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

Concepts of pain mechanisms: the contribution of functional imaging of the human brain

Functional imaging of the conscious human brain has a solid physiological basis in synaptically induced rCBF responses. We still do not know how these responses are generated, but recent studies have shown that the rCBF response is parametrically positively correlated with functional measures of neuronal activity. Technical advances in both fMRI and PET imaging have improved the spatial and temporal resolution of imaging methods. Further advances may be expected in the near future. Consequently, we now have an important tool to apply to the study of normal and, most importantly, pathological pain. There is a tendency to expect too much of this exciting technique, but the problems we wish to address are complex and will require considerable time, effort, and patience. We now know that the CNS adapts to both peripheral and central nervous system injury, sometimes in beneficial ways, but sometimes with reorganization that is maladaptive. An understanding of the pathophysiology of neuropathic pain is further complicated by the new knowledge, emphasized by functional brain imaging, that pain and pain modulation is mediated, not by a simple pathway with one or a few central targets, but by a network of multiple interacting modules of neuronal activity. Simplified phrenological thinking, with complete psychological functions separate and localized, is appealing, but wildly misleading. It is far more realistic and productive to apply qualitative and quantitative spatial and temporal analyses to the distributed activity of the conscious, communicating human brain. This will not be quick and easy, but there is every reason for optimism in our search for a thorough and useful understanding of both normal and pathological pain.

Exploring the pain "neuromatrix"

A considerable number of functional imaging studies have demonstrated the involvement of multiple central regions during the experience of pain. These regions process information in circuits that can broadly be assumed to process the affective, sensory, cognitive, motor, inhibitory, and autonomic responses stimulated by a noxious event. The concept of a "neuromatrix" for pain processing is, therefore, well supported. There is, however, scant evidence for any particular regional or circuit dysfunction during clinical pain. To be clinically useful, functional imaging may have to step beyond the generalities of the neuromatrix.

Gastric distention correlates with activation of multiple cortical and subcortical regions

BACKGROUND & AIMS

The pathophysiology of functional dyspepsia may involve abnormal processing of visceral stimuli at the level of the central nervous system. There is accumulating evidence that visceral and somatic pain processing in the brain share common neuronal substrates. However, the cerebral loci that process sensory information from the stomach are unknown. The aim of this study was to localize the human brain regions that are activated by gastric distention.

METHODS

Brain (15)O-water positron emission tomography was performed in 15 right-handed healthy volunteers during baseline and distal gastric distentions to 10 mm Hg, 20 mm Hg, threshold pain, and moderate pain. Pain, nausea, and bloating were rated during baseline and distentions (0-5 scale). Statistical subtraction analysis of brain images was performed between distentions and baseline.

RESULTS

Symptoms increased with distending stimulus intensity (maximum pain, 2.1 +/- 0.4; nausea, 2.2 +/- 0.4; bloating, 3.7 +/- 0.2). Paralleling increases in distention stimulus and symptoms, progressive increases in activation (P < or = 0.05), were observed in the thalami, insula bilaterally, anterior cingulate cortex, caudate nuclei, brain stem periaqueductal gray matter, cerebellum, and occipital cortex.

CONCLUSIONS

Symptomatic gastric distention activates structures implicated in somatic pain processing, supporting the notion of a common cerebral pain network.

Gender differences and hormonal modulation in visceral pain

Women seek healthcare and are diagnosed more frequently with chronic somatic and visceral pain conditions relative to men. These conditions tend not to be life-threatening disorders, but rather ones that decrease people's quality of life, impinge on work and recreational activities, and increase healthcare resource utilization. With increased awareness of basic gender differences in biology and responsiveness to therapies, there has been renewed interest in factors which may account for the gender disparity in chronic visceral pain conditions. Basic and clinical evidence primarily from patients with irritable bowel syndrome has provided initial insights into visceral pain sensitivity, perception, and responsitivity.

