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Nerve Stimulator-Assisted Evoked Motor Response Predicts the Latency and Success of a Single-Injection Sciatic Block

Sukhani, Radha MD; Nader, Antoun MD; Candido, Kenneth D. MD; Doty, Robert Jr. MD; Benzon, Honorio T. MD; Yaghmour, Edward MD; Kendall, Mark MD; McCarthy, Robert PharmD

doi: 10.1213/01.ANE.0000122823.50592.C9

Variable onset latency of single-injection sciatic nerve block (SNB) may result from drug deposition insufficiently close to all components of the nerve. We hypothesized that this variability is caused by the needle tip position relative to neural components, which is objectified by the type of evoked motor response (EMR) elicited before local anesthetic injection. One-hundred ASA I–II patients undergoing reconstructive ankle surgery received infraglutealparabiceps SNB using 0.4 mL/kg (maximum 35 mL) of levobupivacaine 0.625%. The endpoint for injection was the first elicited EMR: inversion (I), plantar flexion (PF), dorsiflexion (DF), or eversion (E) at 0.2–0.4 mA. The frequencies of the EMRs were: I 40%, PF 43%, E 14%, and DF 3%. SNB was considered complete if both tibial and common peroneal nerves were blocked and failed if either analgesia to pinprick was not observed at 30 min or anesthesia at 60 min. Patients with an EMR of I demonstrated shorter mean times (±95% confidence interval [CI]) to complete the block with 8.5 (95% CI, 6.2–10.8) min compared to 27.0 (95% CI, 20.6–33.4) min after PF (P < 0.001) and 30.4 (95% CI, 24.9–35.8) min after E (P < 0.001). No rescue blocks were required in group I compared with 24% (P = 0.001) and 71% (P < 0.001) of patients in groups PF and E, respectively. We conclude that EMR type during nerve stimulator-assisted single-injection SNB predicts latency and success of complete SNB because the observed EMR is related to the positioning of the needle tip relative to the tibial and common peroneal nerves.

IMPLICATIONS: The type of evoked motor response (EMR) during nerve stimulator-assisted single-injection sciatic nerve block impacts the latency and success of complete block. Compared with EMRs plantar flexion, dorsi-flexion, or eversion, EMR inversion is associated with the shortest latency to a complete block of the sciatic nerve.

Department of Anesthesiology, Northwestern University/Feinberg School of Medicine, Chicago, IL

Accepted for publication February 3, 2004.

Address correspondence and reprint requests to Radha Sukhani, MD, Department of Anesthesiology, 251 E. Huron St., F5-704, Chicago, IL 60611. Address e-mail to

The sciatic nerve is the largest nerve in the body and is composed of two branches: the tibial nerve (medially) and the common peroneal nerve (laterally), which subdivides into superficial and deep peroneal nerves. Prolonged and variable onset latency and failure to achieve complete sensory and motor block are among the major limitations of sciatic nerve block (SNB). Although single-injection SNB techniques are clinically desirable, two-injection methods have been used to reliably achieve block of the tibial and peroneal nerves (1,2). Nerve stimulator-assisted needle placement has also been used to improve the rate of successful block by reducing the distance of the needle to the nerve and obtaining an evoked motor response (EMR) at stimulating currents ≤0.5 mA before the injection of the local anesthetic (LA) (3,4).

The location of the stimulating needle with respect to the tibial and peroneal nerves is reflected by a specific EMR when applying a stimulating current. These responses represent the contraction of muscle groups innervated by either the tibial or peroneal nerves alone or both nerves simultaneously and may include: plantar flexion (PF; tibial nerve), eversion (E; superficial/deep peroneal nerves, with the superficial peroneal nerve mainly responsible), dorsiflexion (DF; deep peroneal nerve), and inversion (I; both tibial and deep peroneal nerves) (5).

