The vitelline, or omphalomesenteric, duct connects the yolk sac with the primitive tubular gut in the early embryonic stages and normally involutes at about the 7th week of fetal life, leaving no trace of its existence. Failure of the vitelline duct to disappear in its entire extension results in a variety of remnants, which include a diverticulum (Meckel diverticulum) attached to the ileum, omphalomesenteric cysts (enterocysts), omphalomesenteric fistulae that drain through the umbilicus, and fibrous bands from the diverticulum to the umbilicus that predispose to bowel obstruction. The most common form is Meckel diverticulum.
|VARIANTS OF VITELLINE DUCT REMNANTS|
Meckel diverticulum is the most frequent congenital anomaly of the gastrointestinal tract and is classically described by the rule of twos. It is prevalent in approximately 2% of the population, is usually located within 2 feet of the ileocecal valve, and measures approximately 2 inches in length. It is two times as prevalent in males as in females, with approximately 2% of patients developing a complication, usually within the first 2 years of life. This diverticulum is always attached to the antimesenteric side of the ileal wall, and it varies in length (from 1 to 10 cm) and also in width (from 1 to 4 cm in diameter), though its shape usually resembles that of a finger of a glove. The artery supplying the diverticulum, the vitelline artery, is a branch of the superior mesenteric artery. It crosses over the ileal wall along the diverticulum to its tip.
Meckel diverticulum is a “true” or “complete” diverticulum composed of all the layers of the small bowel (the mucosal, muscular, and serosal layers), in contrast to an acquired intestinal diverticula, in which the mucosa and submucosa herniate through the muscle layer, covered only by serosa. The mucosal lining of a Meckel diverticulum corresponds to that of the ileum, but occasionally islands of heterotopic (jejunal, duodenal, or gastric) mucosa and nodules of pancreatic tissue may be present and can give rise to serious complications (see below).
The opening of a Meckel diverticulum is funnel-like and, as a rule, wide enough not to give rise to occlusions, as do the “false diverticula” with a narrow neck. In the majority of individuals with Meckel diverticulum, the rest of the former vitelline duct becomes completely obliterated, but in some cases, a nonpatent fibrous cord may remain, which attaches the blind end of the diverticulum to the umbilical site of the abdominal wall. Occasionally, the diverticulum or the fibrous cord may be affixed to another intestinal loop or to another viscus. Rarely, a rudiment of the vitelline duct persists permanently in the form of a solid fibrous cord without development of a diverticulum, resulting in fixation of an ileal loop to the umbilicus. This can lead to intestinal obstruction and strangulation of bowel loops.
The persistence of the entire vitelline duct as a permanent tube leads to an umbilicointestinal fistula, which should be easily discovered soon after birth. The umbilical cord in such relatively rare cases is usually thicker at its base at birth than is normal, and when its external structures have regressed and sloughed off, a reddish mass with a small opening in its center will be noted in the umbilicus. The fistula may discharge intestinal con- tents depending on the caliber of the duct and the changes in the abdominal pressure. The discharge can vary from occasional drainage of small amounts of mucus to continuous loss of enteric contents. In such cases, an umbilical polyp frequently forms at the external opening of the fistula. The most serious complication of an umbilicointestinal fistula, however, is prolapse of the ileum through a fistulous tube of large caliber. Increased abdominal pressure, when the infant cries or coughs, may cause such a prolapse, which presents itself as a dark-red, protruding, sausagelike mass, with a portion of the bowel turned inside out, and intestinal mucosa appearing at the external mouth of the fistula.
Another anatomic variant of a vitelline duct remnant is an umbilical sinus. In these cases, the vitelline duct may remain open only at its outer portion, resulting in a sinus rather than a fistula. In such instances, the proximal part of the duct closer to the ileum is usually transformed into a fibrous cord attached on one end to the sinus and on the other to the ileum. Finally, the vitelline duct may undergo fibrosis on the outer end as well as on the inner end, while a central portion persists as a patent part that develops into a cyst (enterocyst), causing, in later life, a variety of symptoms.
Meckel diverticulum, as well as other variants of vitelline duct remnants, may remain quiescent and may not be discovered for a lifetime, but they can also give rise to a variety of clinical syndromes. Acute inflammation of a diverticulum may be produced by nonspecific infections, foreign bodies, parasites, or trauma. This can range from mild inflammatory changes to gangrene with subsequent perforation and peritonitis. Clinical and pathologic findings in these cases resemble appendiceal inflammation and are often difficult to differentiate. The onset of symptoms with this diverticulitis is usually sudden. Severe pain of colicky character is localized around the umbilicus and accompanied by nausea, persistent vomiting, and fever. The abdomen may be distended, with an area of tenderness around the umbilicus or in the right or left lower quadrant. An inflamed Meckel diverticulum may be very difficult to distinguish from acute appendicitis on physical examination or preoperative imaging and may require exploratory laparotomy. Other diagnoses to consider include acute cholecystitis, colonic diverticulitis, and acute salpingitis.
Ectopic tissue is present in up to 21% of patients with Meckel diverticulum and commonly involves gastric tissue; however, duodenal and pancreatic tissues can also be present. Gastrointestinal bleeding can occur as a result of peptic ulceration caused by heterotopic gastric mucosa in the lining of a diverticulum. The ulcer usually forms adjacent to or away from the diverticulum, and not on the mucosa or ectopic tissue within the diverticulum, and may resemble the marginal jejunal ulcer occurring after gastrojejunal anastomosis. Bleeding is more likely to occur in symptomatic patients and can be chronic and insidious or present with massive hemorrhage. The diagnosis should be suspected in children who develop painless rectal bleeding or in younger adults who present with obscure gastrointestinal bleeding. The gastric mucosa, even when ectopic, is able to concentrate and secrete 99mtechnetium-labeled pertechnetate; this test is often diagnostic and can be used to localize ectopic gastric mucosa in symptomatic Meckel diverticulum. Mesenteric arteriography can also be used to localize a bleeding diverticulum. Additionally, endoscopic procedures such as double-balloon enteroscopy and capsule endoscopy have been described. If diagnostic testing is unrevealing or if the patient is hemodynamically unstable, abdominal exploration may be necessary to determine whether a Meckel diverticulum is the source of bleeding.
Intestinal obstruction can be a complication of a Meckel diverticulum; it may result from intussusception, volvulus, or incarceration in an abdominal hernia. Intussusception occurs when the diverticulum serves as the lead point of ileal or ileocolonic intussusception. A nodule of heterotopic tissue or a tumor situated near the fundus of the diverticulum may become the predisposing factor of an inversion, which, however, can be complete only when the diverticulum is in no way adherent to other structures. Volvulus can occur as a result of twisting of the intestines around a fibrous cord or band related to a Meckel diverticulum. Strangulation of the diverticulum may also occur when it enters the sac of an inguinal hernia (Littre hernia) and most commonly is seen in inguinal hernias followed by femoral and then umbilical hernias.
Benign tumors (myoma, lipoma, adenoma, and neurogenic neoplasm), as well as malignant ones, develop occasionally in a Meckel diverticulum. Different types of carcinoma, sarcoma, and carcinoid tumors have been observed, as in other parts of the small intestine.
|COMPLICATIONS OF MECKEL DIVERTICULUM AND VITELLINE DUCT REMNANTS|
Management of a Meckel diverticulum depends on the clinical presentation. Asymptomatic patients diagnosed incidentally on imaging may not undergo elective resection; however, management of a diverticulum identified on abdominal exploration in an asymptomatic patient is more controversial. Symptomatic Meckel diverticulum should be resected; either open laparotomy or laparoscopy techniques can be employed.