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Analgesia: Research Report

Parasacral Sciatic Nerve Block: Does the Elicited Motor Response Predict the Success Rate?

Hagon, Bénédicte S. MD; Itani, Omar MD; Bidgoli, Jawad Hosseini MD; Van der Linden, Philippe J. MD, PhD

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doi: 10.1213/01.ane.0000266437.41544.b3
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Several approaches to sciatic nerve block have been described. The most frequently performed are probably the Labat’s classical posterior approach (1) and its modification by Winnie et al. (2). A double injection technique, involving identification of both the tibial and peroneal nerves, has been shown to provide a more rapid onset of the block and a better success rate than a single injection technique (3). However, compared with single injection, double injection may increase the incidence of neurologic complications and patient discomfort (4). Mansour described another posterior approach to the sciatic nerve, the “parasacral approach” (5), which provides a high success rate with a single injection (6).

When performing a sciatic nerve block using the parasacral approach, both the tibial (plantar flexion of the foot and/or toes) or the peroneal (dorsiflexion of the foot and/or extension of toes) motor response may be elicited. Injection of local anesthetic after elicitation of one or the other motor response might influence the success rate of the block. We tested this hypothesis in a prospective, randomized, double-blind study in adult patients undergoing elective lower limb surgery.


After approval of the ethics committee of the CHU Brugmann, we performed a prospective, randomized, double-blind, single-center study. The study included two equal groups totaling 40 patients who gave written informed consent.

Forty sealed envelopes were prepared and contained the assigned group by randomization. Patients were randomized in two groups according to the specific nerve motor response elicited when performing the sciatic nerve block using Mansour’s technique (5): the tibial motor response (plantar flexion of the foot and/or toes) in the first group (tibial group; n = 20) and the common peroneal motor response (dorsiflexion of the foot and/or extension of the toes) in the second group (peroneal group; n = 20).

Inclusion criteria were patients older than 18 yr, weighing more than 50 kg, ASA physical status I–III, and undergoing orthopedic surgery of the lower limb under regional anesthesia. Exclusion criteria were patient refusal, patients with a contraindication for regional anesthesia (neurological or neuromuscular disease, coagulation disorders or drug-induced anticoagulation, skin infection at the site of puncture, allergy to local anesthetics, and psychiatric disorders), and pregnancy.

All patients received alprazolam 0.5 mg by mouth 1 h before arriving in the operating room. Standardized monitoring included three-lead electrocardiogram, noninvasive arterial blood pressure, and pulse oximetry. Before performance of the nerve block, IV access was established. All blocks were performed by a single anesthesiologist. The patient was positioned in the lateral decubitus position, with the leg to be blocked uppermost and rolled forward with the knee flexed at a 90 ° angle (Sim’s position). After identifying the posterior superior iliac spine and the lowest point of the ischial tuberosity, a line was drawn between these two points. A mark was made on this line, at 6 cm inferior to the posterior superior iliac spine. After skin disinfection (iodine alcohol 1%), local infiltration of the skin was performed with 2–3 mL of lidocaine 1%, and a 100 mm, 21-gauge (stimuplex A 100, B Braun, Mensulgen, Germany) short-beveled stimulating needle attached to a nerve stimulator (HNS11 B Braun) was inserted perpendicularly to the skin and advanced until either plantar flexion or dorsiflexion of the foot and/or toes was obtained. If the motor response did not correspond to the randomization, the needle was then redirected either medially to elicit plantar flexion (tibial nerve response) or laterally to elicit the dorsiflexion (peroneal nerve response).

The stimulating current was set initially at 2 mA (frequency 1 Hz; time 100 μs). When the desired (according to the randomization) motor response was obtained, the intensity of the stimulating current was gradually decreased to a value of 0.3–0.5 mA. The needle was considered to be adequately positioned when the stimulating current was 0.5 mA or less. A 20 mL solution containing 10 mL lidocaine 2%, adrenaline 1/80,000, and 10 mL ropivacaine 0.75% was slowly injected through the needle, with careful intermittent aspirations every 5 mL. A femoral nerve block with 20 mL of the same solution was also performed in each patient to perform the surgery.

After the procedure, a blinded anesthesiologist, responsible for evaluating the block, analyzed the progression and the intensity of the sensory and motor block every 5 min for 30 min. The intensity of the sensory block of each nerve (tibial nerve, sole of the foot; common peroneal nerve, dorsum of the foot for the superficial peroneal nerve and the region between the two first toes for the deep peroneal nerve) was classified as follows: normal sensation = 1; loss of cold sensation = 2; loss of sensation to manual pinching = 3; loss of tactile sensation = 4.

The intensity of the motor block of each nerve (tibial nerve: plantar flexion of the big toe; peroneal common nerve: dorsiflexion of the big toe) was classified as follows: normal force = 1; decreased force against resistance = 2; decreased force against gravity = 3; no movement = 4.

The sensory block was considered as adequate if the patient did not experience pain from the pinprick test in the corresponding distribution territory (sensory block ≥3). The motor block was considered successful if no movement was observed (motor block = 4). If the sensory and the motor block did not reach the predetermined levels within 30 min after injection, the technique was considered as having failed.

Complications during the performance of the block (paresthesias, pain at injection site, or aspiration of blood) were carefully documented.

Time to perform the block (time between the insertion of the needle and the end of local anesthetic injection), and minimal intensity current (intensity of the current when the needle was considered to be adequately positioned) were collected.

Sample size was calculated based on a success rate of 66% (7). A 50% decrease in success rate was considered clinically significant. For a power of 0.8 and a statistical significance of 0.05, a sample size of 34 patients was calculated to be appropriate.

