ANATOMY AND PHYSIOLOGY
The small intestine comprises the duodenum, jejunum, and ileum and extends from the pylorus proximally to the cecum distally. Its main function is to digest and absorb nutrients. Absorption is achieved by the large surface area of the small intestine, secondary to its long length and extensive mucosal projections of villi and microvilli. A broad-based mesentery suspends the small intestine from the posterior abdominal wall once the retroperitoneal duodenum emerges at the ligament of Treitz and becomes the jejunum. Arterial blood is supplied from branches of the superior mesenteric artery, and venous drainage is via the superior mesenteric vein (Fig. 4-1). The mucosa has sequential circular folds, called plicae circulares. The plicae circulares are more numerous in the proximal bowel than in the distal bowel. The mucosal villi and microvilli create the surface through which carbohydrates, fats, proteins, and electrolytes are absorbed (Figs. 4-2 and 4-3).
|
Figure 4-1 • Anatomy of the small intestine demonstrating vascular anatomy of the varying segments. Note longer vasa recta in jejunum versus ileum. From Lawrence PF, Bell RM, Dayton MT, et al. Essentials of General Surgery. 4th ed. Philadelphia: Lippincott Williams & Wilkins, 2005. |
|
Figure 4-2 • Structure of small intestinal villi. |
|
Figure 4-3 • Diagram of a columnar epithelial intestinal absorptive cell with luminal microvilli. |
SMALL BOWEL OBSTRUCTION
Although the potential causes of small bowel obstruction (SBO) are varied, the presentation of this disorder is usually quite consistent because of a common mechanism. Obstruction of the small bowel lumen causes progressive proximal accumulation of intraluminal fluids and gas. Peristalsis continues to transport swallowed air and secreted intestinal fluid through the bowel proximal to the obstruction, resulting in small bowel dilation and eventual abdominal distention. Depending on the location of the obstruction, vomiting occurs early in proximal obstruction and later in more distal blockage (Fig. 4-4). Crampy abdominal pain initially occurs as active proximal peristalsis exacerbates bowel dilation. With progressive bowel wall edema and luminal dilation, however, peristaltic activity decreases and abdominal pain lessens. At presentation, patients exhibit abdominal distention and complain of mild diffuse abdominal pain.
|
Figure 4-4 • Variable manifestations of small bowel obstruction depend on the level of blockage. |
ETIOLOGY
The first and second most common causes of SBO are adhesions and hernias, respectively (Table 4-1). Most adhesions are caused by postoperative internal scar formation. Discovering the actual mechanism of obstruction is important for therapeutic planning, because the mechanism of obstruction relates to the possibility of vascular compromise and bowel ischemia. For example, a closed-loop obstruction caused by volvulus with torsion is at higher risk for vascular compromise than an SBO from a simple adhesive band (Fig. 4-5).
|
TABLE 4-1 Causes of Small Bowel Obstruction |
||||||||||
|
||||||||||
|
Figure 4-5 • Volvulus. From Willis MC. Medical Terminology: A Programmed Learning Approach to the Language of Health Care. Baltimore: Lippincott Williams & Wilkins, 2002. |
A second mechanism causing bowel ischemia is incarceration in a fixed space. Incarceration and subsequent strangulation impedes venous return, causing edema and eventual bowel infarction. Other mechanisms of obstruction that rarely compromise vascular flow are intraluminal obstruction by a gallstone or bezoar and intussusception caused by an intramural or mucosal lesion at the leading edge.
HISTORY
Patients usually present with complaints of intermittent crampy abdominal pain, abdominal distention, obstipation, nausea, and vomiting. Vomiting of feculent material usually occurs later in the course of obstruction. Constant localized pain or pain out of proportion to
P.30
physical findings may indicate ischemic bowel and is a clear indication for urgent surgical exploration.
PHYSICAL EXAMINATION
A distended abdomen with diffuse midabdominal tenderness to palpation is usually found on physical examination. Typically, there are no signs of peritonitis. If constant localized tenderness is apparent, indicating localized peritonitis, then ischemia and gangrene must be suspected. An essential aspect of the physical examination is to check for abdominal wall hernias, especially in postsurgical patients. Elevation in temperature should not be present in uncomplicated cases. Tachycardia may be present from hypovolemia secondary to persistent vomiting or from toxemia caused by intestinal gangrene.
DIAGNOSTIC EVALUATION
Upright radiographs classically demonstrate distended loops of small bowel with multiple air–fluid interfaces. Occasionally, the radiograph shows the etiology of the obstruction, the site of obstruction, and whether the obstruction is partial or complete. Dilated small bowel in the presence of a dilated colon suggests the diagnosis of paralytic ileus, not SBO. A small bowel contrast study may be necessary to demonstrate transit of contrast into the colon, thereby ruling out SBO. Free air indicates perforation of the intra-abdominal gastrointestinal tract, whereas biliary gas and an opacity near the ileocecal valve indicate gallstone ileus.
