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Development of an ultrasound-guided transgluteal injection of the pudendal nerve in cats: a cadaveric study

Published:November 29, 2021DOI:https://doi.org/10.1016/j.vaa.2021.11.004

      Abstract

      Objective

      To develop an ultrasound-guided interfascial plane technique for injection of the pudendal nerve near its sacral origin in cats.

      Study design

      Prospective, randomized, anatomical study.

      Animals

      A group of 12 feline cadavers.

      Methods

      Gross and ultrasound anatomy of the ischiorectal fossa, the pudendal nerve relationship with parasacral structures, and the interfascial plane were described. Computed tomography was employed to describe a cranial transgluteal approach to the pudendal nerve. Bilateral ultrasound-guided injections were performed in eight cadavers using low [(LV) 0.1 mL kg–1] or high volume [(HV) 0.2 mL kg–1] of ropivacaine–dye solution. Dissections were performed to determine successful staining of the pudendal nerve (>1 cm) and inadvertent staining of the sciatic nerve, and any rectal, urethral, or intravascular puncture. Pudendal nerve staining in groups LV and HV were compared using Fisher's exact and Wilcoxon rank-sum test as appropriate (p = 0.05).

      Results

      The pudendal nerve and its rectal perineal and sensory branches coursed through the ischiorectal fossa, dorsomedial to the ischiatic spine. The pudendal nerve was not identified ultrasonographically, but the target plane was identified between the sacral transverse process, the ischiatic spine, the pelvic fascia and the rectum, and it was filled with dye solution. Both branches of the pudendal nerve were completely stained 75% and 87.5% in groups LV and HV, respectively (p = 1.00). The dorsal aspect of the sciatic nerve was partially stained in 37% of injections in group HV. Rectal or urethral puncture and intravascular injection were not observed.

      Conclusions and clinical relevance

      In cats, ultrasound-guided cranial transgluteal injection successfully stained the pudendal nerve in at least 75% of attempts, regardless of injectate volume. Group HV had a greater probability of sciatic nerve staining.

