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Prior Lumbar Discectomy Surgery Does Not Alter the Efficacy of Neuraxial Labor Analgesia

Bauchat, Jeanette R. MD; McCarthy, Robert J. PharmD; Koski, Tyler R. MD; Cambic, Christopher R. MD; Lee, Amy I. MD; Wong, Cynthia A. MD

doi: 10.1213/ANE.0b013e3182575e1b
Obstetric Anesthesiology
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SDC

BACKGROUND: Lumbar discectomy surgery is a common neurosurgical procedure. Neuraxial labor analgesia may be less effective in parturients with a history of discectomy surgery because of postsurgical scarring and anatomical distortion. In this prospective observational case-controlled study, we compared bupivacaine consumption per hour of labor analgesia as an indirect measure of labor analgesic effectiveness between women with prior discectomy surgery and those who did not have back surgery.

METHODS: All women with prior discectomy surgery who requested neuraxial labor analgesia at a high-volume, single university-affiliated women's hospital during the study period were approached. Control subjects were matched for anesthesiologist skill level. The primary outcome was bupivacaine consumption per hour of labor analgesia. Characteristics associated with the epidural catheter placement including the number of interspaces attempted, time to placement, and number of epidural catheters replaced for inadequate analgesia were recorded. Subject characteristics, labor outcomes, and analgesia outcomes were analyzed using the Wilcoxon ranked sum or Fisher exact test. Epidural placement data were analyzed using the Wilcoxon signed rank, McNemar's, or sign test.

RESULTS: Data were analyzed for 42 women in the discectomy group and 42 women in the control group. Bupivacaine consumption per hour of labor analgesia was not different between groups (median [interquartile range, IQR]: discectomy 12.7 mg/h [11.0 to 15.3] and control 13.2 mg/h [11.3 to 15.7]; difference in medians [95% confidence interval, CI]: −0.55 mg/h [−1.33 to 1.39]; P = 0.43). The interval from initiation of neuraxial analgesia and delivery and mode of delivery did not differ between groups. The median difference (95% CI) in the time to place the epidural catheter between the discectomy and control subjects was 0 minute (−1 to 2.5); P = 0.38. More than 1 interspace was attempted in 17% discectomy in comparison with 2% of the control subjects—difference (95% CI) 15% (2–26); P = 0.03. The neuraxial technique and estimated level of catheter placement did not differ. Completion of the procedure by a more senior anesthesiologist occurred in 3 discectomy subjects and 2 control subjects (P = 1.0). No epidural catheters were replaced.

CONCLUSIONS: There was no difference in hourly bupivacaine consumption in parturients with prior lumbar discectomy surgery undergoing neuraxial labor analgesia in comparison with controls. Time to placement of the epidural catheter was not different either, but more interspaces were attempted in the discectomy group. Our findings suggest that standard clinical neuraxial analgesic methods are effective in women with discectomy surgery.

Published ahead of print May 14, 2012 Supplemental Digital Content is available in the text.

From the Department of Anesthesiology, Northwestern University, Chicago, IL.

Amy I. Lee is currently affiliated with the Department of Anesthesiology, University of Texas Health Science Center at Houston, Houston, TX.

Funding: Department of Anesthesiology resources.

The authors declare no conflict of interest.

Reprints will not be available from the authors.

This report was previously presented, in part, at the SOAP Annual Meeting Chicago, Illinois, October 2011.

Address correspondence to Jeanette R. Bauchat, MD, Department of Anesthesiology, Northwestern University, 250 East Huron F5–704, Chicago, IL 60611. Address e-mail to jbauchat@gmail.com.

Accepted February 29, 2012

Published ahead of print May 14, 2012

Sixty-seven percent to 84% of people living in industrialized countries experience low back pain, with onset typically between the ages of 30 and 50 years of age.1,2 Lumbar disk herniation accounts for 4% of all low back pain, and these patients may elect to undergo discectomy to treat the pain if conservative management fails.25

There is some evidence to suggest that parturients who undergo spine surgery may be at greater risk of difficulty with placement of neuraxial labor analgesia and ineffective analgesia in comparison with parturients who have not had surgery.610 To our knowledge, only a single small study evaluated the efficacy of neuraxial analgesia in laboring women with a history of discectomy surgery.8 Analgesia requirements were not assessed.

