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Veterinarian Technician June 2005 (Vol 26, No 6)

Making Sense of Large Colon Displacements in Horses

by James N. Moore, DVM, PhD

    Key Points

    • When treating horses with colic, it is important to have a thorough knowledge of the anatomy of the equine gastrointestinal tract.
    • Left dorsal displacement, right dorsal displacement, and large colon volvulus are common abnormalities of the large colon.
    • Large colon volvulus is the most critical problem affecting the horse's large colon.

    Colic is a word that causes immediate concern among horse owners. Although colic simply means abdominal pain, many clients associate the word with a life-threatening condition in­volving the horse's gastrointestinal (GI) tract. Fortunately, most horses with colic respond to appropriate medical treatment. However, some conditions require more extensive therapy and, very often, surgical intervention.

    The often difficult task facing the veterinary staff is distinguishing between simple and complicated cases of equine colic. To determine the severity of the case and understand the approach that should be used, it is important for technicians to have a working knowledge of the anatomy of the equine GI tract as well as the other organs in the abdomen (Figure 1). Because the horse's large colon can become twisted or displaced, causing the horse to feel abdominal pain, this article focuses on this part of the horse's GI tract.

    Equine Gastrointestinal Tract

    The Stomach and Small Intestine

    The horse's stomach is relatively small, holding about 10 to 15 L. It is positioned on the left side of the horse's abdomen beneath the ribcage (Figure 2). Because of the peculiar arrangement of the junction of the esophagus and the stomach, horses cannot vomit. As a result, distention of the stomach with excessive amounts of gas or buildup of fluid from the small intestine can cause the stomach to rupture.1 This is why a nasogastric tube is passed in horses that show signs of colic. The junction between the stomach and the first portion of the small intestine is formed by the muscular pyloric sphincter.

    The horse's small intestine comprises three portions: the duodenum, jejunum, and ileum (Figure 3). All three portions are attached to the dorsal body wall by mesentery, a relatively thin, fan-shaped tissue that contains the intestinal blood vessels, lymphatic system, and nerves. The mesentery for the jejunum and ileum is called the great mesentery, and its rather narrow attachment to the dorsal body wall can be palpated during a rectal examination.2

    The first portion of the duodenum arises from the pylorus, is supported by a short mesentery, and is adjacent to the horse's liver and pancreas. It then passes dorsally along the horse's right side for approximately 1 m before it passes around the base of the cecum and turns toward the dorsal midline. After crossing the midline caudal to the root of the mesentery, it passes a short distance cranially. At this point, the length of the small intestinal mesentery increases dramatically, and the duodenum becomes the jejunum. The average adult horse has approximately 25 m of jejunum lying in coils in the middle of the abdomen. Because of the length of the mesentery, loops of small intestine can become twisted or trapped in natural openings in the body such as the inguinal canal.3

    The last part of the small intestine is the ileum, which is about 30 cm long. Unlike the duodenum and the jejunum, the ileum has a thick muscular wall and an additional mesentery that attaches the ileum to the cecum. Thus the ileum has a typical mesentery that contains blood vessels, lymphatic system, nerves, and a so-called antimesenteric mesentery or ileocecal fold. The ingesta move from the ileum into the cecum through an efficient one-way valve called the ileal sphincter, which is formed by a raised circular fold of tissue.

    The Cecum, Large Colon, and Small Colon

    The cecum is a large fermentation vat that is positioned primarily on the right side of the horse's abdomen (Figure 4). In the average adult horse, the cecum is about 1 m long, is shaped somewhat like a comma, and can hold up to 68 L of ingesta.4 The cecum contains fluid, including swallowed saliva and gastric, pancreatic, and small intestinal secretions. The base of the cecum is in the right paralumbar fossa, the body is nestled between the right and left parts of the large colon, and the apex of the cecum points to the ventral midline. The cecum is attached to the dorsal body wall near the right kidney and the root of the mesentery and has sacculations that are involved in mixing the ingesta so the microorganisms can digest the cellulose in plant material.

    Ingesta leave the cecum and move into the large colon on the right side of the horse's abdomen. The easiest way to remember the anatomy of the large colon is to consider it as two horseshoe-shaped pieces of intestine, with one horseshoe sitting directly on top of the other. The ingesta move first through the lower or ventral piece of intestine, turn dorsally, and then move through the upper or dorsal piece of intestine. Thus ingesta start in the right ventral colon, move through the left ventral colon, turn and move up into the left dorsal colon, and finally return to the horse's right side in the right dorsal colon (Figure 5). The diameters of the right and left ventral colons are about 25 cm. However, the diameter at the junction of the left ventral and left dorsal colons, called the pelvic flexure, is only about 8 cm. This is a natural site for obstruction of the large colon by dry, impacted ingesta.5 The size of the dorsal colon increases, with the right dorsal colon having a diameter of approximately 50 cm. The right dorsal colon and cecum are fixed firmly to the dorsal body wall near the root of the mesentery, and the ventral and dorsal colons are connected by a fairly short mesentery. Thus, although the majority of the large colon can move in the abdomen, the ventral and dorsal colons must move as a unit.

