Strangulation obstruction of the small intestine is frequently fatal because of simultaneous occlusion of the intestinal lumen and its blood supply, resulting in progressive necrosis of the mucosa, and development of endotoxemia. Among the more common causes of this condition are strangulating lipomas and entrapment within a natural internal opening or a mesenteric defect.
Strangulation obstruction of the small intestine is frequently fatal because of simultaneous occlusion of the intestinal lumen and its blood supply, resulting in progressive necrosis of the mucosa, and development of endotoxemia. Among the more common causes of this condition are strangulating lipomas and entrapment within a natural internal opening or a mesenteric defect. It has been widely recognized that horses with this type of obstruction are very susceptible to postoperative complications, most notably endotoxic shock, postoperative ileus, and intra-abdominal adhesions. Despite these complications, short-term survival rates (generally defined as discharge from the hospital) have dramatically improved over the last decade. However, long-term survival remains a major concern, particularly because of complications associated with adhesions.
Horses with small intestinal strangulating obstruction typically have moderate and persistent signs of colic, until the latter stages of the disease process, when they become profoundly depressed. Horses will have progressive signs of endotoxemia, including congested mucus membranes, delayed capillary refill time, and an elevated heart rate (60-80bpm). In addition, reflux is typically obtained following passage of a stomach tube, and loops of distended small intestine are usually detected on rectal palpation of the abdomen. However, these latter findings are variable, depending upon the duration of obstruction, and the location of the obstruction. For example, horses with ileal obstructions tend to reflux later in the course of the disease process than horses with a jejunal obstruction. Furthermore, a horse that has an entrapment of small intestine in the epiploic foramen or a rent in the gastrosplenic ligament may not have palpable loops of small intestine because of the cranial location of these structures. Abdominocentesis is indicated in horses with suspected strangulation of the small intestine because analysis of abdominal fluid can provide critical information on the integrity of the intestine. For instance, a horse that has signs compatible with a small intestinal obstruction, and additionally has serosanguinous abdominal fluid with an elevated protein level (>2.5mg/dl) is likely to require surgery. A horse that also has an elevated white blood cell count (>10,000cells/μl) in the abdominal fluid likely has extensive strangulation, or long-standing strangulation.
Treatment should initially be aimed at controlling pain. Short-term analgesics such as xylazine (0.3 – 0.5mg/kg, IV, prn) and butorphanol (0.05mg/kg, IV, prn) are very useful because of their potency, and because recurrent colic will usually be detected within the time of the examination (30-60 minutes). The more potent α2 agonist detomidine (5-10μg/kg, IV, prn) is required for horses that are violently painful, or which have pain recurring within a brief period of time (e.g., 5-15 minutes). Long-term analgesics such as flunixin meglumine (1.1mg/kg, IV, q12h) are very useful for more prolonged periods of analgesia and for reducing production of endotoxin-induced prostanoids. However, flunixin meglumine is best utilized following short-duration analgesics so that recurrent colic can be detected early. In addition, the dose interval is very important considering the deleterious effect of this drug on both the gastrointestinal mucosa and kidneys, particularly in dehydrated horses.
The second major goal of treatment is to ameliorate signs of shock. Horses with small intestinal strangulating obstruction are typically at least 6% dehydrated (Table 1), requiring large volumes of isotonic fluids (e.g., 30L in a 500kg horse) to correct fluid loss or sequestration. Half of the deficit can be administered rapidly (up to 100ml/kg/hr), followed by a reduced rate for the remainder of the deficit volume (3-5L/ hr). In order to achieve more rapid correction of intravascular volume depletion, hypertonic saline or an oncotic agent such as hetastarch can be pre- or co-administered with isotonic fluids. However, these agents do not replace the need for isotonic fluids.
Treatments aimed at reducing the effects of the endotoxin-induced inflammatory cascade include flunixin meglumine and pentoxifylline. The latter is a phosphodiesterase inhibitor that reduces elaboration of tumor necrosis factor (TNF)-?, although this action is questionable at the currently recommended dosage (12mg/kg). This drug also has rheologic effects that may reduce the onset of micro-circulatory disease, including laminitis. Additional treatments that may be used, particularly if endotoxemia is detected in the early stages, are agents that bind endotoxin. These include hyperimmune serum (antibodies directed against the core polysaccharide of either J5 Eschericia coli or the Re mutant of Salmonella) and polymixin B (6,000U/kg, IV).
