Abstract
The objective of this prospective experimental cadaveric study was to develop an ultrasound-guided technique to perform an anaesthetic pudendal nerve block in male cats. Fifteen fresh cadavers were used for this trial. A detailed anatomical dissection was performed on one cat in order to scrutinise the pudendal nerve and its ramifications. In a second step, the cadavers of six cats were used to test three different ultrasonographic approaches to the pudendal nerve: the deep dorso-lateral, the superficial dorso-lateral and the median transperineal. Although none of the approaches allowed direct ultrasonographical identification of the pudendal nerve branches, the deep dorso-lateral was found to be the most advantageous one in terms of practicability and ability to identify useful and reliable landmarks. Based on these findings, the deep dorso-lateral approach was selected as technique of choice for tracer injections (0.1 ml 1% methylene blue injected bilaterally) in six cat cadavers distinct from those used for the ultrasonographical study. Anatomical dissection revealed a homogeneous spread of the tracer around the pudendal nerve sensory branches in all six cadavers. Finally, computed tomography was performed in two additional cadavers after injection of 0.3 ml/kg (0.15 ml/kg per each injection sites, left and right) contrast medium through the deep dorso-lateral approach in order to obtain a model of volume distribution applicable to local anaesthetics. Our findings in cat cadavers indicate that ultrasound-guided pudendal nerve block is feasible and could be proposed to provide peri-operative analgesia in clinical patients undergoing perineal urethrostomy.
Introduction
Ultrasound-guided pudendal nerve block is used commonly in human medicine for the treatment of pudendal neuralgia and associated chronic pelvic pain, 1 and to provide analgesia during short ambulatory procedures, such as prostate biopsy. 2
The somatic innervation of the feline urinary tract has been investigated recently; electrophysiological studies have demonstrated that the anatomical features of the feline pudendal nerve trunk exhibit remarkable similarities to those in humans, being divided into the sensory, caudal rectal (external anal sphincter) and perineal (external urethral sphincter) components.3–7 The sensory component gives origin to two separate branches: the cranial sensory nerve and the dorsal nerve of the penis. In cats, as well as in humans, fibres derived from the sensory branch of the pudendal nerve provide differential innervation of the proximal and distal urethra, and evoke pudendal–anal reflexes when stimulated electrically.7–9
In respect of these findings, the anaesthetic block of the feline pudendal nerve may have relevant clinical applications by representing a specific tool for the treatment of perineal and urethral pain in cats.
To our knowledge, pudendal nerve anaesthetic block has never been described in cats.
The aims of this study were to identify the anatomical and ultrasonographical landmarks for locating the pudendal nerve in feline cadavers, and to develop an ultrasound-guided technique to be further used in clinical patients undergoing perineal urethrostomy.
Materials and methods
Animals and study design
The study was designed as a prospective experimental cadaveric trial.
The experimental setup for the anatomical and ultrasonographical investigations, and the tracer injections included 13 fresh feline cadavers obtained from male castrated experimental cats euthanased for reasons not related to this trial. Two client-owned male castrated cats, also euthanased for reasons unrelated to the trial, were included in the computed tomography (CT) study; for the use of these cadavers, informed owner consent was obtained. Congenital or acquired abnormalities of the pelvic and perineal regions, as well as a history of urinary dysfunction, were considered exclusion criteria.
The experimental trials were commenced within 30 mins of euthanasia.
Anatomical preliminary study
The pelvis and the gluteal region of one cat cadaver were dissected by an experienced prosector (KH) in order to isolate and study the topography and branching pattern of the feline pudendal nerve. The specimen was preserved by immersion fixation in a 4% formaldehyde solution. The cat was sectioned in a left paramedian plane so as to allow all the pelvic structures, notably the lumbosacral plexus and the pudendal nerve to be dissected from medial.
Ultrasonographical study
The cadavers of six cats were used to identify anatomical and ultrasonographical landmarks allowing the pudendal nerve to be located. Three different ultrasonographical approaches were tested.
