Thigh adductor muscle contraction resulting from electrical stimulation of the obturator nerve can occur during transurethral resection (TUR) of tumors located in the inferolateral wall of the bladder.1 This can lead to complications such as bladder perforation, dissemination of cancer cells,2–4 and bleeding.5 To prevent this undesirable muscle response, we commonly perform an obturator nerve block1,2,6 or administer a muscle relaxant before tumor resection.4,7
However, it is possible that an obturator nerve block may fail to suppress the adductor response (failed block). The failure of obturator nerve block is only recognized after thigh adductor muscle contraction is elicited by electric cautery. This is dangerous because the high-intensity electric cautery current produces vigorous leg movements while the bladder wall is being cut, which can easily lead to perforation of the bladder. A previous study8 reported no thigh adductor muscle contraction in 51 of 114 (45%) TUR patients with inferolateral bladder tumors, even without obturator nerve block. However, it is a common practice to administer obturator nerve block or a muscle relaxant to all patients who undergo TUR for inferolateral bladder tumors as a safety precaution. Conversely, there may be patients who do not require a block, because resection does not uniformly stimulate a response.
To address these issues, we have devised a method to deliver several single-twitch electrical stimuli generated by a neuromuscular monitoring device to a specific area of the bladder wall via a resectoscope. We call this method trans-resectoscope stimulation. One of the authors (HI) first used this method in clinical practice in 2008.
The primary aim of this study was to evaluate whether preoperative trans-resectoscope stimulation is useful in predicting the need to block contraction of the thigh adductor muscle (i.e., the need for obturator nerve block or muscle relaxant). The secondary aim was to test whether omitting obturator nerve block or muscle relaxant in patients with no muscle contraction from preoperative trans-resectoscope stimulation successfully predicted lack of thigh adductor muscle contraction during surgery.
This study was conducted in accordance with the Declaration of Helsinki. After obtaining approval from the IRBs of Yokohama City University Hospital (Yokohama, Japan) and Sagamihara Kyodo Hospital (Kanagawa Prefecture, Japan) and written informed patient consent, patients with ASA physical status I and II who were scheduled to undergo TUR for bladder tumor between January 2011 and February 2012 were included in the study. Patients with a known allergy to local anesthetics or other medications used in this study were excluded. We also excluded patients with metastatic disease, patients with neuromuscular disease, and those who underwent segmental cystectomy.
Patients were monitored by continuous electrocardiography, noninvasive arterial blood pressure measurement, and pulse oximetry after entering the operating room. The choice of general or spinal anesthesia was left to the discretion of the assigned anesthesiologist. In the case of spinal anesthesia, 0.5% hyperbaric bupivacaine (1.5–2.0 mL) was injected at the third or fourth lumbar interspace with the patient in the lateral decubitus position. The patient was immediately returned to the supine position, and an anesthesia level of T10 or higher was confirmed by loss of cold sensation. In the case of general anesthesia, propofol (1.5–2.0 mg/kg) and fentanyl (50–100 μg) were administered IV, and a laryngeal mask airway (LMA) was inserted without the use of muscle relaxants. Anesthesia was maintained with inhaled sevoflurane and IV fentanyl. In both cases, after completion of anesthesia induction, patients were placed in the lithotomy position without an obturator nerve block, regardless of the tumor location.
The trans-resectoscope stimulation method consisted of stimulating the inner wall of the bladder by a single-twitch electrical current generated by an neuromuscular monitoring device (TOF-Guard; Organon Teknika NV, Turnhout, Belgium). This stimulating current was delivered using the resectoscope loop which was later used for the surgery. For this purpose, the cathode of the neuromuscular monitoring device was connected to the electric cable of the resectoscope that would normally be connected to the high-frequency electric cautery current generator, and the anode was placed on the patient’s skin near the electric cautery counter electrode plate (Fig. 1).
