Gender differences in pain perception and patterns of cerebral activation during noxious heat stimulation in humans

Sex differences in tail-flick latency of non-stressed and stressed rats

The present study was conducted to assess whether there are sex differences between male and female subjects in their response to noxious stimuli under non-stressed and stressed conditions. Tail-flick latency assay was used as an experimental tool on 12 adult male and 12 adult female Sprague-Dawley rats before immobilization (i.e., non-stressed condition) and after 30, 120, 240, and 360 min of continuous immobilization (i.e., stressed conditions). It was found that the non-stressed female rats exhibited significantly longer response latency to noxious thermal stimuli than the non-stressed male rats. Stressed conditions caused by immobilization of the animal inside a ventilated restrainer significantly prolonged the tail-flick latency thresholds of both sexes. Female and male rats did not develop any adaptation after 120, 240, and 360 min of immobilization compared to their 30 min of immobilization, as demonstrated by the tail-flick assay. Moreover, the difference in the tail-flick latency between male and female rats was reduced as the duration of immobilization in the restrainer lengthened. At 30 min of immobilization, the significant difference between the sexes reduced to p < .05 compared to p < .01 before immobilization, and to statistically non-significant differences after 120, 240, and 360 min of immobilization. Results from this study suggested that female rats had longer tail-flick latency than male rats in non-stressed environment, and the difference in tail-flick latency after immobilization between the sexes became less significant with time. In addition, 360 min of immobilization was not enough to adapt to the restrainer, as shown by the tail-flick assay.

Sex differences in pain perception

BACKGROUND

A number of studies have demonstrated a higher prevalence of chronic pain states and greater pain sensitivity among women compared with men. Pain sensitivity is thought to be mediated by sociocultural, psychological, and biological factors.

OBJECTIVE

This article reviews laboratory studies that provide evidence of sex differences in pain sensitivity and the response to analgesic drugs in animals and humans. The biological basis of such differences is emphasized.

METHODS

The literature from this relatively new field was surveyed, and studies that clearly illustrate the differences in pain mechanisms between the sexes are presented. Using the search terms sex, gender, and pain, a review was conducted of English-language literature published on MEDLINE between January 1980 and August 2004.

RESULTS

Although differences in pain sensitivity between women and men are partly attributable to social conditioning and to psychosocial factors, many laboratory studies of humans have described sex differences in sensitivity to noxious stimuli, suggesting that biological mechanisms underlie such differences. In addition, sex hormones influence pain sensitivity; pain threshold and pain tolerance in women vary with the stage of the menstrual cycle. Imaging studies of the brain have shown differences between men and women in the spatial pattern and intensity of response to acute pain. Among rodents, females are more sensitive than males to noxious stimuli and have lower levels of stress-induced analgesia. Male rodents generally have stronger analgesic response to mu-opioid receptor agonists than females. Research on transgenic mice suggests that normal males have a higher level of activity in the endogenous analgesic system compared with normal females, and a human study has found that mu-receptors in the healthy female brain are activated differently from those in the healthy male brain. The response to kappa-opioids, which is mediated by the melanocortin-1 receptor gene in both mice and humans, is also different for each sex.

CONCLUSION

Continued research at the genetic and receptor levels may support the need to develop gender-specific drug therapies.

Pain and emotion interactions in subregions of the cingulate gyrus

Acute pain and emotion are processed in two forebrain networks, and the cingulate cortex is involved in both. Although Brodmann's cingulate gyrus had two divisions and was not based on any functional criteria, functional imaging studies still use this model. However, recent cytoarchitectural studies of the cingulate gyrus support a four-region model, with subregions, that is based on connections and qualitatively unique functions. Although the activity evoked by pain and emotion has been widely reported, some view them as emergent products of the brain rather than of small aggregates of neurons. Here, we assess pain and emotion in each cingulate subregion, and assess whether pain is co-localized with negative affect. Amazingly, these activation patterns do not simply overlap.

Brain activity associated with painfully hot stimuli applied to the upper limb: a meta-analysis

The capacity of pain to alert against potential injury or focus attention on damaged tissue is enhanced by the intrinsically aversive nature of the experience. Finding methods to relieve pain will ultimately be facilitated by deeper understanding of the processes that contribute to the experience, and functional brain imaging has contributed substantially toward that end. An impressive body of literature has identified a distributed network of pain-related activity in the brain that is subject to considerable modulation by different stimulus parameters, contextual factors, and clinical conditions. The fundamental substrates of the pain network are yet to be distilled from the highly variable results of studies published thus far. Qualitative reviews of the pain-imaging literature have been contributory, but lack the greater surety of quantitative methods. We employ the activation likelihood estimation (ALE) meta-analytic technique to establish the most consistent activations among studies reporting brain responses subsequent to the application of noxious heat. A network of pain-related activity was replicated for stimuli to either upper limb that included two discernible regions of the mid-anterior cingulate cortex, bilateral thalami, insula, and opercula cortices, posterior parietal cortex, premotor cortex, supplementary motor area, and cerebellum. The findings of the meta-analysis resonate with other streams of information that continue to enhance our understanding of pain in the brain. The results also point toward new areas of research that may be fruitful for the exploration of central pain processing.

Gender differences in the functional neuroanatomy of emotional episodic autobiographical memory

Autobiographical memory is based on interactions between episodic memory contents, associated emotions, and a sense of self-continuity along the time axis of one's life. The functional neuroanatomy subserving autobiographical memory is known to include prefrontal, medial and lateral temporal, as well as retrosplenial brain areas; however, whether gender differences exist in neural correlates of autobiographical memory remains to be clarified. We reanalyzed data from a previous functional magnetic resonance imaging (fMRI) experiment to investigate gender-related differences in the neural bases of autobiographical memories with differential remoteness and emotional valence. On the behavioral level, there were no significant gender differences in memory performance or emotional intensity of memories. Activations common to males and females during autobiographical memory retrieval were observed in a bilateral network of brain areas comprising medial and lateral temporal regions, including hippocampal and parahippocampal structures, posterior cingulate, as well as prefrontal cortex. In males (relative to females), all types of autobiographical memories investigated were associated with differential activation of the left parahippocampal gyrus. By contrast, right dorsolateral prefrontal cortex was activated differentially by females. In addition, the right insula was activated differentially in females during remote and negative memory retrieval. The data show gender-related differential neural activations within the network subserving autobiographical memory in both genders. We suggest that the differential activations may reflect gender-specific cognitive strategies during access to autobiographical memories that do not necessarily affect the behavioral level of memory performance and emotionality.

Visceral hypersensitivity

Visceral hypersensitivity is considered one of the causes of functional gastrointestinal disorders. The objectives of this review are to provide a practical description of neuroanatomy and physiology of gut sensation, to describe the diverse tests of visceral sensation and the potential role of brain imaging to further our understanding of visceral sensitivity in health and disease. Changes in motor function in the gut may influence sensory levels, eg, during contractions or as a result of changes in viscus compliance. New insights on sensory end organs, such as intraganglionic laminar endings, and basic neurophysiologic studies showing afferent firing during changes in stretch rather than tension illustrate the importance of different types of stimuli, not just tension, to stimulate afferent sensation. These insights provide the basis for understanding visceral sensation in health and disease, which will be extensively discussed in subsequent articles.

