Efficacy of Outpatient Ketamine Infusions in Refractory Chronic Pain Syndromes: A 5-Year Retrospective Analysis

Abstract

We evaluated whether outpatient intravenous ketamine infusions were satisfactory for pain relief in patients suffering from various chronic intractable pain syndromes. Retrospective chart review. Following Institutional Review Board approval, we retrospectively analyzed our database for all ketamine infusions administered over 5 years from 2004 to 2009. Data reviewed included doses of intravenous ketamine, infusion duration, pain scores on visual analog scale (VAS) pre- and post-procedure, long-term pain relief, previous interventions, and side effects. All patients were pretreated with midazolam and ondansetron. We identified 49 patients undergoing 369 outpatient ketamine infusions through retrospective analysis. We excluded 36 infusions because of missing data. Among our patients, 18 (37%) had a diagnosis of complex regional pain syndrome (CRPS). Of the remaining 31 (63%) patients, eight had refractory headaches and seven had severe back pain. All patients reported significant reduction in VAS score of 5.9 (standard error [SE] 0.35). For patients with CRPS, reduction in VAS score was 7.2 (SE 0.51, P < 0.001); for the others, the reduction was 5.1 (SE 0.40, P < 0.001). The difference of 2.1 between groups was statistically significant (SE 0.64, P = 0.002). In 29 patients, we recorded the duration of pain relief. Using the Bernoulli model, we found (90% confidence interval) that the probability of lasting pain relief in patients with refractory pain states was 59-85% (23-51% relief over 3 weeks). We conclude that in patients with severe refractory pain of multiple etiologies, subanesthetic ketamine infusions may improve VAS scores. In half of our patients, relief lasted for up to 3 weeks with minimal side effects.