ASP Pain Terminology

Introduction
Changes in the 1994 list. There was substantial correspondence from 1986 to 1993 among members of the Task Force on Taxonomy and other colleagues. The previous definitions all remain unchanged, except for very slight alterations in the wording of the definitions of Central Pain and Hyperpathia. Two new terms have been introduced here: Neuropathic Pain and Peripheral Neuropathic Pain.

The terms Sympathetically Maintained Pain and Sympathetically Independent Pain have also been employed; however, these terms are used in connection with syndromes I-4 and I-5, now called Complex Regional Pain Syndromes, Types I and II. These were formerly labeled Reflex Sympathetic Dystrophy and Causalgia, and the discussion of Sympathetically Maintained Pain and Sympathetically Independent Pain is found with those categories.

Changes have been made in the notes on Allodynia to clarify the fact that it may refer to a light stimulus on damaged skin, as well as on normal skin. Also, in the tabulation of the implications of some of the definitions, the words lowered threshold have been removed from the features of Allodynia because it does not occur regularly. Small changes have been made to better describe Hyperpathia in the definition and note. A sentence has been added to the note on Hyperalgesia to refer to current views on its physiology, although as with other definitions, that for Hyperalgesia remains tied to clinical criteria. Last, the note on neuropathy has been expanded.

The 1986 list. A list of pain terms was first published in 1979 (Pain, 6, 249-252). Many of the terms were already established in the literature. One, allodynia, quickly came into use in the columns of Pain and other journals. The terms have been translated into Portuguese (Rev. Bras. Anest., 30, 5, [1980] 349-351,) into French (H. Dehen, Lexique de la douleur, La Presse Mdicale 12, 23, [1983] 1459-1460), and into Turkish (as Agri Terimlri, translated by T. Aldemir, J. Turkish Soc. Algology, 1 [1989] 45-46). A supplementary note was added to these pain terms in Pain (14 [1982] 205-206).

The original list was adopted by the first Subcommittee on Taxonomy of IASP?. Subsequent revisions and additions were prepared by a subgroup of the Committee, particularly Drs. U. Lindblom, P.W. Nathan, W. Noordenbos, and H. Merskey. In 1984, in particular response to some observations by Dr. M. Devor, a further review was undertaken both by correspondence and during the 4th World Congress on Pain of IASP. Those taking part in that review included Dr. Devor, the other colleagues just mentioned, and Dr. J.M. Mumford, Sir Sydney Sunderland, and Dr. P.W. Wall. Following that review, it was agreed to take advantage of the publication of the draft collection of syndromes and their system for classification, to issue an updated list of terms with definitions and notes on usage.

The versions now presented are based upon some subsequent discussions by correspondence. The form of the definitions and notes at this point has been the responsibility of the editor (H.M.). It would be difficult now to single out individual contributions, but the editor remains heavily indebted to those five members of the original Subcommittee on Taxonomy who sustained this work in the form of an Ad Hoc group and whose names are listed at the beginning of this report. Their knowledge and patience was repeatedly provided freely and with good will.

The revised current list follows. The original comments provided as an introduction to the terms are given in the following two paragraphs, which indicate both the process by which the terms were first delivered and the justification for them.

"The usage of individual terms in medicine often varies widely. That need not be a cause of distress provided that each author makes clear precisely how he employs a word. Nevertheless, it is convenient and helpful to others if words can be used which have agreed technical meanings. Following correspondence and meetings during the period 1976-1978, the present committee agreed on the definitions which follow, and the notes have been prepared by the chairman in the light of members' comments. The definitions are intended to be specific and explanatory and to serve as an operational framework, not as a constraint on future development. They represent agreement between diverse specialties including anesthesiology, dentistry, neurology, neurosurgery, neurophysiology, psychiatry, and psychology. A starting point for some of these definitions was provided by the reports of a workshop on Oro-Facial Pain held at the U.S. National Institute of Dental Research in November 1974.

