Resection of tumours of the small colon and rectum

Tumours of the small colon and rectum of horses are rare and not well described in the current literature. Most available case reports regarding descending colon abnormalities focus on rectal tears and include descriptions of direct, blind suturing of the defect, rectal prolapse, ventral celiotomy and flank laparoscopy to correct the defects. The case report by Salazar et al. (2010) in this issue describes the use of endoscopy and simultaneous laparoscopic assistance to remove a hamartoma from the mucosal surface of the rectum in a foal, a unique combination not previously documented. Although lymphoma is considered the most common intestinal neoplasia in the horse (Taylor et al. 2006; Zimmel 2010), documented tumours of the small colon and rectum also include lipomas, leiomyomas, leiomyosarcomas, polyps, ganglioneuromas and hamartomas (Porter et al. 2007; Salazar et al. 2010; Zimmel 2010). Leiomyomas and leiomyosarcomas originate from the smooth muscle lining the gastrointestinal tract, while ganglioneuromas are made up of neurons and their processes, Schwann cells and supporting cells. Polyps are often histologically categorised as benign fibromas (DeBowes 1991) and are clinically inconsequential until their size causes blockage or decreases the size of the intestinal lumen to the point of obstruction. Hamartomas, such as that described by Salazar et al. (2010), are defined as an overgrowth of normal tissue in a normal location (Cullen et al. 2002). It is believed that hamartomas occur due to an embryological defect and their growth often ceases once the surrounding tissue has reached maturity. These masses are well differentiated on histological examination, which distinguishes them from malignant neoplasms and there is no evidence to suggest that they are prone to becoming malignant with time. Much like polyps, it is only when their growth causes stricture, obstruction, or impedance on other structures in the body that hamartomas become problematic for the horse. The small colon, more correctly referred to as the descending colon, begins to the left of the root of the mesentery and lies caudodorsally in the left abdomen. It has a length of approximately 3.5 m and is 7–10 cm in width (Rakestraw and Hardy 2006). At the pelvic inlet, the small colon transitions into the rectum, which is 30 cm in length in mature horses then transitions into the anus (Freeman 2006). Surgically, the majority of the small colon can be exteriorised via ventral celiotomy, with only the most proximal and distal portions being difficult to access. The rectum generally can not be exteriorised except by using a very caudal surgical approach. Current literature predominantly includes descriptions of rectal and mesenteric tears and a variety of approaches to the affected areas. Ventral celiotomy was used by Dart et al. (1992) to perform resection and anastomosis on portions of the small colon in 4 cases: small colon strangulation by lipoma, enterolith, fibrous stricture and subserosal haematoma. All 4 horses had portions of the small colon successfully resected, although the authors did acknowledge that had the lesions been more proximal or distal, near the transverse colon or pelvic inlet, respectively, resection may have proved more challenging. Direct, nonvisual suturing of rectal tears is possible with the use of specialised, long-handled instruments (Freeman 2006); however, there is a steep learning curve for this technique. It is possible that masses with large, easily palpable stalks or pedicles may be resected in this manner, although there are currently no case reports or descriptions of such a procedure. Kay et al. (2008) describe a technique for rectal prolapse to allow for direct visualisation of rectal tears that could also be used for exteriorisation of a rectal mass. Stay sutures are placed through the external anal sphincter, traction applied and the sphincter transected along its dorsal vertical border. With the traction maintained, the mass can be manipulated more caudally and into a more advantageous surgical field. Once all surgical manipulations are complete, the incision in the sphincter may be closed with absorbable suture, or left open to heal by second intention. Six mares with rectal tears 20–50 cm proximal to the rectal sphincter were *Corresponding author email: dean.hendrickson@colostate.edu 62 EQUINE VETERINARY EDUCATION Equine vet. Educ. (2011) 23 (2) 62-63 doi: 10.1111/j.2042-3292.2010.00162.x