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A Prospective, Randomized Comparison Between Single- and Multiple-Injection Techniques for Ultrasound-Guided Subgluteal Sciatic Nerve Block

Yamamoto, Hiroto MD; Sakura, Shinichi MD; Wada, Minori MD; Shido, Akemi MD

doi: 10.1213/ANE.0000000000000462
Regional Anesthesia: Research Report

BACKGROUND: It is believed that local anesthetic injected to obtain circumferential spread around nerves produces a more rapid onset and successful blockade after some ultrasound-guided peripheral nerve blocks. However, evidence demonstrating this point is limited only to the popliteal sciatic nerve block, which is relatively easy to perform by via a high-frequency linear transducer. In the present study, we tested the hypothesis that multiple injections of local anesthetic to make circumferential spread would improve the rate of sensory and motor blocks compared with a single-injection technique for ultrasound-guided subgluteal sciatic nerve block, which is considered a relatively difficult block conducted with a low-frequency, curved-array transducer.

METHODS: Ninety patients undergoing knee surgery were divided randomly into 2 groups to receive the ultrasound-guided subgluteal approach to sciatic nerve block with 20 mL of 1.5% mepivacaine with epinephrine. For group M (the multiple-injection technique), the local anesthetic was injected to create circumferential spread around the sciatic nerve without limitation on the number of needle passes. For group S (the single-injection technique), the number of needle passes was limited to 1, and the local anesthetic was injected to create spread along the dorsal surface of the sciatic nerve, during which no adjustment of the needle tip was made. Sensory and motor blockade were assessed in double-blind fashion for 30 minutes after completion of the block. The primary outcome was sensory blockade of all sciatic components tested, including tibial, superficial peroneal, and sural nerves at 30 minutes after injection.

RESULTS: Data from 86 patients (43 in each group) were analyzed. Block execution took more time for group M than group S. The proportion of patients with complete sensory blockade of all sciatic components at 30 minutes after injection was significantly larger for group M than group S (41.9% vs 16.3%, P = 0.018). Complete motor blockade of foot and toes extension also was observed more frequently in group M than in group S (67.4% vs 34.9%, P = 0.005 and 51.2% vs 25.6%, P = 0.027, respectively).

CONCLUSIONS: When ultrasound-guided subgluteal sciatic nerve block is conducted, multiple injections of local anesthetic to make a circumferential spread around the sciatic nerve improve the rate of sensory and motor blocks compared with a single injection.

Published ahead of print September 26, 2014.

From the Department of Anesthesiology, Shimane University School of Medicine, Izumo City, Japan.

Published ahead of print September 26, 2014.

Accepted for publication August 1, 2014.

Funding: Funding was provided from departmental and institutional sources.

The authors declare no conflicts of interest.

This report was previously presented, in part, at Euroanaesthesia 2012, Paris, France, June 9–12, 2012, and the 32nd Annual ESRA Congress, Glasgow, United Kingdom, September 4–7, 2013.

Reprints will not be available from the authors.

Address correspondence to Shinichi Sakura, MD, Department of Anesthesiology, Shimane University School of Medicine, 89-1, Enya-cho, Izumo City 693–8501, Japan. Address e-mail to ssakura@med.shimane-u.ac.jp.

Ultrasound permits visualization of the nerve tissue, needle, and local anesthetic spread in real time and, thus, has dramatically changed peripheral nerve block techniques.1 One of the changes that may help improve block characteristics is that the operator may adjust the location of both the needle tip and the deposition of local anesthetic to create a desired pattern of local anesthetic spread.2 It is believed that local anesthetic injected to obtain circumferential spread or a “doughnut sign” around nerves produces a more rapid onset and successful blockade after some ultrasound-guided peripheral nerve blocks.3 However, even though this practice has been adopted by many practitioners,4 ourselves included,5–8 the evidence demonstrating this point is limited to only the popliteal sciatic nerve block.9