After having performed the first 26 randomized blocks, the blinded anesthesiologist noticed an abnormally high incidence of failure (13 of 26). After consultation with the scientific committee of the anesthetic department, a statistical analysis was performed to determine the relationship between the observed failure score and the randomization. At this time, 14 tibial group patients and 12 peroneal group patients were included in the study. A significant statistical difference in success rate between the two groups was recorded. Therefore, the department’s scientific committee decided, with the approval of the ethics committee, to discontinue the study.

The different variables were compared between groups using the Student’s t-test for continuous variables or the fisher exact test for nominal data. A P < 0.05 was considered significant.


There was no difference in any of the demographic and surgical variables between groups (Table 1). Time to perform the block was not significantly different between groups (tibial group, 180 ± 89 s; peroneal group, 222 ± 63 s).

Table 1
Table 1:
Demographic and Surgical Data

There was no significant difference in the level of minimal stimulation and the success rate of the block (successful block, 0.42 ± 0.05 mA; failed block, 0.41 ± 0.07 mA) (Table 2).

Table 2
Table 2:
Minimal Stimulation (mA) and Success Rate of the Block

Thirty minutes after completion, successful block was obtained in 11 of the 14 patients in the tibial group and 2 of the 12 patients in the peroneal group (P = 0.002). In one patient in the tibial group, no motor response was elicited.

The percentage of successful complete sensory (≥3) or motor (= 4) blocks for the different nerve distributions are presented in Figure 1. The number of successful sensory and motor blocks was higher after elicited tibial motor response than after peroneal motor response.

Figure 1.
Figure 1.:
Successful complete sensory and motor blocks for the different nerve distributions in each group.

Performance of the sciatic block was not associated with any adverse event. Both motor and sensory block resolved completely 24 h after injection.


The present randomized, double-blind study demonstrated that the success rate of sciatic nerve block using the parasacral approach is significantly influenced by the elicited motor response. Eliciting the tibial motor response is associated with a 79% success rate, whereas eliciting a common peroneal response is associated with a 17% success rate. Various factors markedly affect the success rate of peripheral nerve blocks, such as the concentration and the volume of the injected solution (8), the use of additives (9), the type of approach to the sciatic nerve (10), or the intensity of the current at which peripheral nerve stimulation is achieved (11,12). In the present study, all these factors remained the same in the two groups, indicating that the type of evoked motor response clearly explains the observed results. Our results are in contrast with those of other studies, which reported that the success rate of sciatic block using the Mansour technique is not influenced by the elicited motor response (7,13,14). These differences could be related to different methodological factors, such as the volume of local anesthetic used, the criteria used to define the success rate of the block, and the combination with a lumbar plexus block through a posterior approach, which may have contributed to partial sciatic nerve block. In the Gaertner et al. study (13), the injection of local anesthetic through the needle was completed by a second injection of a small dose of lidocaine 2% with epinephrine through a catheter introduced 2 cm beyond the tip of the needle. This second injection, together with the frequent combination of a lumbar plexus block, may have masked a difference in the success rate of the parasacral block according to the evoked motor response. In the Cuvillon et al. study (7), no difference in success rate was observed when either the tibial or peroneal nerve was injected. However, while using the same volume of local anesthetic (20 mL), their success rate with tibial nerve stimulation was only 66% compared with 79% in our study. In the Ripart et al. study (14), the use of a higher volume of local anesthetic (27 ± 4 mL), in combination with a lumbar plexus block in some cases, may explain their high success rate and its absence of difference in regards to the elicited motor response. Finally, it must be emphasized that none of these studies was specifically designed to evaluate the influence of the elicited motor response on the success rate of the parasacral block, and that their observations were made ad posteriori.

Our results are in accordance with those of Taboada et al., who found that the elicited motor response, when performing a sciatic nerve block, could influence the onset time and the success rate of the block. In their first study (15), using the lateral approach to the sciatic nerve in the popliteal fossa, they demonstrated that the total number of nerves blocked (deep and superficial peroneal, posterior tibial, and sural nerves) after elicited plantar flexion was significantly more than after elicited dorsiflexion. In their second study (16), using Labat’s classic approach to sciatic nerve block, they observed that elicited plantar flexion of the foot predicts a more frequent success rate than elicited dorsiflexion. At either level (lateral popliteal and Labat’ posterior approaches), the size of the two sciatic nerve components while in close proximity, and the thickness of the epineurium make it harder for the local anesthetic to penetrate the larger component of the sciatic nerve when the needle tip is located on the smaller nerve. Increasing the success rate will require a larger volume of local anesthetic or the stimulation of the larger component of the sciatic nerve, i.e., the tibial nerve. Our results demonstrated that this is true at a more proximal level of the sciatic nerve. According to the results of the study of Gaertner et al. (13), injection of the local anesthetic through a catheter positioned 2 cm beyond the tip of the needle might have eliminated the problem by improving the diffusion of the solution.

The decreased sensory and motor blockade with peroneal nerve stimulation, compared with tibial nerve stimulation, is difficult to explain. It may have been related to the position of the needle when a peroneal motor response was elicited; the needle may have been at the lateral aspect of the peroneal nerve and a major portion of the injected local anesthetic diffused away from the peroneal and tibial nerves. This hypothesis, however, needs to be confirmed. It must be noted that similar results were observed by Taboada et al. (16), using Labat’s classic posterior approach to the sciatic nerve.

In conclusion, we demonstrated that, when a small volume of local anesthetic was used in parasacral sciatic nerve block, a tibial motor response resulted in a higher success rate compared with a peroneal motor response.


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