Sympathetic Blocks as a Predictor for Response to Ketamine Infusion in Patients with Complex Regional Pain Syndrome: A Multicenter Study

Background. Ketamine infusions are frequently employed for refractory complex regional pain syndrome (CRPS), but there are limited data on factors associated with treatment response. Sympathetic blocks are also commonly employed in CRPS for diagnostic and therapeutic purposes and generally precede ketamine infusions. Objectives. We sought to determine whether demographic and clinical factors, and technical and psychophysical characteristics of sympathetic blocks are associated with response to ketamine infusion. Methods. In this multi-center retrospective study, 71 patients who underwent sympathetic blocks followed by ketamine infusions at 4 hospitals were evaluated. Sympathetically maintained pain (SMP) was defined as ≥ 50% immediate pain relief after sympathetic block and a positive response to ketamine was defined as ≥ 30% pain relief lasting over 3 weeks. Results. Factors associated with a positive response to ketamine in univariable analysis were the presence of SMP (61.0% success rate vs 26.7% in those with sympathetically independent pain; P = .009) and post-block temperature increase (5.66 6 4.20 in ketamine responders vs 3.68 6 3.85 in non-responders; P = .043). No psychiatric factor was associated with ketamine response. In multivariable analysis, SMP (OR 6.54 [95% CI 1.83, 23.44]) and obesity (OR 8.75 [95% 1.45, 52.73]) were associated with a positive ketamine infusion outcome. Conclusions. The response to sympathetic blocks may predict response to ketamine infusion in CRPS patients, with alleviation of the affective component of pain and predilection to a positive placebo effect being possible explanations.