Abdominal computed tomography (CT) scans can often demonstrate the transition point, where the dilated bowel proximal to the point of obstruction transitions to the decompressed bowel more distally. Also, in cases where the bowel has twisted on its mesentery, a "swirl sign" can be seen as the mesenteric
P.31
vasculature twists on itself, creating a distinctive swirling radiographic pattern.
Laboratory examination often reveals a hypokalemic alkalosis owing to dehydration from repeated emesis. White blood cell count and amylase may be mildly elevated. Lactic acidosis is cause for concern and may indicate intestinal necrosis.
TREATMENT
In decades past, most patients presenting with an SBO were taken directly to the operating room for exploration to rule out intestinal necrosis. However, over the past few decades it became apparent that most patients can be safely managed medically in the absence of peritonitis or other worrisome clinical findings. Supportive therapy allows for spontaneous resolution of the obstruction and return of normal bowel function. For patients who are candidates for a trial of nonsurgical therapy, initial treatment consists of nasogastric decompression to relieve proximal gastrointestinal distention and associated nausea and vomiting. Fluid resuscitation and supportive hydration is necessary because patients are typically intravascularly depleted from diminished oral fluid intake and vomiting.
P.32
The decision to operate is based on the nature of the obstruction and the patient's clinical condition. On initial presentation, if ischemia or perforation is suspected, immediate operation is necessary. Otherwise, patients can be observed with serial physical examinations, serum tests, and abdominal radiographs for evidence of resolution. If the patient's condition worsens or fails to improve with supportive therapy, operative intervention is indicated.
CROHN'S DISEASE
Crohn's disease is a transmural inflammatory disease that may affect any part of the gastrointestinal tract, from the mouth to the anus. Ileal involvement is most common. The disease is characterized by skip lesions that involve discontinuous segments of abnormal mucosa. Granulomata are usually seen microscopically, but not always. Areas of inflammation are often associated with fibrotic strictures, enterocutaneous fistulae, and intra-abdominal abscesses, all of which usually require surgical intervention.
EPIDEMIOLOGY
Crohn's disease occurs throughout the world, although the actual incidence exhibits a geographic and ethnic variability. The incidence in the United States is approximately 10 times that of Japan. Ashkenazi Jews have a far higher incidence of disease than do African Americans.
ETIOLOGY
The cause of Crohn's disease remains unknown. Because of the presence of granulomata, mycobacterial infection has been postulated as the causative agent. Recent investigations with Mycobacterium paratuberculosis have proved inconclusive. An immunologic basis for the disease has also been advanced; however, although humoral and cellular immune responses are involved in disease pathogenesis, no specific immunologic disturbance has been identified.
PATHOLOGY
The small intestine is affected in at least 70% of all patients with Crohn's disease. The ileum is typically diseased, with frequent right colon involvement. On gross inspection, the serosal surface of the bowel is hypervascular, and the mesentery characteristically shows signs of "creeping fat." The bowel walls are edematous and fibrotic. The mucosa has a cobblestone appearance, with varying degrees of associated mucosal ulceration (see Color Plates 2 and 3).
Histologically, a chronic lymphocytic infiltrate in an inflamed mucosa and submucosa is seen. Fissure ulcers penetrate deep into the mucosa and are often associated with granulomata and multinucleated giant cells. Granulomata are seen more frequently in distal tissues, which explains why granulomata are seen more often in colonic disease than in ileal disease.
HISTORY
Patients with Crohn's disease of the small bowel present complaining of diarrhea, abdominal pain, anorexia, nausea, and weight loss. The diarrhea is usually loose and watery, without frank blood. Dull abdominal pain is usually in the right iliac fossa or periumbilical region.
P.33
Children often present with symptoms of malaise and have noticeable growth failure. Strictures may cause partial SBO, resulting in bacterial overgrowth and subsequent steatorrhea, flatus, and bloating.
PHYSICAL EXAMINATION
Patients may appear to be either generally healthy or may have significant cachexia. Abdominal examination may reveal right iliac fossa tenderness. In acutely ill patients, a palpable abdominal mass may be present, indicating abscess formation. Enterocutaneous fistulae may be present. Oral examination may reveal evidence of mucosal ulceration, whereas perianal inspection may show skin tags, fissures, or fistulae. Extraintestinal manifestations include erythema nodosum, pyoderma gangrenosum, ankylosing spondylitis, and uveitis.
