Anesth Analg 2000;90:1396-1401
© 2000 International Anesthesia Research Society
REGIONAL ANESTHESIA AND PAIN MEDICINE
Lumbar Sympathetic Block for Sympathetically Maintained Pain: Changes in Cutaneous Temperatures and Pain Perception
Kha M. Tran, MD*,
Steven M. Frank, MD
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Srinivasa N. Raja, MD,
Hossam K. El-Rahmany, MD,
Lauren J. Kim, BA, and
Brian Vu, MD
*The Johns Hopkins University School of Medicine,
Department of Anesthesiology and Critical Care Medicine, The Johns Hopkins Medical Institutions. Baltimore, Maryland; and
Department of Anesthesiology and Perioperative Care, University of California, San Francisco, California
Address correspondence and reprint requests to Srinivasa N. Raja, MD, 600 North Wolfe St., Johns Hopkins Hospital, Osler 292, Baltimore, MD 21287. Address e-mail to sraja{at}jhmi.edu
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Abstract
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Lumbar sympathetic block (LSB) is used in the management of sympathetically maintained pain states. We characterized cutaneous temperature changes over the lower extremities after LSB. Additionally, we examined the effects of iohexol, a radio-opaque contrast medium, on temperature changes and pain relief. After institutional review board approval and written, informed consent, 28 LSBs were studied in 17 patients. Iohexol or normal saline was injected in a randomized, double-blinded fashion before bupivacaine. Lower extremity cutaneous temperatures were measured. Pain, allodynia, interference with daily function, and perceived pain relief were reported in a subset of 15 LSBs for 1 wk after the block. The distal lower extremity ipsilateral to the LSB had the greatest magnitude (8.7° ± 0.8°C) and rate (1.1° ± 0.2°C/min) of temperature change. The great toe temperature was within 3°C of core temperature within 35 min after LSB. There were no differences in temperature change between the groups. The iohexol group had greater relief of pain until the morning of the first postblock day (P = 0.002) and longer perceived relief of pain (P = 0.01). The maximum temperature of the great toe correlated with allodynia relief (P = 0.0007). Thus clinicians should expect ipsilateral toe temperatures to increase to within ~3°C of core temperature. Iohexol does not alter the efficacy of LSB and may improve relief of symptoms. The magnitude of temperature change may predict relief of allodynia.
Implications: Cutaneous toe temperatures approaching core temperature provide a useful monitor of lumbar sympathetic block and may predict relief of sympathetically maintained pain. Iohexol will not compromise temperature changes or pain relief.
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Introduction
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The sympathetic nervous system may play a role in the maintenance of certain neuropathic pain states. Sympathetic blockade with local anesthetics and surgical sympathectomy have been described as treatments for such conditions as postamputation stump pain, postherpetic neuralgia, and complex regional pain syndromes I and II (reflex sympathetic dystrophy and causalgia) (1). A lumbar sympathetic block (LSB) is performed by percutaneous injection of local anesthetic around the lumbar sympathetic ganglia and results in a temporary sympathectomy (2). LSB can be used to both diagnose and treat sympathetically mediated neuropathic pain (3).
Before the injection of a local anesthetic, clinicians may inject radio-opaque contrast media to confirm needle placement and predict appropriate spread of the anesthetic over the sympathetic ganglia. Iohexol or N,N'-Bis (2, 3-dihydroxypropyl)-5-[N-(2,3-dihydroxypropyl)- acetamido]-2,4,6,-triiodo-isopthalamide is an often-used water-soluble, nonionic contrast dye. Clinical experience suggests that the onset of the sympathetic block could be delayed when iohexol is used, possibly because of a dilutional effect or because the iohexol could act as a barrier for the local anesthetic to reach the lumbar sympathetic ganglion. However, the effects of iohexol on LSB have not been previously studied.
