Hyperalgesia is a phenomenon that creates an increased nervous system response to any stimulus. This reaction is so intense that the human brain interprets these signals as intense pain. It can be experienced in focal areas or in a more general, body-wide manner. There have been a number of conditioning studies that have proven that it is possible to suffer from learned hyperalgesia.
There are two types of focal hyperalgesia that normally are the result of some type of physical injury. Primary hyperalgesia exists when pain sensitivity occurs directly in the damaged tissues, whereas the secondary form is a pain sensitivity that will occur in the peripheral nerves that were not damaged during the injury. Normally this is because there was some damage to the surrounding sensory receptors, or nociceptors. There also is opioid-induced hyperalgesia which can develop as a of long-term opioid use. This normally occurs because opioids were used to treat chronic pain.
The specific mechanisms that cause hyperalgesia vary and, in some cases, are poorly understood. It is most commonly caused by platelet-activating factor (PAF), which is the body's normal response to allergies or muscle inflammation. Essentially, immune cells interact with the peripheral nervous system and release cytokines and chemokines, which are pain-producing hormones. Hyperalgesia also can be caused by pain fibers in the body being stimulated in the same pattern as inflammation. This will create an amplification of the pain fibers' synapses through the nerves in the spinal cord.
People who have been exposed to opioids such as oxycodone or heroin for an extended period of time are at risk of opioid-induced hyperalgesia. Opioids are commonly used to treat chronic pain, and when a person reports higher levels of pain than physical findings indicate, he or she will often have the dosage increased instead of being treated for hyperalgesia. The constant hyper-stimulation of opioid receptors will then result in an altered level of homeostasis in the pain-signaling pathways.
Hyperalgesia demonstrates a similarity to other types of pain disorders that are better understood, such as allodynia. This condition, in all of its forms, is not completely understood and does not have its own standardized treatment, so most people will be treated like they have allodynia. This means that they have the possibility of using a variety of drugs, including tricyclic antidepressants, pregabalin, tramadol and others. From a clinical perspective, creating a treatment regiment is very difficult because it relies on testing a number of drug combinations until one works. In the case of opioid-induced hyperalgesia, there is a chance that simply decreasing the dosage levels of the opioids can result in an improvement.