An alternative terminology for pain assessment Werner Ceusters Department of Biomedical Informatics University at Buffalo Buffalo, NY 14203, USA Email: ceusters@buffalo.edu Abstract — Background: the International Association for unpleasant sensory and emotional experience associated the Study of Pain (IASP) publishes since 1986 a relatively with actual or potential tissue damage, or described in frequently updated list of pain terms with corresponding terms of such damage’. definitions and clarificatory notes currently known as the For terms to be eligible as representational units in a ‘IASP   Taxonomy’.   The   last   update,   i.e.   the   May   2012   version   realism-based ontology such as OPMQoL, they must not of this taxonomy, was subjected to an analysis with the goal to assess whether the definitions of the IASP terms that are used only (1) denote entities that can be classified following the to describe findings of somatosensory testing and pain principles of Ontological Realism [7], but also (2) be assessment satisfy the conditions for these terms to become defined using Aristotelian definitions which specify the part of a realism-based ontology. Results: the taxonomy was necessary and sufficient conditions for class membership, found to be built on definitions that are not in every case based and further lead to a taxonomy based on single inheritance on necessary and sufficient conditions, nor satisfy the single [8]. The goal of the work reported on here was to assess the inheritance principle for realism-based ontologies. adherence of the IASP pain assessment definitions to this Furthermore, although the documentation about introduced second condition and to find ways for remediation if non- changes provided by the IASP makes it clear that the compliance was found. terminology authors tried to solve ambiguities and unclarities present in previous versions, they did not succeed completely and introduced even some inconsistencies. The analysis Allodynia: pain due to a stimulus that does not normally provoke pain. demonstrates that the main cause for this is not the choice of Note: The stimulus leads to an unexpectedly painful response. differentiating characteristics, but rather insufficient attention Analgesia: absence of pain in response to stimulation which would to the wide variability in stimulus/response combinations that normally be painful. these characteristics reveal. Conclusions: the IASP taxonomy is Dysesthesia: an unpleasant abnormal sensation, whether spontaneous or not fit to form the basis for a realism-based ontology. A new evoked. Note: Special cases of dysesthesia include hyperalgesia and allodynia. representation framework for describing pain assessment Hyperalgesia: increased pain from a stimulus that normally provokes findings more accurately using the same set of differentiae is pain. proposed and its correspondence with the traditional Hyperesthesia: increased sensitivity to stimulation, excluding the terminology explained. special senses. Hyperpathia: a painful syndrome characterized by an abnormally Keywords—pain terminology, ontological realism painful reaction to a stimulus. Hypoalgesia: diminished pain in response to a normally painful I. BACKGROUND stimulus. The Ontology for Pain-Related Mental Health and Quality Hypoesthesia: decreased sensitivity to stimulation, excluding the special senses. of Life (OPMQoL) is being developed as part of the Paresthesia: an abnormal sensation, whether spontaneous or evoked. NIDCR-funded project R01DE021917 with the goal to Note: paresthesia is to be used to describe an abnormal sensation that integrate five datasets gathered in four different countries is not unpleasant. from patients suffering from one or other form of orofacial Table 1 - Pain terms analyzed pain [1, 2]. Part of the data in these datasets describe findings that are based on the various kinds of responses