Discussions of pain involve many terms. The meaning and connotation of these different terms may vary widely. For example, some authors use the term pain to relate to a stimulus, others to a thing, and still others to a response. Such inconsistent usage creates difficulties in communication. As Merskey¹ noted, it would be most convenient and helpful if there were some consensus on technical meanings and usage. Based on this belief, the editors of the two editions of the International Association for the Study of Pain (IASP) Classification of Chronic Pain included a set of definitions of commonly used pain terms^2,3 (note that a third adaptation of chronic pain for the International Classification of Diseases 11th revision [ICD-11] does not include any listing of definitions).⁴ In the second edition of this text, Bonica reproduced a list of the terms and in some cases provided annotations. We adopt a similar strategy. We follow the convention of IASP; we begin with the definition of pain and then proceed alphabetically. Terms preceded by an asterisk come directly from the IASP descriptions of pain terms.³

^*Pain: An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage (emphasis added). It should be noted that some have argued that inclusion of the phrase “described in terms of such damage” is problematic because it assumes an ability to verbally communicate that may not be present in very young children and individuals with limitations in their verbal abilities.⁵ They also suggest that the use of the term unpleasant may trivialize the experience which may greatly exceed the unpleasant nature of the experience. Moreover, the original IASP definition fails to incorporate advanced knowledge as to the important role of cognitive, social, and contextual factors. Williams and Craig⁵ suggest a revised definition, “Pain is a distressing experience associated with actual or potential tissue damage with sensory, emotional, cognitive, and social components,”^(p2420) to take these factors into consideration.

Pain, acute/chronic^†: Definitions of acute, chronic, recurrent, and cancer pain are not specifically included in the IASP list of pain terms. We believe, however, that it is important to clarify these because they are commonly used in the literature.

Traditionally, the distinction between acute and chronic pain has relied on a single continuum of time, with some interval since the onset of pain used to designate the onset of acute pain or the transition point when acute pain becomes chronic. The two most commonly used chronologic markers used to denote chronic pain have been 3 months and 6 months, most recently by IASP as lasting longer than 3 months⁴ since the initiation of pain; however, these distinctions are arbitrary. Moreover, these criteria do not take into consideration intensity of pain, the severity and nature of its impact on functioning or treatment-seeking behaviors, or whether pain must be present every day or how frequent it occurs in this interval. These features are important because they may influence estimates of the prevalence of pain and effects on physical activity and treatment requirements and may explain some of the inconsistencies reported.

Another criterion for chronic pain is “pain that extends beyond the expected period of healing.” This is relatively independent of time because it considers pain as chronic even when it has persisted for a relatively brief duration. Unfortunately, how long the expected process of healing will (or should) take is ambiguous. One suggestion has been to differentiate “chronic pain” from “impactful chronic pain.”^6,7

Some hold that pain that persists for long periods of time in the presence of ongoing pathology should be considered an extended “acute” pain state. In this case, treatment targets the underlying pathology. This is not to encourage a Cartesian dualistic perspective of pain that treats mind and body as independent entities with distinctive functions. Historically, such distinction led to a faulty assumption of acute pain as “real,” whereas chronic pain without known pathology was suspect and viewed as being merely “functional.” As the IASP definition clearly states, any pain, acute or chronic, regardless of the presence of identifiable tissue damage, is an unpleasant experience, inherently influenced by various cognitive, affective, and environmental factors. We hold that the weighing of psychological and environmental factors is often greater in chronic pain than acute pain, and the importance of these factors escalates over time, contributing to the experience of pain and associated disability.⁸

We propose conceptualizing acute and chronic pain on two dimensions: time and physical pathology. Figure 2.1 schematically depicts this two-dimensional conceptualization of acute and chronic pain. From this perspective, any case falling above the diagonal line (short duration or high physical pathology) is acute pain, whereas cases falling below the diagonal line (low physical pathology or long duration) suggest chronic pain. The perspective presented in Figure 2.1 leads to the following definitions of acute and chronic pain.