    The ingesta leave the right dorsal colon and enter the transverse colon, a short segment of intestine that is fixed tightly to the dorsal body wall cranial to the root of the mesentery. The transverse colon has a diameter of approximately 8 cm and connects directly to the descending or small colon (Figure 6). The small colon is approximately 3 m long, contains sacculations, and is the portion of the intestinal tract in which fecal balls are formed. The last portion of the horse's GI tract is the rectum, which begins at the pelvic inlet and ends at the anus. The rectum is about 25 cm long.

    Spleen

    Although the spleen is not part of the GI tract, it is involved in one of the most common displacements involving the large colon. The spleen lies against the horse's left abdominal wall with its wide dorsal portion near the left kidney and its more narrow end directed ventrally (Figure 7). The dorsal portion of the spleen is attached to the left kidney by a strong band of tissue called the renosplenic or nephrosplenic ligament. This ligament attaches to the medial side of the spleen forming a natural "shelf" over which the large colon can become displaced and then trapped by the most dorsal part of the spleen.

    Common Abnormalities of the Large Colon

    Three common abnormalities of the large colon are left dorsal displacement, right dorsal displacement, and large colon volvulus. The displacements can be treated either medically or surgically, whereas the latter condition is life threatening and requires emergency abdominal surgery.

    Left Dorsal Displacement

    Left dorsal displacement of the colon occurs when either the pelvic flexure or the entire left colon moves up and over the renosplenic ligament.6 It has been theorized that the colon initially becomes distended with gas, the spleen contracts in response to abdominal pain, the colon becomes displaced dorsally, and the spleen refills with blood and "traps" the colon (Figure 8). Left dorsal displacement usually is associated with a moderate degree of abdominal pain or a prolonged course of intermittent painful episodes. Because the blood supply to the displaced colon is not impaired, the horse does not become sick. As a result, the color of the oral mucous membranes remains normal, and the heart rate is increased only slightly. The diagnosis is made during rectal examination when the veterinarian palpates either the pelvic flexure over the renosplenic ligament or the longitudinal bands of the left ventral colon running dorsocranially to the left kidney or it is determined that the spleen is displaced away from the left body wall. The condition also may be identified using ultrasonography.7 Because the spleen becomes engorged with blood and displaced toward the middle of the abdomen, blood may appear when paracentesis is performed because the needle or cannula may pierce the spleen.

    Several techniques have been developed to treat horses with left dorsal displacement of the colon. Sometimes food is withheld from the horse because natural evacuation of the intestinal contents often allows the colon to return to its normal position. Another technique is to "roll" the horse while it is under short-term anesthesia (usually 1.1 mg/kg xylazine or 0.02 mg/kg detomidine and 2.2 mg/kg ketamine). This procedure, which involves elevating the horse's rear limbs and rolling the horse 360°, is done in an attempt to displace the colon from its abnormal position over the renosplenic ligament and let it return to its normal position. Another method is to administer drugs, such as phenylephrine, that cause the spleen to contract and then "jog" the horse. The goal of this treatment is to reduce the size of the dorsal portion of the spleen that is "trapping" the displaced colon and then determine if jogging will help the colon move back into its normal position. The rate of successful correction of this condition by any of these methods is approximately 75%.8

    Finally, some veterinarians perform surgery to return the colon to its correct position. Surgical intervention is performed via ventral midline celiotomy. Once the displacement is identified, the spleen is retracted medially and the colon lifted to free it from the edge of the spleen. The advantage of the surgical procedure is that the viability of the colon can be assessed. Overall, the prognosis associated with left dorsal displacement is very good.8,9

    Right Dorsal Displacement

    Right dorsal displacement differs from left dorsal displacement because the colons (in the former type) get trapped between the cecum and the right body wall. It has been hypothesized that the condition is initiated by im­paction at the pelvic flexure, which causes the pelvic flexure to displace cranially. The left and right ventral colons then distend with gas and flip caudally ventral to the cecum. As a result, the right ventral and right dorsal portions of the colon twist on themselves near the base of the cecum, and the pelvic flexure ends up near the diaphragm (Figure 9).10 Although there may be some interference with venous drainage from the affected colon, usually the arterial supply remains intact.

