Anesth Prog 37:155-160 1990

Strategies for Classifying Chronic Orofacial Pain Patients Dennis C. Turk, PhD of University Pittsburgh School of Medicine, Pittsburgh, Pennsylvania

has been the absence of an agreed upon classification of chronic pain syndromes that is used on a systematic basis. The lack of availability of any universally accepted system of classification has resulted in a confusion and an inability of investigators as well as practitioners to compare observations and results of research. Bonica1 referred to the state of affairs in chronic pain as a "tower of Babel." Recently, several attempts have been made to rectify the absence of a classification system for chronic. Most notably, the Intemational Association for the Study of Pain (IASP) published a comprehensive taxonomy of chronic pain syndromes.2 On a more specific level, the International Headache Society (IHS) published a classification system for headache and craniofacial disorders.3 In the area of orofacial pain, several classification systems have been proposed.4-6 Classification refers to the process of discovering the best means of grouping individuals, objects, or events together when no prior format is available. The primary purpose of classification is to describe the structure and relationships of constituent individuals, objects, or events to each other and to similar individuals, objects, or events and to simplify these relationships in such a way that general statements can be made about classes. The two primary strategies for classification, theoretical and empirical, have a different underlying premise. The theoretical approach involves the testing of a priori theoretical formulations, whereas the empirical approach is inductive, seeking to identify naturally cooccurring sets of variables that characterize subgroups. Which of the two approaches is adopted will lead to quite different classification systems. Moreover, an infinite number of classification systems can be developed depending upon the rationale about common factors and the variables believed to discriminate among individuals, objects or events.

To communicate, understand, and prescribe treatment, it is essential that some consensually validated criteria be used to describe groups of patients who share a set of relevant attributes. Several classification systems have been developed to described relatively homogeneous subgroups of chronic pain patients. These systems have been based on theoretical perspectives of chronic pain syndromes tied to physical pathology. Alternative systems based on a priori psychological categories or empirically derived classifications also have been proposed. Some of the strengths and weaknesses of deductive and inductive approaches to classification are described, and the advantages of polydiagnostic and multiaxial approaches are described as alternatives to the traditional classification. Research on an empirically derived multiaxial classification for chronic pain is described and related to chronic orofacial pain.

ALTERNATIVE STRATEGIES FOR CLASSIFYING CHRONIC OROFACIAL PAIN To communicate, to understand, to conduct research, and to prescribe treatment for a disease, it is essential that some consensually validated criteria are used to describe groups of individuals who share a set of relevant attributes. One major factor that has inhibited the advancement of knowledge of chronic pain and consequently its treatment

THEORETICAL CLASSIFICATION A deductively derived taxonomy begins with a theoretically based statement about how individuals should differ and subsequently be categorized. Classification of diseases are usually based on a preconceived combination of characteristics (eg, symptoms, signs, test results), no

Address correspondence to Dr. Dennis C. Turk, Pain Evaluation and Treatment Institute, University of Pittsburgh School of Medicine, Baum Boulevard at Craig Street, Pittsburgh, PA 15213. © 1990 by the American Dental Society of Anesthesiology

ISSN 0003-3006/90/$3.50

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single characteristics being both necessary and sufficient for every member of the category, yet the group as a whole possesses a certain unity.7 Most classification systems used in medicine (eg, ICD-9) are based on consensus of a group of professionals and in that sense reflect the elimination of certain diagnostic features on which agreement could not be obtained. In theoretical classifications, preconceived categories are developed and individuals are "forced" into the most appropriate one, even if not all characteristics defining the category are present. EMPIRICAL CLASSIFICATION An alternative approach to classification is predicated on empirical procedures.The inductive approach begins with ordering and arranging objects or events into groups or sets based on some criteria. Individual elements (eg, symptoms, individual difference variables) are proposed as the basis for grouping. Those who advocate the use of empirically derived taxonomies maintain that the relationships of contiguity and similarity should be sought by quantitative analysis of the overall similarity of the individuals, based on the widest possible range of physical and functional characteristics of the individuals themselves. A number of empirical methods are available to determine categories statistically (eg, cluster analysis) that share a relationship derived directly from data v hypothesized relationships as seen with deductive, theoretical systems. Quanfifiability, repeatability, and objectivity are the hallmarks of the inductive approach. No a priori categories are included within the inductive approach. It is important to acknowledge, however, that because not every possible relevant factors can be measured, the use of an inductive approach is dependent upon what the investigator chooses to observe and assess. Thus, it is not a totally objective process that is completely atheoretical.

