Defining Pain

The first task of the authors of any taxonomy is to know what they are talking about. Sometimes knowledge is taken for granted. A taxonomy of pain needs some understanding of the term itself. We all assume that we know the meaning of the word pain—and indeed we do. Nevertheless, for a long time there was no unanimity about how to define pain. There is still no absolute unanimity, but a consensus appears to have formed in favor of the definition of pain offered by the International Association for the Study of Pain (IASP) in 1979 and subsequently published in the Classification of Chronic Pain produced by the IASP. The definition of pain—“an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage”—was based on an earlier one that had achieved some recognition; it was intended to deal with the situation that although pain was normally understood to be the consequence of physically damaging stimulation or a disorder in the body, many patients appeared to have pain but did not have overt tissue damage.

Morris observed that the key to the IASP definition is to dissolve any necessary connection between pain and tissue damage. It depends on use of the word pain regardless of whether physical change is apparent. It is important to recognize that pain is always a subjective psychological state. At the same time, the note on this definition emphasized that pain “most often has a proximate physical cause.” The IASP definition has been adopted fairly broadly and helps minimize the idea that there is some sort of pain that patients imagine and that is not the same as the pain of “real injury or disease.” In the personal opinion of this writer, much pain that is primarily organic in origin has an organic basis that is incompletely explained. Sometimes this happens for reasons of mere convenience; that is, every-day transient pain is not usually investigated, nor does it need to be. At other times it may happen because of difficulties in diagnosis, even with chronic severe disorders. The lack of physical proof should never be taken on its own as a sufficient indicator of a psychological cause of pain.

The Nature of Classification

Taxonomy means the arrangement of rules. Taxonomy as a term is derived from two Greek words— tasso and nomia —meaning “arrangement” and “rules.” In other words, it deals with the principles of classification and not with the content of classifications. It is about how to set up a classification and not about the detail of what goes into it. It ordinarily applies to the science of classification of living organisms. Classifications are also produced for nonliving organisms and material that was never alive.

There are two types of classification, natural and artificial. A natural classification deals with the material of physics and biology and anything else in the natural world, such as types of stars or forms of animals—in other words, the material world. An artificial classification deals with arrangement of the products of human activity, for example, a telephone directory.

In an artificial classification there is no necessary connection between the basis on which the classification is produced and the inherent nature of the subject matter. Thus, the list of names in a telephone directory by alphabetical order is arbitrary but works extremely well.

An ideal classification should not only be comprehensive but should also locate each item within it in a place of its own without overlap. The periodic table in chemistry is a wonderful example of scientific beauty and a perfect or almost perfect classification wherein every element belongs in its own place relative to the other elements. In biology, a superior form of classification is a phylogenetic one based on evolutionary relationships.

Medical classifications are established on a very different basis. In the International Statistical Classification of Diseases and Diagnostic Guidelines, 10th Revision (ICD-10), the classification is arranged by causal agents, such as infectious diseases or neoplasms; by systems of the body, such as cardiovascular or musculoskeletal; by symptom pattern and type of symptoms, as in psychiatric illnesses; and even by whether the condition or event is related to the artificial intervention of an operation. Illnesses or categories may be grouped by time of occurrence, such as congenital or perinatal disorders, and at the basic level are grouped as symptoms, signs, and abnormal clinical and laboratory findings.

In the ICD-10 there is code 080 for delivery in an uneventful case, including spontaneous breech delivery. Major groups are subdivided by system (e.g., neurology), by symptom pattern (e.g., epilepsy or migraine), by the presence of hereditary or degenerative disease (e.g., Huntington’s disease and hereditary ataxia), by location of the disorder (e.g., extrapyramidal disorders), by anatomic and physiologic characteristics (e.g., extrapyramidal and movement disorders, such as Parkinson’s disease and dystonia), by location (e.g., polyneuropathies), and by infectious and chemical causes. With these approaches, categories overlap repeatedly. Pain is found in the group of symptoms, signs, and clinical and laboratory findings as “R52—pain not elsewhere classified.” This particular code excludes some 19 others that reflect pain in different parts of the body and excludes “psychogenic” pain (code F45.4) and renal colic (N33). Thus, pain occurs at various levels of diagnosis and categorization in the ICD-10.

The overlap found in medicine is inevitable. There must always be some provision for conditions that are not well described and will overlap with others that are well described. The purposes of medicine require attention to the many different aspects of disease that enter into the classifications. That should be apparent from the examples cited.

