A back-translational study of descending interactions with the induction of hyperalgesia by high-frequency electrical stimulation in rats and humans

Distress is positively associated with induced secondary hyperalgesia in people with suppressed HIV

Pain and distress are frequently reported by people with HIV. Although pain is widely acknowledged to contribute to distress, distress may also contribute to pain and its persistence. Facilitation of nociceptive signalling in the spinal dorsal horn is one pathway by which distress could exacerbate pain. The current study investigated the relationships between distress, secondary hyperalgesia (SH), and persistent pain in people with HIV, reporting pain (n=19) or no pain (n=26). We hypothesised that self-reported distress would be positively associated with the surface area (primary measure) and magnitude (secondary measure) of induced SH, and that participants reporting persistent pain would display greater induced SH than those reporting no pain. We found that distress was positively associated with the surface area (p=0.02) and the magnitude (p=0.01) of induced SH. However, participants with persistent pain showed no difference in the surface area of SH compared to pain-free participants (p=0.87), and those with pain displayed a marginally lower magnitude of SH (p=0.05). These findings lay a foundation for testing whether reducing distress also reduces SH, but also raise questions about the relevance of heterotopic spinal facilitation mechanisms to clinical persistent pain, in people with suppressed HIV.

Perspective

Distress is prospectively and positively associated with the surface area and magnitude of induced secondary hyperalgesia in people with suppressed HIV.

Successful identification and management of a stroke-like migraine attacks after radiation therapy syndrome occurring 36 years after cranial irradiation

Stroke-like migraine attacks after radiation therapy syndrome is a rare, late-onset neurological disorder observed in patients with a history of cranial radiation therapy. It presents with stroke-like symptoms, including hemiparesis, hemianopsia, seizures, and migraines, often occurring years or decades post-therapy. We report a case of stroke-like migraine attacks after radiation therapy syndrome in a 51-year-old male, manifesting 36 years after treatment for oligodendroglioma with whole-brain irradiation. The patient developed sudden worsening of left hemiparesis, focal seizures, and bilateral frontotemporal headaches. Brain MRI revealed hyperintensity on T2 fluid-attenuated inversion recovery sequence and gyral enhancement in the right parieto-occipital cortex. The patient responded favourably to a 5-day course of steroid pulse therapy (1000mg/day), with significant clinical improvement and resolution of MRI abnormalities. This case highlights one of the longest documented latencies for stroke-like migraine attacks after radiation therapy syndrome and emphasizes the importance of considering this diagnosis in patients after irradiation, even after extended latency periods, to prevent misdiagnosis and inappropriate treatment.

Provoked cytokine response is not associated with distress or induced secondary hyperalgesia in people with suppressed HIV

Psychological distress predicts the onset and worsening of persistent pain, but the mechanisms that underpin this influence are poorly understood. Pro-inflammatory signalling is a plausible mechanistic link, given its known connections to distress, pain, and neural upregulation. Sustained distress may prime the inflammatory system to respond more strongly to a phasic noxious challenge, supporting neuroimmune upregulation of central nociceptive signalling and persistent pain. This cross-sectional study tested the hypotheses that in vitro endotoxin-provoked expression of typically pro-inflammatory cytokines (IL1β, IL6) is a partial mediator between distress and persistent pain, and that it is associated with the secondary hyperalgesia response to an experimental noxious challenge, in people with suppressed HIV. Study participants were 99 adults (mean (range) age: 43(28-64y/o; 72 females) with either no pain (n=54) or persistent pain (n=45), mostly of black South African ethnicity, low socio-economic status, and with high social support. The results replicated previous reports that distress is associated with persistent pain status and pain severity, and also showed an association between distress and the anatomical extent of pain. However, distress was not associated with provoked cytokine expression, nor was provoked cytokine expression associated with secondary hyperalgesia. The conflict between our findings and the evidence on which our hypotheses were based could reflect masking of an effect by differentially trained immune systems or a more complex relationship arising from diverse psychoneuroimmunological interactions in this sample. Our sample's combination of HIV status, African genetic ancestry, financial impoverishment, and rich social interconnectedness is poorly represented in current research and represents an opportunity to deepen insight into psychoneuroimmunological interactions related to distress and persistent pain.