      Keywords

      Introduction

      Diseases affecting the feline urogenital and anorectal tracts are common, and they can have a significant impact on quality of life and the human–animal bond. An example is chronic inappropriate urination leading to relinquishment and, potentially, euthanasia. Recurrence and mortality rates for feline lower urinary tract diseases are reported to be approximately 58% and 5%, respectively (
      • Kaul E.
      • Hartmann K.
      • Reese S.
      • Dorsch R.
      Recurrence rate and long-term course of cats with feline lower urinary tract disease.
      ).
      A few locoregional anesthesia options have been reported to facilitate urogenital procedures in the feline patient, such as urinary catheterization, cystoscopy or surgery, of which epidural injection is the most commonly advocated (
      • O'Hearn A.K.
      • Wright B.D.
      Coccygeal epidural with local anesthetic for catheterization and pain management in the treatment of feline urethral obstruction.
      ;
      • Otero P.
      • Campoy L.
      Epidural and spinal anesthesia.
      ;
      • Otero P.E.
      • Verdier N.
      • Zaccagnini A.S.
      • et al.
      The use of a nerve stimulation test to confirm sacrococcygeal epidural needle placement in cats.
      ;
      • Pratt C.L.
      • Balakrishnan A.
      • McGowan E.
      • et al.
      A prospective randomized, double-blinded clinical study evaluating the efficacy and safety of bupivacaine versus morphine-bupivacaine in caudal epidurals in cats with urethral obstruction.
      ). However, epidural injections may have variable efficacy, potential contraindications such as coagulation disorders, septicemia, pyoderma at the site of injection, severe hypovolemia or hypotension, and anatomic abnormalities, and there is some risk for severe side effects (
      • Song R.B.
      • Cross J.R.
      • Golder F.J.
      • Callan M.B.
      Suspected epidural morphine analgesia induced chronic urinary and bowel dysfunction in a cat.
      ;
      • Bauquier S.H.
      Hypotension and pruritus induced by neuraxial anaesthesia in a cat.
      ;
      • Evangelista M.C.
      • Steagall P.
      • Garofalo N.A.
      • et al.
      Morphine-induced pruritus after epidural administration followed by treatment with naloxone in a cat.
      ).
      Recent studies in humans showed that pudendal nerve blockade is associated with better surgical anesthesia, lower postoperative pain scores and higher satisfaction scores than following epidural injection for colorectal, perineal and lower urinary tract surgeries (
      • Kendigelen P.
      • Tutuncu A.C.
      • Emre S.
      • et al.
      Pudendal versus caudal block in children undergoing hypospadias surgery: a randomized controlled trial.
      ;
      • Fadel M.G.
      • Peltola L.
      • Pellino G.
      • et al.
      The role of pudendal nerve block in colorectal surgery: a systematic review.
      ). Various ultrasound-guided techniques for pudendal nerve blockade have been described in research on human anesthesia (
      • Bendtsen T.F.
      • Parras T.
      • Moriggl B.
      • et al.
      Ultrasound-guided pudendal nerve block at the entrance of the pudendal (Alcock) canal: description of anatomy and clinical technique.
      ;
      • Gaudet-Ferrand I.
      • De La Arena P.
      • Bringuier S.
      • et al.
      Ultrasound-guided pudendal nerve block in children: A new technique of ultrasound-guided transperineal approach.
      ;
      • Fadel M.G.
      • Peltola L.
      • Pellino G.
      • et al.
      The role of pudendal nerve block in colorectal surgery: a systematic review.
      ). In veterinary medicine, blind or nerve stimulator-guided approaches to the pudendal nerve have been described in various species, with few studies utilizing ultrasound guidance (
      • Ahmed A.F.
      • Al-Sobayil F.A.
      • Al-Halag M.A.
      Topographical anatomy and desensitization of the pudendal nerve in adult male dromedary camels.
      ;
      • Gallacher K.
      • Santos L.C.
      • Campoy L.
      • et al.
      Development of a peripheral nerve stimulator-guided technique for equine pudendal nerve blockade.
      ;
      • El-Khamary A.
      • Nazih M.
      • El-Sherif M.
      • et al.
      Ultrasound-guided pudendal nerve block in male donkeys.
      ).
      • Adami C.
      • Angeli G.
      • Haenssgen K.
      • et al.
      Development of an ultrasound-guided technique for pudendal nerve block in cat cadavers.
      described an ultrasound-guided approach to the pudendal nerve in cats; however, the technique only targeted the sensory branch of the pudendal nerve excluding the caudal rectal and deep perineal branches. These branches supply the bulbourethral gland, external urethral sphincter, and sensory membranous urethral fibers (
      • Yoo P.B.
      • Woock J.P.
      • Grill W.M.
      Somatic innervation of the feline lower urinary tract.
      ). This dual innervation of the feline lower urinary tract suggests that targeting the pudendal nerve trunk in the ischiorectal fossa before its bifurcation into the sensory branch (SN) and the rectal perineal (RP) branches would provide improved anesthesia of the region.
      The goal of this study was to develop an ultrasound-guided interfascial plane technique that would stain the pudendal nerve and that can be used not only for anesthetizing the lower urinary tract and anorectal region in cats for surgical purposes but also for the medical and palliative management of the lower urinary tract dysfunctions. Our aims were 1) to describe the gross and ultrasonographic anatomy of the pudendal nerve and surrounding structures in the ischiorectal fossa in feline cadavers, 2) to develop an optimal ultrasound-guided interfascial plane approach to the pudendal nerve, and 3) to define the optimal injectate volume to achieve complete staining of both the SN and RP branches of the pudendal nerve with one injection.
      Our hypotheses were 1) the pudendal nerve would not be identified ultrasonographically in feline cadavers, but that the injection plane would be identified in the ischiorectal fossa; 2) with an in-plane approach, injection of a dye solution into the interfascial plane would stain the main trunk and both the SN and RP branches of the pudendal nerve; and 3) the number of branches stained and accidental staining of the sciatic nerve would differ according to the volume injected.