In patients undergoing surgery in the lower extremities, sciatic nerve block at the subgluteal level provides anesthesia or analgesia of wider range than that at the popliteal level. Because the proximal sciatic nerve is the largest peripheral nerve, the pattern of local anesthetic spread might affect the resultant blockade in a greater degree than the other sciatic nerve locations, including popliteal sciatic nerve. Because the nerve is situated more deeply, however, visualization of the nerve is less ideal with a low-frequency, curved-array transducer,5–8,10–12 which requires a more advanced technique to create a spread as initially planned. In addition, there is concern regarding a possible increase in traumatic injury as well as intraneural injections and longer performance time when multiple redirections of a needle are required to deposit a local anesthetic solution around this deep nerve.

Accordingly, we conducted the present study to test the hypothesis that a multiple-injection technique (no limit on number of needle passes) to create circumferential spread of local anesthetic would result in a greater rate of sensory and motor blockade compared with a single-injection technique after ultrasound-guided subgluteal sciatic nerve block. In addition, we also compared block execution times and the frequency of intraneural injection for the 2 techniques.

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METHODS

The protocol of this prospective, comparative, randomized, and double-blinded trial was approved by the ethics committee at Shimane University School of Medicine (Izumo City, Japan) and registered at UMIN Clinical Trials Registry (Code JPRN-UMIN000005673). Consecutive patients (ASA physical status I–II) undergoing minor knee surgery, including meniscectomy, meniscal repair, or synovectomy, were assessed for eligibility to enter this study. Patients who had contraindications to peripheral nerve blocks, a history of diabetes mellitus, or neurologic disease or who required general anesthesia before nerve blocks were excluded. After they supplied written informed consent, 90 patients in whom the sciatic nerve was clearly visualized with ultrasound in the screening scan were enrolled and divided into 2 study groups (groups M [multiple-injection technique] and S [single-injection technique]) by random number table.

All patients fasted for approximately 6 hours before entering the operating room, where an IV infusion of acetated Ringer’s solution was initiated at a rate of 1−3 mL/kg/h. Standard noninvasive monitors were applied, and oxygen was administered via a facemask. Fentanyl 50−100 μg, with or without midazolam 1−2 mg, was given IV for anxiolysis as necessary, while ensuring that patients remained responsive to verbal cues.

The subgluteal approach to the sciatic nerve block was performed with a handheld ultrasound device (S-Nerve ultrasound system, Fujifilm Sonosite Inc., Bothell, WA) with a low-frequency, 5-2 MHz, curved-array transducer (C60e). Patients were placed laterally with the side to be anesthetized uppermost and the hip and knee of the operated side flexed at approximately 45 degrees. The ultrasound transducer was positioned perpendicular to the skin on the line connecting the ischial tuberosity and greater trochanter, and a clear transverse image of the hyperechoic sciatic nerve between the ischial tuberosity and greater trochanter was obtained. After skin sterilization with an iodine solution and skin infiltration with 1% mepivacaine, a short bevel 100-mm, 21-gauge insulated nerve block needle (CCR, Hakko, Chikuma, Japan) connected to a nerve stimulator (RasinPlex HRD-10, Hakko, Chikuma, Japan) was inserted parallel and in line with the ultrasound transducer, which was covered with a sterile plastic cover and gel from posterolateral to anteromedial. While keeping the sciatic nerve in the middle of the ultrasound screen, the operator slowly advanced the needle until it was immediately adjacent to the nerve. A nerve stimulator set at 1.0 mA with pulse duration of 0.1 ms and stimulating frequency of 2 Hz was then turned on to elicit foot plantarflexion, foot inversion, dorsiflexion, or foot eversion. When the response was observed, the current was reduced gradually to obtain minimal-evoked current eliciting the motor response.

