Cortex areas involved in the processing of normal and altered pain

Cortical nociceptive processes are reduced by visual alpha-band entrainment in the human brain

BACKGROUND

Acute noxious stimuli induce a suppression of cortical alpha activity, yet little is known about whether increasing alpha activity affects the processing of noxious stimuli. We have previously shown that visual alpha stimulation reduces experimental pain. Here, we demonstrate that increasing alpha power causes a reciprocal suppression of acute nociceptive processing.

METHODS

We attempted to increase cortical alpha activity through visual entrainment at 8 Hz, 10 Hz and 12 Hz to investigate the influence on the electrophysiological pain response. Moderately painful laser-heat stimuli were delivered following 10 minutes of visual entrainment across the alpha range.

RESULTS

Alpha power increased significantly relative to the 1 Hz control condition following 8 Hz and 10 Hz visual stimulation. Significant reductions in the P2 peak amplitude of the laser-evoked potential were found following visual entrainment at 10 Hz; the frequency stimulation resulting in the largest reduction in pain perception. Source analysis revealed that, following the 10 Hz stimulation, sources of increased alpha power and decreased nociceptive processing overlapped in precuneus and posterior cingulate cortex, with further reductions in nociceptive processing in insula cortex.

CONCLUSIONS

As far as we are aware, this is the first study to provide direct evidence that experimental induction of increased alpha power suppresses the cortical processing of acute pain.

SIGNIFICANCE

While it is known that visual stimulation can increase the brain's oscillatory alpha rhythms, here, we show that this increase in alpha power occurs alongside reduced cortical processing of nociception, as measured with EEG. This establishes an objective marker of alpha entrainment-based analgesia that may be useful in the development of neuromodulatory treatments for clinical pain.

Prevention of pruritus with ethyl-chloride in skin prick test: a double-blind placebo-controlled prospective study

Background

Ethyl-chloride (EC) spray was recently shown to be an effective antipruritic agent, when given 15 min after histamine skin-prick test (SPT), without changing the wheal and flare reaction. We aimed to investigate the antipruritic effect of EC on SPT, when given prior to SPT.

Methods

A double-blind placebo-controlled prospective study. Overall, 44 volunteers underwent histamine SPT on both arms to trigger local pruritus. Prior to test, they were randomly treated with EC spray on one arm and saline spray (placebo) on the other. Subjects as well as researchers were blinded to the type of applied sprays. The wheal and flare reaction was measured after the SPT and subjects reported the intensity of pruritus following EC/placebo using a validated pruritus questionnaire (indexes 1–3) and a visual analog scale (VAS).

Results

Significant improvement in pruritus was reported following treatment with EC compared with placebo for all four studied parameters. Index 1 in EC 3.7 ± 2.3 versus 5 ± 3.5 (p = 0.007) in placebo, index 2 in EC 2.6 ± 2.1 versus 3.8 ± 2.8 (p = 0.002) in placebo, index 3 of EC 6.3 ± 3.8 versus 8.8 ± 5.8 (p = 0.03) and VAS in EC 3.7 ± 1.9 versus 4.4 ± 2.3 (p = 0.003). There were no significant differences between EC and placebo in terms of the wheal and flare indurations area.

Conclusions

Ethyl-chloride has an effective antipruritic agent, when given before histamine SPT. Its use did not change the wheal and flare reaction, making it ideal for prevention of pruritus, secondary to allergy skin test, without masking the results.

Pain response measured with arterial spin labeling

The majority of functional MRI studies of pain processing in the brain use the blood oxygenation level-dependent (BOLD) imaging approach. However, the BOLD signal is complex as it depends on simultaneous changes in blood flow, vascular volume and oxygen metabolism. Arterial spin labeling (ASL) perfusion imaging is another imaging approach in which the magnetically labeled arterial water is used as an endogenous tracer that allows for direct measurement of cerebral blood flow. In this study, we assessed the pain response in the brain using a pulsed-continuous arterial spin labeling (pCASL) approach and a thermal stimulation paradigm. Using pCASL, response to noxious stimulation was detected in somatosensory cortex, anterior cingulate cortex, anterior insula, hippocampus, amygdala, thalamus and precuneus, consistent with the pain response activation patterns detected using the BOLD imaging approach. We suggest that pCASL is a reliable alternative for functional MRI pain studies in conditions in which blood flow, volume or oxygen extraction are altered or compromised.