      Material and methods

      Animals

      The study used 12 previously frozen adult feline cadavers (four female and eight male) with body condition scores of 3–6 out of 9 (
      • LaFlamme D.P.
      Development and validation of a body condition score system for cats: a clinical tool.
      ). The cats were euthanized for reasons unrelated to the study and immediately frozen. Cadavers were excluded if there was any evidence of trauma or anatomical abnormality in the region of interest. Cadavers were thawed at room temperature (approximately 20 °C) for 48 hours before use. The use of feline cadavers for this study did not require ethical review or approval by the North Carolina State University Institutional Animal Care and Use Committee.
      This study was divided into three phases (Fig. 1): phase I, the anatomical description of the landmarks of the pudendal nerve in the ischiorectal fossa by gross anatomic dissection; phase II, the description of the sonoanatomy of the transgluteal pudendal approach and injectate distribution analysis using computed tomographic (CT) examination and gross anatomical dissection; and phase III, evaluation and comparison of injectate distribution and nerve staining using either low (0.1 mL kg–1; group LV) or high volume (0.2 mL kg–1; group HV) injection solution.
      Figure 1
      Figure 1Flowchart of the study design, showing the number of feline cadavers used in each of the three phases. Cadavers in phase III were injected bilaterally in the ischiorectal fossa with two volumes of a dye–ropivacaine solution. CT, computed tomography; HV, high volume injection (0.2 mL kg−1); LV, low volume injection (0.1 mL kg−1).

      Phase I. Gross anatomical description

      For the gross anatomical investigation and description, two male cadavers and one preserved male prosection were used. Each hemipelvis was dissected by performing a circumferential incision through the skin and subcutaneous fascia around the base of the tail, cranially at the level of the greater trochanter, then extending caudally to the level of the ischiatic tuberosity and to connect ventrally just dorsal to the anus. The skin and subcutaneous tissues were dissected away from the tail base and perineum and were retracted caudally. The tail was elevated dorsally, and the ischiorectal fossa was bluntly entered caudolateral to the rectum, medial to the ischiatic spine and dorsal to the internal obturator muscle. The superficial gluteal, gluteofemoral, levator ani and coccygeal muscles were transected in a cranial-to-caudal direction immediately adjacent to the tail musculature to allow optimum viewing from this approach, exposing the pudendal nerve and its SN and RP branches. Dissections in phase I were performed by two investigators (JB and LC) with the consultation of a board-certified surgeon (KGM).

      Phase II. The transgluteal pudendal approach; sonoanatomy and injectate distribution

