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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.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ADVANCES IN KNOWLEDGE References

 
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

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ADVANCES IN KNOWLEDGE References

 
Lumbar sympathectomy and lumbar sympathetic block are accepted treatment procedures in patients with reflex sympathetic dystrophy (RSD) (1,2) and patients with peripheral arterial occlusive disease whose vascular status does not permit reconstructive surgery (3). Patients with RSD have posttraumatic pain that is disproportionate to the injury and spreads beyond the distribution of any one peripheral nerve (4). RSD is clinically characterized by sensory, motor, and autonomic symptoms, including vascular abnormalities (5). It has been reported that pathophysiologic alterations of the ongoing sympathetic activity play a crucial role in vasomotor disturbances (6).

Several procedures are used to temporarily or irreversibly block the lumbar sympathetic trunk. Surgical sympathectomy can be performed during open surgery (2) or with endoscopic guidance (1,2,7). Alternatively, imaging-guided percutaneous sympathectomy, which includes chemical sympathectomy (8) and thermal ablation with radiofrequency (9), may be used. In irreversible sympathetic blocks (sympathectomy), permanent damage to the sympathetic chain occurs, causing an increase in the blood flow in the skin of patients with peripheral arterial occlusive disease (3) and a decrease in the overactivating sympathetic innervation in patients with RSD (1,2). Repetitive temporary blocks of the lumbar sympathetic trunk (10) and lumbar sympathetic plexus catheter placement (11) with administration of local anesthetics have been shown to produce short- and long-term pain relief in patients with RSD.

Several imaging modalities have been used for needle guidance in lumbar sympathetic blocks. Fluoroscopy (8) is widely available. However, ineffective injection into the psoas muscle can occur when the needle is not advanced far enough. In patients with little or no fat tissue around the sympathetic trunk, there is the potential danger of puncturing the aorta or inferior vena cava. Sonography (12), computed tomography (CT) (3), and magnetic resonance (MR) imaging (13) have also been used to guide needle placement. At our institution, CT guidance is the preferred method for needle placement. Because the sympathetic trunk cannot be identified with CT, the needle tip is targeted at the expected location of the sympathetic trunk, which is anatomically located within the fat tissue between the lumbar vertebral spine, psoas muscle, and aorta or inferior vena cava (14). Although CT guidance is more precise than fluoroscopic guidance, the needle tip may not be placed perfectly, especially when the location of the sympathetic trunk is unusual.

Paravertebral ganglia stimulate the sympathetic innervation of the lower extremities. This sympathetic stimulation activates vasoconstriction and sudomotor activity, such as sweating (15). Sympathetic skin response (SSR) is a polysynaptic reflex, with its afferent component being initiated by a number of different stimuli (pain or fear) that evoke hypothalamic activation (16). The efferent component of the SSR runs through sympathetic-cholinergic fibers from the paravertebral sympathetic trunk to the sweat glands (16). Increased sweating causes a measurable increase in electrical conductivity that is measurable as an SSR within seconds of the application of a sensory stimulus. After a lumbar sympathetic block or when spinal lesions (17), plexus lesions, or peripheral nerve lesions are present, a reduced or completely absent SSR is recorded. Thus, the purpose of our study was to prospectively evaluate the accuracy of the SSR for monitoring CT-guided lumbar sympathetic blocks, with a palpable temperature increase in the injected foot 30 minutes after injection serving as the reference standard.

	MATERIALS AND METHODS

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Patients
This study was approved by the institutional review board. Written informed consent was obtained from each patient. Seventy consecutive lumbar sympathetic blocks were prospectively performed between December 2001 and March 2004 in 13 consecutive patients (six female, seven male; mean age, 45 years; age range, 17–65 years). The indication for temporary repetitive lumbar sympathetic block was RSD of the left or right foot in six and seven patients, respectively. All patients had severe foot pain. In 12 patients, swelling and color changes were visible in the foot. Hyperhydrosis (nine patients) and hypertrichosis (five patients) were other signs of RSD in our study population. Patients who had late-stage RSD with muscle atrophy were excluded from this study. Underlying causes of RSD were ankle sprain (six patients) or fracture (six patients, four ankles were treated surgically) and previous surgery without other trauma (one patient). The mean interval between trauma and the beginning of sympathetic trunk block was 13 months (range, 2–36 months).