Ankle-brachial blood pressure differences in the beach-chair position of the shoulder surgery

BACKGROUND

During shoulder surgery, blood pressure is frequently measured at the ankle. Anesthetic complications may result when ankle blood pressure is higher than brachial blood pressure and anesthesiologists misinterpret ankle blood pressure as brachial blood pressure. Therefore, we investigated whether ankle blood pressure is significantly higher than brachial blood pressure before anesthesia induction, during induction, after tracheal intubation, before beach chair position, and in the beach chair position.

METHODS

Thirty patients requiring general anesthesia for shoulder surgery were included in this study. Ankle and brachial blood pressure were simultaneously measured before induction, during induction, after intubation, before beach chair position, and in the beach chair position.

RESULTS

Ankle blood pressure was higher than brachial blood pressure before induction, during induction, after intubation, before beach chair position, and in the beach chair position. Ankle-brachial blood pressure differences in the beach chair condition were much higher than in four other conditions. The correlation coefficient between mean ankle-brachial blood pressure differences before the beach chair position and mean ankle-brachial blood pressure differences in the beach chair position was 0.616. Brachial systolic blood pressure could be predicted by regression equations (R(2) = 0.306-0.771).

CONCLUSIONS

These results suggest that anesthesiologists should consider these ankle-brachial blood pressure differences when monitoring anesthesia in the beach chair position.

Irritable bowel syndrome: methods, mechanisms, and pathophysiology. Methods to assess visceral hypersensitivity in irritable bowel syndrome

Irritable bowel syndrome (IBS) is a common functional gastrointestinal disorder, characterized by recurrent abdominal pain or discomfort in combination with disturbed bowel habits in the absence of identifiable organic cause. Visceral hypersensitivity has emerged as a key hypothesis in explaining the painful symptoms in IBS and has been proposed as a "biological hallmark" for the condition. Current techniques of assessing visceral perception include the computerized barostat using rectal distensions, registering responses induced by sensory stimuli including the flexor reflex and cerebral evoked potentials, as well as brain imaging modalities such as functional magnetic resonance imaging and positron emission tomography. These methods have provided further insight into alterations in pain processing in IBS, although the most optimal method and condition remain to be established. In an attempt to give an overview of these methods, a literature search in the electronic databases PubMed and MEDLINE was executed using the search terms "assessment of visceral pain/visceral nociception/visceral hypersensitivity" and "irritable bowel syndrome." Both original articles and review articles were considered for data extraction. This review aims to discuss currently used modalities in assessing visceral perception, along with advantages and limitations, and aims also to define future directions for methodological aspects in visceral pain research. Although novel paradigms such as brain imaging and neurophysiological recordings have been introduced in the study of visceral pain, confirmative studies are warranted to establish their robustness and clinical relevance. Therefore, subjective verbal reporting following rectal distension currently remains the best-validated technique in assessing visceral perception in IBS.

Randomised clinical trial: pregabalin attenuates experimental visceral pain through sub-cortical mechanisms in patients with painful chronic pancreatitis

BACKGROUND

Pregabalin has a broad spectrum of analgesic and antihyperalgesic activity in both basic and clinical studies. However, its mechanisms and sites of action have yet to be determined in humans.

AIMS

To assess the antinociceptive effect of pregabalin on experimental gut pain in patients with visceral hyperalgesia due to chronic pancreatitis and to reveal putative changes in corresponding central pain processing as assessed by evoked brain potentials.

METHODS

Thirty-one patients were randomly assigned to receive increasing doses of pregabalin or placebo for three consecutive weeks. Perceptual thresholds to electrical stimulation of the sigmoid with recording of corresponding evoked brain potentials were obtained at baseline and study end. The brain source localisations reflecting direct neuronal activity were fitted by a five-dipole model projected to magnetic resonance imaging of the individuals' brains.