Temporal and spatial dynamics of human forebrain activity during heat pain: analysis by positron emission tomography

To learn about the sequence of brain activation patterns during heat pain, we acquired positron emission tomographic (PET) brain scans at different times during repetitive heat stimulation (40 or 50 degrees C; 5-s contact) of each subject's left forearm. Early scans began at the onset of 60 s of stimulation; late scans began after 40 s of stimulation, which continued throughout the 60-s scan period (total stimulus duration 100 s). Each subject (14 normal, right-handed subjects; 10 male, 4 female; ages 18-42) used a visual analog scale to rate the perceived stimulus intensity (0 = no heat, 7 = pain threshold, 10 = barely tolerable pain) after each scan. The 40 degrees C stimulation received an average intensity rating of 2.19 +/- 1.22 (mean +/- SD) and the 50 degrees C an average rating of 8.93 +/- 1.33. During the scan sessions, subjects did not report a difference between early and late scans. To examine the effect of the duration of stimulation specifically, 8 of these subjects rated the perceived intensity of each of 20 sequential 5-s duration contact heat stimuli (40 or 50 degrees C; 100 s of stimulation). We used a graphical method to detect changes in perceived unpleasantness. There was no difference in perceived intensity or unpleasantness during the 40 degrees C stimulation. However, during 50 degrees C stimulation, perceived unpleasantness increased and subjects perceived the last five, but not the second five, stimuli as more intense than the first five stimuli. These psychophysical changes could be mediated by brain structures with increasing activity from early to late PET scans or that are active only during late scans. These structures include the contralateral M1/S1 cortex, bilateral S2 and mid-insular cortex, contralateral VP thalamus, medial ipsilateral thalamus, and the vermis and paravermis of the cerebellum. Structures that are equally active throughout stimulation (contralateral mid-anterior cingulate and premotor cortex) are less likely to mediate these psychophysical changes. Some cortical, but not subcortical, structures showed significant or borderline activation only during the early scans (ipsilateral premotor cortex, contralateral perigenual anterior cingulate, lateral prefrontal, and anterior insular cortex); they may mediate pain-related attentive or anticipatory functions. Overall, the results reveal that 1) the pattern of brain activation and the perception of heat pain both change during repetitive noxious heat stimulation, 2) cortical activity can be detected before subcortical responses appear, and 3) timing the stimulation with respect to the scan period can, together with psychophysical measurements, identify brain structures that are likely to participate in the perception of pain.

The girl who cried pain: a bias against women in the treatment of pain

There is now a well-established body of literature documenting the pervasive inadequate treatment of pain in this country. There have also been allegations, and some data, supporting the notion that women are more likely than men to be undertreated or inappropriately diagnosed and treated for their pain.

One particularly troublesome study indicated that women are more likely to be given sedatives for their pain and men to be given pain medication. Speculation as to why this difference might exist has included the following: Women complain more than men; women are not accurate reporters of their pain; men are more stoic so that when they do complain of pain, “it's real”; and women are better able to tolerate pain or have better coping skills than men.

Regional cerebral blood flow in fibromyalgia: single-photon-emission computed tomography evidence of reduction in the pontine tegmentum and thalami

OBJECTIVE

To determine whether regional cerebral blood flow (rCBF) is abnormal in any cerebral structure of women with fibromyalgia (FM), following a report that rCBF is reduced in the thalami and heads of caudate nuclei in FM.

METHODS

Seventeen women with FM and 22 healthy women had a resting single-photon-emission computed tomography (SPECT) brain scan to assess rCBF and a T1-weighted magnetic resonance imaging (MRI) scan to enable precise anatomic localization. Additionally, all participants underwent 2 manual tender point examinations and completed a set of questionnaires evaluating clinical features. SPECT scans were analyzed for differences in rCBF between groups using statistical parametric mapping (SPM) and regions of interest (ROIs) manually drawn on coregistered MRI.

