- Email: [email protected]
Review of Pain-Measurement Tools
PAIN MANAGENENT/CONCEPTS
Reviewof Pain-Measurement Tools From the Department of Emergency Medicine, Vancouver Hospital and Hearth Sciences Centre, University of British Columbia, Vancouver, British Columbia*; the:Division of Emergency Medicine, Department of Medicine, University c,f Toronto and St Michael's Hospital, Toronto, Ontario'; Children's Hospital and Dalhousie University, Halifax, Nova Scotia'; and the Department of Emergency Medicine, ChedokeMcMaster Hospitals, Hamilton, Ontario, Canada. '~
Kendall He, MD* Julie Spence, MD* Michael F Murphy, MD§lj
From the First International Symposium on Pain Research in Emergency Medicine, Montreal, October 1994. Copyright © by the American College of Emergency Physicians.
APRIL 1996
27:4
ANNALS OF EMERGENCY MEDICINE
[Ho K, Spence J, Murphy MF: Review of pain-measurement tools. Ann EmergMed Apri11996;27:427-432.] INTRODUCTION Traditionally, emergency physicians have not used objective measures to assess level of pain or relief of pain. Although pain is common, measurement of the intensity of pain and determination of the extent to which that pain affects an individual can be far from simple. Unlike onedimensional parameters such as heart rate that can be easily quantified, pain is a complex and multidimensional phenomenon. 1 First, pain can be generated by different mechanisms, including tissue ischemia, muscle contraction, and direct tissue damage from trauma. In addition, the peripheral and central nervous systems not only passively carry the nociceptive signal for cognitive processing but also actively modify it along the way. 2,3 Finally, interpretation of the nociceptive signal includes psychologic, physiologic, emotional, and behavioral dimensions. Because of these characteristics, the perception of and the response to pain vary greatly; the intensity of pain cannot be inferred from the degree of tissue damage observed. 4 There are no accurate physiologic or clinical signs that can be used to objectively measure pain. Reliance on health care worker assessment of patient pain results in underestimation of the intensity of that pain. According to the National Institutes for Health (NIH), patient selfreporting is the "most reliable indicator of the existence and intensity of pain. ''5 Subjective pain measures that may need to be quantified include intensity, time course, quality, impact, and personal meaning. The ideal tool in the assessment of pain should include the identification of the presence of pain, as well as the progress of pain with time or treatment. 6 Also, this tool should be applicable to any individual, regardless of psychologic, emotional, or cultural background. To that end, unidimensional pain scales have been developed. Because of their ease of use, these
427
PAIN-MEASUREMENT TOOLS Ho, 5pence& Murphy
scales have become popular tools with which to quantitare pain relief and pain intensity. The three scales most frequently used to assess acute pain are the numeric rating scale (NRS), the visual analogue scale (VAS), and the adjective rating scale (ARS). Although popular for study purposes, unidimensional pain scales present certain problems. They were initially developed for use in experimental pain trials m which pain was limited and controlled. Clinical pain is different in that it may become persistent, unbearable, and beyond the individual's control. It is also often associated with a strong emotional component not seen in the experimental setting. Pain scales tend to focus only on pain intensity, with increased risk of oversimplification of the experience, r Furthermore, actual measurements are relative only to the individual being assessed. Identical stimuli applied to different individuals can yield markedly different scores. Thus the numbers, although reproducible to a high degree for each individual, are simply estimates of the perception of the pain, based on past personal experience. Quantification of the experience requires the individual to abstract and quantitate the sensation, s The use of unidimensional pain scales is more appropriate in the setting of acute pain than chronic pain. Chronic pain is usually associated with other dements such as degree of support and depression. The assessment of chronic pain often requires more complex evaluation t o o l s In this article we attempt to catalog the tools that have been reported in the literature. We will emphasize the unidimensional tools, for they are more relevant to areas of research likely to be explored in the emergency setting. They are categorized according to method of evaluation, together with general comments about the characteristics of each class of tools. UNIDIMENSIONAL ASSESSMENT TOOLS BASED ON SELF-REPORT
Before any pain-rating tool is used, the goal of the study must be identified because certain pain tools have clear advantages. Unidimensional pain scales are frequently used when a single, clearly defined question is to be answered. They are easy to understand and use and place a minimal burden on the patient. Scales with a wide range of possible scores ensure sensitivity to intervention and permit statistical evaluation. The pain-assessment tools in this category measure the intensity of pain on the basis of the patient's self-report. No attempt is made to assess the behavioral or physiologic response to pain or