      After studying the pudendal nerve and anatomical landmarks in the ischiorectal fossa, two cadavers (one male, one female; four hemipelves) were used to study the sonoanatomy of the transgluteal pudendal approach. The pelvic and gluteal regions were clipped bilaterally and prepared with hot water, followed by 70% isopropyl alcohol solution. Cadavers were positioned in sternal recumbency with the pelvic limbs abducted and extended caudally (frog-legged position). To facilitate the visualization, a 3 mL syringe case was inserted into the rectum. The injection area was visualized with a micro-convex transducer (C11x/8-5 MHz; SonoSite Inc., WA, USA) connected to a portable ultrasound machine (Edge II; SonoSite Inc.). The transducer was initially positioned over the dorsolateral aspect of the gluteal region in a transverse orientation to the long axis of the pelvis, at the center of an imaginary line between the sacral body and the ischiatic tuberosity, with the orientation marker directed dorsally (Fig. 2a). At this site, caudal to the greater trochanter, the pudendal nerve runs parallel to the long axis of the pelvis in the ischiorectal fossa. The transducer was then rotated, turning the orientation marker approximately 60 degrees cranially (Fig. 2b) to visualize the sacral transverse process craniomedially, the ischiatic spine and the pelvic fascia caudolaterally, and the rectum ventrally, within the same acoustic window (Fig. 3a & b).
      Figure 2
      Figure 2Position of the cat and ultrasound transducer orientation for the cranial transgluteal approach to the pudendal nerve in the ischiorectal fossa. (a) Cat is in sternal recumbency with the pelvic limbs abducted caudally and transducer initially in a transverse orientation to the long axis of the pelvis, at the center of an imaginary line between the ischiatic tuberosity and the sacral body. (b) Computed tomography image of feline pelvis with superimposed representation of initial transducer orientation followed by a 60 degrees cranial rotation of the orientation marker to visualize the sacral transverse process and the ischiatic spine in the same acoustic window. Cd, caudal; Cr, cranial; D, dorsal; V, ventral.
      Figure 3
      Figure 3(a) Ultrasound image of the ischiorectal fossa in a feline cadaver for an in-plane cranial transgluteal approach to the pudendal nerve. (b) The same ultrasound image with superimposed schematic drawings identifying the relevant anatomical structures. The dotted line represents the pelvic fascia, identified as a hyperechoic linear structure that is attached to the ischiatic spine. The white arrow represents the needle trajectory to inject the pudendal nerve in the ischiorectal fossa after penetrating the pelvic fascia. Cd, caudal; Cr, cranial; Isch, ischiatic spine; L, lateral; MGM, middle gluteal muscle; SCM, dorsal sacrococcygeal muscle; SGM, superficial gluteal muscle; V, ventral.
      Both caudal and cranial approaches were tested. For the caudal approach, a 22 gauge, 80 mm insulated echogenic needle (Stimuplex Ultra 360; B Braun Medical Inc., PA, USA) was introduced in-plane, caudal to the transducer, directed cranial-to-medial, and advanced for 4–5 mm (Fig. 4a). For the cranial approach, the echogenic needle was introduced in-plane cranial to the transducer, and advanced caudal-to-lateral toward the ischiatic spine, until the pelvic fascia was perforated (Fig. 4b). CT (Siemens SOMATOM Perspective 64-slice CT Scanner; Siemens Medical Systems USA Inc., PA, USA) was used to study the needle position in relation to the anatomical structures. A solution for injection was prepared by adding 0.5 mL of yellow (phase II) or green (phase III) permanent tissue marking dye (Davidson Marking System dyes; Bradley Products Inc., MN, USA) to 20 mL of 0.5% ropivacaine (Ropivacaine Hydrochloride injection, 0.5%; AuroMedics Pharma LLC, NJ, USA). The yellow dye solution was then diluted with iohexol (Omnipaque, 350 mg mL−1; GE Healthcare China Co. Ltd, Shanghai, China) in a proportion of 9:1. Based on CT results, the cranial transgluteal approach was selected for injection of 0.2 mL kg−1 of the iohexol–dye solution. A second CT examination was immediately performed on the two cadavers (four injections in total) to evaluate injectate distribution in the interfascial plane, while maintaining the muscle planes in situ. Images were evaluated by a board-certified radiologist (EK). The cadavers were dissected as previously described to evaluate iohexol–dye solution spread. The SN and RP branches of the pudendal nerve and the sciatic nerve were identified. Any rectal or urethral puncture and intravascular or intramuscular injection were noted.
      Figure 4
      Figure 4Computed tomography views of cat cadavers level with the sacrum illustrating ultrasound-guided approaches to the pudendal nerve in the ischiorectal fossa. (a) Caudal approach involved insertion of the needle (white line) in-plane caudal to the transducer, in a cranial-to-medial direction toward the rectum. (b) Cranial transgluteal approach involved insertion of the needle in-plane cranial to the transducer, and advanced in a caudal-to-lateral direction toward the ischiatic spine. After touching the ischiatic spine, the needle was redirected slightly medially and further advanced approximately 2–3 mm. Injection of iohexol–dye solution (0.2 mL kg−1) can be observed (dotted line) in the ischiorectal fossa on the right hemipelvis. D, dorsal; Isch, ischium; L, left; Sacr, sacrum; R directional, right; R, rectum; V, ventral.

