (3 EMG bursts or more, each lasting 0.25 to 2.0 sec), tonic (one EMG burst > 2.0 sec), or mixed (both types of bursts) episodes (Lavigne et al., 1996). Tonic activities were analyzed for both groups, even though most episodes in control individuals are phasic or mixed, designated as Rhythmic Masticatory Muscle Activity (RMMA; Lavigne et al., 2001b). Scoring was performed blind to participant status.

Questionnaires
Participants answered questions concerning awareness of sleep bruxism, sleep habits, anxiety, stress, fatigue, nervousness, current facial pain intensity, painful jaw upon awakening, and fatigue of masticatory muscles at different moments. Participants answered a selection questionnaire during a telephone interview. Then, a questionnaire was completed during diagnosis, in the dental clinic. Finally, questionnaires were completed in the sleep laboratory before bedtime in the evening, and after awakening in the morning.

Statistical Analyses
Sleep and sleep bruxism variables that were not normally distributed were normalized with a logarithm. For these variables, groups were compared by two-sample t tests or one-way ANOVA, followed by Tukey pair-wise mean comparisons (Systat 11). Answers to questionnaires were analyzed by Fisher’s exact test and odds ratio. Answers on a VAS scale were evaluated with the Mann-Whitney U test. A p < 0.05 was considered statistically significant. TwoStep Cluster analyses (SPSS 14.0) were performed with sleep bruxism variables to reveal natural groupings (or clusters) among sleep bruxers and control individuals separately. The algorithm tested a range of the number of clusters. The optimal number of clusters was determined by the software, with Schwarz’s Bayesian Criterion (Norusis, 2006; SPSS, 2006).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Participants were young, with a mean age around 25 yrs old (bruxers, mean ± SE = 26.5 ± 0.6; control individuals, 24.5 ± 0.9, p = 0.07). There was a significantly higher proportion of womenamong bruxers (60%) and of men among control individuals (62.8%, p = 0.02).

Participants were included in or excluded from further studies based on an analysis of results from the second night in the sleep laboratory. Bruxers were included when they displayed values higher than 2 of the 3 previously established cut-offs: 4 sleep bruxism episodes/hr, 25 sleep bruxism bursts/hr, 1 episode with grinding noise. Control individuals were included when they had values lower than or equal to 2 of the 3 cut-offs. Based on these EMG criteria, 54 bruxers were included for further studies, while 46 were excluded. Similarly, 34 control individuals were included for further studies, while nine were excluded.

Sleep and sleep bruxism variables for these four subgroups (included bruxers, excluded bruxers, included control individuals, and excluded control individuals) are presented in Table 1Go. Sleep variables revealed few differences among subgroups. The number of micro-arousals per hour was marginally higher in included bruxers than in included control individuals (47% higher), but this did not reach statistical significance in these young participants. However, sleep bruxism variables revealed clear differences among the four subgroups. All subgroups differed regarding the number of episodes/hr, since all paired comparisons were statistically significant. Excluded control individuals showed more episodes/hr than did excluded bruxers (median 4.3 and 2.1, respectively, p < 0.05). All paired comparisons involving phasic episodes/hr, phasic bursts/hr, and bursts/hr were significant, except the comparison between included bruxers and excluded control individuals. No significant difference in the number of episodes with noise was observed between excluded bruxers and excluded control individuals, while other comparisons were significant. Paired comparisons among groups for mixed episodes/hr, tonic bursts/hr, and bursts/episodes were significant mostly between included bruxers and excluded bruxers, and included bruxers and included control individuals. All groups had few tonic episodes/hr (median near 0, p = 0.81).


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  		Table 1. Differences in Sleep and Bruxism between Subgroups
 
TwoStep cluster analysis for bruxers identified three subgroups of bruxers who differed in sleep bruxism frequency: low, moderate, and high (Table 2Go). Box plots for the three clusters are shown on the FigGo. Dotted lines indicate cut-off values of the SB-RDC. Groups were well-defined, and most individuals in the low cluster (n = 49) had values lower than cut-off for episodes/hr (cut-off = 4), bursts/hr (cut-off = 25), and episodes with noise (cut-off = 1). Interestingly, 45 of the 46 excluded bruxers were classified in this cluster. The importance of each variable in the three clusters is presented in the APPENDIX.