42 8
the psychologic, emotional, or cultural influences on the interpretation of pain. Visual analog scale Because the most commonly used and validated tool in this category is the visual analog scale (VAS)8,9, we will discuss it the most thoroughly. The VAS was introduced in the early part of the century as a psychologic assessment tool for the measurement pain and mood. The popularity of the VAS stems from its simple construction and ease of use. It is best used in the sequential evaluation of pain; the broad range of scores allows subsequent statistical analysis. The scale comprises a 10-cm line with descriptive phrases at either end. In most pain studies the range is from "no pain" to "severe pain" or "worst pain ever." Selected phrases should be easily understood and not so extreme as to not be chosen. Patients estimate their level of pain by placing a mark on the line; the distance from the "no pain" point is then measured. No intermediate marks should be placed along the length of the line; this leads to clustering of responses. 8't° A 10-cm horizontal line is the most widely used version of the VAS, and the most widely validated, t o, 1 Once a length has been chosen, it should remain constant throughout a study. Variation in responses has been seen in individuals who are asked to mark lines of different lengths. Many variations on this theme exist, including slide rules 12, numeric markings on top of the line 13,14, vertical lines 15,10, and labeling of the line with descriptive words. ~4 None of these variations has a clear advantage, although the version chosen should be used for the duration of a study, s'11'17-19 Data are commonly analyzed with a 100-point scale corresponding to the 10-cm line. There is no mechanism for validating individual scores because they are a subjective interpretation of a sensation. Apparent discrepancies in absolute self-reported pain scores may be seen in patients with high scores in situations in which the health care worker believes there should be little pain. 5 Motivational reasons (eg, a patient believing he or she will be treated more promptly), as well as emotional reasons (eg, fear of serious illness or of losing a job), often account for these apparent discrepancies. The initial rating assigned by the patient is therefore less meaningful than changes in that rating in response to interventions, r' i0 The VAS has been found to be a valid and reliable means of assessing pain, depression, anxiety, and mood. 8 Different aspects of a painful experience may be rated with separate VASs, permitting more complete understanding of the patient's suffering, r,2°'21 The VAS has been compared with other pain scales and validated through cross-modal-
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PAIN-MEASUREMENT TOOLS No, 5per~ce& Murphy
ity matching. 7,s, lo,1 l, 18,22,23 A strong correlation exists between successive measurements of pain made with the use of a VAS, confirming reproducibility of the tool in the clinical setting. 8,1° The VAS also shows a uniform distribution of response when used in unselected patient populations. It appears to be less useful in the study of chronic pain. 7,18,24,25 A failure rate of approximately 7% is seen in the adult population. 7,8,26 The use of the VAS in the pediatric population has also been studied extensively Children older than 5 years have been shown to be able to use the scale. 27 This ability appears to be independent of sex, background, education, or health status. 2s tn younger patients, aged 2 to 5 years, failure rates of 7% to 11% have been quoted. Surprisingly, these rates are no worse than those in the adult population. 29 In this younger age group, modified VASs are often used. The color scale, on which the child is asked to interpret pain as red, black, or purple, is one example of a modified VAS.27 Another, the Faces Pain Scale, contains pictures of faces ranging in expression from happy to a sad face with tears. 2s A more detailed discussion of pediatric pain assessment appears in "Pediatric Pain Control" [Terndrup TEl 1996;27:466-470]. Pain assessment in older patients may be difficult because hearing, visual, and psychomotor deficits may interfere with pain scale comprehension and use. 5,1s Problems with memory and abstraction also make the application of VAS challenging, s J1 Verbal descriptors and pain thermometers may be preferred tools for pain assessment. Because of a relatively high failure rate, up to 25%, visual analogue scales should be avoided if a highly reproducible response rate is required for analysis, r,ls Statistical analysis of data derived from VAS scores is somewhat controversial. The information is more complex than simple ordinal data and yet is not clearly classified as interval data. 8 Parametric techniques have been used to analyze psychologic data which are similar to those derived from pain scales. 3° The use of parametric techniques does not alter the risk of type I or II errors, whereas nonparametric tests may increase the risk of type II errors. VAS scores do allow calculation of statistically significant changes in pain scores. There is no clearly defined manner in which to relate this to clinical outcome, limiting the utility of the test. 3~,32 The importance of distinguishing statistical from clinical significance will be discussed later in these proceedings. Verbal descriptors or outcome data may be used in conjunction with VAS scores to more clearly define outcomes. 33'34