RESULTS

As compared to placebo, pregabalin significantly increased the pain threshold to electrical gut stimulation from baseline (P=0.02). No differences in evoked brain potential characteristics were seen, neither after pregabalin nor placebo treatment (all P>0.05). In agreement with this, brain source locations remained stable during study treatment (all P>0.05).

CONCLUSION

Pregabalin was superior to placebo for attenuation of experimental visceral pain in chronic pancreatitis patients. We suggest its antinociceptive effects to be mediated primarily through sub-cortical mechanisms.

The pain system in oesophageal disorders: mechanisms, clinical characteristics, and treatment

Pain is common in gastroenterology. This review aims at giving an overview of pain mechanisms, clinical features, and treatment options in oesophageal disorders. The oesophagus has sensory receptors specific for different stimuli. Painful stimuli are encoded by nociceptors and communicated via afferent nerves to the central nervous system. The pain stimulus is further processed and modulated in specific pain centres in the brain, which may undergo plastic alterations. Hence, tissue inflammation and long-term exposure to pain can cause sensitisation and hypersensitivity. Oesophageal sensitivity can be evaluated ,for example, with the oesophageal multimodal probe. Treatment should target the cause of the patient's symptoms. In gastro-oesophageal reflux diseases, proton pump inhibitors are the primary treatment option, surgery being reserved for patients with severe disease resistant to drug therapy. Functional oesophageal disorders are treated with analgesics, antidepressants, and psychological therapy. Lifestyle changes are another option with less documentation.

Neuroimaging of the human visceral pain system–A methodological review

During the last decades there has been a tremendous development of non-invasive methods for assessment of brain activity following visceral pain. Improved methods for neurophysiological and brain imaging techniques have vastly increased our understanding of the central processing of gastrointestinal sensation and pain in both healthy volunteers as well as in patients suffering from gastrointestinal disorders. The techniques used are functional magnetic resonance imaging (fMRI), positron emission tomography (PET), electroencephalography (EEG)/evoked brain potentials (EPs), magnetoencephalography (MEG), single photon emission computed tomography (SPECT), and the multimodal combinations of these techniques. The use of these techniques has brought new insight into the complex brain processes underlying pain perception, including a number of subcortical and cortical regions, and paved new ways in our understanding of acute and chronic pain. The pathways are dynamic with a delicate balance between facilitatory and inhibitory pain mechanisms, and with modulation of the response to internal or external stressors with a high degree of plasticity. Hence, the ultimate goal in imaging of pain is to follow the stimulus response throughout the neuraxis.

Brain activity measured by fMRI is based on subtracting regional changes in blood oxygenation during a resting condition from the signal during a stimulus condition, and has high spatial resolution but low temporal resolution. SPECT and PET are nuclear imaging techniques where radiolabeled molecules are injected with visualization of the distribution, density and activity of receptors in the brain allowing not only assessment of brain activity but also study of receptor sites. EEG is based on assessment of electrical activity in the brain, and recordings of the resting EEG and evoked potentials following an external stimulus are used to study normal visceral pain processing, alterations of pain processing in different patient groups and the effect of pharmacological intervention. EEG has high temporal resolution, but relative poor spatial resolution, which however to some extent can be overcome by applying inverse modelling algorithms and signal decomposition procedures. MEG is based on recording the magnetic fields produced by electrical currents in the brain, has high spatial resolution and is especially suitable for the study cortical activation.

The treatment of chronic abdominal pain is often ineffective and dissapointing, which leads to search for optimized treatment achieved on the basis of a better understanding of underlying pain mechanisms. Application of the recent improvements in neuroimaging on the visceral pain system may likely in near future contribute substantially to our understanding of the functional and structural pathophysiology underlying chronic visceral pain disorders, and pave the road for optimized individual and mechanism based treatments.

The purpose of this review is to give a state-of-the-art overview of these methods, with focus on EEG, and especially the advantages and limitations of the single methods in clinical gastrointestinal pain esearch including examples from relevant studies.