EEG source analysis and fMRI reveal two electrical sources in the fronto-parietal operculum during subepidermal finger stimulation

Using functional magnetic resonance imaging (fMRI) and electroencephalographic (EEG) source dipole analysis in 10 normal subjects, two electrical source dipoles in the contralateral fronto-parietal operculum were identified during repetitive painful subepidermal stimulation of the right index finger. The anterior source dipole peaking at 79 +/- 8 ms (mean +/- SD) was located in the frontal operculum, and oriented tangentially toward the cortical surface. The posterior source dipole peaking at 118 +/- 12 ms was located in the upper bank of the Sylvian fissure corresponding to the second somatosensory cortex (S2). The orientations of the posterior source dipoles displayed large variability, but differed significantly (P < 0.05) from the orientations of the anterior source dipoles. Electrical sources and fMRI clusters were also observed in ipsilateral fronto-parietal operculum. However, due to low signal-to-noise ratio of ipsilateral EEG sources in individual recordings, separation of sources into anterior and posterior clusters was not performed. Combined fMRI and source dipole EEG analysis of individual data suggests the presence of two distinct electrical sources in the fronto-parietal operculum participating in processing of somatosensory stimuli. The anterior region of the fronto-parietal operculum shows earlier peak activation than the posterior region.

Human brain mechanisms of pain perception and regulation in health and disease

CONTEXT

The perception of pain due to an acute injury or in clinical pain states undergoes substantial processing at supraspinal levels. Supraspinal, brain mechanisms are increasingly recognized as playing a major role in the representation and modulation of pain experience. These neural mechanisms may then contribute to interindividual variations and disabilities associated with chronic pain conditions.

OBJECTIVE

To systematically review the literature regarding how activity in diverse brain regions creates and modulates the experience of acute and chronic pain states, emphasizing the contribution of various imaging techniques to emerging concepts.

DATA SOURCES

MEDLINE and PRE-MEDLINE searches were performed to identify all English-language articles that examine human brain activity during pain, using hemodynamic (PET, fMRI), neuroelectrical (EEG, MEG) and neurochemical methods (MRS, receptor binding and neurotransmitter modulation), from January 1, 1988 to March 1, 2003. Additional studies were identified through bibliographies.

STUDY SELECTION

Studies were selected based on consensus across all four authors. The criteria included well-designed experimental procedures, as well as landmark studies that have significantly advanced the field.

DATA SYNTHESIS

Sixty-eight hemodynamic studies of experimental pain in normal subjects, 30 in clinical pain conditions, and 30 using neuroelectrical methods met selection criteria and were used in a meta-analysis. Another 24 articles were identified where brain neurochemistry of pain was examined. Technical issues that may explain differences between studies across laboratories are expounded. The evidence for and the respective incidences of brain areas constituting the brain network for acute pain are presented. The main components of this network are: primary and secondary somatosensory, insular, anterior cingulate, and prefrontal cortices (S1, S2, IC, ACC, PFC) and thalamus (Th). Evidence for somatotopic organization, based on 10 studies, and psychological modulation, based on 20 studies, is discussed, as well as the temporal sequence of the afferent volley to the cortex, based on neuroelectrical studies. A meta-analysis highlights important methodological differences in identifying the brain network underlying acute pain perception. It also shows that the brain network for acute pain perception in normal subjects is at least partially distinct from that seen in chronic clinical pain conditions and that chronic pain engages brain regions critical for cognitive/emotional assessments, implying that this component of pain may be a distinctive feature between chronic and acute pain. The neurochemical studies highlight the role of opiate and catecholamine transmitters and receptors in pain states, and in the modulation of pain with environmental and genetic influences.

CONCLUSIONS

The nociceptive system is now recognized as a sensory system in its own right, from primary afferents to multiple brain areas. Pain experience is strongly modulated by interactions of ascending and descending pathways. Understanding these modulatory mechanisms in health and in disease is critical for developing fully effective therapies for the treatment of clinical pain conditions.

Functional imaging and the neural systems of chronic pain

Pain remains a serious health care problem affecting millions of individuals, costing billions of dollars, and causing an immeasurable amount of human suffering. In designing improved therapies, there is still much to learn about peripheral nociceptor, nerves, and the spinal cord, and brain stem modulatory systems. Nevertheless, it is the brain that presents us with an incredible opportunity to understand the experience we call pain. Functional neuroimaging is helping to unlock the secrets of the sensory and emotional components of pain and its autonomic responses. These techniques are helping us to understand that pain is not a static disease with the pathologic findings localized to the periphery but is instead a highly plastic condition affecting multiple central neural systems. Functional neuroimaging is transforming our understanding of the neurobiology of pain and will be instrumental in helping us to design more rational treatments ultimately aimed at reducing the impact of pain on our patients. It is opening windows into the function of the brain that were previously closed.

Neuronal correlates of gastric pain induced by fundus distension: a 3T-fMRI study

Visceral hypersensitivity in gastric fundus is a possible pathogenesis for functional dyspepsia. The cortical representation of gastric fundus is still unclear. Growing evidence shows that the insula, but not the primary or secondary somatosensory region (SI or SII), may be the cortical target for visceral pain. Animal studies have also demonstrated that amygdala plays an important role in processing visceral pain. We used fMRI to study central projection of stomach pain from fundus balloon distension. We also tested the hypothesis that there will be neither S1 nor S2 activation, but amygdala activation with the fundus distension. A 3T-fMRI was performed on 10 healthy subjects during baseline, fullness (12.7 +/- 0.6 mmHg) and moderate gastric pain (17.0 +/- 0.8 mmHg). fMRI signal was modelled by convolving the predetermined psychophysical response. Statistical comparisons were performed between conditions on a group level. Gastric pain activated a wide range of cortical and subcortical structures, including thalamus and insula, anterior and posterior cingulate cortices, basal ganglia, caudate nuclei, amygdala, brain stem, cerebellum and prefrontal cortex (P < 0.001). A subset of these neuronal substrates was engaged in the central processing of fullness sensation. SI and SII were not activated during the fundus stimulation. In conclusion, the constellation of neuronal structures activated by fundus distension overlaps the pain matrices induced musculocutaneous pain, with the exception of the absence of SI or SII activation. This may account for the vague nature of visceral sensation/pain. Our data also confirms that the insula and amygdala may act as the central role in visceral sensation/pain, as well as in the proposed sensory-limbic model of learning and memory of pain.

