Pupil responses and pain ratings to heat stimuli: Reliability and effects of expectations and a conditioning pain stimulus

Pupillometry as a Potential Objective Measurement of Pain Assessment in Healthy Volunteers

Background

Pain leads to activation of the autonomic nervous system and thus, among other things, to pupillary reflex dilation (PRD). Previous studies have already confirmed a correlation between the perception of pain and the pupillary reaction, measured using pupillometry. However, the previous study populations were under the influence of medication for analgesia in perioperative setting or suffered from pain. This study examines the relationship between pupillary reaction and pain perception in healthy controls and addresses the question of whether endogenous pain inhibition, clinically tested by conditioned pain modulation (CPM), can be quantified using pupillometry.

Methods

Forty-two healthy volunteers (21 females, 21 males, mean age 27.9 ± 5.8 years, range 20-39 years) were included in this study. The PRD, as a measure of the pupillary reaction (variance from the base diameter in percent), was investigated during baseline, heat application and during CPM testing and results compared to the reported pain intensity on the numerical rating scale (NRS).

Results

The volunteers showed higher variances under painful conditions compared to the measurement at rest corresponding to higher sympathetic activity during pain. Volunteers with a higher variance, ie a stronger pupillary reaction, gave higher pain ratings than subjects with a lower pupil variance. However, there was no correlation between the NRS and PRD. PRD and pain ratings during CPM were significantly lower compared to heat pain application alone. However, there was no correlation between the calculated CPM effect and the PRD.

Conclusion

Pupillometry is capable of objectively reflecting the pain response, eg pain relief through CPM testing. However, the CPM effect calculated from the subjective pain ratings and the objective PRD measurements is not associated suggesting that both measure different aspects of pain perception. It must be discussed whether the CPM effect can be the correct measure for the functionality of the pain system.

Pupil dilation reveals the intensity of touch

Touch is important for many aspects of our daily activities. One of the most important tactile characteristics is its perceived intensity. However, quantifying the intensity of perceived tactile stimulation is not always possible using overt responses. Here, we show that pupil responses can objectively index the intensity of tactile stimulation in the absence of overt participant responses. In Experiment 1 (n = 32), we stimulated three reportedly differentially sensitive body locations (finger, forearm, and calf) with a single tap of a tactor while tracking pupil responses. Tactile stimulation resulted in greater pupil dilation than a baseline without stimulation. Furthermore, pupils dilated more for the more sensitive location (finger) than for the less sensitive location (forearm and calf). In Experiment 2 (n = 20) we extended these findings by manipulating the intensity of the stimulation with three different intensities, here a short vibration, always at the little finger. Again, pupils dilated more when being stimulated at higher intensities as compared to lower intensities. In summary, pupils dilated more for more sensitive parts of the body at constant stimulation intensity and for more intense stimulation at constant location. Taken together, the results show that the intensity of perceived tactile stimulation can be objectively measured with pupil responses - and that such responses are a versatile marker for touch research. Our findings may pave the way for previously impossible objective tests of tactile sensitivity, for example in minimally conscious state patients.

Comparison between pupillometry and numeric pain rating scale for pain assessments in communicating adult patients in the emergency department

OBJECTIVE

The adequate assessment of pain in the emergency department (ED) can be challenging. Two dynamic pupillary measures used in conscious subjects after a surgical procedure were previously shown to correlate to the magnitude of ongoing pain. The objective of this study was to test the ability of dynamic measures derived from pupillometry to evaluate pain intensity in conscious adult patients admitted to the ED.

METHODS

This prospective, interventional, single-centre study was performed between August 2021 and January 2022 (NCT05019898). An assessment of self-reported pain intensity was performed on ED admission by the triage nurse using a numeric rating scale (NRS). This was followed by two dynamic measures derived from pupillometry that were previously correlated with pain perception: the pupillary unrest under ambient light (PUAL) and the pupillary light reflex (PLR).

RESULTS

Among the 313 analysed patients, the median age was 41 years, and 52% were women. No correlation was found between self-reported pain ratings and PUAL (r = 0.007) or PLR (baseline diameter r = -0.048; decrease r = 0.024; latency r = 0.019; slope = -0.051). Similarly, the pupillometry measures could not discriminate patients with moderate to severe pain (defined as NRS ≥4).