The purpose of this prospective case-controlled observational study was to compare the hourly rate of bupivacaine consumption during neuraxial labor analgesia as an indirect marker of analgesic efficacy in women with a history of discectomy surgery. We hypothesized that women with a history of discectomy surgery may require more bupivacaine per hour to achieve labor analgesia because of underlying postsurgical changes around the epidural space and nerve roots. In addition, we compared the ease of placement and success of neuraxial analgesia between groups.

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METHODS

This study was approved by the IRB of Northwestern University. All participants gave informed written consent. Consecutive women with a history of discectomy surgery requesting neuraxial labor analgesia were recruited during preanesthetic consultation or after admission to the Labor & Delivery Unit at Prentice Women's Hospital of Northwestern University during the preanesthetic interview. The surgical documentation of discectomy procedures performed at Northwestern University were reviewed; otherwise, surgical information was obtained by patient interview.

The next healthy parturient requesting neuraxial analgesia who agreed to participate in the study served as a control subject. The neuraxial procedure was initially attempted by the same resident, fellow, or attending anesthesiologist to control for the skill level of the provider. Parturients with a history of uncorrected scoliosis, spondylolisthesis, history of chronic back pain outside of pregnancy, or other chronic pain syndromes (>6 months duration outside of pregnancy, requiring chronic pain medication or other pain interventions) were not approached. All procedures were directly supervised by an attending anesthesiologist. Preexisting neurologic symptoms (i.e., low back pain, radiculopathy, or sensory/motor deficit) were considered present if the parturient complained of symptoms at the time of labor presentation.

A baseline Visual Analog Scale (VAS) score for pain (100 mm unmarked line with the end points labeled no pain and worst pain imaginable) was obtained at the time of request for analgesia. A combined spinal–epidural or epidural technique was chosen at the discretion of the anesthesiologist. Analgesia was initiated in the sitting position. In control subjects, the procedure was preferentially performed at the estimated L3-4 or L4-5 interspace. In subjects with prior discectomy, the interspace was chosen on the basis of the parturient's surgical history.

The epidural space was located with a 17-gauge Tuohy epidural needle using a loss-of-resistance to air or saline technique. For epidural analgesia, the epidural catheter (Arrow Flextip® Plus, Reading, PA) was threaded to 4 to 5 cm in the epidural space, and a 3-mL test dose of lidocaine 15 mg/mL with epinephrine 5 μg/mL was administered through the epidural catheter. An initial loading dose of fentanyl 100 μg and bupivacaine 1.25 mg/mL in increments of 5 mL up to 10 mL were injected to patient comfort. The combined spinal–epidural analgesia procedure mimicked the epidural procedure, except that the dura was punctured with a 27-gauge Sprotte needle (Pajunk Sprotte®, Norcross, GA) before threading the epidural catheter. The intrathecal dose consisted of fentanyl 15 μg combined with bupivacaine 2.5 or 1.25 mg, or fentanyl 25 μg without bupivacaine. A test dose was given as described above. The VAS score was assessed 30 minutes after the test dose in all subjects.

Data regarding the neuraxial technique were collected by a research nurse, and included the number of interspaces attempted (defined as a new puncture at a different lumbar interspace) and the time to epidural catheter placement (defined as interval from local anesthetic injection skin wheal injection to test dose administration). Epidural analgesia failure (defined as the need for replacement of the epidural catheter at any time during labor or operative delivery) was also recorded.

Analgesia was maintained with patient-controlled epidural anesthesia (PCEA) with bupivacaine 0.625 mg/mL and fentanyl 1.95 μg/mL (background infusion 15 mL/h, patient-controlled bolus dose 5 mL, lockout period 10 minutes, hourly maximum 30 mL/h).

