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Ethnic Differences Regarding Tactile and Pain Thresholds in the Trigeminal Region
The Journal of Pain, Vol 8, No 4 (April), 2007: pp 363-369 Available online at www.sciencedirect.com
Ethnic Differences Regarding Tactile and Pain Thresholds in the Trigeminal Region Osamu Komiyama,* Misao Kawara,* and Antoon De Laat† *Department of Clinical Oral Physiology, Nihon University School of Dentistry at Matsudo, Sakaecho-nishi Matsudo, Chiba, Japan. † Department of Oral and Maxillofacial Surgery, School of Dentistry, Oral Pathology and Maxillo-Facial Surgery, Catholic University of Leuven, Leuven, Belgium.
Abstract: The aim of the present study was to evaluate ethnic differences of the tactile detection threshold (TDT), the filament-prick pain detection threshold (FPT), the pressure pain detection threshold (PPT), and the pressure pain tolerance detection threshold (PTOL) in the orofacial region of symptom-free subjects. Twenty-two men and 22 women in Belgium and in Japan (age range from 20 to 31 years) participated. The TDT and the FPT were measured on the cheek skin (CS) overlying the masseter muscles (MM), on the maxillary gingiva (MG), and at the tip of the tongue (TT), using Semmes-Weinstein monofilaments. The PPT and PTOL were measured at the central part of the MM, using a pressure algometer. A general linear model was used in each case to capture ethnic and gender effects. Japanese women had the lowest TDT at CS, in contrast to Belgian men, who had the highest value; a significant ethnic and gender effect was found (P ⴝ .026 and P < .001, respectively). Similar results were found for FPT at CS with significant ethnic and gender effects (P < .001 for both). There was no significant ethnic effect regarding intra-oral TDT and FPT or regarding PPT and PTOL. Perspective: Our findings clearly indicate that future studies of tactile and pain measurements need to standardize and control for gender and ethnicity. Further, a comprehensive evaluation of results from various stimulation modalities may better clarify the pain mechanisms and gender/ethnic characteristics, as well as comparisons between normal subjects and patients. © 2007 by the American Pain Society Key words: Ethnic difference, tactile detection threshold, filament-prick pain detection threshold, pressure pain threshold, pressure pain tolerance detection threshold, gender.
D
ifferent consultation rates for painful conditions, a different pain report, disability score, and quality of life have been observed for different ethnic groups.4,9,15,16,26,32,35,36,41,44 In experimental studies, ethnicity has been demonstrated to be an important variable in determining response to painful stimuli.8,17 Zatzick and Dimsdale43 concluded that Caucasians generally demonstrate higher threshold and endurance times for experimental pain stimuli. Watson et al39 showed that ethnicity plays an important role in heat pain threshold and pain report: South Asian men demReceived June 21, 2006; Accepted October 25, 2006. Supported by a 2006 Collaborative Study Grant of the International Association for the Study of Pain (IASP) and a Nihon University Individual Research Grant for 2006. Address reprint requests to Dr. Antoon De Laat, Department of Oral and Maxillofacial Surgery, School of Dentistry, Oral Pathology and MaxilloFacial Surgery, Catholic University of Leuven, Kapucijnenvoer 7, B-3000 Leuven, Belgium. E-mail: [email protected] 1526-5900/$32.00 © 2007 by the American Pain Society doi:10.1016/j.jpain.2006.12.002
onstrated lower pain thresholds and higher pain report when compared with matched white British men. Campbell et al3 examined ethnic differences in response to multiple experimental pain stimuli, including heat pain, cold pressor pain, and ischemic pain, and demonstrated differences in laboratory pain responses between African Americans and whites across multiple stimulus modalities. The effect sizes for these differences in pain tolerance were moderate to large for suprathreshold measures. In the oral and facial region, the tactile detection threshold and the filament-prick pain threshold have been evaluated with Semmes-Weinstein monofilaments, and the pressure pain threshold and the pressure pain tolerance detection threshold have been evaluated with the use of a pressure algometer.7,14,19,29 Komiyama and De Laat24 reported that women showed a significantly lower tactile detection threshold and filament-prick pain threshold at the cheek skin in comparison to men. Further, measurements of pressure pain threshold and pres363
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sure pain tolerance detection threshold confirmed the previously reported higher thresholds in men. One of the consequences of globalization and migration worldwide is that patients presenting in the clinical situation are of a varying ethnicity. Although numerous studies have reported ethnic differences in the prevalence and severity of clinical pain, little is known about how ethnicity affects the perception of experimental pain. In addition, it is not known whether sensory and mechanical pain thresholds in the orofacial region differ between ethnic groups if determined by using identical stimuli. Consequently, the present study was conducted to evaluate ethnic and gender differences in tactile detection threshold, filament-prick pain detection threshold, pressure pain threshold, and pressure pain tolerance detection threshold at multiple measuring points in the orofacial region of symptom-free subjects.
Methods Subjects Subjects were recruited from university students and staff. Twenty-two men and 22 women (age range from 20 to 31 years) (Table 1) participated in both Belgium (Belgian Caucasian subjects) and Japan (Japanese subjects). Ethnicity was determined by the interview of their genetic roots. All were asymptomatic for pain in the
head and neck region. This was defined as absence of jaw dysfunction and headaches and absence of subjective pain or soreness of the masticatory muscles. Subjects also could not participate when they were currently taking medication or received other treatment that could not be interrupted for the study, if general health problems (eg, metabolic disease, neurological disorders, vascular disease, and so forth) or periodontal disease was present, or in case of a history of drug abuse or recent facial or cervical trauma. Since a previous study20 indicated that pain thresholds were lower in the menstrual phase, women were not tested during their menstrual phase, and smokers were excluded to avoid altered intra-oral perception.6 The subjects were informed about the study in a standardized way, and they signed an informed consent form. The institutional ethics committee approved the study (ML2569 in Belgium and EC05015 in Japan). The 2 ethnic groups were tested in 2 different laboratories (1 in Japan and 1 in Belgium) by the same Japanese experimenter, using written instruction boards in Flemish for the Belgian subjects and in Japanese for the Japanese subjects to guarantee the comparability of the instructions for the test procedures. All measurements were taken in a single quiet room per location, in which temperature and humidity were kept stable.1
Descriptive Data (Mean ⴞ Standard Deviation) and Results of Statistical Analysis (P value) of Subject Characteristics and Measurements
Table 1.