TRADITIONAL VIEWS OF CHRONIC PAIN How clinicians think about chronic pain will affect their decision to adopt a deductive or inductive approach to classification and whether they will use a unidimensional or multidimensional approach in either the development of their a priori categories, as in the deductive approach, or the set of variables they choose to use in developing an empirical classification system. Each of the proposed pain taxonomies developed focuses primarily upon the location, cause, and defining features of the symptoms. Thus, they tend to be deductive systems predicated on preconceived categories tied to the sensory features of the symptoms. The emphasis on sensory-physiological factors and the deductive approach

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in pain assessment and classification is no surprise. Traditionally, biomedical assessment of chronic pain patients have adhered to a somatic perspective of pain, whereby pain is viewed as a sensory phenomenon directly linked to the location and extent of physical pathology. From the unidimensional sensory perspective, when sufficient organic pathology cannot be identified, the patient's report of pain is attributed to psychological factors. Thus, classification moves from a unidimensional approach based on physical factors to a unidimensional classification based on psychological factors. Numerous attempts have been made to categorize individuals reporting persistent pain based on personality or psychopathological characteristics. Some have used a deductive approach to classifying pain patients according to preconceived categories8 whereas other have used inductive strategies to develop empirical subgroups.9 It is well recognized that pain is a complex phenomenon with many factors contributing to the perception and experience of sufferers and that people with very similar medical findings show widely variable pain responses.10 The dichotomous organic-psychogenic distinction fails to consider the complexity and the potential interrelationships between physical and psychosocial factors in chronic pain.

CLASSIFICATION OF CHRONIC OROFACIAL PAIN We can turn more specifically to examine assessment and classification of orofacial pain syndromes. Dental practitioners commonly characterize chronic orofacial pain patients such as temporomandibular disorder (TMD) sufferers by a common set of signs (eg, pain upon palpation of masticatory muscles, limitation or deviation of mandibular opening, joint sounds). There is growing consensus that persistent pain in the TM region probably reflects not a single clinical entity but a variety of conditions."'12 Thus, the diagnostic classification of TMD is probably a generic label for several related but distinct orofacial pain states.5," A number of factors have been proposed for the classification of subgroups of TMD patients. The most common approach is to view TMD as consisting of two subgroups based on presumed etiology, myogenic or arthralgic. The myogenic group is often further subdivided into muscular hyperarousal due to stress and muscular abnormalities associated with parafunctional oral habits (eg, "bruxing").'3 The arthralgic category is subdivided further on the basis of specific structural abnormalities.5"4 The approach described for classifying orofacial pain patients is a deductive one based on a priori theories of causality. Several problems are contained within available oral dysfunction/structural abnormalities (OD/SA) assessment

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Anesth Prog 37:155-160 1990 systems. Many rely on subjective reports of patients, others use unstandardized examination and diagnostic procedures that involve clinician interpretation based on uncali-

