If you’re new to pain management – iii


To understand pain and pain management, there are some terms that are used that we need to define. The definitions I use come from the IASP 2008 Proposed Taxonomy Changes, and the previously published Taxonomy of Pain.

Pain is – An unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage.

Noxious stimulus

an actually or potentially tissue-damaging event. The notes state: there are some types of tissue damage that are not detected by any sensory receptors, and thus do not cause pain. (See “nociceptive stimulus”)

Nociceptor

A sensory receptor that is capable of transducing and encoding noxious stimuli. This means that receptors such as tactile or thermoreceptors can respond to noxious stimuli when the stimulus is above the normal threshold but only nociceptors are capable of encoding the relevant properties of those stimuli (e.g. sharpness, heat intensity in the painful range).

Nociceptive neuron

– this is a new definition for 2008 and is a central or peripheral neuron that is capable of encoding noxious stimuli. What this means is that low threshold neurons (such as some of those in the spinal cord) can transmit information that is above the normal threshold (ie noxious), but only high threshold or ‘wide dynamic range’ neurons can transmit information about the properties of that stimulus – eg the sharpness of a stimulus, the intensity of heat in the painful range.)

Nociception

– this is a term we’ve used for quite a while, but this taxonomy defines it specifically as the neural processes of encoding and processing noxious stimuli. This term has been specified to help distinguish between the neural process that forms the basis of many but not all painful experiences – and the experience of pain itself. The taxonomy states ‘Pain is a subjective phenomenon, whereas nociception is the object of sensory physiology.’

Nociceptive stimulus

– this term has been defined to distinguish between a stimulus that is noxious (ie harmful) and a stimulus that is transmitted via nociceptors and therefore potentially experienced as painful. There are some types of tissue damage that are not detected by sensory receptors and therefore are not painful, while stimuli that are able to adequately stimulate nociceptors are a subset of noxious stimuli.

Nociceptive pain

– this is a new term specifically defined as pain arising from the stimulation of nociceptors.

Neuropathic pain

– this is defined as pain arising as a direct consequence of a lesion or disease affecting the somatosensory system. By incorporating the term ‘somatosensory’ system, it can cover the range of neuropathic conditions from post-stroke pain right through to post-herpetic neuralgia.

Neuropathic pain is divided into to primary types – peripheral and central, and must affect the somatosensory system.

Sensitisation

– this is a term that has been used widely over the past few years. It is defined as ‘Increased responsiveness of neurons to their normal input or recruitment of a response to normally subthreshold inputs.’ In other words, the amount of stimulus needed to activate neural firing is lower than normal, or it responds more rapidly to normal input. This is a neurophysiological term that can only be applied when both input and output of the neural system under study are known. Clinically, we can only infer it from the presence of allodynia (increased sensitivity to normally nonnocicpetive stimulation), or hyperalgesia (increased sensitivity to normally painful stimulation).

Like neuropathic pain, sensitisation can occur in the peripheral nervous system as well as the central nervous system. In the CNS, it can also refer to increased responsiveness due to dysfunction of endogenous pain control systems.

Allodynia

– I used this term above, it is formally defined as pain in response to a non-nociceptive stimulus. Because we don’t know which inputs activate nociceptors, it should only be used for dynamic tactile allodynia to tangential stroking stimuli, e.g. brushing the skin, because this is the only established example.

Hyperalgesia

– is now defined as increased pain sensitivity. It may include both a decrease in threshold and an increase in suprathreshold response – that is, it may mean a lower than normal threshold for responding to stimulation – and an increase in the amount of information transmitted once the threshold has been reached.

Pain threshold

– this is one of my favourites to make sure it’s well understood! It’s different from pain tolerance which comes next... Pain threshold refers to the minimal intensity of a stimulus that is perceived as painful.

Pain tolerance

is the maximum intensity of a stimulus that evokes pain and that a subject is willing to tolerate in a given situation. We know that tolerance varies between people and in the same person in different situations, so this definition clarifies that context (and I’m guessing therefore the meaning of the pain, and probably the consequences of pain behaviour) of the stimulation is important.

I decided to take the time to go through these definitions because as you read some of next week’s posts on neurophysiology, it’s important to think about the way these terms are used now, compared with some of the historical definitions.

The first Taxonomy of Pain was mentioned by Loeser and Black in 1975 (Loeser & Black, Pain. 1975 Mar;1(1):81-4.), and it’s been revised since then more than once. The second edition of the taxonomy from IASP was published in 1994 (Merskey H, Bogduk N (Eds.), Classification of Chronic Pain: Descriptions of Chronic Pain Syndromes and Definitions of Pain Terms, 2nd ed. Seattle: IASP Press, 1994.) and now the latest review commonly called the ‘Kyoto Protocol’ from a meeting of the IASP Council in Kyoto, November 29-30, 2007.

Another reason for being quite pedantic about this sort of terminology is that words carry baggage – loads of assumptions about cause, implications about mechanisms, judgements about why and how are carried along with the names we use for things.

For example, think of the term ‘organic’ when it’s used in relation to pain – it assumes that there is some sort of understood and locatable pathology that can be found, while nonorganic suggests that the person experiencing the pain has ‘something wrong with their psyche’. This kind of term harks back to the old body and mind distinction that is so irrelevant in pain and pain management (yet it appears in so many medico-legal reports).

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