Amplitudes of laser evoked potential recorded from primary somatosensory, parasylvian and medial frontal cortex are graded with stimulus intensity

Intensity encoding of painful stimuli in many brain regions has been suggested by imaging studies which cannot measure electrical activity of the brain directly. We have now examined the effect of laser stimulus intensity (three energy levels) on laser evoked potentials (LEPs) recorded directly from the human primary somatosensory (SI), parasylvian, and medial frontal cortical surfaces through subdural electrodes implanted for surgical treatment of medically intractable epilepsy. LEP N2* (early exogenous/stimulus-related potential) and LEP P2** (later endogenous potential) amplitudes were significantly related to the laser energy levels in all regions, although differences between regions were not significant. Both LEP peaks were also significantly correlated with the pain intensity evoked by the laser stimulus, excepting N2* over the parasylvian region. Peak latencies of both LEP peaks were independent of laser energy levels. N2* and P2** amplitudes of the maxima in all regions showed significant positive linear correlations with laser energy, excepting N2* over the parasylvian region. The lack of correlation of parasylvian cortical N2* with laser energy and pain intensity may be due to the unique anatomy of this region, or the small sample, rather than the lack of activation by the laser. Differences in thresholds of the energy correlation with amplitudes were not significant between regions. These results suggest that both exogenous in endogenous potentials evoked by painful stimuli, and recorded over SI, parasylvian, and medial frontal cortex of awake humans, encode the intensity of painful stimuli and correlate with the pain evoked by painful stimuli.

The 1- to 2-Hz oscillations in muscle force are exacerbated by stress, especially in older adults

Although force fluctuations during a steady contraction are often heightened in old adults compared with young adults and are enhanced in young adults during the stress response, the mechanisms underlying the augmentation are uncertain. The purpose of the study was to compare the effect of a stressor on the plasma concentrations of selected stress hormones and on the force fluctuations experienced by young and old adults during the performance of a precision grip. Thirty-six men and women (19–86 yr) participated in a protocol that comprised anticipatory (30 min), stressor (15 min), and recovery periods (25 min). The stressor was a series of noxious electrical stimuli applied to the dorsal surface of the left hand. Subjects sustained a pinch-grip force with the right hand at 2% of the maximal voluntary contraction force. The fluctuations in pinch-grip force, the interference electromyogram (EMG) of six muscles, and the spectra for the force and EMG were quantified across the 70-min protocol. The stressor increased the force fluctuations, largely due to an enhancement of the power at 1–2 Hz in the force spectrum ( r2 = 0.46). The effect was greatest for the old adults compared with young and middle-aged adults. The plasma concentrations of the stress hormones (adrenocorticotropin, epinephrine, and norepinephrine) were elevated to similar levels for all three age groups, and the changes were not associated with modulation of the force fluctuations. Furthermore, the heightened EMG activity exhibited by the old adults during all periods was not related to the changes in the force fluctuations or the 1- to 2-Hz force oscillations. The absence of a change in the mean pinch-grip force during the protocol and the lack of an association between elevation of the plasma concentrations for the stress hormones and modulation of the force fluctuations suggest that the enhanced force fluctuations caused by the stressor was due to an increase in the low-frequency output of the spinal motor neurons.

Cutaneous Painful Laser Stimuli Evoke Responses Recorded Directly From Primary Somatosensory Cortex in Awake Humans

Negative and positive laser evoked potential (LEP) peaks (N2*, P2**) were simultaneously recorded from the primary somatosensory (SI), parasylvian, and medial frontal (MF: anterior cingulate and supplementary motor area) cortical surfaces through subdural electrodes implanted for the surgical treatment of intractable epilepsy. Distribution of the LEP N2*and P2**peaks was estimated to be in cortical areas (SI, parasylvian, and MF) identified by anatomic criteria, by their response to innocuous vibratory stimulation of a finger (v-SEP), and to electrical stimulation of the median nerve (e-SEP). The maximum of the LEP N2*peak was located on the CS, medial (dorsal) to the finger motor area, as determined by cortical stimulation, and to the finger somatosensory area, as determined from the e-SEP and v-SEP. This finding suggests that the generator source of the LEP N2*peak in SI was different from that of e-SEP or v-SEP in Brodmann's areas 3b or 1. In parasylvian and MF, polarity reversal was often observed, indicating tangential current sources in these regions. In contrast to e-SEP and v-SEP, the LEP N2*latency over SI was not shorter than that over the parasylvian region. The amplitude of N2*was larger over SI than over MF and the latencies of the LEP peaks in those 2 regions were different. These findings provide evidence for a significant LEP generator in the postcentral gyrus, perhaps SI cortex, that is situated outside the tactile homunculus in SI and that receives its input arising from nociceptors simultaneously with parasylvian and MF cortex.

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.

Gender differences in the perception of chest pain

This study investigated gender differences in pain perception as characterized by pain symptoms in patients diagnosed with unstable angina pectoris. Twenty-nine women and 32 men were asked to characterize their chest pain using a semi-open questionnaire assessing pain intensity (by numerical rating), pain location, pain characteristics, complaints following chest pain, factors that evoked or reduced chest pain, and whether the chest pain was related to heart disease. Significant gender differences were found. Women scored the intensity of their chest pain significantly higher than men (Chi-square 14.8, P < 0.0001), and related their chest pain less to heart disease (Chi-square 24.6, P < 0.0001). The women described an atypical clinical picture of chest pain that was significantly different from men's. The results are discussed in light of pschological theories regarding gender differences in pain perception. These findings imply the need for special attention to the unique clinical pictures that appear for women and men.

Gender and laterality differences in thermosensation throughout the perceptible range

Several studies suggest that females exhibit greater sensitivity to experimentally induced thermal pain than males. These investigations have focused mainly on the sensory-discriminative rather than the affective aspect of pain. Moreover, potential gender differences for the affective components of innocuous thermal sensations have yet to be examined. The primary aim of the present study was to evaluate gender differences in the sensory and the affective dimensions of the entire thermosensory system, including warmth, coolness, heat pain and cold pain. The secondary aim was to evaluate laterality differences in these same perceptual dimensions and ranges. Twenty healthy females and 20 healthy males immersed their hands in water baths maintained at temperatures ranging from 10 to 47 degrees C, and rated their perceived thermal intensity, (un)pleasantness, and pain intensity. There was a progressive growth in the thermal intensity ratings as bath temperatures either increased or decreased from the adapting temperature of 33 degrees C. No gender differences emerged for these thermal intensity ratings. However, a significant sex effect emerged for the pain intensity ratings (P<0.01), and a significant sex x temperature interaction for the affective ratings (P<0.01). Females provided higher unpleasantness and pain intensity ratings for the more extreme temperatures (10, 15 and 47 degrees C), compared to males. Moreover, women perceived the milder temperature baths as more pleasant than men did. For a given painful temperature, unpleasantness ratings were higher than pain intensity ratings. This relationship between unpleasantness ratings and pain ratings was not significantly different between the sexes. No laterality differences emerged for the thermal intensity ratings. However, perceived pain intensity was significantly higher for the left as compared to the right hand (P<0.01). Ratings of unpleasantness also tended to be higher for the left vs. right hand, but this difference fell just short of statistical significance (P=0.06). These findings indicate that sex differences in thermosensory perception are not general, but occur only for the painful and affective components. Of particular note is the sex difference for affective but not intensive ratings of innocuous temperatures, revealing sex differences in thermal perception outside the nociceptive system.

Effects of electric stimulation on C and A delta fiber-mediated thermal perception thresholds 11No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the authors(s) or upon any organization with which the author(s) is/are associated

OBJECTIVE

To determine if interferential current (IFC) or transcutaneous electric nerve stimulation (TENS) alters C and A delta fiber-mediated thermal perception thresholds.

DESIGN

Single-blind, randomized controlled trial.

SETTING

Laboratory.