Acute pain: Acute pain is the physiologic response to and experience of noxious stimuli that can become pathologic, is normally sudden in onset, is time-limited, and motivates behaviors to avoid potential or actual tissue injury.⁹ Pain is elicited by the injury of body tissues and activation of nociceptive transducers at the site of local tissue damage. The local injury alters the
response characteristics of the nociceptors and perhaps their central connections and the autonomic nervous system in the region. In general, the state of acute pain lasts for a relatively limited time and remits when the underlying pathology resolves (however, see the following definition of central sensitization). This type of pain often serves as the impetus to seek health care, and it occurs following trauma, some disease processes, and invasive interventions.

Chronic pain: May be elicited by an injury or disease but is likely to be perpetuated by factors that are both pathogenetically and physically remote from the originating cause. Chronic pain extends for a long period of time and/or represents low levels of underlying pathology that does not explain the presence and extent of pain (e.g., mechanical back pain, fibromyalgia [FM] syndrome). There have been suggestions that chronic pain in the apparent absence of pathology may be attributable to modification of nerves and sensitization of the peripheral or central nervous system. There have also been suggestions that genetic factors and prior life experiences might predispose some to develop chronic pain problems following an initiating insult that resolves in others who do not have the predisposition. Just as the brain is modified by experience, especially in early life, the brain may alter the way noxious information is processed to reduce or augment its impact on subjective awareness.

Chronic pain frequently is the impetus for people to seek health care. Currently available treatments are rarely capable of totally eliminating the noxious sensations and thereby “curing” chronic pain. Because the pain persists, it is likely that environmental, emotional, and cognitive factors will interact with the already sensitized nervous system, contributing to the persistence of pain and associated illness behaviors (see following description of pain behaviors). It is also possible that, just as the brain is modified by experience, especially in early life, the brain may alter the way noxious information is processed to reduce or augment its impact on subjective awareness.

The acute-chronic pain continuum is based solely on duration. There is an implication that those with chronic pain will have progressed from an acute pain state to a chronic pain state and that once the threshold to chronic pain is crossed, it becomes fixed and relatively immutable with the implication that worsening, and deterioration over time is inevitable. The reality in contrast is there in the presence of considerable variability within individuals who have transitioned into the classification based on arbitrary time points. A range of psychosocial, behavioral, and contextual factors as well as physical ones will influence the adaptation and responses to pain.⁸

Cancer pain: Pain associated with cancer includes pain associated with disease progression as well as treatments (e.g., chemotherapy, radiotherapy, surgery) that may damage the nervous system. Although some contend that pain associated with neoplastic disease is unique, in the majority of instances, we view it as fitting within our description of acute and chronic pain, as depicted in Figure 2.1. Moreover, pain associated with cancer can have multiple causes, namely, disease progression, treatment, and co-occurring diseases (e.g., arthritis). Regardless of whether the pain associated with cancer stems from disease progression, treatment, or a co-occurring disease, it may be either acute or chronic. Thus, we do not advocate a separate classification of cancer pain as distinct from acute and chronic pain.

Some concerns have also been raised regarding the common usage of chronic malignant and chronic benign pain⁴; often, pain unrelated to cancer is implicitly view as “benign” to distinguish it from cancer-related pain. Certainly, people who have pain associated with neoplastic disease experience a unique and disease-specific situation, but from a mechanistic perspective, there may be little to substantiate continued use of this dichotomy. Moreover, patients who have chronic noncancer pain who are told that their pain is “benign” may feel denigrated because, from their perspective, the inference is that their pain is not a serious concern.