Assessing signs of central sensitization: A critical review of physiological measures in experimentally induced secondary hyperalgesia

BACKGROUND AND OBJECTIVES

Central sensitization (CS) is believed to play a role in many chronic pain conditions. Direct non-invasive recording from single nociceptive neurons is not feasible in humans, complicating CS establishment. This review discusses how secondary hyperalgesia (SHA), considered a manifestation of CS, affects physiological measures in healthy individuals and if these measures could indicate CS. It addresses controversies about heat sensitivity changes, the role of tactile afferents in mechanical hypersensitivity and detecting SHA through electrical stimuli. Additionally, it reviews the potential of neurophysiological measures to indicate CS presence.

DATABASES AND DATA TREATMENT

Four databases, PubMed, ScienceDirect, Scopus and Cochrane Library, were searched using terms linked to 'hyperalgesia'. The search was limited to research articles in English conducted in humans until 2023.

RESULTS

Evidence for heat hyperalgesia in the SHA area is sparse and seems to depend on the experimental method used. Minimal or no involvement of tactile afferents in SHA was found. At the spinal level, the threshold of the nociceptive withdrawal reflex (RIII) is consistently reduced during experimentally induced SHA. The RIII area and the spinal somatosensory potential (N13-SEP) amplitude are modulated only with long-lasting nociceptive input. At the brain level, pinprick-evoked potentials within the SHA area are increased.

CONCLUSIONS

Mechanical pinprick hyperalgesia is the most reliable behavioural readout for SHA, while the RIII threshold is the most sensitive neurophysiological readout. Due to scarce data on reliability, sensitivity and specificity, none of the revised neurophysiological methods is currently suitable for CS identification at the individual level.

SIGNIFICANCE

Gathering evidence for CS in humans is a crucial research focus, especially with the increasing interest in concepts such as 'central sensitization-like pain' or 'nociplastic pain'. This review clarifies which readouts, among the different behavioural and neurophysiological proxies tested in experimental settings, can be used to infer the presence of CS in humans.

A Parallel Human and Rat Investigation of the Interaction Between Descending and Spinal Modulatory Mechanisms

BACKGROUND

Healthy individuals demonstrate considerable heterogeneity upon dynamic quantitative sensory testing assessment of endogenous pain modulatory mechanisms. For those who stratify into a 'pro-nociceptive profile' cohort, consisting of inefficient conditioned pain modulation (CPM) and elevated temporal summation of pain (TSP), the optimal approach for balancing the net output of pain modulatory processes towards anti-nociception remains unresolved. In this translational healthy human and rat study, we examined whether descending modulation countered spinal amplification during concurrent application of a CPM and TSP paradigm alongside pupillometry since pontine activity was previously linked to functionality of endogenous pain modulatory mechanisms and pupil dilation.

METHODS

Perceptual (quantitative sensory testing) and spinal neuronal (in vivo electrophysiology) assessment was performed in healthy humans and rats respectively upon application of parallel CPM/diffuse noxious inhibitory controls (cuff algometry) and TSP/wind-up (pinprick) paradigms alongside pupillometry.

RESULTS

In humans, repetitive pinprick stimulation produced TSP while concurrent application of a noxious conditioning stimulus did not affect pain ratings to a single pinprick stimulus, repetitive stimulation or the wind-up ratio. In rats, repetitive pinprick produced neuronal wind-up while concurrent application of a noxious conditioning stimulus inhibited neuronal responses to a single stimulus and repetitive stimulation but not the wind-up ratio. For pupillometry experiments, dilatory responses did not increase during application of a TSP or CPM paradigm in humans, while reliable rat responses were not obtained.

CONCLUSIONS

Under the conditions of our study, spinal amplification mechanisms surpassed descending inhibitory controls while pupillometry did not offer a reliable indicator of endogenous pain modulatory mechanism function.

SIGNIFICANCE

In this translational healthy human and rat study, activity in descending inhibitory controls did not counter spinal amplification processes underpinned by wind up. Despite pupil dilation being previously linked to modulatory mechanisms, dilatory responses did not offer a reliable indicator of functionality. For pro-nociceptive individuals exhibiting inefficient conditioned pain modulation and/or high temporal summation of pain, dampening faciliatory mechanisms rather than augmenting top-down inhibitory processes may be a more effective pain-relief strategy.