CONCLUSIONS

Pupillometry does not appear to be an effective tool to evaluate pain in the ED environment. Indeed, too many factors influencing the sympathetic system-and thus the dynamic pupillary measures-are not controllable in the ED.

SIGNIFICANCE

Pupillometry does not appear to be an effective tool to evaluate pain in the ED environment. There are several possible explanations for these negative results. The factors influencing the sympathetic system-and thus the PD fluctuations-are controllable in the postoperative period but not in the ED (e.g. full bladder, hypothermia). In addition, numerous psychological phenomena can impact pupillometry measurements such as emotional reactions or cognitive tasks. These phenomena are particularly difficult to control in the ED environment.

Does past/current pain change pain experience? Comparing self-reports and pupillary responses

Introduction

For decades, a substantial body of research has confirmed the subjective nature of pain. Subjectivity seems to be integrated into the concept of pain but is often confined to self-reported pain. Although it seems likely that past and current pain experiences would interact and influence subjective pain reports, the influence of these factors has not been investigated in the context of physiological pain. The current study focused on exploring the influence of past/current pain on self-reporting and pupillary responses to pain.

Methods

Overall, 47 participants were divided into two groups, a 4°C-10°C group (experiencing major pain first) and a 10°C-4°C group (experiencing minor pain first), and performed cold pressor tasks (CPT) twice for 30 s each. During the two rounds of CPT, participants reported their pain intensity, and their pupillary responses were measured. Subsequently, they reappraised their pain ratings in the first CPT session.

Results

Self-reported pain showed a significant difference (4°C-10°C: p = 0.045; 10°C-4°C: p < 0.001) in the rating of cold pain stimuli in both groups, and this gap was higher in the 10°C-4°C group than in the 4°C-10°C group. In terms of pupillary response, the 4°C-10°C group exhibited a significant difference in pupil diameter, whereas this was marginally significant in the 10°C-4°C group (4°C-10°C: p < 0.001; 10°C-4°C: p = 0.062). There were no significant changes in self-reported pain after reappraisal in either group.

Discussion

The findings of the current study confirmed that subjective and physiological responses to pain can be altered by previous experiences of pain.

Toward Composite Pain Biomarkers of Neuropathic Pain-Focus on Peripheral Neuropathic Pain

Chronic pain affects ~10-20% of the U.S. population with an estimated annual cost of $600 billion, the most significant economic cost of any disease to-date. Neuropathic pain is a type of chronic pain that is particularly difficult to manage and leads to significant disability and poor quality of life. Pain biomarkers offer the possibility to develop objective pain-related indicators that may help diagnose, treat, and improve the understanding of neuropathic pain pathophysiology. We review neuropathic pain mechanisms related to opiates, inflammation, and endocannabinoids with the objective of identifying composite biomarkers of neuropathic pain. In the literature, pain biomarkers typically are divided into physiological non-imaging pain biomarkers and brain imaging pain biomarkers. We review both types of biomarker types with the goal of identifying composite pain biomarkers that may improve recognition and treatment of neuropathic pain.

Tonic pain alters functional connectivity of the descending pain modulatory network involving amygdala, periaqueductal gray, parabrachial nucleus and anterior cingulate cortex

INTRODUCTION

Resting state functional connectivity (FC) is widely used to assess functional brain alterations in patients with chronic pain. However, reports of FC accompanying tonic pain in pain-free persons are rare. A network we term the Descending Pain Modulatory Network (DPMN) is implicated in healthy and pathologic pain modulation. Here, we evaluate the effect of tonic pain on FC of specific nodes of this network: anterior cingulate cortex (ACC), amygdala (AMYG), periaqueductal gray (PAG), and parabrachial nuclei (PBN).

METHODS

In 50 pain-free participants (30F), we induced tonic pain using a capsaicin-heat pain model. functional MRI measured resting BOLD signal during pain-free rest with a 32°C thermode and then tonic pain where participants experienced a previously warm temperature combined with capsaicin. We evaluated FC from ACC, AMYG, PAG, and PBN with correlation of self-report pain intensity during both states. We hypothesized tonic pain would diminish FC dyads within the DPMN.

