Internal validity of inter‐digital web pinching as a model for perceptual diffuse noxious inhibitory controls‐induced hypoalgesia in healthy humans

Hot and ice‐water immersions are commonly used for heterotopic noxious counter‐stimulation (HNCS) in investigations on endogenous pain modulation. However, coincident sympathetic thermoregulatory activity does not allow to differentiate between perceptual hypoalgesia related to baroreflex sensitivity (BRS) or diffuse noxious inhibitory controls (DNIC). The present study analysed the internal validity of another supposedly less confounded tonic pain model (inter‐digital web pinching; IWP) regarding its potential as DNIC trigger. We performed a randomized controlled study in 24 healthy gender‐matched drug‐free volunteers aged 21–54 (median 25) years. The study protocol comprised the assessment of mechanical and thermal perceptual wind‐up before and after an IWP (15N) or hot water immersion trial (HIT; 47.5°C) of 2min duration. Wind‐up was induced either by 10 repetitive (1Hz) contact heat (max. 49°C; 5×5mm thermode) or ballistic impact stimuli (0.5g at 9m/s) on the phalanges of the non‐dominant hand. Cardiovascular activity, pain experience and corrugator muscle activity were continuously monitored. Although both HNCS forms produced a similar pain experience (45% of scale), a more pronounced cardiovascular activity was observable for the HIT (P<0.01). This indicates a higher baroreceptor activity and stronger contamination of painful water immersion by BRS‐related hypoalgesia. Regardless of pain modality, wind‐up was significantly reduced by HNCS, although this was stronger for painful water immersion than for noxious pinching (P<0.01). The HNCS types allow a differentiation between BRS‐related and DNIC‐like hypoalgesia. IWP proved its validity for DNIC induction, being practically non‐confounded by BRS.     

New and Low-Cost Auto-Algometry for Screening Hypertension-Associated Hypoalgesia

Objective:   The aim of the present work was to measure the pain threshold in hypertensive patients with a new auto-algometry method.
Design and Setting:   Auto-algometry consists of asking the subjects to push their fingers against a fixed round-tip needle until they feel a pain sensation. An electronic force transducer permits the measurement of the force applied by the subjects and storage of the data on a personal computer. Eight tests are performed twice on each subject on the tip and back of four fingers. For each test, the maximal applied force (grams) is defined as pain threshold. The overall discomfort during the entire procedure is reported by the subjects on a 0 (no discomfort) to 10 (intolerable pain) scale.
Patients and Interventions:   A group of hypertensive patients ( n  = 22) and a group of normotensive subjects ( n  = 22) underwent the auto-algometry examination.
Results:   The pain threshold was higher in hypertensive patients compared with normotensive subjects. All discomfort scores referred by the subjects fell within the 4–6 range.
Conclusion:   The data obtained from this study indicate that the auto-algometer as described here can detect hypoalgesia associated with hypertension.     

Volume loading hypoalgesia in SHR, WKY and F1 offspring of a SHR × WKY cross

The effects of volume loading on a nociceptive reflex, arterial blood pressure and heart rate were studied in spontaneously hypertensive rats (SHRs), Wistar Kyoto normotensive rats (WKYs) and the F₁ offspring of a SHR × WKY cross. Volume loading resulted in significantly greater inhibition of the tail-flick reflex to painful radiant heat in SHRs compared to WKYs. The F₁ offspring of a SHR × WKY cross showed levels of hypoalgesia to volume loading that were intermediate to those of SHRs and WKYs. There were no differences between these strains in their hypotensive and bradycardic responses to volume loading. These findings are discussed in terms of cardiovascular-somatosensory interactions.     

