Cause of Intussusception
Intussusception is an invagination of one segment of the gastrointestinal tract into the lumen of an adjoining segment. The intussusceptum is the invaginated segment of the alimentary tract, whereas the intussuscipiens is the enveloping segment. Invagination may occur in an antegrade (aborad) or retrograde (orad) direction, but is most commonly in the antegrade direction. Any portion of the alimentary tract may be involved, but enterocolic intussusceptions account for almost two thirds of the published cases in dogs and cats. Enterocolic intussusceptions can be further divided into three types: (1) cecocolic (or cecal inversion), with the inverted cecum forming the apex; (2) ileocolic, with the ileum forming the apex; and (3) ileocecal, with the ileum forming the apex. Of these three forms of enterocolic intussusception, the ileocolic intussusception is the one most frequently encountered in clinical practice. A number of conditions are reported to predispose to intussusception, including intestinal parasitism, viral enteritis, foreign bodies, and masses, but in dogs and cats most intussusceptions are idiopathic.
The initiating events in an intussusception are often difficult to identify retrospectively, but all intussusceptions appear to share three important features: (1) inhomogeneity in a bowel segment — a region in which the gastrointestinal tract undergoes a sudden anatomic change in diameter (e. g., ileocolic junction) or a bowel segment that is either flaccid or indurated; (2) mechanical linkage of nonadjacent segments — which can be intraluminal (e. g., linear foreign bodies, parasites) or extramural (e. g., fibrous adhesions or bands); and (3) peristaltic activity of the gut.
Invagination begins as a result of peristaltic contraction. Once the invagination has begun, its progress may be rapid, involving as much as several centimeters of intestinal tract within just a few hours. Invagination and intussusception result in luminal obstruction, which may be partial or complete. Obstruction usually results in distension of the bowel segment proximal to the intussusception. The degree of distension is dependent upon the completeness and duration of the obstruction, volume of fluid secretion, degree of vascular compromise, and volume of gas production from bacterial fermentation. Because the mesentery and blood supply are included in the invaginating segment, vascular compromise can occur, which initially leads to intramural hemorrhage and edema and eventually to ischemia and necrosis of the bowel. Full-thickness necrosis may ensue, but perforations are rare.
The most important clinical signs with ileocolic intussusceptions are intermittent vomiting, progressive loss of appetite, mucoid bloody diarrhea, and a palpable cylinder-shaped mass in the cranial abdomen. Abdominal pain is not a consistent finding in affected animals. Clinical signs may persist for several weeks, and affected animals eventually succumb to the effects of starvation rather than dehydration, electrolyte imbalances, or acid-base disturbances.
Diagnosis of Intussusception
With some ileocolic intussusceptions, the intussuscepted bowel may protrude through the anus and must be differentiated from a rectal prolapse. This is accomplished by passing a blunt probe between the protruding segment and the anal sphincter. If the probe can be passed cranial to the pubis without reaching a fornix, then the protruding bowel is the apex of an intussusception rather than rectal prolapse.
Survey abdominal radiographic findings may be suspicious for intussusception. Barium contrast studies (barium enema or upper gastrointestinal series) are often diagnostic, but abdominal ultrasonography is the preferred method of diagnosis. The appearance of a targetlike mass (consisting of two or more hyperechoic and hypoechoic concentric rings in transverse section) or the appearance of multiple hyperechoic and hypoechoic parallel lines in longitudinal section is virtually diagnostic of an intussusception. The ultrasound scan might also identify a mass associated with the intussusception. Endoscopy may be performed in suspected cases of suspected neoplasia, otherwise endoscopy does not confer any additional benefits over abdominal ultrasound or CT scanning.
Treatment of Intussusception
The surgical management of ileocolic intussusception involves either reduction or resection, and anastomosis, or both. Secretory diarrhea may persist after relief of the obstruction, and affected animals may need continuous crystalloid and colloidal therapy. If possible, the ileocecal sphincter should be preserved to reduce reflux and contamination of the distal small bowel. Cecocolic intussusceptions or inversions should also be treated with surgical resection. Surgical resection of the cecocolic intussusceptions is generally curative. Enteroplication procedures have been recommended, but they do not appear to reduce recurrence rates.
Prognosis of Intussusception
The most common complications after treatment of intussusception are recurrence, dehiscence of the anastomosis, ileus, intestinal obstruction, peritonitis, and short bowel syndrome. The recurrence rate in dogs is reported to be between 11% and 20%. In dogs in which no surgical procedure was performed to prevent recurrence, intussusception recurred in 25% of dogs that underwent manual reduction alone and in 19% of dogs that underwent resection and anastomosis. Enteroplication does not appear to reduce recurrence rates any further. Indeed, 19% of dogs undergoing enteroplication in one study experienced severe complications that required a second surgery. Intestinal obstruction was a complication of the enteroplication in those patients.