In a study of the success of popliteal SNB, an EMR of I was associated with the highest rate of complete sensory block of the foot (5). Sensory block was incomplete when the elicited EMR was PF or E. The limitations of the study were the use of high-stimulating currents (>0.5 mA) and the correlation of successful block only with sensory block. We previously reported that the latency was shorter and success of SNB was greater for EMR of I compared with EMR of PF after single-injection infragluteal-parabiceps SNB (6). A limitation of that study was that an EMR of I was sought via needle redirection, and only EMRs of I or PF were accepted as end-points before the LA injection.

The present study was undertaken to evaluate the incidence of the four aforementioned EMRs as an initial response after needle placement using the infragluteal-parabiceps approach to SNB at a stimulating current 0.2–0.4 mA and to compare the latency and success among EMR groups after the administration of a standardized LA drug. We hypothesized that an EMR of I, representing stimulation of both tibial and peroneal nerves, is associated with the shortest latency and most frequent incidence of complete SNB.

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IRB approval was obtained for this prospective, single-blinded study. One-hundred patients (≥18 yr old) scheduled to receive a SNB as a component of their anesthesia management for elective reconstructive ankle surgery gave written informed consent for study participation. Exclusion criteria were as follows: hemostatic abnormalities, chronic pain syndromes, foot deformities restricting normal foot movements, severe liver or renal disease, allergy to amide LAs, and preexisting neurological disorders.

All SNBs were performed a minimum of 60 min before the start of surgery. Blocks were performed by resident trainees supervised by faculty using the infragluteal-parabiceps approach (6). The leg was supported to permit unrestricted movement and observation of the foot in response to nerve stimulation. SNB was performed using a 100-mm 22-gauge insulated needle (Stimuplex®, B-Braun/McGaw Medical, Bethlehem, PA). The needle was connected to the negative lead of a constant current nerve stimulator (Stimuplex HNS-11, B-Braun/McGaw Medical). Stimulation frequency was 2 Hz; pulse width was 100 ms, and the current was set to deliver 1.0 mA. The needle was advanced until a brisk EMR of PF, I, E, or DF was obtained at 0.2–0.4 mA. The first elicited EMR at 0.2–0.4 mA determined group assignment. Levobupivacaine 0.625% (Chirocaine®, Purdue Pharma, Stamford, CT) with epinephrine 1:200,000 was then injected incrementally in 3-mL aliquots to a total volume of 0.4 mL/kg (minimum, 20 mL; maximum, 35 mL).

Sensory and motor block assessments were performed by an investigator blinded to the elicited EMR. Assessments were initiated at the completion of LA injection (time 0), every 2 min for 10 min, and at 5-min intervals until 30 min had elapsed. Sensory block assessments were performed in the distributions of the superficial peroneal nerve (dorsum of the foot), deep peroneal nerve (web space between first and second toes), posterior tibial nerve (sole of foot), and sural nerves (lateral aspect of foot). A 3-level scale was used to determine the intensity of sensory block to pinprick stimulation: 0 = normal sensation, 1 = analgesia (pinprick felt as dull), and 2 = anesthesia (pinprick not felt). Motor block onset and intensity was also graded on a 3-level scale: 0 = normal strength, 1 = paresis (diminished movement), and 2 = paralysis (no movement). Motor block was assessed using the following movements: PF (tibial nerve), DF (deep peroneal nerve), and toe movements (both tibial and peroneal nerves).

Complete block was defined as sensory and motor scores equal to 2 in the distributions of both the tibial and common peroneal nerves. Patients who did not demonstrate onset of block (i.e., sensory and motor scores = 0) in the tibial or peroneal nerve distributions at the 30-min assessment were given the option of undergoing rescue popliteal SNB, neuraxial block, or general anesthesia. Patients who demonstrated a sensory score of 1 at 30 min were assessed for an additional 30 min. If a sensory score of 2 did not develop by this time, the aforementioned anesthetic management options were offered.

Additional study variables included age, height, weight, block performance time, lowest stimulus intensity current before LA injection, LA volume, paresthesias during needle positioning or drug injection, and the presence of complications such as hematoma or systemic toxicity. Assessments for delayed neurological complications were performed via telephone at 24 h, 2 wk, and 1 mo. Patients were questioned using common pain descriptors regarding the presence of paresthesias, dysesthesias, prolonged anesthesia, or unexpected motor deficits (7).

