The effect of ketamine as an additive in epidural block on the intractable herpetic neuralgia: a case report
A 35-year-old, 162 cm, 61 kg female patient was referred to our pain clinic due to severe painful rashes in the right lower extremity (L3 to L5 dermatome). Her past medical history included acute myeloid leukemia for 15 months and peripheral blood stem cell transplantation had been carried out eleven months previously. The rash had developed 25 days ago and clinical evaluation confirmed acute herpes zoster neuropathy. The viral skin lesions resolved successfully with intravenous acyclovir during the acute phase. However, the leg pain remained severe and significantly increased despite treatment with pregabalin, opioids, tricyclic antidepressants, corticosteroids, benzodiazepines and non-steroidal anti-inflammatory drugs. She suffered from continuous burning pain (8 points on verbal rating scale [VRS], 0 = no pain, 10 = worst pain imaginable) with intermittent electrical shock-like painful sensation (10 VRS) occurring approximately every 30 min, each lasting several seconds. The affected area was sensitive to touch and she felt numbness and tingling. She also reported intermittent discoloration on her affected dermatomes from red to dark red. She had been treated with pregabalin 450 mg, oxycodone 320 mg, tramadol 300 mg, fentanyl patch 100 µg, amitriptyline 10 mg, fluoxetine 20 mg in a day and morphine intravenous patient-controlled analgesia (PCA) with a basal rate of 4 mg/h and a bolus dose of 2 mg to control the pain. Despite escalating doses of medication and intravenous PCA infusion, her pain remained at 8 VRS. We considered lumbar epidural PCA infusion for pain relief. With patient approval, an epidural catheter was inserted into the L2-3 interlaminar epidural space and was confirmed on fluoroscopy. A sensory block below the L3 dermatome was established. However, continuous epidural PCA infusion of 0.375% ropivacaine and morphine sulfate 0.4 mg/h did not alleviate pain sufficiently (7 VRS). Approximately one week after epidural PCA administration, we tried single-shot ketamine (Ketomine®, Dai Han Pharmaceutical, Seoul, Korea) 5 mg, which was diluted in normal saline to 3 ml, through the lumbar epidural catheter. Within a few hours, she reported substantial pain relief without significant adverse effect (5 VRS). After three days, she received a second single-shot of ketamine 5 mg via the epidural route. The pain area was gradually reduced in size and allodynia was improved (3 VRS). Epidurally administered ketamine, in addition to ropivacaine and morphine, provided sufficient and prolonged analgesia. Five days later, the epidural PCA could be stopped and she was discharged in a stable condition with a VRS below 3. She has been followed in a VRS 2 to 3 by our pain clinic for the past four months and is undergoing tapering of her medications including pregabalin 75 mg and oxycodone 10 mg a day without further nerve block.