Endogenous pain modulation in chronic temporomandibular disorders: Derivation of pain modulation profiles and assessment of its relationship with clinical characteristics

Background: Endogenous pain modulation (EPM) reflects the brain's ability to modulate incoming nociceptive inputs, and deficient EPM was implicated as a chronic pain mechanism. EPM status has been investigated in temporomandibular disorders (TMD) patients with conflicting results, and its relationship with clinical characteristics in this population is not well known. Objectives: (a) Determine EPM responses in chronic TMD cases and pain-free controls; (b) Derive pain modulation profiles (PMP) based on individual EPM responses; and (c) Categorise clinical characteristics of TMD cases and pain-free controls based on their individual PMP. Methods: Twenty-two chronic TMD cases and 17 age-matched pain-free controls, all females, were comprehensively characterised regarding clinical characteristics and underwent EPM testing using temporal summation of pain (TSP) and conditioned pain modulation (CPM) protocols over the face and hand. Individuals were categorised into PMPs (I-IV) based on predetermined cut-off points for TSP and CPM responses. Results: Between-group comparisons showed similar TSP and CPM responses (P > 0.23) in the face, while TMD cases showed significantly increased TSP (P = 0.04) but similar CPM responses (P > 0.17) in the hand relative to controls. Similar distribution across PMPs and clinical characteristics when categorised into PMPs was found for both groups. Body mass index was associated with increased TSP and reduced CPM in the face in TMD cases. Conclusion: Endogenous pain modulation responses over the face were similar between groups. TMD cases showed increased hand TSP compared to controls while both groups showed no significant hand CPM. PMP classification showed similar results between groups, and further refinement of PMP determination is warranted.