The management of breakthrough pain was standardized. At the request for additional analgesia, the patient's VAS score and sensory level were assessed, and a manual epidural bolus (top-up) was administered. The first epidural manual rebolus dose consisted of bupivacaine 1.25 mg/mL administered in 5-mL aliquots to a total of 10 to 15 mL; the concentration of the PCEA epidural solution was increased to bupivacaine 1.1 mg/mL with fentanyl 1.7 μg/mL. Twenty minutes after the manual bolus, the VAS and sensory level were reassessed to ensure adequate pain relief. If there was a second request for analgesia, the same manual bolus was administered, and the background epidural infusion rate was increased to 20 mL/h. Subjects' VAS and sensory level were reassessed 20 minutes later, and if the rebolus failed to provide pain relief, the epidural catheter was deemed a “failure” and was replaced. Patients were visited on postpartum day 1 and complaints of worsening or new-onset neurologic symptoms were documented and followed-up as per routine.

Analgesic data collected included total bupivacaine consumption and number of episodes of breakthrough pain requiring a manual rebolus. Bupivacaine consumption was normalized by dividing the total bupivacaine dose by the total labor time (time interval from initiation of PCEA labor analgesia until 1 hour after vaginal delivery or decision for cesarean delivery). The initial intrathecal dose and epidural loading dose of bupivacaine were not included in the total bupivacaine consumption.

The primary outcome was bupivacaine consumption per hour of labor analgesia. In a previous study using a similar labor analgesic technique, average bupivacaine consumption per hour of labor was 12.3 ± 2.5 mg.11 Assuming that a difference of 2.2 mg per unit hour would represent a significant and clinically important increase in bupivacaine consumption, one would require a sample size of 41 subjects with prior discectomy and 41 control subjects to detect this difference with a power of 0.95 at α = 0.05 using a 2-sample 2-sided Wilcoxons ranked sum test. These results are based on 5000 Monte Carlo samples from the null distributions. The sample size estimate was based on grouped and not paired samples because pairs of subjects were matched for anesthesia provider placing epidural catheter and not on characteristics associated with analgesic use. Sample size calculations were performed using PASS 2008 (version 8.0.15; release date September 8, 2010; NCSS LLC, Kaysville, UT).

The primary outcome variable was compared between parturients with a history of discectomy surgery and those with no history of back surgery using the Wilcoxon ranked sum test. The median shift and 95% confidence interval of the shift were calculated using the exact method. Subject characteristics and labor and analgesic outcomes were compared between groups using the Wilcoxon ranked sum test or the Fisher exact test. Data regarding the placement of the epidural catheter were evaluated as paired data. The time required to place the epidural catheter was compared using the Wilcoxon signed rank test. Other characteristics of epidural placement were evaluated using McNemar's test for binominal data or the sign test for ordinal data. All reported P values are 2-sided, and confidence intervals are 95%. Data were analyzed using R version 2.14.1 (release date December 22, 2011; R Foundation for Statistical Computing).

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RESULTS

The study was conducted between July 2007 and November 2010. Forty-six women with prior disk surgery were approached for study participation, and 42 gave written consent for study participation. Fifty women were asked to participate in the study as control subjects, and 42 gave written consent. The subjects' characteristics including parity, gestational age, and body mass index were similar, with the exception that women with prior discectomy were older than those in the control group (Table 1). Labor and infant outcomes (fetal weight, mode of delivery, and time to delivery) were not different between groups. The surgical details of the discectomy group are presented in Table 2. No parturient had underlying motor deficits at the time of the study. No patients complained of new or worsening neurologic symptoms on postpartum day 1.

Table 1

Table 1

Table 2

Table 2

Median hourly bupivacaine consumption (interquartile range, IQR) was not different between the discectomy (12.7 mg/h [11.0 to 15.3]) and control groups (13.2 mg/h [11.3 to 15.7]); the difference in medians was −0.5 mg/h (95% confidence interval [CI], −1.3 to 1.4; P = 0.43) (Fig. 1). The VAS at analgesic request, 30 minutes after initiation of analgesia, and the number of subjects requiring an increase in bupivacaine concentration to achieve effective analgesia did not differ (Table 3).

