CONTINUOUS SUBCUTANEOUS ADMINISTRATION OF THE N-METHYL-D-ASPARTIC ACID (NMDA) RECEPTOR ANTAGONIST KETAMINE IN THE TREATMENT OF POSTHERPETICNEURALGIA

The effect of continuous subcutaneous (s.c.) infusion of ketamine on n erve injury pain was examined in patients with post-herpetic neuralgia . Five patients that reported pain relief after acute intravenous inje ction of ketamine were included in this open prospective study. Ketami ne was administered continuously in increasing doses using a portable infusion pump (CADD-PLUS, Pharmacia), and the treatment period for eac h infusion rate (0.05, 0.075, 0.10, or 0.15 mg/kg/h) was 7 days and ni ghts. Relief of continuous pain, as evaluated daily by visual analogue scales, was observed at the infusion rate of 0.05 mg/kg/h, but was mo st marked during infusion of 0.15 mg/kg/h. All the patients reported t hat ketamine reduced the severity of continuous pain as well as reduce d the severity and number of attacks of spontaneous pain. Changes in e voked pain (allodynia and wind-up-like pain) were recorded before chan ge of infusion rate. Allodynia was maximally reduced 59-100% after 1 w eek infusion of 0.05 mg/kg/h, and wind-up-like pain was maximally redu ced 60-100% after 1 week infusion of 0.15 mg/kg/h. Itching and painful indurations at the injection site was the most bothersome side-effect and for this reason 1 patient discontinued treatment after 2 weeks. O ther common side-effects were nausea, fatigue and dizziness. The prese nt results show that continuous, spontaneous and evoked pain in patien ts with post-herpetic neuralgia is reduced by continuous s.c. infusion of ketamine, but is associated with intolerable side effects.