C- and Adelta-fibre mediated thermal perception: response to rate of temperature change using method of limits

Studies investigating the effect of rate of temperature change on thermal thresholds have used a variety of different methods and threshold combinations, and many display incomplete reporting of statistical analyses. It has been suggested that C- and Adelta-fibre mediated thresholds differ in their reaction to different rates of temperature change. Ten healthy female volunteers (aged 18-26 years; mean 21 +/- S.D. 2.53) undertook cold sensation (CS), warm sensation (WS), cold pain (CP) and heat pain (HP) threshold determinations on the thenar eminence of the dominant hand. Rates of temperature change of 0.5, 1, 2.5 and 4 degrees C/s were used, with a modified method of limits. Adaptation temperature was 32 degrees C and thermode size 3 cm x 3 cm. Results showed a significant increase in WS, HP and CP thresholds with increased rates of temperature change (all p < 0.001), but no significant change for CS (p = 0.653). These results suggest that thresholds with a C-fibre component (WS, HP and CP) and those that are Adelta-fibre mediated (CS) behave differently. A traditional explanation of measurement artefact alone is insufficient in rationalizing these results, with additional factors potentially involved. Slow rates of temperature change were shown to reduce mean intra-individual differences in recorded threshold values, and also to abolish ceiling effects with HP threshold determinations. Clinically, therefore, using slow rates of temperature change with method of limits has a range of benefits over and above simply minimizing measurement artefact.     

C- and Aδ-fibre mediated thermal perception: response to rate of temperature change using method of limits

Studies investigating the effect of rate of temperature change on thermal thresholds have used a variety of different methods and threshold combinations, and many display incomplete reporting of statistical analyses. It has been suggested that C- and A &#105 -fibre mediated thresholds differ in their reaction to different rates of temperature change. Ten healthy female volunteers (aged 18-26 years; mean 21 &#45 S.D. 2.53) undertook cold sensation (CS), warm sensation (WS), cold pain (CP) and heat pain (HP) threshold determinations on the thenar eminence of the dominant hand. Rates of temperature change of 0.5, 1, 2.5 and 4°C/s were used, with a modified method of limits. Adaptation temperature was 32°C and thermode size 3cm &#50 3cm. Results showed a significant increase in WS, HP and CP thresholds with increased rates of temperature change (all p < 0.001), but no significant change for CS ( p = 0.653). These results suggest that thresholds with a C-fibre component (WS, HP and CP) and those that are A &#105 -fibre mediated (CS) behave differently. A traditional explanation of measurement artefact alone is insufficient in rationalizing these results, with additional factors potentially involved. Slow rates of temperature change were shown to reduce mean intra-individual differences in recorded threshold values, and also to abolish ceiling effects with HP threshold determinations. Clinically, therefore, using slow rates of temperature change with method of limits has a range of benefits over and above simply minimizing measurement artefact.     

Sex differences in nociceptive withdrawal reflex and pain perception

Experimentally induced pain often reveals sex differences, with higher pain sensitivity in females. The degree of differences has been shown to depend on the stimulation and assessment methods. Since sex differences in pain develop anywhere along the physiological and psychological components of the nociceptive system, we intended to compare the nociceptive flexion reflex (NFR) as a more physiological (spinal) aspect of pain procession to the verbal pain report of intensity and unpleasantness as the more psychological (cortical) aspect. Twenty female and twenty male healthy university students were investigated by use of nociceptive flexion reflex threshold (staircase method) after electrical stimulation of the N. suralis. Furthermore, we assessed supra-threshold reflex responses (latency, amplitude and area) by applying 10 stimuli 5 mA above reflex threshold. Following each stimulation, the subjects provided pain ratings of intensity and unpleasantness on a visual analogue scale. Females exhibited marked lower nociceptive flexion reflex thresholds than males, while the supra-threshold reflex response tailored to the individual reflex threshold did not show any significant differences. The verbal pain ratings, corrected for NFR threshold, were not found to differ significantly. The large sex differences in nociception that were present in NFR threshold but not in the pain ratings corroborate the hypothesis that spinal processes contribute substantially to sex differences in pain procession.     

Comparison of unpleasant and pain thresholds of thermal stimuli in the orofacial regions: a psychophysical study using quantitative sensory testing in healthy young men

Abstract

Purpose/Aim: To gain a better understanding of the psychophysics of thermal pain perception in a clinical setting, this study investigated whether thermal thresholds of unpleasantness are different from pain thresholds of cold and heat stimuli. Of particular interest was the relationship between unpleasantness and pain thresholds for cold vs heat stimuli.