RESULTS

Of all hypothesized FC dyads, only PAG and subgenual ACC was weakly altered during pain (F=3.34; p=0.074; pain-free>pain d=0.25). After pain induction sACC-PAG FC became positively correlated with pain intensity (R=0.38; t=2.81; p=0.007). Right PBN-PAG FC during pain-free rest positively correlated with subsequently experienced pain (R=0.44; t=3.43; p=0.001). During pain, this connection's FC was diminished (paired t=-3.17; p=0.0026). In whole-brain analyses, during pain-free rest, FC between left AMYG and right superior parietal lobule and caudate nucleus were positively correlated with subsequent pain. During pain, FC between left AMYG and right inferior temporal gyrus negatively correlated with pain. Subsequent pain positively correlated with right AMYG FC with right claustrum; right primary visual cortex and right temporo-occipitoparietal junction Conclusion: We demonstrate sACC-PAG tonic pain FC positively correlates with experienced pain and resting right PBN-PAG FC correlates with subsequent pain and is diminished during tonic pain. Finally, we reveal PAG- and right AMYG-anchored networks which correlate with subsequently experienced pain intensity. Our findings suggest specific connectivity patterns within the DPMN at rest are associated with subsequently experienced pain and modulated by tonic pain. These nodes and their functional modulation may reveal new therapeutic targets for neuromodulation or biomarkers to guide interventions.

Self-Report of Severity of Ocular Pain Due to Light as a Predictor of Altered Central Nociceptive System Processing in Individuals With Symptoms of Dry Eye Disease

Dry eye disease (DED) is a diagnosis given to individuals with a heterogeneous combination of symptoms and/or signs, including spontaneous and evoked ocular pain. Our current study evaluated whether and which ocular pain assessments could serve as screening tools for central sensitization in individuals with DED. A cohort of individuals with DED symptoms (n = 235) were evaluated for ocular pain, DED signs (tear production, evaporation), evoked sensitivity to mechanical stimulation at the cornea, and evidence of central sensitization. Central sensitization was defined for this study as the presence of pain 30 seconds after termination of a thermal noxious temporal summation protocol (ie, aftersensations) presented at a site remote from the eye (ventral forearm). We found that combining ratings of average intensity of ocular pain, ratings of average intensity of pain due to light, response to topical anesthetic eye drops, and corneal mechanical pain thresholds produced the best predictive model for central sensitization (area under the curve of .73). When examining ratings of intensity of ocular pain due to light alone (0-10 numerical rating), a cutoff score of 2 maximized sensitivity (85%) and specificity (48%) for the presence of painful aftersensations at the forearm. Self-reported rating of pain sensitivity to light may serve as a quick screening tool indicating the involvement of central nociceptive system dysfunction in individuals with DED. PERSPECTIVE: This study reveals that clinically-relevant variables, including a simple 0 to 10 rating of ocular pain due to light, can be used to predict the contribution of central sensitization mechanisms in a subgroup of individuals with DED symptoms. These findings can potentially improve patient stratification and management for this complex and painful disease.

Central pain modulatory mechanisms of attentional analgesia are preserved in fibromyalgia

ABSTRACT

Fibromyalgia is a prevalent pain condition that is associated with cognitive impairments including in attention, memory, and executive processing. It has been proposed that fibromyalgia may be caused by altered central pain processing characterised by a loss of endogenous pain modulation. We tested whether attentional analgesia, where cognitive engagement diminishes pain percept, was attenuated in patients with fibromyalgia (n = 20) compared with matched healthy controls (n = 20). An individually calibrated, attentional analgesia paradigm with a 2 × 2 factorial design was used with brain and brainstem-focussed functional magnetic resonance imaging. Patients with fibromyalgia had both lower heat pain thresholds and speeds in a visual attention task. When this was taken into account for both attentional task and thermal stimulation, both groups exhibited an equivalent degree of attentional analgesia. Functional magnetic resonance imaging analysis showed similar patterns of activation in the main effects of pain and attention in the brain and brainstem (with the sole exceptions of increased activation in the control group in the frontopolar cortex and the ipsilateral locus coeruleus). The attentional analgesic effect correlated with activity in the periaqueductal gray and rostral ventromedial medulla. These findings indicate that patients with fibromyalgia can engage the descending pain modulatory system if the attentional task and noxious stimulus intensity are appropriately titrated.