The purpose of phase 1 was to calculate the adequate neuromuscular monitoring device’s stimulating current. In 7 consecutive patients, the acceleration sensor of the neuromuscular monitoring device was attached to the left knee to measure the acceleration of left leg adduction when left thigh adductor muscle contraction occurred. After insertion of the resectoscope into the patient’s bladder and filling the bladder with D-sorbitol irrigation fluid, we performed trans-resectoscope stimulation to the left inferolateral wall of the bladder with a current intensity of 60 mA, the maximum level of our neuromuscular monitoring device. If left thigh adductor muscle contraction occurred, we gradually decreased the electrical current from 60 to 10 mA in decrements of 10 mA. Acceleration of the left knee at each electrical intensity level (units: percentage of maximum response) was measured 3 times, and the mean value was calculated at each level.
This study was performed after completion of phase 1 on a separate group of 45 consecutive patients who were scheduled for TUR for bladder tumors for which urologists requested obturator nerve block. After the attending urologist inserted the resectoscope into the bladder and filled it with D-sorbitol irrigation fluid, we performed several trans-resectoscope stimulations (50 mA) to the area of planned resection. If no response to any of these stimulations was observed (i.e., the trans-resectoscope stimulation result was negative), the urologist was allowed to proceed with tumor resection without obturator nerve block or administration of a muscle relaxant. If 1 or more of these trans-resectoscope stimulations was positive, we performed obturator nerve block or administered a muscle relaxant in the case of general anesthesia.
Obturator nerve block was performed under ultrasound guidance combined with nerve stimulation while the patient was in the lithotomy position. We applied a modified Fujiwara9,10 approach for our patients. The adductor longus muscle was identified by palpation. A high-frequency ultrasound probe was placed just proximal to the adductor longus muscle, and the adductor longus, brevis, and magnus muscles were identified in the ultrasound image. Then, we advanced the needle with nerve stimulation at a current intensity of 1.0 mA (2 Hz) to the fascia between the adductor longus and brevis muscles. When an adductor twitch was elicited, we injected 5 to 10 mL of 0.375% ropivacaine. We also advanced the needle to the fascia between the adductor brevis and magnus muscles and injected 5 to 10 mL of 0.375% ropivacaine when an adductor twitch was elicited. If the trans-resectoscope stimulation result was still positive 5 to 10 minutes after obturator nerve block, we repeated the block or switched to general anesthesia with a muscle relaxant. Tumor resection was performed only after we confirmed a negative trans-resectoscope stimulation result in the area of resection in all patients. The time spent on trans-resectoscope stimulation was defined as the period between the time when the neuromuscular monitoring device was attached to the cable of the resectoscope and the time when the anesthesiologist made a decision regarding whether to block contraction of the adductor muscle. The positive or negative initial trans-resectoscope stimulation result, the time spent on trans-resectoscope stimulation to the area of resection, and thigh adductor muscle contraction during resection were recorded. When there were bilateral inferolateral bladder tumors in 1 patient, we counted them as 2 obturator nerve block–requested tumors.
Data and Statistical Analysis
We assumed there was a threshold for the electrical current and predicted that the acceleration of leg movement would decrease sharply (to less than half) when the electrical current of trans-resectoscope stimulation decreased below the threshold. A sample size of 4 patients was required to detect a difference of 50% in the acceleration between the thresholds (80% power), with common standard deviations of 25% at a significance level of 0.05. We recruited 7 patients because we assumed that 45% of patients would not experience thigh adductor muscle contraction based on previous reports.8
In phase 1, we defined acceleration with the neuromuscular monitoring device electrical current intensity set at 60 mA as the maximum response. Acceleration is shown as a percentage of the maximum response. Acceleration data are shown as mean ± SD. Repeated-measures analysis of variance followed by Dunnett multiple comparison of means test were performed to determine the significance of the changes in acceleration. Fisher exact test was used to compare the ratio of positive initial trans-resectoscope stimulations between general and spinal anesthesia. We also calculated relative risk and 95% confidence interval (CI) for positive initial trans-resectoscope stimulations between general and spinal anesthesia. The 95% CI for the risk of thigh adductor muscle contraction during TUR with a negative preoperative trans-resectoscope stimulation result was calculated using the Clopper-Pearson method. Data were analyzed using SPSS for Windows Version 17.0 (SPSS, Chicago, IL). The significance level was set at P < 0.05.
