Acute intestinal obstruction
Etiology and Classification
In 75% of patients, acute intestinal obstruction results from previous abdominal surgery secondary to adhesive bands or internal or external hernias. The incidence of acute intestinal obstruction requiring hospital admission within the first few post-operative 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. The reason for this is unknown. Other causes of intestinal obstruction not related to previous abdominal surgery include lesions intrinsic to the wall of the intestine, e.g., diverticulitis, carcinoma, 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 substances, 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. Unnecessary operations in such patients should be avoided.
Pathophysiology
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 occurs of sodium and consequently water 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.
The most feared complication of acute intestinal obstruction is the presence of a "closed loop." Closed-loop obstruction of the small intestine 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 whose blood supply is often occluded by the hernia or band as well. 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.
Symptoms
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.
Physical Findings
Abdominal distention is the hallmark of all forms of intestinal obstruction. It is least marked in cases of obstruction high in the small intestine and most marked in colonic obstruction. In early obstruction of the small and large intestine, tenderness and rigidity are usually minimal; the temperature is rarely more than 37.8°C (100°F). The appearance of shock, tenderness, rigidity, and fever indicates that contamination of the peritoneum with infected intestinal content has occurred. Hernial orifices should always be carefully examined for the presence of a mass. Auscultation may reveal loud, high-pitched borborygmi coincident with colicky pain, but this finding is often absent late in strangulating or nonstrangulating obstruction. A quiet abdomen does not eliminate the possibility of obstruction, nor does it necessarily establish the diagnosis of adynamic ileus. The presence of a palpable abdominal mass usually signifies a closed-loop strangulating small-bowel obstruction; the tense fluid-filled loop is the palpable lesion.
Laboratory and X-Ray Findings
Laboratory and radiographic studies are used to help differentiate the two important clinical aspects of this disorder: strangulation vs. nonstrangulation and partial vs. complete obstruction. Leukocytosis, with shift to the left, usually occurs when strangulation is present, but a normal white blood cell count does not exclude strangulation. Elevation of the serum amylase level is encountered occasionally in all forms of intestinal obstruction. Roentgenographic images demonstrating distention of fluid- and gas-filled loops of small intestine usually arranged in a "stepladder" pattern with air-fluid levels and an absence or paucity of colonic gas are pathognomonic for small-bowel obstruction. Complete obstruction is suggested when passage of gas or stool per rectum has ceased and when gas is absent in the distal intestine by x-ray. A general haze due to peritoneal fluid and sometimes a "coffee bean"–shaped mass are seen in strangulating closed loop obstruction. A thin barium upper gastrointestinal series may help to differentiate partial from complete obstruction. However, thick barium given by mouth should be avoided when the obstruction is considered to be high grade or complete since retained barium sulfate may become inspissated. CT is the most commonly used modality to evaluate postoperative patients for intestinal obstruction because of its ability in differentiating adynamic ileus, partial obstruction, and complete obstruction (Fig. 293-1). However, the sensitivity and specificity of CT for strangulating obstruction are low (50 and 80%, respectively).
| ||
| ||
|
Common causes of colonic obstruction can be seen on abdominal roentgenographic series. These films may demonstrate a "bird's beak" sign when a sigmoid volvulus has occurred or an enlarged cecum when a cecal torsion or bascule is present. Colonic obstruction with a competent ileocecal valve is easily recognized because distention with gas is mainly confined to the colon. Gastrografin enema may help in demonstrating a complete colonic obstruction. Furthermore, barium should never be given by mouth to a patient with a possible colonic obstruction until that possibility has been excluded.
Acute Intestinal Obstruction: Treatment
Small-Intestinal Obstruction
The overall mortality rate for obstruction of the small intestine is about 10%. While the mortality rate for nonstrangulating obstruction is 5–8%, the mortality rate for a strangulating obstruction ranges from 20 to 75%. Since strangulating small-bowel obstruction is always complete, surgical interventions should always be undertaken in such patients after suitable preparation. Before operating, fluid and electrolyte balance should be restored and decompression instituted by means of a nasogastric tube. Replacement of potassium is especially important because intake is nil and losses in vomitus are large. There are few, if any, indications for the use of a long intestinal tube. Operative intervention may be undertaken successfully by laparoscopic techniques with a decreased incidence of wound complications. However, laparoscopic lysis of adhesions is associated with a longer operative time and higher conversion to open rate when compared to other laparoscopic procedures. Alternatively, lysis of adhesions can occur through an open abdominal incision. In general, more than 50% of adhesions that occur are found at the previous incision site. Purely nonoperative therapy is safe only in the presence of incomplete obstruction and is best utilized in patients without increasing abdominal pain or leukocytosis. The overall recurrence of small-bowel obstruction is 16%. Population-based studies show that although the surgical management of small-bowel obstruction is associated with longer hospital stays, the rate of readmission for obstruction is lower. However, regardless of treatment type, following the index admission, only 20% of patients required readmission within a 5-year follow-up period.
Colonic Obstruction
The mortality rate for colonic obstruction is about 20%. As in small-bowel obstruction, nonoperative treatment is contraindicated unless the obstruction is incomplete. Incomplete obstruction can be treated with colonoscopic decompression and placement of a metallic stent if a malignant lesion is present. The success rate approaches 90% depending on the location of the obstruction, with left-sided lesions being more successfully stented than right-sided lesions. In general, the colonic stent is considered to be a temporary solution or a "bridge to surgery," which allows for colonic preparation before surgical intervention. When obstruction is complete, early operation is mandatory, especially when the ileocecal valve is competent, because of the concern for cecal perforation. Cecal perforation is more likely if the cecal diameter is more than 10 cm on plain abdominal film.
Decisions regarding the operative management of colonic obstruction are based on the cause of the obstruction and the patient's overall well-being. For obstruction on the left side of the colon, operative management strategies include either decompression by cecostomy or transverse colostomy or resection with end-colostomy formation (Hartmann's procedure). Primary resection of obstructing left-sided lesions with on-table washout of the colon has also been accomplished safely. For a lesion of the right or transverse colon, primary resection and anastomosis can be performed safely because distention of the ileum with consequent discrepancy in size and hazard in suture are usually not present. Furthermore, the bacterial and stool content is less on the right side of the colon, decreasing the chance of infection.
Adynamic Ileus
This type of ileus usually responds to nonoperative decompression and treatment of the primary disease. The prognosis is usually good. Correction of electrolyte abnormalities should be performed (i.e., potassium, magnesium). Successful decompression of a colonic ileus has been accomplished by repetitive colonoscopy. Neostigmine is also effective in cases of colonic ileus that have not responded to other conservative treatment. Rarely, adynamic colonic distention may become so great that cecostomy is required if cecal gangrene is feared.
