IASP Pain Terminology – IASP – Dec 2017
The following pain terminology is updated from “Part III: Pain Terms, A Current List with Definitions and Notes on Usage” (pp 209-214) Classification of Chronic Pain, Second Edition, IASP Task Force on Taxonomy, edited by H. Merskey and N. Bogduk, IASP Press, Seattle, ©1994.
This “dictionary” is not new, but these terms are still used and pain is no different than it was years ago.
I’m posting this because it makes the subtle distinctions clear between often misused words: allodynia and the latest fashionable, yet hypothetical syndrome of “hyperalgesia”, when pain supposedly worsens due to “too much” opioid medication.
|Pain||Interdisciplinary Treatment*||Nociceptive Stimulus*|
Note: An asterisk (*) indicates that the term is either newly introduced or the definition or accompanying note has been revised since the 1994 publication.
Pain due to a stimulus that does not normally provoke pain.
Note: The stimulus leads to an unexpectedly painful response. This is a clinical term that does not imply a mechanism.
Allodynia may be seen after different types of somatosensory stimuli applied to many different tissues.
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.
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 nonpainful, 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.
Increased pain from a stimulus that normally provokes pain.
Note: Hyperalgesia reflects increased pain on suprathreshold stimulation.
This is a clinical term that does not imply a mechanism.
Yet, whenever I see this term used they are referring to the “mechanism” of taking opioids, claiming that the opioids themselves are the mechanism causing our pain.
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.
Hyperalgesia may be seen after different types of somatosensory stimulation applied to different tissues.
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,
- radiating sensation, and
may be present, and the pain is often explosive in character.
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.
Most people, even doctors and researchers, don’t use all these terms correctly, and the term hyperalgesia is regularly abused.
I see it used liberally in the media without mention that hyperalgesia has never been proven to exist in humans, only lab rats, which are certainly not a valid model for the supposedly biopsychosocial syndrome of chronic pain.
When the objective is to vilify opioids or those that need them, we are caught in a no-win situation:
- First, we are chastised for our use of opioids and told we should not take opioids expecting to reduce our pain to zero.
- Then, if we still have any pain, terms like “hyperalgesia” are tossed out as the “real reason” we hurt even though we take opioids.
No matter how the argument goes, we are blamed for our pain and vilified if we dare take opioid medication for it.
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: increased response stimulus and response mode may be the same or different Hypoalgesia raised threshold: lowered response stimulus and response mode are the same