[Lateral asymmetry of the human pain sensitivity.]

Interaction between local blood flow and tolerance to prolonged pain in the elderly

PURPOSE

We aimed to explore the link between local vasodilation and pain perception in elderly subjects, testing the hypothesis that altered local cutaneous blood flow participates in the decrease in pain tolerance with age.

METHOD

Sixty-eight young and 83 older participants performed a pain tolerance test in which they hold their hand in an airtight box in which air temperature was regulated at 65 °C until the pain became unbearable. Participants continuously estimated pain intensity. Skin temperature and local blood flow in the box-exposed hand were continuously monitored.

RESULTS

In the young group, 97% of subjects resisted pain until the end of the test, whereas only 53% in the elderly group managed to do so, indicating that pain tolerance is impaired in the elderly. Among all participants, the skin temperature associated with the first pain sensation was below the threshold for nociceptor activation (43 °C). Interestingly, blood flow in the elderly group was correlated with pain judgment, whereas no such correlation was observed in the young.

CONCLUSION

Our results suggest that the local vasodilator response induced by local heating may be involved in pain perception and may influence thermal pain tolerance with aging. These results could contribute to a better understanding of vascular deficits and the development of chronic pain in vascular pathologies.

Observing physicians acting with different levels of empathy modulates later assessed pain tolerance

OBJECTIVES

The patient-physician relationship is essential for treatment success. Previous studies demonstrated that physicians who behave empathic in their interaction with patients have a positive effect on health outcomes. In this study, we investigated if the mere perception of physicians as empathic/not empathic modulates pain despite an emotionally neutral interaction with the patients.

METHODS

N = 60 women took part in an experimental study that simulated a clinical interaction. In the paradigm, each participant watched two immersive 360° videos via a head-mounted display from a patient's perspective. The physicians in the videos behaved either empathic or not empathic towards a third person. Importantly, these physicians remained emotionally neutral in the subsequent virtual interaction with the participants. Finally, participants received a controlled, painful pressure stimulus within the narratives of the videos.

RESULTS

The physicians in the high compared with the low empathy videos were rated as more empathic and more likable, indicating successful experimental manipulation. In spite of later neutral behaviour of physicians, this short observation of physicians' behaviour towards a third person was sufficient to modulate pain tolerance of the participants.

CONCLUSIONS

The finding of this study that the mere observation of physicians' behaviour towards a third person modulates pain, despite a neutral direct interaction with the participants, has important clinical implications. Further, the proposed paradigm enables investigating aspects of patient-physician communication that are difficult to examine in a clinical setting.

Neural Correlates of Empathy with Pain Show Habituation Effects. An fMRI Study

Background

Neuroimaging studies have demonstrated that the actual experience of pain and the perception of another person in pain share common neural substrates, including the bilateral anterior insular cortex and the anterior midcingulate cortex. As many fMRI studies include the exposure of participants to repeated, similar stimuli, we examined whether empathic neural responses were affected by habituation and whether the participants' prior pain experience influenced these habituation effects.

Method

In 128 trials (four runs), 62 participants (31 women, 23.0 ± 4.2 years) were shown pictures of hands exposed to painful pressure (pain pictures) and unexposed (neutral pictures). After each trial, the participants rated the pain of the model. Prior to the experiment, participants were either exposed to the same pain stimulus (pain exposure group) or not (touch exposure group). In order to assess possible habituation effects, linear changes in the strength of the BOLD response to the pain pictures (relative to the neutral pictures) and in the ratings of the model’s pain were evaluated across the four runs.

Results

Although the ratings of the model’s pain remained constant over time, we found neural habituation in the bilateral anterior/midinsular cortex, the posterior midcingulate extending to dorsal posterior cingulate cortex, the supplementary motor area, the cerebellum, the right inferior parietal lobule, and the left superior frontal gyrus, stretching to the pregenual anterior cingulate cortex. The participant’s prior pain experience did neither affect their ratings of the model’s pain nor their maintenance of BOLD activity in areas associated with empathy. Interestingly, participants with high trait personal distress and fantasy tended to show less habituation in the anterior insula.