ETHNICITY Gender Number Age (years) Height (cm) Weight (kg) Measurement Tactile detection threshold (TDT)
JAPANESE Male 22 24.7 ⫾ 3.6 170.1 ⫾ 5.6 63.3 ⫾ 8.6
Female 22 25.0 ⫾ 3.6 162.3 ⫾ 5.6 53.5 ⫾ 6.3
BELGIUM Male 22 24.4 ⫾ 3.1 181.3 ⫾ 8.3 73.8 ⫾ 11.8
Female 22 23.9 ⫾ 3.6 168.5 ⫾ 7.0 57.5 ⫾ 5.6
STATISTICAL ANALYSIS Effect Gender Ethnicity Interaction .879 .346 .604 <.001 <.001 .085 <.001 <.001 .078
Site Cheek skin 2.35 ⫾ 0.23 1.91 ⫾ 0.15 2.49 ⫾ 0.15 2.07 ⫾ 0.29 .026 Maxillary gingiva 3.65 ⫾ 0.26 3.56 ⫾ 0.25 3.62 ⫾ 0.33 3.41 ⫾ 0.33 .221 Tongue tip 1.89 ⫾ 0.20 1.77 ⫾ 0.09 1.91 ⫾ 0.17 1.87 ⫾ 0.18 .088 Thenar skin 2.60 ⫾ 0.22 2.31 ⫾ 0.25* 2.74 ⫾ 0.17 2.63 ⫾ 0.22 .052 Filament-prick pain Cheek skin 5.38 ⫾ 0.35* 4.86 ⫾ 0.39 5.91 ⫾ 0.30 5.17 ⫾ 0.45 <.001 detection threshold Maxillary gingiva 5.40 ⫾ 0.26 5.15 ⫾ 0.24 5.42 ⫾ 0.22 5.25 ⫾ 0.29 .025 (FPT) Tongue tip 4.87 ⫾ 0.38 4.67 ⫾ 0.32 5.11 ⫾ 0.26 4.97 ⫾ 0.23 .053 Thenar skin 5.60 ⫾ 0.49 4.99 ⫾ 0.46 5.94 ⫾ 0.31 5.37 ⫾ 0.48 <.001 Numeric rating scale Cheek skin 1.8 ⫾ 1.1 1.3 ⫾ 0.7 2.1 ⫾ 1.3 1.9 ⫾ 1.0 .300 value (NRS) for FPT Maxillary gingiva 2.2 ⫾ 1.4 1.1 ⫾ 0.7 2.9 ⫾ 0.9 2.3 ⫾ 1.1 .003 Tongue tip 2.4 ⫾ 1.5 1.2 ⫾ 0.8 3.0 ⫾ 0.9 2.1 ⫾ 1.1 .009 Thenar skin 1.8 ⫾ 1.1 1.0 ⫾ 0.8 2.2 ⫾ 1.1 1.7 ⫾ 1.1 .006 Pressure pain threshold Masseter muscle 186.5 ⫾ 52.2 142.1 ⫾ 37.6 197.6 ⫾ 63.2 140.6 ⫾ 38.7 .002 (PPT) Thenar muscle 331.6 ⫾ 86.4 285.0 ⫾ 66.6 394.6 ⫾ 136.2 254.0 ⫾ 77.4 .002 NRS for PPT Masseter muscle 2.7 ⫾ 1.4 2.0 ⫾ 1.2 3.4 ⫾ 1.0 2.7 ⫾ 0.9 .024 Thenar muscle 1.9 ⫾ 1.1 1.6 ⫾ 1.1 2.6 ⫾ 0.8 2.2 ⫾ 0.9 .123 Pressure pain tolerance Masseter muscle 401.0 ⫾ 81.9 265.8 ⫾ 62.3 434.1 ⫾ 99.6 301.0 ⫾ 65.8 <.001 threshold (PTOL) NRS for PTOL Masseter muscle 5.9 ⫾ 2.0 5.2 ⫾ 1.9 7.1 ⫾ 1.1 5.9 ⫾ 1.0 .060
Values of TDT and FPT are expressed as the number of the filament; values of PPT and PTOL are in kilopascals. Bold values indicate statistical significance as calculated using a general linear model (P ⬍ .05). *P ⬍ .05 compared between Japanese and Belgian subjects within the same sex, using Bonferroni correction for multiple comparisons.
.661 .410 .301 .031 .267 .593 .673 .885 .432 .328 .421 .701 .670 .030 .971
<.001 .143 .224 .003 <.001 .683 .001 .003 .065 .001 .056 .008 .599 .734 .011 .006 .223
.667
.019
.620
ORIGINAL REPORT/Komiyama et al
Tactile Detection Threshold and Filament-Prick Pain Detection Threshold The tactile detection threshold (TDT) and the filamentprick pain threshold (FPT) were measured (1) on the cheek skin overlying the central part of the left and right masseter muscles midway between the upper and lower borders and 1 cm posterior to the anterior border, (2) on the right and left maxillary gingival at a point 5 mm cranial to the line connecting the cervical rim of the canine and the lateral incisor, (3) at a point 5 mm proximal to the anterior tip of the tongue on the midline, and (4) on the skin overlying the palm side of the thenar muscle on the point connecting the longitudinal axis of the thumb and index finger (“thenar skin”). The sequence of the measurement sites was randomized in TDT and FPT measurement series. Semmes-Weinstein monofilaments with 20 different diameters were used (Premier Products, Kent, WA). The numbers of the filaments (1.65 to 6.65) correspond to a logarithmic function of the equivalent forces of 0.0045 to 447 grams. At first, TDT was examined by means of a staircase method. The subjects were instructed to close their eyes during the whole test procedure and to raise their hand as soon as they felt the stimulus in the test site. The filament was applied vertically on the test site and slowly pressure was applied until the filament bowed. The time needed to bow the filament was standardized to approximately 1.5 seconds. The stimulus was maintained for approximately 1.5 seconds and then removed in 1.5 seconds. Quick applications and bouncing of the filaments against the skin were avoided. At each site, the test started with the number (No.) 4.31 filament. If the subject raised his or her hand, it was considered a positive response, and the next filament applied was one step lower (No. 4.17). This procedure was repeated with decreased filament diameters until the subject no longer felt the pressure. This was considered as a negative answer. Again, the filament with a higher pressure was applied. This procedure continued until eight positive and eight negative peaks were recorded and the threshold (TDT) was calculated as the average of these values (number of the filament). If the subject still had a positive response while applying the lowest fiber (No. 1.65), this filament was considered the threshold. Two “null” (placebo) trials were performed after 5th and 11th peaks. During these control trials, the filament did not make contact with the tissue. If the subject did not report a sensation during the blank stimuli, the test was continued. If he or she did report a sensation, the test was discontinued and the subject was questioned about what kind of stimulus was perceived. The whole procedure was explained again to the subject, and afterward, the test was restarted.24 After the TDT measurements, the FPT was examined. The stimuli were applied in the same way as for the TDT, but the subjects were instructed to open their eyes to avoid fear of a painful stimulation and to raise their hand as soon as they felt not only pressure but also pain in the test area. If the subject had no positive response for
365 the thickest fiber (No. 6.65), this number was recorded as the threshold. No placebo stimulus was applied. There was a time lag of 3 minutes between the measurements on a similar site to avoid sensitization. Furthermore, after the examination of each measurement site, each subject was asked to rate the pain intensity experienced at the FPT on a numeric rating scale (NRS) where 0 cm indicated “no pain” and 10 cm indicated “worst pain imaginable.”
