Discussion of pain involves many terms. The meaning and connotation of these different terms may vary widely. 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 Merskey noted , it would be most convenient and helpful if there were some consensus on technical meanings and usage. International Association for the Study of Pain (IASP) Classification of Chronic Pain includes a set of definitions of commonly used pain terms . . We also follow the convention of IASP and use the below mentioned definitions in our clinical practice.

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, acute/ chronic : Definitions of acute, chronic, recurrent, and cancer pain are not included in the IASP list of pain terms. We believe, however, that it is important to clarify these as 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 two most commonly used chronologic markers used to denote chronic pain have been 3 months and 6 months since the initiation of pain; however, these distinctions are arbitrary.

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 process of healing will take is ambiguous.

Acute pain : Pain 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 characteristic 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 generally remits when the underlying pathology resolves.

Chronic pain : It is usually elicited by an injury but may be perpetuated by factors that are both pathogenetically and physically remote from the originating cause . This type of pain prompts patients frequently to seek health care, and it is rarely effectively treated. Because the pain persists, it is likely that environmental and affective factors eventually interact with the tissue damage, contributing to the persistence of pain and illness behaviours. 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 effect on subjective awareness.

Cancer pain : Pain associated with cancer includes pain associated with disease progression as well as treatments. . Pain associated with cancer can have multiple causes ? namely, disease progression, treatment (e.g., neuropathic pain resulting from radiation therapy), 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.

Recurrent pain : recurrent pain is 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.

Transient pain : Transient pain is elicited by activation of nociceptors in the absence of any significant local tissue damage. This type of pain is ubiquitous in everyday life and rarely is a reason to seek health care. . This type of pain ceases as soon as the stimulus is removed.

Addiction: A behavioural pattern of psychoactive substance abuse, addiction is characterized by overwhelming involvement with the use of a drug (i.e., compulsive use), the securing of its supply, and the high tendency to relapse. The compulsive use of the drug results in physical, psychological, and social harm to the drug to the user and use continues despite this harm

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

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

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 sudomotor activity in the region of the pain, or allodynia or hyperalgesia.

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

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

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

Impairment : Any loss of use of, or abnormality of psychological physiologic, or anatomic 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 wilful 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.

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

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.

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

Pain behaviour : Verbal or nonverbal actions understood by observers to indicate that a person may be experiencing pain and suffering. These actions may include audible complaints, facial expressions, abnormal postures or gait, use of prosthetic devices, avoidance of activities, overt expressions, and verbal or nonverbal complaints of pain, distress, and suffering.

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, pain threshold is defined as a level of stimulus intensity that a person recognizes as painful 50% of the time.

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

Placebo : A substance or procedure without therapeutic effect that is provided as a treatment. It is frequently used to control patients expectations for the efficacy in testing a treatment intervention.

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

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.

Symptom magnification : Conscious or unconscious exggeration 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, hope-lessness, intolerability, and interminableness. Suffering implies a threat to the wholeness of an individual?s self-concept, self-identify, and integrity.

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

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

Mechanism-Based Classification of Pain

The conventional classifications of pain disorders based on anatomy, duration, and systems have drawn criticism for their deficiency in sensibility for guiding treatment or research A mechanism-based classification of pain, proposing a potential list of pain mechanisms is given as below(Table 1.),

Table 1. Categories of pain and possible mechanisms

Transient pain

Nociceptor specialization

Tissue injury pain

Primary afferent

Sensitization

Recruitment of silent nociceptors

Alteration in phenotype

Hyperinnervation

Central sensitisation recruitment, summation, amplification

Nervous system injury pain

Primary afferent

Acquisition of spontaneous and stimulus-evoked activity by nociceptor axons and

Somata at loci other than peripheral terminals

Alteration in phenotype

Central nervous system mediated

Central sensitisation

Deafferentation of second-order neurons

Disinhibition reorganization