brated methods. In addition, it is often assumed that the criteria believed to be significant for diagnosis are consistently present. Dworkin, LeResche, and DeRouen15 note the failure of many OD/SA indices to report on the reliability of diagnostic procedures used to derive classification systems. They demonstrated the relatively low levels of interrater and test-retest reliability of typical criteria used. The important question of validity of many assessment systems has rarely been addressed. Recently, Van der Weele and Dibbets16 presented data that seriously challenges the validity of one of the most commonly used TMD indices, that proposed by Helkimo.'7 Additionally, there is an assumption among the developers of these classification systems that the results of the procedures can be simply summed,4 that is, findings within the overall assessment scheme are of equivalent diagnostic use. This assumption has yet to be confirmed. Compounding the diagnostic problem is the observation that signs and symptoms of different subcategories of TMD overlap considerably and more than one clinical problem can be present in the same patient at the same time.5'15 As noted, when clinicians who adopt a physical conceptualization are unable to establish the physical basis for reports of pain, they tend to attribute the report to an alternative unidimensional view based on psychological factors. Several investigators'8"9 have used inductive approaches to differentiate subgroups of TMD patients on traditional psychological measures such as the MMPI and the Symptom Checklist-90. The validity of groups differentiated on traditional psychological measures has yet to be demonstrated, nor have replications of subgroups based on these measures appeared in the literature.20 Other investigators21'22 have suggested that TMD patients actually differ on both the presence of derangements of the TMJ and psychological characteristics. However, the integration of physical assessments and psychological responses are treated as separate, presumably independent indices. A problem in classification of chronic pain is the failure of investigators to examine psychosocial and behavioral factors (eg, patients' perceptions of the impact of pain on their lives, how significant others respond to them, and limitation of activities). These factors have been found to be significant in heterogeneous groups of chronic pain patients23'24 as well as TMD patients.20'25 In sum, attempts to identify and treat subgroups of TMD patients have been deductive. That is, they are based on a priori theoretical assumptions regarding etiology and no attempts have been made to consider the development of an empirically derived taxonomy that integrates reliable physical findings with relevant psychosocial and behav-

ioral data. The distinction between subgroups has been assumed to be either physical or psychological. It is plausible, however, that patients with and without identified OD/SA may have other equally important characteristics in common, and physical findings may be only one, albeit important, factors that differentiates TMD subgroups. Thus, there may be an interrelationships between physical and psychological factors that contribute the report of symptoms and the disability associated with TMD.

POLYDIAGNOSTIC CLASSIFICATION A polydiagnostic approach has been proposed whereby multiple classifications are used simultaneously. 26 For example, it might be suggested that physical treatment should be directed toward the disease classification and other treatments focusing on a psychological taxonomy classification might be supplemental. In orofacial pain, the polydiagnostic approach would allow individuals to carry diagnoses based on different taxonomies, for example of a diagnosis of "TMJ internal derangement" without myalgia (TMJID-B21) and a classification of "hypochondrical, moderately distressed" from the empirical classification study of Butterworth and Deardorff.18 Adopting such an approach would serve the valuable function of encouraging diagnosticians to think concurrently in terms of different relevant diagnostic systems. Polydiagnosis might be of particular value when considering chronicity of disease and the changing pattern of response overtime. That is, both physical and psychological factors might contribute to the experience. At initial diagnosis, each of these two components might be differentially weighted for individual patients. Over time, both the physical and psychological may become synchronistic with physical factors leading to structural changes and psychological factors being exacerbated by the duration of the presence of chronic pain. The extent of the changing contribution of physical and psychological factors might be viewed over the course of a pain syndrome.

MULTIAXIAL ASSESSMENT A multiaxial diagnostic approach has been proposed for classification of pain syndromes2 and pain patients.23 This approach involves the systematic consideration of several parameters of illness simultaneously and requires the establishment of an appropriate scale for each parameter or axis. The multiaxial diagnostic approach attempts to present a comprehensive view of the patient's condition by articulating several parameters of a disease. A multiaxial approach differs from the polydiagnostic strategy described in that the patient's scores or rating on