PARTICIPANTS

One hundred forty healthy women volunteers (mean age +/- standard deviation, 20.6+/-2.7 y).

INTERVENTIONS

Subjects were randomly and exclusively assigned to 1 of 7 groups (n=20 in each): 0, 5, and 100 Hz of IFC; 5 and 100 Hz of TENS; placebo and control stimulation. Stimulation was applied through 2 electrodes placed over the median nerve. Warm sensation, cold sensation, hot pain, and cold pain perception thresholds were measured from the thenar eminence by using a quantitative sensory testing device and a method of limits algorithm.

MAIN OUTCOME MEASURES

Warm sensation, cold sensation, hot pain, and cold pain thresholds (degrees C) before, during, and after stimulation.

RESULTS

There was a statistically significant effect of time for all 4 thermal perception thresholds (separate 2-way analyses of variance with repeated measures, all P<.001). There were no statistically significant differences between experimental groups, nor any interaction effects (all P>.05).

CONCLUSIONS

Neither IFC nor TENS altered C and A delta fiber-mediated thermal perception thresholds. The results suggest that any analgesic mechanisms with these modalities are likely to be complex.

Gender differences in patterns of cerebral activation during equal experience of painful laser stimulation

A previous functional imaging study demonstrated greater female response in the anterior insula and thalamus and left prefrontal activation in men and right prefrontal activation in women during equal heat intensity but unequal pain experience. For the current study, subjective intensities of noxious heat delivered to the back of the right hand were equalized across subjects, and regional cerebral blood flow was recorded by using positron emission tomography. The female subjects required less laser energy before reporting pain, but the difference was not significant. Correlation of regional cerebral blood flow with subjective pain experience in the whole group showed significant bilateral responses in the parietal, lateral premotor, prefrontal, secondary somatosensory, anterior cingulate and insula cortices, as well as the thalamus. There was significantly greater activation in the left, contralateral, prefrontal, primary and secondary somatosensory, parietal, and insula cortices in the male subjects compared with the female subjects and greater response in the perigenual cingulate cortex in the female subjects. Our study is the first to associate consistent pain experience with gender differences in central response. These differences may relate to differential processing of acute pain with implications for clinical disorders that show a female dominance. The subtle behavioral differences and inconsistent findings across studies, however, suggest the need for caution and further experimentation before speculating further.

Pathogenesis of the migraine attack

BACKGROUND

There is clinical experimental evidence that extracranial arterial vasodilation, extracranial neurogenic inflammation, and decreased inhibition of central pain transmission are involved in the pathogenesis of the migraine headache. The migraine aura is likely caused by a neurophysiologic phenomenon akin to Leão's cortical spreading depression, a wave of short-lasting neuronal excitation that travels over the cerebral cortex, followed by prolonged depression of cortical neuronal activity.

METHOD

A concept of the pathogenesis of the migraine attack is presented, in which the relation of the mechanism of the migraine aura and that of the migraine headache is considered parallel rather than sequential in nature.

CONCLUSIONS

The process driving the pathogenesis of the migraine attack and susceptible to the migraine trigger factors may be located in the brain stem.

Sex differences in analgesic responses: evidence from experimental pain models

BACKGROUND AND OBJECTIVE

Sex-related influences on the experience of pain have received considerable empirical attention. Women are at greater risk for several forms of clinical pain and exhibit greater perceptual responses to experimental pain. In recent years, investigators have turned their attention to the influence of sex-related factors on analgesic responses. The purpose of this review is to examine the literature on sex differences in analgesic responses, emphasizing findings from experimental studies.

METHODS

First, important methodological issues in laboratory pain research are presented, and sex differences in responses to experimentally-induced pain are briefly addressed. Next, previous data from non-human animal research and human experimental and clinical research related to sex differences in analgesia are discussed. Also, preliminary results are presented from an ongoing study in our laboratory examining analgesic responses in women and men.

RESULTS AND CONCLUSIONS

Both previous research and preliminary findings from our laboratory suggests that opioids produce greater analgesic responses in women than men. Potential mechanisms underlying sex differences in analgesia are proposed, and important directions for future research are suggested.

Sex-related differences in IBS patients: central processing of visceral stimuli

BACKGROUND & AIMS

Women have a higher prevalence of irritable bowel syndrome (IBS) and possible differences in response to treatment, suggesting sex-related differences in underlying pathophysiology. The aim of this study was to determine possible sex-related differences in brain responses to a visceral and a psychological stressor in IBS.

METHODS

Regional cerebral blood flow measurements using H(2)(15)O positron emission tomography were compared across 23 female and 19 male nonconstipated patients with IBS during a visceral stimulus (moderate rectal inflation) and a psychological stimulus (anticipation of a visceral stimulus).

RESULTS

In response to the visceral stimulus, women showed greater activation in the ventromedial prefrontal cortex, right anterior cingulate cortex, and left amygdala, whereas men showed greater activation of the right dorsolateral prefrontal cortex, insula, and dorsal pons/periaqueductal gray. Similar differences were observed during the anticipation condition. Men also reported higher arousal and lower fatigue.

CONCLUSIONS

Male and female patients with IBS differ in activation of brain networks concerned with cognitive, autonomic, and antinociceptive responses to delivered and anticipated aversive visceral stimuli.

Keeping pain out of mind: the role of the dorsolateral prefrontal cortex in pain modulation

Frontal lobe activity during pain is generally linked to attentional processing. We addressed the question of whether 'bottom-up' processing and 'top-down' modulation of nociceptive information dissociate anatomically within the frontal lobe by using PET scanning during painful thermal stimulation of normal and capsaicin-treated skin. We showed recently that pain following normally non-painful heat stimuli on chemically irritated skin (heat allodynia) uniquely engages extensive areas of the bilateral dorsolateral prefrontal (DLPFC), ventral/orbitofrontal (VOFC) and perigenual anterior cingulate (ACC) cortices. Here, we applied principal component analysis (PCA) and multiple regression analysis to study the covariance structure of the volumes of interest (VOI) activated specifically during heat allodynia in 14 male healthy subjects and evaluated the relationship of these VOI to ratings of pain intensity and affect. Results yielded a primary principal component (PC) that correlated positively with intensity and unpleasantness and accounted for activity in the medial thalamus, bilateral anterior insula, ventral striatum, perigenual ACC and bilateral VOFC. Activities in the right and left DLPFC loaded on separate PC and correlated negatively with perceived intensity and unpleasantness. The inter-regional correlation of midbrain and medial thalamic activity was significantly reduced during high left DLPFC activity, suggesting that its negative correlation with pain affect may result from dampening of the effective connectivity of the midbrain-medial thalamic pathway. In contrast, right DLPFC activity was associated with a weakened relationship of the anterior insula with both pain intensity and affect. We propose that the DLPFC exerts active control on pain perception by modulating corticosubcortical and corticocortical pathways.