After placement of the block, the patient must be assessed for sympathectomy. Increasing skin temperature, decreasing pain, and anhidrosis in the distal extremity indicate a sympathectomy (4). Measuring great toe temperature is a simple practice to monitor an LSB, but no guidelines exist as to what magnitude of change would predict a clinically appropriate block. The magnitude and rate of temperature change of various skin sites on the lower extremity have not been examined, and their correlation with pain relief has not been characterized.
We characterized the time course of skin-surface temperature changes along the lower extremity after LSB. We also examined the correlation between temperature change and pain relief and the effects of iohexol on the efficacy of LSB and subsequent pain relief.
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Methods
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After institutional review board approval and written, informed consent, 28 LSBs were studied in 17 patients diagnosed with complex regional pain syndromes I or II by the International Association for the Study of Pain criteria. A sympathetic component to the pain was determined by phentolamine infusion or previous LSB, ensuring absence of somatic blockade. Before the block, patients were assigned in a randomized, double-blinded manner to receive either iohexol or normal saline before the administration of a local anesthetic. The operators, patients, and investigators measuring the outcomes described below were blinded to the nature of the injection (i.e., iohexol or normal saline). Patients were placed in the prone position, and their backs were sterilely prepped and draped. The L3 vertebra was identified by fluoroscopy, the skin several centimeters lateral to the vertebra was infiltrated with 1% lidocaine, and a 5- or 7-inch 22-gauge spinal needle was then introduced 5 to 7 cm lateral to the spinous process. The needle was advanced under fluoroscopic guidance to the anterolateral edge of L3. After needle position was confirmed with both anteroposterior and lateral fluoroscopic views and after negative aspiration for blood 1.5 mL of iohexol or normal saline was administered. No subsequent fluoroscopic images were obtained, and the needle was not moved, until the completion of the drug injections, thus blinding all involved to the nature of the injection. After 60 s, a test dose of 3 mL 2% lidocaine with 1:200,000 epinephrine was injected to detect intravascular or intrathecal injection. Ninety seconds later, 15 mL of the local anesthetic, 0.25% bupivacaine, was injected.
Skin-surface temperatures were monitored with small, adhesive thermocouple probes (Mon-a-ThermTM; Mallinckrodt, St. Louis, MO) attached to the plantar aspect of the great and little toes, dorsal aspect of the foot, and lateral and medial aspects of the calf and thigh. Core temperatures were measured at the tympanic membrane with soft, cotton-tipped thermocouple probes (Mon-a-ThermTM). All temperatures were measured and recorded on a computer hard disk every 2 min with an electronic thermometry and data acquisition system (Iso-thermexTM ; Columbus Instruments, Columbus, OH).
For each monitoring site on the ipsilateral leg that demonstrated a significant change in temperature, the 5%, 25%, 75%, and 95% changes in temperature were determined by using the following formula: 
with Tmax being the maximum postblock temperature and Tmin being the preblock temperature of the site in question. The time required for each of the sites to reach these temperature points was determined and designated t
5, t25, t75, and t95.
For each monitoring site on the ipsilateral leg that demonstrated a significant change in temperature, the rate of temperature change between t25 and t75 was calculated by using the following formula:
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A subset of 11 patients undergoing 15 LSBs agreed to report their subjective symptoms before and after the procedure. Four outcomes were measured in these patients to assess the ability of the block to relieve their symptoms: 1) continuing pain, 2) mechanical allodynia, 3) percent of relief from pain, and 4) amount of pain-related interference with daily function. Patients reported continuing pain and mechanical allodynia on a 0 to 100 visual analog scale (VAS). Mechanical allodynia was elicited by lightly brushing the painful area with a cotton swab. Relief from pain after the LSB was reported as a percentage with 0% indicating no relief and 100% indicating total relief. Pain-related interference with daily function was rated on a 0 to 3 (none, mild, moderate, severe) scale. The investigators made assessments of the patients’ symptoms at two times: a baseline measurement before the LSB, and another measurement 1 h after the LSB. After these first two measurements, the patients were taught how to assess and record their own symptoms. They were given diaries to record their continuing pain, mechanical allodynia, interference with daily function, and percent relief in the morning and evening of the seven consecutive days after the LSB.