Pupillary light reflex as a diagnostic aid from computational viewpoint: A systematic literature review

This work presents a detailed and complete review of publications on pupillary light reflex (PLR) used to aid diagnoses. These are computational techniques used in the evaluation of pupillometry, as well as their application in computer-aided diagnoses (CAD) of pathologies or physiological conditions that can be studied by observing the movements of miosis and mydriasis of the human pupil. A careful survey was carried out of all studies published over the last 10 years which investigated, electronic devices, recording protocols, image treatment, computational algorithms and the pathologies related to PLR. We present the frontier of existing knowledge regarding methods and techniques used in this field of knowledge, which has been expanding due to the possibility of performing diagnoses with high precision, at a low cost and with a non-invasive method.

Effects of pupillary reflex dilation-guided opioid administration on remifentanil and morphine consumption during laparoscopic surgery: A randomised controlled trial

BACKGROUND

Analysis of pupillary reflex dilation (PRD) assesses the balance of nociception--antinociception. Laparoscopic surgery induces haemodynamic variations that are misleading. During laparoscopy, PRD guidance helps differentiate haemodynamic changes because of excess nociception from secondary changes related to the reflex release of endocrine factors.

OBJECTIVE

The present study evaluated the effect of PRD-guided antinociception on the administration of intra-operative remifentanil and immediate postoperative morphine consumption in patients undergoing elective laparoscopic surgery.

DESIGN

The study was a single-blind, randomised controlled trial.

SETTING

The study took place at two sites at the University Hospital of Nancy from March 2014 to November 2017.

PATIENTS

A total of 100 patients who underwent scheduled laparoscopic surgery were included.

INTERVENTIONS

Patients were randomly given remifentanil guided by PRD (PRD-guided) or standard anaesthesia care (control).

MAIN OUTCOME MEASURES

The primary outcome was intra-operative remifentanil consumption. Secondary outcomes included morphine consumption in the immediate postoperative period and the number of intra-operative haemodynamic events.

RESULTS

Data from 95 patients were analysed. Intraoperative remifentanil consumption was lower in the PRD-guided group than in the control group: median [IQR], 0.09 [0.07 to 0.11] vs. 0.14 [0.12 to 0.16] μg kg min, with a mean difference (95% confidence Interval, CI) of 0.048 (0.035 to 0.060) μg kg min; P < 0.0001. Morphine consumption was 0.13 [0.1 to 0.5] vs. 0.15 [0.11 to 0.4] mg kg (P = 0.52) in the PRD-guided and control groups, respectively. The number of hypertensive and tachycardia events was greater in the PRD-guided group than in the control group: Hypertensive events 60.4 vs. 32.6%, relative risk 1.85 (95% CI, 1.24 to 2.84), P = 0.004; tachycardia events 31.6% vs. 4.3%, relative risk 2.09 (95% CI, 1.45 to 2.84), P < 0.001.

CONCLUSIONS

When PRD is used to differentiate between haemodynamic events arising from noxious stimuli and those events because of other nonsurgical stimuli, then intra-operative remifentanil administration is reduced intra-operatively during laparoscopic surgery but there was no change in postoperative morphine consumption.

TRIAL REGISTRATION

Clinicaltrials.gov NCT02116868.

Eye Movements in Response to Pain-Related Feelings in the Presence of Low and High Cognitive Loads

The affective dimension of pain contributes to pain perception. Cognitive load may influence pain-related feelings. Eye tracking has proven useful for detecting cognitive load effects objectively by using relevant eye movement characteristics. In this study, we investigated whether eye movement characteristics differ in response to pain-related feelings in the presence of low and high cognitive loads. A set of validated, control, and pain-related sounds were applied to provoke pain-related feelings. Twelve healthy young participants (six females) performed a cognitive task at two load levels, once with the control and once with pain-related sounds in a randomized order. During the tasks, eye movements and task performance were recorded. Afterwards, the participants were asked to fill out questionnaires on their pain perception in response to the applied cognitive loads. Our findings indicate that an increased cognitive load was associated with a decreased saccade peak velocity, saccade frequency, and fixation frequency, as well as an increased fixation duration and pupil dilation range. Among the oculometrics, pain-related feelings were reflected only in the pupillary responses to a low cognitive load. The performance and perceived cognitive load decreased and increased, respectively, with the task load level and were not influenced by the pain-related sounds. Pain-related feelings were lower when performing the task compared with when no task was being performed in an independent group of participants. This might be due to the cognitive engagement during the task. This study demonstrated that cognitive processing could moderate the feelings associated with pain perception.