Conclusion

Neural structures showed a decrease of the BOLD signal, indicating habituation over the course of 45 minutes. This can be interpreted as a neuronal mechanism responding to the repeated exposure to pain depictions, which may be regarded as functional in a range of contexts.

Measuring mechanical pain: the refinement and standardization of pressure pain threshold measurements

Pain thresholds are widely used in behavioral research, but unlike other pain modalities, a standardized assessment of pressure pain remains a challenge. In this research, we describe the application of an automatic pressure algometer with a linear increase in force. Ergonomically designed fixation devices were developed to increase the accuracy and to shorten the time of each measurement. Ten healthy volunteers were included in a pilot study to test the algometry method. Pressure pain thresholds (PPTs) were investigated over 2 experimental days in three nonconsecutive runs at 29 measurement sites. During the experiment, subjects reported their subjective sleepiness, level of state-anxiety, psychological status and the perceived pain intensity of each measurement. Pain intensity ratings indicate that instructions were followed. State-anxiety and subjective sleepiness levels were low throughout the experiment. The method has proven to be suitable for standardized PPT measurements across the body in an ergonomic, safe, and user-friendly fashion.

Tonic pain grabs attention, but leaves the processing of facial expressions intact-evidence from event-related brain potentials

Emotion and attention are key players in the modulation of pain perception. However, much less is known about the reverse influence of pain on attentional and especially emotional processes. To this end, we employed painful vs. non-painful pressure stimulation to examine effects on the processing of simultaneously presented facial expressions (fearful, neutral, happy). Continuous EEG was recorded and participants had to rate each facial expression with regard to valence and arousal. Painful stimulation attenuated visual processing in general, as reduced P100 and late positive potential (LPP) amplitudes revealed, but did not interfere with structural encoding of faces (N170). In addition, early perceptual discrimination and sustained preferential processing of emotional facial expressions as well as affective ratings were not influenced by pain. Thus, tonic pain demonstrates strong attention-demanding properties, but this does not interfere with concurrently ongoing emotion discrimination processes. These effects point at partially independent effects of pain on emotion and attention, respectively.

Stoicism and Sensation Seeking: Male Vulnerabilities for the Acquired Capability for Suicide

Our aim was to investigate two personality traits (i.e., stoicism and sensation seeking) that may account for well-established gender differences in suicide, within the framework of the interpersonal theory of suicide. This theory proposes that acquired capability for suicide, a construct comprised of pain insensitivity and fearlessness about death, explains gender differences in suicide. Across two samples of undergraduates (N = 185 and N = 363), men demonstrated significantly greater levels of both facets of acquired capability than women. Further, we found that stoicism accounted for the relationship between gender and pain insensitivity, and sensation seeking accounted for the relationship between gender and fearlessness about death. Thus, personality may be one psychological mechanism accounting for gender differences in suicidal behavior.

Affective pain modulation in fibromyalgia, somatoform pain disorder, back pain, and healthy controls

Previous research suggested that patients with fibromyalgia (FM) experience a higher pain intensity (clinical pain) than do patients with musculoskeletal pain after negative emotional priming compared to positive priming. To further examine affective pain modulation in FM, we applied an experimental pain induction to compare 30 patients with FM with 30 healthy (pain-free) participants (HC), and 30 patients with back pain (BP). For another group of 30 patients with somatoform pain disorder (SF), we predicted the same pain modulation as for FM. As primes we presented positive, neutral, negative, and pain-related pictures and assessed pain intensity in response to a fixed pressure weight. Overall, picture valence modulated pain intensities (in the order of pain-related > negative pictures > neutral), but the pain intensities between neutral and positive pictures did not differ significantly. SF reported significantly higher pain intensities than did BP and HC; FM were in between, but did not differ significantly from the three other groups. There was no interaction of priming and group. Affective modulation of pain was not specifically altered in FM and SF, but SF were more sensitive to pressure pain than BP and HC.