Temporary repetitive sympathetic trunk blocks, as performed in our study, have led to long-term improvement in patients with RSD (10). In the physical medicine and rheumatology department of our hospital, RSD is treated with intense physical therapy and analgesic medication for about 3 weeks. During this period, each patient is sent to the radiology department to undergo an average of six CT-guided lumbar sympathetic blocks in the affected side of the body.

Injection Procedure
Fifty-five injections were performed by a musculoskeletal radiologist (M.R.S.) with 4 years of experience with lumbar sympathetic blocks. Twelve injections were performed by another musculoskeletal radiologist (J.H.) with more than 10 years of experience with this procedure. Three injections were performed by two different radiologists, each with more than 5 years of experience with this procedure.

All lumbar sympathetic blocks were performed with CT fluoroscopic guidance and a single-section spiral CT unit (Somatom Plus 4; Siemens Medical Solutions, Erlangen, Germany). CT fluoroscopic images were acquired with a rotation speed of 0.75 second per rotation, a tube current of 50 mA, a tube voltage of 120 kV, and a collimation of 5 mm. Six images per second were obtained with a 256 x 256 matrix and displayed in full spatial resolution (1024 x 1024 matrix) on an in-room monitor. Patients were placed in the prone position. Between 10 and 15 transverse images (section thickness, 5 mm) were acquired at the L2 through L4 vertebral level for injection planning. Preferentially, injections were performed at the L3 vertebral level (n = 46). If fatty tissue was too sparse at the expected position of the sympathetic trunk, the L2 (n = 1) or L4 (n = 23) vertebral level was chosen instead.

After local skin anesthesia (mepivacain hydrochloride 2%, Scandicain; AstraZeneca, London, England), an 18-gauge 4-cm needle was advanced in an anteromedial direction toward the assumed position of the lumbar sympathetic trunk. A 12.7-cm or 17.8-cm 22-gauge needle was then passed through the 18-gauge needle and advanced to the fat tissue located between the psoas muscle, vertebral column, and aorta or inferior vena cava. Needle length was chosen on the basis of the patient's size. While advancing the needle, 2–3 mL of mepivacain hydrochloride 2% was injected to reduce pain. As soon as the needle tip reached the fat tissue surrounding the lumbar sympathetic trunk, a small amount (<1 mL) of iodinated contrast material (iopamidol 200 mg/mL, Iopamiro 200; Bracco, Milan, Italy) was injected to help us identify injection into the psoas muscle or intravascular injection into a small blood vessel. A 5-mL dose of 0.5% bupivacaine (Carbostesin; AstraZeneca) was mixed with 0.5 mL of iopamidol and injected quickly (approximate rate, 1 mL/sec). Such a fast injection rate was used because it did not induce pain (according to patient reports) and it allowed us to more precisely define the injection time.

CT Measurements
Needle tip position before administration of the bupivacaine-iopamidol mixture was documented on fluoroscopic CT images. The horizontal distance of the needle tip and the medial and lateral borders of the bupivacaine-iopamidol mixture from the midline of the vertebral body was measured. These measurements were obtained by an author (M.R.S.) with 10 years of experience in cross-sectional imaging. Additionally, the distances from the bupivacaine-iopamidol injection site to the nearest point of the vertebral body and from the injection site to the anterior edge between the psoas muscle and the vertebral body were measured. All measurements were obtained with a picture archiving and communication system workstation (ID report 2.1; Cerner, Kansas City, Mo). Measurements were accurate to the nearest millimeter.

Recording of SSR
Skin electrodes for SSR measurement were placed at the plantar and dorsal sides of both feet. SSR was measured with a standard electromyographic system (EMG 12; Dantec, Skovlunde, Denmark). SSRs were activated with electrical stimuli set at a level below the pain threshold (5–20 mA) at the right wrist in 11 of 13 patients. Electrodes for stimulation were fixed at the palmar and dorsal sides of the right wrist. The remaining two patients were hyperexcitable; therefore, an acoustic stimulus (loud clapping of the hands) was sufficient to obtain measurable responses. Within a few seconds, sweat glands of both feet were activated by the described stimuli, which caused a measurable increase in the electrical conductivity of the skin. If the sympathetic trunk was completely blocked, conductivity remained at baseline levels after stimulation.