Material and methods: Thirty healthy male volunteers (mean age 26.1?years, range 23 to 32?years) participated. Thermal detection, cold pain (CPT) and heat pain (HPT) thresholds were measured at 5 trigeminal sites by the method of limits using quantitative sensory testing (QST), followed by cold unpleasant (CUT) and heat unpleasant (HUT) thresholds.

Results: The temperatures at which individuals first reported thermal sensations as unpleasant or painful substantially differed among subjects. CUT exhibited a higher mean value with less variability than CPT, and HUT presented a lower mean than HPT (p?<?.001). As with CPT, CUT did not show any significant difference between the test sites. On the other hand, HUT, like HPT, exhibited site differences (p?<?.001). There was moderate correlation between CUT and CPT, whereas HUT and HPT were strongly correlated. The relationship between unpleasant and pain thresholds of cold vs heat stimuli was significantly different even when controlling for test site variability (p?<?.001).

Conclusion: These findings indicate that unpleasant and pain thresholds to thermal stimuli differ in healthy young men. Of particular note is the distinct relationship of unpleasant and pain thresholds of cold vs heat stimuli, revealing the thermal difference in temperature transition from unpleasantness to pain.     

Spatial summation of heat pain in males and females

Sex differences in pain sensitivity have been found to vary between considerable and negligible. It has appeared that the pain stimulation method is critical in this context. It was assumed this might be due to the different degrees of spatial summation associated with the different pain stimulus modalities. Hence, sex differences were investigated in spatial summation of heat pain in 20 healthy women and 20 healthy men of similar age. Pain thresholds were assessed by a tracking procedure and responses to supra-threshold pain stimulation by numerical ratings. Heat stimuli were administered by a thermode with contact areas of 1, 3, 6 and 10 cm2. Pain thresholds were significantly higher with smaller areas stimulated than with larger ones. No significant effect of area was found for the ratings of the supra-threshold stimuli, the intensities of which were tailored to the individual pain threshold. Consequently, spatial summation of heat pain appeared to result mainly in a shift of the pain threshold on the ordinate and not a change of slope of the stimulus-response function in the pain range. In neither of the two pain parameters were there any sex differences. Therefore, the present study demonstrated that sex differences in spatial summation of heat pain are unlikely.     

Comparing test–retest reliability and magnitude of conditioned pain modulation using different combinations of test and conditioning stimuli

This study aimed to compare the reliability and magnitude of conditioned pain modulation (CPM) by applying different test stimuli (TS) and conditioning stimuli (CS). Twenty-six healthy male participants were recruited in the study of two identical sessions. In each session, four TS (electrical, heat, handheld, and cuff pressure algometry) were applied before and during CS (cold pressor test (CPT) or cuff algometry). The same procedure was repeated with 45-min intervals, but with the other CS. Five thresholds were measured including four pain detection thresholds from four TS and pain tolerance threshold from cuff TS (cuff PTT). Intraclass correlation coefficient (ICC (3,1)) and coefficient of variation (CV) were calculated as measures of reliability. The reliability of TS before and during CS was good for all combinations (ICC: 0.60–0.96, CV: 2.2–22.9%), but the reliability of the CPM effect varied (ICC: 0.04–0.53, CV: 63.6–503.9%). The most reliable combinations were considered to be the handheld pressure pain threshold with CPT (ICC: 0.49, CV: 63.6%) and the cuff pressure pain threshold with CPT (ICC: 0.44, CV: 107.6%). Significant CPM effects were found for all combinations, except the combinations of electrical and heat pain thresholds with cuff CS, which indicates the novel classification of the CPM mechanism. The combinations of handheld pressure and heat pain threshold with CPT would provide the minimum sample size to detect the significant CPM changes in further studies. It is beneficial to provide and compare both ICC and CV to design further clinical trials.     