II. METHODS that patients may report when subjected to stimuli to test Based on the definitions of the terms studied – note that their somatosensory status and that are typically described table 1 contains only part of the relevant notes and that the using   terms   such   as   ‘allodynia’,   ‘hyperesthesia’,   and   so   reader should for complete understanding of the analysis forth. Although these terms were already in practice since at method consult reference [6] - an analysis framework was least the early 19th century [3], standard definitions for these designed by introducing nine hierarchically organized terms were first proposed in 1979 [4] and are since then variables reflecting the type of stimulus, the presence or regularly updated by the International Association for the absence of a response, and the type of response when Study of Pain (IASP), in print for the last time in 1994 [5], present, when a patient is subjected to a pain assessment with more regular electronic updates on the IASP webpage investigation. The allowed values for these variables were [6] the last one in May 2012 (Table 1). These definitions are defined, depending on what the variable stands for, either on further   based   on   the   IASP   definition   for   ‘pain’ as   ‘an a nominal or ordinal scale (Table 2).  49   Variable Values 4. if a response is present, it may be either (4a) selective, Stimulus application Y(es) i.e. exclusively being unpleasant, painful, or of modus modus M level Threshold B(elow), O(n), A(bove) M in isolation, or (4b) combining either a modus M and Pain level Threshold B(elow), O(n), A(bove) non-painful unpleasant response, or a modus M and Response to stimulus Y(es), N(o) painful response; modus M Response Y(es), N(o) modus M Intensity L(ess), C(oncordant), H(igh) 5. all pain responses are unpleasant, thus following the Unpleasant response Y(es), N(o) IASP   definition   for   ‘pain’ as ‘an unpleasant sensory Pain Response Y(es), N(o) and emotional experience associated with actual or Pain Intensity L(ess), C(oncordant), H(igh) potential tissue damage, or described in terms of such Table 2 - Basic analysis framework variables, values and definitions damage’,   but   an   unpleasant   response   does   not   need   to   be painful. The next step consisted of identifying and representing As a third step, each combination was assessed for whether all theoretically possible stimulus/response combinations, a it could figure as an exemplar for each of the terms of Table part of which is displayed in Table 3. 1. Table 4 provides an example of this step for the IASP- Although the maximal theoretical number of possible definition   of   ‘allodynia’   without   taking   the   note   into   combinations would be 1296 (1*3*3*2*2*3*2*2*3), the account. A complication at this phase was that the actual number is only 130 because of the hierarchical definitions and notes left certain questions with respect to organization of the variables which implements the inclusion and exclusion criteria unanswered. It was thus for following dependencies typical for somatosensory and pain many definitions required to find meaningful subgroups and assessment studies [9]: for some of these subgroups the IASP documentation did 1. each stimulus, whether to test either somatosensory not provide enough information to assess whether they status (e.g. temperature, pressure, pin prick, and so represent intended interpretations, although from a forth,  henceforth  called  ‘modus  M’)  or  pain  sensitivity,   terminological and ontological perspective perfectly falls under one of three disjoint categories: (1) below plausible. Table 5 shows the subgroups identified as well as threshold, (2) on threshold, or (3) above threshold; the counts of stimulus/response combinations that fall under 2. modus M and pain stimuli may be given selectively or them. When subgroups were defined, the