In our investigation, baseline measurements were obtained 1 minute after contrast material was injected (immediately before bupivacaine injection). Measurements were obtained every minute after bupivacaine was injected until the SSR disappeared on the injected side. If the SSR persisted, measurements were obtained until 12 minutes after bupivacaine injection. The SSRs were quantified as follows: The electromyographic system allowed calculation of the maximal amplitude after markers were placed at baseline and at peak of response. In our study, the percentage ratio of the SSR amplitude in the ipsilateral foot to that in the contralateral foot (SSR ratio) was calculated for each measurement. The ratio between SSR measurements obtained before and those obtained after bupivacaine injection was not calculated because SSR decreases physiologically and bilaterally in response to repetitive stimuli because of a habituation of the patient to the stimulus in a certain amount (16).

In one patient who had no SSR decrease and no palpable temperature increase 12 and 30 minutes, respectively, after bupivacaine injection, the needle tip was redirected medially, and bupivacaine injection was repeated. For statistical evaluation, this procedure was considered unsuccessful because data obtained before repositioning of the needle were used.

Clinical Follow-up and Reference Standard
All patients were examined before each lumbar sympathetic block and 30 minutes after the end of injection by the radiologist who performed the injection. Additionally, all patients were examined 24 hours after the lumbar sympathetic block by the rheumatologist who treated them; this examination included inspection of both feet. Examinations were performed by one of the six rheumatologists (clinical experience, 3–25 years) who treated the patients. During these examinations, palpable temperature differences between the right and left legs and feet were recorded. The following grades of temperature difference were noted: 1, distinct temperature difference; 2, minor but unequivocal temperature difference; 3, questionable equivocal temperature difference; 4, probably no temperature difference; and 5, no temperature difference. A distinct temperature difference (grade 1) or a minor temperature increase (grade 2) that was palpable on the injected side 30 minutes after injection indicated a successful lumbar sympathetic block and served as the reference standard. The mean temperature increase measured by Kim et al (18) was 6.2° C after a successful lumbar sympathetic block. Such a temperature difference can be detected with palpation. If the temperature was symmetrical (grades 4 and 5) or only equivocally different (grade 3) after injection, the lumbar sympathetic block was considered unsuccessful.

An interval of at least 2 days was maintained between interventions. This ensured that no increased temperature of the affected foot remained from previous lumbar sympathetic blocks.

Statistical Analysis
Predictors of injection success—such as SSR ratio until 7 minutes after injection, level of injection, needle tip position and bupivacaine distribution, radiologist performing the injection, and clinical signs (hyperhydrosis, hypertrichosis)—were analyzed with logistic regression adjusted for clustering of procedures within patients. This analysis was performed with Stata software (version 8.2; Stata, College Station, Tex). Receiver operating characteristic curves, area under the curve, sensitivity, and specificity were calculated with SPSS software (version 11.0; SPSS, Chicago, Ill). Results were considered significant if the two-sided comparison-wise P value was less than .05.

	RESULTS

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Success
Fifty-eight (83%) of 70 lumbar sympathetic blocks were clinically successful, with either a distinct (n = 57) or a minor but unequivocal (n = 1) palpable temperature increase in the ipsilateral foot. In the remaining 12 (17%) blocks, there was no unequivocal temperature difference between the feet at palpation and the procedure was considered unsuccessful. In all of the unsuccessful injections, no temperature difference was detected (subjectively by the patient or objectively by the treating rheumatologist) in the 24-hour follow-up period. In one patient with no substantial change in the SSR ratio 12 minutes after bupivacaine injection, the needle tip was redirected 5 mm medially and a second dose of bupivacaine was injected (Fig 1). This injection was integrated into the group of unsuccessful (true-negative) injections because we were interested in the success rate of procedures that were not redirected. Immediate needle replacement was not performed in any of the other patients.