Recurrent pain: Episodic or intermittent occurrences of pain, with each episode lasting for a relatively short period of time but recurring across an extended period of time (e.g., migraine headaches, tic douloureux, sickle cell crisis, dysmenorrhea). Our distinction between acute and chronic pain using the integration of the dimensions of time and pathology does not specifically include recurrent pain. In the case of recurrent pain, patients may experience episodes of pain interspersed with periods of being completely pain-free. Although recurrent pain may seem acute because each pain episode (e.g., headache) is of relatively short duration, the pathophysiology of many recurrent pain disorders (e.g., migraine) is not well understood. Syndromes characterized by recurrent acute pain share features in common with both acute and chronic pain. The fact that these syndromes extend over time, however, suggests that psychosocial and behavioral factors, not only physical pathology, may be major contributors to emotional and behavioral responses. IASP⁴ now includes recurrent pain lasting longer than 3 months within its definition of chronic pain. However, it is not clear whether multiple episodes lasting several days within 3 months would meet the chronic pain criterion or whether the pain must last at least 3 months. That is, would multiple migraines in a 3-month period be chronic even if there were pain-free periods within the 3 month period?

Transient pain: Pain elicited by activation of nociceptors in the absence of any significant local tissue damage. This type of pain is ubiquitous in everyday life and is rarely a reason to seek health care. It is seen in the clinical setting and only in incidental or procedural pain, such as during a venipuncture or injection. This type of pain ceases as soon as the stimulus is removed. There are situations where sources of transient pain may be treated by providers with preventive analgesic or topical medication.

Acceptance: A choice to acknowledge pain experiences (intensity, thoughts, emotions) and to cease efforts to control them while simultaneously engaging in valued behaviors, particularly when control efforts have let to restrictions.

Addiction: A behavioral pattern of substance, including prescribed medication, abuse characterized by overwhelming
involvement with the use of a drug (i.e., compulsive use), the securing of its supply, and a high tendency to relapse. The compulsive use of the drug results in physical, psychological, and/or social harm to the user, and use continues despite this harm. (See also physical dependence.)

^*Allodynia: Pain due to a stimulus that does not normally provoke pain.

Analgesia: Absence of the spontaneous report of pain or pain behaviors in response to stimulation that would normally be expected to be painful. The term implies a defined stimulus and a defined response. Analgesic responses can be tested in nonhuman as well as humans.

^*Anesthesia dolorosa: Spontaneous pain in an area or region that is anesthetic.

Breakthrough pain: A transient increase in pain to greater than moderate intensity superimposed on baseline pain that is fairly well managed. Breakthrough pain includes (1) incident pain that may arise from some activity or physical function (e.g., coughing, ambulating), (2) pain that routinely increases as the duration of analgesic medication is reaching its limit (end-of-dose failure), and (3) spontaneous exacerbation of a stable level of pain for nonspecific reasons.

Catastrophizing: A cognitive and emotional process that involves magnification of pain-related stimuli, feelings of helplessness, and a negative orientation to pain and life circumstances. Catastrophizing has been shown to be an important predictor of response to both acute and chronic pain.¹⁰

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

Central sensitization: Increase in the excitability and responsiveness of neurons in the spinal cord. Central sensitization may explain the persistence of pain beyond the removal or resolution of the initiating stimulus.

Chronic widespread pain: A complex condition with a range of disabling physical and psychological symptoms that does not fit neatly into any medical specialty and has a myriad of possible causes and triggers, both physical and psychological. A set of disparate disorders is often lumped into chronic widespread pain including nonradicular back pain, FM, irritable bowel syndrome, pelvic pain, temporomandibular disorders (TMD), and tension-type headache. This diagnosis is based on the presence and distribution of symptoms in the absence of another defined pathologic process: The features in the history or clinical examination are generally more important than laboratory investigations.

^*Complex regional pain syndrome type 1 (formerly reflex sympathetic dystrophy): A syndrome that usually develops after an initiating noxious event, is not limited to the distribution of a single peripheral nerve, and is apparently disproportionate to the inciting event. It is associated at some point with evidence of edema, changes in skin blood flow, abnormal pseudomotor activity in the region of the pain, or allodynia or hyperalgesia. Specific criteria for the diagnosis of complex regional pain syndrome (CRPS) have been proposed.¹¹

^*Complex regional pain syndrome type 2 (formerly causalgia): A syndrome of sustained burning pain, allodynia, and hyperpathia following a traumatic nerve lesion, often combined with vasomotor dysfunction and later trophic changes.