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Other unidimensional tools Another popular tool with which to assess acute pain is the numeric rating scale (NRS). The patient is asked to choose one number, from a numeric range, that best reflects the intensity of pain. A frequently used range in the assessment of ischemic cardiac pain is 0 to 10 (NRS-11). Another customary range is 0 to 100 (NRS-101). ~'12 This scale appears to yield better results than the VAS for patients with chronic pain 11 and illiterate patients. 35 A variation on the NRS is the 11-point box scale. The patient is asked to circle one of the 11 boxes arranged horizontally on a straight line, each containing a number from 0 to 10. ~2,35 The verbal rating scale (VRS) comprises carefully chosen phrases, arranged in an order to describe increasing perception of the intensity of pare. The patient is asked to choose the phrase that best describes his or her pain. s2,15,35 Pain drawings have also been advocated as an informative tool. Instead of assessing the overall intensity of pain, this method identifies the distribution of pain and the characteristics of the pain syndromes experienced. 36-3s On a line drawing of a person, the patient is asked to mark the areas where he or she feels pain. The patient is then asked to use different shadings to indicate the quality of pain (eg, stabbing, pins and needles, cramps, numbness). Other methods of self-report include the single-digit sign, in which the patient is asked to indicate the location of maximal pain with a single finger. 39,4° (This method is of little value in research.) With the rapid increase of computerization in medicine, evaluation tools involving computer animation may emerge. 4~ Observation of behavioral and physiologic responses Although behavioral patterns and vital signs are inferior to self-report in the accurate assessment of pain 42, in some circumstances self-report is not feasible: in the postoperative period, when the patient is still under the influence of anesthetics; in the absence of adequate verbal skills, such as in neonates and children younger than 3 yearsl'43; and in the cases of mentally challenged patients and some geriatric patients with regression of linguistic ability. ~4 In such circumstances the severity of pain can be estimated only from the patients' physiologic and behavioral responses. Physiologic responses to pain, which are numerous, are seen in the respiratory, cardiovascular, gastrointestinal, urinary, neuroendocrine, and metabolic systems. ~5 Many of these responses can be attenuated or eliminated through the provision of adequate analgesia. A nonspecific increase in minute ventilation occurs with painful stimuli. However, with chest and upper-
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HO,5pence & Mlxrphy
abdominal injuries and wounds, abdominal distension may occur, possibly resulting in reductions in vital capacity, tidal volume, residual volume, functional residual capacity, and 1-second forced expiratory volume. In neonates it has been shown that SaO 2 decreases during painful procedures. 46 The most widely appreciated, and perhaps most easily identified, responses to acute painful stimuli occur in the cardiovascular system as a result of increased Sympathetic tone. Heart rate:, stroke volume, blood pressure, cardiac work, and M v o 2 a r e decreased. Patients in pain often reduce their physical activity, resulting in increased venous stasis and platelet aggregation and, ultimately, in increased risk of deep-vein thrombosis. Nausea, vomiting, and ileus are considered gastrointestinal autonomic concomitants of pain. General hypomotility of the urinary tract and difficulty in urination are likely mediated by the same autonomic stimulation. Suprasegmental reflex responses lead to increased sympathetic tone and catecholamine secretion. Increased catabolic hormone secretion leads to increases in serum levels of adrenocorticotropic hormone, antidiuretic hormone, growth hormone, cyclic AMP, glucagon, aldosterone, and renin/angiotensin I2. The effects are predictable and include sodium and water retention, as well as increased levels of blood glucose, free fatty acids, ketone bodies, and lactate. Physiologic :measurements to assess the intensity of pain such as topographic mapping of the brain 4r and analyses of blood or urine for determination of changes in circulating neurotransmitters and other chemicals have been made. These modalities still need much refinement and are not currently appropriate for clinical application. Two observational tools based on behavioral response have been proposed for the assessment of pain in neonates and young children. 1,48 For the neonatal group, five behavioral responses were chosen for observation: brow bulge, eye squeeze, nasolabial furrow, open mouth, and crying. Each response is given a score of 1 if present, with a maximal score of 5. 4s For the pediatric group, 10 behavioral responses were assessed, s A pediatric pain chart is another model proposed for the observation of behavior after surgery. 49 Mechanical methods of reporting pain intensity have been proposed, such as squeezing a piece of calibrated equipment to demonstrate the intensity of pain. 5° However, such methods generally prove inferior to verbalreport methods. Multidimensional assessment tools Whereas the unidimensional tools measure mainly the intensity of pain,
