A growing number of animal and human studies show that under certain circumstances, opioids can elicit unexpected changes in pain sensitivity, resulting in hyperalgesia (abnormally intense pain in response to painful stimulation) and allodynia (abnormally intense pain in response to non-painful stimulation).
Interestingly, previous studies have found evidence for hyperalgesia only subsequent to brief exposure to opioid treatment. No reports of hyperalgesia following the prolonged administration of opioids for the treatment of chronic or cancer pain have yet been published. However, using the cold pressor test (CPT), several studies have suggested that opioid addicts (OAs) may exhibit hyperalgesia as shown by their ability to maintain their hand in ice-cold water for a significantly shorter duration than were healthy controls.
Recent evidence reported on increased pain sensitivity in animals following parenteral opioid administration and in humans subsequent to intravenously of short-acting opioids and possibly in drug addicts.
A new study, to be published in Drug and Alcohol Dependence (available online 17 October 2005) was designed with the objectives to test if drug addicts, while actively consuming opioids, differ from normal subjects in their latency of pain onset, pain intensity, or time required for hand withdrawal in response to the CPT; and, if indeed so, to determine whether the discontinuation of opioid consumption will reset their altered pain responses.
For the study, 60 heroin or methadone OAs who attended a 4-week inpatient detoxification program were exposed to the cold pressor test (CPT) upon entrance to the program, at 7 and 28 days subsequent to the cessation of opioid consumption (verified by repeated urine toxicology tests).
Latency of pain onset (s), pain intensity (0-100 VAS), and tolerance (time for hand withdrawal) in response to the CPT were measured. The main findings of this study were that in response to the CPT, OAs had a longer mean latency for pain and a lower mean pain intensity rating, in comparison with healthy controls, whereas their mean time for hand withdrawal (tolerance) was shorter than that of the controls. In addition, their altered pain perception did not reset 4 weeks subsequent to the cessation of opioid use.
The present study provides further evidence that OAs exhibit short tolerance to the CPT, as manifested by their quicker hand withdrawal, which can be interpreted as hyperalgesia.