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.

SPECT scan in somatization disorder patients: an exploratory study of eleven cases

OBJECTIVE

There are no previous studies using single photon emission computed tomography (SPECT) scans in somatization disorder (SD) patients. The aim of this paper is to assess SPECT imaging abnormalities in SD patients and study any relation to laterality.

METHOD

Eleven SD patients from the Somatization Disorder Unit of Miguel Servet University Hospital, Zaragoza, Spain, not fulfilling criteria for any other psychiatric disorder and showing normal computed tomography (CT) and magnetic resonance imaging (MRI) images were studied with SPECT. Patients with DSM-IV axis I comorbidity were ruled out because it has been demonstrated that SPECT scans can show abnormalities in patients with depression and anxiety disorders. The technique used for SPECT was 99mTc-D,1,hexamethylpropyleneamide-oxime (99mTc-HMPAO) in four patients and 99mTc-bicisate in the other seven. The SPECT scans were evaluated without knowledge of clinical data and entirely by visual inspection.

RESULTS

Seven out of 11 (63.6%) SD patients showed hypoperfusion in SPECT imaging. In four cases there was hypoperfusion in the non-dominant hemisphere and the predominance of pain symptoms took place in the contralateral hemibody. In the other three patients hypoperfusion was bilateral. The anatomical regions affected were cerebellum (four cases), frontal and prefrontal areas (three cases), temporoparietal areas (two cases) and the complete hemisphere (one case).

CONCLUSIONS

A proportion of SD patients may present hypoperfusion in SPECT images, uni- or bilaterally, in different brain areas. Possible aetiological explanations for this finding are discussed. Controlled studies are necessary to confirm or refute this hypothesis.

Pressure pain thresholds asymmetry in left- and right-handers: Associations with behavioural measures of cerebral laterality

Pressure pain threshold (PPT) asymmetry of the left and right third digits was assessed in 12 right-handed and 12 left-handed subjects using an automatised pressure algometer. A clear PPT asymmetry was found in right-handed participants, while left-handed participants revealed no PPT asymmetry. The PPT asymmetry of right-handed participants was due to a reduced PPT or increased pain sensitivity at the left hand. Behavioural laterality tests revealed a right ear or left hemisphere advantage for the processing of verbal material (consonant-vocal syllables) and a left visual field or right hemisphere advantage for the processing of emotional faces in all participants. PPT asymmetry was not associated with cerebral laterality assessed with these tests. We conclude that PPT asymmetry is associated with handedness, but neither PPT asymmetry nor handedness are closely associated with measures of cerebral laterality. Copyright 1999 European Federation of Chapters of the International Association for the Study of Pain.

Right-lateralised central processing for pain of nitroglycerin-induced cluster headache

Recent functional brain imaging studies with positron emission tomography (PET) suggest a preference of the right hemisphere, especially the anterior cingulate cortex (ACC), in affective processing of the clinical pain syndromes. We have investigated the central processing of cluster headache (CH) attacks provoked by sublingual nitroglycerin (NTG). In the cerebrum, provoked CH activated the ACC and the temporopolar region of the right hemisphere in addition to other regions. The regions activated in the ACC (Brodmann area (BA) 24 and 32) are involved in affective/cognitive processing of pain and willed attention. Our study discloses the preferential role of the right hemisphere in attributing emotional valence and attention to the suffering of pain. The findings support the theory of a right hemispheric specialisation in the mediation of withdrawal-related negative affect. The divergence of the distributed central processing between provoked cluster headache attack and experimentally induced acute pain indicates different central mechanisms for different types of pain.