      Phase III. Evaluation of injectate distribution of groups LV and HV

      A total of eight cadavers (three female and five male) weighing 3.20 (2.56, 3.95) kg [median (interquartile range, IQR)] were used in phase III. Based on the results obtained in phase II, the cranial transgluteal approach to the pudendal nerve trunk was selected. All cadavers were prepared and positioned as previously described. Each hemipelvis was randomly assigned to group LV or HV by flipping a coin. Acquisition of the ultrasound images and all the injections were performed by a single investigator experienced in ultrasound-guided regional anesthesia (LC). After bilateral cranial transgluteal injections with green dye–ropivacaine solution, the cadavers were immediately dissected, as previously described, by a board-certified surgeon blinded to the group (KGM). The dye–ropivacaine solution longitudinal spread along the SN and RP branches of the pudendal nerve was measured with a caliper (Stainless steel digital caliper; Neiko, ZJ, China). Nerve staining was scored (0–2; 0, no nerve staining; 1, stained < 1 cm length or not stained circumferentially; 2, complete circumferential staining ≥ 1 cm;
      • Micieli F.
      • Chiavaccini L.
      • Mennonna G.
      • et al.
      An ultrasound-guided subparaneural approach to the sciatic nerve in the dog: a cadaver study.
      ). Both branches of the pudendal nerve were required to achieve a score of 2 for an injection to be considered successful. Any staining of the sciatic nerve was recorded. Any intramuscular diffusion and any rectal, vascular or urethral staining were also noted.

      Statistical analysis

      Continuous data were checked for normality using the Shapiro–Wilk normality test and graphically with quantile-quantile plot and histogram population distribution. Data were reported as median (IQR). Based on preliminary data, we estimated that all injections will stain the pudendal nerve at least in part, but only 15% will reach complete staining (score 2) in group LV versus 80% in group HV. We calculated that a total of eight cadavers (16 hemipelves) would be sufficient for part III of the study. The staining quality, staining of the SN and RP branches of the pudendal nerve, staining of the sciatic nerve, diffusion in muscle, inadvertent rectal, urethral or vascular puncture were compared between groups using Fisher's exact test. Length of nerve staining was compared between groups using Wilcoxon rank-sum test. All statistical analyses were performed using Stata/IC Version 14.1 for Mac (StataCorp LP, TX, USA).

      Results

      Phase I. Gross anatomical description

      The pudendal nerve was identified distal to the origin from the second and third sacral spinal nerves, coursing caudomedially, parallel to the rectum. Caudal to the lumbosacral trunk, approximately at the level of the ischiatic spine, the pudendal nerve bifurcated into the RP and SN branches. The SN branch coursed caudally and then ventrally where it divided into two branches: the cranial sensory branch innervating the proximal urethra and the dorsal nerve of the penis (vulva) that coursed distally toward the glans and distal urethra. The RP branch was also identified along with the deep perineal and caudal rectal branches coursing caudally to the perineal musculature and external anal sphincter, respectively. Macroscopically, the pudendal nerve was surrounded by a sheath of connective tissue identified as the pelvic fascia. It was determined that the optimal target for injection was the fascial plane proximal to the bifurcation of the pudendal nerve, where the nerve courses caudomedially to the lumbosacral trunk and medially to the ischiatic spine.