The influence of rate of temperature change and peak stimulus duration on pain intensity and quality

An important aspect of experimental pain research is that the assessment methods can investigate the different aspects of pain perception. The aim of the present study was to investigate the influence of rate of temperature change and peak stimulus duration on heat evoked pain intensity and quality. All stimuli were applied within the medial aspect of the anterior forearm. The rate of temperature change was varied from 1 to 16 C/s without any effect on the pain threshold. The pain threshold decreased with an increasing peak stimulus duration from 0.1 to 2 s, but not from 2 to 3 s. The pain intensity for suprathreshold stimuli (46 C, 48 C, 50 C) increased for decreasing rates and increasing duration. The pain intensity was highly correlated with the energy of the stimulus. When the rates of temperature change (1-16 C/s) are varied, no differences between pricking and burning pain were present at either low stimulus intensity (46 C) or high stimulus intensity (50 C). At low stimulus intensity (46 C), the pricking pain was not influenced by the duration (0.1-3 s), but the burning pain was intensified when the duration was increased from 1.5 to 3 s. At high intensity stimuli (50 C), the pricking pain intensified with an increased duration, whereas burning pain did not. The heat pain threshold is influenced by the peak stimulus duration, and not by the rate of temperature change. If suprathreshold stimuli are used, both the rate of temperature change and the peak stimulus duration can strongly affect the pain intensity and the pain quality. Therefore, the same stimulus modality can be used to assess the modulation of different pain intensities and of the pricking and burning pain qualities simply by varying the stimulus configuration.     

Sex differences in thermal detection and thermal pain threshold and the thermal grill illusion: a psychophysical study in young volunteers

Background

Sex-related differences in human thermal and pain sensitivity are the subject of controversial discussion. The goal of this study in a large number of subjects was to investigate sex differences in thermal and thermal pain perception and the thermal grill illusion (TGI) as a phenomenon reflecting crosstalk between the thermoreceptive and nociceptive systems. The thermal grill illusion is a sensation of strong, but not necessarily painful, heat often preceded by transient cold upon skin contact with spatially interlaced innocuous warm and cool stimuli.

Methods

The TGI was studied in a group of 78 female and 58 male undergraduate students and was evoked by placing the palm of the right hand on the thermal grill (20/40 °C interleaved stimulus). Sex-related thermal perception was investigated by a retrospective analysis of thermal detection and thermal pain threshold data that had been measured in student laboratory courses over 5 years (776 female and 476 male undergraduate students) using the method of quantitative sensory testing (QST). To analyse correlations between thermal pain sensitivity and the TGI, thermal pain threshold and the TGI were determined in a group of 20 female and 20 male undergraduate students.

Results

The TGI was more pronounced in females than males. Females were more sensitive with respect to thermal detection and thermal pain thresholds. Independent of sex, thermal detection thresholds were dependent on the baseline temperature with a specific progression of an optimum curve for cold detection threshold versus baseline temperature. The distribution of cold pain thresholds was multi-modal and sex-dependent. The more pronounced TGI in females correlated with higher cold sensitivity and cold pain sensitivity in females than in males.

Conclusions

Our finding that thermal detection threshold not only differs between the sexes but is also dependent on the baseline temperature reveals a complex processing of “cold” and “warm” inputs in thermal perception. The results of the TGI experiment support the assumption that sex differences in cold-related thermoreception are responsible for sex differences in the TGI.

     

Test–retest reliability of subjective supra-threshold scaling of multiple pressure-pain sensations among healthy individuals: a study using hydraulic pressure algometry

Abstract

Background: Supra-threshold scaling of multiple pressure-pain sensations involves delivery of varied stimulus intensities, either via stimulus-dependent or response-dependent manner, and recording of subjective pain ratings by participants. The focus of this study was to determine the intra- and inter-session reliability of pain intensity and pain unpleasantness ratings related to pressure-pain thresholds (PPTs) of just noticeable pain (JNP), weak pain (WP) and moderate pain (MP) among healthy individuals.

Methods: Fourteen healthy participants (eight women, six men) participated in three sessions of testing at varied intervals over the course of 72?h. In session one, a multiple random staircase method using hydraulic pressure algometry was used to measure PPT of JNP, WP and MP on thumbnail bed. In session 2, ratings of pain intensity and pain unpleasantness were recorded when stimuli at levels corresponding to PPT of JNP, WP and MP were repeatedly applied before and after 20?min of no intervention.

Results: Interclass correlation coefficient (ICC) values for pain ratings of JNP, WP and MP in intra-session reliability were 0.810, 0.826 and 0.881, respectively, whereas the values were 0.817, 0.792 and 0.910, respectively, for inter-session reliability. ICC values for pain unpleasantness were also highly consistent and repeatable. Temporal summation of pain intensity and pain unpleasantness were not related to the repeated application of pressure stimuli.

Conclusions: The findings indicate that the pain intensity and pain unpleasantness ratings for stimuli at levels equal to the thresholds of JNP, WP and MP have good intra- and inter-session reliability.