Gender differences in pain ratings and pupil reactions to painful pressure stimuli

In order to investigate gender differences in pain perception, the present study employed both a psychophysical and a psychophysiological measure. In experiment 1, 20 subjects rated the painfulness of 4 different levels of tonic pressure applied to their fingers using a verbally anchored categorization procedure. In general agreement with studies of pain threshold and tolerance, female subjects reported greater pain at high levels of stimulation, with no gender difference being evident at low pressure levels. In experiment 2, 16 different subjects were exposed to the same painful pressure stimuli while measuring their pupil reactions using infrared video pupillometry. The pupil dilations seen during the last 10 sec of the 20-sec pressure application turned out to be a highly significant indicator of pain intensity. When female and male subjects were compared on this measure, a similar divergent pattern as in the psychophysical data emerged, with female subjects showing greater pupil dilations at high pressure levels only. The fact that gender differences in pain perception can be demonstrated using an autonomic indicator of pain that is beyond voluntary control suggests that these differences reflect low-level sensory and/or affective components of pain rather than attitudinal or response-bias factors.

Thresholds for the perception of pressure, sharpness, and mechanically evoked cutaneous pain: effects of laterality and repeated testing

Twenty-four healthy human subjects provided thresholds for their perception of pressure, sharpness, and pain. Mechanical forces were applied to the dorsal surface of the digits with flat-tipped probes of various sizes. Thresholds (expressed as force) increased with increasing probe size, as previously described. There was no evidence of a laterality difference for any of the thresholds. There was a trend for increasing thresholds with repeated testing, but this trend was not statistically significant for the group as a whole. Examination of individual subjects' thresholds over time revealed that 27% showed significant increases in pain threshold over the 15 days of testing. In contrast, only 6% of subjects showed significant increases in sharpness or pressure thresholds over the same period. Thus, whereas most subjects exhibited stable pain thresholds, approximately one-fourth showed significant increases in pain threshold over time. We conclude that for evaluating regional dysesthesia or hemidysesthesia, a right-left difference in pain thresholds will provide a more sensitive and reliable measure than absolute pain threshold.

Neuroendocrine brain asymmetry and physical complaints

It was examined whether there is an association between physical health and alterations of normal brain asymmetry in cerebral control of neuroendocrine activity. In particular, it was studied whether right-handed subjects showing a high amount of physical complaints and recurrent diseases differ in direction and degree of brain asymmetry in cerebral control of cortisol secretion from subjects with only few physical complaints. Lateralized brain activation was achieved by presenting an emotionally aversive film to the subjects' left or right visual hemifield, using a technique for lateralizing visual input that allows prolonged viewing while permitting free ocular scanning. Results revealed a clear association between a high degree of physical complaints and altered brain asymmetry in cerebral control of neuroendocrine activity.

Body site variation of heat pain sensitivity

Thirty-two healthy human subjects provided thresholds for the perception of slight and moderate heat pain. Four body sites were tested bilaterally: thenar eminence of the hand, plantar surface of the foot, dorsolateral forearm, and lateral calf. Thresholds for the glabrous skin of the hand and foot were significantly greater than thresholds for the hairy skin of the arm and leg, the average difference being 1.3 degree C. Laterality was not a statistically significant factor. Thresholds increased progressively over 2-4 weeks of repeated testing, resulting in values averaging 0.6 degree C higher in the later sessions. The difference between moderate and slight pain thresholds averaged 1.1 degree C, and was consistent across body sites and with repeated testing. The threshold values were normally distributed across subjects. Considerable intersubject variability was observed for both slight and moderate pain thresholds, more so on glabrous than on hairy skin sites. In comparison, the distribution of right-left difference values was narrower, demonstrating less intrasubject versus intersubject variability. The highly significant difference in thresholds between glabrous and hairy skin sites demonstrates the importance of skin type for heat pain sensitivity. In contrast, there was no significant difference in heat pain sensitivity between comparable sites on the upper versus lower extremities, or between left and right sides.