Noxious counterirritation in patients with advanced osteoarthritis of the knee reduces MCC but not SII pain generators: A combined use of MEG and EEG

Chronic pain is mainly a result of two processes: peripheral and central sensitization, which can result in neuroplastic changes. Previous psychophysical studies suggested a decrease of the so-called pain-inhibiting-pain effect (DNIC) in chronic pain patients. We aimed to study the DNIC effect on the neuronal level using magnetoencephalography and electroencephalography in 12 patients suffering from advanced unilateral knee osteoarthritis (OA). DNIC was induced in patients by provoking the typical OA pain by a slightly hyperextended joint position, while they received short electrical pain stimuli. Although the patients did not report a reduction of electrical pain perception, the cingulate gyrus showed a decrease of activation during provoked OA pain, while activity in the secondary somatosensory cortex did not change. Based on much stronger DNIC induction at comparable intensities of an acute counterirritant pain in healthy subjects this result suggests a deficit of DNIC in OA patients. We suggest that the strength of DNIC is subject to neuronal plasticity of descending inhibitory pain systems and diminishes during the development of a chronic pain condition.

Learning potentiates neurophysiological and behavioral placebo analgesic responses

Expectation and conditioning are supposed to be the two main psychological mechanisms for inducing a placebo response. Here, we further investigate the effects of both expectation, which was induced by verbal suggestion alone, and conditioning at the level of N1 and N2-P2 components of CO2 laser-evoked potentials (LEPs) and subjective pain reports. Forty-four healthy volunteers were pseudorandomly assigned to one of three experimental groups: Group 1 was tested with verbal suggestion alone, Group 2 was tested with a conditioning procedure, whereby the intensity of painful stimulation was reduced surreptitiously, so as to make the volunteers believe that the treatment was effective, Group 3 was a control group that allowed us to rule out phenomena of sensitization and/or habituation. Pain perception was assessed according to a Numerical Rating Scale (NRS) ranging from 0=no pain sensation to 10=maximum imaginable pain. Both verbal suggestions (Group 1) and conditioning (Group 2) modified the N2-P2 complex, but not the N1 component of LEPs. However, the suggestion-induced LEP changes occurred without subjective perception of pain decrease. Conversely, the N2-P2 amplitude changes that were induced by the conditioning procedure were associated with the subjective perception of pain reduction. Compared to natural history, conditioning produced more robust reductions of LEP amplitudes than verbal suggestions alone. Overall, these findings indicate that prior positive experience plays a key role in maximizing both behavioral and neurophysiological placebo responses, emphasizing that the placebo effect is a learning phenomenon which affects the early central nociceptive processing.

Duration of the cue-to-pain delay increases pain intensity: a combined EEG and MEG study

Expectation of pain is an important adaptive process enabling individuals to avoid bodily harm. It reflects the linking of past experience and environmental cues with imminent threat. In the present study, we examined changes in perceived pain contingent upon variation of the interval between an auditory cue and a subsequent painful laser stimulus. The duration of the cue-to-stimulus delay was systematically varied between 2, 4 and 6 s. Pain intensity and evoked brain responses measured by EEG and MEG recordings were analysed. Pain ratings from 15 subjects increased with longer cue-to-pain delays, accompanied by an increase in activity of the midcingulate cortex (MCC), as modelled from evoked EEG potential maps. On the other hand, MEG-based source activity in secondary somatosensory (SII) cortex remained unaffected by manipulation of the cue-to-stimulus interval. We conclude that activity in limbic structures such as MCC play a key role in the temporal dynamics of recruitment of expectation towards pain. Although this reaction is adaptive if the individual is able to avoid the stimulus, it is maladaptive if such opportunity is not present.

Neuronal Control of Skin Function: The Skin as a Neuroimmunoendocrine Organ

This review focuses on the role of the peripheral nervous system in cutaneous biology and disease. During the last few years, a modern concept of an interactive network between cutaneous nerves, the neuroendocrine axis, and the immune system has been established. We learned that neurocutaneous interactions influence a variety of physiological and pathophysiological functions, including cell growth, immunity, inflammation, pruritus, and wound healing. This interaction is mediated by primary afferent as well as autonomic nerves, which release neuromediators and activate specific receptors on many target cells in the skin. A dense network of sensory nerves releases neuropeptides, thereby modulating inflammation, cell growth, and the immune responses in the skin. Neurotrophic factors, in addition to regulating nerve growth, participate in many properties of skin function. The skin expresses a variety of neurohormone receptors coupled to heterotrimeric G proteins that are tightly involved in skin homeostasis and inflammation. This neurohormone-receptor interaction is modulated by endopeptidases, which are able to terminate neuropeptide-induced inflammatory or immune responses. Neuronal proteinase-activated receptors or transient receptor potential ion channels are recently described receptors that may have been important in regulating neurogenic inflammation, pain, and pruritus. Together, a close multidirectional interaction between neuromediators, high-affinity receptors, and regulatory proteases is critically involved to maintain tissue integrity and regulate inflammatory responses in the skin. A deeper understanding of cutaneous neuroimmunoendocrinology may help to develop new strategies for the treatment of several skin diseases.