43O
multidimensional methods also take into account the other factors that influence its perception. These factors include the quality and the temporal sequence of pain, the affective contributions, and the patient's belief system. Although this comprehensive approach is important, use and interpretation of these tools are difficult because of their complexity. 51 In addition, the length of time required to complete the evaluations usually make them impractical in the emergency setting. The best-known example of this category is the McGill Pain Questionnaire (MPQ). 52 It identifies key words that describe the different qualities of pain and classes them into four major groups: sensory, affective, evaluative, and miscellaneous. Each word is assigned a rank value, and the sum of the rank values of the words chosen becomes the pain rating index (PRI), The patient is also asked to give a value from 0 to 5 at the time of the questionnaire to represent the overall pain intensity; this number is termed the present pain intensity (PPI). Finally, the number of words chosen to describe the pain is also counted. Over time, the MPQ has shown good consistency and reliability in the evaluation of acute and chronic pain 53,54 and has been translated to and validated in many other languages+55,56 Some other examples of assessment tools used to measure the multidimensional aspects of pain include the Minnesota Multiphasic Personality Inventory (MMPI) 57, the West Haven-Yale Multidimensional Pain Inventory5s, the Checklist for Interpersonal Pain Behavior 59, and the Pain Beliefs Questionnaire. 6° Because these questionnaires are often long and tedious to administer, abbreviated versions simplify the task without losing the multidimensional assessment. Some examples include the short-form MPQ 61, the MtdiMuh 62, and the Faschingbauer abbreviated MMP163 (short versions of the MMPI) and the Memorial Pain Assessment Card. 22 Manniche64 recently described a low back pain rating scale with which to measure pain intensity, disability, and physical impairment. This index scale allows rapid assessment and appears to have achieved a high level of interobserver agreement. Pain is a sensation with physiologic, psychologic, emotional, and behavioral dimensions. When deciding on a pain-assessment tool, the investigator must determine which aspect of pain he or she wishes to evaluate while keeping in mind the characteristics of the group of patients, their backgrounds, and their communication skills. Making the proper choice will facilitate the acquisition of meaningful data and the formulation of valid conclusions.
ANNALS OF EMERGENCY MEDICINE
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PAIN-MEASUREMENT TOOLS
~o, Spence &"M~lrpt~y
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11. Jansen MP, KarolyP, BraverS: The measurementof clinical pain intensity: A comparisonof methods. Pain1986;27:117-126.
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41. Swanston M, AbrahamC, Macrae WA, et ah Pain assessmentwith interactivecomputeranimation. Pain1993;53:347-351.
13. Spans H, Pugh GC: Measurementof VAS pain scoreswith a 'magic screen' (letter). Anaesthesia 1992;47:359-369.
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14. Ekb/omA, Hansson P: Pain intensity measurementsin patients with acute pain receiving afferent stimulation. J Neurol NeurosurgPsychiatry1988;51:481-486.
43. Manna SL, JacobsenPB, ReddWH: Assessmentof acute pediatric pain: Do child self-report, parent ratings, and nurse ratings measurethe same phenomenon?Pain 1992;48:45-52.
15. BarkerM, Hughes B: Using a tool for pain assessment. Nursing TimesI990;86:50-52.
44. SengstakenEA, King SA: The problemsof pain and its detection amonggeriatric nursing home residents. J Am GeriatrSoc 1993;41:541-544.
16. McCafferyM, BeebeA: Pain: Clinical manual for nursing practice. St. Louis: Mosby, 1989:22. 17. Dixon JS, Bird HA: Reproducibility along a 10 cm vertical visual analoguescale. Ann Rheum Dis 1981;40:87-89. 18. Herr KA, Mobily PR: Comparisonof selected pain assessmenttools for usa with the elderly. Appl Nurs Res 1993;6:39-46. 19. Sriwatanakul K, KelvieW, LasagnaL, et el: Studies with different types of visual analogue scales for measurementof pain. ClinPharmacolThor1983;34:234-239. 20. Gramling SE, Elliott TR: Efficient pain assessmentin clinicaJ settings. BehavRes Ther 1992;30:71-73. 21. FishmanB, Pasternak5, Wallenstein SL, et el: The Memorial Pain AssessmentCard. Cancer 1987;60:1151-1158.
45. Miller RD (ed):Anesthesia.St Louis: Mosby, 1994:2328. 46. EokelaML: Pain relief can reduce hypoxemiain distressed neonatesduring routine treatment procedures. Pediatrics1994;93:379-383. 47. ChenACN: Human brain measuresof clinical pain: A review. I. Topographicmappings. Pain 1993;54:115-132. 48. RushforthJA, LeveneMh Behavioural responseto pain in healthy neonates.Arch Dis Child FetaINeonatal Ed 1994;70(suppl):E174-F176. 49. EdwardsRB: Paediatricpain chart (letter). Anaesthesia1994;49:91-92. 50. GracelyRH, Dubner B: Reliability and validity of verbal descriptorscales of painfulness. Pain 1987; 29:175-185.
22. Price DD, Bush FM, Long S, et el: A comparisonof pain measurementcharacteristicsof mechanical visual analogueand simple numerical scares. Pain 1994:56: 217-226.