The perineal and gluteal areas of the cadavers were shaved and cleaned; thereafter, the cats were positioned in sternal recumbency with the hind limbs flexed and placed caudally (frog position). A portable ultrasound device (M-Turbo, SonoSite) with a 7.5-MHz linear probe was used for ultrasonographical identification of pelvic anatomical structures. In order to facilitate orientation, the urethra was catheterised in all cadavers and a wet tampon was inserted into the rectum.
For each of the three approaches tested, the degree of technical difficulty was scored in every cat by an experienced radiologist (GA) on the basis of three different criteria: visibility of sonographical landmarks, accessibility for needle insertion and safety upon injection (intended as low risk of iatrogenic accidental rectal, urethral or vascular puncture). For each of these criteria, a score ranging from 1 to 3 was assigned, where 1 indicated ‘poor’, 2 indicated ‘intermediate’ and 3 indicated good’. The medians of the scores obtained from each cat for every approach were calculated. For each of the three approaches, the sum of the medians was defined as ‘total score’ and ranged from 3 to 9. The degree of technical difficulty of the approach was scored as ‘high’ if the total score was ≤4, as ‘moderate’ for scores ranging from 5 to 6 and as ‘low’ for scores higher than 6.
This preliminary anatomical study was conducted in order to identify the most promising approach in terms of practicability and safety. Only the selected method was then investigated further by means of tracer injections, CT and injection volume determination.
Deep dorso-lateral approach
The target site of injection for the deep dorso-lateral approach (DDLA) was the pudendal sensory component at the level of its bifurcation into the cranial sensory nerve and the dorsal nerve of the penis.
The probe was positioned on a parasagittal plane, lateral to the intervertebral junction between the first and second coccygeal vertebrae (base of the tail) and cranio-medial to the sciatic tuber with the cranial margin of the probe lying about 0.5–1 cm caudal to the caudal extremity of the sacrum (Figure 1). These landmarks were identified by palpation. To visualise the rectum, the probe was tilted up to a 45°angle with respect to the parasagittal plane in order to orient the ultrasound’s beam medio-ventrally. Thereafter, starting from this position the urethra was identified by titling the probe in the opposite direction toward the previously described parasagittal plane until the ultrasound beam was aligned with the longitudinal axis of the urethra; in this way, the bulbo-urethral gland, the pubic bone shadow and the urethra were visualised simultaneously within the same plane. At this point, the urinary catheter was gently moved back and forth for about 0.5 cm; the observance of a hyperechoeic double line moving within the urethral lumen during this manoeuver was considered indicative of correct identification of the urethra. From this position, the probe was slowly moved cranially along the urethral longitudinal axis until the probe was aligned with the body of the first coccygeal vertebra; in this new position, the identification of the prostate was also attempted.
Probe positioning and needle insertion for the deep dorso-lateral approach to the pudendal nerve
Superficial dorso-lateral approach
The superficial dorso-lateral approach (SDLA) aimed to target the pudendal nerve more proximally than with the DDLA, ie, at the level of its sacral emergence.
The probe was positioned caudo-lateral to the sacro-coccygeal junction with its cranial margin lying just caudally to the caudal extremity of the sacrum (Figure 2). These landmarks were identified by palpation. To visualise the rectum, the probe was tilted at a 45° angle with respect to the parasagittal plane in order to orient the ultrasound’s beam medio-ventrally. The probe was then slowly tilted back, as to be repositioned to a parasagittal plane parallel to the spinal cord, until the ultrasound beam was aligned with the longitudinal axis of the urethra.
Probe positioning and needle insertion for superficial dorso-lateral approach to the pudendal nerve
Median transperineal approach
Similarly to the DDLA, the median transperineal approach (MTA) aimed to target the pudendal nerve at the level of the bifurcation of its sensory component into the cranial sensory nerve and the dorsal nerve of the penis. The urethra and the rectum were located by positioning the probe on a sagittal plane just ventrally to the external anal sphincter, half way between the anus (dorsal to the probe) and the testicles (ventral to the probe) (Figure 3).