Conditioned pain modulation: Altered endogenous pain modulation is considered as a mechanism involved with diverse chronic pain syndromes (e.g., TMD, FM, chronic tension-type headache, and irritable bowel syndrome). It is assessed by measuring phasic pain response after a conditioned tonic pain stimulus. Conditioned pain modulation is at least partially mediated by the diffuse noxious inhibitory control (DNIC) system characterized by inhibition of wide dynamic range neurons in the dorsal horn of the spinal cord by heterosegmental noxious afferent input.¹²

Cost-benefit analysis: Evaluation of the costs and effects of an intervention in a common, usually monetary unit. The standardization of unit has an advantage because it permits comparisons across dissimilar intervention programs. On the other hand, the conversion of treatment effects to monetary units may not always be feasible. Estimation of the cost to outcome ratio is possible, as are comparisons between interventions using the rates of improvement (e.g., return to work) with common denominators.

Cost-effectiveness analysis: Estimation of treatment outcome entails criteria other than monetary terms, such as lives saved or return to work. An intervention is cost-effective when it satisfies one of the following conditions:

Disability: Any restriction or loss of capacity to perform an activity in the manner or within the range considered normal for a human being, such as climbing stairs, lifting groceries, or talking on a telephone. It is a task-based concept that involves both the person and the environment. Disability is essentially a social and not a medical term or classification. Level of disability should be determined only after a patient has reached maximum medical improvement following appropriate treatment and rehabilitation.

^*Dysesthesia: An unpleasant abnormal sensation, whether spontaneous or evoked.

^*Hyperalgesia: An increased response to a stimulus that is normally painful.

^*Hyperesthesia: Increased sensitivity to stimulation, excluding special senses.

^*Hyperpathia: A painful syndrome characterized by an abnormally painful reaction to a stimulus, especially a repetitive stimulus, as well as an increased threshold.

^*Hypoalgesia: Diminished pain in response to a normally painful stimulus.

Hypochondriasis: An excessive preoccupation with bodily sensations and fears that they represent serious disease despite reassurance to the contrary.

Impairment: Any loss of use of, or abnormality of, psychological, physiologic, or anatomical structure or function that is quantifiable. It is not equivalent to disability. Impairment is to disability as disease is to illness.

Malingering: A conscious and willful feigning or exaggeration of a disease or effect of an injury in order to obtain a specific external gain. It is usually motivated by external incentives such as financial compensation, avoiding work, or obtaining drugs.

Maximum medical improvement: The state beyond which additional medical treatment is unlikely to produce an improvement in function.

Minimum clinically important difference (MCID): The magnitude of reduction in pain or related problems that a patient would consider minimally important. In considering the determination of clinically important differences, two different aspects of the interpretation of clinical trial results must be distinguished. One is establishing the difference in the magnitude of response between the treatment and control groups that will be considered large enough to establish the scientific or therapeutic importance of the results. The other is establishing what change in the outcome measure represents a meaningful difference for patients. This later consideration has come to be referred to as the minimum clinically important difference. The development of criteria for determining what are important changes in an
individuals’ scores on the outcome measures used in chronic pain trials would provide clinicians and researchers with essential methods for evaluating treatment responses of individuals in clinical trials and clinical practice. Such individual-level criteria make it possible to conduct responder analyses that classify each trial participant as “improved,” “stable,” or “worse” on the basis of validated criteria of important change. (See description of patient global impression of change.)

Multidisciplinary (interdisciplinary) pain center: An organization of health care professionals and basic and applied scientists that includes research, teaching, and patient care related to acute and chronic pain. It includes a wide array of health care professionals including physicians, psychologists, nurses, physical therapists, occupational therapists, and other specialty health care providers. Multiple therapeutic modalities are available. These centers provide evaluation and treatment and are usually affiliated with major health science institutions.