RESULTS

Compared with control subjects, the rCBF in FM patients was significantly reduced in the right thalamus (P = 0.006), but not in the left thalamus or head of either caudate nucleus. SPM analysis indicated a statistically significant reduction in rCBF in the inferior pontine tegmentum (corrected P = 0.006 at the cluster level and corrected P = 0.023 for voxel of maximal significance), with consistent findings from ROI analysis (P = 0.003). SPM also detected a reduction in rCBF on the perimeter of the right lentiform nucleus. No correlations were found with clinical features or indices of pain threshold.

CONCLUSION

Our finding of a reduction in thalamic rCBF is consistent with findings of functional brain imaging studies of other chronic clinical pain syndromes, while our finding of reduced pontine tegmental rCBF is new. The pathophysiologic significance of these changes in FM remains to be elucidated.

Severe pain confounds neuropsychological test performance

There is little information on the effect of pain on neuropsychological test performance. We have undertaken this study to explore which tests are affected by pain, the magnitude of these changes, and other confounders of neuropsychological performance in a population of patients having spine surgery. Twenty-four elderly English speaking Caucasian patients (age > 60 years) were enrolled pre-operatively in this Institutional Review Board approved study. Pain scores using an 11-point Numeric Pain Intensity scale and performance on a neuropsychological battery (Controlled Oral Word Association, Rey Complex Figure, Trails A and B) were assessed at two times, before and one day after surgery. Scores were calculated using the standard algorithms and change scores were calculated by subtracting the baseline from follow-up scores. After surgery, performance on the Rey Complex Figure ( r = -0.577, p = 0.004) and Trails Part A (r = 0.527, p = 0.01) declined with increasing post-operative pain scores. Women reported higher pain scores post-operatively than men (p = 0.046), and performed worse than men for change in performance on Trails Part A (p = 0.027). These data suggest that pain can influence performance on certain cognitive tests, and that some gender differences in these effects may occur. Interpretation of performance measures should take into account possible effects of pain, although our understanding of pain effects and ability to predict them in individual people, currently are quite limited.

Functional imaging of brain responses to pain. A review and meta-analysis (2000)

Brain responses to pain, assessed through positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) are reviewed. Functional activation of brain regions are thought to be reflected by increases in the regional cerebral blood flow (rCBF) in PET studies, and in the blood oxygen level dependent (BOLD) signal in fMRI. rCBF increases to noxious stimuli are almost constantly observed in second somatic (SII) and insular regions, and in the anterior cingulate cortex (ACC), and with slightly less consistency in the contralateral thalamus and the primary somatic area (SI). Activation of the lateral thalamus, SI, SII and insula are thought to be related to the sensory-discriminative aspects of pain processing. SI is activated in roughly half of the studies, and the probability of obtaining SI activation appears related to the total amount of body surface stimulated (spatial summation) and probably also by temporal summation and attention to the stimulus. In a number of studies, the thalamic response was bilateral, probably reflecting generalised arousal in reaction to pain. ACC does not seem to be involved in coding stimulus intensity or location but appears to participate in both the affective and attentional concomitants of pain sensation, as well as in response selection. ACC subdivisions activated by painful stimuli partially overlap those activated in orienting and target detection tasks, but are distinct from those activated in tests involving sustained attention (Stroop, etc.). In addition to ACC, increased blood flow in the posterior parietal and prefrontal cortices is thought to reflect attentional and memory networks activated by noxious stimulation. Less noted but frequent activation concerns motor-related areas such as the striatum, cerebellum and supplementary motor area, as well as regions involved in pain control such as the periaqueductal grey. In patients, chronic spontaneous pain is associated with decreased resting rCBF in contralateral thalamus, which may be reverted by analgesic procedures. Abnormal pain evoked by innocuous stimuli (allodynia) has been associated with amplification of the thalamic, insular and SII responses, concomitant to a paradoxical CBF decrease in ACC. It is argued that imaging studies of allodynia should be encouraged in order to understand central reorganisations leading to abnormal cortical pain processing. A number of brain areas activated by acute pain, particularly the thalamus and anterior cingulate, also show increases in rCBF during analgesic procedures. Taken together, these data suggest that hemodynamic responses to pain reflect simultaneously the sensory, cognitive and affective dimensions of pain, and that the same structure may both respond to pain and participate in pain control. The precise biochemical nature of these mechanisms remains to be investigated.

