Post-traumatic stress disorder and temporomandibular dysfunction : a review and clinical implications

BACKGROUND AND OBJECTIVES: Post-traumatic stress disorder is a disabling disorder that can be developed after a person has experienced or exposed to a traumatic event. The evidence shows the coexistence between post-traumatic stress disorder and chronic painful conditions as the temporomandibular dysfunction. This study aimed to review the literature to describe the coexistence relation between post-traumatic stress disorder and temporomandibular dysfunction. CONTENTS: A non-systematic search was carried on the Pubmed, BVS and LILACS databases on studies evaluating the relationship between post-traumatic stress disorder and temporomandibular dysfunction. Clinical studies published in the last 10 years that presented a diagnostic criterion validated for temporomandibular dysfunction and the post-traumatic stress disorder were selected. Six studies were included in the review. The results showed that the post-traumatic stress disorder often occurs in patients with temporomandibular dysfunction and, at the same time, a higher prevalence of temporomandibular dysfunction is found in individuals with post-traumatic stress disorder. There is a trend for a larger connection between post-traumatic stress disorder and muscular pain than pain in the temporomandibular joint. The presence of post-traumatic stress disorder modulates the level of physical, psychological and behavioral involvement in individuals with temporomandibular dysfunction, and can predict the onset of this painful condition. CONCLUSION: The review shows a complex coexistence between post-traumatic stress disorder and painful temporomandibular dysfunction.


INTRODUCTION
Temporomandibular disorder (TMD) comprises a group of musculoskeletal conditions involving the temporomandibular joint (TMJ), masticatory muscles and associated structures 1 .It is an important public health problem affecting 5 to 12% of the world population and represents the most common chronic painful condition in the orofacial region 2,3 .
REVIEW ARTICLE DOI 10.5935/2595-0118.20180012Br J Pain.São Paulo, 2018 jan-mar;1(1):55-9 Post-traumatic stress disorder (PTSD) is a relatively common anxiety disorder and can be defined as a set of symptoms that manifests itself after exposure to a stressful or traumatic event 4 .According to the 5 th edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM), PTSD diagnosis requires that the subject be exposed to a stressful event such as death, death threat, actual or possible serious injury, actual or possible sexual violence in the following forms: direct exposure, trauma testimony, knowledge that a family member or friend was exposed to trauma, or indirect exposure to trauma details 4 .PTSD is characterized by symptoms of negative and inopportune thoughts about trauma, nightmares, emotional distress, increased reactivity to stressful stimuli, and dodge/avoidance behavior 4 .These symptoms' persistence is what characterizes the pathological picture; i.e., PTSD reflects a failure of physiological and psychological adaptation, in which normal acute reactions to a stressor are not corrected over time 5 .Evidence shows that chronic painful TMD pictures often coexist with PTSD.Epidemiological studies have indicated that PTSD is more prevalent in TMD patients when compared to the general population 6,7 and PTSD subjects also presented more TMD than subjects without the disorder 8 .In addition, the comorbidity relationship between TMD and PTSD may be a complicating factor for TMD handling, as it reduces the efficacy of commonly used therapeutic interventions 9 .The clinically observable relationship's reasons between TMD and PTSD are not fully understood, but some theoretical models have been proposed to explain the relationship between chronic pain and PTSD [10][11][12] .Among them, the mutual maintenance model 10 is the most widespread and suggests that components related to PTSD and painful condition exacerbate and maintain one another.Considering the negative impact of PTSD in the TMD patients handling, this study aims to describe the relationship of coexistence between TMD and PTSD, as well as the clinical implications involved in this relationship.

CONTENTS
A non-systematic literature search for studies on the relationship between PTSD and TMD published between 2007 and June 2017 was performed in Pubmed, BVS and LILACS databases.The search included the following keywords: temporomandibular joint; temporomandibular joint disorders; temporomandibular joint dysfunction syndrome; craniomandibular disorders combined with stress disorders, traumatic; stress disorders, posttraumatic; combat disorders and psychological trauma.The articles' titles and abstracts were evaluated to identify the inclusion criteria.There was no restriction for participants' age and gender.The papers should present primary results and written in Portuguese or English.The following criteria were adopted to verify the coexistence relationship between PTSD and TMD: 1. Cross-sectional, case-control, randomized clinical trial or cohort studies; 2. TMD diagnosis based on validated criteria (RDC/TMD 13 , DC/TMD 14 or AAOP guidelines 15 ); 3. Medical PTSD diagnosis, or based on a structured questionnaire defined by DSM 4,16 .

RESULTS
The databases search resulted in 116 articles.After sorting the titles and abstracts, 12 articles were selected for a complete reading of the texts.From these, six meet the inclusion criteria [6][7][8][17][18][19] . One paer was excluded because it did not evaluate the relationship between TMD and PTSD 20 and five because they did not use diagnostic criteria validated for TMD, only evaluating signs and symptoms of dysfunction in the studied population [21][22][23][24][25] .The flowchart with the article selection process is shown in figure 1, and the information of the studies included in the review is shown in table 1.

