• Steven P Cohen, Chachrit Khunsriraksakul, Yongjae Yoo, Evan Parker, Christelle D K Samen-Akinsiku, Nirav Patel, Seffrah J Cohen, Xiaoning Yuan, Jianguo Cheng, and Jee Youn Moon.
• Departments of Anesthesiology & Critical Care Medicine, Neurology, Physical Medicine & Rehabilitation, and Psychiatry & Behavioral Sciences, Johns Hopkins School of Medicine, Baltimore, Maryland, USA.
• Pain Med. 2023 Mar 1; 24 (3): 316324316-324.

BackgroundKetamine 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.ObjectivesWe sought to determine whether demographic and clinical factors, and technical and psychophysical characteristics of sympathetic blocks are associated with response to ketamine infusion.MethodsIn 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.ResultsFactors 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 ± 4.20 in ketamine responders vs 3.68 ± 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.ConclusionsThe 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.© The Author(s) 2022. Published by Oxford University Press on behalf of the American Academy of Pain Medicine. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.

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