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*Department of Anesthesiology and Intensive Care Medicine, Private Hospital Group of Center Alsace, Colmar, France;
Department of Anesthesiology and Intensive Care Medicine, Nancy University Hospitals, Nancy, France;
Department of Anesthesiology, Hôpital de la Conception, Marseille, France
Address correspondence and reprint requests to Professor Hervé Bouaziz Service d’anesthésie-réanimation chirurgicale Hôpitaux de ville, CHU de Nancy 29, Avenue du Maréchal-de-Lattre-de-Tassigny 54035 Nancy Cedex, France. Address e-mail to h.bouaziz{at}chu-nancy.fr
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Abstract |
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IMPLICATIONS: The aim of this study was to objectively evaluate the efficacy of obturator nerve anesthesia after a parasacral block. These findings demonstrate the unreliability of parasacral block to achieve obturator nerve anesthesia. A selective obturator nerve block should be considered in the clinical setting when such a block is desirable.
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Introduction |
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The aim of our study was to objectively evaluate the efficacy of the obturator nerve block after a parasacral approach to the sciatic nerve block.
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Methods |
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On arrival in the anesthesia induction room, standard monitoring was established (pulse oximetry, electrocardiography, and automated sphygmomanometer; Cardiocap II; Datex, Helsinki, Finland). A 20-gauge cannula was placed in a peripheral vein in the arm and oxygen was delivered via a Venturi face mask at a rate of 3 L/min.
All patients were tested for normal sensory function in the lower limb in question and a baseline measurement of adductor muscle strength was performed using a mercury sphygmomanometer as described by Lang et al. (3). Briefly, patients were asked to extend the hips and knees, then to squeeze a blood pressure cuff, previously inflated to 40 mm Hg, between their knees. The maximal sustained pressure generated was recorded as an index of adductor muscle strength (maximum sustained pressure = pressure generated on the mercury sphygmomanometer – 40 mm Hg).
A parasacral sciatic nerve block was then performed by experienced anesthesiologists using the technique described by Mansour (1). With the patient in semi-prone position, and the side to be blocked uppermost, the dependent limb was straightened and the opposite was flexed at both hip and knee. A line was drawn between the posterior superior iliac spine and the lowest point of the ischial tuberosity. Six cm inferior to the posterior superior iliac spine along this line, a 100-mm 21-gauge insulated short bevel needle (Stimuplex® A 100, B Braun, Melsungen, Germany) was introduced and advanced perpendicular to all planes. Using a nerve stimulator (Stimuplex HNS 11, B Braun) we accepted the first elicited foot movement (eversion, inversion, dorsi or plantar flexion) as evidence of proximity to the sciatic roots. The stimulation frequency was set at 2 Hz and the duration of stimulation at 0.1 ms. The intensity of the stimulating current, initially set to deliver 2 mA, was gradually decreased to 
0.5 mA after the appropriate muscular response was observed. Similar to the study by Morris et al. (2), increments of 0.75% ropivacaine (up to a total of 30 mL) were injected through a stationary needle.
Blocks were evaluated every 5 min in the first 30 min from the time of withdrawal of the needle. Patients were not able to watch the investigator performing the sensory or motor testing. Sensory block was evaluated in the three regions of nerve distribution (tibial, peroneal, and posterior cutaneous) using a temperature discrimination test with modified alcohol (Cooper®; Coopération Pharmaceutique Francaise, Melun, France) as follows: 0 = normal sensation, 1 = blunted sensation, 2 = no perception. Thus, the onset time of sensory block was defined as the time elapsed from the removal of the stimulating needle (end of injection) until the complete loss of sensory function in a specific territory. Because of its well-described variability (4), the cutaneous distribution area of the obturator nerve was not examined. Complete sensory block was considered when temperature discrimination had a score of two in the three cutaneous distributions of the sciatic nerve tested.
The objective assessment of adductor motor strength was performed at the same time points. Patients who did not have a complete sensory block in the sciatic nerve distribution at 30 min after the end of the parasacral block were excluded from the study at this time.
After this initial 30 min evaluation period, a selective obturator nerve block was performed as follows: with the patient supine and legs slightly abducted, a 50-mm 22-gauge short bevel insulated needle (Stimuplex® A 50, B Braun) was inserted almost perpendicularly to the skin, 2 cm caudal and 2 cm lateral to the pubic tubercle. The needle was advanced until the adductor muscles of the thigh twitched at 0.5 mA (0.1 ms, 2 Hz) and 7 mL of 0.75% ropivacaine was injected. Adductor muscle strength was evaluated every 5 min for a further 15-min period.
Finally, a femoral nerve block was performed with 10 mL 0.75% ropivacaine for complete anesthesia of the lower limb. Patients were then asked to score discomfort induced by each of the individual blocks using a visual analog scale (VAS, 0 = no discomfort, 100 = worst imaginable discomfort).