The sample size estimated for this study (n = 100) was determined to detect a difference with an α = 0.05 in the frequency of complete block between the four EMR response groups. Assumptions were based on our previous observed effect size (W) of 0.33 and a desired power of 0.80 (6). The frequencies of complete block and gender distribution among groups were compared using a χ2 statistic and the Fisher’s exact test. Kaplan-Meier survival curves were constructed, and the log-rank test was used to compare the time to onset of complete block among groups. Patients who did not achieve a sustained EMR at a stimulating current 0.2–0.4mA were excluded from the analysis at time zero, whereas those who did not reach a sensory score of 1 at 30 min were excluded at 30 min. The Kruskal-Wallis test was used to compare age, body mass index, block completion time, LA volume, and stimulating current at LA injection. The Kruskal-Wallis test and the Kruskal-Wallis Z-test for post hoc comparisons were used to compare block latencies to a sensory score of 2 or a motor score of 2 in the distributions of the nerve branches. Because the DF group included only three patients, it was excluded from the statistical analysis. A P < 0.05 was required to reject the null hypothesis.

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The demographic and clinical variables for the groups are shown in Table 1. The frequencies of the initial EMR responses were 40% (I), 43% (PF), 4% (E), and 3% (DF). The SNB procedure was aborted in 2 cases (EMR of PF) for failure to sustain an EMR at ≤0.4 mA. Paresthesias during needle placement were encountered in 2 patients (EMR of PF and E). In both instances, needle readjustment resolved the paresthesia before LA injection.

Table 1

Table 1

Complete sensory and motor block was achieved in 100% of patients who exhibited an EMR of I compared with only 76%, 33%, and 29% of the PF, DF, and E groups, respectively (Table 1). Figure 1 shows the percentage of patients who achieved complete block as a function of time. Compared with an EMR of PF and E, subjects with an EMR of I had a more rapid onset of complete block.

Figure 1

Figure 1

Figure 2 depicts latency to onset of surgical anesthesia (sensory score = 2) in the posterior tibial, superficial peroneal, deep peroneal, and sural nerve distributions. Time to sensory score 2 was shorter when the EMR was I compared with PF or E in the sural and tibial nerve distributions. Latencies in the deep peroneal and superficial peroneal distributions were also shorter with an EMR of I compared with an EMR of PF. Complete motor block latencies (motor score = 2) for PF and DF were shorter with an EMR of I compared with PF and for PF with an EMR of I compared with an EMR of E (Fig. 3).

Figure 2

Figure 2

Figure 3

Figure 3

All patients with incomplete blocks (14 cases) received rescue popliteal SNBs. Two cases of aborted blocks (EMR of PF) also received popliteal SNBs. No patients demonstrating an EMR of I required rescue interventions. There were no immediate complications including systemic toxicity attributed to the LA injection. No neurological deficits were identified at the 2-wk or 4-wk follow-ups (96% completed follow-up). The duration of analgesia (perception of pain at surgical site) was 19 ± 6 h in patients who achieved complete SNB and was similar among the EMR groups.

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Important determinants of the latency to sensory and motor block after a peripheral nerve block are the physical properties and dose (volume and concentration) of LA, the proximity of the needle to the nerve, and anatomical characteristics of the adjacent tissue layers. The sciatic nerve is the largest in the body (2 cm in width) with two branches, the tibial and common peroneal nerves, which separate at the level of the piriformis muscle or more distally (2,8,9). The separation of the branches may contribute to a prolonged latency and failure to achieve complete block when using a single-injection SNB technique, despite ensuring needle-to-nerve proximity (1,10). Studies comparing single- versus double-injection techniques of SNB suggest that ensuring close proximity of the injecting needle to both branches of the sciatic nerve enhances success and decreases latency to complete block (1,2,11). Multiple injection techniques, however, produce significant patient discomfort and may increase the risk of neurologic injury (11).