      Phase II. The transgluteal pudendal approach; sonoanatomy and injectate distribution

      The sacral transverse process and the ischiatic spine were identified in the same acoustic window as hyperechoic structures with distal acoustic shadowing. Dorsal sacrococcygeal and gluteal muscles were visualized dorsally and laterally to the sacral transverse process. The pelvic fascia was visualized as a hyperechoic linear structure of lower echogenicity than the bone and medial to the ischiatic spine. The rectum was identifiable by a thin wall, mixed echogenic contents, and luminal gas shadowing (Fig. 3). The pudendal nerve was not sonographically identifiable, but the interfascial plane within the ischiorectal fossa was easily identified approximately 1.5–2.0 cm from the skin by these landmarks: the sacral transverse process, the ischiatic spine, the pelvic fascia and the rectum. The needle position of the caudal approach was evaluated by CT on a single hemipelvis. The needle trajectory was directly toward the rectum; therefore, no injection was performed (Fig. 4a). The cranial transgluteal approach allowed easier, ergonomic targeting of the pudendal nerve before the bifurcation with a trajectory aiming toward the ischiatic spine rather than the rectal wall (Fig. 4b). Iohexol–dye solution was detected in the interfascial plane surrounding the pudendal nerve in the ischiorectal fossa in four out of four sites injected (Fig. 4b). Of these, two injections stained the pudendal nerve trunk prior to bifurcation and had minimal sciatic nerve staining. The remaining two injections partially stained the pudendal nerve trunk prior to bifurcation with concurrent partial staining of the sciatic nerve. In one injection, the iohexol–dye solution spread distally along the sciatic nerve.

      Phase III. Evaluation of injectate distribution

      A total of 16 ultrasound-guided cranial transgluteal injections were performed in this phase. The volumes injected were 0.11 (0.11, 0.11) mL kg−1 in group LV and 0.20 (0.20, 0.21) mL kg−1 in group HV. Both the SN and RP branches of the pudendal nerve were successfully stained in six out of eight (75%) and seven out of eight (87.5%) injections in groups LV and HV, respectively (Fig. 5). In group LV, one injection resulted in partial staining (score 1) of the RP branch and no staining (score 0) of the SN branch. The failed injection in group LV resulted from the spread of the dye–ropivacaine solution into the internal obturator muscle and the floor of the pelvis. In group HV, partial staining (score 1) of both the SN and the RP branches of the pudendal nerve was obtained after one injection. Success rates between the groups were not statistically significant (p = 1.00). The SN and RP branches were stained 2.00 (1.00, 2.00) cm in group LV and 2.25 (2.00, 2.75) cm in group HV (p = 0.06). In three of the eight injections in group HV, partial staining of the sciatic nerve was observed at the level of the lesser ischiatic notch, compared with none in group LV (p = 0.20). Spread of the dye–ropivacaine solution into muscle was observed in four (50%) and three (37.5%) injections in groups LV and HV, respectively (p = 1.00). Rectal puncture, urethral puncture and intravascular injection were never observed.
      Figure 5
      Figure 5Lateral view of a dissected left hemipelvis of a male feline cadaver after injection of 0.2 mL kg−1 green dye–ropivacaine solution by cranial transgluteal approach. Skin and superficial musculature are reflected to reveal the pudendal nerve and branches passing through the ischiorectal fossa. Cd, caudal; Cr, cranial; CR, caudal rectal branch; CSN, cranial sensory nerve; D, dorsal; DNP, dorsal nerve of the penis; DP, deep perineal branch; EAS, external anal sphincter; RP, rectal perineal branch; ScN, sciatic nerve; SN, sensory branch; V, ventral. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