Pressure Pain Threshold and Pressure Pain Tolerance Detection Threshold A pressure algometer (Somedic, Hörby, Sweden) was used to test the sensitivity to stimuli applied to the masseter muscles. The pressure pain threshold (PPT) was defined as the amount of pressure (in kilopascals) that the subject first perceived to be painful.38 The PPT was determined with a constant application rate of 30 kPa/s and a probe diameter of 1 cm. The subject pushed a button to stop the pressure stimulation when the threshold was reached. These measurements were done at least 5 minutes after the FPT measurements. Measuring points were (1) the central part of the right and left masseter muscles midway between the upper and lower borders and 1 cm posterior to the anterior border and (2) the palm side of the thenar muscle on the point connecting the longitudinal axis of the thumb and index finger (“thenar muscle”). These test points were identical to the ones used for measuring TDT and FPT. At the start of each session, the subjects were familiarized with the measurement procedure and the equipment through a demonstration on the right forearm, and they were instructed to keep their teeth slightly apart to avoid contraction of the jawclosing muscles during the stimulation. To avoid movement of the head, it was stabilized to the head support of the chair. The PPT measurements were done 3 times at each of 3 measuring points (right and left masseter muscle, thenar muscle), and the sequence of the measuring points in each series was randomized by using a computer-generated list. There was a time lag of 2 minutes between the series of measurements. The mean value of the 3 measurements was used for further statistical analysis. The pressure pain tolerance detection threshold (PTOL) was defined as the pressure producing the maximum amount of pain the subjects were willing or able to accept. The PTOL was measured in the same way as the PPT. However, the test was applied just once at the end of the whole experimental session and only on the masseter muscles, and there was a time lag of 5 minutes between the right and left measurements to avoid sensitization. The sequence of the measurement sides was randomized. After each examination, the average pain intensity during the PPT measurement and the PTOL measurement was assessed on an NRS, where 0 cm indicated “no pain” and 10 cm indicated “the worst pain imaginable.”
Statistical Analysis Descriptive statistics were used to summarize all measurements. Since there was no side difference in a previ-
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Ethnic Differences of Tactile and Pain Thresholds 24
ous similar study, the values for cheek skin and maxillary gingiva on TDT and FPT and for masseter muscle on PPT and PTOL were used as mean values of the left and right sides. The mean values and standard deviations of TDT, FPT, PPT, PTOL, and NRS in each sex, each ethnic group, and each test area were calculated. Two-way ANOVA was used to analyze the gender and ethnic differences in age, weight, and height. To obtain a normal distribution, the natural logarithm of TDT and PPT values, the square root of the PTOL values, and logit (log(x/(1⫺x))) transformation of NRS values were used as a response variable. The response variables were assumed to be continuous, and a general linear model was used in each case to capture the ethnic and gender effects. Moreover, age, weight, height, and NRS values were considered for an adequate fit of the covariance matrix. The Bonferroni method was used to adjust probability values for multiple comparisons within the same gender or ethnic group. The significance level for each test was set to ␣ ⫽ 5%. All analyses were performed within the SPSS version 12 for Windows (SPSS, Chicago, IL).
Results Table 1 shows the descriptive data of the subject characteristics and measurements as well as the probability values resulting from the statistical analysis to summarize the data. There was no significant age difference between sexes or ethnic groups. There were, by contrast, significant differences between the sexes and ethnic groups regarding height and weight (ANOVA, P ⬍ .001).
Tactile Detection Threshold Regarding the TDT at cheek skin, significant effects were found for ethnic group (P ⬍ .026) and sex (P ⬍ .001).There was no significant age, height, or weight effect. No significant ethnic and gender effects were found at the maxillary gingiva or the tongue tip. A significant ethnic effect (P ⫽ .003) was found for the TDT at thenar skin, but there were no significant age, height, weight, or gender effects. There was no interaction between ethnic and gender effect at all measurement sites. Multiple comparison showed, within the same sex, that Japanese women had a lower TDT at thenar skin compared with Belgian women (Bonferroni: P ⫽ .004).
Filament-Prick Pain Threshold As for FPT at cheek skin, significant gender (P ⬍ .001) and ethnic (P ⬍ .001) effects were apparent. No significant age, height, weight, or NRS effect were observed. Regarding the FPT at the maxillary gingiva, a significant gender effect (P ⫽ .025) but no significant ethnic effect was observed. By contrast, a significant ethnic effect (P ⫽ 0.001) but no gender effect was seen at the tongue tip. Significant gender (P ⬍ 0.001) and ethnic (P ⫽ 0.003) effects were found at thenar skin. For all measurement sites, there was no interaction between the ethnic and gender effect. Multiple comparison showed, within the same
sex, that Japanese men had a lower FPT at the cheek skin in comparison to Belgian men (Bonferroni P ⫽ .002). Regarding the NRS values for FPT at cheek skin, no significant ethnic, gender, age, height, or weight effects were present. Significant ethnic (P ⫽ .003) and gender (P ⫽ .001) effects were found in NRS values for FPT at the maxillary gingival. No effects of age, height, or weight were present. Also at the tongue tip, a significant gender effect (P ⫽ .009) was found, but no effects were found for ethnic group, age, height, weight, or any interaction. At the thenar skin, significant gender (P ⫽ .006) and ethnic effects (P ⫽ .008) were found, but no effects were found for age, height, weight, or any interaction.
Pressure Pain Threshold For the masseter muscle, the analysis revealed no significant ethnic, age, height, weight, or NRS effect. The only significant effect was gender (P ⫽ .002). Similarly, for thenar muscle, the only difference was gender (P ⫽ .002), and no interaction between ethnic and gender effect was observed. Regarding the NRS value for PPT at the masseter muscle, significant ethnic and gender effects could be found (P ⫽ .024 and P ⫽ .011, respectively); no age, height or weight effect, or interaction between gender and ethnic group was present. At the thenar muscle, only a significant ethnic effect (P ⫽ .006) was found.
Pressure Pain Tolerance Detection Threshold Only the gender effect appeared to be significant (P ⬍ .001). There were no significant ethnic, age, height, weight, or NRS effects nor an interaction between ethnic and gender effects. The NRS value for PTOL was significantly different between the ethnic groups (P ⫽ .019), but no gender, age, height, or weight effects were found. There was also no interaction between ethnic and gender effect.
