Pain Sensitization and Descending Pain Inhibition in Fibromyalgia

Background: Both facilitation of ascending nociceptive pathways and impaired inhibition of descending ones may contribute to pain sensitization in fibromyalgia (FM). The slowly repeated evoked pain (SREP) protocol is a potential diagnostic marker for this sensitization. Though its mechanisms are unclear, SREP appears linked to ascending facilitation, while the role of descending inhibitory dysfunction in SREP sensitization remains to be clarified. Objective: To quantify descending pain inhibition in FM compared to healthy individuals and to assess its relationship with pain sensitization via SREP. Additionally, associations between descending pain inhibition and clinical symptoms were examined. Methods: In 55 women with FM and 45 healthy women, descending pain inhibition was estimated using the conditioned pain modulation (CPM) paradigm, with interdigital web pinching as the conditioning stimulus. The use of SREP protocol consisted of applying pressure stimuli to the nail of the third finger of the non-dominant hand. Clinical symptoms were assessed using questionnaires. Results: SREP sensitization was stronger and CPM smaller in FM patients than in healthy women. In FM patients, SREP sensitization was inversely associated with CPM, and both related to clinical symptoms. Patients who did not show CPM reported greater severity of FM symptoms and higher anxiety and fatigue levels than those who showed CPM. Conclusions: Impaired endogenous pain inhibition contributes to pain sensitization in FM and may partly explain SREP sensitization. This reduced pain inhibition could also underlie the clinical symptoms commonly seen in FM. Future research may clarify altered balance between ascending and descending pain processes in FM.