Student’s t-tests and Fisher’s exact tests were used to compare data between the groups of patients. Continuous data collected over time were analyzed by using two-way analysis of variance for both between- and within-group comparisons. Fisher’s exact tests were used to compare changes in the incidence of moderate or severe daily function interference within the groups and over time. The relationships between temperature and time variables to relief of symptoms were analyzed by using linear regression. Data were reported as mean ± SEM. P < 0.05 defined statistical significance.
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Results
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There were no significant differences in the demographic characteristics of patients in the two treatment groups or the subset that gave subjective pain scores (Table 1). The precipitants of pain were trauma (n = 7), surgery (n = 6), IM injection (n = 2), and other (n = 2). Eleven patients had 5 or fewer LSBs, 4 patients had between 6 and 10 LSBs, and 2 had more than 10 LSBs.
When the iohexol and normal saline groups were analyzed together, all temperature sites on the ipsilateral leg, except for the medial thigh, demonstrated a significant change in temperature over the course of the procedure (P < 0.03). The magnitude of increase in temperature was greater at the more distal temperature sites on the leg (Fig. 1). For example, the lateral thigh increased 1.9° ± 0.2°C to reach 33.7° ± 0.3°C, whereas the great toe temperature increased 8.7° ± 0.8°C to reach 33.3° ± 0.7°C. On the contralateral leg, only the great toe and foot had significant increases in temperature. These increases were both <2°C, resulting in a final great toe temperature of 26.1° ± 0.7°C and a final foot temperature of 30.3° ± 0.5°C. There were no significant differences in magnitude of temperature change at any skin-surface site between the iohexol and normal saline groups (P > 0.2). Core temperature measured at the tympanic membrane decreased from 36.7° ± 0.1°C to 36.3° ± 0.2°C during the course of the treatment with no difference between the groups. Final ipsilateral great toe temperature was between 2° and 3°C less than core temperature after LSB. Ambient temperature was 23.3° ± 0.2°C for the procedures.
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Figure 1. Difference between minimum and maximum temperature for each skin-surface temperature monitoring site. The differences are much greater in the distal portion of the extremity ipsilateral to the lumbar sympathetic block (LSB). *P < 0.05 for pre-LSB versus post-LSB temperatures.
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When the iohexol and normal saline groups were analyzed together, there were no significant differences between either t5 or t25 for any of the ipsilateral sites (P > 0.3). For all sites, t5 ranged from 9 to 12 min, and t25 ranged from 15 to 20 min. The more distal sites (great and little toes), however, had significantly shorter t75 and t95 than the other sites (P < 0.0001). For the more proximal sites, t75 ranged from 34 to 38 min, whereas for the great and little toes, t75 was <25 min. For the proximal sites t95 ranged from 45 to 50 min, but for the great and little toes, t95 was <35 min. When analyzed separately, there were no significant differences in t5, t25, t75, or t95 between the iohexol and normal saline groups at any of the temperature monitoring sites.
When the iohexol and normal saline groups were analyzed together, the great toe and little toe on the ipsilateral leg increased in temperature faster than the other sites (Fig. 2) (P < 0.0001). The great toe temperature increased 0.9° ± 0.2°C/min and the little toe temperature increased 1.1° ± 0.2°C/min whereas the other sites increased only 0.1° to 0.2° ± 0.2°C/min. When the iohexol and normal saline groups were compared at each temperature monitoring site, there were no significant differences in rate of temperature change.
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Figure 2. Rate of change in temperature for sites that had significant changes. The distal sites changed temperature faster than the proximal sites. *P < 0.05 for great and little toe versus other sites.