51. GracelyRH: Evaluationof multi-dimensional pain scales (editorial). Pain 1992;48:297-300.
23. DeConnoF, CaraceniA, GambaA, et ah Pain measurementin cancerpatients: A comparison of six methods. Pain 1994;57:161-166.
52. Melzack R: The McGill Pain Questionnaire,in Melzack R (ed): PainMeasurementand Assessment. New York: RavenPress, 1983:41-47.
24. Linton SJ, GotestamKG: A clinical comparisonof two pain scales: Correlation,remembering chronic pain, and a measureof camp/lance. Pain 1983;17:57-65.
53. Holroyd KA, Holm JE, Keefe EJ, et al: A multbcenterevaluationof the McGill Pain Questionnaire: results from more than 1700 chronic pain patients. Pain 1992; 48:301-311.
25. Turk DC, RudyTE, Sorkin BA: Neglectedtopics in chronic pain treatment outcomestudies: Determination of success. Pain 1993;53:3-16.
54. Lowe NK, Walker SN, MacOallum RC: Confirmingthe theoretical structure of the MoGill Pain Questionnaire in acute clinical pain. Pain1991;46:53-60.
26. Jansen MP, Karoly, P, O'Riordan EF, et ah The subjective experienceof acute pain: An assessmentof the utility of 10 indices. ClinJPain 1989;5:153-159.
55. BoureauF, Luu M, DoubrereJF: Comparativestudy of the validity of four FrenchMcGili Pain Questionnaire(MPQ)versions. Pain1992;50:59-65.
27. Tyler DC, Tu A, Douthit J, et eh Toward validation of pain measurementtools for children: A pilot study, Pain 1993;52:301-309.
56. Strand LI, Wisnes AR: The developmentof a Norwegian pain questionnaire.Pain 1991;46:6165.
28. McGrath PA: An assessmentof children's pain: A review of behavioral, physiological and direct scaling techniques. Pain1987;31:147-176.
57. LoveAW, PeckCL: The MMPI and psychologicalfactors in chronic low back pain: A review. Pain 1987; 28:1-12.
29. Thompson KL, Varni JW: A developmental cognitive-biobehaviouralapproachto pediatric pain assessment.Pain1986;25:283-296.
58. Kerns RD, Turk DC, RudyTE: The West Haven-Yale Multidimensional Pain Inventory.Pain 1985;23:345-356.
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59. Vlaeyen JWS, Pernot BF, Kole-Snijders AM, et ah Assessment of the components of observed chronic pain behaviour:The Checklist for Interpersonal Pain Behavieur. Pain1990; 43:337-347.
Reprint no. 47/1171810
Address for reprints:
60. Edwards LO, PearoeSA, Turner-StokesL, et ah The Pain Beliefs Questionnaire:An investigation of beliefs in the causes and consequencesof pain. Pain1992;51:267-272.
Kendall He, MD
61. Melzack R: The short-form McGill Pain Questionnaire. Pain1£87;30:191-197.
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43 2
Department of Emergency Medicine
Canada V5Z 1M9
ANNALS OF EMERGENCY MEDICINE
27:4
APRIL 1996
Reviewof Pain-Measurement Tools From the Department of Emergency Medicine, Vancouver Hospital and Hearth Sciences Centre, University of British Columbia, Vancouver, British Columbia*; the:Division of Emergency Medicine, Department of Medicine, University c,f Toronto and St Michael's Hospital, Toronto, Ontario'; Children's Hospital and Dalhousie University, Halifax, Nova Scotia'; and the Department of Emergency Medicine, ChedokeMcMaster Hospitals, Hamilton, Ontario, Canada. '~
Kendall He, MD* Julie Spence, MD* Michael F Murphy, MD§lj
From the First International Symposium on Pain Research in Emergency Medicine, Montreal, October 1994. Copyright © by the American College of Emergency Physicians.