Gender differences in regional brain response to visceral pressure in IBS patients

In two experiments including a total of 30 irritable bowel syndrome patients, symptom-mimicking rectal pressure stimuli elicited changes in regional neural activation as measured by positron electron tomography (PET) cerebral blood flow images. Although most stimuli were not rated as painful, rectal pressure increased regional cerebral blood flow (rCBF) in areas commonly associated with somatic pain, including the anterior cingulate, insula, prefrontal cortex, thalamus, and cerebellum. Despite similar stimulus ratings in male and female patients, regional activations were much stronger for males. In both experiments, rectal pressure activated the insula bilaterally in males but not in females. Insula activation was associated most strongly with objective visceral pressure, whereas anterior cingulate activation was associated more with correlated ratings of subjective discomfort. The insula is discussed as a visceral sensory cortex. Several possible reasons for the insula gender effect are proposed.

A Review of Functional Imaging of the Brain and Pain

Functional imaging of the brain is a current reality using positron emission tomography and functional magnetic imaging. This article reviews many of the reports that have emerged in the past several years using these techniques in the analysis of pain experience. The areas of the brain that appear to be functioning during the experience of pain are discussed, and the variances in findings between studies are described. The implications of the findings are noted. Although much has been learned through these techniques, it is clear that further research is needed before clinicians can use these diagnostic studies for therapeutic purposes.

Meta-Analysis of Thirty-Four Independent Samples Studied Using PET Reveals a Significantly Attenuated Central Response to Noxious Stimulation in Clinical Pain Patients

Chronic pain disorder is widely understood as a "biopsychosocial" phenomenon, meaning that it is influenced by psychology and certain life events. This broad understanding of chronic pain suggests that central responses during pain experience should be altered in patients compared with pain-free volunteers. A total of 34 studies are reviewed, revealing a widespread "neuromatrix" of activated regions. These regions include the brain stem, thalamus, and lentiform nucleus, and the insula, prefrontal, parietal, and anterior cingulate cortices. Meta-analysis of these studies does not reveal any single region or pattern of activity to be of particular influence during chronic pain but does reveal a generally reduced response to noxious stimulation in patients with concomitant clinical pain. The relevance of this finding remains unclear with the most parsimonious explanation being increased response variability in patients. More specific findings can be revealed when using a hypothesis-generated approach; further investigation of genetic and developmental predisposition is suggested.

Cerebral Activity in the Perception of Visceral Pain

Specific patterns of cerebral activity have recently been identified in relation to acute and chronic visceral pain experiences in humans. Activity may occur in a brain region as a consequence of receiving and processing neurally encoded information perceived to be arising from the viscera. (This includes, but is not necessarily limited to, information actually arising from the viscera.) Observed cerebral activity may also represent the mobilization and output of instructions emanating from the cerebrum, which helps create and respond to visceral events. Functional neuroanatomy underlying the transmission of both kinds of neuronally encoded information is outlined. Recent research and ideas on cerebral activity associated with visceral pain perceived to be arising from within the abdomen, chest, or pelvis are considered. Finally, the relationships of pain perception and cerebral activity to depression, autonomic function, and sex are discussed.