Discussion The present study characterized significant ethnic and gender effects for quantitative sensory measurements. Overall, women were more sensitive compared with men, and Japanese subjects also were more sensitive compared with the Belgian subjects after intra-oral and extra-oral stimulation by means of various modalities. However, regarding the NRS evaluation for painful stimulation, Japanese had a lower value compared with the Belgians, and men had a higher value compared with women. Regarding the TDT at cheek skin, significant effects were found for ethnic group and sex. A significant ethnic effect was also found for the TDT at thenar skin. Multiple comparisons showed, within the same sex, that Japanese women had a lower TDT at thenar skin compared with Belgian women. The Semmes-Weinstein monofilaments used for determining the TDT in the present study caused a minute indentation of the skin during the measurement. The force required to produce a certain amplitude or gradient of this skin displacement varies with the stiff-
ORIGINAL REPORT/Komiyama et al 21
ness of the tissue. Female and male skin may vary regarding physical properties. It has been pointed out that female skin appears to have a higher elasticity and extensibility.5,25 Although no apparent difference in epidermal innervation was observed between European Caucasian and Japanese/Chinese skin at the architectural or biochemical level,34 other studies suggested differences of skin properties between different ethnic groups40 that may in part explain the observed TDT differences between the sexes and the ethnics. Further studies need to clarify these issues. As for FPT at cheek skin, tongue tip, and thenar skin, a significant ethnic effect was apparent. Multiple comparisons showed, within the same sex, that Japanese men had a lower FPT at the cheek skin than Belgian men. Campbell et al3 also reported that no ethnic group differences emerged for threshold measures, but African Americans had a lower tolerance for heat pain, cold pressor pain, and ischemic pain in comparison to whites. Watson et al39 reported that South Asians demonstrated a lower cold pain thresholds and reported more unpleasantness at all temperatures. In addition, Gazerani and Arendt-Nielsen13 showed significantly greater pain responses in South Indians compared with Caucasians after capsaicin injection to the forehead skin. Our resent results, using mechanical sensory and pain thresholds, support these previous findings39 on the difference in response to experimental pain stimuli, with Caucasians generally demonstrating higher thresholds and endurance times compared with Asians and Africans. Previous studies have shown that darker skin types have different barrier protective effects, which may alter pain perception.33 The epidermis and dermis are innervated with nociceptive A delta and C fibers, which transmit sensations of pain, temperature, and touch. There is compelling evidence that epidermal receptors with specialized physiological properties exist and generate activity in afferent peripheral nerve fibers when stimulated.22,28 Ethnic groups with different skin types may vary with respect to these epidermal receptors.42 This may explain in part the observed differences in pain sensitivity between the 2 ethnic groups in this study. A significant ethnic effect was also observed in the NRS values for FPT at the maxillary gingiva and the thenar skin and at the cheek skin. Also regarding the NRS value for PPT and PTOL, significant ethnic effects could be found. From these findings, it is clear that Japanese reported a lower NRS value compared with Belgians when a painful stimulation was given, which could reflect a different response or attitude to the word “pain,” based on the different cultures. Hobara18 reported that Japanese of both sexes were less prone to overt pain expression in comparison to Euro-Americans. These findings reflect the traditional Japanese cultural emphasis on stoicism and the desirability of concealing pain and emotions, whereas the Euro-American culture puts a greater emphasis on the expression of personal feelings. The present results are similar to the findings of Nayak et al27 regarding cultural differences between participants in the United States and India. It is possible that the tradi-
367 tional stoicism, a well-known characteristic of many Asian cultures, may result in greater acceptance of pain and enhanced coping skills.2 It is very important for pain professionals to be aware of how patients with different cultural backgrounds experience and express pain. For FPT, PPT, and PTOL, significant gender effects were apparent. The variability of the threshold depended on the measurement place, and this was in line with our previous study,24 as were the observed gender differences. Women may be consequently considered more sensitive compared with men regardless of their ethnicity. Prior research of pain thresholds using other forms of noxious stimulation suggest that a variety of factors may contribute, including hormonal alterations,11,31 blood pressure,10 and psychological factors.11 In addition, other investigators have reported that sex role expectancies30 and anxiety may moderate gender differences.12 Kim et al23 evaluated genetic influences on variability in human pain sensitivity associated with gender, ethnicity, and temperament. Their observations demonstrate that gender, ethnicity, and temperament contribute to individual variation in thermal and cold pain sensitivity by interactions with loci in the vanilloid receptor subtype 1 gene and opioid receptor subtype 1 gene single nucleotide polymorphisms. Our results on ethnic and gender differences in this study could also possibly be explained by these gene differences. All data were collected by the same Japanese male investigator. Zatzick and Dimsdale43 suggested that the race and sex of the investigator might influence the outcome of laboratory studies. Edwards and Fillingim,8 however, found no evidence that the sex and ethnicity of the investigator influenced the outcome. From the present study design, we cannot conclude whether ethnicity of the experimenter influenced the results. In the present study, the TDT, FPT, PPT, and PTOL at multiple measuring points in the orofacial region of normal subjects were evaluated and ethnic and gender differences were characterized. The number of subjects was limited to 88, distributed over the 4 groups, which means that some of the nonsignificant findings may have resulted from inadequate power. Also, most subjects were recruited from university staff and students, possibly creating a bias toward a distinct societal category.37 Nevertheless, our findings clearly indicate that future studies of tactile and pain measurements in general need to standardize and control for gender and ethnicity. These results therefore contribute to the evaluation of patients with atypical odontalgia, allodynia in the trigeminal region, and other orofacial pain. Further, a comprehensive evaluation and the combination of results from various stimulation modalities may better clarify the pain mechanisms and gender/ethnic characteristics, as well as comparisons between normal subjects and patients.14
Acknowledgments The authors thank Dr. Kris Bogaerts and Mr. Dimitris Rizopoulos, Biostatistical Center, School of Public Health,
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Catholic University of Leuven, Leuven, Belgium, for the statistical analysis and advice, and Dr. Hans Isselee,
Department of Rehabilitation, St Jan Hospital, Bruges, Belgium, for the use of the algometer.