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Compared with the normal saline group, pain intensity scores were lower during the first day postblock in the iohexol group (Fig. 3). In this group, the continuing pain scores were significantly lower than the baseline pain scores both 1 h postblock and in the morning of the first postblock day (P < 0.04). Over these times, baseline pain scores changed from 57 ± 11 VAS units, to 15 ± 6 VAS units 1 h after the block, and to 22 ± 6 VAS units the next morning. Although both groups started at a similar baseline (P > 0.5), at the two subsequent time points, the continuing pain scores of the iohexol group were significantly lower than those of the normal saline group. Continuing pain scores in both the iohexol and normal saline groups were not significantly different from each other or from baseline levels for the remainder of the week.
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Figure 3. Change in subjective perception of pain symptoms after lumbar sympathetic block (LSB). The iohexol group experienced a greater decrease in continuing pain and a longer perception of relief from pain symptoms. *P < 0.05 versus pre-LSB, and #P < 0.05 iohexol versus normal saline groups.
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Mechanical allodynia scores changed similarly over time in the iohexol and normal saline groups. The only statistically significant difference occurred between the baseline and 1 h postblock ratings in the normal saline group.
The magnitude of reported pain relief was similar between groups. Both groups reported significant relief of pain from baseline 1 h postblock (P < 0.0002) and for several days after. The iohexol group, however, had significant relief until the morning of the sixth postblock day, whereas the normal saline group had relief only until the morning of the third postblock day.
The incidence of moderate or severe interference with daily function was not different between the iohexol or normal saline groups at any time. When the groups were analyzed together, the incidence of moderate or severe interference with daily function was significantly decreased from baseline (93%) only through the morning of the first postblock day (53%) (P < 0.04). At all subsequent time periods there was no difference from baseline.
In the subset of patients who gave subjective pain relief data, linear regression was performed to determine if either the magnitude or rate of change in cutaneous temperature correlated with degree of pain relief after LSB. One hour after LSB, the maximum temperature of the great toe correlated with the percent change in allodynia (P < 0.0007) (Fig. 4).
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Figure 4. Maximum cutaneous temperature of the great toe after lumbar sympathetic block and change in allodynia by linear regression. P > 0.05, and r2 = 0.9.
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Discussion
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We have characterized lower extremity cutaneous temperature changes and their relationship to pain symptoms in patients treated with LSB for sympathetically maintained pain in the lower extremity. We also examined the effect of radio-opaque contrast media (iohexol) on LSB. After unilateral LSB, the ipsilateral distal lower extremity demonstrated the greatest and most rapid changes in temperature. The maximum skin temperature attained in the distal lower extremity was predictive of increased relief of mechanical allodynia. Use of iohexol did not affect the magnitude or rate of cutaneous temperature change and was associated with greater pain relief for a longer period of time when compared with control injections of normal saline.
The treatment of sympathetically maintained neuropathic pain is a great challenge. The component of pain that is mediated by the sympathetic nervous system varies (5), and the placebo effect complicates the picture. Price et al. (6) demonstrated that either normal saline or local anesthetic injections result in comparable pain relief in the initial postblock period. However, patients receiving anesthetic have relief of pain for an average of three days and 18 hours, whereas those receiving saline have 19.9 hours of relief. Steps that clinicians can take to improve chances of a complete sympathectomy include improving both localization of the sympathetic ganglia and placement of the anesthetic, while monitoring the progress of the sympathectomy. Rocco et al. (7), through cadaveric dissection, have found the most common location of the lumbar sympathetic ganglia. Fluoroscopy can be used to assure placement of the needle. Monitoring the patient for signs of sympathetic blockade allows clinicians to readjust or add more anesthetic should signs of a sympathectomy not occur within reasonable time. Combining Rocco et al.’s (7) approach, with the use of fluoroscopy and monitoring of sympathectomy, clinicians should be able to maximize the chance of a complete sympathectomy.