Study types Studied population Main results
Bertoli et al. 6 Cross-sectional 445 patients with painful TMD 91% women 46% reported at least one traumatic stressor event.
12.6% with PTSD.PTSD prevalence is higher in the muscle pain group (14.9%) than in the joint pain group (9.9%); although not statistically different.
Positive association between PTSD and disability, psychological dysfunction, difficulty coping with pain and sleep problems.

Burris et al. 17
Cross-sectional 411 patients with orofacial pain reporting traumatic life events prior to pain 100% women 23,6% with PTSD.PTSD was associated with greater severity of pain, daily interferences, psychological dysfunction, reduced levels of daily activity and sleep problems.
Porto et al. 7 Cross-sectional 81 patients with myofascial pain type TMD 73% women 43.2% reported at least one traumatic stressor event.
Weber et al. 18 Cross-sectional 610 patients with TMD reporting traumatic life events 85% women PTSD predicts pain severity, psychological dysfunction, and interference in daily activities in TMD patients.
Muhvić-Urek et al. 8 Case-control 50 war veterans with PTSD 50 healthy subjects 100% men TMD prevalence was 48% in the veterans and 8% in the control group.The most common diagnosis was myofascial pain among veterans and disc displacement in the control group.
Fillingim et al. 19 Cohort 3,263 healthy subjects monitored for 2.8 years PTSD was associated with a higher incidence of painful TMD (risk ratio = 1.38).TMD = temporomandibular disorder; PTSD = post-traumatic stress disorder.
The relationship between PTSD and TMD was investigated through cross-sectional, case-control, and cohort studies.Among TMD patients, 43 to 46% reported at least one traumatic stressor event 6,7 .PTSD prevalence in TMD patients ranged from 12.6 to 23.6% in the studies 6,7,17 .The sample of TMD patients was composed mostly of women, with an average age ranging from 37 to 47 years old.One paper evaluated only subjects with masticatory muscle myofascial pain 7 and three studies investigated populations with painful muscle and joint TMD 6,17,18 .In these papers 6,7,17,18  The positive association between PTSD symptoms and variables such as pain intensity, disability, psychological dysfunction, difficulties in coping with pain, interference with daily activities and sleep problems were found in three studies 6,17,18 .
Finally, a cohort study reported that PTSD predicted the incidence of painful TMD in healthy populations with a risk ratio of 1.38 19 .