View larger version (95K): [in this window] [in a new window] [Download PPT slide]   Figure 1a: Transverse low-dose (50-mA) CT images in a 27-year-old man with RSD in the left foot 1 year after left ankle fracture. (a) Needle tip (arrow) is positioned 17 mm laterally from midline and located in fat tissue (arrowheads) between aorta, vertebral column, and left psoas muscle. (b) Distribution of bupivacaine and contrast material (arrow) anterior to the anterior margin (arrowheads) of psoas muscle. Medial and lateral borders of bupivacaine distribution were 14 and 38 mm, respectively, from midline. Distances of bupivacaine distribution to anterior edge between psoas muscle and vertebral body (two-headed arrow) and from bupivacaine distribution to vertebral body (white line) were 7 and 6 mm, respectively. (d) After repositioning, needle tip (arrow) is 12 mm from midline. (e) After second injection, bupivacaine and contrast material (arrowheads) are medial to the initially injected mixture (arrow). Medial and lateral borders of combined injections were 3 and 40 mm, respectively, lateral from the midline. (c) Graph shows normal SSR persisting 12 minutes after bupivacaine injection in the right (upper curve; amplitude, 1.920 mV) and left (lower curve; amplitude, 2.700 mV) feet (SSR ratio, 140.6%). (f) Graph shows normal SSR in the right foot (upper curve amplitude, 1.630 mV) 1 minute after second bupivacaine injection. There is no SSR in the injected side (lower curve amplitude, 0.012 mV). SSR ratio between left and right SSR measurements decreased to 0.7%. mV/D = millivolts per division, s/D = seconds per division.

View larger version (93K): [in this window] [in a new window] [Download PPT slide]   Figure 1b: Transverse low-dose (50-mA) CT images in a 27-year-old man with RSD in the left foot 1 year after left ankle fracture. (a) Needle tip (arrow) is positioned 17 mm laterally from midline and located in fat tissue (arrowheads) between aorta, vertebral column, and left psoas muscle. (b) Distribution of bupivacaine and contrast material (arrow) anterior to the anterior margin (arrowheads) of psoas muscle. Medial and lateral borders of bupivacaine distribution were 14 and 38 mm, respectively, from midline. Distances of bupivacaine distribution to anterior edge between psoas muscle and vertebral body (two-headed arrow) and from bupivacaine distribution to vertebral body (white line) were 7 and 6 mm, respectively. (d) After repositioning, needle tip (arrow) is 12 mm from midline. (e) After second injection, bupivacaine and contrast material (arrowheads) are medial to the initially injected mixture (arrow). Medial and lateral borders of combined injections were 3 and 40 mm, respectively, lateral from the midline. (c) Graph shows normal SSR persisting 12 minutes after bupivacaine injection in the right (upper curve; amplitude, 1.920 mV) and left (lower curve; amplitude, 2.700 mV) feet (SSR ratio, 140.6%). (f) Graph shows normal SSR in the right foot (upper curve amplitude, 1.630 mV) 1 minute after second bupivacaine injection. There is no SSR in the injected side (lower curve amplitude, 0.012 mV). SSR ratio between left and right SSR measurements decreased to 0.7%. mV/D = millivolts per division, s/D = seconds per division.

View larger version (7K): [in this window] [in a new window] [Download PPT slide]   Figure 1c: Transverse low-dose (50-mA) CT images in a 27-year-old man with RSD in the left foot 1 year after left ankle fracture. (a) Needle tip (arrow) is positioned 17 mm laterally from midline and located in fat tissue (arrowheads) between aorta, vertebral column, and left psoas muscle. (b) Distribution of bupivacaine and contrast material (arrow) anterior to the anterior margin (arrowheads) of psoas muscle. Medial and lateral borders of bupivacaine distribution were 14 and 38 mm, respectively, from midline. Distances of bupivacaine distribution to anterior edge between psoas muscle and vertebral body (two-headed arrow) and from bupivacaine distribution to vertebral body (white line) were 7 and 6 mm, respectively. (d) After repositioning, needle tip (arrow) is 12 mm from midline. (e) After second injection, bupivacaine and contrast material (arrowheads) are medial to the initially injected mixture (arrow). Medial and lateral borders of combined injections were 3 and 40 mm, respectively, lateral from the midline. (c) Graph shows normal SSR persisting 12 minutes after bupivacaine injection in the right (upper curve; amplitude, 1.920 mV) and left (lower curve; amplitude, 2.700 mV) feet (SSR ratio, 140.6%). (f) Graph shows normal SSR in the right foot (upper curve amplitude, 1.630 mV) 1 minute after second bupivacaine injection. There is no SSR in the injected side (lower curve amplitude, 0.012 mV). SSR ratio between left and right SSR measurements decreased to 0.7%. mV/D = millivolts per division, s/D = seconds per division.