Significance: This study showed that both pain intensity and pain unpleasantness of JNP, WP and MP have good intra- and inter-session reliability and agreement. Furthermore, the temporal summation of pain or unpleasantness is not related to repeated application of pressure stimuli.

Abbreviations: JNP: Just noticeable pain; WP: Weak pain; MP: Moderate pain; PPTs: pressure-pain thresholds; HPA: Hydraulic pressure algometry; MRSM: multiple random staircase method     

Site-dependent and subject-related variations in perioral thermal sensitivity

The Marstock method of limits was used to obtain thresholds for detection of cooling, warming, cold pain and heat pain for 34 young adults, upon eight spatially matched sites on the left and right sides of the face, the right ventral forearm and the scalp. Male and female subjects were tested by both a male and a female experimenter. Neither the experimenter nor the gender of the subject individually influenced the thresholds. The thermal thresholds varied greatly across facial sites: sixfold and tenfold for cool and warmth, respectively, from the most sensitive sites on the vermilion to the least sensitive facial site, the preauricular skin. Warm thresholds were 68% higher than cool thresholds, on average, and 12% higher on the left compared to the right side of the face. The mean cold pain threshold increased from 21.0°C on the hairy upper lip to 17.8°C on the preauricular skin. Sites on the upper lip were also most sensitive to noxious heat with pain thresholds of 42–43°C. The scalp was notably insensitive to innocuous and noxious changes in temperature. For the sensations of nonpainful cool and warmth, the more sensitive a site, the less the estimates of the thresholds differed between subjects. In contrast, for heat pain, the more sensitive a site, the more the estimates differed between subjects. Subjects who were relatively more sensitive to cool tended to be relatively more sensitive to warmth. Subjects’ sensitivities to nonpainful cool and warmth were less predictive of their sensitivities to painful cold and heat, respectively. Short-term within-subject variability increased with the magnitude of the thresholds. The lower the threshold, the more similar were repeated measurements of it, within a 5–25?s period.     

Site-dependent and subject-related variations in perioral thermal sensitivity

The Marstock method of limits was used to obtain thresholds for detection of cooling, warming, cold pain and heat pain for 34 young adults, upon eight spatially matched sites on the left and right sides of the face, the right ventral forearm and the scalp. Male and female subjects were tested by both a male and a female experimenter. Neither the experimenter nor the gender of the subject individually influenced the thresholds. The thermal thresholds varied greatly across facial sites: sixfold and tenfold for cool and warmth, respectively, from the most sensitive sites on the vermilion to the least sensitive facial site, the preauricular skin. Warm thresholds were 68% higher than cool thresholds, on average, and 12% higher on the left compared to the right side of the face. The mean cold pain threshold increased from 21.0 degrees C on the hairy upper lip to 17.8 degrees C on the preauricular skin. Sites on the upper lip were also most sensitive to noxious heat with pain thresholds of 42-43 degrees C. The scalp was notably insensitive to innocuous and noxious changes in temperature. For the sensations of nonpainful cool and warmth, the more sensitive a site, the less the estimates of the thresholds differed between subjects. In contrast, for heat pain, the more sensitive a site, the more the estimates differed between subjects. Subjects who were relatively more sensitive to cool tended to be relatively more sensitive to warmth. Subjects' sensitivities to nonpainful cool and warmth were less predictive of their sensitivities to painful cold and heat, respectively. Short-term within-subject variability increased with the magnitude of the thresholds. The lower the threshold, the more similar were repeated measurements of it, within a 5-25 s period.     

Sex hormones, central nervous system and pain

The aim of the present review, which highlights some relationships between sex hormones, the CNS and pain, is to provide reference points for discussion on one of the most intriguing aspects of pain pathophysiology: the presence of sex differences in the response threshold to phasic painful stimuli and in the incidence of chronic pain syndromes. The first part of the review deals with sex steroids and their mechanisms of action. In the second part, the connections between sex steroids, the CNS and pain are illustrated to introduce possible areas of discussion in the study of sex differences in experimental and clinical pain.     

