Perceptual and motor responses directly and indirectly mediate the effects of noxious stimuli on autonomic responses

Nocebo Effect on Pain-Related Autonomic Responses in a State of Experimentally-Induced Sensitization

BACKGROUND

Enhanced pain-related autonomic responses were reported after experimentally-induced secondary mechanical hyperalgesia (SMH) in healthy individuals as well as in a variety of chronic pain cohorts. Stimulus-induced autonomic responses can also be modulated by positive and negative expectations towards the applied stimulus. This study aimed to investigate the influence of negative expectations on pain-related autonomic responses after experimentally-induced SMH.

METHODS

Forty healthy participants (20 females) were recruited and assigned to a NOCEBO or a NAïVE group. Phasic skin conductance responses (SCR) and tonic background skin conductance level (SCL) were recorded in response to 10 pinprick stimuli applied to both volar forearms. On one arm, all stimuli were applied (EXP-arm) before (PRE) and after (POST) an experimental heat pain model to induce SMH. The other arm served as the control (CTRL-arm). The NOCEBO group was instructed that the stimuli will be 'more intense and painful' in the POST-assessment. The NAïVE group did not receive any instructions. Pain ratings were matched to a numeric rating scale 4 across all assessments to control for subjective pain perception.

RESULTS

Only the combination of induced SMH and negative expectation (i.e., EXP-arm in the NOCEBO group) increased the pinprick-evoked phasic SCRs (p < 0.001) from PRE to POST. Tonic background SCL increased from PRE to POST (p < 0.01) independent of stimulation area (i.e., EXP-arm or CTRL-arm) or group (i.e., NOCEBO or NAïVE).

CONCLUSIONS

These results demonstrate facilitatory effects of top-down modulatory processes (i.e., negative expectations) on pain-related autonomic responses after experimentally-induced SMH.

SIGNIFICANCE

This study showed a facilitatory effect of negative expectation on enhanced pain-related autonomic responses in a state of experimentally-induced sensitisation in healthy participants. Hence, pain-related autonomic responses are shaped by both bottom-up (nociceptive input) and top-down (expectation) modulatory processes. This leads to the clinical implication that increased pain-related autonomic responses reported in individuals with chronic pain might not solely reflect pain hypersensitivities through nociceptive sensitisation, but also exaggerated negative expectation.

Priming of the autonomic nervous system after an experimental human pain model

Modulated autonomic responses to noxious stimulation have been reported in experimental and clinical pain. These effects are likely mediated by nociceptive sensitization, but may also, more simply reflect increased stimulus-associated arousal. To disentangle between sensitization- and arousal-mediated effects on autonomic responses to noxious input, we recorded sympathetic skin responses (SSRs) in response to 10 pinprick and heat stimuli before (PRE) and after (POST) an experimental heat pain model to induce secondary hyperalgesia (EXP) and a control model (CTRL) in 20 healthy females. Pinprick and heat stimuli were individually adapted for pain perception (4/10) across all assessments. Heart rate, heart rate variability, and skin conductance level (SCL) were assessed before, during, and after the experimental heat pain model. Both pinprick- and heat-induced SSRs habituated from PRE to POST in CTRL, but not EXP (P = 0.033). Background SCL (during stimuli application) was heightened in EXP compared with CTRL condition during pinprick and heat stimuli (P = 0.009). Our findings indicate that enhanced SSRs after an experimental pain model are neither fully related to subjective pain, as SSRs dissociated from perceptual responses, nor to nociceptive sensitization, as SSRs were enhanced for both modalities. Our findings can, however, be explained by priming of the autonomic nervous system during the experimental pain model, which makes the autonomic nervous system more susceptible to noxious input. Taken together, autonomic readouts have the potential to objectively assess not only nociceptive sensitization but also priming of the autonomic nervous system, which may be involved in the generation of distinct clinical pain phenotypes.NEW & NOTEWORTHY The facilitation of pain-induced sympathetic skin responses observed after experimentally induced central sensitization is unspecific to the stimulation modality and thereby unlikely solely driven by nociceptive sensitization. In addition, these enhanced pain-induced autonomic responses are also not related to higher stimulus-associated arousal, but rather a general priming of the autonomic nervous system. Hence, autonomic readouts may be able to detect generalized hyperexcitability in chronic pain, beyond the nociceptive system, which may contribute to clinical pain phenotypes.

In anticipation of pain: expectancy modulates corticospinal excitability, autonomic response, and pain perception

ABSTRACT

Pain is a ubiquitous experience encompassing perceptual, autonomic, and motor responses. Expectancy is known to amplify the perceived and autonomic components of pain, but its effects on motor responses are poorly understood. Understanding expectancy modulation of corticospinal excitability has important implications regarding deployment of adaptive and maladaptive protective behaviours in anticipation of pain. We developed a protocol to compare corticospinal excitability to expected high pain, expected low pain, and critically low pain when high pain was expected. Expecting high pain suppressed corticospinal excitability and heightened perceptual and autonomic responses to the low-pain stimulus, as with increased noxious stimulation (ie, expected high pain). Multilevel modelling revealed that perceived pain mediated the effect of both noxious stimulation and this expectancy-modulated pain on autonomic responses, but corticospinal excitability did not. These results demonstrate that merely expecting pain influenced all pain components. Findings shed new light on the aetiology of expectancy-modulated pain, whereby expecting pain mobilises the motor system to protect the body from harm by a protective withdrawal reflex, associated with reduced corticospinal excitability, and activates similar processes as increased nociceptive stimulation. This has significant practical implications for the treatment of pain, particularly in scenarios where avoidance of pain-related movement contributes to its maintenance.

Dispositional Mindfulness and Acute Heat Pain: Comparing Stimulus-Evoked Pain With Summary Pain Assessment

OBJECTIVE

Dispositional mindfulness is associated with reduced pain in clinical and experimental settings. However, researchers have neglected the type of pain assessment, as dispositional mindfulness may have unique benefits for reduced pain sensitivity when relying on summary pain assessments, in contrast to assessing the pain of each noxious stimulus. Here, we test the association between dispositional mindfulness and pain using both trial-by-trial pain assessments and overall summary ratings after acute pain tasks.

METHODS

One hundred thirty-one healthy adult volunteers (mean age = 29.09 [8.00] years, 55.7% female) underwent two experimental thermal pain paradigms. We tested whether dispositional mindfulness measured with the Mindful Attention Awareness Scale was related to a) heat-evoked pain sensitivity, as measured by pain threshold, pain tolerance, average pain, trial-by-trial ratings, and heat-evoked skin conductance response, and b) summary judgments of sensory and affective pain assessed using the McGill Pain Questionnaire (MPQ).

RESULTS

Mindful Attention Awareness Scale ratings were associated with decreased pain on the MPQ sensory (B = -0.18, SE = 0.05, 95% confidence interval = -0.29 to -0.07, t = -3.28, p = .001) and affective (B = -0.11, SE = 0.03, 95% confidence interval = -0.18 to -0.05, t = -3.32, p = .001) dimensions but not with experimental thermal pain assessments, including threshold, tolerance, heat-evoked pain, or skin conductance response (p values ≥ .29).

CONCLUSIONS

In this study, dispositional mindfulness mitigated acute thermal pain only when pain was assessed using the MPQ. These findings may reflect differences in immediate versus retrospective judgments or the type of pain assessed by each measure. Future research should examine regulation processes that may explain these differential analgesic benefits, such as attention, rumination, or reappraisal.