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each of the axes are integrated to form the diagnostic classification. The categories of the multiaxial system may, of course, be theoretically or empirically derived. The determination of the most appropriate number of axes in a multiaxial system depends on the scope of clinical information required to reach a comprehensive diagnosis of the patient. The IASP multiaxial classification of chronic pain is a theoretical one. That is, the categories were based on consensus, and once the category was established each was rated along five quasi-independent predetermined axes deemed to be sufficiently comprehensive to classify all chronic pain syndromes (ie, axis I = region, axes II = system, axis III = temporal characteristics of pain: pattern of occurrence, axis IV = patient's statement of intensity: time since onset of pain, and axis V = etiology. Note that axes III and IV are based on patient report). Review of the chronic pain literature reveals that a number of factors, in addition to physical pathology, play a role in reports of pain and disability associated with chronic pain. Specifically, research suggests that patients' perceptions of pain, the impact of pain on their lives, and control of pain and life27; dysphoric mood; responses of significant others27'28; and levels of activity28 all contribute to the patient's disability. Thus, we need to consider classification of pain patients and not only classification of diseases according to location, system involved, time course, severity, and etiology. Turk and Rudy'2324 have proposed a classification system for chronic pain based on the empirical integration of physical, psychosocial, and behavioral data in assessing chronic pain and have labeled this approach a Multiaxial Assessment of Pain (MAP). The primary hypothesis was that certain modal patterns in psychological assessment data recur in chronic pain patients and that these patterns represent homogeneous subgroups of chronic pain patients, at least to some extent, independent of medical

diagnosis. To assess the psychosocial and behavioral factors, Turk and Rudy24 used the West Haven-Yale Multidimensional Pain Inventory (MPI).29 The MPI consists of a set empirically derived scales designed to assess chronic pain patients' appraisals of pain severity and impact, mood state, response from significant others, and interference with activities. In the first study24 a cluster analysis was performed on the MPI scales to group heterogeneous samples of chronic pain patients according to similarities between their profile patterns. Based on this analysis, three distinct patient profiles were identified, and the three-cluster solution was replicated on a second sample. The three profiles identified were labeled "dysfunctional" (DYS, 43% of the sample), "interpersonally distressed" (ID, 28% of the sample), and "adaptive copers" (AC, 29.5% of the sample). The DYS group was distinguished from the combined

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sample of chronic pain patients by higher than average levels of pain severity, life interference, and affective distress, and lower than average levels of life control and activity. The ID group was described by perceived levels of social support. The AC group was distinguished from the heterogeneous sample by significantly lower levels of pain severity, interference, affective distress, and greater perceptions of life control. Turk and Rudy24 noted that the groups identified did not differ significantly from each other in duration of pain, gender, or age. The groups did differ on the extent of physical pathology, with the DYS group demonstrating significantly more physical findings than the other two groups; however, the groups remained significantly different from each (ie, distinct) even when physical pathology was controlled by using it as a covariate.

GENERALIZABILITY OF THE MAP TAXONOMY The robustness of the MAP taxonomy was examined by studying several different pain syndromes, specifically, chronic low back pain (BP), headache (H), and TMD.30 As would be expected, the mean scores on each of the MPI scales were significantly different for each of the three patient samples. However, when mean differences were removed, the scale intercorrelations between the BP, TMD, and H groups were equivalent. When group mean scores were controlled for, a higher percentage of BP pain patients were classified as dysfunctional (62%) than the other two patient samples (46% and 44% for the TMD and H samples, respectively). A greater percentage of TMD and H patients than BP patients were classified as adaptive copers (32%, 31%, and 20% for the TMD, H, and BP samples, respectively), and no between group differences were observed for the interpersonally distressed (18%, 22%, and 25% for the BP, TMD, and H samples, respectively). All three syndrome groups, however, were represented in each of the MAP clusters. Thus, it is possible that TMD, H, and BP patients who are classified within the same subgroup may be more similar to each other than patients with the same diagnosis but who are classified within different subgroups. A related study of TMD patients31 contrasted the three subgroups of TMD patients identified in the described study. They concluded that the between cluster differences were independent of age, duration of pain, common TMD symptoms (eg, crepitation, joint sounds, intercisal opening, pain on palpation of muscles of mastication), and computed axial tomography. These results raise some question as to the predominance of physical factors as the primary determinants for establishing subgroups of TMD

