We endeavored to assess the short-term effects of intrathecal fentanyl and lidocaine in chronic-pain patients by ascertaining whether the opioid fentanyl, by virtue of its lack of sensory and motor paralysis, conferred any diagnostic advantages over lidocaine, a local anesthetic whose effects include sensory and motor paralysis. Neuraxial administration of fentanyl has been touted as an improved diagnostic tool to distinguish between peripheral and central pain, because the absence of sensory and motor effects may avert the patient's presumption of the onset of analgesia based on these cues. Twenty-two patients with persistent low-back pain, whose investigations had determined that they were not surgical candidates, were studied using a counterbalanced, placebo-controlled, and double-blinded crossover design. Each patient received three separate lumbar intrathecal injections of equal volume (1.4 ml): cerebrospinal fluid, fentanyl 25 μg, and lidocaine 70 mg. Pain and symptom assessments were performed preinjection (baseline), and at regular intervals up to and including 4 h postinjection. Pain was evaluated by verbal patient response using a numerical pain-rating system of 0 to 10. Duration of analgesia, sensation of warmth, and adverse effects were noted. Statistical analyses were performed using nonparametric tests. Subjects' average age was 56 years, with a median low-back pain duration of 16 years. There were no significant differences in the baseline median-pain scores among injection types. The baseline and best cerebrospinal fluid-pain scores were significantly different, suggesting a placebo effect. The best pain scores for fentanyl and lidocaine were superior to their own baseline levels and to the best cerebrospinal fluid scores. The median duration of analgesia was significantly longer with both fentanyl and lidocaine than with cerebrospinal fluid. Although fentanyl and lidocaine both produced substantial comparable improvement in pain scores, fentanyl provided more prolonged analgesia than lidocaine. Unique sensory effects of fentanyl included pruritus and a sensation of warmth affecting the trunk and lower limbs. Aggravation of low-back pain occurred in some patients following regression of the effects of the injection. The absence of sensory and motor paralysis with fentanyl conferred no diagnostic advantages when compared with lidocaine, in that pain response was similar. Surprisingly, given the lack of objective findings in the sample, most patients (91%) responded to the injections as if the pain involved nociception, and not solely placebo or central effects. The utility of spinal injections for the diagnosis of persistent low-back pain is questioned.
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