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ORIGINAL REPORT/Komiyama et al 34. Reilley DM, Ferdinando D, Johnston C, Shaw C, Buchanan KD, Green MR: The epidermal nerve fibre network: Characterization of nerve fibres in human skin by confocal microscopy and assessment of racial variations. Br J Dermatol 137:163-170, 1997 35. Riley JL III, Wade JB, Myers CD, Sheffield D, Papas RK, Price DD: Racial/ethnic differences in the experience of chronic pain. Pain 100:291-298, 2002 36. Riley JL III, Gilbert GH, Heft MW: Race/ethnic differences in health care use for orofacial pain among older adults. Pain 100:119-130, 2002 37. Rollman GB: The need for ecological validity in studies of pain and ethnicity. Pain 113:3-4, 2005 38. Svensson P, Arendt-Nielsen L, Nielsen H, Larsen JK: Effect of chronic and experimental jaw muscle pain on painpressure thresholds and stimulus-response curves. J Orofac Pain 9:347-356, 1995 39. Watson PJ, Latif RK, Rowbotham DJ: Ethnic differences
369 in thermal pain responses: A comparison of South Asian and White British healthy males. Pain 118:228-294, 2005 40. Wesley NO, Maibach HI: Racial (ethnic) differences in skin properties: The objective data. Am J Clin Dermatol 4:843-860, 2003 41. Weisse CS, Sorum PC, Dominguez RE: The influence of gender and race on physicians’ pain management decisions. J Pain 4:505-510, 2003 42. Yosipovitch G, Meredith G, Chan YH, Goh CL: Do ethnicity and gender have an impact on pain thresholds in minor dermatologic procedure? A study on thermal pain perception thresholds in Asian ethnic groups. Skin Res Technol 10; 38-42, 2004 43. Zatzick D, Dimsdale JE: Cultural variations in response to painful stimuli. Psychosom Med 52:547-557, 1990 44. Zborowski M: Cultural components in response to pain. J Soc Issues 8:16-30, 1952
Ethnic Differences Regarding Tactile and Pain Thresholds in the Trigeminal Region Osamu Komiyama,* Misao Kawara,* and Antoon De Laat† *Department of Clinical Oral Physiology, Nihon University School of Dentistry at Matsudo, Sakaecho-nishi Matsudo, Chiba, Japan. † Department of Oral and Maxillofacial Surgery, School of Dentistry, Oral Pathology and Maxillo-Facial Surgery, Catholic University of Leuven, Leuven, Belgium.
Abstract: The aim of the present study was to evaluate ethnic differences of the tactile detection threshold (TDT), the filament-prick pain detection threshold (FPT), the pressure pain detection threshold (PPT), and the pressure pain tolerance detection threshold (PTOL) in the orofacial region of symptom-free subjects. Twenty-two men and 22 women in Belgium and in Japan (age range from 20 to 31 years) participated. The TDT and the FPT were measured on the cheek skin (CS) overlying the masseter muscles (MM), on the maxillary gingiva (MG), and at the tip of the tongue (TT), using Semmes-Weinstein monofilaments. The PPT and PTOL were measured at the central part of the MM, using a pressure algometer. A general linear model was used in each case to capture ethnic and gender effects. Japanese women had the lowest TDT at CS, in contrast to Belgian men, who had the highest value; a significant ethnic and gender effect was found (P ⴝ .026 and P < .001, respectively). Similar results were found for FPT at CS with significant ethnic and gender effects (P < .001 for both). There was no significant ethnic effect regarding intra-oral TDT and FPT or regarding PPT and PTOL. Perspective: Our findings clearly indicate that future studies of tactile and pain measurements need to standardize and control for gender and ethnicity. Further, a comprehensive evaluation of results from various stimulation modalities may better clarify the pain mechanisms and gender/ethnic characteristics, as well as comparisons between normal subjects and patients. © 2007 by the American Pain Society Key words: Ethnic difference, tactile detection threshold, filament-prick pain detection threshold, pressure pain threshold, pressure pain tolerance detection threshold, gender.
D
ifferent consultation rates for painful conditions, a different pain report, disability score, and quality of life have been observed for different ethnic groups.4,9,15,16,26,32,35,36,41,44 In experimental studies, ethnicity has been demonstrated to be an important variable in determining response to painful stimuli.8,17 Zatzick and Dimsdale43 concluded that Caucasians generally demonstrate higher threshold and endurance times for experimental pain stimuli. Watson et al39 showed that ethnicity plays an important role in heat pain threshold and pain report: South Asian men demReceived June 21, 2006; Accepted October 25, 2006. Supported by a 2006 Collaborative Study Grant of the International Association for the Study of Pain (IASP) and a Nihon University Individual Research Grant for 2006. Address reprint requests to Dr. Antoon De Laat, Department of Oral and Maxillofacial Surgery, School of Dentistry, Oral Pathology and MaxilloFacial Surgery, Catholic University of Leuven, Kapucijnenvoer 7, B-3000 Leuven, Belgium. E-mail: [email protected] 1526-5900/$32.00 © 2007 by the American Pain Society doi:10.1016/j.jpain.2006.12.002
onstrated lower pain thresholds and higher pain report when compared with matched white British men. Campbell et al3 examined ethnic differences in response to multiple experimental pain stimuli, including heat pain, cold pressor pain, and ischemic pain, and demonstrated differences in laboratory pain responses between African Americans and whites across multiple stimulus modalities. The effect sizes for these differences in pain tolerance were moderate to large for suprathreshold measures. In the oral and facial region, the tactile detection threshold and the filament-prick pain threshold have been evaluated with Semmes-Weinstein monofilaments, and the pressure pain threshold and the pressure pain tolerance detection threshold have been evaluated with the use of a pressure algometer.7,14,19,29 Komiyama and De Laat24 reported that women showed a significantly lower tactile detection threshold and filament-prick pain threshold at the cheek skin in comparison to men. Further, measurements of pressure pain threshold and pres363
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sure pain tolerance detection threshold confirmed the previously reported higher thresholds in men. One of the consequences of globalization and migration worldwide is that patients presenting in the clinical situation are of a varying ethnicity. Although numerous studies have reported ethnic differences in the prevalence and severity of clinical pain, little is known about how ethnicity affects the perception of experimental pain. In addition, it is not known whether sensory and mechanical pain thresholds in the orofacial region differ between ethnic groups if determined by using identical stimuli. Consequently, the present study was conducted to evaluate ethnic and gender differences in tactile detection threshold, filament-prick pain detection threshold, pressure pain threshold, and pressure pain tolerance detection threshold at multiple measuring points in the orofacial region of symptom-free subjects.
Methods Subjects Subjects were recruited from university students and staff. Twenty-two men and 22 women (age range from 20 to 31 years) (Table 1) participated in both Belgium (Belgian Caucasian subjects) and Japan (Japanese subjects). Ethnicity was determined by the interview of their genetic roots. All were asymptomatic for pain in the
head and neck region. This was defined as absence of jaw dysfunction and headaches and absence of subjective pain or soreness of the masticatory muscles. Subjects also could not participate when they were currently taking medication or received other treatment that could not be interrupted for the study, if general health problems (eg, metabolic disease, neurological disorders, vascular disease, and so forth) or periodontal disease was present, or in case of a history of drug abuse or recent facial or cervical trauma. Since a previous study20 indicated that pain thresholds were lower in the menstrual phase, women were not tested during their menstrual phase, and smokers were excluded to avoid altered intra-oral perception.6 The subjects were informed about the study in a standardized way, and they signed an informed consent form. The institutional ethics committee approved the study (ML2569 in Belgium and EC05015 in Japan). The 2 ethnic groups were tested in 2 different laboratories (1 in Japan and 1 in Belgium) by the same Japanese experimenter, using written instruction boards in Flemish for the Belgian subjects and in Japanese for the Japanese subjects to guarantee the comparability of the instructions for the test procedures. All measurements were taken in a single quiet room per location, in which temperature and humidity were kept stable.1
Descriptive Data (Mean ⴞ Standard Deviation) and Results of Statistical Analysis (P value) of Subject Characteristics and Measurements
Table 1.
