Running head: SELF- OF TOLERANCE CORRESPONDS TO PAIN SENSITIVITY 1

Self-Perception of Pain Tolerance Corresponds to Pain Sensitivity

Kim A. Fasano

Mentors: Joel Bialosky, PT, PhD, FAAOMPT, OCS/ Mark Bishop, PT, PhD

University of Florida

SELF-PERCEPTION OF PAIN TOLERANCE CORRESPONDS TO PAIN SENSITIVITY 2

Abstract

Self-Perception of Pain Tolerance Corresponds to Pain Sensitivity

Background: High pain sensitivity, measured through quantitative sensory testing (QST), is a risk factor for . It is unknown if self-perception of pain tolerance is an acceptable proxy measure of pain sensitivity. This study’s primary purpose was to determine if self- perception of pain tolerance corresponds to QST measures. A secondary purpose was to characterize individual self-perception of pain tolerance based on key demographic and psychological factors. We hypothesized participants perceiving themselves as less pain tolerant would demonstrate greater pain sensitivity through QST.

Method: Pain free participants completed questionnaires of demographic factors, , and catastrophizing. Participants were characterized by self-reported pain tolerance as having higher, lower, the same, or uncertain pain tolerance compared to “most people I know.” All participants underwent QST measures of thermal and mechanical pain threshold, tolerance, and suprathreshold measures. ANOVA and pairwise comparisons were used to analyze the data.

Results: The study included 138 participants, with a mean age of 23.5 (sd=4.8), 63.3% female.

The “higher pain tolerance” group showed less fear of pain than the “lower pain tolerance” group

(p=.02). The “higher” and “uncertain” pain tolerance group experienced less temporal summation than the other groups (p=.01). Trends favored the “uncertain” pain tolerance group for all QST measures

Conclusions: Lower self-perception of pain tolerance corresponds to higher levels of fear and temporal summation suggesting vulnerability to chronic pain.

Public Health: Self-perception of pain tolerance may allow healthcare professionals to quickly screen patients for chronic pain susceptibility, and provide individualized healthcare.

Funding: NCCAM (R01AT006334-01) Bishop PI

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Introduction

Chronic pain affects an estimated 45% of the United States population, with healthcare costs estimated at more than 100 billion dollars annually (Staud, 2009). Many chronic pain syndromes including fibromyalgia, irritable bowel syndrome, chronic fatigue syndrome, migraine , chronic , and complex regional pain syndrome, are associated with hypersensitivity to pain, and reduced endogenous pain inhibition (Staud, 2009). While acute pain is a symptom of physical tissue damage, this linkage does not exist in chronic pain conditions (Cohen, Quintner, & Buchanan, 2013). Therefore, different diagnostic strategies must be used when classifying chronic pain conditions, including the consideration of internal and external factors (Fillingim, 2014). Inter-individual differences in pain sensitivity may suggest differences in susceptibility to chronic pain conditions (Staud, 2009).

Quantitative sensory testing (QST) is a method that can quantitatively measure variability in pain sensitivity (Cruz-Almeida & Fillingim, 2013). QST refers to “a group of procedures that assess the perceptual responses to systematically applied and quantifiable sensory stimuli for the purpose of characterizing somatosensory function or dysfunction.” (Cruz-

Almeida & Fillingim, 2013, p. 1-2). This can include modalities such as thermal, mechanical, electrical, or chemical stimulation. Differences in pain sensitivity measured by QST are clinically relevant, as this variability may represent behavioral measures of the central nervous system that play a role in the transition from acute to chronic pain, and the maintenance of chronic pain

(Cruz-Almeida & Fillingim, 2013).

Inter-individual pain responses are highly variable and can be influenced by many different factors, and psychosocial variables (Cruz-Almeida & Fillingim, 2013). Young et al.

(2011) explained that there is a high degree of variability in pain responses, as well as differences in reporting of pain due to interactions between genetic and environmental responses. Young et al. (2011) reported that 30-60% of variation in pain responses may be accounted for by genetic factors. Additionally, race and ethnicity have been shown to explain SELF-PERCEPTION OF PAIN TOLERANCE CORRESPONDS TO PAIN SENSITIVITY 4 differences in the pain response (Rahim-Williams, JL, Williams, & Fillingim, 2012). For example,

African Americans, and Hispanics tend to report more pain than Caucasians within a clinical setting (Young et al., 2011). In a controlled setting, variability in pain sensitivity is also explained by gender, with women reporting more pain than men (Nielsen et al. 2008). This body of literature suggests that the pain experience is highly variable and this variability is influenced by multiple factors. Pain sensitivity is an indicator of the ability to modulate pain, and therefore may predict the risk for individual development of chronic pain (Staud, 2009).