^*Neuralgia: Pain in the distribution of a nerve or nerves.

^*Neuritis: Inflammation of a nerve or nerves.

^*Neurogenic pain: Pain initiated or caused by a primary lesion, disease, dysfunction, or transitory perturbation in the somatosensory nervous system.¹³ It may be spontaneous or evoked, as an increased response to a painful stimulus (hyperalgesia), a painful response to a painful stimulus (hyperalgesia), or a painful response to a normally nonpainful stimulus (allodynia).

Neuropathic pain: Pain arising as a direct consequence of a lesion or disease affecting the somatosensory system.¹⁴

^*Neuropathy: A disturbance of function or pathologic change in a nerve: in one nerve, mononeuropathy; in several nerves, mononeuropathy multiplex; if diffuse and bilateral, polyneuropathy.

Nocebo: Negative treatment effects induced by a substance or procedure containing no toxic or detrimental substance.

Nociception: Activation of sensory transduction in nerves by thermal, mechanical, or chemical energy impinging on specialized nerve endings. The nerve(s) involved conveys information about tissue damage to the central nervous system.

^*Nociceptor: A receptor preferentially sensitive to tissue trauma or to a stimulus that would damage tissue if prolonged.

^*Noxious stimulus: A stimulus that is capable of activating receptors for tissue damage.

Pain behavior: Verbal or nonverbal actions understood by observers to indicate that a person may be experiencing pain and suffering. These actions may include audible emissions (e.g., signs, moans); facial expressions (e.g., grimacing); abnormal postures or gait (e.g., limping, bracing, moving in a guarded fashion); motor behavior (e.g., rubbing a body part); use of prosthetic devices; avoidance of activities; and verbal indications of pain, distress, and suffering. An important feature is the observable nature of these behaviors that can be subjected to the conditioning process. Once conditioned, the same behavior is exhibited as a learned response rather than expression of actual pain experience. Thus, pain behavior can either reflect internal experience of pain or is exhibited as a learned behavior in response to certain cues.

Pain clinic: Facilities focusing on diagnosis and management of patients with pain problems. It may specialize in specific diagnoses or pain related to a specific area of the body.

Pain relief: Report of reduced pain after a treatment. It does not require reduced response to a noxious stimulus and is not a synonym for analgesia. The term applies only to humans.

Pain threshold: The least level of stimulus intensity perceived as painful. In psychophysics, this is defined as a level of stimulus intensity that a person recognizes as painful 50% of time.

^*Pain tolerance level: The greatest level of noxious stimulation that an individual is willing to tolerate.

Pain sensitivity range: The difference between the pain threshold and the pain tolerance level.

^*Paresthesia: An abnormal sensation whether spontaneous or evoked.

Patient global impression of change (PGIC): Patients’ overall evaluation of improvement or worsening of symptoms over the course of treatment. This measure is often a single-item rating by patients on a scale, often 5-point or 7-point scale that ranges from “very much improved” to “very much worse” with “no change” as the midpoint.

^*Peripheral neurogenic pain: Pain initiated or caused by a primary lesion or dysfunction or transitory perturbation in the peripheral nervous system.

Physical dependence: A pharmacologic property of a drug (e.g., opioid) characterized by the occurrence of an abstinence syndrome following abrupt discontinuation of the substance or administration of an antagonist. It does not imply an aberrant psychological state or behavior or addiction.

Placebo: An inert substance or procedure without a specified therapeutic ingredient that is provided as a treatment. It is frequently used to control patients’ expectations for the efficacy in testing a treatment.

Placebo effects: Refers to the positive benefit(s) from a placebo (i.e., inert) preparation or procedure when such benefit is generally achieved only with an active treatment intervention. Active treatments also are likely to have a placebo component that augments the active component associated with the treatment.