DIAGNOSTIC EVALUATION
Blood studies often show a mild iron-deficiency anemia and a depressed albumin level. If hypoalbuminemia is severe, peripheral edema may be present.
Small intestine Crohn's disease is diagnosed by bariumcontrast enteroclysis. This small intestine enema technique provides better mucosal definition than standard small bowel follow-through studies. This technique illustrates aphthoid ulcers, strictures, fissures, bowel wall thickening, and fistulae. Fistulograms are helpful to define existing fistula tracks, and CT can localize abscesses. Once radiographic evidence of disease is found, colonoscopy should be performed to evaluate the colonic mucosa and to obtain biopsies of the terminal ileum.
DIFFERENTIAL DIAGNOSIS
In addition to the diagnosis of Crohn's disease, one should consider the possibility of acute appendicitis, Yersinia infection, lymphoma, intestinal tuberculosis, and Behçet disease.
COMPLICATIONS
Crohn's disease carries a high morbidity and low mortality. Small bowel strictures secondary to inflammation and fibrosis are common complications that present as obstructions. Fistulae from small bowel to adjacent loops of small bowel, large bowel, bladder, vagina, or skin also occur. Ileal Crohn's disease can result in gallstone formation because of the interruption of the enterohepatic circulation of bile salts. Kidney stones may also form because of hyperoxaluria. Normally, calcium and oxalate bind in the intestine and are excreted in the feces. With ileal Crohn's disease, steatorrhea causes ingested fat to bind intraluminal calcium, thus allowing free oxalate to be absorbed. Finally, adenocarcinoma is a rare complication that usually arises in the ileum.
TREATMENT
Mild disease can be controlled with a 4- to 6-week course of sulfasalazine or mesalazine. Alternatively, oral corticosteroids can be used with equivalent results. Metronidazole may also be useful. Patients with bile salt–induced diarrhea after ileal resection may benefit from cholestyramine.
Severe disease is treated with hospitalization, bowel rest, hydration, intravenous nutrition, corticosteroids, and metronidazole. Patients with chronic active disease may benefit from a course of mercaptopurine.
Surgery for Crohn's disease should only be performed for complications of the disease (Table 4-2). Operation should be conservative and should address only the presenting indication, using gentle surgical technique. Resections should be avoided, as overly aggressive intervention can produce surgically induced short bowel syndrome and malnutrition. Some common surgical problems encountered in Crohn's disease and its treatments include ileocolic disease, which is managed by conservative ileocecal resection to grossly normal margins (Fig. 4-6); stricture, managed by stricturoplasty, which entails incising the antimesenteric border of the stricture along the intestinal long axis and then closing the enterotomy transversely (Fig. 4-7); and abscess/fistula, which is managed by open or percutaneous drainage of the abscess and resection of the small bowel segment responsible for initiating the fistula with primary anastomosis (Fig. 4-8).
|
TABLE 4-2 Indications for Surgery in Crohn's Disease |
|||||
|
P.34
|
Figure 4-6 • Ileocecal resection for Crohn's disease. |
|
Figure 4-7 • Stricturoplasty of a localized stricture with transverse closure. |
|
Figure 4-8 • Fistula resection with end-to-end anastomosis. |
MECKEL'S DIVERTICULUM
This most common congenital anomaly of the small intestine is an antimesenteric remnant arising from a failure of vitelline duct obliteration during embryonic development. Meckel's diverticula are true diverticula affecting all three intestinal muscle layers. Diverticula are usually <12 cm in length and are found within 100 cm of the ileocecal valve.
Associated abnormalities of the vitelline duct depend on the degree of duct obliteration that occurs during development. Complete ductal obliteration leaves a thin fibrous band connecting ileum to umbilicus, whereas complete duct persistence results in a patent ileoumbilical fistula. Partial obstruction results in cyst or blind sinus formation (Fig. 4-9). Heterotopic tissue (gastric, pancreatic) is found in 30% to 50% of diverticula.
|
Figure 4-9 • Vitelline duct remnants. |
In the United States, Meckel's diverticulum is associated with the "Rule of 2s": it occurs in 2% of the population, is located within 2 ft of the ileocecal valve, is usually 2 in long, contains two types of heterotopic tissue (gastric, pancreatic, duodenal, or intestinal), and is the most common cause of rectal bleeding in infants younger than 2 years.
COMPLICATIONS
Bleeding within the diverticulum may occur from peptic ulceration arising from heterotopic gastric mucosa. In infants, Meckel's diverticulum is the most common cause of major lower gastrointestinal bleeding.
Bowel obstruction may result from one of two mechanisms. Intussusception can occur when an inverted diverticulum functions as a lead point, or small bowel volvulus can occur around a fixed obliterated vitelline duct extending from the ileum to the umbilicus.
