Altered pro-nociceptive and anti-nociceptive mechanisms are often implicated in painful conditions and have been increasingly studied over the past decade. For some painful conditions alterations are well-established, but in low back pain (LBP) populations there remains considerable debate whether these mechanisms are altered. The present systematic review aimed to address this issue by identifying studies assessing Conditioned Pain Modulation (CPM) and/or Temporal Summation of Pain (TSP) in LBP patients, comparing to either a healthy control group or using a method with reference data available. Qualitative synthesis and quantitative meta-analysis of group differences were performed. For CPM and TSP, 20 and 29 original articles were eligible, with data for metaanalysis obtainable from 18 (1500 patients, 505 controls) and 27 (1507 patients, 1127 controls) studies, respectively. Most studies were of poor-to-fair quality with significant heterogeneity in study size, population, assessment methodology and outcome. Nonetheless, CPM was impaired in LBP patients compared to controls (standardized mean difference =-0.44 [-0.64,-0.23], P<0.001), and the magnitude of this impairment was related to pain chronicity (acute/recurrent versus chronic, P=0.003), duration (R=-0.62, P=0.006) and severity (R=-0.54, P=0.02). TSP was facilitated in LBP patients compared to controls (standardized mean difference = 0.50 [0.29, 0.72], P<0.001), and the magnitude of this facilitation was weakly related to pain severity (R=0.41, P=0.04) and appeared to be influenced by test modality (P<0.001). Impaired CPM and facilitated TSP was present in LBP patients compared to controls, though the magnitude of differences was small which may direct future research on the clinical utility.

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Persistent pain conditions, including low back pain (LBP), are often accompanied by alterations in pronociceptive and antinociceptive mechanisms, as quantified by temporal summation of pain (TSP) and conditioned pain modulation (CPM). It remains unclear whether altered pain sensitivity, CPM, and/or TSP are a consequence of pain presence or determine the degree of pain development. Pressure pain sensitivity, TSP, and CPM were assessed across an episode of exercise-induced LBP maintained for several days. Thirty healthy individuals participated in 3 experimental sessions: before (day 0), 2 days after fatiguing back muscle exercise with exercise-induced LBP present (day 2), and after pain resolution (day 7). Both handheld and cuff pressure-pain thresholds, along with TSP (10-cuff pain stimuli at .5 Hz) and CPM (cuff pain detection threshold prior versus during painful pressure conditioning) were assessed, alongside questionnaires pertaining to pain, disability, mood, sleep, menstruation, physical activity, and catastrophizing. The exercise-induced LBP model produced mild pain and disability, and reductions in pressure pain thresholds over both the lumbar and distant testing sites (p < .007). No pain-related changes were observed for TSP (p > .44) or CPM (p > .17). The baseline TSP was associated with the peak pain intensity of the exercise-induced LBP (p < .003). Perspective: Pressure-pain sensitivity was impacted by the presence of exercise-induced LBP, whereas TSP seemed to be more stable and was instead associated with the intensity of pain developed. No significant pain-related changes or associations were observed for CPM, suggesting this measure may have less usefulness in mild musculoskeletal pain conditions.

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, T. (2019). Assessment of conditioned pain modulation in healthy participants and patients with chronic pain: manifestations and implications for pain progression.