Pain is: “An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage” 1-3, as proposed by the International Association for the Study of Pain (IASP). Pain is subjective. Each individual learns the application of the word through experiences related to injury in early life. Noxious stimuli are liable to damage tissue. Pain is an experience we associate with actual or potential tissue damage. It is always an unpleasant and therefore emotional experience. In several cases pain is reported in the absence of tissue damage or any likely pathophysiological cause. Usually there is no way to distinguish the experience of pain perception from that due to tissue damage. If the pain experience is reported as pain in the same way as caused by tissue damage, it should be accepted as pain. “Pain is what the patient says it is, existing whenever the experiencing person says it does” 4. This definition avoids relating 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 due to a stimulus which does not normally provoke pain is termed allodynia. It involves a change in the quality of a sensation, whether tactile, thermal or of any sort. The original modality is normally non-painful, but the response is painful. There is a loss of specificity of sensory modality 2,5.
Analgesia is the term used for the absence of pain in response to stimulation which normally would be painful. Analgesia implies a defined stimulus and a defined response 2,5.
Anaesthesia dolorosa is pain in an area or region which is anaesthetic 2,5.
Causalgia is a syndrome of sustained burning pain, allodynia, and hyperpathia after nerve lesion, often combined with vasomotor and sudomotor dysfunction and later trophic changes 2.
Pain initiated or caused by a primary lesion or dysfunction in the central nervous system is termed central pain 2,5.
Dysesthesia is an unpleasant abnormal sensation, whether spontaneous or evoked. Compared with pain and with paresthesia, special cases of dyseshesia 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 2,5.
Hyperalgesia is an increased response to a stimulus which is normally painful 5. It reflects increased pain on suprathreshold stimulation. It is an increased response at a normal threshold or at an increased threshold, in patients with neuropathy. Hyperalgesia is a consequence of perturbation of the nociceptive system with peripheral or central sensitization, or both 2.
Hyperesthesia is an increased sensitivity to stimulation, excluding the special senses 5. The stimulus and locus should be specified. It may refer to various modes of cutaneous sensibility including touch and thermal sensation without pain, as well as to pain. The term is used to indicate both diminished threshold to any stimulus and an increased response to stimuli that are normally recognised. Hyperesthesia includes both, allodynia and hyperalgesia, but the more specific terms should be used wherever they are applicable 2.
Hyperpathia is a painful syndrome characterized by an abnormally painful reaction to a stimulus, especially a repetitive stimulus, as well as an increased threshold 5. It may occur with allodynia, hyperesthesia, hyperalgesia, or dysethesia. 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 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 2.
Diminished pain in response to a normally painful stimulus is termed hypoalgesia 5. It is defined as diminished sensitivity to noxious stimulation, making it a particular case of hypoesthesia. However, it now refers only to the occurrence of relatively less pain in response to stimulation that produces pain. Hypesthesia covers the case of diminished sensitivity to stimulation that is normally painful 2.
The implications of some of the above definitions may be summarized as in table 1.
Table 1: Types of pain summarized.
Hypoesthesia is a decreased sensitivity to stimulation, excluding the special senses. In case of hypoesthesia the stimulation and locus must be specified 2.
Neuralgia is pain in the distribution of a nerve or nerves 5. Common usage, especially in Europe, often implies a paroxysmal quality, but neuralgia should not be reserved for paroxysmal pains 2.
Inflammation of a nerve or nerves is termed neuritis 5. This term must not be used unless inflammation is proven to be present 2.
The term neurogenic pain is used for pain initiated or caused by a primary lesion, dysfunction, or transitory perturbation in the peripheral or central nervous system 2,5.
Pain initiated or caused by a primary lesion or dysfunction in the nervous system is termed neuropathic pain. 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 2.
Neuropathy is a disturbance of function or pathological change in a nerve. When it occurs in one nerve it is termed: mononeuropathy. When several nerves are involved the term: mononeuropathy multiplex is to be used. The term: polyneuropathy is to be used in case of diffuse and bilateral peripheral nerve problem 2,5.
Neural plasticity means the nociceptive input leading to structural and functional changes that may cause altered perceptual processing and contribute to pain chronicity 5.
A nociceptor is a receptor preferentially sensitive to a noxious stimulus or to a stimulus which would become noxious if prolonged 5. The terms like pain receptor, pain pathways, etc. must be avoided 2.
Nociception is the 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 5.
A noxious stimulus is one which is damaging to normal tissues. Although the definition of a noxious stimulus has been retained, the term is not used in this list to define other terms 2. A noxious stimulus is also defined as a stimulus capable of activation receptors for tissue damage 5.