Understanding variability in the pain response is clinically relevant, because it would allow health care professionals to predict patients’ susceptibility to chronic pain (Nielsen et al.,

2008), leading to more individualized care, and therapeutic options (Young et al., 2011). QST can help to determine the mechanisms involved in the transition from acute to chronic pain, by representing behavioral measures of nociceptive processing within the central nervous system

(Cruz-Almeida & Fillingim, 2013). Clinically, this would allow healthcare professionals to identify individuals at risk for developing chronic pain, and QST measures could serve as treatment targets for effective interventions. However, quantitative sensory testing is often expensive, timely, and not readily available to clinicians (Edwards & Fillingim, 2007). It is unknown if self- perception of pain tolerance is an acceptable proxy measure of pain sensitivity. With this in mind, it would be beneficial to determine if perceived, self-reported pain tolerance accurately predicts QST measures of pain sensitivity. This study’s primary purpose was to determine if self-perception of pain tolerance corresponds to QST measures. A secondary purpose was to characterize individual self-perception of pain tolerance based on key demographic and psychological factors. We hypothesized participants perceiving themselves as less pain tolerant would demonstrate greater pain sensitivity through QST.

Method

Individuals between 18 and 40 years of age were recruited by posted advertisements via study fliers posted on campus at the University of Florida, and surrounding community. Reasons SELF-PERCEPTION OF PAIN TOLERANCE CORRESPONDS TO PAIN SENSITIVITY 5 for exclusion included: Participation in a conditioning program of trunk extensors in the past six months, report of back or leg pain in the past 3 months, chronic medical conditions that affect pain perception, history of previous injury, consumption of any drugs that may affect pain perception or hydration from 24 hours before participation, until completion of investigation, any contraindication to MRI.

Measures

Eligible individuals agreeing to participate and providing informed consent as approved by the University of Florida Institutional Review Board completed a standard intake form with demographic information relating to: gender, age, employment status, marital status, educational level, and health history. Participants were further asked to choose which statement best characterizes their pain tolerance: a) Higher than most people I know; I can tolerate a high amount of pain b) Lower than most people I know; I can tolerate low amounts of pain, but not high c) Relatively the same as other people; I can tolerate moderate amounts of pain d) I don’t know how much pain I can tolerate. Participants also completed questionnaires related to psychological measures known to affect pain reporting.

Fear of pain questionnaire (FPQ-III): Participants completed this 30-item, 5-point rating scale developed to measure fear about specific situations that normally produce pain with higher scores indicating greater fear of pain. Total scores were used in order to measure participants’ general fear of pain. FPQ-III has been established as a trait measure with predictive value for fear of pain (Roelofs, 2005).

The scale (PCS): uses a 13-item, 5-point Likert scale with higher scores indicating elevated levels of catastrophizing. The PCS structure is an established measure of pain catastrophizing (Quartana, Campbell, & Edwards, 2009).

Quantitative Sensory Testing (QST)

All QST procedures were administered by a trained research assistant using a standardized instructional set. The range of stimuli temperatures (40-51°C) was applied SELF-PERCEPTION OF PAIN TOLERANCE CORRESPONDS TO PAIN SENSITIVITY 6 beforehand in one-degree steps to each participant, in order to familiarize participants with the stimulus range, and to alleviate anxiety. QST included two stimulus modalities, heat and mechanical pressure. Thermal stimuli were applied to various parts of the body using the TSA-II

Neurosensory Analyzer (Medoc Advanced Medical Systems Inc., Ramat Yishai Isreal) with a handheld, peltier-element-based (30mm x 30mm) stimulator. Pressure stimuli were applied to various parts of the body using a hand held Fischer .

The following QST measures were performed and described below:

Thermal (heat) threshold: a heat stimulus was delivered to the participants’ dominant forearm, starting at 35°C and increased at a rate of .5°C/ second. Participants verbally indicated pain threshold (“when the sensation first transitions from just heat to pain”) by saying stop. The research assistant recorded the temperature in degrees Celsius.

Thermal (heat) tolerance: a heat stimulus was delivered to the participants’ forearm starting at 35°C and increased at a rate of .5°C/ second to a maximum temperature of 51°C.