The relationship of gender to pain

Gender differences have been identified in the perception of pain intensity for both acute and chronic pain and with responses to analgesics. Women seem to show lower pain thresholds, a greater ability to discriminate painful sensations, higher pain ratings, and a lower tolerance for pain. Although some pain syndromes, such as facial pain, are more common in women, gender-related responses to pain are not completely consistent. The study of gender differences in relation to pain is relatively new, yet promising. This article reviews the evidence for how gender may play a role in reports of pain intensity, measurements of patient responses, and differences in response to pain therapies. Literature that addresses pain perception and response in acute and chronic nonmalignant and cancer pain states, experimentally induced pain, and responses to analgesics are reviewed in terms of their relationship to gender. Although there are conflicting results for experimental and clinical studies, there is agreement among investigators that certain factors, such as perceptual ability and physiologic mechanisms, do explain gender-related differences to pain and its treatment. Gender is an important variable and should be taken into account in both research and the clinical practice of pain management.

Selective opiate modulation of nociceptive processing in the human brain

Fentanyl, a mu-opioid receptor agonist, produces analgesia while leaving vibrotactile sensation intact. We used positron emission tomography (PET) to study the mechanisms mediating this specific effect in healthy, right-handed human males (ages 18-28 yr). Subjects received either painful cold (n = 11) or painless vibratory (n = 9) stimulation before and after the intravenous injection of fentanyl (1.5 microgram/kg) or placebo (saline). Compared with cool water (29 degrees C), immersion of the hand in ice water (1 degrees C) is painful and produces highly significant increases in regional cerebral blood flow (rCBF) within the contralateral second somatosensory (S2) and insular cortex, bilaterally in the thalamus and cerebellum, and medially in the cerebellar vermis. Responses just below the statistical threshold (3.5 < Z < 4.0) are seen in the contralateral anterior cingulate, ipsilateral insular cortex, and dorsal medial midbrain. The contralateral primary sensory cortex (S1) shows a trend of activation. Except for slight changes in intensity, this pattern is unchanged following a saline placebo injection. Fentanyl reduces the average visual analogue scale ratings of perceived pain intensity (47%) and unpleasantness (50%), reduces pain-related cardioacceleration, and has positive hedonic effects. After fentanyl, but not placebo, all cortical and subcortical responses to noxious cold are greatly reduced. Subtraction analysis [(innocuous water + fentanyl) - (innocuous water + no injection)] shows that fentanyl alone increases rCBF in the anterior cingulate cortex, particularly in the perigenual region. Vibration (compared with mock vibration) evokes highly significant rCBF responses in the contralateral S1 cortex in the baseline (no injection) and placebo conditions; borderline responses (3.5 < Z < 4. 0) are detected also in the contralateral thalamus. Fentanyl has no effect on the perceived intensity or unpleasantness of vibratory stimulation, which continues to activate contralateral S1. Fentanyl alone [(mock vibration + fentanyl) - (mock vibration + no injection)] again produces highly significant activation of the perigenual and mid-anterior cingulate cortex. A specific comparison of volumes of interest, developed from activation peaks in the baseline condition (no injection), shows that fentanyl strongly attenuates both the contralateral thalamic and S1 cortical responses to noxious cold stimulation (P < 0.048 and 0.007, respectively) but fails to affect significantly these responses to vibrotactile stimulation (P > 0.26 and 0.91, respectively). In addition, fentanyl, compared with placebo, produces a unique activation of the mid-anterior cingulate cortex during fentanyl analgesia, suggesting that this region of the cingulate cortex participates actively in mediating opioid analgesia. The results are consistent with a selective, fentanyl-mediated suppression of nociceptive spinothalamic transmission to the forebrain. This effect could be implemented directly at the spinal level, indirectly through cingulate corticofugal pathways, or by a combination of both mechanisms.

