Hyperalgesia: increased sensitivity to pain

Sometimes traumatic injuries damage nerve fibers that transmit tactile sensations to the brain. In these and other cases, the perception of pain may intensify due to sensitization of the nervous system; when this happens, we are talking about hyperalgesia.

In this article we will describe what is hyperalgesia, what causes it and how it is 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.

    What is hyperalgesia? What are the causes?

    Hyperalgesia is defined as a sustained increase in pain sensitivity. In people with this disorder, the sensory threshold at which pain is felt is reduced, so that stimuli that would not be very painful for most people can be for those with hyperalgesia.

    It can be caused by different causes, such as damage to the nociceptors (cells that detect pain signals) or prolonged use of opiates like morphine and heroin. Depending on the specific cause of hyperalgesia and how it is treated, 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 elicit inflammatory or allergic-type responses, increasing the release of pain-related chemicals. These reactions can become chronic under certain circumstances.

      Relationship with allodynia

      Hyperalgesia is closely related to allodynia, which is the appearance of pain sensations in response to objectively painless stimuli, Such as brushing your hair or coming into contact with water at a slightly high temperature.

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

      Hyperalgesia and allodynia have been associated with changes in the central and peripheral nervous system that cause exaggerated pain perception. We think that fibromyalgia, migraine and complex regional pain syndrome they are also linked to similar dysfunctions.

        Types of hyperalgesia

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

        1. Primary

        Primary hyperalgesia appears as a result of an injury. It consists of an increase in the sensitivity of the nerve endings of the nociceptors to 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 hyperalgesia, in secondary hyperalgesia painful sensations occur in areas other than the lesion; however, it can be used both to talk about excessive pain in the areas surrounding the damaged one and in areas further away.

        In this case, the hyperalgesia is not due to sensitization of the nociceptor fibers but is attributed exclusively to central nervous system dysfunctions. Yet stimulation is necessary for the person to feel pain; in case that didn’t happen, we would be talking about allodynia.

        3. Opioid induced

        If maintained over the long term, the consumption of opiates (morphine, heroin, methadone, hydrocodone, oxycodone, etc.) can cause nervous sensitization to painful stimuli. In fact, it appears that even the 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 on contact with hot or cold stimuli.

        5. Mechanics

        Mechanical hyperalgesia occurs as a result of sensations of pressure, vibration, puncture, friction, etc., which activate the mechanical nociceptors of the peripheral nervous system.

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

        6. Engine

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

        Processing and handling

        Although the treatment of hyperalgesia should be tailored to the specific causes of the impairment, in general usually treated with pain relievers; the same goes for allodynia, neuropathic pain and other disorders related to abnormal pain perception.

        In this way, they usually use nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen and aspirin, glucocorticoids (cortisol, prednisone …) or anticonvulsants like pregabalin and gabapentin, as well as NMDA and atypical receptor antagonists. opiates, for example tramadol.

        Often times, the most suitable drug for each patient is difficult to find with hyperalgesia, so it is likely that different pain relieving drugs will need to be tried before the pain can be effectively treated.

        In case of hyperalgesia due to the consumption of substancesAs with patients with chronic hypersensitivity due to morphine or other opioid abuse, research has found that, paradoxically, dose reduction can be helpful in relieving pain sensations.

          Bibliographical references:

          • Chu, LF; Angst, MS and Clark, D. (2008). Opioid-induced hyperalgesia in humans: molecular mechanisms and clinical considerations. Clinical Journal of Pain, 24 (6): 479-96.
          • Sandkühler, J. (2009). Models and mechanisms of hyperalgesia and allodynia. Physiological Examinations, 89: 707-758.

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