Temporal stability of conditioned pain modulation in healthy women over four menstrual cycles at the follicular and luteal phases

Conditioned pain modulation (CPM) is a phenomenon that may be tested with a dynamic quantitative sensory test that assesses the inhibitory aspect of this pain modulatory network. Although CPM has been adopted as a clinical assessment tool in recent years, the stability of the measure has not been determined over long time intervals. The question of stability over time is crucial to our understanding of pain processing, and critical for the use of this tool as a clinical test. The primary objective of this study was to evaluate the stability of a CPM paradigm over time in healthy women. The secondary objective was to determine the potential influence of menstrual cycle phase on CPM. CPM was assessed 8 times in 22 healthy women during the follicular and luteal phases of 4 different cycles. The CPM effect was evidenced by a reduction in the pain rating of a test stimulus (6.3 ± 0.2) with the introduction of a conditioning stimulus (5.0 ± 0.3; P < 0.001). The intraclass correlation coefficient for the CPM effect was modest (0.39; CI = 0.23-0.59), suggesting that there is significant variation in CPM over long time intervals. CPM did not vary across phases in the menstrual cycle. Prior to the adoption of CPM as a clinical tool to predict individual risk and aid diagnosis, additional research is needed to establish the measurement properties of CPM paradigms and evaluate factors that influence CPM effects.