APRIL 1996
27:4
ANNALS OF EMERGENCY MEDICINE
[Ho K, Spence J, Murphy MF: Review of pain-measurement tools. Ann EmergMed Apri11996;27:427-432.] INTRODUCTION Traditionally, emergency physicians have not used objective measures to assess level of pain or relief of pain. Although pain is common, measurement of the intensity of pain and determination of the extent to which that pain affects an individual can be far from simple. Unlike onedimensional parameters such as heart rate that can be easily quantified, pain is a complex and multidimensional phenomenon. 1 First, pain can be generated by different mechanisms, including tissue ischemia, muscle contraction, and direct tissue damage from trauma. In addition, the peripheral and central nervous systems not only passively carry the nociceptive signal for cognitive processing but also actively modify it along the way. 2,3 Finally, interpretation of the nociceptive signal includes psychologic, physiologic, emotional, and behavioral dimensions. Because of these characteristics, the perception of and the response to pain vary greatly; the intensity of pain cannot be inferred from the degree of tissue damage observed. 4 There are no accurate physiologic or clinical signs that can be used to objectively measure pain. Reliance on health care worker assessment of patient pain results in underestimation of the intensity of that pain. According to the National Institutes for Health (NIH), patient selfreporting is the "most reliable indicator of the existence and intensity of pain. ''5 Subjective pain measures that may need to be quantified include intensity, time course, quality, impact, and personal meaning. The ideal tool in the assessment of pain should include the identification of the presence of pain, as well as the progress of pain with time or treatment. 6 Also, this tool should be applicable to any individual, regardless of psychologic, emotional, or cultural background. To that end, unidimensional pain scales have been developed. Because of their ease of use, these
427
PAIN-MEASUREMENT TOOLS Ho, 5pence& Murphy
scales have become popular tools with which to quantitare pain relief and pain intensity. The three scales most frequently used to assess acute pain are the numeric rating scale (NRS), the visual analogue scale (VAS), and the adjective rating scale (ARS). Although popular for study purposes, unidimensional pain scales present certain problems. They were initially developed for use in experimental pain trials m which pain was limited and controlled. Clinical pain is different in that it may become persistent, unbearable, and beyond the individual's control. It is also often associated with a strong emotional component not seen in the experimental setting. Pain scales tend to focus only on pain intensity, with increased risk of oversimplification of the experience, r Furthermore, actual measurements are relative only to the individual being assessed. Identical stimuli applied to different individuals can yield markedly different scores. Thus the numbers, although reproducible to a high degree for each individual, are simply estimates of the perception of the pain, based on past personal experience. Quantification of the experience requires the individual to abstract and quantitate the sensation, s The use of unidimensional pain scales is more appropriate in the setting of acute pain than chronic pain. Chronic pain is usually associated with other dements such as degree of support and depression. The assessment of chronic pain often requires more complex evaluation t o o l s In this article we attempt to catalog the tools that have been reported in the literature. We will emphasize the unidimensional tools, for they are more relevant to areas of research likely to be explored in the emergency setting. They are categorized according to method of evaluation, together with general comments about the characteristics of each class of tools. UNIDIMENSIONAL ASSESSMENT TOOLS BASED ON SELF-REPORT
Before any pain-rating tool is used, the goal of the study must be identified because certain pain tools have clear advantages. Unidimensional pain scales are frequently used when a single, clearly defined question is to be answered. They are easy to understand and use and place a minimal burden on the patient. Scales with a wide range of possible scores ensure sensitivity to intervention and permit statistical evaluation. The pain-assessment tools in this category measure the intensity of pain on the basis of the patient's self-report. No attempt is made to assess the behavioral or physiologic response to pain or
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the psychologic, emotional, or cultural influences on the interpretation of pain. Visual analog scale Because the most commonly used and validated tool in this category is the visual analog scale (VAS)8,9, we will discuss it the most thoroughly. The VAS was introduced in the early part of the century as a psychologic assessment tool for the measurement pain and mood. The popularity of the VAS stems from its simple construction and ease of use. It is best used in the sequential evaluation of pain; the broad range of scores allows subsequent statistical analysis. The scale comprises a 10-cm line with descriptive phrases at either end. In most pain studies the range is from "no pain" to "severe pain" or "worst pain ever." Selected phrases should be easily understood and not so extreme as to not be chosen. Patients estimate their level of pain by placing a mark on the line; the distance from the "no pain" point is then measured. No intermediate marks should be placed along the length of the line; this leads to clustering of responses. 