Probe positioning for median transperineal approach to the pudendal nerve
Tracer injection
Based on the ultrasonographical findings, the DDLA was selected as the technique of choice to proceed with the methylene blue injection.
A disposable external needle guide (Biopsy starter kit, SonoSite) was mounted onto the ultrasound probe and used to facilitate a targeted needle insertion. The tracer injection was always performed by the same team, which consisted of an anaesthesist and a radiologist (CA and GA, respectively); a 24-gauge atraumatic needle (SonoPlex cannula with SPROTTE tip, Pajunk) was used.
The needle was inserted at the cranial margin of the probe, just caudal to the caudal limit of the sacrum, and advanced until its tip reached the dorsal aspect of the urethra; 0.1 ml of methylene blue was then injected and the needle was withdrawn. The same procedure was repeated on the other side of the body in order to inject a total volume of 0.2 ml 1% methylene blue.
The spread of dye and nerve coloration, as well as the presence of signs compatible with urethral, rectal or vascular iatrogenic puncture, were evaluated by gross dissection, performed by two experienced anatomical specialists (KH and MS), who were not involved in either the ultrasonographic evaluations or in the tracer injection.
CT and injection volume determination
A pelvic helical CT (Somatom Volume Zoom, Siemens) was performed in two cadavers in order to obtain a model of local anaesthetic distribution.
A sequence with 3 mm wide slices thickness and 1.5 mm of recon increment was obtained before and after injection; a volume of 0.15 ml/kg, consisting of a mixture of contrast medium (Iobitridol 767.8 mg/ml solution Xenetix 350, Guerbet) and saline (1:1), was injected in both sides to yield a total injection volume of 0.3 ml/kg. The injection was performed according to the DDLA approach. The evaluation of the CT images was performed by a radiologist experienced in this field (GA).
Results
Anatomical preliminary study
The anatomical dissection revealed that the feline pudendal nerve is a multi-fasciculated trunk, which arises at mid-sacral level from the convergence of two sacral spinal nerves (emerging from the first and second sacral vertebrae). The rectal perineal branch originates from the most caudal of these two sacral nerves before they join to form the main trunk of the pudendal sensory branch. From its sacral emergence, the sensory main trunk runs ventro-caudally to reach the dorsal aspect of the urethra, approximately at mid-distance between the penis and the neck of the urinary bladder, dorsal to the prostate and cranial to the bulbo-urethral glands. At this point, the nerve bifurcates into two separate branches: the cranial sensory nerve, which runs cranially along the urethra to reach the prostate, and the dorsal nerve of the penis, which runs within the same plane but in the opposite direction, to innervate the distal urethra and the penis (Figure 4).
Anatomical dissection of the feline pelvic region. IN = ischiatic nerve; U = urethra; R = rectum; PSC = pudendal sensory component; CSN = cranial sensory nerve; DNP = dorsal nerve of the penis; RPN = rectal perineal nerve; UB = urinary bladder; P = pubic bone. Red uppercase letters indicate anatomical structures used as ultrasonographical landmarks
Ultrasonographical study
DDLA
Although none of the approaches investigated allowed for direct ultrasonographical identification of the feline pudendal nerve, the DDLA clearly revealed helpful ultrasonographical landmarks, namely the urethra (6/6 cats), the bulbo-urethral glands (5/6 cats), the caudal border of the pubic bone (6/6 cats) and the rectum (6/6 cats), thus allowing the site of injection to be located (Figure 5). The ultrasonographical identification of the prostate, however, was technically difficult and poorly reliable, as it was only possible in two cats.
