Vanessa L. Cook
Medical Management of Large (Ascending) Colon Colic
Colic caused by large colon disorders is relatively common. It is most frequently seen in younger to middle-aged performance horses that are receiving large amounts of grain or sweet feed. Carbohydrates are usually hydrolyzed to simple sugars and actively absorbed in the small intestine. However, if large amounts of hydrolyzable carbohydrates are consumed at one time, the digestive processes of the small intestine are overwhelmed, and undigested carbohydrates pass into the cecum and large colon. There they are subjected to bacterial fermentation, with the resulting ingesta being less dense and containing more gas bubbles than in horses on a lower carbohydrate diet. Such changes in the consistency of the colonic contents could predispose horses to displacements of the large colon and even colonic volvulus. Additionally, feeding large meals one to two times daily results in transient postprandial dehydration and activation of the renin-angiotensin-aldosterone system to conserve water. The resultant resorption of water from the colonic contents could cause impactions. It is therefore recommended that a single feeding of grain not exceed 0.2% of the horse’s body weight (approximately 2 lb for a 1000-lb horse). If a high calorie intake is required, that quantity can be offered three or four times daily rather than increasing the amount given at one time.
Most horses with colic attributable to the large colon can be managed medically. The exception is horses with a large colon volvulus, for which early identification and rapid referral for surgery are critical to survival. Because so many horses with large colon displacements or impactions respond favorably to intensive medical therapy, far fewer undergo surgical correction now than they did 10 to 15 years ago. Medical management is usually less expensive and allows the horse to return to work more rapidly. Medical treatment also offers an option for horses for which referral or surgical intervention is not possible because of economics, distance to a referral facility, or personal choice of the client. In such situations, more aggressive medical therapy can be attempted to achieve a successful outcome.
Should Medical Treatment be Attempted in This Horse?
Before it is elected to continue medical treatment, every possible effort should be made to rule out an ischemic intestinal lesion, such as a large colon volvulus, that requires surgical correction. There are several diagnostic tools that can aid in making this critical distinction. Transabdominal ultrasound imaging should be the starting point for this determination. In particular, when large colon gas distension is felt on rectal examination, ultrasound can be used to differentiate between a large colon volvulus that necessitates surgery and a displacement that could be managed medically. Gas and ingesta in the large colon prevent sound waves from traversing the colon; therefore ultrasound is used primarily to assess the thickness of the large colon wall. Colon wall thickness is best imaged in the cranioventral part of the abdomen, just caudal to the xiphoid, in the region where abdominocentesis would be performed. The wall of the colon is normally less than 5 mm thick but can increase to more than 9 mm with colon wall edema. Observation of colon wall edema, coupled with other corroborating signs such as a increased heart rate and severe pain, is 100% specific for a large colon volvulus. Acute volvulus that is evaluated early in the course of the disease may have a wall thickness of less than 5 mm, but that value increases rapidly with the occlusion of venous drainage. Consequently, a repeat ultrasound examination an hour later will reveal an increase in wall thickness if a volvulus is present.
Another important diagnostic technique to differentiate a severe intestinal problem from one that can be managed medically is abdominocentesis. Color of the fluid alone can be indicative of a strangulating obstruction because red blood cells leach into the peritoneal fluid from ischemic intestine. Serosanguineous peritoneal fluid is 99% specific for a lesion that requires surgical correction. Determination of peritoneal fluid total protein concentration should also be made. A concentration greater than 2.5 g/dL can indicate ischemia or inflammation, such as peritonitis or enteritis. Changes in the abdominal fluid are much more sensitive for a small than a large intestinal lesion, and peritoneal fluid can appear grossly normal in a horse with a large colon volvulus. Despite this, abdominocentesis is still helpful, in that finding normal peritoneal fluid will help to confirm that the horse truly has a large colon problem, and that a small intestinal lesion such as an epiploic foramen entrapment is not being overlooked.