Frontiers in pruritus research: scratching the brain for more effective itch therapy

This Review highlights selected frontiers in pruritus research and focuses on recently attained insights into the neurophysiological, neuroimmunological, and neuroendocrine mechanisms underlying skin-derived itch (pruritogenic pruritus), which may affect future antipruritic strategies. Special attention is paid to newly identified itch-specific neuronal pathways in the spinothalamic tract that are distinct from pain pathways and to CNS regions that process peripheral pruritogenic stimuli. In addition, the relation between itch and pain is discussed, with emphasis on how the intimate contacts between these closely related yet distinct sensory phenomena may be exploited therapeutically. Furthermore, newly identified or unduly neglected intracutaneous itch mediators (e.g., endovanilloids, proteases, cannabinoids, opioids, neurotrophins, and cytokines) and relevant receptors (e.g., vanilloid receptor channels and proteinase-activated, cannabinoid, opioid, cytokine, and new histamine receptors) are discussed. In summarizing promising new avenues for managing itch more effectively, we advocate therapeutic approaches that strive for the combination of peripherally active antiinflammatory agents with drugs that counteract chronic central itch sensitization.

Cortical changes to experimental sensitization of the human esophagus

Topographical organization in the neocortex shows experience-dependent plasticity. We hypothesized that experimental sensitization of the esophagus results in changes of the topographical distribution of the evoked potentials and the corresponding dipole source activities to painful stimulation. An endoscopic method was used to deliver 35 electrical stimuli at the pain threshold to a fixed area of the mucosa in 10 healthy volunteer men and women. The stimulations were repeated after 30 min (reproducibility experiment), and after 60 min following perfusion of 200 ml 0.1 N hydrochloric acid (sensitization experiment). During stimulation the electroencephalogram was recorded from 64 surface electrodes. The sensitization resulted in a decrease in the pain threshold (F=6.2; P=0.004). The topographic distribution of the evoked potentials showed reproducible negative (N1, N2) and positive (P1, P2) components. After acid perfusion a reduced latency and a change in localization was seen for the P1 subdivided into frontal and occipital components (F=29.5, P<0.001; F=53.7, P<0.001). Furthermore the sensitization resulted in a reduction of the latency for P2 (F=6.2, P=0.009). The source analysis showed consistent dipolar activity in the bilateral opercular-insular cortex before and after acid perfusion. For the anterior cingulate dipole there was a reduction in latency (P=0.03) and a posterior shift (P=0.0002) following acid perfusion. The findings indicate that short-term sensitization of the esophagus results in central neuroplastic changes involving the cingulate gyrus, which also showed pathological activation in functional diseases of the gut, thus reflecting the importance of this region in visceral pain and hyperalgesia.

The involvement of the posterior cingulate gyrus in phasic pain processing of humans

Cingulate cortex (CC) processing of phasic pain starts in caudal parts. This is evidenced by electrical brain source analysis (BESA(R)) and current reconstruction imaging (CURRY(R)) which identify the activated neuronal assemblies in the individual brain morphology, including the boundary element method for volume current calculation. Data were obtained from 30 subjects who experienced, in repeated sessions, infrared laser stimuli applied on temple, hand, and foot, or electrical shocks applied intracutaneously on finger tip. CC activity started around 230 ms (+/-20 ms; inter-subject SD.) and was localized in mid or posterior CC, in general in Brodmann's area 23 (intra-subject SD <5 mm; inter-subject SD <15 mm), independent of the kind and body site of noxious stimulation.

