Dynamics of Descending Inhibition and Neuroendocrine Analgesia with special reference to the Trigeminal region

Abstract: In patients with pain, it is of relevance to use clinical assessments that evaluate the pain modulation. In this thesis, the dynamics of descending inhibition or central sensitization and neuroendocrine analgesia were investigated: in healthy volunteers; in patients with chronic closed lock of the temporomandibular joint (TMJ), before and after discectomy; and finally in patients with possible neuropathic pain, (atypical odontalgia). Pain assessment was made with the eleven point numerical rating scale. The Pain thresholds were determined to evaluate central sensitization, and cold pressor tests of 2-4 °C were used to provoke descending noxious inhibitory controls. Plasma β- endorphin was determined by radioimmunoassay. The focus of evaluation was on the change induced by provocation or pain relieving surgery. In study I, we found, that with an exception for the electrical pain threshold over the central maxillary incisor, healthy volunteers increased their electrical and pressure pain thresholds during cold pressor test. There were region and stimuli specific changes between the trigeminal and the spinal region, regarding the change in pain thresholds, but no differences in respect to gender. In study II, we demonstrated that female patients, with chronic closed lock, i.e. limited jaw function and movement-evoked pain from the TMJ, had central sensitization and an elevated neuroendocrine opioid level in plasma. Study III, showed, that in 91% of female patients with chronic closed lock, the movement-evoked pain had disappeared at a median (range) of 8 (6-24) months after TMJ-discectomy. In particular, a clinically substantial reduction in pain intensity was required for a decline in plasma β-endorphin. Central sensitization showed signs to decrease in relation to relief in pain intensity. However, on the whole, central sensitization was not healed. In study IV, we noted that patients with atypical odontalgia had HPA-axis hyperactivity and altered coping together with deficient neuroendocrine opioid release and descending facilitation in the maxillary branch of the trigeminal nerve during cold pressor test. In conclusion, the outcome measure of pain relivieving surgery is feasibly a clinically substantial improvement in maximal pain intensity, here confirmed by a recovery in plasma β-endorphin. Afference from the pulp was absent from descending inhibition in healthy, and possibly promoted by descending facilitation in atypical odontalgia, indicating that the dentoalveolar region is vulnerable to development of chronic pain.