Obturator nerve block (ONB) has been commonly used to treat hip joint pain and relieve adductor muscle spasms associated with central neurologic problems. ONB is also frequently used in urologic surgery to suppress the obturator reflex during transurethral resection of the lateral bladder wall. If resectors directly stimulate the obturator nerve as it passes in close proximity to the bladder wall, sudden, violent adductor muscle contraction can result. This is potentially dangerous, increasing the risk of serious complications such as bladder wall perforation, vessel laceration, incomplete tumor resection, and obturator hematomas [1,2]. General anesthesia, neuromuscular blocking drugs [3,4] as well as subarachnoid and epidural anesthesia have not demonstrated to be beneficial for the prevention of this contraction of the adductor muscle. Selective ONB is considered as the safest and most effective method to resolve these problems. Because of a lack of clear anatomic landmarks, the block complexity, patients' discomforts and inconsistent results, the classical method (Labat's technique) remained forgotten until 1967, when this method was simplified [5]. Presently, this classical pubic approach is still commonly used for ONB using a nerve stimulator, although the success rate varies (60.5-91.7%) [6,7]. In 1993, the interadductor approach was described, in which a needle is inserted behind the upper end of the adductor longus muscle [8]. Recently, a simple and reliable new method (the inguinal approach) was described by Choquet et al. [9], in which the needle is inserted at the mid-point of the femoral arterial pulse and the inner side of the adductor longus muscle on the inguinal crease.

This randomized clinical study was undertaken to compare the success rate of the inguinal and pubic approach in ONB with spinal anesthesia for transurethral resection of bladder lateral wall masses.

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After approval from our institutional review board and the written informed consent from patients was obtained, 102 patients scheduled to transurethral resection of bladder lateral wall masses under spinal anesthesia who required ONB were included in this study. Patients with advanced cardiac and respiratory insufficiency, allergy to local anesthetics, preexisting neurologic deficits, prior operations involving the hip and the inguinal region and bleeding tendency were excluded. ONB was perforemed in the patients who were asked ONB by urologist because of invasive lateral bladder wall tumor. And these patients were randomly allocated to the conventional pubic approach group (Group P, n = 51) or new inguinal approach group (Group I, n = 51). On arrival to the operating room, standard anesthetic monitors were applied and 0.9% normal saline 300 ml was given intravenously. A spinal block was performed with a 25 G Quincke needle at the L3-4 or L4-5 interspace in a lateral decubitus position. After confirming free-flow and clear CSF, 0.5% hyperbaric bupivacaine (8 mg) was administered. In a supine position, sympathetic and sensory blocks were checked with an alcohol swab and pin-prick test. When the sensory level block reached above T12, ONB was performed according to group assignment. All ONB were performed by 2 investigators that alternated procedures and they were not involved in further treatment of those patients. ONB was performed with a 10 cm Teflon-insulated needle (21G Stimuplex® A, B. Braun Melsungen AG, Germany) using a nerve stimulator (B. Braun Melsungen. AG, Germany). Nerve stimulation was applied using a current of 0.5 mA at 1 Hz only.

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One patient in the group P was excluded from the analysis of the study due to operative procedure changes. There were no significant differences in patient characteristics, local anesthetic dosage and sensory block level between the two groups (Table 1). The success rate of ONB was higher in group I compared to group P (96.1% vs. 84.0%, P = 0.046) and the frequency of needle attempts was also lower in group I than in group P (P = 0.01: Table 2). Mild obturator signs occurred during the operation in 3 of the 10 failed cases (2/8 vs. 1/2), but the procedures were completed without general anesthesia. None of the patients with successful ONB showed obturator signs during operation. There were no ONB-related complications such as hematoma, postoperative sensory or motor changes.

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We compared the success rate of an inguinal approach and a pubic approach for obturator block with spinal anesthesia for transurethral resection of bladder lateral wall masses.

The obturator nerve originates from the lumbar plexus of L2 to L4 and contains both motor and sensory nerve fivers. It runs close to the prostatic urethra, bladder neck and inferolateral bladder wall within the pelvic cavity [10,11]. When the bladder is distended with irrigation fluid during transurethral procedure, the obturator nerve becomes very close to the lateral wall of the bladder and electrical currents can stimulate the obturator nerve accompanied by activation of adductor muscles of the thigh. Motor neurons carry Aα fibers which are thicker in diameter. For an effective obturator block, the local anesthetic concentration must exceed that for pain and temperature sensations, which are carried by thin Aδ and C fiber, by 2-fold. Thus the concentration of lidocaine used must be greater than 1% for an effective motor block [12]. Lidocaine is commonly used since it has rapid onset and can last up to 40 minutes, making it suitable for blocking obturator signs at the usual dose of 10 to 20 ml [13].

Although various imaging techniques have been used in an effort to increase the success rate of ONB [14-18], a blind approach using nerve stimulators is still a common technique. The success rate of ONB with the classical pubic method varies from 60.5% to 91.7% [6,7]. Even in the most experienced hands, this block can be missed. For the pubic approach, the pubic tubercle is the most definitive landmark. Sometimes the identification of the pubic tubercle is difficult in obese patients or patients with a blunt pubic bone. When identification of the tubercle is difficult, the needle may pass above the pubic ramus and may cause damage to the surrounding structures (bladder, rectum, spermatic cord). Furthermore, this approach is performed in a highly vascularized region [19]. Although no one showed ONB-related complications in our study, careful approach is mandatory with this technique to avoid the damage to surrounding structures and vessels. In comparison, the inguinal approach is performed at a distance from the pelvis and large vessels, thus this technique may minimize the risk of the aforementioned complications and allows compression in the case of hematoma [9]. Even though this inguinal approach has the limitation of reaching obturator branches contributing to hip joint innervation which occurs frequently before entry of the nerve into the thigh, it generally has little effect on the procedure during TURB [9].

In our study, we defined a successful block when any adductor muscle contraction occurred within three needle attempts and muscle contraction did not occur during the operation. The success rate of ONB was significantly higher with the inguinal approach than with the pubic approach (96.1% vs. 84.0%, P = 0.046). The safety and clinical applicability of ultrasound-guided ONB is frequently reported, although there is a study where the success rate with ultrasound-guided ONB was similar to that reported in studies using nerve stimulation [20]. However, ONB using nerve stimulation was the most used technique until ultrasound was commonly available in the clinical setting.

There are limitations to this study. First, this was a relatively small sample size. In our preliminary study, we expected the difference in means for success to be about 30%. However, we found that the difference of success between the two approaches was 12.1% (P = 0.046). Thus, further study is needed to increase the reliability of this finding.

In conclusion, the inguinal approach for ONB seems to be technically easier as it offers certain anatomical advantages in comparison with the pubic approach, although both approaches can provide adequate blockade for the completion of the transurethral resection of lateral bladder wall tumors.

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