count for the together, thus resulting in 4 stimulation modes: (1) sub- (direct or indirect) parent terms were obtained by applying a threshold (for both pain and modus M), (2-3) modus M- Boolean OR operation on the combinations (and not the or pain-selective, and (4) bimodal (i.e. on or supra- mere addition as subgroups are not necessarily mutually threshold for both modus M and pain); exclusive). This information was in a fourth step used to 3. if there is no response to a stimulus, then there are no compute the exact overlap between these terms in function values for the intensity of modus M sensation and pain; of positive and negative co-occurrence. S Stimulus given Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y MT Modus M threshold A A A A A A A A A A A A A A A A A A A A PT Pain Threshold A A A A A A A A A A A A A A A A A A A A R Response N Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y MR Modus M response N N N N N Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y MI Modus M response Intensity - - - - - L L L L L C C C C C H H H H H U Unpleasant response N Y Y Y Y N Y Y Y Y N Y Y Y Y N Y Y Y Y PR Pain Response - N Y Y Y - N Y Y Y - N Y Y Y - N Y Y Y PI Pain Response Intensity - - L C H - - L C H - - L C H - - L C H Table 3 - Different  stimulus/response  combinations  possible  for  bimodal  above  (but  not  ‘on’)  threshold stimulation. Legend for values: Y = Yes, N = No, B = Below threshold stimulus, O = On threshold stimulus, A = Above threshold stimulus, H = Higher than expected response intensity, C = response intensity Concordant with stimulus, L = Lower than expected response intensity. S Stimulus given Y Y Y Y Y Y Y Y Y Y MT Modus M threshold B B O O O O A A A A PT Pain Threshold B B B B B B B B B B R Response Y Y Y Y Y Y Y Y Y Y MR Modus M response N Y N Y Y Y N Y Y Y MI Modus M response Intensity - H - L C H - L C H U Unpleasant response Y Y Y Y Y Y Y Y Y Y PR Pain Response Y Y Y Y Y Y Y Y Y Y PI Pain Response Intensity H H H H H H H H H H ALLO-D Y Y Y Y Y Y Y Y Y Y Table 4 - Possible stimulus/response combinations for Allodynia (following the IASP definition strictly). Legend for values: Y = Yes, N = No, B = Below threshold stimulus, O = On threshold stimulus, A = Above threshold stimulus, H = Higher than expected response intensity, C = response intensity Concordant with stimulus, L = Lower than expected response intensity.  50   Acronym Term (plus meaning) N Figure 1 - IASP pain assessment terminology hierarchy CONC Normal case 9 ABSENCE SENSATION SENSITIVITY SYNDROME ALLO-D allodynia (definition): unexpected evoked pain 10 ALLO-N allodynia (note): unexpected more intense evoked pain 30 Paresthesia ANAL analgesia: unexpected absence of evoked pain 40 DYS-E evoked dysesthesia 80 DYS-EP painful evoked dysesthesia 50 PAIN Dysesthesia Hyperesthesia Hypoesthesia DYS-EU non-painful evoked dysesthesia 30 HYPERA hyperalgesia: unexpected more intense evoked pain 20 HYPERE hyperesthesia = increased sensitivity to stimulation 81 HYPERE-I unexpected more intense evoked sensation 42 Analgesia Hypoalgesia Allodynia Hyperalgesia Hyperpathia HYPERE-IP unexpected more intense evoked pain 20 HYPERE-IM unexpected more intense evoked modus M 26 HYPERE-P unexpected presence of evoked sensation 49 nothing which is a kind of one can also be a kind of the other. HYPERE-PU unexpected evoked unpleasant sensation other than 30 In addition, already a superficial reading of these terms and pain accompanying notes reveals ambiguities and inconsistencies. HYPERE-PP unexpected pain 10 The   definition   of   ‘allodynia’, for instance, indicates that the HYPERE-PM unexpected modus M 13 term should be used for pain evoked after applying a stimulus HYPERP hyperpathia 30 HYPOALG hypoalgesia 20 which is below the normal pain threshold. The corresponding HYPOE hypoesthesia = decreased sensitivity to stimulation 58 note however suggests that also a response on an above- HYPOE-P decreased sensitivity to pain stimulation 40 threshold stimulus may count as such when the stimulus leads HYPOE-PL less pain to pain stimulation 20 to   more   pain   than   expected.   