Does a dose-response relation exist between spinal pain and temporomandibular disorders?
From: BMC Musculoskelet Disord. 2009 Mar 2;10:28
Temporomandibular disorders are musculoskeletal pain conditions characterised by pain and dysfunction in the jaw-face muscles and/or the temporomandibular joint. Musculoskeletal pain conditions occurring at various locations may share pathophysiological mechanisms. Co-morbidity between temporomandibular disorders, headaches and neck/shoulder pain has been reported in temporomandibular disorders patient samples as well as in samples drawn from the general population. Low back pain, one of the most common pain conditions in humans, has been associated with other pains such as neck pain and headaches, which has been interpreted as a tendency for symptoms to cluster in some individuals. The source of these patterns is not known, but neurobiological sensitization processes, genetically determined vulnerability and psychological factors are commonly given as possible explanations. Results of a 3-year prospective study showed a significantly increased risk of developing a new pain condition with presence of a pain condition at baseline. A more recent prospective study based on patients with non-painful temporomandibular disorders indicated a dose-response relationship between the number of pain sites at baseline (head, back, chest, stomach) and the risk of onset of dysfunctional temporomandibular disorders pain among women. Frequency of headaches was found to have a dose-response relationship with occurrence of musculoskeletal symptoms (e.g. pain in neck, shoulders and low back) in a Norwegian population.
The authors have recently shown that patients with long-term spinal pain (neck, shoulder and/or low back) significantly more often have signs and symptoms of temporomandibular disorders than do matched controls. The associations remained statistically significant also after exclusion of those who reported jaw pain. It is not known whether co-morbidity between temporomandibular disorders and neck pain, shoulder pain and/or low back pain occurs within the whole range of variation in symptom frequency and severity. Most analyses in this field have involved dichotomized samples, not taking variations of symptom severity into consideration. The aim of the present study was to test whether a reciprocal dose-response relation exists between frequency and severity of neck pain, shoulder pain and/or low back pain and temporomandibular disorders. The authors tested the following null hypotheses:
1. Occurrence of frequent temporomandibular disorders symptoms and headaches does not differ significantly between study groups with varying frequency and severity of neck pain, shoulder pain and/or low back pain.
2. Presence of frequent neck pain, shoulder pain and/or low back pain does not differ significantly between study groups with varying frequency and severity of temporomandibular disorders symptoms.
The operational definition of ‘spinal pain’ was pain in the neck, shoulders and/or low back. Symptoms in the jaw-face region, head, neck, shoulder and low back regions were assessed by questionnaire. Presence of symptoms was stated for frequency (never; not now, but previously; once or twice a month; once or twice a week; several times a week; daily), duration (< 1 month; 1 month–1 year; 1–5 years; > 5 years) and intensity. The subjects were also asked to estimate the impact of jaw symptoms, headaches, neck-shoulder pain and low back pain on activities of daily living. Intensity and activities of daily living was assessed using the 11-point Numerical Rating Scale.
Presence and severity of temporomandibular disorders was evaluated for the separate symptoms and according to the Helkimo Anamnestic dysfunction Index. This classification grades the severity of symptoms in the jaw-face region into mild (i.e. temporomandibular joint sounds during opening and closing of the jaw and/or tiredness/stiffness in the jaws) or severe (i.e. pain, temporomandibular joint locking and/or difficulties in opening the mouth wide).
The present study showed a dose-response relation between frequency and severity of spinal pain and temporomandibular disorders. The pattern was evident in both directions, the prevalence of frequent temporomandibular disorders symptoms and headaches increasing with increasing frequency/severity of spinal pain, and the prevalence of frequent pain in the neck, shoulders and/or low back increasing with increasing frequency and severity of temporomandibular disorders symptoms. The test for trends showed significant dose-response associations in both directions. The two tested null hypotheses were therefore rejected.