      Discussion

      The present study demonstrated that the pudendal nerve could not be ultrasonographically identified in feline cadavers, under the described conditions. However, the fascial plane containing the pudendal nerve in the ischiorectal fossa was identified using three landmarks: the sacral transverse process craniodorsally, the ischiatic spine of the pelvis caudoventrally and the rectum ventrally. The results of the study demonstrated that the injection of a dye–ropivacaine solution into the interfascial plane using a cranial transgluteal in-plane technique stains both the SN and RP branches of the pudendal nerve and part of the pudendal nerve trunk. The study results did not prove a statistically significant difference in the quality of staining between injections of high and low volumes.
      The gross anatomical dissection enabled identification of the pudendal nerve as it coursed through the ischiorectal fossa and divided into SN and RP branches. Cats have been extensively used as animal models for urogenital studies involving the pudendal nerve, consequently, several descriptions exist of the feline pudendal nerve anatomy and innervation of the lower urinary tract (
      • Martin W.D.
      • Fletcher T.F.
      • Bradley W.E.
      Innervation of feline perineal musculature.
      ;
      • Mariano T.Y.
      • Boger A.S.
      • Gustafson K.J.
      The feline dorsal nerve of the penis arises from the deep perineal nerve and not the sensory afferent branch.
      ;
      • Yoo P.B.
      • Woock J.P.
      • Grill W.M.
      Somatic innervation of the feline lower urinary tract.
      ). However, discrepancies persist.
      • Martin W.D.
      • Fletcher T.F.
      • Bradley W.E.
      Innervation of feline perineal musculature.
      described the feline pudendal nerve emerging from the sacral plexus and dividing into three branches: the common trunk that then divides into caudal rectal and deep perineal nerves, and the dorsal nerve of the penis (or clitoris). More recently,
      • Mariano T.Y.
      • Boger A.S.
      • Gustafson K.J.
      The feline dorsal nerve of the penis arises from the deep perineal nerve and not the sensory afferent branch.
      reported that the deep perineal nerve gives rise to the dorsal nerve of the penis. Anatomical findings in the present study agree with
      • Yoo P.B.
      • Woock J.P.
      • Grill W.M.
      Somatic innervation of the feline lower urinary tract.
      description, showing that the pudendal nerve divides into two branches, the SN and the RP branches, soon after its origin from the first and second sacral nerves. The SN branch gives rise to the cranial sensory nerve and the dorsal nerve of the penis, while the RP branch gives rise to the deep perineal and caudal rectal nerves.
      Beyond the anatomical debate, when utilizing the cranial transgluteal approach to the interfascial plane in the ischiorectal fossa, the ultrasonographic features and landmarks were easily identified in all specimens, and the injected dye–ropivacaine solution stained both the SN and RP branches of the pudendal nerve. This cranial transgluteal technique differs from that described by
      • Adami C.
      • Angeli G.
      • Haenssgen K.
      • et al.
      Development of an ultrasound-guided technique for pudendal nerve block in cat cadavers.
      , which only targeted the SN branch (the cranial sensory nerve and the dorsal nerve of the penis), excluding the caudal rectal and deep perineal branches. The visualization of landmarks in a single acoustic window and overall ease of operation and staining of both pudendal nerve branches make the cranial transgluteal technique described in the present study more widely applicable.
      A limitation of the current study is the lack of visualization of the pudendal nerve with the use of a micro-convex transducer. It is possible that with the use of a higher frequency transducer, the pudendal nerve may be visible. However, in an adult human, the pudendal nerve is only 3.6 mm in diameter and difficult to visualize even with a higher resolution transducer.
      • Bellingham G.A.
      • Bhatia A.
      • Chan C.W.
      • Peng P.W.
      Randomized controlled trial comparing pudendal nerve block under ultrasound and fluoroscopic guidance.
      identified the pudendal nerve only in 57% of human adult patients scanned by an experienced operator, and most of the time the nerve was identified in the interfascial plane medial to the internal pudendal artery. In the cadavers in the present study, the pudendal nerve had a cross-sectional diameter of less than 0.7 mm. A high-frequency transducer under ideal conditions has a lateral resolution of 0.2 mm. Therefore, the visualization of the pudendal nerve may be hard even in optimal conditions and by an experienced operator. The caudal rectal branch of the internal pudendal artery which is found in proximity to the pudendal nerve prior to bifurcation may provide an additional landmark in live animals.
