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 by a sensitization of the nervous system; when this happens we speak of hyperalgesia.
In this article we will describe what hyperalgesia is, 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 causes it?
Hyperalgesia is defined as a sustained increase in sensitivity to pain . In people who suffer this alteration the sensory threshold above which pain is experienced is lowered, so that stimuli that would not be very painful for most people can be painful for those with hyperalgesia.
It can be caused by different things, such as injury to nociceptors (cells that detect pain signals) or the prolonged use of opiates such as morphine and heroin. Depending on the specific cause of the hyperalgesia and the way it is handled, it will be a transient or chronic phenomenon.
In most cases hyperalgesia is due to the sensitization of peripheral nerve fibers from focal lesions, which cause inflammatory or allergic responses, increasing the release of pain-related chemicals. These reactions may become chronic under certain circumstances.
Relationship to 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 brushing one’s hair or coming into contact with water at a slightly elevated 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 speak of allodynia when the pain should not appear, and of hyperalgesia when it is more intense than would be expected.
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 are also related to similar dysfunctions.
Types of hyperalgesia
There are different types of hyperalgesia depending on the causes of its appearance and the type of stimuli that cause the pain. Below we will describe the most relevant ones.
1. Primary
Primary hyperalgesia appears as a consequence of an injury . It consists of an increase in the sensitivity of the nerve endings of the 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 in primary hyperalgesia, in secondary hyperalgesia the painful sensations occur in regions other than that of the injury; 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 nociceptor fibers but is exclusively attributed to central nervous system dysfunctions . Even so, stimulation is necessary for the person to feel pain; if this were not to occur we would speak of allodynia.
3. Opiate-induced
If maintained over the long term, the use of opiates (morphine, heroin, methadone, hydrocodone, oxycodone, etc.) can cause nerve sensitization to painful stimuli. In fact, it seems 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 the 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, 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. Motorcycle
Normal muscle and joint movements, for example those involved in behaviors such as walking or getting up from a chair, can cause severe pain in people with hyperalgesia.
Treatment and handling
Although the treatment of hyperalgesia must be adapted to the specific causes of the disorder, in general it is usually treated using analgesic drugs ; the same is true for allodynia, neuropathic pain and other disorders related to abnormal pain perception.
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 such as tramadol are often used in this way.
Often the most appropriate medication for each patient is difficult to find in cases of hyperalgesia, so it is likely that different pain medications will have to be tried before pain can be effectively treated.
In the case of hyperalgesia due to substance use , as it happens in chronic patients hypersensitized due to morphine or other opiate abuse, research has revealed that, paradoxically, dose reduction may be useful to relieve pain sensations.
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.
- Sandkühler, J. (2009). Models and mechanisms of hyperalgesia and allodyinia. Physiological Reviews, 89: 707-758.