The authors have previously shown that patients with long-term pain in the neck, shoulders and/or low back have a sevenfold risk of reporting pain and dysfunction in the jaw-face region and a fivefold risk of having clinical signs of temporomandibular disorders, compared with matched controls. This finding was recently supported in a cross-sectional analysis based on almost 30,000 adults in the USA, indicating a strong relationship between reported pain in the neck, shoulders and/or low back and jaw-face pain. The present study shows a stepwise positive correlation between severity of spinal pain and pain and dysfunction in the jaw-face region. This dose-response-like pattern should not be interpreted as a sign of exposure and outcome. However, it strengthens previous results of an association between temporomandibular disorders and spinal pain and may point to common underlying biological or psychological mechanisms. It should be emphasized that the results are derived from a cross-sectional study and do not show causality. Owing to the study design we have no information about the temporal sequence of the examined disorders, an essential element in assessing causality. Studies with a prospective design have indicated that presence of a pain condition increases the risk of contracting temporomadibular disorder pain. In a recent prospective study the risk for onset of facial pain, meeting research diagnostic criteria for temporomadibular disorders, was almost four times higher among adolescents with back pain at baseline, than among those without back pain. Papageorgiou et al. followed a cohort without low back pain at baseline and noted that musculoskeletal pain at other sites predicted future episodes of low back pain. These results are interesting, but so far there is no sufficient evidence to conclude that back pain precedes temporomandibular disorders, or vice versa. Psychological factors are often co-morbid with chronic pain conditions. The temporal sequence of pain and depression is however not clear. In a review addressing this question the majority of studies indicated that depression was a consequence rather than an antecedent of pain. Longitudinal studies on these issues are therefore warranted.
It has been suggested that generalized pain (i.e. fibromyalgia) is at one end of a continuum. Vierck presents temporomandibular pain as an example of a focal pain condition where the nociceptive sensory input may contribute to development of generalized hypersensitivity and related susceptibility to further load. In line with this hypothesis one experimental study reports signs of mechanical allodynia in the hindpaw following nociceptive stimuli applied to the masseter muscle of rats. Other experimental studies have shown that perceived muscle pain intensity and distribution is influenced by the stimulation rate (temporal summation) and the number of stimulated afferents (spatial summation). Temporal summation has been shown in temporomandibular disorders patients, as well as in other chronic pain conditions, suggesting a generalized hyperexcitability of the central nociceptive system. In a large population sample grouped with respect to frequency of reported headaches a dose-response pattern was demonstrated between headache frequency and 1-year prevalence of musculoskeletal symptoms (with locations including neck, shoulders, elbows, wrist/hands, chest/abdomen, upper back, low back, hips, knees, ankles/feet). The contribution of input from the craniofacial nervous system in spreading pain may therefore be of significance and more experimental and clinical studies are needed.
Recent studies have shown that genetic polymorphism, with influence on the metabolism of catecholamines, is highly associated with pain sensitivity and the risk for developing temporomandibular disorders. Central sensitization may be one possible explanation for co-morbidity between pain conditions at different locations, as well as presence of allodynia and hyperalgesia. Reflex connections between nociceptors and the fusimotor-muscle spindle system may also be involved in the pathophysiologic mechanisms related to pain and dysfunction.
The allocation of subjects in the present study to different pain in the neck, shoulders and/or low back groups was based on the participants’ reports of pain frequency in the questionnaire. For example, if a subject reported daily shoulder pain, but infrequent low back pain, the grouping was done according to the frequency of shoulder pain. Subjects who had been referred to a rehabilitation programme and who were on sick leave were considered to have more severe spinal pain than subjects with frequent pain but not on sick leave. Symptom description in self-report questionnaires may be a limitation in a strict dose-response discussion; however, frequency as well as intensity and duration of pain and dysfunction are important variables in health care seeking behaviour. Similarly, in this study, pain severity in the separate neck-shoulder pain and low back pain groups demonstrates stepwise increased mean values of reported pain intensity and impact on activities of daily living. In the sub-sample test with symptoms of temporomandibular disorders as independent variable, we included none of the patients from the rehabilitation center. The severity of the temporomandibular disorders symptoms is reflected by the reported interference of jaw symptoms with daily living. The formation of groups, aiming at discrete severity categories (dose), therefore seems valid also with regard to the mean intensity level and the impact of the symptoms on daily living.
The study shows a reciprocal positive dose-response pattern between frequency and severity of neck-shoulder pain and low back pain and temporomandibular disorders. The results indicate a strong co-morbidity between these two conditions, suggesting that they may share risk factors or that they may influence each other. The authors agree with the recently advocated view of a need for hypothesis-based studies on specific pain -pain co-morbidities, but also on pain-dysfunction co-morbidities. The present results are of significance for physicians and dentists, both of whom are expected to manage patients with pain and dysfunction. Collaboration as well as a costing system for cooperation in the diagnosis and management of the two conditions is warranted. Researchers of pain conditions should include the jaw-face region in their efforts to comprehend the pain patient’s case history.
