Fibromyalgia (FM)–are there different mechanisms in the processing of pain? A double blind crossover comparison of analgesic drugs

OBJECTIVE: Pain was analyzed in patients with fibromyalgia (FM) in a
randomized, double blind, crossover study using intravenous
(i.v.) administration of different drugs.

METHODS: In 18
patients with FM muscle pain to i.v. administration of
morphine (0.3 mg/kg), lidocaine (5 mg/kg), ketamine (0.3
mg/kg), or saline was studied. Spontaneous pain intensity,
muscle strength, static muscle endurance, pressure pain
threshold, and pain tolerance at tender points and non-tender
point areas were followed. Drug plasma concentrations and
effects on physical functioning ability score (FIQ) were
recorded. A personality inventory (KSP) was used to related
pain response to personality traits.

RESULTS: Thirteen
patients responded to one or several of the drugs, but not to
placebo. Two patients were placebo responders responding to
all 4 infusions. Three were nonresponders responding to no
infusions. Seven of the responders had a reduction in pain for
1-5 days. Pressure pain threshold and pain tolerance increased
significantly in responders. Plasma concentrations were
similar in responders and nonresponders. FIQ values improved
significantly after the ketamine infusion. Responders scored
higher on KSP scales for somatic anxiety, muscular tension,
and psychasthenia compared with healthy controls.

CONCLUSION:
FM diagnosed according to the American College of Rheumatology
criteria seems to include patients with different pain
processing mechanisms. A pharmacological pain analysis with
subdivision into responders and nonresponders might be
considered before instituting therapeutic interventions or
research.

Sorensen J, Bengtsson A, Ahlner J, Henriksson KG, Ekselius L, Bengtsson M

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