Gender differences in limbic responsiveness, by SPECT, following a pharmacologic challenge in healthy subjects

Limbic system functioning is integral to the control and modulation of affect, motivation, reward, and memory. Neuropsychiatric disturbances involving disruptions in these cognitive and emotional dimensions exhibit different prevalence rates for men and women. Gender-specific differences in this integrated brain area may therefore be important in understanding both normal behavioral functioning and the etiologic underpinnings of neuropsychiatric disorders. To further explore such differences in limbic system function, we assessed regional cerebral blood flow, by SPECT, in men and women following the administration of procaine. Procaine is a local anesthetic that preferentially stimulates limbic structures. Psychiatrically and medically healthy, age-matched women (n = 15, 33.2 +/- 6.9 years) and men (n = 15, 32.8 +/- 6.9 years) were administered 1.38 mg/kg procaine or saline intravenously in two separate sessions. Using voxel-based analyses (P < 0.001), males significantly activated the bilateral insular cortex following procaine, whereas females more strongly activated the bilateral anterior and mesial temporal cortex. Both groups demonstrated significant anterior cingulate activation. Subjective responses to procaine did not significantly differ between the men and women. To our knowledge, this is the first report demonstrating gender-specific responses in limbic activation following a pharmacologic challenge. These findings suggest that men and women can activate different limbic structures following the same provocative pharmacologic stimulus, despite sharing a similar subjective experience. Studies assessing pharmacologic challenges of limbic system structures should consider gender as a critical variable in assessing biologic responsiveness.

Sex differences and incentive effects on perceptual and cardiovascular responses to cold pressor pain

OBJECTIVE

Sex differences in pain perception have been widely reported, with women typically displaying greater pain sensitivity than men, but the mechanisms underlying these differences remain unclear. One possible explanation suggests that men are more motivated to tolerate and suppress expressions of pain because of the masculine sex role, whereas the feminine sex role encourages pain expression and produces lower motivation to tolerate pain among women.

METHODS

To examine the influence of motivation on perceptual and cardiovascular responses to pain among women and men, different levels of monetary incentive (high vs. low incentive) were provided to a group of 81 healthy young adults undergoing the cold pressor pain procedure. It was anticipated that men would have greater endogenous motivation and would therefore be less affected by the external incentive.

RESULTS

Men had higher pain thresholds and tolerances and lower pain ratings than women, but the incentive condition produced no significant effect on pain responses. Resting blood pressure was positively correlated with pain tolerance among the low incentive group, whereas blood pressure reactivity to the cold pressor predicted pain tolerance in the high incentive group.

CONCLUSIONS

Thus, monetary incentive did not influence pain responses, but the relationship between cardiovascular measures and pain responses was influenced by the incentive manipulation. Potential explanations for the observed results are presented, and the implications for applying the biopsychosocial model to pain research are discussed.

[Do men and women have different perceptions of pain?]

OBJECTIVE

To evaluate possible differences between men and women's perception of a painful stimulus (venous puncture).

DESIGN

Transversal, analytic and observational study.Setting. Primary care.Participants. 709 people over 14 who attended three health centres for blood extraction. 98 cases were rejected (most because they received more than one puncture).

MAIN MEASUREMENTS

Age, sex, tobacco habit, perception of pain (through visual analogic scale from 1 to 10), complaints expressed (oral and/or by gestures) and patient's prior expectations.

RESULTS

Mean age: 47.4; 60.1% women. The intensity of pain had an average of 1.35 (SD, 1.60; mode, 0; median, 0.8). 14.7% expressed a complaint. Median of pain was 0.95 (Q3-Q1, 1.9-0.4) in men, whereas it was 0.70 (Q3-Q1, 1.725-0.3) in women (U=40.48; P=.095). No correlation was found between valuation of pain and age (R=0.055). No differences were found for smokers (median, 0.925; Q3-Q1, 1.725-0.35) or non-smokers (0.75; 1.825-0.325) (P.05). There were statistically significant valuations that varied according to the prior expectations of pain. Equally, pain was assessed as more intense by those who expressed a complaint (median, 2.1; Q3-Q1, 3.85-0.475) versus those who did not (0.625; 1.55-0.3) (P<.001).

CONCLUSIONS

We found no significant differences between men and women's perception of painful stimulus. In any case, differences were clinically irrelevant. There is wide individual variability, which suggests that the essential factors are psycho-social. We have to assess and treat properly a patient in pain, regardless of his/her gender.

Odorant differentiated pattern of cerebral activation: comparison of acetone and vanillin

Whether different odorous compounds (odorants) are processed by different cerebral circuits is presently unknown. A first step to address this complicated issue is to investigate how the cerebral regions mediating signals from olfactory (i.e., unimodal) odorants, differ from those mediating the olfactory + trigeminal (i.e., bimodal) odorants. [15O]-H2O-PET scans were conducted in 12 healthy females during three separate conditions: birhinal, passive smelling of: 1) the unimodal odorant vanillin; 2) the bimodal odorant acetone; and 3) odorless air. Significant activations were calculated contrasting vanillin to air, acetone to air, and deactivations, running these contrasts in the opposite direction. Smelling of vanillin activated bilaterally the amygdala and piriform cortex. These regions were only engaged slightly by acetone. Instead, strong activations were found in the anterior and central insula and claustrum, the posterior portion of anterior cingulate, the somatosensory cortex (SI for face), cerebellum, ventral medial (VMPo) and dorsal medial (MDvc) thalamus, the lateral hypothalamus, and pons/medulla. In parallel, the somatosensory (SI, below central representation of face), secondary visual and auditory cortices, as well as the supplementary motor area and the parahippocampal gyri were deactivated. No deactivations were observed with vanillin, although the odor components of acetone and vanillin were rated similarly intense (75 +/- 17 mm vs. 61 +/- 22 mm, NS). The differentiated pattern of cerebral activation during odorant perception seems to be dependent on the signal transducing cranial nerves involved. In contrast to vanillin, which solely activates the olfactory cortex, acetone engages predominantly trigeminal projections from the nasal mucosa. Acetone's limited activation of the olfactory cortex may result from a cross-modal interaction, with inhibition of acetone's odor component by its trigeminal component.

A unique representation of heat allodynia in the human brain

Skin inflammation causes innocuous heat to become painful. This condition, called heat allodynia, is a common feature of pathological pain states. Here, we show that heat allodynia is functionally and neuroanatomically distinct from normal heat pain. We subtracted positron emission tomography scans obtained during painful heating of normal skin from scans during equally intense but normally innocuous heating of capsaicin-treated skin. This comparison reveals the specific activation of a medial thalamic pathway to the frontal lobe during heat allodynia. The results suggest that different central pathways mediate the intensity and certain qualitative aspects of pain. In making this differentiation, the brain recognizes unique physiological features of different painful conditions, thus permitting adaptive responses to different pain states.

mu-opioid receptor-mediated antinociceptive responses differ in men and women

Sex differences in the experience of clinical and experimental pain have been reported. However, the neurobiological sources underlying the variability in pain responses between sexes have not been adequately explored, especially in humans. The endogenous opioid neurotransmitters and mu-opioid receptors are centrally implicated in responses to stress, in the suppression of pain, and in the action of opiate analgesic drugs. Here we examined sex differences in the activation of the mu-opioid system in response to an intensity-controlled sustained deep-tissue pain challenge with positron emission tomography and a mu-opioid receptor-selective radiotracer. Twenty-eight young healthy volunteers (14 men and 14 women) were studied during saline control and pain conditions using a double-blind, randomized, and counterbalanced design. Women were scanned during the early follicular phase of their menstrual cycles after ovulatory cycles. Significant sex differences in the regional activation of the mu-opioid system in response to sustained pain were detected compared with saline controls. Men demonstrated larger magnitudes of mu-opioid system activation than women in the anterior thalamus, ventral basal ganglia, and amygdala. Conversely, women demonstrated reductions in the basal state of activation of the mu-opioid system during pain in the nucleus accumbens, an area previously associated with hyperalgesic responses to the blockade of opioid receptors in experimental animals. These data demonstrate that at matched levels of pain intensity, men and women during their follicular phase differ in the magnitude and direction of response of the mu-opioid system in distinct brain nuclei.

