Pain analysis : A study in patients with chronic low back pain or fibromyalgia

Abstract: Chronic low back pain (CLBP} and fibromyalgia (FM} are two common chronic pain conditions in which the pain processing mechanisms are not well understood. To identify the types of pain, we used different intravenous {t.v.} and epidural pharmacological test procedures as well as questionnaires with pain intensity ratings, pain drawings, self-assessment of functional status, disability and depression. 93 patients with CLBP and 49 women with FM were included in 7 studies. CLBP patients were classified, by response to t.v. morphine, t.v. lidocaine and epidurally administered fentanyl and local anaesthetic, into the different pain types; nociceptive, neuropathic, and idiopathic {non-responding) pain. In about 85% of CLBP patients it was possible to classify the pain of which about 300;6, admitted for assessment of suitability for surgery, had non-responding {idiopathic) pain. CLBP patients chosen for lumbar fusion surgery were prospectively evaluated as to whether preoperative pha:nnacological pain classification associated with the outcome of surgical treatment. A significantly poorer (P<0.05) outcome in the non-responding group suggests that pharmacological pain analysis might be useful as a predictor of surgical outcome. Patients with persistent pain after low back surgery were examined by standard clinical and radiological methods and different questionnaires. The pharmacological test battery was used to classify the patients into different pain groups. With the phannacologtcal classification taken as astandard, the clinical judgement seemed to have difficulty mainly in the differentiation between neuropathic pain and idiopathic pain. The pharmacological pain classification can support the surgeon in deciding when further surgery should be avoided and also be helpful in avoiding an inappropriate diagnostic label. The patients with "failed back surgery syndrome" were also assessed with t.v. phentolamine to identify those with sympathetically maintained pain (SMP). It was concluded that SMP is either an uncommon cause of persistent pain in this type of failed back surgery patients or, the phentolamine test as we performed it was unable to identify SMP. Patients with FM diagnosed according to tlle American College of Rheumatology (ACR) criteria were classified pharmacologtcally into responders and non-responders by response to t.v. morphine, i.v. lidocaine, i.v. ketamine and t.v. saline (placebo). The effects on muscle strength, static muscle endurance, pressure pain threshold, and pain tolerance at tender points and non-tender point areas were also assessed. Ketamine, an NMDA-receptor antagonist. significantly reduced pain intensity, and increased pressure pain threshold and pain tolerance at tender points and non-tender point areas. The ACR classification in FM seems to allow inclusion of patients with different pain processing mechanisms. An experimental study examined 1) whetller non-painful sites in FM patients showed evidence of lowered pressure pain thresholds and 2) lowered pain thresholds as a response to either single or repeated electrical stimulation of the skin and into a non-painful muscle, and 3) the responses to injection of an algesic substance, hypertonic saline, into the underlying, non-painful muscle. Compared to age-matched controls FM patients had lower pressure-pain thresholds, but unaltered electrical skin responstvity and greater pain duration and wider spatial distribution of pain following the injection of hypertonic saline. The results suggests an upregulation in the central nociceptive system in FM patients.

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