High-frequency electrical stimulation increases cortical excitability and mechanical sensitivity in a chronic large animal model

ABSTRACT

Translational models of the sensitized pain system are needed to progress the understanding of involved mechanisms. In this study, long-term potentiation was used to develop a mechanism-based large-animal pain model. Event-related potentials to electrical stimulation of the ulnar nerve were recorded by intracranial recordings in pigs, 3 weeks before, immediately before and after, and 3 weeks after peripheral high-frequency stimulation (HFS) applied to the ulnar nerve in the right forelimb (7 pigs) or in control animals (5 pigs). Event-related potential recordings and peripheral HFS were done during anesthesia. Two weeks before and after the HFS, behavioral responses reflecting mechanical and thermal sensitivity were collected using brush, noxious limb-mounted pressure algometer, and noxious laser stimuli. The HFS intervention limb was progressively sensitized to noxious mechanical stimulation in week 1 and 2 compared with baseline ( P = 0.045) and the control group ( P < 0.034) but not significantly to laser or brush stimulation. The first negative (N1) peak of the event-related potential was increased 30 minutes after HFS compared with before ( P < 0.05). The N1 peak was also larger compared with control pigs 20 to 40 minutes after HFS ( P < 0.031) but not significantly increased 3 weeks after. The relative increase in N1 30 minutes after HFS and the degree of mechanical hyperalgesia 2 weeks post-HFS was correlated ( P < 0.033). These results show for the first time that the pig HFS model resembles the human HFS model closely where the profile of sensitization is comparable. Interestingly, the degree of sensitization was associated with the cortical signs of hyperexcitability at HFS induction.

Preoperative susceptibility to developing secondary hyperalgesia is associated with post-thoracotomy pain at 2 months

BACKGROUND

Identifying the subset of patients at risk for developing persistent pain after surgery is clinically important as they could benefit from targeted prevention measures. In this prospective study, we investigated if the preoperative assessment of the individual susceptibility to developing experimentally induced secondary hyperalgesia is associated with post-thoracotomy pain at 2 months.

METHODS

Forty-one patients scheduled to undergo a posterolateral thoracotomy were recruited before surgery and followed prospectively for 2 months. The day before surgery, we experimentally induced secondary hyperalgesia at one of the two forearms and measured the change of perception to mechanical pinprick stimuli and the area of hyperalgesia. On postoperative Day 4, Day 15 and at the 2-month follow-up, patients were asked about their pain intensity at rest and during coughing and the area of secondary hyperalgesia around the scar as well as the change in perception to mechanical pinprick stimuli was measured.

RESULTS

Of the 41 patients that were recruited only 20 could be analysed. Forty per cent reported pain at the 2-month follow-up. All of them reported cough-evoked pain and 10 per cent also reported pain at rest. A binary logistic regression model with both the magnitude and extent of experimentally induced secondary hyperalgesia was statistically significant (chi-squared = 12.439, p = 0.002, McFadden R2 = 0.462) and showed excellent discriminative power (AUC = 0.938) for the presence or absence of cough-evoked pain at the 2 month follow-up.

CONCLUSION

Our findings indicate that the individual susceptibility to developing experimentally induced secondary hyperalgesia preoperatively may identify patients who are potentially vulnerable to develop persistent post-thoracotomy pain.

SIGNIFICANCE

Our data suggests that preoperatively assessed experimentally induced secondary hyperalgesia displays excellent discriminative power for the presence or absence of cough-evoked pain 2 months after thoracotomy.

Studying the Effect of Expectations on High-Frequency Electrical Stimulation-Induced Pain and Pinprick Hypersensitivity

Negative expectations can increase pain, but can they promote the development of central sensitization? This study used an inert treatment and verbal suggestions to induce expectations of increased high-frequency electrical stimulation (HFS)-induced pain and assessed their effects on pain ratings during HFS and HFS-induced pinprick hypersensitivity. Fifty healthy volunteers were randomly allocated to either a control group (N = 25) or a nocebo group (N = 25). Participants in both groups received a patch containing water on the right forearm. The nocebo group was told that the patch contained capsaicin that sensitized their skin, while the control group was told that the patch contained water that had no effect on skin sensitivity. Before and after patch attachment, single electrical stimuli were delivered to the area of the patch to measure the perceived intensity to these stimuli. After patch removal and after the participant rated expected pain and fear for HFS, HFS was delivered to the same skin site, followed by the assessment of pinprick sensitivity. The nocebo group rated the perceived intensity for the single electrical stimulus after removal of the patch as more intense compared with the control group, indicating that our manipulation worked. Yet, this effect did not transfer to expected pain for HFS, nor did it affect pain intensity ratings during HFS. HFS increased pinprick sensitivity but no group differences were found. Because of the lack of differences in expected pain and pain intensity ratings for HFS between groups, no firm conclusions can be drawn regarding their effect on pinprick hypersensitivity. PERSPECTIVE: This study shows that sham treatment combined with verbal suggestions induces a nocebo effect but does not necessarily change expectations and experience of upcoming pain.