Methylene blue (MB) is a diaminophenothiazine with potent antioxidant properties and with inhibitory effects on both nitric oxide synthases and on the enzyme guanylate cyclase. It was recently demonstrated that MB reduces chronic discogenic low back pain, pain after lateral sphincterotomy as well as pain on injection of propofol. The underlying mechanisms of MB treatment in the above clinical pain conditions are not completely understood. Given that NMDA receptor is implicated in hyperalgesia and many of the NMDA effects are mediated through the production of nitric oxide we hypothesized that administration of a nitric oxide antagonist would decrease the pain scores and hyperalgesia in neuropathic pain.
The aims of the study
We performed a prospective clinical trial consisting of a double-blinded, crossed-over, randomized clinical trial (RCT). The aim is to determine the clinical effectiveness of methylene blue in the treatment of neuropathic pain.
Material and methods
Patients with peripheral or central neuropathic pain were randomized to receive one of two treatments: Methylene blue 2mg/kg (10 mg/mL Methyltioninklorid, Apoteket, Umeå, Sweden) or methylene blue 0.02 mg/kg both infused i.v. over 60 minutes. The pain was measured at baseline and at 60 min after the start of infusion (NRS scale) and also on a diary in the next 24 hours. ECG, pulse, blood pressure, were continuously recorded. Plasma concentrations of 8-isoprostane-prostaglandin F2α (8-iso-PGF2α) an indicator of oxidative injury, were measured according to a highly specific and validated radioimmunoassay (RIA) method at our laboratory. The detection limit for 8-iso-PGF2α is 23 pmol/L (normal values under 80 pmol/L). Cytokines (TNF-α, IL-4, IL-6, IL-7, IL-8, IL-10, IL-17A) and nNOS were detected in plasma by antibody-based proximity ligation before and after the infusion of MB and placebo. Proximity ligation assay has been used to develop homogeneous immunoassays to detect cytokines in the sub-pg/mL range.
Statistics Assuming that MB treatment would reduce the NRS 25%, power analysis with
a = 0.05, b = 0.8, showed that we would need to study 10 patients in each group. The computer program GraphPad was used for all data analyses. Parametric data were analysed by two-way ANOVA. Unpaired Student t-tests, Chi-squared tests and Fisher’s exact probability tests were used as appropriate for comparison between groups. All values are expressed as mean (SD). Statistical significance was accepted at p < 0.05.
The mean age of the patients was 65.4±14, mean duration of pain 4.6±2.6. A decrease of VAS at 60 min in comparison with baseline was observed in MB (p=0.047) group, at 6 hours after baseline (p=0.01), 10, 14, 18 h. Differences between MB and placebo were observed on the day of the infusion (p=0.04), day 2 (p=0.008). Adverse reactions
A statistically significant in the changes of the VAS scores was obtained in the patients with neuropathic pain after the treatment at 60 min and at 10, 14, 18 hours in the MB group. The study suggests that the infusion of MB may be an alternative for the treatment of neuropathic pain. MB has been in use as a medicinal continuously throughout the past century and it may now return to play an important therapeutic role.