Spectroscopic differences in posterior insula in patients with chronic temporomandibular pain

Background and aims: Chronic pain including temporomandibular disorder (TMD) pain involves a complex interplay between peripheral and central sensitization, endogenous modulatory pathways, cortical processing and integration and numerous psychological, behavioral and social factors. The aim of this study was to compare spectroscopic patterns of N-Acetyl-aspartate (NAA), total creatine (tCr), choline (Cho), myo-inositol (MI), glutamate (Glu), and the combination of Glu and glutamine in the posterior insula in patients with chronic generalized or regional chronic TMD pain (gTMD and rTMD, respectively) compared to healthy individuals (HI) in relation to clinical findings of TMD pain. Methods: Thirty-six female patients with chronic rTMD or gTMD with at least 3 months duration were included in the study. Ten healthy women were included as controls. All participants completed a questionnaire that comprised assessment of degrees of depression, anxiety, stress, catastrophizing, pain intensity, disability and locations. A clinical Diagnostic Criteria for Temporomandibular Disorders examination that comprised assessment of pain locations, headache, mouth opening capacity, pain on mandibular movement, pain on palpation and temporomandibular joint noises was performed. Pressure-pain threshold (PPT) over the masseter muscle and temporal summation to pressure stimuli were assessed with an algometer. Within a week all participants underwent non-contrast enhanced MRI on a 3T MR scanner assessing T1-w and T2-w fluid attenuation inversion recovery. A single-voxel H-1-MRS examination using point-resolved spectroscopy was performed. The metabolite concentrations of NAA, tCr, Cho, MI, Glu and Glx were analyzed with the LC model. Metabolite levels were calculated as absolute concentrations, normalized to the water signal. Metabolite concentrations were used for statistical analysis from the LC model if the Cramer-Rao bounds were less than 20%. In addition, the ratios NAA/tCr, Cho/tCr, Glu/tCr and MI/tCr were calculated. Results: The results showed significantly higher tCr levels within the posterior insula in patients with rTMD or gTMD pain than in HI (p = 0.029). Cho was negatively correlated to maximum mouth opening capacity with or without pain (r(s) = -0.42, n = 28, p = 0.031 and r(s) = -0.48, n = 28, p = 0.034, respectively) as well as pressure-pain threshold on the hand (r(s) = -0.41, n = 28, p = 0.031). Glu was positively correlated to temporal summation to painful mechanical stimuli (r(s) = 0.42, n = 26, p = 0.034). Conclusions: The present study found that increased concentrations of Cho and Glu in the posterior insular cortex is related to clinical characteristics of chronic TMD pain, including generalized pain. These findings provide new evidence about the critical involvement of the posterior insular cortex and the neurobiology underlying TMD pain in both regional and generalized manifestations. Implications: The findings in this study have indirect implications for the diagnosis and management of TMD patients. That said, the findings provide new evidence about the critical involvement of the posterior insular cortex and the neurobiology underlying TMD pain in both regional and generalized manifestations. It is also a further step towards understanding and accepting chronic pain as a disorder in itself.