Evoked temporal summation in dogs to assess pain central sensitization and modulation – A feasibility study

Central sensitization and pain endogenous controls imbalance were characterized in humans affected by chronic pain. An early plastic change, the wind-up phenomenon, was described in cats using a validated mechanical device inducing temporal summation of pain (TSP). The aim of this study was to validate this method in dogs and to describe pro- or anti-nociceptive profiles occurring during canine osteoarthritis (OA).
Healthy (N=4) and OA (N=31) dogs were assessed using TSP. Six stimulation profiles were tested for their reliability and specificity, at mid-forearm (0.38Hz at 2–4N, 0.25Hz or 0.50Hz at 4N) or tail-basis (0.25Hz or 0.50Hz at 2N). Endogenous facilitatory, or inhibitory, controls were assessed using a pressure pain threshold (PPT) before and after TSP, or conditioning pain modulation (CPM) via an ischemic model, respectively. The percentage of change of the associated PPTs was calculated. Statistical analysis included intraclass coefficient of correlation, Spearman’s correlations, Fisher and Mann U tests, with α=0.05.
The response to mechanical TSP scores were moderately reliable, while the inter-trials variability decreased with the tail-basis stimulus compared to mid-forearm for healthy dogs (P=0.029). Particularly, a frequency of 0.5Hz and 2N halved the number of tolerated stimulations by OA dogs compared to healthy dogs (P=0.008). This profile led to spinal hyperexcitability, with facilitated OA dogs having a functional CPM for only 45% of them versus 73% for the non-facilitated dogs. No correlations were found between radiographic score, orthopedic score, response to mechanical TSP or age with the facilitation or inhibition rates (P>0.173).
The response to mechanical TSP was reliable and specific, and OA dogs developed central sensitization as reflected by the wind-up phenomenon. An imbalance between endogenous facilitatory or inhibitory controls was described suggesting neuroplasticity. The characterization of endogenous pain controls profiles will attempt to better manage OA pain by preventing inhibitory system fatigue.