Differences in pain expression between male and female newborn infants

The study of neonatal gender differences in pain expression is important since neonatal pain behavior occurs prior to any learned reaction pattern. The objective of this study was to verify the presence of gender differences in pain expression in preterm and term newborn infants. Sixty-five consecutive neonates (37 female and 28 male infants) with gestational age between 28 and 42 weeks and with 25-120 h of life were studied. Healthy term neonates required a capillary puncture for PKU screening and clinically stable premature infants needed a capillary puncture for glucose dosage. The Neonatal Facial Coding System (NFCS) and the Neonatal Infant Pain Scale (NIPS) were evaluated at bedside prior to the puncture, when patients were at rest, during foot heating; during capillary puncture; and at 1, 3, and 5 min after heel lancing. Results were analyzed by repeated-measures ANOVA followed by the Multiple Comparison Method of Bonferroni. A significant difference among the mean NFCS scores during the six study periods was noted for the whole group of neonates (P<0.000001). Also, a significant interaction between the NFCS score profile in female and male neonates at the different study periods was observed (P=0.025). Regarding NIPS, ANOVA showed only a significant difference among the mean NIPS scores during the six study periods for the whole group of neonates (P<0.000001). No significant interactions between gestational age and time, nor between gestational age and gender were noted, for both NFCS and NIPS. In conclusion, recently born female neonates of all gestational ages expressed more facial features of pain than male infants, during the capillary puncture and 1 min afterwards. Maybe differences in pain processing and/or pain expression among genders may explain this finding.

Sex differences in pain indices, exercise, and use of analgesics

To understand better reported sex differences in sensitivity to pain, this study examined daily pain frequency and intensity, use of analgesics, physical activity, and both subjective and physiological response to acute pain in 18 men and 24 women, healthy people who provided information about their daily pain symptoms and physical activity before completing a cold pressor task. Compared to men, women reported more frequent and intense pain symptoms, as well as more frequent use of analgesics and lower physical activity. Women evinced higher physiological arousal during the cold pressor task but similar subjective pain. The findings highlight the different ways men and women cope with pain and the effect on their responses to acute pain.

Loss of synaptic depression in mammalian anterior cingulate cortex after amputation

Two forms of activity-dependent long-term depression (LTD) in the CNS, as defined by their sensitivity to the blockade of NMDA receptors, are thought to be important in learning, memory, and development. Here, we report that NMDA receptor-independent LTD is the major form of long-term plasticity in the anterior cingulate cortex (ACC). Both L-type voltage-gated calcium channels and metabotropic glutamate receptors are required for inducing LTD. Amputation of a third hindpaw digit in an adult rat induced rapid expression of immediate early genes in the ACC bilaterally and caused a loss of LTD that persisted for at least 2 weeks. Our results suggest that synaptic LTD in the ACC may contribute to enhanced neuronal responses to subsequent somatosensory stimuli after amputation.

Loss of synaptic depression in mammalian anterior cingulate cortex after amputation

Two forms of activity-dependent long-term depression (LTD) in the CNS, as defined by their sensitivity to the blockade of NMDA receptors, are thought to be important in learning, memory, and development. Here, we report that NMDA receptor-independent LTD is the major form of long-term plasticity in the anterior cingulate cortex (ACC). Both L-type voltage-gated calcium channels and metabotropic glutamate receptors are required for inducing LTD. Amputation of a third hindpaw digit in an adult rat induced rapid expression of immediate early genes in the ACC bilaterally and caused a loss of LTD that persisted for at least 2 weeks. Our results suggest that synaptic LTD in the ACC may contribute to enhanced neuronal responses to subsequent somatosensory stimuli after amputation.