Testing the sensitivity hypothesis in practice: tools and methods, assumptions and pitfalls

Visceral hypersensitivity is widely regarded as the reason for the development of functional gastrointestinal disorders, including functional dyspepsia and irritable bowel syndrome. The principles and techniques involved in testing the hypothesis that visceral sensitivity is important are discussed, together with the controversies in the assumptions, methods, and interpretations of the data acquired to date.

mu-opioid receptor-mediated antinociceptive responses differ in men and women

Sex differences in the experience of clinical and experimental pain have been reported. However, the neurobiological sources underlying the variability in pain responses between sexes have not been adequately explored, especially in humans. The endogenous opioid neurotransmitters and mu-opioid receptors are centrally implicated in responses to stress, in the suppression of pain, and in the action of opiate analgesic drugs. Here we examined sex differences in the activation of the mu-opioid system in response to an intensity-controlled sustained deep-tissue pain challenge with positron emission tomography and a mu-opioid receptor-selective radiotracer. Twenty-eight young healthy volunteers (14 men and 14 women) were studied during saline control and pain conditions using a double-blind, randomized, and counterbalanced design. Women were scanned during the early follicular phase of their menstrual cycles after ovulatory cycles. Significant sex differences in the regional activation of the mu-opioid system in response to sustained pain were detected compared with saline controls. Men demonstrated larger magnitudes of mu-opioid system activation than women in the anterior thalamus, ventral basal ganglia, and amygdala. Conversely, women demonstrated reductions in the basal state of activation of the mu-opioid system during pain in the nucleus accumbens, an area previously associated with hyperalgesic responses to the blockade of opioid receptors in experimental animals. These data demonstrate that at matched levels of pain intensity, men and women during their follicular phase differ in the magnitude and direction of response of the mu-opioid system in distinct brain nuclei.

Central neural contribution to the perception of chest pain in cardiac syndrome X

OBJECTIVE

To investigate the central neural contribution to chest pain perception in cardiac syndrome X (angina-like pain, ECG changes during stress, angiographically normal coronary arteriogram).

SUBJECTS

Eight syndrome X patients and eight healthy volunteers.

METHODS

Dobutamine stress using echocardiography to assess myocardial function, and positron emission tomography to measure changes in regional cerebral blood flow, as an index of neuronal activity.

RESULTS

During similar doses of dobutamine, syndrome X patients and controls showed comparable regional cerebral blood flow changes in the hypothalamus, thalami, right orbito-frontal cortex, and anterior temporal poles, associated with the sensation of a fast or powerful heart beat. In patients, but not controls, the stress also generated severe chest pain associated with increased activity in the right anterior insula/frontal operculum junction. There were ischaemia-like ECG changes in the syndrome X patients, but no left ventricular dysfunction on echocardiography. Activation of the right insula during chest pain clearly distinguished the syndrome X patients from a group of patients with known coronary disease.

CONCLUSIONS

Chest pain and ECG changes were not accompanied by demonstrable myocardial dysfunction in syndrome X patients, but altered central neural handling of afferent signals may contribute to the abnormal pain perception in these patients.

Brain imaging studies of the functional organization of human olfaction

It is believed that sensory functions are organized in a hierarchical and parallel manner. The sense of smell differs in several aspects from other senses: odors can immediately elicit emotional evocations, they are remembered after a long time, and they are difficult to label. This raises the question of whether odorous stimuli may be processed differently from the other sensory stimuli. New data from brain imaging studies suggest that this is not the case and that the specific characteristics of the sense of smell can be attributed to the engagement of limbic structures at an early stage in the signal processing.

Brain metabolic differences as a function of hemisphere, writing hand preference, and gender

A total of 35 university-educated normal men (24 right handwriters and 11 left handwriters) and 36 age- and education-matched women (25 right handwriters and 11 left handwriters) underwent a proton magnetic resonance spectroscopy examination in seven 8 cm(3) voxels including the right and left frontal lobe tips, the right and left mid-temporal lobes, the right and left thalami, and the hypothalamus. Dependent measures were N-acetylaspartate (NAA), choline-containing compounds (Cho) and creatine/phosphocreatine (Cr) metabolite peak area ratios relative to total H(2)O. As expected, thalamic grey matter contained higher NAA ratios than telencephalic voxels (containing white and grey matter) (p < .001). The thalamic Cr/ H(2)O ratio was higher on the right, but the opposite asymmetry was observed for the temporal lobe (p < .05). Women had a higher left frontal NAA/ H(2)O ratio than men, but men had a higher hypothalamic NAA/ H(2)O ratio than women. Right-handers had a higher temporal lobe NAA/H(2)O ratio than left-handers, particularly in the left hemisphere. In addition, several significant 2- and 3-way interactions between writing hand preference, gender, and hemisphere were observed, but only in the frontal lobe.

Cortical mapping of visceral pain in patients with GI disorders using functional magnetic resonance imaging

OBJECTIVE

We sought to identify central loci that activate in response to visceral stimuli (stool and pain). We had a particular interest in observing the anterior cingulate gyrus and frontal cortex in normals and in patients with intestinal disease, including inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS).

METHODS

Subjects underwent rectal balloon distention to a sensation of stool and to a sensation of pain while undergoing blood oxygenation level-dependent functional magnetic resonance imaging. Experiments were conducted in a Magnex 3.0-T whole body magnet with a Bruker Biospec console and a quadrature head coil. Four contiguous 5.0-mm oblique axial slices designed to optimize coverage of areas believed to be responsive to noxious stimulation were acquired. Activations were detected by using cross-correlation maps (p < 0.001) for individual subjects. The experimental groups were compared using both an analysis of variance and profile analysis.

RESULTS

A significantly higher percentage of pixels activated in the anterior cingulate gyrus over both pain and stool conditions for the control group than for the IBS group and for the IBS group than for the IBD group (p < 0.035). Deactivation of left somatosensory cortex was greater for the IBS group than for the IBD group and greater for the IBD group than for the controls (p < 0.0065) in the boxcar condition. Frontal deactivation in controls compared with disease groups bordered on statistical significance. Profile analysis of the three groups across six regions of interest revealed that the control and IBD groups were distinguished by different profiles of response (p < 0.005). Nonparametric evaluation of the data suggests that, among the pixels in the anterior cingulate activating to pain, there are two patterns of response to pain-on/off and graded. This was true for both controls and disease groups.

