Hyperalgesia: increased sensitivity to pain

April 3, 2024

Sometimes traumatic injuries cause damage to the nerve fibers that transmit tactile sensations to the brain. In these and other cases it is possible that the perception of pain intensifies due to a sensitization of the nervous system; When this happens we talk about hyperalgesia.

In this article we will describe what is hyperalgesia, what causes it and how is it treated . We will also explain the different types of hyperalgesia that have been proposed so far, as well as the relationship of this phenomenon with another very similar one: allodynia.

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What is hyperalgesia? What causes it?

Hyperalgesia is defined as a sustained increase in pain sensitivity . In people who suffer this alteration the sensory threshold from which pain is experienced is reduced, so that stimuli that would not be very painful for most people can be for those who have hyperalgesia.

It can be produced by different causes, such as lesions in the nociceptors (cells that detect pain signals) or the prolonged opioid use like morphine and heroin. Depending on the specific cause of hyperalgesia and how it is managed, it will be a transient or chronic phenomenon.

In most cases the hyperalgesia is due to the sensitization of peripheral nerve fibers due to focal lesions, which provoke inflammatory or allergic type responses, increasing the release of chemical substances related to pain. These reactions can become chronic in certain circumstances.

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Relationship with allodynia

Hyperalgesia is closely related to allodynia, which consists of the appearance of pain sensations in response to stimuli that are objectively non-painful , such as the fact of passing a brush through the hair or coming into contact with water at a slightly elevated temperature.

Allodynia and hyperalgesia are often studied jointly because there are notable similarities between the two phenomena. In many cases the difference between both phenomena is limited to the intensity of the stimulation: we speak of allodynia when pain should not appear, and of hyperalgesia when it is more intense than we would expect.

Both hyperalgesia and allodynia have been associated with alterations in the central and peripheral nervous system that cause an exaggerated perception of pain. It is hypothesized that fibromyalgia, migraine and complex regional pain syndrome They are also related to similar dysfunctions.

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Types of hyperalgesia

There are different types of hyperalgesia depending on the causes of their appearance and the type of stimuli that cause pain. Next we will describe the most relevant ones.

1. Primary

Primary hyperalgesia appears as a result of an injury . It consists in an increase in the sensitivity of the nerve endings of nociceptors in the damaged region, although it also involves alterations in the processing of pain signals at the level of the central nervous system.

2. Secondary

Unlike what happens in primary school, in secondary hyperalgesia, painful sensations occur in regions other than the lesion; however, it can be used both to talk about excessive pain in areas surrounding the damaged one and in other more distant areas.

In this case the hyperalgesia is not due to sensitization of the fibers of the nociceptors but is attributed exclusively to dysfunctions of the central nervous system . Even so, stimulation is necessary for the person to feel pain; if it did not occur, we would talk about allodynia.

3. Induced by opiates

If maintained long-term, the consumption of opiates (morphine, heroin, methadone, hydrocodone, oxycodone, etc.) can cause a nervous sensitization to painful stimuli. In fact it seems that even the timely taking of these substances has the potential to produce temporary symptoms of hyperalgesia and allodynia.

4. Thermal

We speak of thermal hyperalgesia when the stimulus that causes pain is related to temperature; in these cases the person feels excessive pain when coming in contact with hot or cold stimuli .

5. Mechanics

Mechanical hyperalgesia appears as a consequence of sensations of pressure, vibration, puncture, friction, etc., that activate the mechanical nociceptors of the peripheral nervous system.

We can distinguish two subtypes of mechanical hyperalgesia: static and dynamic . The first is associated with a single contact with the painful stimulus, while dynamic hyperalgesia occurs when the object is in motion.

6. Motor

Normal muscle and joint movements, for example those that are involved in behaviors such as walking or getting up from a seat, can cause severe pain in people with hyperalgesia.

Treatment and management

Although the treatment of hyperalgesia must be adapted to the specific causes of the alteration, in general It is usually treated using analgesic medications ; the same happens with allodynia, neuropathic pain and other disorders related to the abnormal perception of pain.

Thus, non-steroidal anti-inflammatory drugs (NSAIDs) are commonly used, such as ibuprofen and aspirin, glucocorticoids (cortisol, prednisone ...) or anticonvulsants such as pregabalin and gabapentin, as well as antagonists of NMDA receptors and atypical opiates, for example tramadol.

Frequently, the most appropriate medication for each patient is difficult to find in cases of hyperalgesia, so it is likely that different analgesic drugs have to be tried before the pain can be effectively treated.

In the case of hyperalgesia due to substance use As in chronic hypersensitized patients due to the abuse of morphine or other opiates, research has revealed that, paradoxically, reducing the dose may be useful in relieving pain sensations.

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Bibliographic references:

Chu, L. F .; Angst, M. S. & Clark, D. (2008). Opioid-induced hyperalgesia in humans: molecular mechanisms and clinical considerations. Clinical Journal of Pain, 24 (6): 479-96.