TMD patients come in many different varieties. Some experience pain and dysfunction confined only to the jaw and/or the associated chewing muscles. Other TMD patients have jaw pain plus one or more other painful conditions elsewhere in the body. Scientists have been trying to figure out if these differences in their experience of pain reflect any long-term changes in the brain, in particular a phenomenon called "central sensitization," in which individuals become more sensitive to pain over time. Although "central sensitization" is considered a normal phenomenon, pain sensitive individuals may have brain changes that may make their pattern of central sensitization different than that of individuals without chronic pain.
To test for central sensitization, researchers typically use a 'temporal summation' method, in which volunteers are exposed to a nonpainful stimulus that becomes painful with repeated exposures to it. You might make an analogy to water dripping on rock that over time eventually penetrates the rock. In the temporal summation method used in the TMD experiment described below, volunteers were exposed to a heated pad placed on their palms for multiple times over a fixed time period. Individuals experiencing central sensitization would feel the heated pad becoming painful over time.
It is widely believed that individuals with fibromyalgia (widespread pain with diffuse tender points that are painful when lightly pressed) and perhaps some other pain conditions, including TMD, experience enhanced central sensitization, demonstrated either by reaching peak pain earlier in the temporal summation test or perhaps by having more lasting 'aftersensations' following removal of the heat. In a study conducted at New York University by Dr. Karen Raphael in collaboration with other institutions, researchers compared experimental pain sensitivity in three groups: patients with only muscle-based TMD (n=100), patients with TMD plus fibromyalgia (n=26), and a matched pain-free control group (n=48). Their surmise was that the TMD patients with fibromyalgia would have different pain sensitivity than TMD patients whose pain was confined to the chewing muscles or normal controls.
Results: Surprisingly, when the researchers looked at each individual participant, temporal summation did not often occur as expected, and rates of summation did not differ among participants in different groups. Even at the highest constant temperatures considered safe to test, less than half of participants showed an increase in subjective painfulness of the heated pad over repeated presentations. Participants were more likely to either rate the heat as having a consistent painfulness over time or, for some, even a reduction in painfulness. Examining just those participants who appeared to become centrally sensitized, the rate and pattern of central sensitization did not differ among any of the study groups. However, the subgroups of TMD patients had longer-lasting painful aftersensations than controls when the heated pad was removed. TMD patients with fibromyalgia had similarly painful aftersensations to those TMD patients without fibromyalgia.
Conclusions: This study shows that some of the methods used to assess pain sensitivity do not work as researchers expect. However, the problems may be obscured when averaging responses across all participants in a group. Nevertheless, since TMD patients had more lingering painful aftersensations when the heated pad was removed, the researchers were able to support the concept that TMD patients, both those with and without fibromyalgia, have some form of disturbance of central nervous system-managed pain processing mechanisms.