Lumbar epidural fentanyl: segmental spread and effect on temporal summation and muscle pain

Background. Despite extensive use, different aspects of thepharmacological action of epidural fentanyl have not been clarified.We applied a multi-modal sensory test procedure to investigatethe effect of epidural fentanyl on segmental spread, temporalsummation (as a measure for short-lasting central hyperexcitability)and muscle pain. Methods. Thirty patients received either placebo, 50 or 100µg single dose of fentanyl epidurally (L2–3), ina randomized, double-blind fashion. Heat pain tolerance thresholdsat eight dermatomes from S1 to fifth cranial nerve (assessmentof segmental spread), pain threshold to transcutaneous repeatedelectrical stimulation of the sural nerve (assessment of temporalsummation) and pain intensity after injection of hypertonicsaline into the tibialis anterior muscle (assessment of musclepain) were recorded. Results. Fentanyl 100 µg, but not 50 µg, producedanalgesia to heat stimulation only at L2. Surprisingly, no effectat S1 was detected. Both fentanyl doses significantly increasedtemporal summation threshold and decreased muscle pain intensity. Conclusions. The findings suggest that a single lumbar epiduraldose of fentanyl should be injected at the spinal interspacecorresponding to the dermatomal site of pain. Increased effecton L2 compared with S1 suggests that drug effect on spinal nerveroots and binding to opioid receptors on the dorsal root gangliamay be more important than traditionally believed for the segmentaleffect of epidurally injected fentanyl. Epidural fentanyl increasestemporal summation threshold and could therefore contributeto prevention and treatment of central hypersensitivity states.I.M. injection of hypertonic saline is a sensitive techniquefor detecting the analgesic action of epidural opioids. Br J Anaesth 2003; 90: 467–73