Effect of the Combined Use of Tramadol and Milnacipran on Pain Threshold in an Animal Model of Fibromyalgia

BACKGROUND/AIMS: Acidic saline injections produce mechanical hyperresponsiveness in male Sprague-Dawley rats. We investigated the effect of milnacipran in conjunction with tramadol on the pain threshold in an acidic saline animal model of pain. METHODS: The left gastrocnemius muscle of 20 male rats was injected with 100 microL of saline at pH 4.0 under brief isoflurane anesthesia on days 0 and 5. Rats administered acidic saline injections were separated into four study subgroups. After determining the pre-drug pain threshold, rats were injected intraperitoneally with one of the following regimens; saline, milnacipran alone (60 mg/kg), milnacipran (40 mg/kg) plus tramadol (20 mg/kg), or milnacipran (40 mg/kg) plus tramadol (40 mg/kg). Paw withdrawal in response to pressure was measured at 30 min, 120 min, and 5 days after injection. Nociceptive thresholds, expressed in grams, were measured with a Dynamic Plantar Aesthesiometer (Ugo Basile, Italy) by applying increasing pressure to the right or left hind paw until the rat withdrew the paw. RESULTS: A potent antihyperalgesic effect was observed when tramadol and milnacipran were used in combination (injected paw, p=0.001; contralateral paw, p=0.012). This finding was observed only at 30 min after the combination treatment. CONCLUSIONS: We observed potentiation of the antihyperalgesic effect when milnacipran and tramadol were administered in combination in an animal model of fibromyalgia. Further research is required to determine the efficacy of various combination treatments in fibromyalgia in humans.

Pentazocine-induced fibrous myopathy and localized neuropathy.

A 47 year-old woman who had a 4-year history of intramuscular pentazocine injections in the lower extremities, developed gradual stiffness and weakness of the lower extremities. The thigh and buttock muscles were "wooden-hard" on palpation. The skin was hard, shiny and hairless. Associated clinical and electrophysiological polyradiculopathy and multiple mononeuropathy of the lower extremities were observed. Imaging studies showed calcification and fibrosis of the involved muscles. Muscle biopsy revealed fibrous myopathy. Caution in longterm usage and early recognition of pentazocine toxicity as a neuromuscular complication are important in order to prevent irreversible drug-induced fibrous myopathy and localized neuropathy.