8't° A 10-cm horizontal line is the most widely used version of the VAS, and the most widely validated, t o, 1 Once a length has been chosen, it should remain constant throughout a study. Variation in responses has been seen in individuals who are asked to mark lines of different lengths. Many variations on this theme exist, including slide rules 12, numeric markings on top of the line 13,14, vertical lines 15,10, and labeling of the line with descriptive words. ~4 None of these variations has a clear advantage, although the version chosen should be used for the duration of a study, s'11'17-19 Data are commonly analyzed with a 100-point scale corresponding to the 10-cm line. There is no mechanism for validating individual scores because they are a subjective interpretation of a sensation. Apparent discrepancies in absolute self-reported pain scores may be seen in patients with high scores in situations in which the health care worker believes there should be little pain. 5 Motivational reasons (eg, a patient believing he or she will be treated more promptly), as well as emotional reasons (eg, fear of serious illness or of losing a job), often account for these apparent discrepancies. The initial rating assigned by the patient is therefore less meaningful than changes in that rating in response to interventions, r' i0 The VAS has been found to be a valid and reliable means of assessing pain, depression, anxiety, and mood. 8 Different aspects of a painful experience may be rated with separate VASs, permitting more complete understanding of the patient's suffering, r,2°'21 The VAS has been compared with other pain scales and validated through cross-modal-
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ity matching. 7,s, lo,1 l, 18,22,23 A strong correlation exists between successive measurements of pain made with the use of a VAS, confirming reproducibility of the tool in the clinical setting. 8,1° The VAS also shows a uniform distribution of response when used in unselected patient populations. It appears to be less useful in the study of chronic pain. 7,18,24,25 A failure rate of approximately 7% is seen in the adult population. 7,8,26 The use of the VAS in the pediatric population has also been studied extensively Children older than 5 years have been shown to be able to use the scale. 27 This ability appears to be independent of sex, background, education, or health status. 2s tn younger patients, aged 2 to 5 years, failure rates of 7% to 11% have been quoted. Surprisingly, these rates are no worse than those in the adult population. 29 In this younger age group, modified VASs are often used. The color scale, on which the child is asked to interpret pain as red, black, or purple, is one example of a modified VAS.27 Another, the Faces Pain Scale, contains pictures of faces ranging in expression from happy to a sad face with tears. 2s A more detailed discussion of pediatric pain assessment appears in "Pediatric Pain Control" [Terndrup TEl 1996;27:466-470]. Pain assessment in older patients may be difficult because hearing, visual, and psychomotor deficits may interfere with pain scale comprehension and use. 5,1s Problems with memory and abstraction also make the application of VAS challenging, s J1 Verbal descriptors and pain thermometers may be preferred tools for pain assessment. Because of a relatively high failure rate, up to 25%, visual analogue scales should be avoided if a highly reproducible response rate is required for analysis, r,ls Statistical analysis of data derived from VAS scores is somewhat controversial. The information is more complex than simple ordinal data and yet is not clearly classified as interval data. 8 Parametric techniques have been used to analyze psychologic data which are similar to those derived from pain scales. 3° The use of parametric techniques does not alter the risk of type I or II errors, whereas nonparametric tests may increase the risk of type II errors. VAS scores do allow calculation of statistically significant changes in pain scores. There is no clearly defined manner in which to relate this to clinical outcome, limiting the utility of the test. 3~,32 The importance of distinguishing statistical from clinical significance will be discussed later in these proceedings. Verbal descriptors or outcome data may be used in conjunction with VAS scores to more clearly define outcomes. 33'34
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Other unidimensional tools Another popular tool with which to assess acute pain is the numeric rating scale (NRS). The patient is asked to choose one number, from a numeric range, that best reflects the intensity of pain. A frequently used range in the assessment of ischemic cardiac pain is 0 to 10 (NRS-11). Another customary range is 0 to 100 (NRS-101). ~'12 This scale appears to yield better results than the VAS for patients with chronic pain 11 and illiterate patients. 35 A variation on the NRS is the 11-point box scale. The patient is asked to circle one of the 11 boxes arranged horizontally on a straight line, each containing a number from 0 to 10. ~2,35 The verbal rating scale (VRS) comprises carefully chosen phrases, arranged in an order to describe increasing perception of the intensity of pare. The patient is asked to choose the phrase that best describes his or her pain. s2,15,35 Pain drawings have also been advocated as an informative tool. Instead of assessing the overall intensity of pain, this method identifies the distribution of pain and the characteristics of the pain syndromes experienced. 36-3s On a line drawing of a person, the patient is asked to mark the areas where he or she feels pain. The patient is then asked to use different shadings to indicate the quality of pain (eg, stabbing, pins and needles, cramps, numbness). Other methods of self-report include the single-digit sign, in which the patient is asked to indicate the location of maximal pain with a single finger. 39,4° (This method is of little value in research.) With the rapid increase of computerization in medicine, evaluation tools involving computer animation may emerge. 4~ Observation of behavioral and physiologic responses Although behavioral patterns and vital signs are inferior to self-report in the accurate assessment of pain 42, in some circumstances self-report is not feasible: in the postoperative period, when the patient is still under the influence of anesthetics; in the absence of adequate verbal skills, such as in neonates and children younger than 3 yearsl'43; and in the cases of mentally challenged patients and some geriatric patients with regression of linguistic ability. ~4 In such circumstances the severity of pain can be estimated only from the patients' physiologic and behavioral responses. Physiologic responses to pain, which are numerous, are seen in the respiratory, cardiovascular, gastrointestinal, urinary, neuroendocrine, and metabolic systems. ~5 Many of these responses can be attenuated or eliminated through the provision of adequate analgesia. A nonspecific increase in minute ventilation occurs with painful stimuli. However, with chest and upper-