"The terms and definitions are not meant to provide a comprehensive glossary but rather a minimum standard vocabulary for members of different disciplines who work in the field of pain. We hope that they will prove acceptable to all those in the health professions who deal with pain. Not only are they a limited selection from available terms, but it is emphasized that except for pain itself, they are defined primarily in relation to the skin and the special senses are excluded. They may be used when appropriate for responses to somatic stimulation elsewhere or to the viscera. Except for Pain, the arrangement is in alphabetical order."
It is important to emphasize something that was implicit in the previous definitions but was not specifically stated: that the terms have been developed for use in clinical practice rather than for experimental work, physiology, or anatomical purposes.

Pain Terms
Allodynia
Pain due to a stimulus which does not normally provoke pain. Note: The term allodynia was originally introduced to separate from hyperalgesia and hyperesthesia, the conditions seen in patients with lesions of the nervous system where touch, light pressure, or moderate cold or warmth evoke pain when applied to apparently normal skin. Allo means "other" in Greek and is a common prefix for medical conditions that diverge from the expected. Odynia is derived from the Greek word "odune" or "odyne," which is used in "pleurodynia" and "coccydynia" and is similar in meaning to the root from which we derive words with -algia or -algesia in them. Allodynia was suggested following discussions with Professor Paul Potter of the Department of the History of Medicine and Science at The University of Western Ontario.

The words "to normal skin" were used in the original definition but later were omitted in order to remove any suggestion that allodynia applied only to referred pain. Originally, also, the pain-provoking stimulus was described as "non-noxious." However, a stimulus may be noxious at some times and not at others, for example, with intact skin and sunburned skin, and also, the boundaries of noxious stimulation may be hard to delimit. Since the Committee aimed at providing terms for clinical use, it did not wish to define them by reference to the specific physical characteristics of the stimulation, e.g., pressure in kilopascals per square centimeter. Moreover, even in intact skin there is little evidence one way or the other that a strong painful pinch to a normal person does or does not damage tissue. Accordingly, it was considered to be preferable to define allodynia in terms of the response to clinical stimuli and to point out that the normal response to the stimulus could almost always be tested elsewhere in the body, usually in a corresponding part. Further, allodynia is taken to apply to conditions which may give rise to sensitization of the skin, e.g., sunburn, inflammation, trauma.

It is important to recognize that allodynia involves a change in the quality of a sensation, whether tactile, thermal, or of any other sort. The original modality is normally non-painful, but the response is painful. There is thus a loss of specificity of a sensory modality. By contrast, hyperalgesia (q.v.) represents an augmented response in a specific mode, viz., pain. With other cutaneous modalities, hyperesthesia is the term which corresponds to hyperalgesia, and as with hyperalgesia, the quality is not altered. In allodynia the stimulus mode and the response mode differ, unlike the situation with hyperalgesia. This distinction should not be confused by the fact that allodynia and hyperalgesia can be plotted with overlap along the same continuum of physical intensity in certain circumstances, for example, with pressure or temperature.

See also the notes on hyperalgesia and hyperpathia.

Analgesia
Absence of pain in response to stimulation which would normally be painful. Note: As with allodynia (q.v.), the stimulus is defined by its usual subjective effects.

Anesthesia Dolorosa
Pain in an area or region which is anesthetic.

Causalgia
A syndrome of sustained burning pain, allodynia, and hyperpathia after a traumatic nerve lesion, often combined with vasomotor and sudomotor dysfunction and later trophic changes.

Central Pain
Pain initiated or caused by a primary lesion or dysfunction in the central nervous system.

Dysesthesia
An unpleasant abnormal sensation, whether spontaneous or evoked. Note: Compare with pain and with paresthesia. Special cases of dysesthesia include hyperalgesia and allodynia. A dysesthesia should always be unpleasant and a paresthesia should not be unpleasant, although it is recognized that the borderline may present some difficulties when it comes to deciding as to whether a sensation is pleasant or unpleasant. It should always be specified whether the sensations are spontaneous or evoked.

