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EQUINE VETERINARY EDUCATION / AE / AUGUST 2020


superficial inguinal ring (Schumacher 2012) and subsequent laparotomy under general anaesthesia (GA) (Kummer 2012).


Laparoscopy Horses were restrained in stocks and sedated with a combination of an a2-adrenoreceptor agonist and an opioid. Standard laparoscopic techniques were used to gain visualisation of the ipsilateral deep inguinal area (Kummer 2012), with additional instruments used to facilitate this by moving bowel and allowing traction on the mesorchium if needed (Fig 1). The mesorchium was infiltrated with local anaesthetic (2% mepivacaine) then divided with a vessel- sealing device (LigasureTM


), before being removed via an


ipsilateral flank laparotomy (Hendrickson 2007). Standing open castration was then performed on the contralateral side (Searle et al. 1999).


General anaesthetic technique With horses anaesthetised in dorsal recumbency, the inguinal canal was explored using an inguinal approach first. When the vaginal process could not be identified in the inguinal canal, the surgeon converted to a laparotomy approach. The approach to laparotomy differed between surgeons (parainguinal, suprapubic paramedian or ventral midline (Kummer 2012)). Regardless of approach, the abdomen was explored manually and any spermatic tissue present was extracted and resected. The normally descended scrotal testicle was removed using a closed castration technique. Spermatic structures were resected at surgery to avoid the possibility of functional testicular tissue being left behind. The resected tissue was then examined grossly for the presence of testicular tissue and/or sent for histopathology.


Results


Ten horses were found to be monorchids during castration, one of which had a teratoma on the contralateral side (Fig 2). Age range was 1–12 years (median 3 years and mean 4.2 years). Breeds are listed in Table 1. It is clear from the detailed clinical records, that these horses had been in their owner’s possession from birth. Five horses had standing laparoscopy (four unilateral and one bilateral), whilst five horses had laparotomy using suprapubic paramedian (n = 2), parainguinal (n = 2) and ventral midline approaches (n = 1). The right testicle was absent in three horses, and the left testicle was absent in seven horses. The surgical findings are summarised in Table 1. The vaginal process was present in all horses; the ductus deferens


a)


and epididymis were present in eight horses (Figs 3 and 4) and the ligament of the tail of the epididymis and testicular vessels were identified in five. There were no significant post-operative complications recorded in any of the cases. Hormone tests were performed in eight horses within


2–4 weeks of surgery. Post-hCG administration testosterone levels were <0.2 nmol/L (reference range: gelding <0.05 nmol/L). in four horses, with the results for two horses lost to follow-up. The serum oestrone sulphate concentration in one horse was 0.01 ng/mL (reference range: gelding <0.02 ng/mL). The serum AMH concentration in one horse was 0.1 pg/L, (reference range: gelding <0.14 pg/L). Post-operative hormone assays confirmed the lack of testosterone-producing tissue in six of ten horses consistent with them being geldings. There were no complaints from clients concerning persistent stallion-like behaviour post-surgery suggesting there was no residual functional testicular tissue having removed the normally descended testicle. Histopathology results were available for three horses (Horses 1, 8 and 9). Histology of tissue samples from Horse 1 revealed ciliated columnar epithelium associated with the vas deferens, a fibrotic nodule, but no testicular tissue (Fig 5). In Horse 8, a small nodule of dense fibrous tissue with pigment- laden macrophages representing the remnants of markedly atrophic testicular tissue, skeletal muscle assumed to be cremator muscle, and ciliated columnar epithelium of the epididymis were identified. In Horse 9, the proper ligament of the testis and aplastic testicular tissue were present.


Discussion


In these monorchid horses, associated accessory spermatic structures were always present despite the absence of true testicular tissue on the ipsilateral side. The vaginal process was identified in all horses, which is similar to previous reports (Parks et al. 1989; Strong et al. 1997; Kelmer et al. 2006; Brommer et al. 2011). The presence of the vaginal process implies that at some point, gubernacular, and probably testicular tissue, had been present (Parks et al. 1989), suggesting a degenerative process is more likely than testicular agenesis. The gubernaculum, a mesenchymal column of tissue, develops during testicular development and extends from the caudal pole of the mesonephros and testis to the inguinal region. An invagination of peritoneum, the vaginal process, grows into the extra-abdominal part of the gubernaculum in horses at 48 days (McGeady et al. 2006). The cause of acquired monorchidism has been proposed to be ischaemic injury of the developing testis in utero (Parks


b)


Fig 1: a) Laparoscopic view showing spermatic tissues entering the right internal inguinal ring. b) Gentle traction was applied on the mesorchium to bring it back into the abdomen but no testicular tissue was identifiable. The elongated mesorchium was subsequently resected and sent for histopathology confirming complete absence of testicular tissue.


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