ETHNICITY Gender Number Age (years) Height (cm) Weight (kg) Measurement Tactile detection threshold (TDT)
JAPANESE Male 22 24.7 ⫾ 3.6 170.1 ⫾ 5.6 63.3 ⫾ 8.6
Female 22 25.0 ⫾ 3.6 162.3 ⫾ 5.6 53.5 ⫾ 6.3
BELGIUM Male 22 24.4 ⫾ 3.1 181.3 ⫾ 8.3 73.8 ⫾ 11.8
Female 22 23.9 ⫾ 3.6 168.5 ⫾ 7.0 57.5 ⫾ 5.6
STATISTICAL ANALYSIS Effect Gender Ethnicity Interaction .879 .346 .604 <.001 <.001 .085 <.001 <.001 .078
Site Cheek skin 2.35 ⫾ 0.23 1.91 ⫾ 0.15 2.49 ⫾ 0.15 2.07 ⫾ 0.29 .026 Maxillary gingiva 3.65 ⫾ 0.26 3.56 ⫾ 0.25 3.62 ⫾ 0.33 3.41 ⫾ 0.33 .221 Tongue tip 1.89 ⫾ 0.20 1.77 ⫾ 0.09 1.91 ⫾ 0.17 1.87 ⫾ 0.18 .088 Thenar skin 2.60 ⫾ 0.22 2.31 ⫾ 0.25* 2.74 ⫾ 0.17 2.63 ⫾ 0.22 .052 Filament-prick pain Cheek skin 5.38 ⫾ 0.35* 4.86 ⫾ 0.39 5.91 ⫾ 0.30 5.17 ⫾ 0.45 <.001 detection threshold Maxillary gingiva 5.40 ⫾ 0.26 5.15 ⫾ 0.24 5.42 ⫾ 0.22 5.25 ⫾ 0.29 .025 (FPT) Tongue tip 4.87 ⫾ 0.38 4.67 ⫾ 0.32 5.11 ⫾ 0.26 4.97 ⫾ 0.23 .053 Thenar skin 5.60 ⫾ 0.49 4.99 ⫾ 0.46 5.94 ⫾ 0.31 5.37 ⫾ 0.48 <.001 Numeric rating scale Cheek skin 1.8 ⫾ 1.1 1.3 ⫾ 0.7 2.1 ⫾ 1.3 1.9 ⫾ 1.0 .300 value (NRS) for FPT Maxillary gingiva 2.2 ⫾ 1.4 1.1 ⫾ 0.7 2.9 ⫾ 0.9 2.3 ⫾ 1.1 .003 Tongue tip 2.4 ⫾ 1.5 1.2 ⫾ 0.8 3.0 ⫾ 0.9 2.1 ⫾ 1.1 .009 Thenar skin 1.8 ⫾ 1.1 1.0 ⫾ 0.8 2.2 ⫾ 1.1 1.7 ⫾ 1.1 .006 Pressure pain threshold Masseter muscle 186.5 ⫾ 52.2 142.1 ⫾ 37.6 197.6 ⫾ 63.2 140.6 ⫾ 38.7 .002 (PPT) Thenar muscle 331.6 ⫾ 86.4 285.0 ⫾ 66.6 394.6 ⫾ 136.2 254.0 ⫾ 77.4 .002 NRS for PPT Masseter muscle 2.7 ⫾ 1.4 2.0 ⫾ 1.2 3.4 ⫾ 1.0 2.7 ⫾ 0.9 .024 Thenar muscle 1.9 ⫾ 1.1 1.6 ⫾ 1.1 2.6 ⫾ 0.8 2.2 ⫾ 0.9 .123 Pressure pain tolerance Masseter muscle 401.0 ⫾ 81.9 265.8 ⫾ 62.3 434.1 ⫾ 99.6 301.0 ⫾ 65.8 <.001 threshold (PTOL) NRS for PTOL Masseter muscle 5.9 ⫾ 2.0 5.2 ⫾ 1.9 7.1 ⫾ 1.1 5.9 ⫾ 1.0 .060
Values of TDT and FPT are expressed as the number of the filament; values of PPT and PTOL are in kilopascals. Bold values indicate statistical significance as calculated using a general linear model (P ⬍ .05). *P ⬍ .05 compared between Japanese and Belgian subjects within the same sex, using Bonferroni correction for multiple comparisons.
.661 .410 .301 .031 .267 .593 .673 .885 .432 .328 .421 .701 .670 .030 .971
<.001 .143 .224 .003 <.001 .683 .001 .003 .065 .001 .056 .008 .599 .734 .011 .006 .223
.667
.019
.620
ORIGINAL REPORT/Komiyama et al
Tactile Detection Threshold and Filament-Prick Pain Detection Threshold The tactile detection threshold (TDT) and the filamentprick pain threshold (FPT) were measured (1) on the cheek skin overlying the central part of the left and right masseter muscles midway between the upper and lower borders and 1 cm posterior to the anterior border, (2) on the right and left maxillary gingival at a point 5 mm cranial to the line connecting the cervical rim of the canine and the lateral incisor, (3) at a point 5 mm proximal to the anterior tip of the tongue on the midline, and (4) on the skin overlying the palm side of the thenar muscle on the point connecting the longitudinal axis of the thumb and index finger (“thenar skin”). The sequence of the measurement sites was randomized in TDT and FPT measurement series. Semmes-Weinstein monofilaments with 20 different diameters were used (Premier Products, Kent, WA). The numbers of the filaments (1.65 to 6.65) correspond to a logarithmic function of the equivalent forces of 0.0045 to 447 grams. At first, TDT was examined by means of a staircase method. The subjects were instructed to close their eyes during the whole test procedure and to raise their hand as soon as they felt the stimulus in the test site. The filament was applied vertically on the test site and slowly pressure was applied until the filament bowed. The time needed to bow the filament was standardized to approximately 1.5 seconds. The stimulus was maintained for approximately 1.5 seconds and then removed in 1.5 seconds. Quick applications and bouncing of the filaments against the skin were avoided. At each site, the test started with the number (No.) 4.31 filament. If the subject raised his or her hand, it was considered a positive response, and the next filament applied was one step lower (No. 4.17). This procedure was repeated with decreased filament diameters until the subject no longer felt the pressure. This was considered as a negative answer. Again, the filament with a higher pressure was applied. This procedure continued until eight positive and eight negative peaks were recorded and the threshold (TDT) was calculated as the average of these values (number of the filament). If the subject still had a positive response while applying the lowest fiber (No. 1.65), this filament was considered the threshold. Two “null” (placebo) trials were performed after 5th and 11th peaks. During these control trials, the filament did not make contact with the tissue. If the subject did not report a sensation during the blank stimuli, the test was continued. If he or she did report a sensation, the test was discontinued and the subject was questioned about what kind of stimulus was perceived. The whole procedure was explained again to the subject, and afterward, the test was restarted.24 After the TDT measurements, the FPT was examined. The stimuli were applied in the same way as for the TDT, but the subjects were instructed to open their eyes to avoid fear of a painful stimulation and to raise their hand as soon as they felt not only pressure but also pain in the test area. If the subject had no positive response for
365 the thickest fiber (No. 6.65), this number was recorded as the threshold. No placebo stimulus was applied. There was a time lag of 3 minutes between the measurements on a similar site to avoid sensitization. Furthermore, after the examination of each measurement site, each subject was asked to rate the pain intensity experienced at the FPT on a numeric rating scale (NRS) where 0 cm indicated “no pain” and 10 cm indicated “worst pain imaginable.”