The sciatic nerve at the infragluteal site is comprised of the distinctly separate tibial and common peroneal nerves within a common epineural sheath (6). Evaluation of the EMR during nerve stimulated-assisted SNB may allow not only assurance of proximity of the needle to the nerve, but also identification of the position of the stimulating needle tip with respect to the sciatic nerve branches (5,6). Whereas an EMR of PF, DF, and E are produced by stimulation of the tibial, deep, or superficial peroneal nerves, respectively, EMR of I is produced by stimulation of both the tibial and deep peroneal branches of the common peroneal nerve. Intraneuronal topography of the tibial nerve demonstrates that nerve bundles innervating the tibialis posterior are located laterally, i.e., within the central part of the sciatic nerve (9). Therefore, an EMR of I may signify that the needle tip is central in relation to the sciatic nerve trunk with either simultaneous stimulation of both the tibial and deep peroneal nerves or stimulation of the lateral edge of tibial nerve, which lies in close proximity to the common peroneal nerve. Logically, the LA solution injected close to the center of the sciatic nerve trunk could more predictably block both the tibial and peroneal nerve components, whereas injection at the medial edge of the nerve close to the tibial component (EMR of PF) or at the lateral edge of the nerve close to only the peroneal component (EMR of E) may not achieve the same degree of success.

The association of the four elicited EMR responses with the latency and success of SNB has not been previously examined. Cuvillon et al. (1) compared latency and success of SNB using three strategies for two posterior approaches; the parasacral approach and the Winnie modified Labat approach using either a single or double injection. An EMR of tibial origin (PF) was identified in 68% of patients using the Winnie technique and 80% of patients using the parasacral approach. In the remaining patients, an EMR of peroneal origin (DF) was identified. The success rate for complete block at 30 minutes in the 2 single-injection techniques was 66% for the parasacral approach and <50% using the Winnie technique. At 60 minutes, the success rate was 80% for the single-injection parasacral approach and 50% with the Winnie single-injection approach. In the present study, the success rate using a single-injection infragluteal-parabiceps approach was 80% across all the EMR groups at 60 minutes, similar to that reported by Cuvillon et al. (1) using their single-injection parasacral approach. The increased frequency of complete SNB with a short latency (85% complete blocks within 15 minutes and 95% within 30 minutes) with an EMR of I in the present study is more than the 68% and 66% rates of complete block at 30 minutes using the double-injection Winnie technique, the parasacral approach, or other single-injection techniques (1,10,12,13).

In addition to needle to nerve distance, the other major determinant of latency and success of SNB is LA dose. A wide range of latencies to complete SNB with single-injection (20–30 mL of LA) techniques has been reported using both intermediate and long-acting drugs. In two separate studies, onset time to surgical block using 20 mL of 0.5% and 0.75% ropivacaine and 0.5% levobupivacaine was 25 to 30 minutes (range, 5–60 minutes) (1,13). Although the total dose of LA used in the present study (0.4 mL/kg of 0.625% levobupivacaine) was larger than in aforementioned studies, there was still a wide variability in the time to complete block among the different EMR groups. Although not tested, this suggests that a complete block of the sciatic nerve may be possible using a smaller dose of LA by ensuring an optimal EMR before LA injection.

A limitation of the present study was the use of rescue popliteal SNB at 30 minutes in patients who had no block (sensory-motor score = 0) in the distribution of either the tibial or peroneal nerves. Patients who demonstrated grade 1 sensory-motor block at 30 minutes were followed up for an additional 30 minutes to further assess block progression. The decision to follow this regimen was based on our experience that the absence of block onset in one neural component at 30 minutes is unlikely to result in complete SNB for surgery by 60 minutes.

In conclusion, the data from the present study support our hypothesis that EMR of I elicited at 0.2–0.4mA during neurostimulation-assisted single-injection SNB representing stimulation of both tibial and peroneal branches of the sciatic nerve is superior to an EMR of PF, DF, and E with respect to the latency and success of SNB.

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