The   note   for   ‘dysesthesia’, as HYPOE-PA non painful unpleasant response to pain stimulation 20 many similar notes for other terms which for space reasons are HYPOE-M decreased sensitivity to modus M stimulation 26 not reproduced in Table 1 but can be found in reference [6], HYPOE-BI decreased sensitivity to both kinds of stimulation 8 PAR-D-E evoked paresthesia (definition) 81 indicate that there is considerable overlap between the terms. PAR-D-EP painful evoked paresthesia 30 B. Traditional pain assessment terminology shows PAR-D-EU non-painful unpleasant evoked paresthesia 30 considerable overlap PAR-D-EN non-painful not unpleasant evoked paresthesia 39 PAR-N-E evoked paresthesia (note) 19 All terms of Table 1 could be mapped to the stimulus/response Table 5 - Terms and ontological subgroups for the IASP pain assessment combinations. Table 6 illustrates how the parent terms relate terminology. Legend: N = number of stimulus/response combinations to each other in function of the stimulus/response applicable (max = 130). combinations. The individual cells contain the counts for the This  step  answers  thus   for  each  term  pair  ‘A  B’  the  question   overlap, if any. For example, the overlap cells between which and how many of the possible stimulus/response hyperesthesia and hypoalgesia show - surprisingly - that these combinations can occur in the pair combinations A+/B+, two conditions do not exclude each other: 6 of the 130 A+/B-, A-/B+,A-/B- where   ‘+’   and   ‘-’   indicate   that   the   combinations fall under both definitions, 14 are such that stimulus/response combination can, resp. cannot occur under hypoalgesia is present without hyperesthesia, 75 have the definition of the term. As it became clear at this point that hyperesthesia  without  hypoalgesia,  and  35  don’t  exhibit  either.   overlap was considerable, we designed a new terminology An additional color coding is used to highlight the type of based on definitions that minimize the potential overlap using overlap: white indicates a symmetric overlap for all 4 types of categories that are mutually exclusive. We then compared this co-occurrence as exemplified by the hyperesthesia/ new terminology with the traditional one, again using the hypoalgesia pair; green indicates mutual exclusion of the stimulus/response combinations as benchmark. positive occurrences, the other three colors indicate an asymmetric overlap. An ideal terminology would be such that the classes defined are mutually disjoint. For 12 (n) classes as III. RESULTS is the case here, there are 66 possible overlaps ( n*(n-1)/2 ) A. The IASP terms do not satisfy the criteria for direct between any pair of these classes, not counting overlap of a integration in a realism-based ontology. class with itself. As displayed in Table 6, there is no overlap in only 2 cases of these 66: (1) for hyperpathia versus allodynia Figure 1 - in which terms displayed in SMALL CAPS are the (taking the note into account), and (2) for hyperesthesia and immediate superordinate terms found in the definitions and the paresthesia (when the note is not taken into account). arrows stand for the classical subsumption relation [10] – demonstrates that although the individual definitions follow C. Novel terminology with less overlap the  Aristotelian  form  ‘an A is a B which C’,  the  defined  terms   Table 7 provides an overview of the proposed terminology do not lead all together to a complete directed graph with an which uses 6 variables (Response expectation, Main finding, overarching top, not even if all 29 IASP terms would be Sensation expectation, Sensation intensity, Sensation mode, included. Furthermore,   the   terms   ‘allodynia’ and and Stimulation type) that can take a number of values and ‘hyperalgesia’ have superordinate terms which under their which are strongly related to the variables and values used to standard meanings should represent disjoined classes: design the analysis framework of the 130 stimulus/response although sensation and sensitivity are certainly related, combinations.  