Pressure Pain Threshold and Pressure Pain Tolerance Detection Threshold A pressure algometer (Somedic, Hörby, Sweden) was used to test the sensitivity to stimuli applied to the masseter muscles. The pressure pain threshold (PPT) was defined as the amount of pressure (in kilopascals) that the subject first perceived to be painful.38 The PPT was determined with a constant application rate of 30 kPa/s and a probe diameter of 1 cm. The subject pushed a button to stop the pressure stimulation when the threshold was reached. These measurements were done at least 5 minutes after the FPT measurements. Measuring points were (1) the central part of the right and left masseter muscles midway between the upper and lower borders and 1 cm posterior to the anterior border and (2) the palm side of the thenar muscle on the point connecting the longitudinal axis of the thumb and index finger (“thenar muscle”). These test points were identical to the ones used for measuring TDT and FPT. At the start of each session, the subjects were familiarized with the measurement procedure and the equipment through a demonstration on the right forearm, and they were instructed to keep their teeth slightly apart to avoid contraction of the jawclosing muscles during the stimulation. To avoid movement of the head, it was stabilized to the head support of the chair. The PPT measurements were done 3 times at each of 3 measuring points (right and left masseter muscle, thenar muscle), and the sequence of the measuring points in each series was randomized by using a computer-generated list. There was a time lag of 2 minutes between the series of measurements. The mean value of the 3 measurements was used for further statistical analysis. The pressure pain tolerance detection threshold (PTOL) was defined as the pressure producing the maximum amount of pain the subjects were willing or able to accept. The PTOL was measured in the same way as the PPT. However, the test was applied just once at the end of the whole experimental session and only on the masseter muscles, and there was a time lag of 5 minutes between the right and left measurements to avoid sensitization. The sequence of the measurement sides was randomized. After each examination, the average pain intensity during the PPT measurement and the PTOL measurement was assessed on an NRS, where 0 cm indicated “no pain” and 10 cm indicated “the worst pain imaginable.”
Statistical Analysis Descriptive statistics were used to summarize all measurements. Since there was no side difference in a previ-
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Ethnic Differences of Tactile and Pain Thresholds 24
ous similar study, the values for cheek skin and maxillary gingiva on TDT and FPT and for masseter muscle on PPT and PTOL were used as mean values of the left and right sides. The mean values and standard deviations of TDT, FPT, PPT, PTOL, and NRS in each sex, each ethnic group, and each test area were calculated. Two-way ANOVA was used to analyze the gender and ethnic differences in age, weight, and height. To obtain a normal distribution, the natural logarithm of TDT and PPT values, the square root of the PTOL values, and logit (log(x/(1⫺x))) transformation of NRS values were used as a response variable. The response variables were assumed to be continuous, and a general linear model was used in each case to capture the ethnic and gender effects. Moreover, age, weight, height, and NRS values were considered for an adequate fit of the covariance matrix. The Bonferroni method was used to adjust probability values for multiple comparisons within the same gender or ethnic group. The significance level for each test was set to ␣ ⫽ 5%. All analyses were performed within the SPSS version 12 for Windows (SPSS, Chicago, IL).
Results Table 1 shows the descriptive data of the subject characteristics and measurements as well as the probability values resulting from the statistical analysis to summarize the data. There was no significant age difference between sexes or ethnic groups. There were, by contrast, significant differences between the sexes and ethnic groups regarding height and weight (ANOVA, P ⬍ .001).
Tactile Detection Threshold Regarding the TDT at cheek skin, significant effects were found for ethnic group (P ⬍ .026) and sex (P ⬍ .001).There was no significant age, height, or weight effect. No significant ethnic and gender effects were found at the maxillary gingiva or the tongue tip. A significant ethnic effect (P ⫽ .003) was found for the TDT at thenar skin, but there were no significant age, height, weight, or gender effects. There was no interaction between ethnic and gender effect at all measurement sites. Multiple comparison showed, within the same sex, that Japanese women had a lower TDT at thenar skin compared with Belgian women (Bonferroni: P ⫽ .004).
Filament-Prick Pain Threshold As for FPT at cheek skin, significant gender (P ⬍ .001) and ethnic (P ⬍ .001) effects were apparent. No significant age, height, weight, or NRS effect were observed. Regarding the FPT at the maxillary gingiva, a significant gender effect (P ⫽ .025) but no significant ethnic effect was observed. By contrast, a significant ethnic effect (P ⫽ 0.001) but no gender effect was seen at the tongue tip. Significant gender (P ⬍ 0.001) and ethnic (P ⫽ 0.003) effects were found at thenar skin. For all measurement sites, there was no interaction between the ethnic and gender effect. Multiple comparison showed, within the same
sex, that Japanese men had a lower FPT at the cheek skin in comparison to Belgian men (Bonferroni P ⫽ .002). Regarding the NRS values for FPT at cheek skin, no significant ethnic, gender, age, height, or weight effects were present. Significant ethnic (P ⫽ .003) and gender (P ⫽ .001) effects were found in NRS values for FPT at the maxillary gingival. No effects of age, height, or weight were present. Also at the tongue tip, a significant gender effect (P ⫽ .009) was found, but no effects were found for ethnic group, age, height, weight, or any interaction. At the thenar skin, significant gender (P ⫽ .006) and ethnic effects (P ⫽ .008) were found, but no effects were found for age, height, weight, or any interaction.
Pressure Pain Threshold For the masseter muscle, the analysis revealed no significant ethnic, age, height, weight, or NRS effect. The only significant effect was gender (P ⫽ .002). Similarly, for thenar muscle, the only difference was gender (P ⫽ .002), and no interaction between ethnic and gender effect was observed. Regarding the NRS value for PPT at the masseter muscle, significant ethnic and gender effects could be found (P ⫽ .024 and P ⫽ .011, respectively); no age, height or weight effect, or interaction between gender and ethnic group was present. At the thenar muscle, only a significant ethnic effect (P ⫽ .006) was found.