CONCLUSIONS

Normal controls and subjects with IBD and IBS share similar loci of activations to visceral sensations of stool and pain. Both activation and deactivation of particular regions of interest differentiate the three groups, as do profiles of patterned response across six of the regions of interest for the control and IBD groups.

Structure of the cerebral cortex in men and women

Expanding previous studies of human cerebral cortical sexual dimorphism showing higher neuronal densities in males, we investigated whether gender differences also exist in the extent of neuropil, size of neuronal somata, and volumes of astrocytes. This histo-morphometric study includes select autopsy brains of 6 males and 5 females, 12 to 24 yr old. In each brain, 86 defined loci were analyzed for cortical thickness, neuronal and astrocytic (8 loci) density (stereological counts), and neuronal and astrocytic (8 loci) soma size, enabling calculations of neuropil and astrocytic volumes. The female group showed significantly larger neuropil volumes than males, whereas neuronal soma size and astrocytic volumes did not differ. The expanded data confirmed higher neuronal densities in males than in females without a gender difference in cortical thickness. These findings indicate that fundamental gender differences exist in the structure of the human cerebral cortex, with more numerous, smaller neuronal units in men and fewer, larger ones in women; they may underlie gender-specific abilities and susceptibilities to disease affecting the neocortex. Laterality differences between the sexes were restricted to neuronal soma size showing significantly larger values in the female group in the left hemisphere. This gender difference may support female's right-handedness, language advantage, and tendency for bilateral activation patterns.

Anticipated and experienced pain associated with endodontic therapy

BACKGROUND

The authors compared the levels of anticipated and experienced pain of patients who received endodontic therapy, or ET, with selected patient and dental characteristics.

METHODS

Sensory and affective pain outcome measures (pain and unpleasantness) were evaluated by 333 adult patients immediately before and after dental school faculty or residents performed ET. Dentists provided clinical evaluations and a pulpal diagnosis for each tooth and then rated the level of their patients' pain during treatment.

RESULTS

Before ET, 43 percent of all patients anticipated high outcome levels, yet only 22 percent experienced high pain levels, and only 18 percent experienced high unpleasantness levels. Outcome levels did not differ by tooth type, pulpal diagnosis, ET history or dental care attendance. Women were significantly more likely to anticipate higher pain and unpleasantness levels than were men. Experienced outcome levels, however, did not differ by sex. Anticipated and experienced outcome levels significantly decreased with increasing age. Dentists' evaluation of their patients' pain levels correlated more highly for female than for male patients.

CONCLUSIONS

Pain experienced during ET often is less than anticipated. Younger people anticipate and experience higher pain levels. Women are more likely than men to anticipate, but not necessarily experience, higher pain levels. Dentists are more closely attuned to the pain experiences of their female patients. Clinical Implications. Practitioners could better prepare younger patients and female patients for ET and improve pain communication with male patients. Findings suggest that patients perceive each ET experience as new, implying that dentists should manage patients who have had ET in the past as carefully as those receiving ET for the first time.

Reward circuitry activation by noxious thermal stimuli

Using functional magnetic resonance imaging (fMRI), we observed that noxious thermal stimuli (46 degrees C) produce significant signal change in putative reward circuitry as well as in classic pain circuitry. Increases in signal were observed in the sublenticular extended amygdala of the basal forebrain (SLEA) and the ventral tegmentum/periaqueductal gray (VT/PAG), while foci of increased signal and decreased signal were observed in the ventral striatum and nucleus accumbens (NAc). Early and late phases were observed for signals in most brain regions, with early activation in reward related regions such as the SLEA, VT/PAG, and ventral striatum. In contrast, structures associated with somatosensory perception, including SI somatosensory cortex, thalamus, and insula, showed delayed activation. These data support the notion that there may be a shared neural system for evaluation of aversive and rewarding stimuli.

Gender differences in cortical representation of rectal distension in healthy humans

Cerebral cortical processing of information relayed via visceral afferents is poorly understood. We determined and compared cortical activity caused by various levels of rectal distension in healthy male and female subjects. Twenty-eight healthy, young (20-44 yr) volunteer subjects (13 male, 15 female) were studied with a paradigm-driven functional magnetic resonance imaging (fMRI) technique during barostat-controlled rectal distension at perception threshold and 10 mmHg below and above perception threshold. Male subjects showed localized clusters of fMRI activity primarily in the sensory and parietooccipital regions, whereas female subjects also showed activity in the anterior cingulate and insular regions. A progressive increase in maximum percent fMRI signal change and total volume of cortical activity was associated with the intensity of rectal distension pressure in both genders. Regions of cortical activity for below-threshold stimuli showed less substantial signal intensity and volume than responses for threshold and above-threshold stimuli. Volume of cortical activity during rectal distension in women was significantly higher than that for men for all distensions. We conclude that 1) there are substantial differences in female cortical activation topography during rectal distension compared with males; 2) intensity and volume of registered cortical activity due to rectal stimulation are directly related to stimulus strength; and 3) rectal stimulation below perception level is registered in the cerebral cortex.

The morbidity, time course and predictive factors for persistent post-thoracotomy pain

After thoracotomy, patients often suffer from a persistent pain syndrome called post-thoracotomy pain. To elucidate morbidity, time course, and predictive factors for this syndrome, we analyzed follow-up data for 85 post-thoracotomy patients. We used a four-point scale to assess pain: none, slight, moderate and severe. Of 85 patients, 50 reported pain (39 slight, 11 moderate) one day after surgery. A year after surgery, the patients were polled using a simple questionnaire received by the mail. Sixty patients reported persistent pain (34 slight, 14 moderate, 12 severe) a month after surgery, and 35 patients reported persistent pain (33 slight, two moderate) around the time of the poll (1 year after surgery). Although pain deterioration was observed in 40% (34/85) of patients during month 1 after surgery, pain alleviation was seen in 48% (41/85) of patients during months 2-12. Stepwise regression analysis revealed that female gender and pain at postoperative day 1 were predictive for persistent pain both 1 month and 1 year after thoracotomy. Among 35 patients with persistent pain 1 year after surgery, 24 cases reported paresthesia-dysesthesia, and 14 cases reported hypoesthesia. The present data thus suggests that persistent pain is common and often severe 1 month after surgery but is alleviated after 1 year. Clinical time course and symptoms indicate that nerve impairment rather than simple nociceptive impact may be involved in this syndrome.

Cortical representation of the sensory dimension of pain

It is well accepted that pain is a multidimensional experience, but little is known of how the brain represents these dimensions. We used positron emission tomography (PET) to indirectly measure pain-evoked cerebral activity before and after hypnotic suggestions were given to modulate the perceived intensity of a painful stimulus. These techniques were similar to those of a previous study in which we gave suggestions to modulate the perceived unpleasantness of a noxious stimulus. Ten volunteers were scanned while tonic warm and noxious heat stimuli were presented to the hand during four experimental conditions: alert control, hypnosis control, hypnotic suggestions for increased-pain intensity and hypnotic suggestions for decreased-pain intensity. As shown in previous brain imaging studies, noxious thermal stimuli presented during the alert and hypnosis-control conditions reliably activated contralateral structures, including primary somatosensory cortex (S1), secondary somatosensory cortex (S2), anterior cingulate cortex, and insular cortex. Hypnotic modulation of the intensity of the pain sensation led to significant changes in pain-evoked activity within S1 in contrast to our previous study in which specific modulation of pain unpleasantness (affect), independent of pain intensity, produced specific changes within the ACC. This double dissociation of cortical modulation indicates a relative specialization of the sensory and the classical limbic cortical areas in the processing of the sensory and affective dimensions of pain.