A comparative fMRI study of cortical representations for thermal painful, vibrotactile, and motor performance tasks

Cortical activity due to a thermal painful stimulus applied to the right hand was studied in the middle third of the contralateral brain and compared to activations for vibrotactile and motor tasks using the same body part, in nine normal subjects. Cortical activity was demonstrated utilizing multislice echo-planar functional magnetic resonance imaging (fMRI) and a surface coil. The cortical activity was analyzed based upon individual subject activity maps and on group-averaged activity maps. The results show significant differences in activations across the three tasks and the cortical areas studied. The study indicates that fMRI enables examination of cortical networks subserving pain perception at an anatomical detail not available with other brain imaging techniques and shows that this cortical network underlying pain perception shares components with the networks underlying touch perception and motor execution. However, the thermal pain perception network also has components that are unique to this perception. The uniquely activated areas were in the secondary somatosensory region, insula, and posterior cingulate cortex. The posterior cingulate cortex activity was in a region that, in the monkey, receives nociceptive inputs from posterior thalamic medial and lateral nuclei that in turn are targets for spinothalamic terminations. Discrete subdivisions of the primary somatosensory and motor cortical areas were also activated in the thermal pain task, showing region-dependent differences in the extent of overlap with the other two tasks. Within the primary motor cortex, a hand region was preferentially active in the task in which the stimulus was painful heat. In the primary somatosensory cortex most activity in the painful heat task was localized to area 1, where the motor and vibratory task activities were also coincident. The study also indicates that the functional connectivity across multiple cortical regions reorganizes dynamically with each task.

Pain perception: is there a role for primary somatosensory cortex?

Anatomical, physiological, and lesion data implicate multiple cortical regions in the complex experience of pain. These regions include primary and secondary somatosensory cortices, anterior cingulate cortex, insular cortex, and regions of the frontal cortex. Nevertheless, the role of different cortical areas in pain processing is controversial, particularly that of primary somatosensory cortex (S1). Human brain-imaging studies do not consistently reveal pain-related activation of S1, and older studies of cortical lesions and cortical stimulation in humans did not uncover a clear role of S1 in the pain experience. Whereas studies from a number of laboratories show that S1 is activated during the presentation of noxious stimuli as well as in association with some pathological pain states, others do not report such activation. Several factors may contribute to the different results among studies. First, we have evidence demonstrating that S1 activation is highly modulated by cognitive factors that alter pain perception, including attention and previous experience. Second, the precise somatotopic organization of S1 may lead to small focal activations, which are degraded by sulcal anatomical variability when averaging data across subjects. Third, the probable mixed excitatory and inhibitory effects of nociceptive input to S1 could be disparately represented in different experimental paradigms. Finally, statistical considerations are important in interpreting negative findings in S1. We conclude that, when these factors are taken into account, the bulk of the evidence now strongly supports a prominent and highly modulated role for S1 cortex in the sensory aspects of pain, including localization and discrimination of pain intensity.

Forebrain mechanisms of nociception and pain: analysis through imaging

Pain is a unified experience composed of interacting discriminative, affective-motivational, and cognitive components, each of which is mediated and modulated through forebrain mechanisms acting at spinal, brainstem, and cerebral levels. The size of the human forebrain in relation to the spinal cord gives anatomical emphasis to forebrain control over nociceptive processing. Human forebrain pathology can cause pain without the activation of nociceptors. Functional imaging of the normal human brain with positron emission tomography (PET) shows synaptically induced increases in regional cerebral blood flow (rCBF) in several regions specifically during pain. We have examined the variables of gender, type of noxious stimulus, and the origin of nociceptive input as potential determinants of the pattern and intensity of rCBF responses. The structures most consistently activated across genders and during contact heat pain, cold pain, cutaneous laser pain or intramuscular pain were the contralateral insula and anterior cingulate cortex, the bilateral thalamus and premotor cortex, and the cerebellar vermis. These regions are commonly activated in PET studies of pain conducted by other investigators, and the intensity of the brain rCBF response correlates parametrically with perceived pain intensity. To complement the human studies, we developed an animal model for investigating stimulus-induced rCBF responses in the rat. In accord with behavioral measures and the results of human PET, there is a progressive and selective activation of somatosensory and limbic system structures in the brain and brainstem following the subcutaneous injection of formalin. The animal model and human PET studies should be mutually reinforcing and thus facilitate progress in understanding forebrain mechanisms of normal and pathological pain.