View larger version (90K): [in this window] [in a new window] [Download PPT slide]   Figure 1d: Transverse low-dose (50-mA) CT images in a 27-year-old man with RSD in the left foot 1 year after left ankle fracture. (a) Needle tip (arrow) is positioned 17 mm laterally from midline and located in fat tissue (arrowheads) between aorta, vertebral column, and left psoas muscle. (b) Distribution of bupivacaine and contrast material (arrow) anterior to the anterior margin (arrowheads) of psoas muscle. Medial and lateral borders of bupivacaine distribution were 14 and 38 mm, respectively, from midline. Distances of bupivacaine distribution to anterior edge between psoas muscle and vertebral body (two-headed arrow) and from bupivacaine distribution to vertebral body (white line) were 7 and 6 mm, respectively. (d) After repositioning, needle tip (arrow) is 12 mm from midline. (e) After second injection, bupivacaine and contrast material (arrowheads) are medial to the initially injected mixture (arrow). Medial and lateral borders of combined injections were 3 and 40 mm, respectively, lateral from the midline. (c) Graph shows normal SSR persisting 12 minutes after bupivacaine injection in the right (upper curve; amplitude, 1.920 mV) and left (lower curve; amplitude, 2.700 mV) feet (SSR ratio, 140.6%). (f) Graph shows normal SSR in the right foot (upper curve amplitude, 1.630 mV) 1 minute after second bupivacaine injection. There is no SSR in the injected side (lower curve amplitude, 0.012 mV). SSR ratio between left and right SSR measurements decreased to 0.7%. mV/D = millivolts per division, s/D = seconds per division.

View larger version (96K): [in this window] [in a new window] [Download PPT slide]   Figure 1e: Transverse low-dose (50-mA) CT images in a 27-year-old man with RSD in the left foot 1 year after left ankle fracture. (a) Needle tip (arrow) is positioned 17 mm laterally from midline and located in fat tissue (arrowheads) between aorta, vertebral column, and left psoas muscle. (b) Distribution of bupivacaine and contrast material (arrow) anterior to the anterior margin (arrowheads) of psoas muscle. Medial and lateral borders of bupivacaine distribution were 14 and 38 mm, respectively, from midline. Distances of bupivacaine distribution to anterior edge between psoas muscle and vertebral body (two-headed arrow) and from bupivacaine distribution to vertebral body (white line) were 7 and 6 mm, respectively. (d) After repositioning, needle tip (arrow) is 12 mm from midline. (e) After second injection, bupivacaine and contrast material (arrowheads) are medial to the initially injected mixture (arrow). Medial and lateral borders of combined injections were 3 and 40 mm, respectively, lateral from the midline. (c) Graph shows normal SSR persisting 12 minutes after bupivacaine injection in the right (upper curve; amplitude, 1.920 mV) and left (lower curve; amplitude, 2.700 mV) feet (SSR ratio, 140.6%). (f) Graph shows normal SSR in the right foot (upper curve amplitude, 1.630 mV) 1 minute after second bupivacaine injection. There is no SSR in the injected side (lower curve amplitude, 0.012 mV). SSR ratio between left and right SSR measurements decreased to 0.7%. mV/D = millivolts per division, s/D = seconds per division.

View larger version (7K): [in this window] [in a new window] [Download PPT slide]   Figure 1f: Transverse low-dose (50-mA) CT images in a 27-year-old man with RSD in the left foot 1 year after left ankle fracture. (a) Needle tip (arrow) is positioned 17 mm laterally from midline and located in fat tissue (arrowheads) between aorta, vertebral column, and left psoas muscle. (b) Distribution of bupivacaine and contrast material (arrow) anterior to the anterior margin (arrowheads) of psoas muscle. Medial and lateral borders of bupivacaine distribution were 14 and 38 mm, respectively, from midline. Distances of bupivacaine distribution to anterior edge between psoas muscle and vertebral body (two-headed arrow) and from bupivacaine distribution to vertebral body (white line) were 7 and 6 mm, respectively. (d) After repositioning, needle tip (arrow) is 12 mm from midline. (e) After second injection, bupivacaine and contrast material (arrowheads) are medial to the initially injected mixture (arrow). Medial and lateral borders of combined injections were 3 and 40 mm, respectively, lateral from the midline. (c) Graph shows normal SSR persisting 12 minutes after bupivacaine injection in the right (upper curve; amplitude, 1.920 mV) and left (lower curve; amplitude, 2.700 mV) feet (SSR ratio, 140.6%). (f) Graph shows normal SSR in the right foot (upper curve amplitude, 1.630 mV) 1 minute after second bupivacaine injection. There is no SSR in the injected side (lower curve amplitude, 0.012 mV). SSR ratio between left and right SSR measurements decreased to 0.7%. mV/D = millivolts per division, s/D = seconds per division.