Participants verbally indicated pain tolerance (“when the sensation becomes so strong that you can no longer bear it”) by saying stop. The research assistant recorded the temperature in degrees Celsius. A temperature of 51°C was recorded if the max temperature was achieved prior to the participant saying stop.

Temporal summation of pain (TSP): a series of 6 heat pulses was applied to the glabrous skin of the foot. Each pulse began at a temperature of 39°C, increased to a peak of

50°C, and returned to 39°C at a rate of 10°C/sec. The inter-stimulus interval was < 3 seconds.

Participants rated the the intensity of their second pain following each heat pulse using a visual analog scale (VAS) response to each stimulus, where 0=no pain sensation and 100= the most intense pain sensation imaginable.

Suprathreshold: Suprathreshold pain sensitivity was assessed on the non- dominant calf of each participant through the application of heat stimuli of 3 seconds duration. The thermode was applied with a baseline temperature of 35°C which rose (10°C/sec) to peak of 45, SELF-PERCEPTION OF PAIN TOLERANCE CORRESPONDS TO PAIN SENSITIVITY 7

47, 49, or 50°C. The sequence of application of the temperatures was randomly determined to prevent an order effect. Participants were asked to rate their "first" pain intensity using a 0 to

100 NRS. This was performed two times at each temperature with the average rating at 49°C analyzed. The researcher waited 60 seconds between trials in order to prevent habituation.

Mechanical threshold (PPT): Pressure was applied at a rate of 1 kg/second through a

1 cm2 application tip to the hand (1st dorsal interosseous muscle), between the first and second toe on the dorsal aspect of the foot, and at the lumber spine. Participants verbally indicated pain threshold (“when the sensation first transitions from just pressure to pain”) by saying stop. The research assistant recorded the amount of pressure in kg/cm2.

Data Analysis

Chi Square was used to assess group related differences in self-perception of pain tolerance and categorical demographic variables. Univariate ANOVA was used to consider group related difference in self-perception of pain tolerance and continuous demographic variables, psychological factors, and QST.

Results

Demographic Variables/ Psychosocial Factors

Refer to Appendix Table A/ Table B for participant demographic percentage breakdown, refer to

Appendix Table C for group differences.

A total of 138 individuals participated in the study (mean age 23.5 ± 4.8 years), 63.3% female. Thirty-eight participants identified their pain tolerance as “higher than most people I know.” Eight participants identified their pain tolerance as “lower than most people I know.”

Eighty-three participants identified their pain tolerance as “relatively the same as other people.”

Nine participants identified with “I don’t know how much pain I can tolerate.”

Group differences in self-perceived pain tolerance were not observed in terms of percentage of sex (p=.41), race (p=.42), or household income (p=.98). There was a statistically significant difference between self-perceived pain tolerance and fear of pain (p=.02). Pairwise SELF-PERCEPTION OF PAIN TOLERANCE CORRESPONDS TO PAIN SENSITIVITY 8 comparisons indicated that the “higher pain tolerance” group showed less fear of pain than the

“lower pain tolerance” group (p=.02). The groups did not differ by level of pain catastrophizing

(p=.21).

QST Measures

Thermal (heat) threshold/ tolerance: The groups differed between self-perceived pain tolerance and thermal threshold (p<.01). Pairwise comparisons indicated that the “uncertain” pain tolerance group showed higher threshold levels than the “lower” pain tolerance group

(p=.03), and the “relatively the same as others” group (p=.03). No significant differences were found between self-perceived pain tolerance and thermal tolerance (p=.15).

Suprathreshold: The groups neared significant differences with suprathreshold (p=.06).

Temporal summation of pain (TSP): Group differences were observed in temporal summation (p=.01). Pairwise comparison showed the “higher” and “uncertain” pain tolerance

groups experienced less temporal summation than the other groups (p=.01).

Mechanical threshold (pressure pain threshold, PPT): The groups differed by pain threshold at the lower back (p=.02). The “uncertain” pain tolerance group showed higher threshold levels than the “lower” pain tolerance group (p=.04), and the “relatively the same as others” group (p=.02). The groups did not differ by pressure pain threshold at the hand (p=.29).

Discussion

These results support the hypothesis that participants who perceive themselves as less pain tolerant demonstrate greater pain sensitivity, specifically, lower self-perception of pain tolerance corresponded to higher levels of fear of pain and temporal summation.