The Nature of Classification

Taxonomy means the arrangement of rules. Taxonomy as a term is derived from two Greek words— tasso and nomia —meaning “arrangement” and “rules.” In other words, it deals with the principles of classification and not with the content of classifications. It is about how to set up a classification and not about the detail of what goes into it. It ordinarily applies to the science of classification of living organisms. Classifications are also produced for nonliving organisms and material that was never alive.

There are two types of classification, natural and artificial. A natural classification deals with the material of physics and biology and anything else in the natural world, such as types of stars or forms of animals—in other words, the material world. An artificial classification deals with arrangement of the products of human activity, for example, a telephone directory.

In an artificial classification there is no necessary connection between the basis on which the classification is produced and the inherent nature of the subject matter. Thus, the list of names in a telephone directory by alphabetical order is arbitrary but works extremely well.

An ideal classification should not only be comprehensive but should also locate each item within it in a place of its own without overlap. The periodic table in chemistry is a wonderful example of scientific beauty and a perfect or almost perfect classification wherein every element belongs in its own place relative to the other elements. In biology, a superior form of classification is a phylogenetic one based on evolutionary relationships.

Medical classifications are established on a very different basis. In the International Statistical Classification of Diseases and Diagnostic Guidelines, 10th Revision (ICD-10), the classification is arranged by causal agents, such as infectious diseases or neoplasms; by systems of the body, such as cardiovascular or musculoskeletal; by symptom pattern and type of symptoms, as in psychiatric illnesses; and even by whether the condition or event is related to the artificial intervention of an operation. Illnesses or categories may be grouped by time of occurrence, such as congenital or perinatal disorders, and at the basic level are grouped as symptoms, signs, and abnormal clinical and laboratory findings.

In the ICD-10 there is code 080 for delivery in an uneventful case, including spontaneous breech delivery. Major groups are subdivided by system (e.g., neurology), by symptom pattern (e.g., epilepsy or migraine), by the presence of hereditary or degenerative disease (e.g., Huntington’s disease and hereditary ataxia), by location of the disorder (e.g., extrapyramidal disorders), by anatomic and physiologic characteristics (e.g., extrapyramidal and movement disorders, such as Parkinson’s disease and dystonia), by location (e.g., polyneuropathies), and by infectious and chemical causes. With these approaches, categories overlap repeatedly. Pain is found in the group of symptoms, signs, and clinical and laboratory findings as “R52—pain not elsewhere classified.” This particular code excludes some 19 others that reflect pain in different parts of the body and excludes “psychogenic” pain (code F45.4) and renal colic (N33). Thus, pain occurs at various levels of diagnosis and categorization in the ICD-10.

The overlap found in medicine is inevitable. There must always be some provision for conditions that are not well described and will overlap with others that are well described. The purposes of medicine require attention to the many different aspects of disease that enter into the classifications. That should be apparent from the examples cited.

Which Types of Pain Need Classification

From the point of view of a pain practitioner, only some types of pain need classification, and indeed it would be inappropriate to classify all types of pain in a chronic pain classification. A large proportion of the pain that human beings and other creatures experience in the world is brief and transitory. As a rule, it is accompanied by overt damage that needs its own appropriate treatment or it passes quickly. Pain is the most common symptom in the whole of medicine. Therefore, any attempt to classify all types of pain would inevitably lead to an overall classification of medicine that would have a particular focus that is unnecessary for most medical cases. Illnesses with pain that have needed a special classification are those in which pain is a significant persistent problem. This conclusion still leaves a large field for a classification of pain but saves the pain specialist from having to write the classification for all the rest of medicine as well.

Among specific systems of classification, the ICD-10 is used worldwide for the purpose of documenting mortality and morbidity. In the United States, a slightly modified version of the previous international system of classification, namely, ICD-9CM, is used. (CM stands for Clinical Modification.) This modification was promoted by the U.S. government to provide the additional data required by clinicians, researchers, epidemiologists, medical record librarians, and administrators of inpatient and outpatient community programs. In the United States, ICD-9CM is published by the Department of Health and Human Services, Public Health Services, Health Care Financing Administration.

The international ICD-10 system comprises a table of names and numerical codes for these names. The ICD-10 consists of three volumes. Volume I is a tabular list that contains the report of the International Conference for the 10th Revision, the classification itself at three- and four-character levels, a classification of the morphology of neoplasms, a special tabulation list for mortality and morbidity, definitions, and the nomenclature regulations. Volume II includes an instruction manual, and Volume III is an alphabetical index. The latter also includes expanded instructions on use of the index.

