Title: Surgical Management of a Perineal Hernia in a 24-year-old Mare

Authors: Alessandro Spadari ¹, Federica Meistro¹, Giuditta Saragoni^1,*, Riccardo Rinnovati¹ and Maria Virginia Ralletti¹

Author’s institutional affiliations :¹Department of Veterinary Medical Sciences, University of Bologna, Via Tolara di Sopra 50, 40064 Ozzano dell’Emilia, Italy

* Correspondence: giuditta.saragoni2@unibo.it

Summary : A rare case of bilateral perineal hernia was reported in a 24-year-old Italian Trotter mare. This condition is described as a protrusion of retroperitoneal fat, and/or the abdominal or pelvic viscera through the pelvic diaphragm which supports the rectal wall. It is commonly observed in olduncastrated old male dogs, but occurs very rarely in large ruminants; to the Authors’ knowledge, it has never been described in horses. Chronic weight loss is the main clinical sign, even if the main concern regards the entrapment and the strangulation of the bowel content.

Reaching a diagnosis involves clinical signs, palpation, ultrasound evaluation and rectal examination. The approach chosen in all the cases of perineal hernia described in large ruminants is appositional herniorrhaphy; for this reason, it was utilised as the first surgical choice in the present case. Due to its failure, a new surgical approach was used The second procedure involved the transposition of the semimembranosus muscle. The short-term outcome after the second surgery presented no recurrence or complications. It is therefore necessary to consider perineal hernia among the various differential diagnosis of perineal swellings in horses. Additional cases are required to determine its benefits.

Keywords : perineal hernia; horse; surgery; herniorrhaphy; equine.

Introduction: Perineal hernia is a protrusion of retroperitoneal fat, and/or abdominal or pelvic viscera through the pelvic diaphragm which supports the rectal wall . For the most part, it occurs consequent to weakened perineal muscles which cause an abnormal displacement of the pelvic and caudal abdominal organs into a paranal position . This condition is common in old uncastrated male dogs, but occurs very rarely in large ruminants ; to the Authors’ knowledge, it has never been described in horses. In this species, the most frequently reported hernias are umbilical, scrotal and inguinal . To this day, there has only been one reported case of perineal hernia in equids which involved a seven-year-old female donkey .

In dogs, factors which lead to its development are not fully understood; however, it is likely that they are multifactorial, including tenesmus, associated with chronic prostatic disease or constipation, myopathy, rectal abnormalities, and gonadal hormonal imbalances . In large animals, there are fewer predisposing conditions, namely perineal trauma, chronic diarrhoea, constipation and tenesmus . The presentation of a perineal hernia in both dogs and large animals can be either unilateral or bilateral, and also reducible or non-reducible . Animals presenting this condition can manifest severe abdominal straining during defecation and urination . Reaching a diagnosis involves clinical signs, palpation, ultrasound evaluation and rectal examination .

Medical management of a perineal hernia in dogs may be achieved conservatively by using stool softeners, periodic enemas and digital faecal removal. However, the majority of cases require surgical herniorrhaphy to prevent the life-threatening complications caused by the incarceration of herniated organs . In the literature regarding small animals, several surgical techniques have been reported. Internal obturator muscle transposition is the most successful and commonly used method. However, a supplementary approach may be adopted which includes gluteal muscle transposition, semitendinosus muscle transposition, synthetic implants and biomaterials .

Appositional herniorrhaphy was the first technique to be used on dogs; it consists of using the external anal sphincter and any remnants of the levator ani or coccygeus muscles for primary repair. The sacrotuberous ligament can be utilised as a lateral component of the repair if the levator ani muscle is significantly atrophied . This was the approach chosen in all the cases of perineal hernia described in large ruminants for this reason, it was the first repair technique used in the present case. Due to the failure of the first surgery, an original surgical procedure was carried out. Another surgical approach for a canine perineal hernia is superficial gluteal muscle transposition . Nevertheless, owing to the horse’s anatomy, this muscle is particularly developed in size and quite distant from the perineal area; therefore, it cannot be used. The last surgical choice in dogs is semitendinosus muscle transposition which is considered to be a salvage technique . In this particular case, none of the procedures reported were carried out, while an innovative surgical method which involved the transposition of the semimembranosus muscle was performed. The decision to use this muscle was dictated by its proximity to the surgical defect, by its superficiality and by its width which could easily fill the hernial defect.

Anatomy of the pelvic diaphragm: The pelvic diaphragm is the muscular basis of the perineum. It is formed by the levator ani and the coccygeus muscles. The levator ani arises from the ischial spine and is adjacent to the medial surface of the sacroischiatic ligament. The majority of its fibres end at the anus where they mingle with those of the external anal sphincter. The coccygeus has a similar origin but passes more dorsally to insert on the transverse process of three to five tail vertebrae and also on the tail fascia. It has three different parts; starting from the dorsal aspect, there is the coccygeal part, the anal part and the perineal part .

The innervation of the levator ani and the coccygeus muscles is controlled by the caudal rectal nerve. The space between two parts of the coccygeus muscle dorsal to the anus is closed by the external anal sphincter, the smooth internal anal sphincter and the rectococcygeus muscles. The coccygeal part is composed of muscle fibres related to the tail vertebrae surrounding the anus, the anal part is characterised by a thickened circular muscle layer of rectum surrounding the anus, and the perineal part is a caudal continuation of the longitudinal smooth muscle layer of the rectum dorsal to the anus which inserts onto the fourth or fifth coccygeal vertebra. Two parts of the coccygeus muscle are supplied by the caudal rectal and perineal nerves, while the rectococcygeus is innervated only by the caudal rectal nerve.The opening between the pelvic diaphragm and the caudal margin of the pelvic floor is closed by the urogenital diaphragm, penetrated by the urogenital canal, which changes anatomy according to sex. In males, it is accompanied by the bulbospongiosus muscle which is the continuation of the urethralis muscle and extends to the glans with individual fibres which deepen between the corpus cavernosum and median raphe.

Innervation is provided by the deep perineal nerve. Instead, in mares, it is provided by the corresponding constrictor vestibuli (the origins of which form the ventral border of the levator ani and end in the muscle fibres from the other side ventral to the vestibule) together with the constrictor vulvae muscles (which are made up of striated fibre bundles under the skin of the vulva which fuse dorsally with the external anal sphincter). The first one is innervated solely by the deep perineal nerve, while the second one is also innervated by the caudal rectal nerve. Another muscle present in the area is the paired retractor penis/clitoridis which arises from the second caudal vertebra and descends deep to the levator ani on each side of the rectum. It decussates ventral to the rectum and, from here, it descends to the ventral surface of the penis; however, it does not reach the clitoris in the mare. It is also supplied by the deep perineal nerve. The perineal body is a musculo-fibrous node of tissue between the anus and the vestibule, and consequently is a median structure. It comprises the muscular connection between the external anal sphincter and the constrictor vestibuli, the internal anal sphincter, the subanal decussation of the retractor clitoridis, and a fibrous plate (perineal septum) which passes craniodorsally from the vestibule to the rectum (Figure 1).