Hyperalgesia: increased sensitivity to pain

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 is intensified due to a sensitization of the nervous system; when this happens we speak of 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.

• You may be interested in: "The 13 types of pain: classification and characteristics"

What is hyperalgesia? What causes it?

Hyperalgesia is defined as a sustained increase in pain sensitivity. In people who suffer from this alteration, the sensory threshold from which pain is experienced is reduced, thus 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 damage to the nociceptors (cells that detect pain signals) or the

long-term opioid use as the morphine and the heroin. Depending on the specific cause of the hyperalgesia and the way it is managed, it will be a transient or chronic phenomenon.

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

• Related article: "Chronic pain: what it is and how it is treated from Psychology"

Relationship with allodynia

Hyperalgesia is closely related to allodynia, which consists of the appearance of pain sensations in response to stimuli that are objectively not painful, as can be the fact of passing a brush through the hair or coming into contact with water at a slightly high temperature.

Allodynia and hyperalgesia are often studied together because there are remarkable similarities between the two phenomena. In many cases, the difference between the two phenomena is limited to the intensity of the stimulation: we are talking about allodynia when the pain should not appear, and hyperalgesia when it is more intense than it should be wait.

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 associated with similar dysfunctions.

• You may be interested in: "Fibromyalgia: causes, symptoms and treatments"

Types of hyperalgesia

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

1. Primary

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

2. Secondary

Unlike what happens in primary hyperalgesia, in secondary hyperalgesia the painful sensations are produced in regions other than that of the lesion; However, it can be used both to talk about excessive pain in areas that surround the one that is damaged and in others that are further away.

In this case, the hyperalgesia is not due to the sensitization of the nociceptor fibers but is attributed exclusively to central nervous system dysfunctions. Even so, stimulation is necessary for the person to feel pain; in the event that this does not occur, we would speak of allodynia.

3. opioid induced

If sustained over the long term, the use of opioids (morphine, heroin, methadone, hydrocodone, oxycodone, etc.) can cause nervous sensitization to painful stimuli. In fact, it seems that even occasional intake of these substances has the potential to produce transient 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 into contact with hot or cold stimuli.

5. Mechanics

Mechanical hyperalgesia appears as a consequence of sensations of pressure, vibration, puncture, rubbing, etc., which 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. Motorboat

Normal muscle and joint movements, for example those involved in Behaviors such as walking or getting up from a seat can cause intense 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 usually treated using pain relievers; the same is true of allodynia, neuropathic pain, and other disorders associated with abnormal pain perception.

Thus, non-steroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen and aspirin, glucocorticoids (cortisol, prednisone…) or anticonvulsants such as pregabalin and gabapentin, as well as NMDA receptor antagonists and atypical opioids, for example tramadol.

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

In the case of hyperalgesia due to substance use, as it happens in chronic patients hypersensitized due to the abuse of morphine or other opioids, the Research has revealed that, paradoxically, dose reduction may be helpful in relieving sensations of pain.

• You may be interested in: "Types of psychoactive drugs: uses and side effects"

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.

• Sandkuhler, J. (2009). Models and mechanisms of hyperalgesia and allodyinia. Physiological Reviews, 89: 707-758.