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Sympathetic Skin Response: Monitoring of CT-guided Lumbar Sympathetic Blocks¹

¹ From the Department of Radiology (M.R.S., J.H.), Department of Physical Medicine and Rheumatology (R.O.K., G.J.), and Spinal Cord Injury Center (A.C.), University Hospital Balgrist, Zurich, Switzerland. Received July 22, 2005; revision requested September 27; revision received November 7; accepted December 8; final version accepted January 4, 2006. Address correspondence to M.R.S., MRI am Bahnhofplatz 3, 8001 Zurich, Switzerland.

Purpose: To prospectively evaluate accuracy of sympathetic skin response (SSR) for monitoring computed tomography (CT)-guided lumbar sympathetic blocks, with palpable temperature increase in the foot 30 minutes after injection serving as the reference standard.

Materials and Methods: Institutional review board approval and written informed consent were obtained. Seventy individual lumbar sympathetic blocks were performed in 13 patients (six female, seven male; mean age, 45 years) with reflex sympathetic dystrophy of the foot. A 22-gauge needle was advanced to the sympathetic trunk at midlumbar level with CT fluoroscopic guidance, and 1 mL of iopamidol (200 mg of iodine per milliliter) and 5 mL of 0.5% bupivacaine were injected. SSR was monitored in both feet before and after bupivacaine injection. SSRs were activated with painless low-strength (5–20-mA) electrical stimuli. SSR ratio (SSR in the injected foot versus SSR in the contralateral foot) was calculated before injection and repeatedly at 1-minute intervals thereafter. Needle tip position and distribution of bupivacaine were measured on CT images. Receiver operating characteristic curves for SSR ratio were calculated until 7 minutes after injection. Logistic regression analyses adjusted for clustering were calculated for SSR ratio, injection parameters, needle tip position, and bupivacaine distribution.

Results: Thirty minutes after injection, 83% of procedures were considered clinically successful. An SSR cutoff ratio of 1:10 was used, and sensitivity, specificity, and accuracy of SSR for prediction of clinical success were 84%, 92%, and 86%, respectively, 4 minutes after injection and 95%, 92%, and 94%, respectively, 7 minutes after injection. Needle tip position (P = .19), medial and lateral borders of bupivacaine distribution (P = .11 and .056), and distance between bupivacaine distribution and the vertebral body (P = .41) were not significantly different between successful and unsuccessful injections.

Conclusion: SSR can be used to correctly identify needle tip position in lumbar sympathetic blocks 6 and 7 minutes after injection.

© RSNA, 2006