Ultrasound image of pelvic landmarks used for performing the pudendal nerve block with the deep dorso-lateral approach. The pubic bone (P) casts an acoustic shadow and is identified by its shape and location relative to the bulbo-urethral glands (BU); the BU is seen as relatively hypoechoic, rounded structures located dorsal to the caudal limit of the pubic bone. N = needle; UC = urethral catheter; U = urethra; CR = cranial; CD = caudal
The approach was found feasible and potentially performable by operators with an average level of expertise in ultrasonography; on the basis of the previously described scoring system, its degree of technical difficulty was judged as low (Table 1).
Scoring system adopted to evaluate the degree of technical difficulty of each approach. The scores obtained from each cat cadaver are presented as medians and ranges (maximum—minimum scores assigned). As can be seen from the ranges, within each group [deep dorso-lateral approach (DDLA), superficial dorso-lateral approach (SDLA) and median trans-perineal approach (MTA)] every cat received the same score
SDLA
The SDLA allowed clear visualisation of the rectum (6/6 cats) and the urethra (6/6 cats) only. No other clear ultrasonographical landmarks useful in locating the targeted structure could be identified in any of the cats. The degree of technical difficulty of this approach was judged to be moderate (Table 1).
MTA
Similarly to the SDLA, the MTA allowed correct identification of two useful anatomical structures only, namely the rectum (6/6 cats) and the urethra (6/6 cats); additionally, this approach required a good level of expertise in ultrasonography and, as based on the scoring system adopted in this study, its degree of technical difficulty was judged as high (Table 1).
Tracer injection
The anatomical dissection revealed that in 4/6 cats, both sensory branches of the pudendal nerve (cranial sensory nerve and dorsal nerve of the penis) were stained entirely. In the remaining two cadavers, the sensory branches were coloured for a length of approximately 1 cm, but the dye spread also extended to the main pudendal sensory trunk (proximally to the bifurcation) for about 0.5 cm. At any level of its length the sciatic nerve was coloured by the methylene blue.
Signs compatible with rectal, urethral or vascular puncture were not detected in any cats.
CT and injection volume determination
In both cadavers, CT images revealed a homogeneous distribution of the contrast medium in the area of interest, namely the dorsal and lateral aspects of the mid- urethra, approximately between the neck of the urinary bladder and the penis (Figure 6).
Three-dimensional reconstruction from pelvic helical computed tomography scan (3 mm slice with 1.5 mm of recon increment) performed in a fresh cadaver after injection — using the deep dorso-lateral approach — of 0.3 ml/kg (split in two injection sites) of a 1:1 mixture of iobitridol solution and 0.9 % sodium chloride. The image shows an even distribution of the contrast media (white) in the area of interest, ventral and lateral to the rectum (orange) and around the dorsal and lateral aspects of the urethra (red)
Discussion
To our knowledge, this is the first description of an ultrasound-guided selective perineural injection involving the feline pudendal nerve.
The ultrasonographical findings for the MTA and the SDLA immediately showed that both approaches have important limitations, which would have precluded their application in clinical patients.
More precisely, the MTA was judged inadequate to be attempted in live animals owing to the level of technical difficulty, the poor ultrasonographical visibility, which could imply a high risk of rectal or urethral puncture if attempted in live patients, and, because of the proximity to the external anal sphincter, the impossibility of maintaining the needle entrance site sterile. Furthermore, the small dimensions of the feline perineum allowed for a very narrow window for probe positioning only. Although the use of a miniature-linear ultrasound probe could have overcome this disadvantage, all these limitations consistently jeopardised the practicability of the MTA.
Concerning the SDLA, this approach was attempted to target the pudendal nerve just distal to its emergence from the second sacral vertebra, at the level of the bifurcation into sensory and rectal perineal branches. We hypothesised that the relatively thick main nerve trunk could have been identified directly by ultrasonography, and we attempted to do so in six different cat cadavers. In contrast to our hypothesis, the ultrasonographical findings indicated that the pudendal nerve trunk is still too small to be identified clearly by ultrasonography. A higher-frequency ultrasonography probe might enable an experienced radiologist to identify small nerves. However, not all facilities have at their disposal such sophisticated and expensive equipment, which makes a technique based on its use inapplicable to the clinical setting. Additionally, if the pudendal nerve block is performed in live cats, the operator would, most probably, be an anaesthetist with a considerably lower level of expertise in ultrasonography than a radiologist.
