Do you know a patient who seems to get worsening pain as they escalateto a higher dose of narcotic pain medication? Conventional thinkingis that patients get worse over time due to progression of disease processor increasing tolerance to their medication – both of these are certainlypossible. However, there is another cause of decreasing effectiveness ofopiates, one that is actually caused by the medication itself.
Opiate pain medications are the drug of choice for moderate tosevere acute and chronic pain, however, there is growing evidence thatopiate pain medications can paradoxically make a patient’s pain worse.Opioid–induced hyperalgesia (OIH) is a recognized phenomenon whereopiates have been found to heighten pain sensitivity and worsen painlevels.
One would expect as you increase doses of pain medications thatanalgesia would improve, and this is true if the pain is secondary to apathologic disease process or a tolerance issue. However, pain related toOIH will continue to worsen as doses are escalated – and only improvewhen opiates are decreased or discontinued.
Differentiating pain secondary to OIH from the primary pain conditionmay be difficult. Usually pathologic pain will be more localizedand/or follow a dermatomal pattern, whereas pain from OIH will bemore generalized in distribution.
OIH pain has a neuropathic component, and is usually described asa burning pain with increased sensitivity to light touch. While increasingopiate doses will worsen pain from OIH, decreasing opiate dose hasbeen shown to decrease pain levels in this setting. In contrast, if pain ispathologic (not OIH), attempting to wean pain medications will likelyresult in escalation of pain level.
Studies in rats have shown that repeated doses of morphine can leadto lowering the pain threshold (i.e. less stimulation to cause pain). Trialsin human narcotic addicts have shown that addicts are significantlymore sensitive to pain than non–addicts.
In addition, those narcotic addicts on methadone had higher pain sensitivitythan addicts not on methadone. The mechanism of this processinvolves multiple pain pathways. Interestingly, the OIH pathways aresimilar to mechanisms responsible for both neuropathic pain and opioidtolerance.
Evidence is showing that progression to OIH may vary between opiates.Morphine has been shown to be more likely to induce hyperalgesiathan methadone. Hyperalgesia has been shown to occur in both acute andchronic settings. There are several ongoing studies to further define thisinteresting phenomenon.
Physicians should consider the development of OIH in a patient whois not responding to pain medications or paradoxically gets worse withincreased dose of opiate pain medications. Should OIH be a concern,treatment options to consider include rotating to another opioid, usingadjuvant medications such as anti–depressants and anti–epileptics, orconsidering an opioid wean.
It’s important to remember that worsening disease process andmedication tolerance are not the only reason for narcotic pain medicationsbecoming less effective over time. We should always consider thepossibility of opioid–induced hyperalgesia in a patient who does not seemto respond to conventional treatment and has new, more diffuse paincomplaints.
Although likely to be unpopular with the patient, a trial of weaning thepain medication is prudent and should be strongly considered. If you’reuncertain about patients’ pain symptoms or concerned about OIH, contactyour pain specialist so that we may work with you to improve yourpatients’ pain.