The least experience of pain which a subject can recognize is termed the pain threshold 5. Traditionally the threshold has often been defined, 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 as painful. The stimulus is not pain and cannot be a measure of pain 2.
Pain Tolerance Level
The greatest level of pain which a subject is prepared to tolerate is termed the pain tolerance level 5. 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 2.
Paresthesia is an abnormal sensation, whether spontaneous or evoked 5. It is used to describe an abnormal sensation that is not unpleasant while dyseshesia 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 favoured. 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 2.
Peripheral Neurogenic Pain
Pain initiated or caused by a primary lesion or dysfunction or transitory perturbation in the peripheral nervous system is termed peripheral neurogenic pain 2,5.
Peripheral Neuropathic Pain
Peripheral neuropathic pain is the term used for pain initiated or caused by a primary lesion or dysfunction in the peripheral nervous system 2.
Psychogenic pain is the reporting 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 and is not usually an effective method of describing a patient 5.
Pain localized not to the site of its cause but to an area that may be adjacent to or at a distance from such a site is termed referred pain. For example shoulder pain can be caused by an diaphragmic irritations 6.
Wind up, means the second pain induced by a slow temporal summation of pain mediated by C fibres. It is caused by repetitive noxious stimulation slower than one stimulus every 3 seconds. Consequently the subject may experience a gradual increase in the perceived magnitude of pain 5.
Classification of Pain
Nociceptive pain arises from the stimulation of specific pain receptors. These receptors can respond to heat, cold, vibration, stretch and chemical stimuli released from tissue injury. Non nociceptive pain arises from within the peripheral and central nervous system. Specific receptors do not exist here, with pain being generated by nerve cell dysfunction 7.
Figure 1: Classification of pain.
Musculoskeletal pain can occur as a result of injury in tissues such as: skin, muscle, joints, bones, and ligaments. Specific receptors (nociceptors) for heat, cold, vibration, stretch, inflammation and oxygen starvation are involved. A sharp and well localised pain can often be provoked by touching or moving the area or tissue involved 7.
Organs can be the cause of pain. Specific receptors (nociceptors) for stretch, inflammation, and oxygen starvation (ischaemia) can be involved. The pain perceived is often poorly localised, and may feel like a vague deep ache, sometimes being cramping or colicky in nature. It frequently produces referred pain to the back. Pelvic pain refers pain to the lower back, abdominal pain to the mid-back, and thoracic pain to the upper back 7.
Nerve pain occurs within the nervous system itself. The pain may originate from the peripheral nervous system or from the central nervous system. Nerve pain can be caused by: degeneration (multiple sclerosis, stroke, brain haemorrhage, and oxygen starvation), pressure or strain (trapped nerve), inflammation or infection. The nervous system does not have specific receptors for pain (non nociceptive). Instead, when a nerve becomes injured, its conduction becomes unstable, firing off signals in a completely inappropriate, random, and disordered fashion. This phenomen is also termed ectopic pacemaker or abnormal impuls generated sites (AIGS). The impulses then are interpreted by the brain as pain, and can be associated with signs of nerve malfunction such as hypersensitivity (touch, vibration, hot and cold), tingling, numbness, and weakness. There is often referred pain to an area where that nerve would normally supply (neurotome). Nerve pain is often described as lancinating, shooting, burning, and hypersensitive 7.
Sympathetic pain is caused by possible over-activity response of the sympathetic nervous system, and central or peripheral nervous system mechanisms. The sympathetic nervous system controls blood flow to tissues such as skin and muscle, sweating by the skin, and the speed and responsiveness of the peripheral nervous system. Sympathetic pain occurs more commonly after fractures and soft tissue injuries of the arms and legs but can be present in the pelvic area aswell. These injuries may lead to complex regional pain syndrome. CRPS was previously known as reflex sympathetic dystrophy. There are no specific pain receptors that induce CRPS but a disbalance of the nervous system may operate in CRPS. The hypersensitivity in the skin around the injury and also peripherally is associated with abnormalities of sweating and temperature control in the area. Functio leasi occurs with muscle atrophy, joint problems, contractures, and osteoporosis as a result. It is possible that the syndrome is initiated by trauma to the small peripheral nerves close to the injury 7.
Taxonomies Related to Pain
The taxonomy of chronic pain syndromes is a difficult subject. Bonica referred the language ambiguity as “a modern tower of Babel” 8. Taxonomy and classification of pain is important identifying target groups, conduct research and the approach of patients 5. The classification of pain can be expert based 9, anatomy based, duration based, etiologic 10, body system based 11, mechanism based 12 and/or based on severity using a scale model (VAS)13-16. Multidimensional sytems of pain classification exist and are proposed by the International Association for the Study of Pain Taxonomy. The IASP has published an expert-based multiaxial classification of chronic pain 2,3.