Once the minimal-evoked current was established, nerve stimulation was switched off, and no attempt was made to seek an even smaller electric current to elicit motor response or a particular motor response pattern. Each patient then started to receive an aliquot of 20 mL 1.5% mepivacaine with 1:400,000 epinephrine and continued to receive the solution remaining via 1 of 2 techniques: In group M, local anesthetic was injected to create a circumferential spread of the solution around the sciatic nerve (Fig. 1), for which the needle tip was repositioned with no limitation on the number of needle passes. In group S, all the solution was injected without changing the initial position of the needle tip to create spread along the dorsal surface of the sciatic nerve (Fig. 2). All injections were incrementally made using a 20-mL syringe with the intention of avoiding intraneural injection. Ultrasound video was recorded during sciatic nerve block and used later to determine whether the local anesthetic was injected properly. All patients also received ultrasound-guided femoral nerve block and, if needed, lateral femoral cutaneous nerve block.13 All nerve blocks were performed by anesthesiologists (the authors), who have similar amounts of experience in the blocks and technique used in the study. The patients were sedated with midazolam, fentanyl, or propofol by request during surgery, and no tourniquet was used.

Figure 1

Figure 1

Figure 2

Figure 2

Sensory and motor functions on the operated limb were examined 10, 20, and 30 minutes after injection of local anesthetic. Sensory blockade was evaluated by using pinprick (22G) on the plantar aspect of the foot (tibial nerve), the dorsal aspect of the foot (superficial peroneal nerve), and the posterolateral area of the leg (sural nerve). It was considered complete when the patient did not feel a pinprick sensation. Motor blockade was assessed by the dorsal and plantar flexion of foot and toes and considered complete when the patient showed paralysis. Measurements also included the depth and the size of the sciatic nerve, minimal current to elicit motor response, and block execution time for sciatic nerve block (time from the insertion of the block needle to the end of local anesthetic injection and withdrawal of the needle). Measurements of depth and size were conducted on the screen by the internal measuring program of the ultrasound device. Patients also were evaluated for potential block-related complications, including dysesthesia, paresthesia, and/or motor weakness 2 days and approximately 4 weeks after surgery by an investigator and the surgical team during their office visits. An investigator who was blinded to group assignment collected all data. An investigator who was blinded to all patients and data, except group assignment, reviewed all videos.

Intraneural injection of local anesthetic was defined as apparent swelling of any part of the nerve in the cross-sectional view while local anesthetic was being injected as described previously.7 Patients in whom a poor video image was obtained during performance of the sciatic nerve block were excluded from the study. Those with a block execution time exceeding 10 minutes also were excluded to preclude possible effects of delay in starting the measurements. The exclusion time was determined by adding 3 SDs to the average performance time according to our own experience.7,8

The primary outcome measure was complete sensory blocks of all sciatic components, including tibial, superficial peroneal, and sural nerves at 30 minutes after injection. Our previous study5 showed that subgluteal sciatic nerve block with circumferential spread of local anesthetic produced sensory blockade at 30 minutes after injection in approximately 50% of the tibial nerve, which took more time to block than the other components of the sciatic nerve. Based on the results of previous studies9,14 showing approximately 30% difference in sensory blockade 30 minutes after popliteal sciatic nerve block between circumferential and noncircumferential spread, we assumed that a multiple-injection technique would increase the rate of sensory blockade from 20% to 50%. Thus, power analysis was conducted to calculate sample size to detect the difference of 30% with beta set at 0.2 and alpha set at 0.05. A minimal sample of 78 patients (39 in each group) met these criteria. To accommodate for patient dropouts, 90 patients were enrolled in the study. Data are presented as mean ± SD unless otherwise stated. Continuous variables were compared with Welch 2-group 2-sided t-test. χ2 analysis was used to compare the number of patients whose sensory and motor blockade was completed at 10, 20, and 30 minutes after the block. A P value of <0.05 was considered statistically significant.