Needle Tip Position and Bupivacaine Distribution
The mean needle tip position in patients with a successful lumbar sympathetic block was 18 mm (range, 4–32 mm) lateral to the midline, whereas the mean needle tip position in patients with an unsuccessful lumbar sympathetic block was 21 mm (range, 5–31 mm) lateral to the midline (Fig 2). The medial borders of the bupivacaine distribution in successful and unsuccessful procedures were 9 mm (range, –8 to 25 mm) and 14 mm (range, 2–26 mm), respectively. These differences were not significant (P = .19 and .11, respectively). Despite an obvious overlap, a nearly significant difference (P = .056) between the lateral border of bupivacaine distribution in successful (mean, 31 mm; range, 14–55 mm) and that in unsuccessful (mean, 40 mm; range, 32–49 mm) lumbar sympathetic blocks was found. Mean distances of the bupivacaine distribution to the vertebral body in successful and unsuccessful procedures were 0.6 mm (range, 0–4 mm) and 0.9 mm (range, 0–7 mm), respectively (P = .41). Mean distance of the bupivacaine distribution to the anterior edge between the psoas muscle and the vertebral body was 1.3 mm both in successful (range, 0–6 mm) and in unsuccessful (range, 0–7 mm) injections (P = .91).

View larger version (89K): [in this window] [in a new window] [Download PPT slide]   Figure 2a: Transverse low-dose (50-mA) CT images in a 36-year-old woman with RSD in her left foot 11 months after ankle sprain with rupture of anterior fibulotalar ligament show (a) distance (24 mm) (two-headed arrow) between needle tip position and midline (black line) and (b) distribution of bupivacaine and contrast agent mixture in relation to midline. Distances of lateral (black arrow) and medial (white arrow) borders of the bupivacaine depot from midline (black line) were 36 and 19 mm, respectively. (c) Graph shows SSR in the right (upper curve) and left (lower curve) feet immediately before bupivacaine injection. Both curves demonstrate normal SSRs with amplitudes of 2.140 mV (right side) and 1.382 mV (left side) after stimulus (SSR ratio, 64.6%). Cursors (arrows) must be set manually at the starting point for electronic measurement of curve amplitude. (d) Graph shows measurement of SSR in right (upper curve) and left (lower curve) feet 1 minute after bupivacaine injection. There is normal activity in right foot (amplitude, 1.870 mV). SSR disappeared almost completely in left foot (amplitude, 0.016 mV), which resulted in SSR ratio of 0.9%. mV/D = millivolts per division, s/D = seconds per division.

View larger version (91K): [in this window] [in a new window] [Download PPT slide]   Figure 2b: Transverse low-dose (50-mA) CT images in a 36-year-old woman with RSD in her left foot 11 months after ankle sprain with rupture of anterior fibulotalar ligament show (a) distance (24 mm) (two-headed arrow) between needle tip position and midline (black line) and (b) distribution of bupivacaine and contrast agent mixture in relation to midline. Distances of lateral (black arrow) and medial (white arrow) borders of the bupivacaine depot from midline (black line) were 36 and 19 mm, respectively. (c) Graph shows SSR in the right (upper curve) and left (lower curve) feet immediately before bupivacaine injection. Both curves demonstrate normal SSRs with amplitudes of 2.140 mV (right side) and 1.382 mV (left side) after stimulus (SSR ratio, 64.6%). Cursors (arrows) must be set manually at the starting point for electronic measurement of curve amplitude. (d) Graph shows measurement of SSR in right (upper curve) and left (lower curve) feet 1 minute after bupivacaine injection. There is normal activity in right foot (amplitude, 1.870 mV). SSR disappeared almost completely in left foot (amplitude, 0.016 mV), which resulted in SSR ratio of 0.9%. mV/D = millivolts per division, s/D = seconds per division.