patients.21 The results of the Turk and Rudy30 study suggest that

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a reliable, valid, and objective multivariate classification procedure can be developed for chronic pain patients. The three chronic pain patient profiles replicates earlier work24 and extends this approach and the taxonomy to other prevalent subgroups of chronic pain patients providing support for the generalizability, stability, and robustness of the multiaxial taxonomy. The taxonomy described in the reviewed studies24'30'31 only considered two of the three axes proposed in the MAP system, namely, psychosocial, behavioral, but not physical. One reason that physical pathology was not included was that no procedure was available that has been shown to reliably and validly assess physical pathology across a wide diversity of chronic pain patients.32 In a preliminary attempt to developed a screening measure of physical pathology that could be used to evaluate the range of chronic pain patient, Rudy, Turk, and Brena33 developed and evaluated an instrument they labeled "Medical Examination Diagnostic Information Coding System" (MEDICS). This evaluation approach is based on the weighted summation of pathology on 18 common medical examination and laboratory procedures frequently used in assessing chronic pain patients. In a second study, a reclustering of the MPI scores along with the inclusion of physical findings as assessed on the MEDICS resulted in the DYS group subdividing into two groups of relatively equal size. The two groups that resulted did not have significantly different MPI scores but were significantly different on physical findings. In short, the only factor that discriminated between these two groups was the extent of physical pathology. One subgroup labeled "impaired" was characterized by high levels of pain severity, interference, and affective distress with low levels of low control and higher than average levels of physical findings. The other subgroup identified as "dysfunctional," was virtually identical to the impaired group with the exception of significantly lower levels of physical pathology. The other original groups, ID and AC, remained unchanged by the inclusion of physical findings. Additional research with the MAP classification system is required. The empirically derived MAP system should contribute to our understanding of diverse groups of chronic pain patients, assist in evaluation and the prescription of specific therapeutic interventions, and further our ability to predict treatment outcome.

CONCLUSION The use of each of the proposed classification systems is tied to the psychometric adequacy of the classifications made. It is essential that the reliability, validity, and use of any taxometric system be established. It is important to establish the extent of agreement of multiple diagnosticians in classifying a set of patients within the proposed

system. As an example, in a study of the reliability of one of the primary axes of the IASP classification,2 it was demonstrated32 that the interjudge agreement for the presumed etiology (axis V) was poor with some categories within the axis having reliabilities lower than chance! These results call into question the adequacy of this axis if not the entire IASP taxonomy as it stands and suggest a need to modify the coding to improve reliability. The use of any classification system is best determined by actually using it to classify individuals. Does assignment of an individual to a class facilitate treatment decisions or predictions of future behavior? None of the classification systems described herein have had their use confirmed. The inability of a taxonomy to incorporate previously unnoticed cases suggests the need to revise (and sometime to scrap) the taxonomy. In any event a taxonomy should be seen as a heuristic device. Taxonomies should not be considered as set in stone. The very nature of taxonomies and their development suggests that they will be ever changing. The construction of classification systems and their eventual use are psychological processes, no matter how objective they may seem. What has been described herein are the two primary approaches to developing classification systems, namely, deductive and inductive and the use of polydiagnosis and multiaxial approaches. Some of the advantages and disadvantages of these strategies were noted. The important point, however, is the fundamental need to develop and evaluate taxometric systems that can facilitate communication, understanding, and treatment. If there are, indeed, different subgroups or clusters of patients, then the matching of treatment to patient characteristics would seem a reasonable altemative to treating all patients with the same diagnosis as a homogeneous group.