On the "absoluteness" of category and magnitude scales of pain

The concept of "absolute scaling" (Zwislocki & Goodman, 1980) implies that direct judgments of sensory magnitude not only reflect the relative positions of the stimuli being judged, but also permit us to assess level differences in sensation. In order to explore this notion for different scaling methods, in the present investigation we compared magnitude estimation with category partitioning, a verbally anchored categorization procedure, in scaling painful pressure stimuli covering different intensity ranges. The results indicate that when the same stimulus range was presented after 1 week, both methods appeared to be highly reliable, with category partitioning faring somewhat better than magnitude estimation. When the stimulus range was unobtrusively changed between sessions, both methods reflected the within-subjects shift in absolute level. When two different sets of subjects judged the slightly different stimulus ranges, both methods resulted in scale values consistent with absolute scaling, though only category partitioning was sensitive enough to differentiate the two stimulus ranges. The results are discussed in the context of different possibilities of anchoring direct scaling methods in order to obtain "absolute" level information.

Post-lumbar puncture headache: the relation between experimental suprathreshold pain sensitivity and a quasi-experimental clinical pain syndrome

The relation between general experimental pain sensitivity and the clinical expression of post-lumbar puncture syndrome (PLPS) was examined in a prospective double-blind study of patients who underwent a lumbar puncture with a 20-gauge spinal needle. In 44 neurology patients randomized pain was induced on the middle phalanges of fingers II-IV of both hands using a tension device on the day before lumbar puncture. Pain intensity was measured using the category sub-dividing procedure. The correspondence between experimental pain stimuli and induced pain intensities could best be described by a logarithmic function. PLPS symptoms were documented in a multi-dimensional quantitative fashion using various scales. Pronounced position-dependent headache occurred in 31% of the patients. There proved to be very significant effects of experimental pain sensitivity on intensity and duration of position-dependent headaches, as well as on vegetative symptoms of PLPS (P less than or equal to 0.01). The results provide an explanation for the varying interindividual manifestation of PLPS symptoms, in addition to factors already identified such as the size of needle used in individual patients.

[Algesimetric experiment on pain sensitivity in the pericranial musculature. Circadian rhythms and corresponding psychic variables.]

Many types of headache that occur in the form of attacks show a notably high incidence at certain times of the day. Attacks are often accompanied by emotional instability, irritability, exhaustion and other impairments of well-being. The cause for the frequent daytime occurrence of attacks is not clear. One possible explanation could be the variability in a person's sensitivity to headache over the course of the day. For this reason we set out to investigate whether pain sensitivity to experimentally-induced headache varies in a circadian manner and whether this variation correlates to changes in the current state of well-being over the course of the day. In 24 healthy, voluntary subjects, headache was induced experimentally at 0200, 0600, 1000, 1400, 1800, and 2200 hours. Blood criculation in both superficial temporal and occipital arteries was reduced by applying a cuff to the head and inflating it to 200 mmHg, thereby producing a continuously increasing, bilateral, dull, frontal headache. The subjects scaled the intensity continually, using the category subdividing procedure ranging from pain threshold to pain tolerance level. In addition, 17 dimensions of the current state of well-being and 12 personality traits were recorded at the above-mentioned times in a standardized quantitative manner. At low levels of headache intensity there were no significant differences in pain sensitivity. Sensitivity to very intense headache, however, varied significantly over the course of the day: sensitivity was greatest at 0200 hours; it decreased at a constant rate until 1400 hours, and increased again continuously until 2200 hours (P</=0.05). Sensitivity to low levels of headache intensity did not prove to be significantly correlated to dimensions of current well-being. By comparison, however, the findings showed significant correlations between sensitivity to very high headache intensities and multiple dimensions of the current state of well-being. These findings show that headache sensitivity follows circadian patterns. Daytime variations, however, are only significant for sensitivity to high intensities. Thus these results may explain, in part, why violent attacks of pain occur predominantly at night and in the early morning.