Background: Investigations based on quantitative sensory testing have consistently shown evidence of allodynia in fibromyalgia syndrome (FMS) patients involving both the spinal and supraspinal pain regulatory systems. Functional imaging studies have demonstrated enhanced neural activities in pain-related brain areas as well as impairment of pain inhibition in the descending nociceptive regulatory system. A higher state of excitability of spinal nociceptive neurons as evidenced by lowered nociceptive flexion reflex R-III (NFR) threshold was reported for FMS patients. The NFR procedure has been shown to be a valuable tool to evaluate pharmacologically active therapeutic agents at the spinal level.
Objective: Serotonin-noradrenaline reuptake inhibitors have been shown to reduce pain in FMS patients possibly through descending monoaminergic pain pathways modulation. This randomized double-blind placebo-controlled trial assessed the pharmacodynamic activity of the dual-reuptake inhibitor milnacipran (MLN) at the spinal level by means of the objective spinal NFR.
Study design: Randomized, double-blind, placebo-controlled trial.
Setting: A single academic medical center, outpatient setting.
Methods: Seven-week exposure (100, 150, 200mg/day) in women fibromyalgia patients. Evaluation consisted of extensive quantitative sensory testing including determination of the NFR threshold, self-reported standard questionnaires investigating pain, visual analog scales, fibromyalgia impact, health-related quality of life, depression and anxiety questionnaires, as well as the Patient's Global Impression of Change (PGIC). Analysis of covariance adjusted for baseline value was used for all endpoints.
Results: Seventy-seven (39 placebo, 38 milnacipran all doses) out of 80 randomized patients were available for analysis. The absence of influence of MLN (any dose) on the NFR surprisingly contrasted with the dose-dependent analgesic effect observed in MLN-treated patients with an adjusted change difference of -18.4mm (-30.9; -5.8) in pain reduction between placebo and the maximum dosage (200 mg) MLN groups (P = 0.02). Unchanged depression and anxiety scores confirmed the predominant selectivity of the analgesic effect of MLN on nociceptive pain pathway. Self-reported questionnaires consistently reflected the positive effects of MLN on quality of life and psychological well-being. Odds ratio 5.1 for PGIC responders (i.e. much/very much improved) was significantly in favor of MLN (P = 0.04).
Conclusion: Milnacipran has a predominantly supraspinal analgesic effect as evidenced by the significant clinical benefits and the absence of changes in the nociceptive spinal reflex threshold. Higher dose was associated with higher pain reduction. Reported analgesia was independent of patients' emotional status.