Pain after periodontal scaling and root planing

BACKGROUND

Although periodontal scaling and root planing, or SRP, is one of the most common procedures used in dental practice, there is little information available about the degree of postprocedural pain associated with it. The authors undertook this study to document the intensity and duration of pain after SRP with a view toward helping practitioners and their patients manage postprocedural discomfort.

METHODS

Using the Heft-Parker self-assessment pain scale, 52 adults with moderate periodontitis evaluated their pain before and after SRP conducted with local anesthetic.

RESULTS

After SRP, 28 percent of all patients reported faint-to-weak pain, 18 percent experienced weak-to-mild pain, 28 percent experienced mild-to-moderate pain, 8 percent had moderate-to-strong pain and 8 percent reported strong-to-intense pain. The average time to onset of maximum pain was approximately three hours after SRP, and the average duration of mild or greater pain was about six hours. Upon awakening the morning after SRP, subjects found that pain had returned to pre-SRP levels. Overall, 23 percent of all patients reported self-medicating with analgesics to relieve postprocedural pain. Women self-medicated earlier (P < .05) and more often than men (43 percent vs. 10 percent; P < .05).

CONCLUSIONS

Patients experienced significant duration and magnitude of pain after SRP. This pain peaked between two and eight hours after SRP, lasted about six hours, and returned to pre-SRP levels by the morning after the procedure. Almost 25 percent of all patients self-medicated to relieve pain after SRP, and women took analgesic medication earlier and more often than men.

CLINICAL IMPLICATIONS

Practitioners should consider using appropriate analgesic drugs to alleviate mild-to-moderate pain after SRP. On the basis of this study, it would appear that an analgesic that has a peak effect two to eight hours after the completion of SRP would be the most appropriate medication. Moreover, it is unlikely that analgesic medication would be needed by most patients beyond the day on which SRP was performed.

Pain

Advances in our understanding of the activation of peripheral damage-sensing neurons (nociceptors) over the past year have been complemented by electrophysiological and imaging studies of central nervous system pain-related centres. The manipulation of gene expression in a reversible and cell type specific way combined with imaging and electrophysiological studies holds promise for helping us to identify the spatial and molecular substrates of pain perception with increasing precision and gives hope for improved analgesic therapies.

Cerebral responses to pain in patients suffering acute post-dental extraction pain measured by positron emission tomography (PET)

Previous studies with normal volunteers have demonstrated distributed cortical responses to experimental heat pain within a network of structures. The network includes the insula, anterior cingulate, prefrontal, inferior parietal and somatosensory cortices. Patients suffering from chronic nociceptive pain following rheumatoid arthritis (RA) have shown damped central responses to experimental heat pain applied to the back of the right hand. In this study of patients with acute, left-sided, post-molar-extraction (surgical) pain, we assessed the cortical responses to experimental heat pain, applied to the back of the right hand, using positron emission tomography (PET), and compared the responses with a previously reported control group and the RA group. In response to the experimental heat pain, the surgical group indicated significantly increased regional cerebral blood flow in the prefrontal cortex [Brodman's area (BA) 44] ipsilateral to the heat stimulus. Contralateral increases were detected in the putamen and transverse temporal gyrus (BA 40/41/42) with bilateral increases in the insular cortex. Compared to the control and RA group, there were significantly reduced responses in the anterior cingulate (BA 24), pre-frontal medial, and orbito-frontal (BA 9/10/32/47) cortices. These results suggest that relatively discrete regions of the cerebral cortex are responsible for acute nociceptive processing during an acute inflammatory episode. The reduced frontal and anterior cingulate responses to the experimental heat pain (applied to the right hand) during acute inflammatory pain (left jaw) illustrates cortical modulation of nociceptive processing that may be related to non-somatotopic, bilateral, nociceptive inputs to these areas. Copyright 1999 European Federation of Chapters of the International Association for the Study of Pain.