Left knee movement in response to trans-resectoscope stimulation (60 mA) was obtained in 4 of the 7 patients tested. When the intensity of electrical current was reduced, acceleration of the left knee decreased in a sigmoid fashion. When the electrical current was >40 mA, the acceleration was not significantly different from the maximum response (99.2% ± 5.9% at 50 mA, P = 0.99 and 93.9% ± 6.0% at 40 mA, P = 0.09; Fig. 2). However, when the current was ≤30 mA, the acceleration was significantly lower than the maximum response (54.9% ± 5.8% at 30 mA, 0% ± 0% at 20 mA, and0% ± 0% at 10 mA, all P < 0.001).
Based on these results, we set 50 mA as the intensity of electrical current for trans-resectoscope stimulation for phase 2, allowing 10 mA as a margin of error.
The baseline demographic data of the patients in phase 2 are shown in Table 1. There were 51 obturator nerve block–requested tumors in a total of 45 patients (39 unilateral and 6 bilateral; Table 2). The initial trans-resectoscope stimulation results in the area of planned resection were positive in 22 cases (43%) and negative in 29 cases (57%). There was no thigh adductor muscle contraction during TUR in all cases. The 95% CI for the risk of thigh adductor muscle contraction during TUR with a negative preoperative trans-resectoscope stimulation result was 0% to 5.7%. To the extent detectable with the relatively small sample size, there was no apparent difference between anesthetic techniques in the initial response to the trans-resectoscope stimulation (P = 0.71 in Fisher exact test, relative risk [95% CI] = 0.84 [0.39–1.82]). In the negative cases, no thigh adductor muscle contraction occurred during resection even though the operation was performed without obturator nerve block or a muscle relaxant. Among the 22 cases in which the initial trans-resectoscope stimulation result was positive, a muscle relaxant was administered for 2 cases (both in patients who were receiving general anesthesia with an LMA) according to the choice of the attending anesthesiologist. For the remaining 20 cases, obturator nerve block was performed. The trans-resectoscope stimulation result remained positive after the initial obturator nerve block in 3 cases; an obturator nerve block was therefore repeated for 2 cases, and general anesthesia with LMA was initiated to administer muscle relaxant for the remaining case. For all the 22 cases with a positive initial trans-resectoscope stimulation result, the final trans-resectoscope stimulation result turned negative, and no thigh adductor muscle contraction was observed during resection. The time spent on trans-resectoscope stimulation to each area of resection was 36 (28) and 29 (21–41) seconds in mean (SD) and median (interquartile range), respectively.
In this preliminary study, we have demonstrated that the trans-resectoscope stimulation method predicts the need to block the contraction of the thigh adductor response during bladder tumor resection. Trans-resectoscope stimulation may also be useful in confirming that the obturator nerve block is successful and will be adequate to prevent movement.
Although we commonly perform obturator nerve block or administer a muscle relaxant based on the location of the tumor (i.e., when the tumor is located in the inferolateral wall of the bladder), in this sample we have shown that thigh adductor muscle contraction is not elicited in 57% of trans-resectoscope stimulations to the planned resection area for inferolateral bladder tumors. In these patients, subsequent TUR did not elicit thigh adductor muscle contraction even though neither obturator nerve block nor muscle relaxants were administered. Our results are consistent with those of a previous study8 in which TUR was performed without obturator nerve block on 114 patients with inferolateral bladder tumors, of whom 45% did not experience thigh adductor muscle contraction. Collectively, these findings suggest that neither obturator nerve block nor muscle relaxant is necessary in approximately half of inferolateral bladder tumor TURs.
Anatomical variations in the course of the obturator nerve in the pelvis may be one of the explanations for these findings. The obturator nerve emerges from the medial aspect of the psoas major muscle, passes down into the lesser pelvis, and enters the internal obturator muscle. The nerve passes through the obturator canal and divides into anterior and posterior branches. The obturator nerve runs in close proximity to the inferolateral bladder wall in the lesser pelvis, which is why electrical stimulation to the inferolateral bladder wall could lead to adductor muscle contraction. However, electrical stimulation to the inferolateral bladder wall does not lead to adductor muscle contraction if the distance between the bladder and the obturator nerve in the lesser pelvis is large. A previous cadaver study11 reported that the obturator nerve emerges from the medial aspect of the psoas major muscle adjacent to the level of the intervertebral disk between L5 and S1 in 60%, above the level of L5 in 17%, and below the level of S1 in 24% of cases. Because the obturator nerve is not fixed to the bladder wall, this anatomical variation in emergence point may lead to differences in the distance between the obturator nerve and the bladder in the lesser pelvis. We believe that anatomical variation in the distance between the obturator nerve and the bladder is one of the reasons why electrical stimulation to the inferolateral bladder wall did not lead to adductor muscle contraction in approximately half of our patients.