Pressure Pain Tolerance Detection Threshold Only the gender effect appeared to be significant (P ⬍ .001). There were no significant ethnic, age, height, weight, or NRS effects nor an interaction between ethnic and gender effects. The NRS value for PTOL was significantly different between the ethnic groups (P ⫽ .019), but no gender, age, height, or weight effects were found. There was also no interaction between ethnic and gender effect.
Discussion The present study characterized significant ethnic and gender effects for quantitative sensory measurements. Overall, women were more sensitive compared with men, and Japanese subjects also were more sensitive compared with the Belgian subjects after intra-oral and extra-oral stimulation by means of various modalities. However, regarding the NRS evaluation for painful stimulation, Japanese had a lower value compared with the Belgians, and men had a higher value compared with women. Regarding the TDT at cheek skin, significant effects were found for ethnic group and sex. A significant ethnic effect was also found for the TDT at thenar skin. Multiple comparisons showed, within the same sex, that Japanese women had a lower TDT at thenar skin compared with Belgian women. The Semmes-Weinstein monofilaments used for determining the TDT in the present study caused a minute indentation of the skin during the measurement. The force required to produce a certain amplitude or gradient of this skin displacement varies with the stiff-
ORIGINAL REPORT/Komiyama et al 21
ness of the tissue. Female and male skin may vary regarding physical properties. It has been pointed out that female skin appears to have a higher elasticity and extensibility.5,25 Although no apparent difference in epidermal innervation was observed between European Caucasian and Japanese/Chinese skin at the architectural or biochemical level,34 other studies suggested differences of skin properties between different ethnic groups40 that may in part explain the observed TDT differences between the sexes and the ethnics. Further studies need to clarify these issues. As for FPT at cheek skin, tongue tip, and thenar skin, a significant ethnic effect was apparent. Multiple comparisons showed, within the same sex, that Japanese men had a lower FPT at the cheek skin than Belgian men. Campbell et al3 also reported that no ethnic group differences emerged for threshold measures, but African Americans had a lower tolerance for heat pain, cold pressor pain, and ischemic pain in comparison to whites. Watson et al39 reported that South Asians demonstrated a lower cold pain thresholds and reported more unpleasantness at all temperatures. In addition, Gazerani and Arendt-Nielsen13 showed significantly greater pain responses in South Indians compared with Caucasians after capsaicin injection to the forehead skin. Our resent results, using mechanical sensory and pain thresholds, support these previous findings39 on the difference in response to experimental pain stimuli, with Caucasians generally demonstrating higher thresholds and endurance times compared with Asians and Africans. Previous studies have shown that darker skin types have different barrier protective effects, which may alter pain perception.33 The epidermis and dermis are innervated with nociceptive A delta and C fibers, which transmit sensations of pain, temperature, and touch. There is compelling evidence that epidermal receptors with specialized physiological properties exist and generate activity in afferent peripheral nerve fibers when stimulated.22,28 Ethnic groups with different skin types may vary with respect to these epidermal receptors.42 This may explain in part the observed differences in pain sensitivity between the 2 ethnic groups in this study. A significant ethnic effect was also observed in the NRS values for FPT at the maxillary gingiva and the thenar skin and at the cheek skin. Also regarding the NRS value for PPT and PTOL, significant ethnic effects could be found. From these findings, it is clear that Japanese reported a lower NRS value compared with Belgians when a painful stimulation was given, which could reflect a different response or attitude to the word “pain,” based on the different cultures. Hobara18 reported that Japanese of both sexes were less prone to overt pain expression in comparison to Euro-Americans. These findings reflect the traditional Japanese cultural emphasis on stoicism and the desirability of concealing pain and emotions, whereas the Euro-American culture puts a greater emphasis on the expression of personal feelings. The present results are similar to the findings of Nayak et al27 regarding cultural differences between participants in the United States and India. It is possible that the tradi-
367 tional stoicism, a well-known characteristic of many Asian cultures, may result in greater acceptance of pain and enhanced coping skills.2 It is very important for pain professionals to be aware of how patients with different cultural backgrounds experience and express pain. For FPT, PPT, and PTOL, significant gender effects were apparent. The variability of the threshold depended on the measurement place, and this was in line with our previous study,24 as were the observed gender differences. Women may be consequently considered more sensitive compared with men regardless of their ethnicity. Prior research of pain thresholds using other forms of noxious stimulation suggest that a variety of factors may contribute, including hormonal alterations,11,31 blood pressure,10 and psychological factors.11 In addition, other investigators have reported that sex role expectancies30 and anxiety may moderate gender differences.12 Kim et al23 evaluated genetic influences on variability in human pain sensitivity associated with gender, ethnicity, and temperament. Their observations demonstrate that gender, ethnicity, and temperament contribute to individual variation in thermal and cold pain sensitivity by interactions with loci in the vanilloid receptor subtype 1 gene and opioid receptor subtype 1 gene single nucleotide polymorphisms. Our results on ethnic and gender differences in this study could also possibly be explained by these gene differences. All data were collected by the same Japanese male investigator. Zatzick and Dimsdale43 suggested that the race and sex of the investigator might influence the outcome of laboratory studies. Edwards and Fillingim,8 however, found no evidence that the sex and ethnicity of the investigator influenced the outcome. From the present study design, we cannot conclude whether ethnicity of the experimenter influenced the results. In the present study, the TDT, FPT, PPT, and PTOL at multiple measuring points in the orofacial region of normal subjects were evaluated and ethnic and gender differences were characterized. The number of subjects was limited to 88, distributed over the 4 groups, which means that some of the nonsignificant findings may have resulted from inadequate power. Also, most subjects were recruited from university staff and students, possibly creating a bias toward a distinct societal category.37 Nevertheless, our findings clearly indicate that future studies of tactile and pain measurements in general need to standardize and control for gender and ethnicity. These results therefore contribute to the evaluation of patients with atypical odontalgia, allodynia in the trigeminal region, and other orofacial pain. Further, a comprehensive evaluation and the combination of results from various stimulation modalities may better clarify the pain mechanisms and gender/ethnic characteristics, as well as comparisons between normal subjects and patients.14
Acknowledgments The authors thank Dr. Kris Bogaerts and Mr. Dimitris Rizopoulos, Biostatistical Center, School of Public Health,
368
Ethnic Differences of Tactile and Pain Thresholds
Catholic University of Leuven, Leuven, Belgium, for the statistical analysis and advice, and Dr. Hans Isselee,
Department of Rehabilitation, St Jan Hospital, Bruges, Belgium, for the use of the algometer.
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