Side effects and their management were defined as follows: a) hypotension: a decrease of more than 25% of mean systemic arterial blood pressure, managed by increments of ephedrine 3 mg IV every 2 min; b) bradycardia: heart rate <45 bpm, managed by a 0.5 mg IV bolus of atropine. Had anesthesia of the lower limb been insufficient, general anesthesia would have been delivered and the patient would have been excluded from the study.
Based on previous studies, it was calculated that a minimum of 30 patients would be required to have an 80% power of detecting a 50% reduction in the adductor muscle strength at a significance level of 0.05.
Onset time of sensory blocks and discomfort scores for each individual block were compared using nonparametric tests (Wilcoxon’s and Mann-Whitney U-tests when appropriate). Repeated comparisons of adductor muscle strength were performed using a one-way analysis of variance followed by Fisher’s protected least significant difference test when statistical significance was detected. Data are expressed as mean ± SD unless otherwise specified. P < 0.05 was considered significant.
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Results |
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The sciatic roots were identified in all patients. The depth at which acceptable motor response was elicited was 84 ± 14 mm. The first evoked motor response was in the tibial component of the sciatic nerve in 17 patients and in the peroneal component in 19 patients respectively. The minimal nerve stimulator intensity considered to inject the local anesthetic solution was 0.214 ± 0.03 mA.
Five patients who did not achieve complete sensory block 30 min after the parasacral sciatic nerve block were excluded from the study. When considering the entire cutaneous distribution of the sciatic nerve, complete sensory block at 30 min was achieved with a success rate of 89% (31 of 36 patients). The remainder had a sensory block mean onset time of 18 ± 9 min after a parasacral block. Time required to obtain a loss of cold sensation in the tibial, peroneal, and posterior cutaneous nerve distribution was 14 ± 8, 13 ± 8, and 12 ± 8 min, respectively.
Adductor muscle strength decreased by 11.3% ± 7% compared with baseline values 30 min after completion of the parasacral sciatic nerve block (85 ± 24 versus 97 ± 28 mm Hg, P = 0.002) (Fig. 1). In 10 patients adductor muscle strength was absolutely unchanged compared with the baseline 30 min after the parasacral block despite complete sensory block in the cutaneous distribution of the sciatic nerve.
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Cadaver Experiment
The second part of this study consisted of an anatomical experiment on two fresh cadavers in which the parasacral blockade was simulated on both sides using the previously described landmarks. As a 16-gauge Tuohy needle was advanced, a microdissection was performed until the tip of the needle reached the sacral plexus. The different anatomical layers were closed in inverse order and 30 mL of colored latex solution was injected. Thereafter, the pelvis was dissected to examine the spread of the latex solution in relation to the obturator nerve.
Injection of 30 mL of colored latex into cadavers resulted in spread of the injectate restricted to the sacral plexus; the obturator nerve was spared in all cases (Fig. 2). No leak between the layers was noted.
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Discussion |
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The obturator nerve is a branch of the lumbar plexus formed within the substance of the psoas muscle from the anterior division of the second, third and fourth lumbar nerves (anterior primary rami). It is the nerve of the adductor compartment of the thigh, which it reaches by piercing the medial border of the psoas and passing straight along the sidewall of the pelvis to the obturator foramen. Within the pelvis it is contained in the extraperitoneal space, delimited by the parietal peritoneum and the pelvic fascia; thus, unlike the sacral plexus, the obturator nerve is contained within the pelvic fascia.
After entering the thigh through the obturator foramen, it divides into anterior and posterior divisions. Both carry motor fibers to the adductor muscles of the thigh but only the anterior division provides any cutaneous innervation. The posterior division, however, is important in that it contributes to the innervation of the knee joint. The obturator nerve supplies the obturator externus and the adductor muscles of the thigh, has branches extending to the hip and knee joints, and has a variable cutaneous distribution to the medial aspect of the thigh. When present, the cutaneous branch unites with branches of the saphenous and medial femoral cutaneous nerves to form the subsartorial plexus and contributes to the innervation of the skin over the distal two-thirds of the medial side of the thigh (5).
Infrequent use of lower limb peripheral blockade has been reported in several surveys (6,7). Suggested possible explanations for this include the facts that the blocks are technically demanding and multiple injections are required to anesthetize the entire extremity. However, advantages of these methods include preservation of contralateral sympathetic tone resulting in greater hemodynamic stability and postoperative analgesia of long duration, as well as providing the practitioner with an alternative method of anesthesia for patients in whom neuraxial anesthesia is technically difficult or contraindicated. Classically, lower limb anesthesia can be achieved with a combination of three peripheral nerve blocks: femoral, obturator, and sciatic nerve blockade. Muscle contractions and needle movements experienced during multiple nerve locations may increase patient discomfort and elicit pain (8). Therefore, the possibility of blocking the lower limb with fewer injections seems attractive from both the patient’s and the anesthesiologist’s point of view.
