Objectives: This study evaluates the analgesic responses to intravenous administration of morphine, lidocaine, and ketamine and their relations to duration of chronic pain after whiplash trauma. In addition, experimental muscle pain sensitivity and its correlation to pain duration and pharmacological responses were assessed.
Methods: Thirty-three patients with diagnosed whiplash-associated disorder grade II in the chronic stage, according to the Quebec classification, were included. The pharmacological evaluation was performed in a randomized, double-blind, cross-over design and consisted of a 30-minute period of intravenous administration of morphine (0.3 mg/kg), lidocaine (5 mg/kg), ketamine (0.3 mg/kg), or placebo (isotonic saline). Intensity ratings of habitual pain on a visual analogue scale were taken before, during, and after the infusion. The patients were classified as nonresponders, placebo-responders, or responders (minimum 50% decrease of pain intensity) of the drugs. Pressure pain thresholds and intramuscular and cutaneous electrical stimulation pain thresholds were measured. The pain intensity during experimental muscle pain by intramuscular hypertonic saline was also recorded. Experimental pain assessments were performed on the lower legs outside the habitual painful area.
Results: Thirty patients completed the study; 2 were placebo responders and 10 were nonresponders. Of 18 responders, there were 15 morphine responders, 11 lidocaine responders, and 14 ketamine responders. In the patients with whiplash-associated disorder duration less than 2 years, 7 responded to morphine, 5 to lidocaine, and 8 to ketamine. In the patients with pain duration longer than 2 years, 8 responded to morphine, 6 to lidocaine, and 6 to ketamine. Thus, no pattern with respect to pain duration was found. Seventeen patients participated in the experimental pain assessment, and no significant differences in the variables of the intramuscular and cutaneous stimulation and intramuscular-induced pain with respect to response to the pharmacological challenges or whiplash-associated disorder duration existed.
Discussion: The pharmacological challenges identified subgroups of patients with chronic whiplash-associated disorder that might be considered before instituting therapeutic interventions or research. However, the pattern of responses to the pharmacological challenges did not show any clear relationships with pain duration or the experimental pain tests.