The use of chronic opioids leads to either tolerance or sensitization. This results in loss of efficacy either way and results in pronociception either way. The nociceptive system works to maintain responsiveness of the system. The longer you take opioids to blunt chronic pain, the more likely you are to experience enhanced pain as the body tries to restore normal sensory function.
Preclinical evidence. Thermal hyperalgesia has been recognized back to the 1980’s in the animal sciences. Hyperalgesia is defined as an exaggerated response to painful stimulation. Hyperalgesia is manifested as an exaggerated response to hot temperatures. The animals who experienced hyperalgesia could tolerate less heat than normal animals not suffering from hyperalgesia. Hyperalgesia can be demonstrated following the injection of morphine into the spinal fluid of rats over several days. The duration of the hyperalgesic effect extended beyond the time that the morphine was discontinued which implied changes in the nervous system. [Vanderah et al. J Neurosci. 2000 Sep 15;20(18):7074-9 , Mao J et al. J Neurosci. 2002 Sep 15;22(18):8312-23.] Other opiates such as Fentanyl and Heroin have the same effect. [Celerier et al, J Neurosci. 2001 Jun 1;21(11):4074-80.]
Clinical Implications: Repeated use of opioids can lead to a pathological pain state. Decreased analgesic efficacy can be seen as early as following a single dose. [ Cooper et al. 1997, Vinik and Kissin 1998, Guignard et al 2000.] Patients not given opioids interoperatively required fewer opioids post operatively.
When patients with low back pain were tested for hyperalgesia with a cold pressor test, it was found that patients with low back pain alone had much better pain tolerance than patients who had low back pain and had been treated with opioids. [Chu et al 2006.]
It appears that chronic back pain alone causes little wind up when compared to healthy individuals, but exposure to opioids resulted in dramatic thermal allodynia and temporal summation. The patients simply had more pain. They felt pain with less stimulation and their pain was more severe.
In an effort to relieve pain in these patients by treating them with opioid pain relievers, the patients developed worse pain and lowered pain tolerance in a short period of time.
