Epidural block and the pain cycle

A 68 year old female with carcinoma vulva, presented with severe unrelenting, burning pain in the vulval region. The pain was not controlled by dispersible Morphine 20 mg q4h, and a dose of 25 mg q4h produced intolerable side effects like nausea, vomiting and drowsiness, without additional pain relief. A ganglion impar could not be done due to altered anatomy. A lumbar epidural block with a catheter to deliver a continuous infusion of 0.125% bupivacaine at 2.0 ml/hour was initiated. She had excellent analgesia and she could be taken off morphine. Her epidural catheter dislodged in 3 days and could not be reinserted immediately. When we started her on morphine, her pain was well controlled with Morphine 5 mg q4h. We propose this increase in sensitivity to morphine to be due to breaking of the pain cycle with the epidural block.