Hyperalgesia
An increased response to a stimulus which is normally painful. Note: Hyperalgesia reflects increased pain on suprathreshold stimulation. For pain evoked by stimuli that usually are not painful, the term allodynia is preferred, while hyperalgesia is more appropriately used for cases with an increased response at a normal threshold, or at an increased threshold, e.g., in patients with neuropathy. It should also be recognized that with allodynia the stimulus and the response are in different modes, whereas with hyperalgesia they are in the same mode. Current evidence suggests that hyperalgesia is a consequence of perturbation of the nociceptive system with peripheral or central sensitization, or both, but it is important to distinguish between the clinical phenomena, which this definition emphasizes, and the interpretation, which may well change as knowledge advances.

Hyperesthesia
Increased sensitivity to stimulation, excluding the special senses. Note: The stimulus and locus should be specified. Hyperesthesia may refer to various modes of cutaneous sensibility including touch and thermal sensation without pain, as well as to pain. The word is used to indicate both diminished threshold to any stimulus and an increased response to stimuli that are normally recognized.

Allodynia is suggested for pain after stimulation which is not normally painful. Hyperesthesia includes both allodynia and hyperalgesia, but the more specific terms should be used wherever they are applicable.

Hyperpathia
A painful syndrome characterized by an abnormally painful reaction to a stimulus, especially a repetitive stimulus, as well as an increased threshold. Note: It may occur with allodynia, hyperesthesia, hyperalgesia, or dysesthesia. Faulty identification and localization of the stimulus, delay, radiating sensation, and after-sensation may be present, and the pain is often explosive in character. The changes in this note are the specification of allodynia and the inclusion of hyperalgesia explicitly. Previously hyperalgesia was implied, since hyperesthesia was mentioned in the previous note and hyperalgesia is a special case of hyperesthesia.

Hypoalgesia
Diminished pain in response to a normally painful stimulus. Note: Hypoalgesia was formerly defined as diminished sensitivity to noxious stimulation, making it a particular case of hypoesthesia (q.v.). However, it now refers only to the occurrence of relatively less pain in response to stimulation that produces pain. Hypoesthesia covers the case of diminished sensitivity to stimulation that is normally painful.

The implications of some of the above definitions may be summarized for convenience as follows:
Allodynia: lowered threshold: stimulus and response mode differ
Hyperalgesia: increased response: stimulus and response mode are the same
Hyperpathia: raised threshold: stimulus and response mode may be the increased response: same or different
Hypoalgesia: raised threshold: stimulus and response mode are the same lowered response:
The above essentials of the definitions do not have to be symmetrical and are not symmetrical at present. Lowered threshold may occur with allodynia but is not required. Also, there is no category for lowered threshold and lowered response - if it ever occurs.

Hypoesthesia
Decreased sensitivity to stimulation, excluding the special senses. Note: Stimulation and locus to be specified.
Neuralgia
Pain in the distribution of a nerve or nerves. Note: Common usage, especially in Europe, often implies a paroxysmal quality, but neuralgia should not be reserved for paroxysmal pains.

Neuritis
Inflammation of a nerve or nerves. Note: Not to be used unless inflammation is thought to be present.

Neurogenic Pain
Pain initiated or caused by a primary lesion, dysfunction, or transitory perturbation in the peripheral or central nervous system.

Neuropathic Pain
Pain initiated or caused by a primary lesion or dysfunction in the nervous system. Note: See also Neurogenic Pain and Central Pain. Peripheral neuropathic pain occurs when the lesion or dysfunction affects the peripheral nervous system. Central pain may be retained as the term when the lesion or dysfunction affects the central nervous system.