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abdominal injuries and wounds, abdominal distension may occur, possibly resulting in reductions in vital capacity, tidal volume, residual volume, functional residual capacity, and 1-second forced expiratory volume. In neonates it has been shown that SaO 2 decreases during painful procedures. 46 The most widely appreciated, and perhaps most easily identified, responses to acute painful stimuli occur in the cardiovascular system as a result of increased Sympathetic tone. Heart rate:, stroke volume, blood pressure, cardiac work, and M v o 2 a r e decreased. Patients in pain often reduce their physical activity, resulting in increased venous stasis and platelet aggregation and, ultimately, in increased risk of deep-vein thrombosis. Nausea, vomiting, and ileus are considered gastrointestinal autonomic concomitants of pain. General hypomotility of the urinary tract and difficulty in urination are likely mediated by the same autonomic stimulation. Suprasegmental reflex responses lead to increased sympathetic tone and catecholamine secretion. Increased catabolic hormone secretion leads to increases in serum levels of adrenocorticotropic hormone, antidiuretic hormone, growth hormone, cyclic AMP, glucagon, aldosterone, and renin/angiotensin I2. The effects are predictable and include sodium and water retention, as well as increased levels of blood glucose, free fatty acids, ketone bodies, and lactate. Physiologic :measurements to assess the intensity of pain such as topographic mapping of the brain 4r and analyses of blood or urine for determination of changes in circulating neurotransmitters and other chemicals have been made. These modalities still need much refinement and are not currently appropriate for clinical application. Two observational tools based on behavioral response have been proposed for the assessment of pain in neonates and young children. 1,48 For the neonatal group, five behavioral responses were chosen for observation: brow bulge, eye squeeze, nasolabial furrow, open mouth, and crying. Each response is given a score of 1 if present, with a maximal score of 5. 4s For the pediatric group, 10 behavioral responses were assessed, s A pediatric pain chart is another model proposed for the observation of behavior after surgery. 49 Mechanical methods of reporting pain intensity have been proposed, such as squeezing a piece of calibrated equipment to demonstrate the intensity of pain. 5° However, such methods generally prove inferior to verbalreport methods. Multidimensional assessment tools Whereas the unidimensional tools measure mainly the intensity of pain,
43O
multidimensional methods also take into account the other factors that influence its perception. These factors include the quality and the temporal sequence of pain, the affective contributions, and the patient's belief system. Although this comprehensive approach is important, use and interpretation of these tools are difficult because of their complexity. 51 In addition, the length of time required to complete the evaluations usually make them impractical in the emergency setting. The best-known example of this category is the McGill Pain Questionnaire (MPQ). 52 It identifies key words that describe the different qualities of pain and classes them into four major groups: sensory, affective, evaluative, and miscellaneous. Each word is assigned a rank value, and the sum of the rank values of the words chosen becomes the pain rating index (PRI), The patient is also asked to give a value from 0 to 5 at the time of the questionnaire to represent the overall pain intensity; this number is termed the present pain intensity (PPI). Finally, the number of words chosen to describe the pain is also counted. Over time, the MPQ has shown good consistency and reliability in the evaluation of acute and chronic pain 53,54 and has been translated to and validated in many other languages+55,56 Some other examples of assessment tools used to measure the multidimensional aspects of pain include the Minnesota Multiphasic Personality Inventory (MMPI) 57, the West Haven-Yale Multidimensional Pain Inventory5s, the Checklist for Interpersonal Pain Behavior 59, and the Pain Beliefs Questionnaire. 6° Because these questionnaires are often long and tedious to administer, abbreviated versions simplify the task without losing the multidimensional assessment. Some examples include the short-form MPQ 61, the MtdiMuh 62, and the Faschingbauer abbreviated MMP163 (short versions of the MMPI) and the Memorial Pain Assessment Card. 22 Manniche64 recently described a low back pain rating scale with which to measure pain intensity, disability, and physical impairment. This index scale allows rapid assessment and appears to have achieved a high level of interobserver agreement. Pain is a sensation with physiologic, psychologic, emotional, and behavioral dimensions. When deciding on a pain-assessment tool, the investigator must determine which aspect of pain he or she wishes to evaluate while keeping in mind the characteristics of the group of patients, their backgrounds, and their communication skills. Making the proper choice will facilitate the acquisition of meaningful data and the formulation of valid conclusions.
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Department of Emergency Medicine
Canada V5Z 1M9
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