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RESULTS

No patient was precluded from entering the study as the result of poor ultrasound imaging of the sciatic nerve (Fig. 3). Of 90 patients enrolled, 86 patients (43 in each group) completed the study. Three patients (2 in group M, 1 in group S) were excluded because the block execution took more than 10 minutes: 900 and 1200 seconds for the patients in group M and 1080 seconds for the patient in group S. The fourth patient was excluded from group S because of poor video image quality during the block. Both groups were similar in sex, age, weight, height, physical status, duration of surgery conducted (Table 1), as well as the depth and size of the sciatic nerve on the ultrasound screen (Table 2). Data with nerve stimulation were obtained in all patients but one, in whom the nerve stimulator did not function for unknown reasons. Because the adjacent location of the needle tip near the nerve was evident on ultrasound, however, the patient was not excluded from the study. For all other patients, the minimal electric current and response pattern were similar between the 2 groups. Block execution took more time for group M than group S.

Figure 3

Figure 3

Table 1

Table 1

Table 2

Table 2

Circumferential spread of local anesthetic was confirmed by video in all group M patients (100%) and also was observed in 5 group S patients (11.6%). Intraneural injection was detected in a similar percentage of patients for the 2 groups: 6 (14.0%) and 5 (11.6%) patients in groups M and S, respectively.

Multiple injections produced a significantly greater rate of sensory blockade of all sciatic components at 30 minutes after the completion of the block compared with a single injection (41.9% vs 16.3%, P = 0.018) (Fig. 4). The proportion of patients with sensory blockade of each nerve significantly differed between groups: for the sural nerve at 20 minutes, for the tibial nerve at 30 minutes, and for the superficial peroneal nerve at 10, 20, and 30 minutes. A significantly greater rate of motor blockade was observed for the dorsal flexion of foot and toes extension in group M than in group S at 30 minutes (67.4% vs 34.9%, P = 0.005, and 51.2% vs 25.6%, P = 0.027, respectively) (Fig. 5). Time from the completion of sciatic nerve block to the start of the surgical procedure was shorter in group M than in group S (44 ± 6 minutes vs 48 ± 6 minutes, P = 0.007). No patient received general anesthesia. Intraoperative fentanyl (50–200 μg) injections similarly were required because of slight knee pain for the 2 groups (12 and 19 patients in groups M and S, respectively). No neurologic complications such as prolonged sensory or motor dysfunction were observed postoperatively.

Figure 4

Figure 4

Figure 5

Figure 5

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DISCUSSION

In this prospective, comparative, randomized, and double-blinded study, we demonstrated that, when ultrasound-guided subgluteal sciatic nerve block was performed, a multiple-injection technique to create circumferential spread around the nerve produced sensory anesthesia and motor palsy more rapidly than a single-injection technique. We also showed that a single-injection technique required shorter block performance time but had a similar frequency of unintentional intraneural injection as the other technique.

To the best of our knowledge, this is the first study to determine whether injection patterns influence the effects of sciatic nerve block in the subgluteal region. Several recent studies9,15–19 have focused on possible differences in the effects on resultant block characteristics among different sites for needle placement and local anesthetic. The effects between circumferential and noncircumferential spread around the nerve, however, have been compared only with popliteal sciatic nerve block,9,14 a relatively easy block conducted via the use of a high-frequency linear transducer. Brull et al.9 compared single and circumferential injections made immediately proximal to the bifurcation with only ultrasound guidance. Their results showed that circumferential spread of local anesthetic around the nerve improved the rate of sensory block without changing the motor block or block performance time. The results of the present study, in which a low-frequency curved array transducer was used to target the more proximally and deeply located nerve, confirmed most of their results, but added that the improvement of blockade can also be observed partly in motor function. The size difference of the target nerve may explain the difference in the rate of motor blockade between the 2 studies, because it is reasonable to assume that it takes longer for local anesthetic deposited to travel a longer distance.