VISUAL HAZE DETECTION THRESHOLD ASSESSMENT BY ASCENDING METHOD OF LIMITS AND A TRANSFORMED STAIRCASE PROCEDURE IN APPLE JUICE SIMULATIONS UNDER " STORE LIKE" CONDITIONS

Visual haze detection thresholds were assessed in thirty panelists by two methods: the ascending method of limits and a transformed staircase procedure (UDTR, from Wetherill and Levitt 1965). The 3AFC test was used as the response procedure for both methods. Experimental samples consisted of an apple juice simulation (water with food coloring). Haze was added to the samples using formazin, and was calibrated to eleven levels ranging from 0.75–13.30NTU. Haze detection thresholds under store-like conditions (3.55 ± 0.57 [mean ± S.E.] and 3.16 ± 0.50 NTU by the two methods, respectively) were higher than those found previously under more controlled viewing conditions. No differences were found between methods, genders, or order of method presentation ( p > 0.05); a practice effect showed a slight improvement in detection thresholds between sessions ( p = 0.05). Nonparametric tests confirmed the similarities in the threshold estimates from the two methods. The two methods estimated visual haze detection thresholds equally well. However, the ascending method of limits may be preferred method due to the shorter time involved in administration.     

Pain Intensity Recognition Rates via Biopotential Feature Patterns with Support Vector Machines

Background

The clinically used methods of pain diagnosis do not allow for objective and robust measurement, and physicians must rely on the patient’s report on the pain sensation. Verbal scales, visual analog scales (VAS) or numeric rating scales (NRS) count among the most common tools, which are restricted to patients with normal mental abilities. There also exist instruments for pain assessment in people with verbal and / or cognitive impairments and instruments for pain assessment in people who are sedated and automated ventilated. However, all these diagnostic methods either have limited reliability and validity or are very time-consuming. In contrast, biopotentials can be automatically analyzed with machine learning algorithms to provide a surrogate measure of pain intensity.

Methods

In this context, we created a database of biopotentials to advance an automated pain recognition system, determine its theoretical testing quality, and optimize its performance. Eighty-five participants were subjected to painful heat stimuli (baseline, pain threshold, two intermediate thresholds, and pain tolerance threshold) under controlled conditions and the signals of electromyography, skin conductance level, and electrocardiography were collected. A total of 159 features were extracted from the mathematical groupings of amplitude, frequency, stationarity, entropy, linearity, variability, and similarity.

Results

We achieved classification rates of 90.94% for baseline vs. pain tolerance threshold and 79.29% for baseline vs. pain threshold. The most selected pain features stemmed from the amplitude and similarity group and were derived from facial electromyography.

Conclusion

The machine learning measurement of pain in patients could provide valuable information for a clinical team and thus support the treatment assessment.     

Pain sensations to the cold pressor test in normally menstruating women: comparison with men and relation to menstrual phase and serum sex steroid levels

The role of gonadal hormones on pain sensations was investigated in normally menstruating women (n = 16) using the cold pressor test. Tolerance time, pain threshold, and pain intensity were examined once a week during a 4-wk period, and serum concentrations of 17beta-estradiol and progesterone were determined at each test session, which were classified into the early follicular phase, late follicular phase, early luteal phase, and late luteal phase, as determined by the first day of menses and the actual hormone levels recorded. A group of men (n = 10) of the same age interval was examined for comparison. The data show that pain threshold was reduced during the late luteal phase compared with the late follicular phase, and hormone analyses showed significant positive correlation between the progesterone concentration and lowered pain threshold and increasing pain intensity. Hormone analysis also showed an interaction between S-estradiol and S-progesterone on pain intensity, demonstrating that the increased perceived pain intensity that was associated with high progesterone concentrations was significantly reduced with increasing levels of estradiol. While no statistically significant sex differences in pain measurements were found, women displayed much more pronounced, and statistically significant, session-to-session effects than men, with increased pain threshold and decreased pain intensity with each test session. Hence, these data suggest that the changes in the serum concentration of gonadal hormones that occur during the menstrual cycle influence pain sensations elicited by noxious tonic cold stimulation and show that adaptation to the cold pressor test may be sex dependent.     

Reliability study of thermal quantitative sensory testing in healthy Chinese

Background: Test–retest reliability is important to establish for any diagnostic tool. The reliability of quantitative sensory testing (QST) in the trigeminal region has recently been described in Caucasians as well as differences in absolute thresholds and responses between Caucasians and Chinese. However, the test–retest reliability has not been determined in a Chinese population.

Objective: To provide novel information on the test–retest reliability of thermal QST in the trigeminal and spinal system in healthy Chinese.

Methods: Twenty healthy volunteers (10 women and 10 men) participated. Cold detection threshold (CDT), warm detection threshold (WDT), cold pain threshold (CPT), and heat pain threshold (HPT) were measured at two sites: the surface of the left hand and the left masseter. The testing was performed over three consecutive stimuli trials, three sessions conducted on one day and repeated one week later. Data were analyzed with intra-tester reliability test and four-way analysis of variance (ANOVA) for repeated measures.