Peripheral nerve block of the entire lower limb may be achieved using a combination of the posterior approach to the lumbar plexus (psoas compartment block) and a proximal approach to the sciatic nerve. However, serious complications have been reported after the posterior approach to the lumbar plexus (9).
The combination of a femoral and parasacral sciatic nerve block would seem a feasible alternative, given the results of Morris et al. (2) (i.e., obturator nerve blockade in 93% of patients) could be reproduced.
Although controversial because of the major contribution of the obturator nerve to the innervation of the knee joint, effective obturator nerve anesthesia may be important for both knee surgery (10) and postoperative analgesia after total knee replacement (11).
Because of variability in the cutaneous distribution of the obturator nerve, several authors consider paralysis of the adductor muscles of the thigh as the sole reliable indicator of a successful obturator nerve block (3,12,13).
Clinical studies in which anesthesia of the obturator nerve was assessed by testing motor function clearly showed that the three-in-one block usually spares this nerve (11). The proximity of the sacral plexus to the obturator nerve in the pelvis was the reason why Morris et al. (2) decided to evaluate the extent of anesthesia to the obturator nerve after a parasacral block. They demonstrated, using a subjective assessment of adductor muscle strength, anesthesia of the obturator nerve in 73% to 93% of cases. One strength of the present study is the use of an objective assessment of adductor muscle strength. Our data suggest that the obturator nerve is sporadically affected by the parasacral sciatic nerve block. A possible explanation of these conflicting results may lie with the fact that the sacral plexus itself could be responsible for 34% of loss of adductor muscle strength (14); this could have been mistaken for obturator block in the study by Morris et al. (2). As the adductor magnus muscle derives its nerve supply from both the lumbar plexus (i.e., obturator nerve) and the sacral plexus (i.e., sciatic nerve), a partial adductor weakness may be observed after a sciatic or a sacral plexus block. The methodology we used clearly demonstrated a minimal decrease of adductor muscle strength attributable to the sacral plexus within the first 30 min, followed by a further dramatic decrease after the selective obturator nerve block.
Although the 30 min time point after a parasacral block with 0.75% ropivacaine was arbitrarily chosen as a cutoff point for evaluation of sciatic nerve anesthesia, there are data in the literature showing a shorter onset time (14 ± 17 min, n = 15) of sensory block after sciatic nerve block with 0.75% ropivacaine (15). Furthermore, the onset time of motor block (14 ± 8 min) preceded that of sensory block in this setting, suggesting that sensory evaluation is a reliable indicator of sciatic nerve anesthesia (15).
Although it may be difficult to extrapolate the results after only four blocks in cadavers, injection of the same volume of colored latex resulted in spread of the injectate to the sacral plexus but not to the obturator nerve. This may be explained by the distance from the sacral plexus to the obturator nerve and is also consistent with the presence of the pelvic fascia separating these two neural structures.
Our success rate with the parasacral approach (89%), although less than the 97% of success reported by Morris et al. (2), reflects the clinical practice for most peripheral nerve blocks. As the sciatic nerve block is often associated with one or two other blocks for anesthesia of the entire limb, the feasibility of smaller volumes (20 mL) should be evaluated. Thus, frequent success using smaller volumes would justify this single injection new approach to the sciatic nerve in terms of better patient acceptance compared with more distal, possibly multistimulation, techniques (16). Recently, Cuvillon et al. (17) demonstrated that the parasacral approach using 20 mL 0.75% ropivacaine produced similar success rates compared to double injection, known to result in a more rapid onset and increased success rate compared with single injection (18).
It has been documented that, despite an overall frequent success rate (93%), patients receiving combined sciatic and femoral nerve blocks complained of more discomfort during the anesthetic procedure and showed a poorer acceptance than those receiving brachial plexus anesthesia (8). VAS pain scores reported in the present study during the parasacral sciatic nerve block were higher (26 ± 19) than those reported by Morris et al. (2) (13 ± 11) for the same procedure. However, Morris et al. (2) used incremental boluses of alfentanil to facilitate positioning and delivery of the block, whereas some of our patients received a light sedative premedication 2 hours before arrival to the operating room. A combination of midazolam and sufentanil administered 5 minutes before the block probably would have been more efficient, as demonstrated for upper limb block (19).
In conclusion, our study findings demonstrate the unreliability of parasacral sciatic nerve blocks in achieving a conduction block of the obturator nerve. Given its important distribution area (innervation of the knee), a selective obturator nerve block should be considered in the clinical setting where such a block is desirable.
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Acknowledgments |
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This article has been cited by other articles:
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  N. Valade, J. Ripart, E. Nouvellon, P. Cuvillon, D. Prat-Pradal, J.-Y. Lefrant, and J.-E. de La Coussaye Does Sciatic Parasacral Injection Spread to the Obturator Nerve? An Anatomic Study Anesth. Analg., February 1, 2008; 106(2): 664 - 667. [Abstract] [Full Text] [PDF]
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