Pupil response to noxious corneal stimulation

Purpose

Ocular somatosensory-autonomic reflexes play critical roles in maintaining homeostasis of the eye. The purpose of this study was to investigate the pupil response to nociceptive corneal stimuli.

Methods

A Waterloo-Belmonte pneumatic esthesiometer was used to determine detection thresholds and randomly deliver mechanical and chemical stimuli from levels of detection threshold to twice the threshold in 50% steps to the central cornea of 15 healthy subjects. For each stimulus, imaging of the stimulated/unstimulated eye was performed using two modified/calibrated Logitech c920 digital cameras for 4 seconds each, pre/post stimulus capture. The data were processed with a custom segmentation algorithm to help identify the pupils and pupil diameter was measured using ImageJ software. Pupil dilation response differences between the ipsi- and contralateral eye was analyzed using dependent t-tests. The effect of stimulus intensity, modality and sex of subjects were analyzed using repeated measures.

Results

In mechanical and chemical stimulation experiments, there was no difference in pupil responses between the stimulated eye and the unstimulated eye, (all dependent T-test p > 0.05). On average, pupil diameter increased from baseline as the corneal stimulus intensity increased. This happened regardless of whether mechanical or chemical stimulation occurred (ANOVA p < 0.05). At 200% threshold, pupil diameter was greater than at all stimulus intensities (Tukey HSD, all p < 0.05). Based on stimulus intensity, females had greater pupil diameters than males at levels of 150% threshold and 200% threshold (ANOVA p < 0.05, all Tukey HSD p < 0.05).

Conclusion

This study serves as a basis for the characterization of the local stimulus-response neural circuitry relating nociceptive stimuli to autonomic responses and in combination with our work on completely separate autonomic circuits of bulbar conjunctival vessel dilation and reflex tearing suggests that the monotonic measurements of redness, tearing and pupils provide accurate, separable responses that reflect painful stimulus intensity.

Distinguishing pain from nociception, salience, and arousal: How autonomic nervous system activity can improve neuroimaging tests of specificity

Pain is a subjective, multidimensional experience that is distinct from nociception. A large body of work has focused on whether pain processing is supported by specific, dedicated brain circuits. Despite advances in human neuroscience and neuroimaging analysis, dissociating acute pain from other sensations has been challenging since both pain and non-pain stimuli evoke salience and arousal responses throughout the body and in overlapping brain circuits. In this review, we discuss these challenges and propose that brain-body interactions in pain can be leveraged in order to improve tests for pain specificity. We review brain and bodily responses to pain and nociception and extant efforts toward identifying pain-specific brain networks. We propose that autonomic nervous system activity should be used as a surrogate measure of salience and arousal to improve these efforts and enable researchers to parse out pain-specific responses in the brain, and demonstrate the feasibility of this approach using example fMRI data from a thermal pain paradigm. This new approach will improve the accuracy and specificity of functional neuroimaging analyses and help to overcome current difficulties in assessing pain specific responses in the human brain.

The search for pain biomarkers in the human brain

Non-invasive functional brain imaging is used more than ever to investigate pain in health and disease, with the prospect of finding new means to alleviate pain and improve patient wellbeing. The observation that several brain areas are activated by transient painful stimuli, and that the magnitude of this activity is often graded with pain intensity, has prompted researchers to extract features of brain activity that could serve as biomarkers to measure pain objectively. However, most of the brain responses observed when pain is present can also be observed when pain is absent. For example, similar brain responses can be elicited by salient but non-painful auditory, tactile and visual stimuli, and such responses can even be recorded in patients with congenital analgesia. Thus, as argued in this review, there is still disagreement on the degree to which current measures of brain activity exactly relate to pain. Furthermore, whether more recent analysis techniques can be used to identify distributed patterns of brain activity specific for pain can be only warranted using carefully designed control conditions. On a more general level, the clinical utility of current pain biomarkers derived from human functional neuroimaging appears to be overstated, and evidence for their efficacy in real-life clinical conditions is scarce. Rather than searching for biomarkers of pain perception, several researchers are developing biomarkers to achieve mechanism-based stratification of pain conditions, predict response to medication and offer personalized treatments. Initial results with promising clinical perspectives need to be further tested for replicability and generalizability.