Results: There was a tendency for the first trial in CDT (p?=?0.005), CPT (p?=?0.02), and HPT (p?p?=?0.003) and HPT (p?=?0.045) with higher sensitivity at the masseter muscle. There were significant gender differences with higher sensitivity in women for CPT (p?=?0.001) and HPT (p?=?0.001).

Conclusion: Test site and gender affect thermal thresholds substantially. The test–retest reliability of most thermal threshold measures were acceptable for assessing somatosensory function, however, innocuous thresholds appear to be associated with larger variability than noxious thresholds in a Chinese population.     

Systemic Inflammation Decreases Pain Threshold in Humans In Vivo

Background

Hyperalgesia is a well recognized hallmark of disease. Pro-inflammatory cytokines have been suggested to be mainly responsible, but human data are scarce. Changes in pain threshold during systemic inflammation evoked by human endotoxemia, were evaluated with three quantitative sensory testing methods.

Methods and Results

Pressure pain thresholds, electrical pain thresholds and tolerance to the cold pressor test were measured before and 2 hours after the intravenous administration of 2 ng/kg purified E. coli endotoxin in 27 healthy volunteers. Another 20 subjects not exposed to endotoxemia served as controls. Endotoxemia led to a rise in body temperature and inflammatory symptom scores and a rise in plasma TNF-α, IL-6, IL-10 and IL-1RA. During endotoxemia, pressure pain thresholds and electrical pain thresholds were reduced with 20±4 % and 13±3 %, respectively. In controls only a minor decrease in pressure pain thresholds (7±3 %) and no change in electrical pain thresholds occurred. Endotoxin-treated subjects experienced more pain during the cold pressor test, and fewer subjects were able to complete the cold pressor test measurement, while in controls the cold pressor test results were not altered. Peak levels and area under curves of each individual cytokine did not correlate to a change in pain threshold measured by one of the applied quantitative sensory testing techniques.

Conclusions and Significance

In conclusion, this study shows that systemic inflammation elicited by the administration of endotoxin to humans, results in lowering of the pain threshold measured by 3 quantitative sensory testing techniques. The current work provides additional evidence that systemic inflammation is accompanied by changes in pain perception.     

Association between ABO blood types and pain perception

Aim of the study: Pain perception is associated with different phenotypic characteristics such as sex, eye, and hair color. Hence, it is assumed that ABO blood type can also affect pain perception.

Materials and methods: In order to investigate this hypothesis, an experimental study with healthy volunteers (18–40?years) was designed. The experimental procedure included a blood type test and two rounds of pressure pain threshold assessments separated by a cold pressor test. Pressure pain threshold was assessed bilaterally at the temporalis, masseter, and deltoid muscles, where the muscle sites were randomized. Cold pressor test was conducted by immersion of participants’ non-dominant hand into iced water of 1–4?°C for 2?min.

Results: Thirty-seven healthy volunteers, distributed in the four blood type groups, completed the study. Participants with blood type B scored the highest pressure pain thresholds at the examined craniofacial muscles, while participants with blood type AB tended to score the lowest. Furthermore, participants with blood type AB displayed the highest elevation in pressure pain thresholds after cold pressor test.

Conclusions: Participants with blood type B displayed the lowest mechanical pain sensitivity and the blood type AB group exhibited the strongest conditioned pain modulation effect. These findings emphasize the necessity of considering ABO blood types in future pain research.     

Influence of surface anesthesia on the pressure pain threshold measured with different-sized probes

Transcutaneous pressure with pressure probes of arbitrary diameters have been commonly used for measuring the threshold and magnitude of muscle pain, yet this procedure lacks scientific validation. To examine the valid probe dimensions, we conducted physiological experiments using 34 human subjects. Pin-prick pain, pressure pain threshold (PPT) to pressure probes of various diameters, heat pain threshold, and electrical pain threshold of deep tissues were measured before and after application of surface lidocaine anesthesia to the skin surface over the brachioradial muscle in a double-blinded manner. The anesthesia neither affected PPT with larger probes (diameters: 1.6 and 15?mm) nor increased electric pain threshold of deep structures, whereas it diminished pain count in pin-prick test and PPT with a 1.0?mm diameter probe, suggesting that mechanical pain thresholds measured with 1.6 and 15?mm probes reflect the pain threshold of deep tissues, possibly muscle. Pain thresholds to heat did not change after application of the anesthesia. These results suggest that larger pressure probes can give a better estimation of muscular pain threshold.