In 75% of patients, acute intestinal obstruction results from adhesive bands or internal hernias secondary to previous abdominal surgery or from external hernias. The incidence of acute intestinal obstruction requiring hospital admission within the first few postoperative weeks is 5–25%, and 10–50% of these patients will require surgical intervention. The incidence of postoperative intestinal obstruction may be lower following laparoscopic surgery than open procedures. However, the laparoscopic gastric bypass procedure may be associated with an unexpected high rate of intestinal obstruction, with a higher reoperative rate. Other causes of intestinal obstruction not related to previous abdominal surgery include lesions intrinsic to the wall of the intestine, e.g., diverticulitis, carcinoma, and regional enteritis; and luminal obstruction, e.g., gallstone obstruction, intussusception.
Two other conditions that must be differentiated from acute intestinal obstruction include adynamic ileus and primary intestinal pseudo-obstruction. Adynamic ileus is mediated via the hormonal component of the sympathoadrenal system and may occur after any peritoneal insult; its severity and duration will be dependent to some degree on the type of peritoneal injury. Hydrochloric acid, colonic contents, and pancreatic enzymes are among the most irritating to the peritoneum, whereas blood and urine are less so. Adynamic ileus occurs to some degree after any abdominal operation. Retroperitoneal hematoma, particularly associated with vertebral fracture, may cause severe adynamic ileus, and the latter may occur with other retroperitoneal conditions, such as ureteral calculus or severe pyelonephritis. Thoracic diseases, including lower-lobe pneumonia, fractured ribs, and myocardial infarction, frequently produce adynamic ileus, as do electrolyte disturbances, particularly potassium depletion. Finally, intestinal ischemia, whether from vascular occlusion or intestinal distention itself, may perpetuate an adynamic ileus. Intestinal pseudo-obstruction is a chronic motility disorder that frequently mimics mechanical obstruction. This condition is often exacerbated by narcotic use.
Distention of the intestine is caused by the accumulation of gas and fluid proximal to and within the obstructed segment. Between 70 and 80% of intestinal gas consists of swallowed air, and because this is composed mainly of nitrogen, which is poorly absorbed from the intestinal lumen, removal of air by continuous gastric suction is a useful adjunct in the treatment of intestinal distention. The accumulation of fluid proximal to the obstructing mechanism results not only from ingested fluid, swallowed saliva, gastric juice, and biliary and pancreatic secretions but also from interference with normal sodium and water transport. During the first 12–24 h of obstruction, a marked depression of flux from lumen to blood of sodium and water occurs in the distended proximal intestine. After 24 h, sodium and water move into the lumen, contributing further to the distention and fluid losses. Intraluminal pressure rises from a normal of 2–4 cmH2O to 8–10 cmH2O. The loss of fluids and electrolytes may be extreme, and unless replacement is prompt, hypovolemia, renal insufficiency, and shock may result. Vomiting, accumulation of fluids within the lumen, and the sequestration of fluid into the edematous intestinal wall and peritoneal cavity as a result of impairment of venous return from the intestine all contribute to massive loss of fluid and electrolytes.
A “closed loop” is the most feared complication of acute intestinal obstruction. Closed-loop obstruction results when the lumen is occluded at two points by a single mechanism such as a fascial hernia or adhesive band, thus producing a closed loop, the blood supply of which is also often occluded by the hernia or band. During peristalsis, when a “closed loop” is present, pressures reach 30–60 cmH2O. Strangulation of the closed loop is common in association with marked distention proximal to the involved loop. A form of closed-loop obstruction is encountered when complete obstruction of the colon exists in the presence of a competent ileocecal valve (85% of individuals). Although the blood supply of the colon is not entrapped within the obstructing mechanism, distention of the cecum is extreme because of its greater diameter (Laplace's law), and impairment of the intramural blood supply is considerable, with consequent gangrene of the cecal wall. Once impairment of blood supply to the gastrointestinal tract occurs, bacterial invasion supervenes, and peritonitis develops. The systemic effects of extreme distention include elevation of the diaphragm with restricted ventilation and subsequent atelectasis. Venous return via the inferior vena cava may also be impaired.
Mechanical intestinal obstruction is characterized by cramping midabdominal pain, which tends to be more severe the higher the obstruction. The pain occurs in paroxysms, and the patient is relatively comfortable in the intervals between the pains. Audible borborygmi are often noted by the patient simultaneously with the paroxysms of pain. The pain may become less severe as distention progresses, probably because motility is impaired in the edematous intestine. When strangulation is present, the pain is usually more localized and may be steady and severe without a colicky component, a fact that often causes delay in diagnosis of obstruction. Vomiting is almost invariable, and it is earlier and more profuse the higher the obstruction. The vomitus initially contains bile and mucus and remains as such if the obstruction is high in the intestine. With low ileal obstruction, the vomitus becomes feculent, i.e., orange-brown in color with a foul odor, which results from the overgrowth of bacteria proximal to the obstruction. Hiccups (singultus) are common. Obstipation and failure to pass gas by rectum are invariably present when the obstruction is complete, although some stool and gas may be passed spontaneously or after an enema shortly after onset of the complete obstruction. Diarrhea is occasionally observed in partial obstruction. Blood in the stool is rare but does occur in cases of intussusception.
In adynamic ileus as well as colonic pseudo-obstruction, colicky pain is absent and only discomfort from distention is evident. Vomiting may be frequent but is rarely profuse. Complete obstipation may or may not occur. Singultus (hiccups) is common.