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8. Blumer D, Heilbronn M: The pain-prone disorder. J Nerv Ment Dis 1982;170:381-406. 9. Bradley LA, Prokop CK, Margolis R, Gentry WD: Multivariate analyses of the MMPI profiles of low back pain patients. J Behav Med 1978;1:253-272. 1O. Osterweis M, Kleinman A, Mechanic D: Pain and Disability: Clinical, Behavioral, and Public Policy Perspectives. Washington, DC, National Academy Press, 1987. 11. American Dental Association: Report of the president's conference on the examination, diagnosis and treatment of temporomandibular disorders. J Am Dent Assoc 1983;66:75-77. 12. Rugh JD, Solberg WK: Oral health status in the United States: Temporomandibular disorders. J Dent Educ 1985;49:398-405. 13. Laskin DM: Myofascial pain-dysfunction: Etiology. In: Samat BG, Laskin DM, eds., The Temporomandibular Joint: A Biological Basis for Clinical Practice, Springfield, IL, Charles C. Thomas, 1979;289-299. 14. Moss RA, Garrett J, Chiodo JF: Temporomandibular joint dysfunction and myofascial pain dysfunction: Parameters, etiology and treatment. Psychol Bull 1982;92:331-346. 15. Dworkin SF, LeResche L, DeRouen T: Reliability of clinical measurement in temporomandibular disorders. Clin J Pain 1988;4:89-100. 16. Van der Weele T, Dibbets JMH: Helkimo's index: a scale or just a set of symptoms? J Oral Rehab 1987;14:229-237. 17. Helkimo M: Studies of function and dysfunction of the masticatory system. II. Index of anamnestic and clinical dysfunction and occlusal state. Swed Dent J 1974;67:101-121. 18. Butterworth JC, Deardorff WW: Psychometric profiles of craniomandibular pain patients: Identifying specific subgroups. J Craniomandibular Pract 1987;5:225-232. 19. Lipton JA, Marbach JJ: Predictors of treatment outcome in patients with myofascial pain-dysfunction syndrome and organic temporomandibular joint disorders. J Prosthet Dent 1984;51:387-393. 20. Rugh JD, Solberg WK: Psychological implications in temporomandibular pain and dysfunction. Oral Sci Rev 1976;1:3-30.

Anesth Prog 37:155-160 1990 21. Eversole LR, Stone CE, Matheson D, Kaplan H: Psychometric profiles of facial pain. Oral Surg Oral Med Oral Pathol 1985;60:269-274. 22. Levitt SR, McKinney MW, Lundeen TF: The TMJ Scale: Cross-validation and reliability studies. J Craniomandibular Pract 1988;6:17-25. 23. Turk DC, Rudy TE: Toward a comprehensive assessment of chronic pain patients: A multiaxial approach. Behav Res Ther 1987;24:237-249. 24. Turk DC, Rudy TE: Toward an empirically derived taxonomy of chronic pain patients: Integration of psychological assessment data. J Consult Clin Psychol 1988;56:233-238. 25. Marbach JJ, Lipton JA: Biopsychosocial factors of the temporomandibular pain dysfunction syndrome: Relevance to restorative dentistry. Dent Clin North Am 1987;31:473486. 26. Bemer P, Katschnig H, Lenz G: Poly-diagnostic approach: A method to clarify incongruences among the classification of functional psychoses. Psychiatr J Univ Ottawa

1982;7:244 248. 27. Turk DC, Rudy TE: Assessment of cognitive factors in chronic pain: a worthwhile enterprise? J Consult Clin Psychol 1986;54:760-768. 28. Fordyce WE: Behavioral Methods for Chronic Pain and Illness. St. Louis, Mosby, 1976. 29. Kerns RJ, Turk DC, Rudy TE: The West Haven-Yale Multidimensional Pain Inventory (WHYMPI). Pain 1985;23:245-356. 30. Turk DC, Rudy TE: Robustness of an empirically derived taxonomy of chronic pain patients. Pain, in press. 31. Rudy TE, Turk DC, Zaki HS, Curtin HD: An empirical taxometric altemative to traditional classification of temporomandibular disorders. Pain, 1989;36:311-320. 32. Turk DC, Rudy TE: IASP taxonomy of chronic pain syndromes: Preliminary assessment of reliability. Pain 1987;30:177-189. 33. Rudy TE, Turk DC, Brena SF: Differential utility of medical procedures in the assessment of chronic pain patients. Pain 1988;34:53-60.

Strategies for classifying chronic orofacial pain patients.

To communicate, understand, and prescribe treatment, it is essential that some consensually validated criteria be used to describe groups of patients ...
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