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  		Table 2. Clusters for Sleep Bruxers (n = 99) and Control Individuals (n = 42)
 

Figure 1
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  		Figure. Subgroups identified by cluster analysis based on sleep bruxism frequency. (a) Sleep bruxers were divided into three clusters: low (n = 49), moderate (n = 37), and high (n = 13). (b) Control individuals were divided into two clusters: low (n = 34) and high (n = 8). Box plots combined with symmetrical dot densities are shown. Dotted lines indicate cut-off values of the SB-RDC.

 
Cluster analysis performed in control individuals identified two clusters (Table 2Go). Most individuals in the high cluster had values above cut-off (Fig.Go). Seven of the nine control individuals who were excluded were classified in this cluster. The importance of each variable in the two clusters is given in the APPENDIX.

Answers to questionnaires (Table 3Go) revealed that excluded bruxers were significantly more likely than included bruxers to complain of clenching, with an odds ratio (OR) and 95% confidence interval of 4.9 (1.3–18.6). Awareness of tooth grinding, grinding noise, and tooth wear did not differ between excluded bruxers and included bruxers. Analyses revealed that both subgroups differed regarding complaint of pain. Excluded bruxers were more likely than included bruxers to complain of painful jaw upon awakening and fatigue of masticatory muscles (OR over 3.9, Table 3Go). The level of pain of excluded bruxers was slightly higher than that of included bruxers (median of 10.0 compared with 0.0 on a 0–100 VAS, p = 0.06). Evaluation of the psychological state "During the day before recording" showed no significant difference between subgroups. "Just before recording", excluded bruxers reported stress and nervousness in a higher proportion than did included bruxers (OR 3.5 for stress, Table 3Go), and stress and fatigue in a higher proportion than reported by included control individuals (p < 0.04, not shown). No significant difference in psychological state was observed between included bruxers and control individuals, either during the day before recording or just before recording (p > 0.1, not shown).


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  		Table 3. Differences in Answers* to Questionnaires between Included and Excluded Sleep Bruxers
 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
In a study published in 1996, the selection criteria for sleep bruxers included having exhibited grinding sounds during sleep at least 5 nights a wk in the preceding 6 mos (Lavigne et al., 1996). Moderate to severe sleep bruxers were recruited, as shown by their number of sleep bruxism episodes/hr (mean ± SE, 5.4 ± 0.6) and number of bursts/hr (40.7 ± 6.7). These values are comparable with those of sleep bruxers included for further studies and with those of the moderate cluster. Over the years, to have access to a larger pool of potential participants, investigators have reduced the criterion for reported grinding sounds during sleep from 5 to 3 nights a wk. Based on this criterion, 46 sleep bruxers recorded in the sleep laboratory were excluded from further studies, according to the SB-RDC. Consequently, the sensitivity and specificity of the SB-RDC established in 1996 would be lower if re-evaluated based on the total sample of sleep bruxers (n = 100) and control individuals (n = 46), due to the low sleep bruxers and control individuals with high frequencies of activities. For example, the cut-off of 4 episodes/hr would lead to lower sensitivity (55% instead of 72%) and specificity (84% instead of 94%). Lower values of cutoff (2.5 episodes/hr instead of 4.0) would be required to have sensitivity and specificity around 70%. However, the classification established by the SB-RDC closely matches that of the cluster analyses. Sleep bruxers in the low cluster and control individuals in the high cluster are those excluded by the SB-RDC. These persons displayed frequencies of activities different from those typical of their clinical group. Their inclusion in sleep laboratory studies is therefore not suitable.