Plasticity, neural: Nociceptive input leading to structural and functional changes that may cause altered perceptual processing and contribute to pain chronicity.

Pseudoaddiction: Refers to drug-seeking behavior or misuse by patients who have severe pain and are undermedicated or who have not received other effective pain treatment interventions. Such patients may appear preoccupied with obtaining opioids, but the preoccupation reflects a need for pain relief and not drug addiction. Pseudoaddictive behavior differs from true addictive behavior because when higher doses of opioid are provided, the patient does not use these in a manner that persistently causes sedation or euphoria, the level of function is increased rather than decreased, and the medications are used as prescribed without loss of control over use.

Psychogenic pain: Report of pain attributable primarily to psychological factors usually in the absence of any objective physical pathology that could account for pain. This term is commonly used in a pejorative sense. It often suggests a Cartesian dualism and is not usually a helpful method of describing a patient.

Quality of life/health-related quality of life: Quality of life (QOL) refers to an individual’s perception of his or her position in life in the context of the culture and value systems in which he or she lives and in relation to his or her goals, expectations, standards, and concerns. Concerns with this all-encompassing description have led a number of investigators to use a more circumscribed construct, health-related quality of life (HRQOL). Although HRQOL has been used interchangeably with terms such as health status and functional status, HRQOL is a narrower term than QOL because it does not include aspects of work, environmental conditions, housing, and other variables that are often considered relevant to QOL but that do not involve health directly.⁷

Rehabilitation: Restoration of an individual to maximal physical and mental functioning in light of his or her impairment.

Residual functional capacity: The capacity to perform specific social and work-related physical and mental activities following rehabilitation related to impairment or when a condition has reached a point of maximum medical improvement.

Resilience: Capacity and dynamic process of adaptively overcoming stress and adversity while maintaining normal psychological and physical functioning.¹⁵

Summed pain intensity difference (SPID): A strategy for combining relief magnitude and duration in a single score. It is calculated by the sum of the time-weighted pain intensity difference (difference between current pain and pain at baseline) multiplied by the interval between ratings.

Symptom magnification: Conscious or unconscious exaggeration of symptom severity in an attempt to convince an observer that one is truly experiencing some level of pain. It differs from malingering as it is an effort to be believed, not necessarily to achieve a positive outcome (i.e., secondary gain) such as financial compensation.

Suffering: Reaction to the physical or emotional components of pain with a feeling of uncontrollability, helplessness, hopelessness, intolerability, and interminability. Suffering implies a threat to the intactness of an individual’s self-concept, self-identify, and integrity.

Tolerance, drug: A physiologic state in which a person requires an increased dosage of a psychoactive substance to sustain a desired effect.

Total pain relief (TOPAR): Is used in clinical trials to assess pain relief over time. It is a cumulative measure that is composed of the sum of time-weighted pain relief score multiplied by the interval between ratings. TOPAR is frequently used in clinical trials of medications designed to ameliorate pain.

Wind-up: Slow temporal summation of pain mediated by C fibers due to repetitive noxious stimulation at a rate faster than one stimulus every 3 seconds. It may cause the person to experience a gradual increase in the perceived magnitude of pain.

The lack of a classification of chronic pain syndromes that is used on a consistent basis inhibits the advancement of knowledge and treatment of chronic pain and makes it hard for investigators as well as practitioners to compare observations and results of research. Bonica¹⁶ referred to this language ambiguity as “a modern tower of Babel.”

In order to identify target groups, conduct research, prescribe treatment, evaluate treatment efficacy, and for policy and decision making, it is essential that some consensually validated criteria are used to distinguish groups of individuals who share a common set of relevant attributes. The primary purpose of such a classification is to describe the relationships of constituent members based on their equivalence along a set of basic dimensions that represent the structure of a particular domain. Infinite classification systems are possible, depending on the rationale about common factors and the variables believed to discriminate among individuals. The majority of the current taxonomies of pain are “expert-based” classifications.