51   /CONC CONC /ALLO-D ALLO-D CONC 9 0 /ALLO-N ALLO-N /CONC 0 121 ALLO-D 0 10 10 0 /ALLO-D 9 111 0 120 /ANAL ANAL ALLO-N 0 30 10 20 30 0 /ALLO-N 9 91 0 100 0 100 /DYS-E DYS-E ANAL 0 40 0 40 0 40 40 0 /HYPERA HYPERA /ANAL 9 81 10 80 30 60 0 90 DYS-E 0 80 10 70 30 50 20 60 80 0 /HYPERE HYPERE /DYS-E 9 41 0 50 0 50 20 30 0 50 HYPERA 0 20 0 20 20 0 0 20 20 0 20 0 /HYPERP HYPERP /HYPERA 9 101 10 100 10 100 40 70 60 50 0 110 /HYPOALG HYPERE 0 81 10 71 30 51 26 55 66 15 20 61 81 0 HYPOALG /HYPERE 9 40 0 49 0 49 14 35 14 35 0 49 0 49 HYPERP 0 30 10 20 30 0 0 30 30 0 20 10 30 0 30 0 /HYPOE /HYPERP 9 91 0 100 0 100 40 60 50 50 0 100 51 49 0 100 HYPOE HYPOALG 0 20 0 20 0 20 0 20 20 0 0 20 6 14 0 20 20 0 /PAR-D-E PAR-D-E /HYPOALG 9 101 10 100 30 80 40 70 60 50 20 90 75 35 30 80 0 110 HYPOE 0 58 2 56 6 52 24 34 48 10 4 54 34 24 6 52 20 38 58 0 /PAR-N-E PAR-N-E /HYPOE 9 63 8 64 24 48 16 56 32 40 16 56 47 25 24 48 0 72 0 72 PAR-D-E 0 81 10 71 30 51 26 55 66 15 20 61 81 0 30 51 6 75 34 47 81 0 /PAR-D-E 9 40 0 49 0 49 14 35 14 35 0 49 0 49 0 49 14 35 24 25 0 49 PAR-N-E 0 19 0 19 0 19 14 5 0 19 0 19 9 10 0 19 0 19 6 13 9 10 19 0 /PAR-N-E 9 102 10 101 30 81 26 85 80 31 20 91 72 39 30 81 20 91 52 59 72 39 0 111 Table 6 - Positive/negative contingency table for traditional pain terminology. A color coding is used for the 2-by-2 contingency tables to highlight the type of overlap: white indicates a symmetric overlap for all 4 types of co-occurrence; green indicates mutual exclusion of the positive occurrences, the other three colors indicate an asymmetric overlap. Response Main finding Sensation Sensation Sensation Stimulation expectation expectation intensity mode type Concordant Absence Concordant hypOresponsive Modal Sensation Subthreshold Stimulation Discordant Presence Discordant hypErresponsive Unpleasant Pain-specific Configuration Painful Modus-specific Bimodal CA---SS C A S S S DPDEMSS D P D E M S S S DPDEUSS D P D E U S S S DPDEPSS D P D E P S S S DC---SS D C S S S CA—MSP C A M S P S CPC-PSP C P C P S P S CC---SP C C S P S DA—PSP D A P S P S DPDOUSP D P D O U S P S DPDOPSP D P D O P S P S DPDEMSP D P D E M S P S DPDEPSP D P D E P S P S DC---SP D C S P S CA—USM C A U S M S CPC-MSM C P C M S M S CC---SM C C S M S DA—MSM D A M S M S DPDEMSM D P D E M S M S DPDEUSM D P D E U S M S DPDEPSM D P D E P S M S DC---SM D C S M S CPC-MSB C P C M S B S CPC-PSB C P C P S B S CC---SB C C S B S DA—MSB D A M S B S DA—PSB D A P S B S DPDOMSB D P D O M S B S DPDOUSB D P D O U S B S DPDOPSB D P D O P S B S DPDEMSB D P D E M S B S DPDEPSB D P D E P S B S DC---SB D C S B S Table 7 - Proposed alternative terminology  52   The values for sensation mode are to be interpreted as stimulation’   and   ‘discordant   presence   of   discordant   hyper- follows:   ‘modal’   means   that   there   is   only   a   modal   response   responsive  modal  sensation  after  subthreshold  stimulation’.   which  is  not  unpleasant  or  painful,  ‘unpleasant’  means  that  the   The left column of Table 7 contains for further reference in response is unpleasant but not painful, irrespective of whether Table 8 acronyms for the various possibilities formed by there   is   a   modal   response   as   well,   whereas   ‘painful’   means   means of the concatenation of the individual values for a there  is  only  a  painful  response.  ‘Subthreshold’  for  stimulation certain variable, excluding, for space reasons, the last type reflects a subthreshold stimulation for both pain and (constant)  ‘S’  for  ‘Stimulation’.   