      By reaching both the SN and RP branches of the pudendal nerve, the technique described in the present study may desensitize the distal urogenital and anorectal tracts. In humans, a pudendal nerve block is described for perioperative locoregional analgesia for gynecologic, colorectal and urinary tract surgeries. Studies have found the peripheral pudendal nerve block to be superior to neuraxial analgesia by providing fewer adverse side effects, lower opioid consumption and higher satisfaction scores (
      • Kendigelen P.
      • Tutuncu A.C.
      • Emre S.
      • et al.
      Pudendal versus caudal block in children undergoing hypospadias surgery: a randomized controlled trial.
      ;
      • Aksu C.
      • Akay M.A.
      • Şen M.C.
      • Gürkan Y.
      Ultrasound-guided dorsal penile nerve block vs neurostimulator-guided pudendal nerve block in children undergoing hypospadias surgery: a prospective, randomized, double-blinded trial.
      ;
      • Fadel M.G.
      • Peltola L.
      • Pellino G.
      • et al.
      The role of pudendal nerve block in colorectal surgery: a systematic review.
      ). Pudendal nerve block has also been an area of interest for medical and palliative treatment of conditions such as interstitial cystitis and pudendal neuralgia. In the case of interstitial cystitis, it has been reported that bilateral pudendal nerve blockade provided sustained pain and urinary symptom relief (
      • Lean L.L.
      • Hegarty D.
      • Harmon D.
      Analgesic effect of bilateral ultrasound-guided pudendal nerve blocks in management of interstitial cystitis.
      ). Because feline lower urinary tract disease continues to pose many challenges in veterinary medicine, the development of this technique may provide a novel palliative modality. Further studies using the cranial transgluteal technique in clinical feline patients are warranted.
      During phase II of this study, the caudal approach was discarded without performing injections because this approach was subjectively more technically difficult for an experienced operator and because the needle traveled in a direct path toward the rectum, increasing the risk of accidental rectal puncture. Using the cranial transgluteal approach, staining of the SN and RP branches was not significantly different between high and low volume injections. However, all but a single injection in group HV was successful. The single partially successful injection occurred in a feline cadaver with a low body condition score (3/6) and with notes of difficult visualization of the landmarks. Although the difference was not statistically significant, incomplete staining of the dorsal aspect of the sciatic nerve at the level of the lesser ischiatic notch occurred in group HV but not in group LV. Clinical studies are warranted to determine the clinical relevance of partial staining of the sciatic nerve in cats and any differences in the efficacy of the pudendal nerve block between groups. The pairing of ultrasound guidance with nerve stimulation may improve targeting of the pudendal nerve in live cats. The operator should observe contraction of both the anal sphincter and perineal muscles, without flexion or extension of the tarsus.
      The present study has limitations. The echogenicity of muscles and fascial planes may be altered during freezing and thawing and may not reflect the sonoanatomy of a live cat. Moreover, it cannot be ascertained whether the spread of local anesthetic in live animals will reflect the spread of the dye–ropivacaine solution described in feline cadavers. Finally, the difference in the dye–ropivacaine spread between groups was smaller than anticipated, and the sample size may have been too small to detect a statistically significant difference. However, the difference between the two volumes was unlikely to be clinically relevant.

      Conclusions

      The ultrasound-guided cranial transgluteal technique provided reliable identification of the interfascial plane for the pudendal nerve within the ischiorectal fossa in feline cadavers. However, the pudendal nerve was not directly visualized. In-plane needle introduction was easy, and injectate spread was sufficient within the interfascial plane to cover both the SN and the RP branches of the pudendal nerve. Injection of dye–ropivacaine solution (0.1 and 0.2 mL kg−1) resulted in similar staining of the pudendal nerve branches, and sciatic nerve staining occurred with the higher injectate volume. These results suggest that pudendal nerve anesthesia could be easily achieved using the cranial transgluteal approach. Studies are necessary to determine the usefulness of this technique in live cats.

      Acknowledgements

      This research received no specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

      Authors’ contributions

      JDB: study design, data collection, preparation of manuscript. EKK: data interpretation, preparation of manuscript. KGM: consultation, data collection, preparation of manuscript. LC: study design, data collection and interpretation, statistical analysis, preparation of manuscript. All authors approved the final version of the manuscript.

      Conflict of interest statement

      The authors declare no conflict of interest. A disclosure is noted that LC is an associate editor of this journal but had no access to the review process.

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