View larger version (8K): [in this window] [in a new window] [Download PPT slide]   Figure 2c: Transverse low-dose (50-mA) CT images in a 36-year-old woman with RSD in her left foot 11 months after ankle sprain with rupture of anterior fibulotalar ligament show (a) distance (24 mm) (two-headed arrow) between needle tip position and midline (black line) and (b) distribution of bupivacaine and contrast agent mixture in relation to midline. Distances of lateral (black arrow) and medial (white arrow) borders of the bupivacaine depot from midline (black line) were 36 and 19 mm, respectively. (c) Graph shows SSR in the right (upper curve) and left (lower curve) feet immediately before bupivacaine injection. Both curves demonstrate normal SSRs with amplitudes of 2.140 mV (right side) and 1.382 mV (left side) after stimulus (SSR ratio, 64.6%). Cursors (arrows) must be set manually at the starting point for electronic measurement of curve amplitude. (d) Graph shows measurement of SSR in right (upper curve) and left (lower curve) feet 1 minute after bupivacaine injection. There is normal activity in right foot (amplitude, 1.870 mV). SSR disappeared almost completely in left foot (amplitude, 0.016 mV), which resulted in SSR ratio of 0.9%. mV/D = millivolts per division, s/D = seconds per division.

View larger version (7K): [in this window] [in a new window] [Download PPT slide]   Figure 2d: Transverse low-dose (50-mA) CT images in a 36-year-old woman with RSD in her left foot 11 months after ankle sprain with rupture of anterior fibulotalar ligament show (a) distance (24 mm) (two-headed arrow) between needle tip position and midline (black line) and (b) distribution of bupivacaine and contrast agent mixture in relation to midline. Distances of lateral (black arrow) and medial (white arrow) borders of the bupivacaine depot from midline (black line) were 36 and 19 mm, respectively. (c) Graph shows SSR in the right (upper curve) and left (lower curve) feet immediately before bupivacaine injection. Both curves demonstrate normal SSRs with amplitudes of 2.140 mV (right side) and 1.382 mV (left side) after stimulus (SSR ratio, 64.6%). Cursors (arrows) must be set manually at the starting point for electronic measurement of curve amplitude. (d) Graph shows measurement of SSR in right (upper curve) and left (lower curve) feet 1 minute after bupivacaine injection. There is normal activity in right foot (amplitude, 1.870 mV). SSR disappeared almost completely in left foot (amplitude, 0.016 mV), which resulted in SSR ratio of 0.9%. mV/D = millivolts per division, s/D = seconds per division.

View larger version (19K): [in this window] [in a new window] [Download PPT slide]   Figure 3: Receiver operating characteristic curve shows true-positive (sensitivity) and false-positive (1 – specificity) rates 6 minutes after bupivacaine injection. Threshold of positive rating of SSR was set at a 10% SSR ratio (SSR in ipsilateral foot was 10% of SSR in contralateral foot). Since the curve closely follows the left and top borders of the receiver operating characteristic space, accuracy of SSR to predict the outcome of the lumbar sympathetic block is excellent.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ADVANCES IN KNOWLEDGE References

 
The sympathetic trunk cannot be identified with CT alone. The needle tip position in our study was slightly variable because of various factors, including the localization of fat tissue, which allowed safe injection and small deviations from the originally planned needle track in obese patients. However, we found no significant relationship between the needle tip position and the success of lumbar sympathetic blocking. Only the lateral border of the bupivacaine distribution showed a difference between successful and unsuccessful injections that bordered on statistical significance. The more lateral distribution in unsuccessful injections could represent drainage of bupivacaine away from the needle tip and the sympathetic chain. Unfortunately, there is a considerable overlap in this lateral border of bupivacaine distribution, which does not allow differentiation between successful and unsuccessful injections. According to our results, needle tip position and bupivacaine distribution cannot be used to reliably predict the clinical outcome of lumbar sympathetic blocking, even if CT demonstrates the correct position anterior to the psoas muscle.

According to the literature, fluoroscopically (8) and sonographically (12) guided lumbar sympathetic blocks have been performed. These methods are less likely than CT-guided lumbar sympathetic block to result in the needle tip being positioned precisely at the sympathetic trunk; however, they still allow successful injections. These two methods (8,12) have been used in patients with peripheral arterial occlusive disease, and 38% of the patients in the study of Rosen et al (8) benefited clinically from the procedure. Few reports have discussed MR-guided lumbar sympathetic blocks (13,19). MR imaging and MR guidance have been performed with open MR units, field strengths of 0.5 T (13) and 0.2 T (19), and a range of various MR sequences. Although needle position and distribution of infiltrated substances were clearly documented in both reports, identification of the sympathetic trunk and optimization of needle tip position in relation to the assumed trunk position were not investigated in either study (13,19).