A final technique that can be used to detect a serious problem is measurement of blood lactate concentration. Pyruvate is produced from glucose by glycolysis and is subsequently used in the Krebs cycle to produce adenosine triphosphate. However, in the absence of oxygen, pyruvate is converted to lactate. An increase in blood lactate concentration is an early sensitive indicator of anaerobic metabolism and poor peripheral perfusion. Lactate concentration can be measured rapidly and inexpensively in the field with a handheld lactometer. Blood lactate concentration should be less than 0.8 mmol/L. Slight elevations are common in horses that have been expending energy from active signs of colic. However, in any horse with colic, a blood lactate concentration of less than 4 mmol/L indicates poor tissue perfusion that warrants treatment with large volumes of intravenous fluids. Horses with a large colon volvulus and plasma lactate of more than 6 mmol/L have a poor prognosis for survival. Lactate concentration can also be measured in peritoneal fluid. Normally, peritoneal and blood lactate concentrations are the same, but because lactate readily leaks out of devitalized intestine into the peritoneal fluid, a higher lactate concentration in peritoneal fluid than in blood suggests that the horse has ischemic or inflamed intestine.
If results from these basic tests are within normal limits, the horse is unlikely to have compromised ascending colon that necessitates surgery or euthanasia, and further medical treatment can be attempted. Medically managed horses should be monitored regularly and all diagnostic testing repeated if the horse becomes increasingly painful or its cardiovascular status deteriorates.
Medical Management of Specific Large Colon Disorders
Pelvic Flexure Impaction
Impactions of the large colon usually occur at the pelvic flexure or in the right dorsal colon. Causes include a decrease in water intake and altered function of the pelvic flexure pacemakers. Mares appear to be overrepresented, and certain individuals seem to be predisposed to recurrent impactions.
The traditional treatment for impaction is administration of mineral oil through a nasogastric tube. Mineral oil should never be syringed into a horse’s mouth because it does not trigger the mechanoreceptors in the nasopharynx, and aspiration can result, a complication that is usually fatal. Mineral oil neither helps the impaction to pass nor breaks it up. Its only function is to act as a marker for passage from one end of the intestinal tract to the other, and it therefore serves little purpose, although horse owners usually expect it to be administered. Other substances used to treat impactions include osmotic laxatives, such as magnesium sulfate or sodium sulfate, and surfactants such as dioctyl sodium sulfosuccinate. Overhydration with intravenous crystalloids has also been advocated to promote intraluminal secretions and hence rehydrate the impaction. In a study that compared the efficacy of sodium sulfate, magnesium sulfate, intravenous fluids, water, and an orally administered balanced electrolyte solution on colonic water content and plasma electrolyte concentrations, the oral balanced electrolyte solution was the best treatment for large colon impactions because it was the only treatment that hydrated the contents of the right dorsal colon without causing electrolyte imbalances.
The treatment of choice for a pelvic flexure impaction is isotonic enteral fluid administration, which resolves 99% of impactions. Intravenous fluid therapy is rarely needed unless there is cardiovascular compromise. Enteral fluids are not only effective at rehydrating the impaction but also cause gastric distension, which elicits the gastrocolic reflex and triggers contraction of the colon. These effects promote passage of the impaction. For administration, a stomach tube is passed and tied to the horse’s halter to maintain it in place. Most horses will tolerate the tube, but a muzzle is often necessary to prevent the horse from pulling the tube out between treatments (Figure 74-1). It is important to check for gastric reflux before each administration because an expanding, softening impaction can compress the duodenum and occlude gastric emptying. If reflux is obtained, oral fluids should be discontinued. Usually 4 to 6 L of lukewarm water is administered with electrolytes added to make it isotonic. Various recipes are in use, but the inclusion of both NaCl and KCl appears to be critical to prevent electrolyte alterations. In the previously mentioned investigation, the solution used contained 5.27 g NaCl, 0.37 g KCl, and 3.78 g NaHCO3 per liter of water. A simpler recipe is to alternate treatments of 30 mL (1 oz) NaCl with 30 mL (1 oz) KCl in 4 to 6 L of water. Because of the large volume of fluids being given at one time, it is imperative that they be administered by gravity flow with a funnel on the end of the tube, as opposed to being pumped in (Figure 74-2). It is not unusual for the horse to show increased signs of colic after administration. This is likely because of gastric distension or colonic contraction around the impaction secondary to activation of the gastrocolic reflex and should not be interpreted as failure of the technique. It is often helpful for the horse to be walked for 5 to 10 minutes after each fluid administration. Some horses do not tolerate the nasogastric tube and constantly retch and gag on the tube. In those instances, the esophagus can be perforated, so the tube should be removed and an alternative treatment used.
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