Association between hypoalgesia and hypertension in rats after short-term isolation

By isolating young rats (90-100 g) a state of hypertension and tachycardia was induced after 7 days or a longer period of social deprivation. Clonidine, a drug used to treat hypertension in man, readily reversed the high blood pressure and heart rate in this experimental model of hypertension. In two different tests, an elevated nociceptive threshold was shown to be present in isolated animals as compared to group-housed rats. Naloxone was found to reverse this hypoalgesic state. The opiate antagonist also diminished the high blood pressure in the socially-deprived animals. Moreover, after 7 days of isolation, 24 hr of housing the rats in groups of five made the level of blood pressure and the sensitivity to pain return to control values. In this experimental model, in which hypertension was linked to stressful housing conditions, the data suggest that high blood pressure and hypoalgesia are closely associated.     

Differential physiological effects during tonic painful hand immersion tests using hot and ice water

The cold pressor test (CPT) is an empirically validated test commonly used in research on stress, pain and cardiovascular reactivity. Surprisingly, the equivalent test with water heated to noxious temperatures (hot water immersion test, HIT) has not been thoroughly investigated. The aim of the present study was to characterize the physiological effects and psychophysics of both tests and to analyze whether the autonomic responses are mainly induced by baroreflexes or a consequence of the pain experience itself. The study consisted of a single session including one CPT (4 ± 0.2 °C) and one HIT (47 ± 0.5 °C; cut‐off point 5 min) trial performed on 30 healthy drug free volunteers aged 19–57 (median 24) yrs. The sequence of both trials was alternated and participants were randomly assigned to sequence order and parallelized with respect to gender. Physiological parameters (cardiovascular, respiratory and electrodermal activity) and subjective pain intensity were continuously monitored. In addition, pain detection and tolerance thresholds as well as pain unpleasantness were assessed. Both tests were comparable with regard to the time course and intensity of subjective pain. However, a significantly higher increase of blood pressure could be observed during the CPT when compared to the HIT. The HIT appears less confounded with thermoregulatory baroreflex activity and therefore seems to be a more appropriate model for tonic pain.     

Exercise-Induced Hypoalgesia in Pain-Free and Chronic Pain Populations: State of the Art and Future Directions

Exercise is considered an important component of effective chronic pain management and it is well-established that long-term exercise training provides pain relief. In healthy, pain-free populations, a single bout of aerobic or resistance exercise typically leads to exercise-induced hypoalgesia (EIH), a generalized reduction in pain and pain sensitivity that occurs during exercise and for some time afterward. In contrast, EIH is more variable in chronic pain populations and is more frequently impaired; with pain and pain sensitivity decreasing, remaining unchanged or, in some cases, even increasing in response to exercise. Pain exacerbation with exercise may be a major barrier to adherence, precipitating a cycle of physical inactivity that can lead to long-term worsening of both pain and disability. To optimize the therapeutic benefits of exercise, it is important to understand how EIH works, why it may be impaired in some people with chronic pain, and how this should be addressed in clinical practice. In this article, we provide an overview of EIH across different chronic pain conditions. We discuss possible biological mechanisms of EIH and the potential influence of sex and psychosocial factors, both in pain-free adults and, where possible, in individuals with chronic pain. The clinical implications of impaired EIH are discussed and recommendations are made for future research, including further exploration of individual differences in EIH, the relationship between exercise dose and EIH, the efficacy of combined treatments and the use of alternative measures to quantify EIH.PerspectiveThis article provides a contemporary review of the acute effects of exercise on pain and pain sensitivity, including in people with chronic pain conditions. Existing findings are critically reviewed, clinical implications are discussed, and recommendations are offered for future research.     

Temporal summation in muscles and referred pain areas: An experimental human study

The aim of the present study was to assess temporal summation within saline-induced, localized and referred muscle pain areas. The sensibility to single and repeated electrical stimuli were assessed in the muscle by means of needle electrodes and in the referred pain area by surface stimulation. The study demonstrates that temporal summation of nociceptive input from muscles exists and that the responses to single and repeated nociceptive stimuli of the referred pain area are facilitated. © 1997 John Wiley & Sons, Inc. Muscle Nerve 20: 1311–1313, 1997