Because of the problems regarding direct identification of the sympathetic trunk with all types of imaging modalities, indirect methods, such as SSR measurement (16) in both feet, are possible alternatives for optimization of needle tip position. This method has been used previously in the identification of sympathetic trunk activity (16,17); however, to our knowledge, it has not been used to monitor sympathetic trunk interventions. Our results show that this method can be used to predict the outcome of a lumbar sympathetic block as early as 5 minutes after injection with an accuracy of more than 90%. This is considerably faster than waiting for a palpable temperature increase as a sign of success. If no substantial decrease in the SSR is recorded 5 minutes after injection, the needle tip can be moved to a more favorable site and another injection can be performed. Such repositioning of the needle tip was performed in one injection in our study and resulted in an immediate SSR decrease after repeated injection of bupivacaine in a more medial needle position. If chemical lumbar sympathicolysis is planned, such SSR measurements could improve needle tip position before sclerosing agents are used.

There are other indirect methods that can be used to identify successful sympathectomy. Laser Doppler flowmetry has been used to measure skin blood flow, and sympathetic vasoconstrictor reflexes induced by deep inspiration and skin temperature have been assessed at the affected and contralateral hands before and after surgical sympathectomy (20).

Instead of surgical sympathectomy (1,2,7,20), chemical sympathicolysis (3,8,19,21) with phenol or ethanol injections may be performed to permanently block the sympathetic trunk. Necrosis (22) and strictures (3) of the ureters and pelviureteric junction disruption (21) have been reported as complications of chemical sympathicolysis. In this procedure, 5–15 mL of alcohol (most often phenol) is injected at the assumed site of the sympathetic trunk (3,8,19,22). Konig et al (19) found that the injected substance covered a maximal distance of 14–45 mm in the transverse plane. In our study, bupivacaine distribution was between 55 mm lateral to the midline (maximal lateral border) and 8 mm on the contralateral side (maximal medial border), although we injected only 5 mL of bupivacaine. When up to 15 mL of alcohol is injected, the chance of reaching the ureter with relevant amounts cannot be excluded. To decrease the injection volume, the lumbar sympathetic trunk must be identified as precisely as possible. According to the anatomic data of Rocco et al (23), there is considerable variation in the location of the sympathetic chain, which increases the need for better localization prior to chemical sympathicolysis. One possible solution is our method of SSR recording after injection of small amounts of local anesthetics prior to chemical sympathectomy. According to our results, these measurements can be used to accurately predict the outcome of a sympathetic trunk block. Needle tip position can be optimized on the basis of an objective value before irreversible types of sympathetic trunk interventions are performed.

A limitation of our study was the small number of patients (n = 13). However, the number of injections (n = 70) partially compensated for this problem. Looking at single injections appears reasonable because no persisting SSR decrease caused by previous injections was measurable prior to each injection. Additionally, our purpose was to determine if SSR measurements can be used to predict the quality of the injections, not to predict the outcome of RSD therapy. Another limitation was the qualitative palpable temperature measurement 30 minutes after injection. Measurements obtained with adhesive liquid crystal thermometers would have given an objective value, but in successful injections, marked temperature differences can be palpated clearly, and minor measured temperature differences (measured by adhesive liquid crystal thermometers) would also need to be interpreted as indicature of unequivocal or unsuccessful injections.

In conclusion, SSR can be used to identify the correct needle tip position of lumbar sympathetic blocks with 94% accuracy 6 and 7 minutes after injection. Because needle tip position cannot be used to predict the outcome of lumbar sympathetic blocks, objective monitoring of the SSR may be important prior to irreversible chemical sympathectomy.

	ADVANCES IN KNOWLEDGE

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ADVANCES IN KNOWLEDGE References

 

• Sympathetic skin response can be used to identify correct needle tip position of lumbar sympathetic blocks with 94% accuracy 6 and 7 minutes after injection.
  
• Measurement of sympathetic skin response appears to be a good monitoring tool prior to chemical sympathicolysis.
  

	FOOTNOTES

 

Abbreviations: RSD = reflex sympathetic dystrophy • SSR = sympathetic skin response

Authors stated no financial relationship to disclose.

Author contributions: Guarantors of integrity of entire study, M.R.S., J.H.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; manuscript final version approval, all authors; literature research, M.R.S., A.C.; clinical studies, M.R.S., R.O.K., G.J., J.H.; statistical analysis, J.H.; and manuscript editing, M.R.S., A.C., J.H.

	References

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ADVANCES IN KNOWLEDGE References

 

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