Block performance time for creating a circumferential spread was significantly longer than the other technique by about 40 seconds in the present study. This is clearly accounted for by extra time for needle redirection with the multiple-injection technique in subgluteal sciatic nerve block, which, compared with the popliteal sciatic nerve block, should require more careful manipulation of the needle. An initial aliquot of local anesthetic must have been injected earlier by nearly 40 seconds with the multiple-injection technique than with the other technique. Thus, it is possible, albeit unlikely, that this extra time was partly responsible for the differences observed in sensory and motor functions after the block.

It is surprising that intraneural injection occurred at a similar rate for the 2 infusion techniques. Thus, it is likely that there are other factors besides the number of needle passes that might cause intraneural injection. In our previous study7 where unintentional intraneural injection occurred 16.3% of the time when a multiple-injection technique was used, we showed that intraneural injection hastened the development of sensory and motor blockade. However, the present study was apparently underpowered to determine that intraneural injection with a single-injection technique similarly affected the development of the blockade.

Because similar percentages of patients required fentanyl as the result of slight pain in the knee during surgery, it appears that both injection techniques eventually offered similar quality of sensory blockade for minor surgical procedures in the knee. This may be partly explained by the approximately 4 minutes longer period of time (for unknown reasons) from the completion of sciatic nerve block until surgery in patients receiving a single injection than in those given multiple injections. Interestingly, even when examined immediately after sciatic nerve block completion, local anesthetic solution administered with a single-injection technique was found to encircle the nerve in 11.6% of the patients. The rate might have been even larger if the postinjection spread had been evaluated later in time. Morau et al.14 conducted an ultrasound examination of the sciatic nerve 5 minutes after popliteal block made only with nerve stimulation guidance at 10 cm above the popliteal fossa crease and found that a circumferential spread of local anesthetic occurred in 47% of patients. Thus, it is possible that the pattern of local anesthetic spread counts only for the first 30 minutes after the block.

There are other limitations to this study. First, only patients whose sciatic nerves were clearly visualized were enrolled in the present study. Thus, the present results may not apply to some patients, such as the elderly or obese, in whom subgluteal nerve block is more difficult to perform. In elderly patients, muscles can be atrophic, and the fascia may not be distinguishable from the nerve with ultrasound.20,21 In obese patients, the nerve can be located deeper and may be less clearly visualized.22 Second, all blocks were conducted by anesthesiologists who were well experienced in ultrasound-guided peripheral nerve blocks, including the techniques used in this study. Thus, visualization of the nerve and resultant sensory and motor blockade may be poorer, obscuring the different effects between the 2 techniques in a daily clinical situation. Third, only mepivacaine, a short-acting local anesthetic, was used for sciatic nerve block. Thus, it is possible that the present results may not apply to the block conducted with long-acting local anesthetic such as ropivacaine. Forth, we did not measure the duration of sciatic nerve block in this study. Thus, the 2 approaches might have resulted in different durations of anesthesia and analgesia. Fifth, the investigator reviewing the ultrasound video during block was aware of the group assignment of each patient. Thus, it is possible that data obtained from the video, such as the confirmation of circumferential spread and intraneural injection, were biased.

In conclusion, when ultrasound-guided subgluteal sciatic nerve block is conducted, multiple injections of local anesthetic to create a circumferential spread around the sciatic nerve result in a greater rate of sensory and motor blockade as compared with a single-injection technique.

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DISCLOSURES

Name: Hiroto Yamamoto, MD.

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

Attestation: Hiroto Yamamoto has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Shinichi Sakura, MD.

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

Attestation: Shinichi Sakura 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: Minori Wada, MD.

Contribution: This author helped conduct the study and write the manuscript.

Attestation: Minori Wada has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Akemi Shido, MD.

Contribution: This author helped conduct the study and write the manuscript.

Attestation: Akemi Shido has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

This manuscript was handled by: Terese T. Horlocker, MD.

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