Close to 50% of persons with a clinical history of tooth grinding presented low frequencies of jaw muscle contractions (episodes/hr, bursts/hr) and tooth-grinding events in the sleep laboratory. A high proportion of these participants reported painful jaw and fatigue of masticatory muscles, although they did not complain of or present TMD (temporomandibular muscle or joint pain or dysfunction). Other studies have reported that sleep bruxers frequently present with low pain intensity in jaw and neck muscles, or temporal headaches upon waking (Bader et al., 1997; Lavigne et al., 1997; Camparis et al., 2006; Huynh et al., 2006). The possibility that this episodic pain and headache occurred in relation to the recording in the sleep laboratory may have contributed to a reduction in the motor activity, as suggested by the Pain Adaptation Model (Lund, 1995). Another explanation could be the natural variability in the occurrence of sleep bruxism over time. We noted that the variability in the number of episodes/hr of sleep was 25%, while the variability in the number of episodes with grinding sound was over 50% (Lavigne et al., 2001a).

Conversely, approximately 20% (8/42) of control individuals displayed a high frequency of orofacial activities and were classified in the ‘high control’ cluster. Seven of these control individuals had more than 4 episodes/hr of sleep, eight reported more than 25 bursts/hr of sleep, and four presented at least 2 episodes with grinding sounds. This last finding may be surprising, but can be explained by the fact that some control individuals, who are young (mean age = 25 yrs old), may grind their teeth very rarely. Persons may also sleep alone or with a sleep partner who is not disturbed by their grinding sounds, thus providing an unreliable report of absence of tooth grinding at home.

A correlation of sleep bruxism with stress and anxiety from situational or psychological sources has been suggested (Rugh and Harlan, 1988; Hicks et al., 1990; AASM, 2005). However, this association remains controversial (Harness and Peltier, 1992; Pierce et al., 1995; Watanabe et al., 2003). The present sleep study does not support this association, since the levels of stress and anxiety did not differ between included sleep bruxers and control individuals.

The observation of 3 clusters in sleep bruxers and of 2 in control individuals further supports the suggestion that jaw muscle contractions during sleep are a natural activity, with a wide spectrum of frequency (number of episodes/hr). In a previous study, control individuals presented from 0.1 to 12.6 episodes/hr of jaw muscle contraction during sleep, while sleep bruxers presented from 1.2 to 15.2 episodes/hr (Lavigne et al., 2001b). It was also observed that the probability of jaw muscle contraction during sleep is related to intrinsic physiological cardiac and brain-related arousals called ‘micro-arousals’ (Macaluso et al., 1998; Kato et al., 2001, 2003). From the above observations, we suggested that these contractions are probably distributed over a continuum, from a low-frequency range to intermediate and high ranges, with coincidental tooth grinding (Lavigne et al., 2003).

This study provides confirmation that the SB-RDC developed ten years ago facilitates a high level of discrimination between sleep bruxers and control individuals. The SB-RDC distinguishes low sleep bruxers and high control individuals from the other individuals within their group. Furthermore, pain is frequently reported among sleep bruxers who display low frequencies of jaw muscle contractions.


   ACKNOWLEDGMENTS
 
This study was supported by the Canadian CIHR and the Québec FRSQ. The authors thank Mrs. Christiane Manzini and Nelly Huynh for their help in recruiting participants. We appreciate Dr. Alice Petersen’s contribution to editing of this paper.


   FOOTNOTES
 
A supplemental appendix to this article is published electronically only at http://www.dentalresearch.org.

Received February 14, 2006; Last revision April 18, 2007; Accepted May 8, 2007


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 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
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American Sleep Disorders Association (1997). The international classification of sleep disorders: diagnostic and coding manual.

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Camparis CM, Formigoni G, Teixeira MJ, Bittencourt LR, Tufik S, de Siqueira JT (2006). Sleep bruxism and temporomandibular disorder: clinical and polysomnographic evaluation. Arch Oral Biol 51:721–728.[ISI][Medline]

Harness DM, Peltier B (1992). Comparison of MMPI scores with self-report of sleep disturbance and bruxism in the facial pain population. Cranio 10:70–74.[ISI][Medline]

Hicks RA, Conti PA, Bragg HR (1990). Increases in nocturnal bruxism among college students implicate stress. Med Hypotheses 33:239–240.[ISI][Medline]

Huynh N, Khoury S, Rompré PH, Montplaisir J, Lavigne GJ (2006). Prevalence of headache and neck pain in a sleep bruxism population investigated in a sleep laboratory (abstract). Sleep 29(Suppl):0829.

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