Background & Aims
Conditioned pain modulation (CPM), offset analgesia (OA) and temporal summation of pain (TSP) are commonly applied paradigms used to assess the state of an individual’s endogenous pain modulatory system [1-4]. Previously, test-retest reliability of these measures has not been well characterised together within the same participants. The aim of this analysis was to evaluate the test-retest reliability of these three measures within a single cohort of healthy participants. and to explore whether the magnitude and reliability of CPM, OA and TSP profiles in individuals are associated with activity in the spinal cord, assessed using the nociceptive withdrawal reflex (NWR) or functional connectivity across pain processing regions of the brain, measured using fMRI. Data collection is ongoing for NWR and fMRI measures and therefore data from CPM, OA and TSP reliability is presented here in the first instance.
Methods
Data was collected from 19 participants in 2 sessions (N=30 planned at study completion). CPM and TSP paradigms were conducted using cuff-pressure algometry. For CPM, cuffs were placed around the gastrocnemius muscle of each leg. Baseline pain detection threshold (PDT) and pain tolerance threshold (PTT) were determined. The conditioning cuff was then inflated to 70% of the PTT for 100s, and after 15s the test stimulus cuff was steadily inflated at a rate of 1 kPa/s, during which PDT and PPT were reassessed. TSP was measured by delivering 10 stimuli at the PTT (1s stimulus duration with 1s interval). OA was measured by delivering 3 heat stimuli (rise and fall rate: 6°C/s) via a square thermode on the forearm. Heat stimuli were delivered over 30s: T1 (47°C; 5s), T2 (48°C; 5s) and T3 (47°C; 20s). Participants continuously rated pain intensity during all paradigms. Participants also attended 2 further sessions involving recording of NWR responses and collection of resting state fMRI data.
Results
For CPM, change from baseline to the CPM condition were assessed. Reliability, assessed using intra-class correlation coefficients (ICC (3,1)), was <0.1 for PDT, 0.76 for PTT and 0.77 for area under the curve (AUC) of pain ratings. TSP was assessed with the sum of normalised pain ratings, indicating whether stimuli following the first were more or less painful overall (ICC: 0.1). The mean of the first three normalised ratings (VAS-I) and the last three normalised ratings (VAS-III) was also computed and the ratio (VAS-I/VAS-III) calculated (ICC: 0.13). For OA, reliability of the heat pain PDT and PTT, assessed using average ratings for 3 ramps of increasing temperature, were excellent (ICC: 0.88 ad 0.82, respectively). Two measures were considered to calculate OA effect; ?decrease (change in pain intensity after a temperature decrease) and magnitude (difference between ?decrease and ?increase); both showed very good reliability (ICC: 0.757 and 0.759, respectively).
Conclusions
We have demonstrated the test-retest reliability of CPM, TSP and OA, three measures of endogenous pain modulation, in one group of participants. The paradigm with the highest reliability was OA. CPM showed very good reliability for PTT and AUC, but it was poor for PDT. TSP had the lowest reliability. These data enable valuable conclusions to be drawn regarding whether reliability is consistent across three distinct measures of endogenous pain modulation in each individual, and future analysis will explore correlations between the different measures. Further, we will investigate how the reliability and magnitude of the three measures in each individual may be influenced by spinal and brain mechanisms, through exploring relationships with the NWR and fMRI data also collected in this cohort.
References
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