Hyperalgesia is a medical term describing a condition in which the patient has an increased sensitivity to pain. It is often caused by damage to nociceptors (sensory nerve cells) or peripheral nerves. Opioid-induced hyperalgesia is, therefore, pain sensitivity caused by the use of opioids.
This hypersensitivity can be caused by a response to illness, such as infection. It can be experienced in specific areas of the body or all over. There are two types of hyperalgesia: primary and secondary.
The primary subtype includes pain sensitivity which occurs directly in damaged tissues. The secondary subtype includes pain sensitivity experienced in surrounding, undamaged tissue.
Opioid-induced hyperalgesia (OIH) can develop as a result of long-term opioid therapy in the treatment of chronic pain. Both types of hyperalgesia can occur after either chronic or acute exposure to opioids.
Opioids and Pain Management
Pain is among the most common reasons why people seek medical attention. Pain, simply put, is a physical reaction to certain stimuli. It is our body’s way of telling us something is wrong, or whatever we are doing, we’d better knock it off.
Using opioids for chronic pain has been a common strategy for the last two decades. However, chronic pain often persists or worsens despite long-term opioid therapy. This is a paradoxical problem—the treatment intended to mitigate the condition can actually make it worse.
This year, the Centers for Disease Control and Prevention released a set of guidelines intended to help doctors who prescribe opioids. They discouraged the use of opioids as a first line of defense and also indicated that theye were not a good idea for long-term, chronic pain. In addition to OIH, opioids are highly addictive and can rapidly cause dependence.
Opioid-Induced Hyperalgesia and Tolerance
Opioid-induced hyperalgesia is more formally defined as increased sensory nerve sensitization and is not the same condition as tolerance, addiction, or dependence.
Among the main problems of diagnosing OIH is that its presence tends to overlap with opioid tolerance. That is, both conditions share a decreasing response to an opioid dosage, and probably share other mechanic similarities.
However, tolerance can be somewhat resolved by increasing a dose, whereas OIH results in increased pain. Also, tolerance builds over time, whereas OIH tends to come on quickly. Pain intensity also increases with OIH, whereas that may not be the case with tolerance.
Specifics of Opioid-Induced Hyperalgesia
Opioid-induced hyperalgesia also has other possible characteristics. For example, the pain tends to be more diffuse (spread out) and harmful stimuli tend to be more painful than expected. Pain quite often extends beyond the area of damage. For example, even if the damage is removed or healed, pain in surrounding areas may persist. For this reason, the pain may give the illusion that the condition is getting worse when actually, it is just the response to pain that is increasing.
The most common theory currently accepted about OIH is called the neuroexcitatory (stimulation of neurons) model. To put this model most simply, it is believed that opioids activate a receptor which causes an increase in calcium. This significantly increases the excitability of the neuron. When this receptor (called the NMDA) and its neurons are more active, they increase the transmission of painful impulses in response to stimuli.
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