DISCUSSION
The bidirectional relationship between PTSD and painful TMD is supported by studies evaluating the prevalence, inci-dence, and correlation between the two conditions.Approximately half of the TMD patients reported at least one lifethreatening traumatic event 6,7 and 12,6 to 23,6% fulfilled the criteria for PTSD 6,7,17 .Epidemiological studies show that 30 to 90% of the population report a traumatic events history; however, the PTSD prevalence is less than 10% 27,28 .Therefore, the included studies indicated a higher PTSD prevalence in TMD subjects than in the general population.These results corroborate a recently published systematic review demonstrating that there is sufficient evidence to confirm the association between PTSD and chronic painful conditions 29 .In addition, PTSD seems to be strongly associated with certain conditions of chronic pain in the craniofacial segment than other conditions.In a subgroup analysis, the PTSD prevalence was higher among subjects with a headache and facial pain (9 to 25%) than in subjects with lumbar pain (0.3 to 0.7%) 29,30 .TMD patients reported traumatic stressors of different natures.Knowing that a family member or friend was injured or killed; experiencing automobile accidents and suffering from violent attacks were the most commonly reported traumatic experiences and of important personal significance for TMD patients 17 .PTSD prevalence for different TMD subgroups was investigated by Bertoli et al. 6 .Although not statistically significant, the values indicated a higher PTSD prevalence in subjects with masticatory muscles pain when compared to TMJ pain.An explanation for these results is the greater psychological vulnerability found in subjects with myalgia.These patients present high levels of anxiety, depression, pain-related disability, and report a greater number of stressful events than subjects with arthralgia 31,32 .Additional studies investigating the PTSD prevalence in subgroups of TMD patients are needed to elucidate this outcome.
From another perspective, the TMD prevalence is higher in PTSD subjects when compared to subjects without PTSD 8 .Studies of this nature are carried out mainly in at-risk populations for PTSD, such as war veterans or natural disasters victims.PTSD development after traumatic events is associated with increased risk of musculoskeletal pain and functional somatic syndromes (FSS) 33,34 .
In addition, subjects with both conditions, TMD and PTSD, presented greater pain intensity, psychological dysfunction, interference with daily activities, sleep problems and pain coping difficulties when compared to subjects without PTSD 6,17,18 .These results may be reflective of changes in pain processing found in PTSD subjects.
A recent study with quantitative sensory tests demonstrated increased pain sensitivity (hyperalgesia) possibly related to processes of central sensitization in these subjects 35 .On the other hand, PTSD was associated with reduced sensitivity to noxious stimuli (hypoalgesia) 36 .This pattern type suggests preservation of the sensory pathways and absence of central sensitization and indicates that the pain perception in these patients is modulated centrally by emotional factors 36 .However, it is still uncertain whether alterations in pain processing are pre-existing risk factors for PTSD development or whether they are consequences of exposure to trauma and its neurobiological implications.A set of physiological and psychological processes is triggered when the human being experiences stressful or traumatic events.For many, these physiological changes return to normal once the stressor or trauma is ceased.However, the initial state of hyperexcitation may become chronic for some subjects.Chronic hyperexcitation leads to physiological stress system dysregulation and to the development of several changes.Studies have demonstrated dysfunction in the hypothalamicpituitary-adrenal (HPA) 37 axis and abnormal activation patterns in frontal-limbic brain areas (prefrontal cortex, hippocampus, insula, and amygdala) in PTSD subjects 38 .In addition, the trauma experience may also affect the interpretation of a potentially threatening stimulus.This interpretation bias may then result in dodge behavior, catastrophic symptoms, and amplification of the disease state 34 .
Studies investigating the temporal relationship between TMD and PTSD are scarce in the literature and this review identified only one cohort study 19 .Fillingim et al. 19 reported that the PTSD presence in healthy subjects predicts the risk for developing painful TMD (Risk Rate 1.38).This result corroborates with retrospective studies outcomes.A recent meta-analyzes has shown that exposure to traumatic life events such as physical, sexual, emotional abuse, war combat or PTSD diagnosis increases by 2.7 times the chance of a subject developing FSS such as TMD, fibromyalgia, generalized chronic pain, chronic fatigue syndrome and irritable bowel syndrome 34 .In addition, when trauma results in PTSD, the link between exposure to traumatic events and FSS becomes more pronounced.This illustrates the impact that PTSD development following trauma can have on health.While a person exposed to trauma may or may not have a long-term maladaptive response, a subject who develops PTSD will likely have several physiological, psychological, and behavioral consequences that may limit recovery and may result in the FSS development 34 .Further research with prospective methodologies to delineate a temporal relationship between PTSD and TMD and the factors that influence this relationship are necessary.Understanding the reasons that explain the coexistence between PTSD and painful TMD is difficult due to overlapping symptoms.Both conditions share somatic hypervigilance, dodge behavior, high levels of fear and arousal to traumatic stimuli, suffering and dysregulation of the stress system 28,39 .Some theoretical models have been proposed to explain the relationship between chronic pain and PTSD [10][11][12] , although none presents significant empirical support.The mutual maintenance model proposed by Sharp and Harvey is the most widespread 10 .In this model, the cognitive, affective and behavioral components of chronic pain exacerbate and maintain PTSD; while the physiological, affective and behavioral components of PTSD exacerbate and maintain problems related to chronic pain.For example, pain in PTSD subjects can cause discomfort, increase stress, apathy and be a trigger to remember the trauma.On the other hand, remembering the trauma promotes hypervigilance behavior and dodge of activities related to pain that result in the pain experience amplification.The subject then engages in a vicious cycle where the PTSD symptoms and pain interact producing emotional distress and self-sustaining functional disability 10 .

Clinical implications
Psychiatric disorders related to traumatic events usually go unnoticed when evaluating TMD subjects.Considering the high PTSD prevalence in these patients, especially in cases of masticatory muscle myalgia, it is prudent to adopt screening instruments for PTSD, since there is a chance of this condition being present.Another implication is that treatments for chronic pain are time-consuming and require patient adherence.Subjects diagnosed with PTSD are three times more likely not to adhere to treatment 40 .
In addition, individuals with painful TMD who report high levels of psychiatric symptoms make up a subgroup of patients for whom conventional treatments are ineffective 9 .In such cases, effective interventions for both TMD and PTSD, including psychiatric and psychological therapies, should be considered in the treatment plan.

CONCLUSION
This review described a complex coexistence relationship between PTSD and painful TMD.PTSD frequently occurs in patients with TMD, and at the same time, a higher TMD prevalence is found in PTSD subjects.There is a trend indicating a greater relationship between PTSD and muscle pain than TMJ pain.
PTSD presence modulates the level of physical, psychological and behavioral impairment in TMD subjects and can predict the onset of this painful condition.
8nly one paper investigated the TMD prevalence in PTSD subjects8.TMD prevalence was 48% in PTSD subjects and 8% in healthy subjects.The most common TMD diagnosis was myofascial pain among PTSD subjects, while in the control group it was disc displacement.The PTSD population studied consisted of war veterans with an average age of 42 years and with medical PTSD diagnosis.
26e participants were not formally diagnosed (medical diagnosis) with PTSD but had symptoms consistent with PTSD diagnosis identified by Post-Traumatic Stress Disorder Check List-Civilian (PCL-C)26, a structured questionnaire, developed according to the DSM, and validated to investigate the PTSD prevalence in epidemiological surveys.