Figure 1

Figure 1

Table 3

Table 3

The evaluation of neuraxial placement in the paired discectomy and control subjects is shown in Table 4. The time to place the epidural catheter did not differ between the discectomy and control subjects, median difference (95% CI) 0 minute (−1 to 2.5); P = 0.38. Although the epidural was placed at the first chosen interspace in 34 pairs of discectomy and control subjects, >1 interspace was attempted in 17% discectomy in comparison with 2% of the control subjects, difference (95% CI) 15% (2–26); P = 0.03. The estimated final interspace was also more cephalad in 8 pairs and more caudal in 3 pairs of the discectomy subjects compared to the controls. The skill level of the anesthesia providers who initiated and completed the neuraxial technique was not different between the discectomy and control subject. No epidural catheters were replaced for failed analgesia.

Table 4

Table 4

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CONCLUSIONS

Hourly epidural bupivacaine consumption for maintenance of epidural labor analgesia was not different between parturients with lumbar discectomy surgery and those with no known lumbar spine pathology. In addition, there was no difference in the proportion of parturients who required an increase in bupivacaine concentration to treat breakthrough pain, nor were there any failures to initiate neuraxial analgesia or need to replace epidural catheters. These findings are consistent with those of Villevieille et al., who evaluated neuraxial labor analgesic efficacy and ease of initiation in parturients with a history of back surgery.8 None of the 16 parturients with a history of lumbar discectomy surgery experienced technical failures, but 1 patient experienced “ineffective analgesia.”8 None of the parturients who had discectomy surgery in this study who required a cesarean delivery experienced failure to convert epidural analgesia to anesthesia. This result is consistent with the findings of Hebl et al., who found no difference in efficacy of surgical anesthesia achieved by neuraxial techniques in those who had prior back surgery and those who did not, although the majority of patients had prior laminectomy surgery and only 9 had discectomy surgery.12

Most case series and prospective studies evaluating efficacy and ease of placement of neuraxial labor analgesia in parturients with prior spine surgery were in women with corrective surgery for scoliosis or decompressive laminectomies. Although the ultimate success of neuraxial placement was in the range of 85%–95%, the reported analgesic efficacy rates were only 50% to 60%.610,13,14 Possible reasons for difficult neuraxial initiation or ineffective analgesia in women with prior back surgery include limited back mobility, inflammation and scarring in the epidural space or surrounding nerves, and increased degenerative bone changes surrounding areas of injury or instrumentation.12,15,16 Differences in findings in previous studies in parturients with scoliosis repair and laminectomy, in comparison with our findings in patients with prior discectomy, likely reflect differences in the extent of surgical damage caused by multilevel instrumentation and bone removal or grafting, which alters the epidural space more than does isolated discectomy surgery.

Women in our discectomy group were slightly older than those in the control group. Eighty-six percent of lumbar disk herniations resolve spontaneously with conservative management and resolution is correlated with younger age; thus it is expected that women who require lumbar discectomy surgery would be older.17 Age has not been demonstrated to be an independent predictor of analgesia requirements in laboring women, and therefore it is unlikely that the age difference between our groups affected bupivacaine requirements.

The neuraxial procedure was performed at a more cephalad interspace in 8 parturients with prior discectomy than in the control subjects. The spinal cord normally terminates above the L2 vertebral body, and the majority of discectomy procedures are performed at L4/5 or L5/S1. Therefore, there is usually adequate space above the surgical site for lumbar epidural catheter placement. However, “blind” neuraxial techniques frequently result in use of a more cephalad interspace than is intended or predicted.18,19 Thus, the neuraxial technique is likely placed at closer proximity to the spinal cord in discectomy patients. It is not clear whether using an interspace cephalad to the surgical site is necessary, because Fredman et al. demonstrated successful “blind” epidural catheter placement in 17 of 20 patients at or near the level of prior discectomy surgery (confirmed via fluoroscopy) in patients receiving epidural steroid injections.20

The incidence of low back pain during pregnancy in women with a history of discectomy surgery is 76%, but is comparable to that of women with no history of back pathology.21,22 ”Failed back syndrome”—characterized by low back pain, radiculopathy, or both—occurs in 8.3% to 12% of patients after discectomy surgery, but the effects of pregnancy on this syndrome have not been studied.2325 More severe pain and radiculopathy after spine surgery correlate with radiographic evidence of more extensive scarring and anatomical changes at the surgical site.15,16,26 Although 27% of the discectomy group had low back pain or radiculopathy and potentially more scarring or anatomical changes, this observational study was not sufficiently powered to assess differences in analgesic consumption in the subset of discectomy patients who had exacerbation of back pain from failed back syndrome or pregnancy.