Dipolar source modelling of brain potentials evoked by painful electrical stimulation of the human sigmoid colon

The aim of the study was to compare the cerebral generators following painful stimulation of the sigmoid colon and the abdominal skin in 11 healthy subjects. The evoked potentials (EPs) were recorded from 31 surface electrodes following painful electrical stimuli of the sigmoid colon, and of the referred pain area on the abdomen. Current dipole models estimating the EPs amplitude and topography were calculated. For colon stimulation, the earliest cortical activities were described by dipoles in the bilateral insula and in the anterior cingulate cortex, while both secondary somatosensory areas were activated later. When the skin was stimulated, early bilateral dipoles in the primary and secondary somatosensory areas were estimated, followed by a dipole in the frontal region. This suggests that painful cutaneous and visceral stimuli are processed differently in the brain.

Brain generators of laser-evoked potentials: from dipoles to functional significance

In this work we review data on cortical generators of laser-evoked potentials (LEPs) in humans, as inferred from dipolar modelling of scalp EEG/MEG results, as well as from intracranial data recorded with subdural grids or intracortical electrodes. The cortical regions most consistently tagged as sources of scalp LERs are the suprasylvian region (parietal operculum, SII) and the anterior cingulate cortex (ACC). Variability in opercular sources across studies appear mainly in the anterior-posterior direction, where sources tend to follow the axis of the Sylvian fissure. As compared with parasylvian activation described in functional pain imaging studies, LEP opercular sources tended to cluster at more superior sites and not to involve the insula. The existence of suprasylvian opercular LEPs has been confirmed by both epicortical (subdural) and intracortical recordings. In dipole-modelling studies, these sources appear to become active less than 150 ms post-stimulus, and remain in action for longer than opercular responses recorded intracortically, thus suggesting that modelled opercular dipoles reflect a "lumped" activation of several sources in the suprasylvian region, including both the operculum and the insula. Participation of SI sources to explain LEP scalp distribution remains controversial, but evidence is emerging that both SI and opercular sources may be concomitantly activated by laser pulses, with very similar time courses. Should these data be confirmed, it would suggest that a parallel processing in SI and SII has remained functional in humans for noxious inputs, whereas hierarchical processing from SI toward SII has emerged for other somatosensory sub-modalities. The ACC has been described as a source of LEPs by virtually all EEG studies so far, with activation times roughly corresponding to scalp P2. Activation is generally confined to area 24 in the caudal ACC, and has been confirmed by subdural and intracortical recordings. The inability of most MEG studies to disclose such ACC activity may be due to the radial orientation of ACC currents relative to scalp. ACC dipole sources have been consistently located between the VAC and VPC lines of Talairach's space, near to the cingulate subsections activated by motor tasks involving control of the hand. Together with the fact that scalp activities at this latency are very sensitive to arousal and attention, this supports the hypothesis that laser-evoked ACC activity may underlie orienting reactions tightly coupled with limb withdrawal (or control of withdrawal). With much less consistency than the above-mentioned areas, posterior parietal, medial temporal and anterior insular regions have been occasionally tagged as possible contributors to LEPs. Dipoles ascribed to medial temporal lobe may be in some cases re-interpreted as being located at or near the insular cortex. This would make sense as the insular region has been shown to respond to thermal pain stimuli in both functional imaging and intracranial EEG studies.

Attentional modulation of human pain processing in the secondary somatosensory cortex: a magnetoencephalographic study

The influence of attention on the processing of pain in the secondary somatosensory cortex (SII) was analyzed using magnetoencephalography in response to painful infra-red heat stimuli applied to the left hand in six male healthy subjects, aged 22-28 years. Three experimental paradigms were chosen to deliver attention dependent results under comparable levels of vigilance. Single moving dipole sources for the pain-evoked responses were calculated in the individual cortex anatomy determined by magnetic resonance imaging. Though pain stimuli followed the same intensity pattern in all paradigms, evoked SII activity increased markedly from the low attention task to the mid-level attention task (P < 0.001). In contrast, further increase of attention from mid-level to high was not accompanied by an additional enhancement of SII activity. It therefore is concluded that activation patterns of SII follow a saturation function which cannot be enlarged by maximizing the relevance of the painful stimuli.