In the United States, ICD-9CM coding has particular importance because of the 1988 Medical Catastrophic Coverage Act, which although later repealed, required the use of ICD-9 codes on “Medicare Part B” claims. This requirement continued with ICD-9CM, and to date, ICD-9CM has not been replaced in the United States. Pain specialists in the United States may believe that the ICD-9CM classification does not cover their requirements for appropriate billing of work done and may prefer a pain-based classification.

Of course, classifications have a number of purposes besides billing. The primary one is to exchange standardized information so that “stroke,” “cholecystitis,” and “depressive disorder,” for example, have the same meanings to different colleagues. Meanings should be the same both within the same country and throughout the world. This should facilitate statistical comparisons of the occurrence and management of disease and serve as a basic tool for scientific progress by establishing standards of diagnosis and description that can be compared between workers within countries and internationally.

Such classification can help provide an understanding of disorders, but it does so only by giving shape to the advances of investigators, whether alone, in working groups, or in national and international organizations. Classifications also serve as a means of recognizing work done and providing standards for payment. This is one of the reasons for their relative popularity with both medical professionals and administrators.

Classifications, of necessity, cannot provide “absolute truth.” Thus even when a classification recognizes a disorder as a “condition,” a “disorder,” or a “disease,” it is not the classification that provides the knowledge that justifies these various titles but rather the existing level of scientific knowledge. To the extent that a classification identifies current scientific knowledge and claims it to be acceptable, it may establish unity, but classifications as a rule only follow scientific knowledge.

This also means that just as classifications take material as they find it, they are not expected to provide perfect decisions or standards by which we can state that something is “a disease,” a “disorder,” a “syndrome,” or merely a “symptom.” The one word of these four for which the meaning is not in dispute is symptom , the patient’s statement of a complaint. All four words involve or have involved some dispute regarding whether they reflect the true nature of the phenomena with which physicians deal. Physicians become concerned about whether they recognize something as a disease or “only a syndrome” or “just a symptom.” It is not the function of a classification to determine the answers to such questions. In fact, it can be extraordinarily hard to determine what constitutes a syndrome and whether diseases should have a fixed standard.

The International Association for the Study of Pain Classification

The IASP classification focuses on chronic pain. A small number of pain syndromes that are not necessarily chronic were included for comparative purposes because they might be relevant to pain specialists (e.g., acute herpes zoster, burns with spasm, pancreatitis, prolapsed intervertebral disk) or because the acute version frequently becomes chronic. The classification is based on five axes. The first axis is anatomic localization, which was chosen for both historical and practical reasons. The historical reasons are that there was previously difficulty in establishing a chronic pain classification based on etiology and that there was too much argument or potential argument about causes. It was also recognized that in essence pain is referred to parts of the body and it is always a somatic symptom, whatever its cause. In addition, location provides a useful means of distinction between different conditions. Accordingly, the IASP classification presents a list of relatively generalized syndromes followed by regional ones. Relatively generalized syndromes include peripheral neuropathy, stump pain, phantom pain, complex regional pain syndrome, central pain, syringomyelia, polymyalgia rheumatica, fibromyalgia, rheumatoid arthritis, and so forth. Pain of psychological origin is also included. Relatively localized syndromes are subdivided according to whether they affect the head and neck, limbs, thorax, or abdomen or whether they have a spinal or radicular distribution or origin.

The IASP classification set out to provide categories and codes for all the relevant conditions. Not all pain is continuously chronic. Some pain that is severe and chronic remits between episodes (e.g., migraine and cluster headache), but these types of pain are also included under the rubric of chronic pain. Some chronic pain consists of pain that persists past what has usually been considered to be the normal time needed for healing. However, this is not always the case, and the decision of what constitutes the normal time for healing is much argued. Indeed, it is now understood—but not so well understood in 1986 when the first edition of the classification was published—that pathophysiologic processes may well maintain pain long after the normal expectation of pain from injury has ended. I personally question whether we should even mention the normal time needed for healing when discussing chronic pain.

Be that as it may, the IASP Taxonomy Committee recognized that some pain persists despite no apparent explanation, other pain persists with an explanation (e.g., the pain of osteoarthritis), and still other pain, which is not always continuous, can recur. Patients with these types of pain, by virtue of their intractability, were considered proper subjects for a classification of chronic pain.