Nerves with underlying pathology—when exposed to a second trauma such as surgical ischemia, stretch injury, metabolic injury, or local anesthetic toxicity—are at higher risk of neurologic injury.27,28 A retrospective study by Hebl et al. demonstrated a higher incidence of new-onset neurologic impairment in surgical patients receiving neuraxial anesthesia who had preexisting radiculopathy or neurologic deficits.12 Patients in the discectomy group with underlying low back pain or radiculopathy theoretically may have been at increased risk of new or worsening neurologic deficits due to local anesthetic exposure and peripartum nerve injuries, but our study was not powered to detect differences in postpartum neurological deficits between groups.

There are several limitations to our study, including the lack of surgical details for subjects in our discectomy group. There is evidence that discectomy and microdiscectomy surgeries are equivalent in most surgical outcome measures, but other factors make discectomy procedures heterogeneous including minimally invasive versus open techniques, ligamentum flavum-sparing techniques, translaminar versus transforaminal approaches to disk removal, amount of disk removed, presence of dural tears, and whether antiscarring agents or steroids are applied at the surgical site.2931 This study is underpowered to account for influences of these surgical variations on bupivacaine consumption among discectomy patients.

Another limitation is that the majority of patients requesting labor analgesia received a combined spinal–epidural technique for initiation of analgesia. Combined spinal– epidural analgesia is the preferred technique at our institution because the rapid onset of analgesia is advantageous in a high-volume setting with an epidural analgesia utilization rate of 89%. Epidural analgesia is chosen for specific indications, for example, difficult airway or nonreassuring fetal status, when a functioning epidural is deemed an important component of patient safety. The average labor duration of the discectomy group was 8 hours, which allowed assessment of differences in maintenance of epidural analgesia between groups because the spinal portion of the combined spinal–epidural technique lasts approximately 60 to 100 minutes.11,32 Studies are conflicting as to whether the presence of a dural puncture upon initiation of epidural analgesia improves the overall quality of analgesia due to translocation of epidural medication into the cerebral spinal fluid.33,34 Therefore, our results may not be generalizable to women for whom analgesia is initiated with a traditional epidural and not a combined spinal– epidural technique.

This study demonstrates that parturients with a history of lumbar discectomy surgery can undergo neuraxial labor analgesia without an increased risk of difficult block initiation, epidural catheter failure, or increased analgesic drug consumption in comparison with parturients without a history of back surgery. Studies regarding efficacy or ease of placement of neuraxial labor analgesic techniques have used a heterogenous mix of “spine surgery,” and review of these studies suggests that more extensive procedures performed with instrumentation place patients at greater risk of complications from neuraxial analgesia. Future research should focus on specific surgical procedures so that anesthesiologists may appropriately counsel patients regarding their specific back pathology. Further research is warranted on whether pregnant women who have underlying back pain or neurologic deficits are at greater risk of postpartum neurologic injury.

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RECUSE NOTE

Dr. Cynthia Wong is the section Editor for Obstetric Anesthesiology for Anesthesia & Analgesia. This manuscript was handled by Dr. Steven L. Shafer, Editor-in-Chief, and Dr. Wong was not involved in any way with the editorial process or decision.

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DISCLOSURES

Name: Jeanette R. Bauchat, MD.

Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.

Attestation: Jeanette R. Bauchat has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.

Name: Robert J. McCarthy, PharmD.

Contribution: This author helped design the study, analyze the data, and write the manuscript.

Attestation: Robert J. McCarthy has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Tyler R. Koski, MD.

Contribution: This author helped write the manuscript.

Attestation: Tyler R. Koski approved the final manuscript.

Name: Christopher R. Cambic, MD.

Contribution: This author helped conduct the study.

Attestation: Christopher R. Cambic has seen the original study data and approved the final manuscript.

Name: Amy I. Lee, MD.

Contribution: This author helped conduct the study.

Attestation: Amy I. Lee has seen the original study data and approved the final manuscript.

Name: Cynthia A Wong, MD.

Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.

Attestation: Cynthia A. Wong has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

This manuscript was handled by: Steven L. Shafer, MD.

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