The important points in performing trans-resectoscope stimulation are “adequate bladder volume” and “flow of electrical current.” The relationship between the obturator nerve and the wall of the bladder changes according to the bladder volume.6 The bladder volume during resection depends on the operating surgeon since the amount of fluid in the bladder is associated directly with the view of the operative field. Thus, we should perform trans-resectoscope stimulation when the bladder volume is approximately the same as when tumor resection is to be performed. It has been reported that the position of the counter electrode plate affects the rate of thigh adductor muscle contraction during TUR for bladder tumor.12 Thus, care should also be taken to place the neuromuscular monitoring device anode near the counter electrode plate so that the route of the electrical current is similar during trans-resectoscope stimulation and TUR.
High success rates in preventing adductor muscle contraction during bladder tumor resection with obturator nerve block have been reported in the literature (97% by Tatlisen and Sofikerim8; 91% by Fujiwara et al.9; 81.4% by Wassef13; and 96% by Lee et al.14). Our results suggest that a considerable fraction of these “successful” obturator nerve blocks were performed in cases in which thigh adductor muscle contraction would not have occurred even without obturator nerve block. Further investigation into trans-resectoscope stimulation will be necessary to determine the exact number of patients who need obturator nerve block to prevent thigh adductor muscle contraction to precisely calculate the success rate of obturator nerve block.
We have demonstrated that, among 51 tumors in patients whose urologists requested obturator nerve block, 29 had a negative initial trans-resectoscope stimulation result, and in the remaining 22, the trans-resectoscope stimulation result turned negative after obturator nerve block or muscle relaxation. For these 51 tumors, no thigh muscle contraction occurred during the subsequent TUR. This means that a negative response to the final preoperative trans-resectoscope stimulation has a 95% CI for the risk of thigh adductor muscle contraction during TUR of 0% to 5.7%. Further trials are needed to elucidate whether the trans-resectoscope stimulation method has a clinically acceptable negative predictive value.
The small number of subjects in phase 1 is a limitation of this study. We set 50 mA as the intensity of electrical current for trans-resectoscope stimulation, allowing 10 mA as a margin of error. However, the average current intensity derived from a small number of patients may not be applicable to a large number of patients. Thus, caution should be exercised in determining the intensity of electrical current for trans-resectoscope stimulation in a patient population that is different from our patient population.
In conclusion, the trans-resectoscope stimulation method can predict whether thigh adductor muscle contraction will occur by electrical stimulation without bladder injury. Trans-resectoscope stimulation could be beneficial not only to predict the need to block the contraction of the thigh adductor during tumor resection but also to avoid unnecessary obturator nerve block. Furthermore, trans-resectoscope stimulation is clinically feasible because it requires a single neuromuscular transmitter monitor of the kind regularly used in operating rooms and takes only a mean of 36 seconds to perform.
Name: Takahiro Mihara, MD, PhD.
Contribution: This author helped design and conduct the study, analyze the data, and write the manuscript.
Attestation: Takahiro Mihara 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: Hideki Itoh, MD, PhD.
Contribution: This author helped design and conduct the study.
Attestation: Hideki Itoh has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.
Name: Kozo Hashimoto, MD.
Contribution: This author helped design and conduct the study.
Attestation: Kozo Hashimoto has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.
Name: Takahisa Goto, MD.
Contribution: This author helped design the study, analyze the data, and write the manuscript.
Attestation: Takahisa Goto 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.
We would like to thank D. Ishiwa, H. Ohno, K. Tsuji, M. Yokose, E. Yoshitake, H. Yamato, and Y. Emura at Sagamihara Kyodo Hospital, and S. Ishida, T. Irie, and M. Koga at Yokohama City University Graduate School of Medicine for their efforts and cooperation.
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