Causes of small intestinal strangulating obstruction
Of the many causes of small intestinal strangulating obstruction, strangulating lipomas are by far the most common cause of small intestinal strangulation at North Carolina State University, an institution that evaluates a disproportionately high number of small intestinal obstructions (at least 50% of the colic surgical caseload). Lipomas form between the leaves of the mesentery as horses age, and develop mesenteric stalks as the weight of the lipoma tugs on the mesentery. The lipoma together with its stalk may subsequently wrap around a loop of small intestine or small colon causing strangulation (Fig. 1). Strangulating lipomas should be suspected in aged (> 15-years-old) geldings with acute colic referable to the small intestinal tract. The fact that geldings are at risk suggests an endocrine role in fat deposition and subsequent lipoma formation. Although preventive measures have not been clinically tested, specific attention to the diet of mature and aging geldings to reduce excessive body fat should be considered. Ponies also appear to be at risk of developing strangulating lipomas, possibly because of their propensity to lay down body fat, suggesting similar precautions should be taken. The diagnosis of small intestinal strangulation by a pedunculated lipoma is usually made at surgery, although on rare occasions a lipoma can be palpated per rectum. Treatment involves surgical resection of the lipoma and strangulated bowel, although strangulated intestine is not always non-viable. Studies indicate that approximately 75% of horses are discharged from the hospital following surgical treatment, but the long-term survival (> 6 months) is closer to 50%.
Aside from strangulating lipomas, entrapments in the epiploic foramen or a mesenteric defect or ligamentous defect appear to be the next most common cause of small intestinal strangulating obstruction. The epiploic foramen is a potential opening (because the walls of the foramen are usually in contact) to the omental bursa located within the right cranial quadrant of the abdomen. It is bounded dorsally by the caudate process of the liver and caudal vena cava, and ventrally by the pancreas, the hepatoduodenal ligament, and the portal vein. Epiploic foramen entrapment of small intestine tends to be more prevalent in older horses, possibly because of enlargement of the epiploic foramen as the right lobe of the liver undergoes age-associated atrophy. However, the disease has also been recognized in foals. The diagnosis is definitively made at surgery, although ultrasonographic findings of distended loops of edematous small intestine adjacent to the right middle body wall are suggestive of epiploic foramen entrapment. Entrapped small intestine may enter the foramen from the visceral surface of the liver toward the right body wall, or the opposite direction. Studies differ as to which is the most common form. In treating epiploic foramen entrapment, the epiploic foramen must not be enlarged either by blunt force or with a sharp instrument, since rupture of the vena cava or portal vein and fatal hemorrhage may occur. Prognosis has substantially improved over the last decade, with current short-term survival rates of approximately 75%. However, studies indicate that long-term survival may be as low as 60%.
Recent studies on small intestinal entrapment within mesenteric defects indicate that this form of colic carries a particularly poor prognosis. This stems from either an inability to reduce the intestinal obstruction, severe hemorrhage from the mesentery, or an excessive length of involved intestine (>50% of the length of the small intestine). In addition, other types of mesenteric or ligamentous defects that entrap small intestine continue to be discovered. For example, a recent report documented small intestinal entrapment in a defect in the proximal aspect of the cecocolic ligament in 9 horses, with a long-term survival rate of approximately 50%.
The prognosis for survival in horses with small intestinal strangulating lesions is approximately 75% short-term survival and 40-70% long-term survival, depending upon the condition. Owners should be alerted to the reduced long-term survival compared to the short-term survival so that they do not have unrealistic expectations. The prognosis can be amended based on severity of endotoxemia at presentation (heart rate being consistently the most important prognostic indicator) and extent of strangulation at surgery. This is the key area where veterinarians can make a difference. By early referral (if the owner is willing), and based on the educated guess on the part of the veterinarian, many of the strangulating obstruction cases can probably be referred earlier. This would in some cases result in cases being referred that do not require surgery, but would almost certainly increase the prognosis. For example, a recent preliminary study in our hospital on horses undergoing small intestinal resection has revealed that horses that exhibit clinical signs of endotoxemia for > 3 days were far less likely to survive to discharge than horses that had resolution of such signs within 24-48 hours. Endotoxemia could be dramatically reduced by referring cases in which the veterinarian suspects surgery will ultimately be necessary. The easiest way to do this is to speak with owners about referral if the initial treatment is not effective.