An important finding of this study was that participants who self-reported their pain tolerance as “higher than most people I know” showed less fear of pain than participants who self-reported their pain as “lower than most people I know.” Research suggests that higher levels of fear of pain serves as “a risk factor for the development and persistence of chronic pain” (Turk & Wilson, 2010, p. 3). This study suggests that self-reporting of pain sensitivity SELF-PERCEPTION OF PAIN TOLERANCE CORRESPONDS TO PAIN SENSITIVITY 9 predicts pain-related fear, which can help to identify individuals who are likely to transition from acute to chronic pain. Identifying patients who are at an increased risk for developing chronic pain can allow clinicians to develop optimal treatment plans by assessing and matching to the patients’ self-reported attitudes and beliefs about pain tolerance and fear of pain.

This study also revealed that individuals who self-report their pain sensitivity as “higher”, or “uncertain” experienced less temporal summation or “wind-up” than the other groups (Turk &

Wilson, 2010). Individuals with low endogenous pain inhibition, and high pain sensitivity are characteristic of chronic pain populations (Staud, 2009). For example, fibromyalgia patients show higher levels of perceived pain and temporal summation (Staud, 2009). QST testing for temporal summation plays an important role in the assessment of chronic pain, and may become a biomarker for pain sensitivity and pain modulation (Staud, 2009, p. 7). Self-reporting of pain tolerance may be a quick and affordable alternative to QST, in order to predict temporal summation levels, and in turn understand patients’ susceptibility to chronic pain conditions.

Lastly, this study determined that trends favored the “uncertain” pain tolerance group for all QST measures of pain sensitivity. This may be due to lack of expectation of pain tolerance, leading to outcomes not influenced by factors such as gender role expectations. For example, according to a study by Robinson, Gagnon, Riley III, and Price (2003), self-perception of pain tolerance based on pre-conceived gender roles can affect a participant’s willingness to report pain, with men reporting less pain than women. Perhaps the individuals who were “uncertain” of their pain sensitivity were not influenced by pre-conceived expectations during QST measures.

A limitation is that this study was performed with pain free individuals; therefore, findings may be different for people in pain. Also, the sample included a young population, with relatively few participants indicating their pain sensitivity as “lower than most” or “uncertain.” Future studies should include a larger clinical sample with a greater age range.

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Appendix Table A Participant Ages

N Minimum Maximum Mean Std. Dev

Age (years) 138 18 40 23.5 4.8

Table B Participant Demographic Percentages

Demographic Category Percentage (%) Sex Male 36.7% Female 63.3%

Ethnicity Hispanic or Latino 17.4% Not Hispanic or Latino 82.6% Race Asian 9.6% Black 11.9% White 64.4% Asian Indian 8.1% More than one 5.9% Education Did not finish High School 1.4% High school or GED 37.4% Associate’s Degree 10.8% Bachelor’s Degree 33.8% Master’s Degree 13.7% Doctorate or Post 2.9% Professional Degree

Employment Status Full Time 10.8% Part time 23.0% Unemployed 5.8% Student 59%

Household Income Less than 20,000 53.3% 20,000-35,000 16.8% 35,000-50,000 12.4% 50,000-70,000 5.8% 70,000 or greater 11.7%

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Table C QST and demographic group differences

Higher than most Lower than most Same as most Don’t know

Fear of Pain 15.9 (± 6.4)* 24.0 (± 8.2)* 19.1 (± 6.8) 19.1 (± 6.5)

Pain 9.8 (± 8.3) 10.3 (± 11.4) 13.7 (± 9.4) 12.3 (± 11.7) Catastrophizing

Thermal Heat 45.3 (± 1.9) 43.2 (± 1.7)* 44.2 (± 2.7)* 46.6 (± 2.2) Threshold

Thermal Heat 48.6 (± 1.3) 47.9 (± 1.4) 48.1 (± 1.9) 49.3 (± 1.2) Tolerance

Suprathreshold 20.1 (± 23.0) 27.1 (± 18.3) 31.5 (± 26.0) 14.4 (± 11.4)

Temporal 5.8 (± 5.1)* 8.8 (± 6.3) 9.6 (± 6.2) 5.2 (± 4.4)* Summation

Mechanical Threshold

Lumbar Spine: 25.2 (± 2.0) 20.2 (± 4.0)* 23.4 (± 1.3)* 35.7 (± 3.8)*

Hand: 15.4 (± 7.1) 13.6 (± 4.7) 14.7 (± 6.9) 19.3 (± 10.2)

*= significant at p<0.05