Neuropathy
A disturbance of function or pathological change in a nerve: in one nerve, mononeuropathy; in several nerves, mononeuropathy multiplex; if diffuse and bilateral, polyneuropathy. Note: Neuritis (q.v.) is a special case of neuropathy and is now reserved for inflammatory processes affecting nerves. Neuropathy is not intended to cover cases like neurapraxia, neurotmesis, section of a nerve, or transitory impact like a blow, stretching, or an epileptic discharge. The term neurogenic applies to pain due to such temporary perturbations.

Nociceptor
A receptor preferentially sensitive to a noxious stimulus or to a stimulus which would become noxious if prolonged. Note: Avoid use of terms like pain receptor, pain pathway, etc.

Noxious Stimulus
A noxious stimulus is one which is damaging to normal tissues. Note: Although the definition of a noxious stimulus has been retained, the term is not used in this list to define other terms.
Pain
An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage. Note: The inability to communicate verbally does not negate the possibility that an individual is experiencing pain and is in need of appropriate pain-relieving treatment. Pain is always subjective. Each individual learns the application of the word through experiences related to injury in early life. Biologists recognize that those stimuli which cause pain are liable to damage tissue. Accordingly, pain is that experience we associate with actual or potential tissue damage. It is unquestionably a sensation in a part or parts of the body, but it is also always unpleasant and therefore also an emotional experience. Experiences which resemble pain but are not unpleasant, e.g., pricking, should not be called pain. Unpleasant abnormal experiences (dysesthesias) may also be pain but are not necessarily so because, subjectively, they may not have the usual sensory qualities of pain.

Many people report pain in the absence of tissue damage or any likely pathophysiological cause; usually this happens for psychological reasons. There is usually no way to distinguish their experience from that due to tissue damage if we take the subjective report. If they regard their experience as pain and if they report it in the same ways as pain caused by tissue damage, it should be accepted as pain. This definition avoids tying pain to the stimulus. Activity induced in the nociceptor and nociceptive pathways by a noxious stimulus is not pain, which is always a psychological state, even though we may well appreciate that pain most often has a proximate physical cause.

Pain Threshold
The least experience of pain which a subject can recognize. Note: Traditionally the threshold has often been defined, as we defined it formerly, as the least stimulus intensity at which a subject perceives pain. Properly defined, the threshold is really the experience of the patient, whereas the intensity measured is an external event. It has been common usage for most pain research workers to define the threshold in terms of the stimulus, and that should be avoided. However, the threshold stimulus can be recognized as such and measured. In psychophysics, thresholds are defined as the level at which 50% of stimuli are recognized. In that case, the pain threshold would be the level at which 50% of stimuli would be recognized as painful. The stimulus is not pain (q.v.) and cannot be a measure of pain.

Pain Tolerance Level
The greatest level of pain which a subject is prepared to tolerate. Note: As with pain threshold, the pain tolerance level is the subjective experience of the individual. The stimuli which are normally measured in relation to its production are the pain tolerance level stimuli and not the level itself. Thus, the same argument applies to pain tolerance level as to pain threshold, and it is not defined in terms of the external stimulation as such.

Paresthesia
An abnormal sensation, whether spontaneous or evoked. Note: Compare with dysesthesia. After much discussion, it has been agreed to recommend that paresthesia be used to describe an abnormal sensation that is not unpleasant while dysesthesia be used preferentially for an abnormal sensation that is considered to be unpleasant. The use of one term (paresthesia) to indicate spontaneous sensations and the other to refer to evoked sensations is not favored. There is a sense in which, since paresthesia refers to abnormal sensations in general, it might include dysesthesia, but the reverse is not true. Dysesthesia does not include all abnormal sensations, but only those which are unpleasant.

Peripheral Neurogenic Pain
Pain initiated or caused by a primary lesion or dysfunction or transitory perturbation in the peripheral nervous system.

Peripheral Neuropathic Pain
Pain initiated or caused by a primary lesion or dysfunction in the peripheral nervous system.

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