    Most horses with right dorsal displacements exhibit moderate degrees of pain and slowly become dehydrated. Some horses may show signs of only mild abdominal pain, whereas others appear to be in severe pain. The degree of pain correlates with the degree of gas distention of the colon. The condition is identified during the rectal examination when the bands on the colon running transversely across the pelvic inlet are palpated, and the cecum cannot be identified.

    If the horse is in only mild pain and there is no evidence of gas distention, food can be withheld to see if natural evacuation of the intestinal contents allows the colon to return to its normal position. If the animal is in pain, surgery must be performed while the horse is under general anesthesia and positioned on its back. The pelvic flexure must be identified and the colon relocated to its normal position by rotating it around the cecal base. This procedure corrects the twisting of the right ventral and right dorsal portions of the colon. Manipulation may be difficult because of the degree of displacement and size of the colon. Consequently, a large ventral midline incision often is necessary. The prognosis is very good provided care is used to ensure that the colonic wall is not damaged during surgery.9

    Large Colon Volvulus

    Large colon volvulus is the most critical problem affecting the horse's large colon. Although the term torsion has been used for years to indicate that the colon has twisted on itself, the condition should be called a volvulus because the twisting involves the mesentery between the ventral and dorsal colons. The volvulus occurs very close to the attachment of the right ventral colon to the cecum and results in a great deal of gas distention of the affected colon. Because the blood supply to the dorsal and ventral colons is involved in the twist, much of the colon can rapidly become ischemic (Figure 10).

    The onset of abdominal pain is sudden, and the degree of pain is often severe. The severity of the pain depends on the extent of the twist and the amount of gas distention. The colon becomes extremely enlarged and very evident on rectal examination. The horse's heart rate is rapid, the animal's status deteriorates rapidly, and there is poor peripheral perfusion. Distention of the abdomen usually is marked. On brood mare farms, colonic volvulus most commonly occurs about 90 days after foaling, but the cause of the condition is unknown.10

    Horses with colonic volvulus require surgery to correct the problem and remove ischemic colon if necessary. Because the condition recurs in about 20% of brood mares, special surgical procedures have been devised to reduce the recurrence of the condition.11 The prognosis is directly related to the time that elapses between onset of the condition and surgery. As a result, survival rates exceeding 90% are common for veterinary surgical facilities located near brood mare farms, whereas much lower survival rates are reported when horses must be transported several hours to a surgical facility.

    Conclusion

    One of the most important factors involved in making an accurate diagnosis in horses with signs of colic is having a solid understanding of the anatomy of the horse's GI tract. Technicians should have an appreciation of the complexity of the horse's abdominal contents and how some of the more common abnormalities of the large colon arise.

    *All images presented within this text were created by Thel Melton, University of Georgia, and provided by The Glass Horse Project LLC (www.3dglasshorse.com); with permission.

    1. Todhunter RJ, Erb HN, Roth L: Gastric rupture in horses: A review of 54 cases. Equine Vet J 18(4):288-293, 1986.

    2. White NA: Rectal examination for the acute abdomen, in White NA, Moore JN (eds): Current Techniques in Equine Surgery and Lameness, ed 2. Philadelphia, WB Saunders, 1998, pp 262-269.

    3. van der Velden MA: Surgical treatment of acquired inguinal hernia in the horse: A review of 51 cases. Equine Vet J 20(3):173-177, 1988.

    4. Ross MW: Diseases of the cecum, in Colahan PT, Merritt AM, Mayhew IG, Moore JN (eds): Equine Medicine and Surgery, ed 5. St Louis, Mosby, 1999, pp 735-470.

    5. White NA, Dabareiner RM: Treatment of impaction colics. Vet Clin North Am Equine Pract 13(2):243-259, 1997.

    6. Sivula NJ: Renosplenic entrapment of the large colon in horses: 33 cases (1984-1989). JAVMA 199(2):244-246, 1991.

    7. Santschi EM, Slone DE, Frank WM: Use of ultrasound in horses for diagnosis of left dorsal displacement of the large colon and monitoring its nonsurgical correction. Vet Surg 22(4):281-284, 1993.

    8. Hardy J, Minton M, Robertson JT, et al: Nephrosplenic entrapment in the horse: A retrospective study of 174 cases. Equine Vet J 32(Suppl):95-97, 2000.

    9. Baird AN, Cohen ND, Taylor TS, et al: Renosplenic entrapment of the large colon in horses: 57 cases (1983-1988). JAVMA 198(8):1423-1426, 1991.

    10. Johnston JK, Freeman DE: Diseases and surgery of the large colon. Vet Clin North Am Equine Pract 13(2):317-340, 1997.

    11. Hance SR, Embertson RM: Colopexy in broodmares: 44 cases (1986-1990). JAVMA 201(5):782-878, 1992.

    References »

    NEXT: On the Cover: A Talk with Greg Hanson, RVT, VTS (Anesthesia)

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