Sympathectomy can be monitored several ways. Subjective measures include pain relief, warmth, change in color, and anhidrosis (4). An objective measure is preferable to assess the success of the sympathectomy. Objective tests include measurements of skin temperature (8) and blood flow (9), provocative sweat tests (8), and measurement of electrical skin activity (10). Rubinstein and Sessler (11) have shown in the upper extremity that skin-surface temperature gradients correlate well with blood flow as determined by venous-occlusion volume plethysmography. Kistler et al. (12) have also shown that temperature provides a good index of skin blood flow by using infrared thermography, laser Doppler flowmetry, and photoplethysmography. Skin temperature measurement is an inexpensive, painless, and sensitive measure of cutaneous blood flow. Temperature measurement at the acral regions of the body, such as the great and little toes, is well suited to indicate sympathetic tone because of the unique control of blood flow in these regions that have a predominance of arteriovenous anastomoses (13).
The similar changes in temperature in the iohexol and normal saline groups suggest that the contrast agent did not change the efficacy of the LSB. However, with equivalent sympathectomy, the relief of symptoms was greater in the iohexol group. This has not been previously described. This finding is unlikely to result from a difference in needle positioning because no fluoroscopic images were obtained after the injection of the iohexol or saline. Two potential explanations for the effect of iohexol on subjective pain symptoms in the patients are 1) that the iohexol may exert an independent antinociceptive effect, and 2) that it may alter the action or delivery of bupivacaine.
Addressing the first possibility, Sand (14) has reported increased latency of F responses after lumbar myelography with iohexol. This increased latency can be taken as a sign of decreased neural conduction. Although the response was minimal and did not result in clinically significant changes, this neural depression may partly explain iohexol’s antinociceptive effect. Iohexol also has a mild inhibitory effect on the plasma activity of butyrylcholinesterase (15). This enzyme plays a role in the metabolism of acetylcholine and ester-linked local anesthetics. Whereas direct action of plasma butyrylcholinesterase on the amide-linked anesthetic bupivacaine should not be significant, decreased breakdown of acetylcholine may result in better pain relief (16). Although the effects of iohexol on plasma butyrylcholinesterase are small, it is possible that iohexol, like neostigmine, may enhance pain relief. The physicochemical properties of the iohexol solution, such as its pH, may also have an effect on the action of bupivacaine. The solution, however, was buffered to a pH between 6.8 and 7.7 with hydrochloric acid or sodium hydroxide, so it should not have significantly altered the polarity or action of bupivacaine. Further in vitro and in vivo work should be done to characterize an analgesic mechanism.
The limitations of this study include the small number of patients and the variability inherent in the phenomenon of neuropathic pain. Although there was a difference in the pain relief and continuing pain responses, skin temperature changes were similar in the iohexol and normal saline groups, which may in part be related to the small sample size. Although patients were allowed to participate more than once in the study, no patient contributed more than twice to the subjective pain scores after LSB, thus minimizing bias toward one patient. The subjective nature of pain, the variable components of sympathetically mediated and independent pain, the placebo effect, and personal expectations further complicate any study of neuropathic pain.
In summary, we have characterized the changes in cutaneous temperature that occur after LSB in the lower extremity of patients with sympathetically maintained chronic pain and examined the effects of a radio-opaque contrast medium, iohexol, on these temperature changes and relief of the pain symptoms in these patients. Monitoring temperature at the most distal parts of the lower extremity, such as the great or little toe, is most reliable because these regions change with greater magnitude and rate relative to other regions in the lower extremity. Changes of 8° to 9°C in the great and little toes over a period of approximately 35 min were typical. These changes brought the cutaneous temperatures of the distal extremity to within 3°C of core temperature. The use of iohexol did not impair the ability of the local anesthetic to induce LSB, and the use of iohexol was associated with greater symptom relief. The magnitude of temperature change may have prognostic value, as higher maximum temperatures of the great toe were associated with more relief of mechanical allodynia.
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Footnotes
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The temperature monitoring equipment used in this study was donated by Mallinckrodt and Medical Inc., St. Louis, MO.
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References
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Accepted for publication February 7, 2000.
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Abstract
Introduction