modus   M,   while   ‘bimodal’   indicates   an   above   threshold   Table 8 shows the extent to which the proposed stimulation for both modus M and pain. terminology categories suffer from a far less degree of As is the case for the analysis framework, some values are overlap, overlap being indicated by the cells in light and dark constrained by the values for some other variables. As an red background: only 23 overlaps of the total possible 325. example, when the value for stimulus intensity is ‘subthreshold’,   there   is   either   (1)   no   response   in   which   case   IV. DISCUSSION the value for response expectation is constrained to Our results in Table 5, combined with Table 1, clearly indicate ‘concordant’,  the   value   for   main finding to  ‘absence’,  and  all   that the traditional terminology is based on rather ambiguous other variables have no value, or (2) a response is present, in definitions and application recommendations some of which which case the values for response expectation and sensation lead to interpretations for which it is not clear whether they expectation are  both  constrained  to  ‘discordant’,  the  value  for   are intended or not. This is overwhelmingly obvious for the main finding to   ‘presence’,   and   the   value   for   sensation terms   ‘hyperesthesia’,   ‘hypoesthesia’ and   ‘paresthesia’. The intensity to   ‘hyper-responsive’.   The   constraints   make   once   latter is very broadly defined as an abnormal sensation, again the total number of possibilities lower than can be without   making   it   explicit   what   ‘abnormal’ exactly means: expected: 26, excluding the combinations with the value ‘abnormal’ may indeed be interpreted as anything what is not ‘configuration’  for  main  finding which are constructed by the expected, such as more or less intense pain than expected after boolean AND-ing and OR-ing of concordant and discordant giving a supra-threshold pain stimulus, or more or less intense situations. The terms for this terminology are then all of the pressure sensation than expected when giving a supra- form   ‘(Response expectation) (Main finding) of (Sensation threshold pressure stimulus. expectation) (Sensation intensity) (Sensation mode) sensation It may also be interpreted as feeling an itch - a form of after (Stimulation type) stimulation’ whereby the variables in unpleasant sensation - when giving a pressure stimulus with or italics are replaced by the terms for the allowed values, and without there being a pressure sensation, and so forth. The the words in bold are constant. As an example, the terms for note for   paresthesia,   in   contrast,   tells   us   that   only   ‘not the first two combinations in Table 7 are respectively unpleasant’ sensations should count as qualifying, which ‘concordant absence of sensation after subthreshold limits the number of possibilities considerably. CA---SS CA---SS 1 DPDEMSS DPDEMSS 1 DPDEUSS DPDEUSS 2 DPDEPSS DPDEPSS 2 CA--MSP CA--MSP 10 CPC-PSP CPC-PSP 2 4 DA--PSP DA--PSP 4 8 DPDOUSP DPDOUSP 2 4 4 DPDOPSP DPDOPSP 2 4 DPDEMSP DPDEMSP 2 2 DPDEPSP DPDEPSP 2 4 CA--USM CA--USM 8 CPC-MSM CPC-MSM 2 2 DA--MSM DA--MSM 2 6 DPDEMSM DPDEMSM 2 2 DPDEUSM DPDEUSM 2 8 DPDEPSM DPDEPSM 2 8 CPC-MSB CPC-MSB 20 CPC-PSB CPC-PSB 4 16 DA--MSB DA--MSB 4 20 DA--PSB DA--PSB 4 4 16 DPDOMSB DPDOMSB 4 DPDOUSB DPDOUSB 4 4 16 16 DPDOPSB DPDOPSB 4 4 16 DPDEMSB DPDEMSB 4 DPDEPSB DPDEPSB 4 4 16 Table 8 - Overlap between proposed pain assessment categories  53   It leaves however still many interpretations open, such as VIII. ACKNOWLEDGEMENTS whether the resulting sensation must be alien to the given The work described is funded in part by grant 1R01- stimulus - would an erotic feeling induced by providing a DE021917-01A1 from the National Institute of Dental and pressure stimulus to the hand count as such a non-unpleasant Craniofacial Research. 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