Still concerning the limitations of the SDLA, it should also be considered that, even in the case of successful blind perineural injection, owing to the immediate vicinity of the pudendal nerve to the ischiatic nerve’s sacral emergence, such a proximal block would probably increase the risk of a motor block in live animals.
In the light of these considerations, we judged the MTA and the SDLA impracticable in the clinical setting and not to be proposed for live cats; therefore, we decided not to further investigate their applicability.
Therefore, the DDLA was the only suitable approach in terms of practicability, sterility and ability to visualise useful ultrasonographical landmarks. For this reason, it was selected as the technique of choice to be investigated further in the second and third phases of the study.
Different volumes of dye tracer and contrast medium were used to monitor the injection site and study the volume distribution by gross dissection and CT respectively.
Similarly to other dyes, methylene blue 1% in aqueous solution diffuses rapidly through tissues, especially in cadavers where the cellular membrane integrity soon becomes compromised. For this reason, in order to assess the exact site of injection when performing cadaveric studies, it is common practice to inject the dye tracer at very low volumes, even if the target is a relatively large nerve.10,11
Also, in our study, the use of volumes of more than 0.1 ml of methylene blue per nerve would have resulted in non-selective spread of the dye tracer; however, the use of such a demandingly low volume of dye allowed a more accurate and precise evaluation of the tracer distribution at dissection. Additionally, it should be considered that, as we were able to colour the nerve with 0.1 ml of methylene blue only, a block failure under clinical conditions with higher volumes of local anaesthetics would be unlikely.
Although in clinical practice a volume of 0.1 ml of local anaesthetic may be considered adequate for blocking small nerve branches, among peripheral nerves the pudendum presents unique features. Unlike the limb nerves, which are clearly confined and delimited by muscular planes and bones, the pudendal nerve is located within the pelvic cavity without being bounded sharply by surrounding anatomical structures, which could limit the spatial distribution of the local anaesthetic. Furthermore, the sternal recumbency position adopted for performing the pudendal nerve block may further facilitate a dorso-ventral distribution of the local anaesthetic owing to gravity. For this reason, when performing the CT study, the purpose of which was to obtain a model of distribution of the solution used in clinical patients, we increased the injection volume to 0.15 ml/kg per each nerve.
The use of a mixture of contrast medium and colour tracer would have overcome the problem of using different volumes of injectate; unfortunately, performing the CT and the anatomical dissection within the same experimental session would not have been possible.
Potential complications occurring during and after pudendal perineural injection are puncture of the rectum or of the urethra, development of infections at the injection site, and puncture of big vessels, such as the pudendal vein and artery, with consequent haematoma formation. The presence of faeces or gas within the rectum may interfere with the correct ultrasonographical visualisation of the landmarks; in addition, the consequent medio-lateral displacement of the rectal walls greatly increases the risk of iatrogenic rectal puncture during needle insertion. For these reasons, it is recommended to empty the rectum before attempting the nerve block in live cats and to avoid suturing the anus prior to injection.
Owing to the possibility of monitoring needle advancement through the tissues, and observing the spread of local anaesthetics at the target site while directly visualising vessels and other anatomical structures, ultrasonographical guidance is likely to greatly decrease the risk of block-related complications compared with a blind technique, and also offers the advantage of a more accurate and specific nerve block.
Nonetheless, on the basis of our findings, it can be assumed that successful ultrasound-guided pudendal nerve block in cats requires good needle-handling skills and experience in ultrasound-guided interventions.
Conclusions
This study suggests that a feline pudendal nerve block under ultrasonographical guidance is feasible and effective. Future trials will be needed to assess the usefulness and the applicability of this technique in the clinical setting.
Footnotes
Funding
This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.
Conflict of interest
The authors do not have any potential conflicts of interest to declare.