Acupuncture produces central activations in pain regions

Acupuncture is largely used for pain control in several pathological conditions. Its effects on the central nervous system are not well defined. We investigated the effect of the application of acupuncture to 13 normal subjects (males, 21-32 years). H2(15)O bolus PET scans were read before the application of the needles (Rest, R) and after 25 min of needle insertion. Data were acquired by scanning in 3-D mode. The acupuncture application, true acupuncture (TA), was alternated to a placebo needle application (PA) in two different sequences (seven and six subjects, respectively), either R,PA,R, TA or R,TA,R,PA, a period of 15 min being left after every first TA or PA to allow for the recovery of basal conditions. Here we show that classic acupuncture activates the left Anterior Cingulus, the Insulae bilaterally, the Cerebellum bilaterally, the left Superior Frontal Gyrus, and the right Medial and Inferior Frontal Gyri. Most of the activated areas are shared with areas activated in acute and chronic pain states as described in the literature. Thus acupuncture appears to act by activating areas also involved in pain. This indicates that acupuncture could relief pain by unbalancing the equilibrium of distributed pain-related central networks.

Effects of stress and relaxation on capsaicin-induced pain

A sizable body of research has been devoted to understanding the relationship between pain sensitivity and the psychological state of the individual. Considerable disagreement as to the direction of the association still exists. This study examines the effects of 2 experimental manipulations, cognitive/emotional stress and relaxation, on capsaicin-induced pain. Subjects were pretrained in relaxation and then randomized to experimental stress produced by a 20-minute Stroop test, relaxation (tape), or a control condition (neutral video), followed by a capsaicin injection in the forearm. Cardiovascular measures were taken at regular intervals, and cortisol, norepinephrine (NE), and self-reports of arousal (relaxation index) were taken immediately before and after the experimental task. The manipulation significantly interacted with sex to predict capsaicin-induced maximum pain. Women in the stress condition reported greater pain than both men in the stress condition and women in the relaxation condition. Pain was correlated negatively with task-induced changes in NE and cortisol and positively with self-reported arousal (decreased relaxation). However, separate analyses showed that some physiologic indexes of heightened arousal (increased blood pressure and NE) predicted lower pain only in men, whereas subjective increases in arousal predicted higher pain only in women. Multiple hierarchical regression analyses confirmed that physiologic and self-reported arousal predicted pain independently and in opposite directions, and a model including both accounted for 56% of the overall variance. These findings suggest that a unidimensional model of arousal may be insufficient to explain the effects of stress on pain and that these effects operate differently in men and women.

Hemispheric lateralization of somatosensory processing

Processing of both painful and nonpainful somatosensory information is generally thought to be subserved by brain regions predominantly contralateral to the stimulated body region. However, lesions to right, but not left, posterior parietal cortex have been reported to produce a unilateral tactile neglect syndrome, suggesting that components of somatosensory information are preferentially processed in the right half of the brain. To better characterize right hemispheric lateralization of somatosensory processing, H(2)(15)O positron emission tomography (PET) of cerebral blood flow was used to map brain activation produced by contact thermal stimulation of both the left and right arms of right-handed subjects. To allow direct assessment of the lateralization of activation, left- and right-sided stimuli were delivered during separate PET scans. Both innocuous (35 degrees C) and painful (49 degrees C) stimuli were employed to determine whether lateralized processing occurred in a manner related to perceived pain intensity. Subjects were also scanned during a nonstimulated rest condition to characterize activation that was not related to perceived pain intensity. Pain intensity-dependent and -independent changes in activation were identified in separate multiple regression analyses. Regardless of the side of stimulation, pain intensity--dependent activation was localized to contralateral regions of the primary somatosensory cortex, secondary somatosensory cortex, insular cortex, and bilateral regions of the cerebellum, putamen, thalamus, anterior cingulate cortex, and frontal operculum. No hemispheric lateralization of pain intensity-dependent processing was detected. In sharp contrast, portions of the thalamus, inferior parietal cortex (BA 40), dorsolateral prefrontal cortex (BA 9/46), and dorsal frontal cortex (BA 6) exhibited right lateralized activation during both innocuous and painful stimulation, regardless of the side of stimulation. Thus components of information arising from the body surface are processed, in part, by right lateralized systems analogous to those that process auditory and visual spatial information arising from extrapersonal space. Such right lateralized processing can account for the left somatosensory neglect arising from injury to brain regions within the right cerebral hemisphere.

Cerebral activation in patients with irritable bowel syndrome and control subjects during rectosigmoid stimulation

OBJECTIVE

Patients with irritable bowel syndrome (IBS) show evidence of altered perceptual responses to visceral stimuli, consistent with altered processing of visceral afferent information by the brain. In the current study, brain responses to anticipated and delivered rectal balloon distension were assessed.

METHODS

Changes in regional cerebral blood flow were measured using H2(15)O-water positron emission tomography in 12 nonconstipated IBS patients and 12 healthy control subjects. Regional cerebral blood flow responses to moderate rectal distension (45 mm Hg) and anticipated but undelivered distension were assessed before and after a series of repetitive noxious (60-mm Hg) sigmoid distensions.

RESULTS

Brain regions activated by actual and simulated distensions were similar in both groups. Compared with control subjects, patients with IBS showed lateralized activation of right prefrontal cortex; reduced activation of perigenual cortex, temporal lobe, and brain stem; but enhanced activation of rostral anterior cingulate and posterior cingulate cortices.

CONCLUSIONS

IBS patients show altered brain responses to rectal stimuli, regardless of whether these stimuli are actually delivered or simply anticipated. These alterations are consistent with reported alterations in autonomic and perceptual responses and may be related to altered central noradrenergic modulation.

Advancing the science of symptom management

Since the publication of the original Symptom Management Model (Larson et al. 1994), faculty and students at the University of California, San Francisco (UCSF) School of Nursing Centre for System Management have tested this model in research studies and expanded the model through collegial discussions and seminars.

AIM

In this paper, we describe the evidence-based revised conceptual model, the three dimensions of the model, and the areas where further research is needed.

BACKGROUND/RATIONALE

The experience of symptoms, minor to severe, prompts millions of patients to visit their healthcare providers each year. Symptoms not only create distress, but also disrupt social functioning. The management of symptoms and their resulting outcomes often become the responsibility of the patient and his or her family members. Healthcare providers have difficulty developing symptom management strategies that can be applied across acute and home-care settings because few models of symptom management have been tested empirically. To date, the majority of research on symptoms was directed toward studying a single symptom, such as pain or fatigue, or toward evaluating associated symptoms, such as depression and sleep disturbance. While this approach has advanced our understanding of some symptoms, we offer a generic symptom management model to provide direction for selecting clinical interventions, informing research, and bridging an array of symptoms associated with a variety of diseases and conditions. Finally, a broadly-based symptom management model allows the integration of science from other fields.