DIAGNOSTIC EVALUATION
For Meckel's diverticula containing heterotopic gastric mucosa, the technetium 99 (99Tc) scan is helpful for diagnosis: pertechnetate anions are taken up by ectopic gastric parietal cells and indicate diverticulum location. Diverticula that do not contain heterotopic gastric tissue can occasionally be visualized using standard barium-contrast studies.
TREATMENT
Definitive treatment for Meckel's diverticulum complications is surgical resection. In adult patients incidentally
P.35
found to have an asymptomatic Meckel's diverticulum during laparotomy, the diverticulum should be left in situ, as the chance of producing surgical morbidity and mortality are respectively 23 and five times higher for resection than when only symptomatic diverticula are removed.
SMALL BOWEL TUMORS
Tumors of the small bowel are rare, accounting for 1% to 5% of all gastrointestinal tumors. Most tumors are benign. Common benign neoplasms of the small bowel include tubular and villous adenomas, lipomas, leiomyomas, and hemangiomas. Telangiectasias of Rendu- Osler-Weber syndrome, neurofibromas of neurofibromatosis, hamartomatous polyps of Peutz-Jeghers syndrome, and heterotopic tissue as in Meckel's diver-ticulum are also found. Possible explanations for this lack of malignancy include short exposure to ingested carcinogens secondary to rapid transit time, low bacterial counts resulting in fewer endogenously produced carcinogens, and the intraluminal secretion of IgA by small bowel mucosa.
Benign lesions are usually asymptomatic and are incidental findings. Of symptomatic lesions, obstruction is the most common presentation, followed by hemorrhage. In the workup of gastrointestinal bleeding, however, unless other evidence exists, small bowel lesions should be low on the list of differential diagnoses, because >90% of bleeding lesions occur between the esophagus and distal duodenum and between the ileocecal valve and anus. Small bowel lesions should be suspected if careful skin examination reveals café-au-lait spots (neurofibromatosis), telangiectasia (Rendu- Osler-Weber syndrome), or mucocutaneous pigmentation (Peutz-Jeghers syndrome).
Malignant tumors of the small bowel typically present with obstruction or bleeding. The four major malignant tumors are adenocarcinoma, gastrointestinal stromal tumors, carcinoid, and lymphoma.
DIAGNOSTIC EVALUATION
Visual endoscopic identification of small bowel tumors is usually possible for lesions of the proximal duodenum and terminal ileum. The remainder of the small bowel requires examination by barium-contrast studies. For larger lesions, CT may be helpful.
In situations involving active hemorrhage, 99Tc sulfur colloid or 99Tc-labeled red blood cell studies may show the bleeding site. However, a bleeding rate of 1 mL/min is required for accurate localization.
When available diagnostic modalities are insufficient, exploratory laparotomy may be necessary. In addition to external inspection at laparotomy, operative endoscopy can be used for intraluminal evaluation.
CARCINOID TUMORS
Carcinoid tumors are the most common endocrine tumors of the gastrointestinal tract, constituting more than half of all such lesions. They account for up to 30% of all small bowel tumors. Carcinoid tumors arise from neuroendocrine enterochromaffin cells. Hence tumors can secrete serotonin and other humoral substances, such as histamine, dopamine, tachykinins, peptides, and prostaglandins. The metabolite of serotonin, 5-hydroxyindoleacetic acid, is excreted in the urine and is easily detected.
All carcinoids are considered malignant because of their potential for invasion and metastasis. Patients with metastatic disease manifest the carcinoid syndrome, which consists of the systemic effects (flushing, diarrhea, sweating, and wheezing) of secreted vasoactive substances. Presence of the carcinoid syndrome indicates hepatic metastasis, because systemic
P.36
effects occur when venous drainage from a tumor escapes hepatic metabolism of vasoactive substances.
Approximately 85% of carcinoid tumors are found in the intestine; of these, approximately 50% are found in the appendix, making it the most common site of occurrence, followed by the ileum, jejunum, rectum, and duodenum (see Color Plate 4). Other sites of disease include the lungs and occasionally the pancreas and biliary tract. Appendiceal carcinoids rarely metastasize, whereas lesions of the ileum have the highest association with carcinoid syndrome. Jejunoileal carcinoids are frequently multicentric.
HISTORY
The clinical presentation of patients with carcinoid tumors differs depending on tumor location. Primary tumors may present as SBO, because tumors can incite an intense local fibrosis of the bowel that causes angulation and kinking of the involved segment. As noted, metastatic disease with hepatic spread manifests as the carcinoid syndrome. Occult primary lesions do not cause systemic effects because 5-hydroxytryptamine (serotonin) is metabolized by the liver. Other presenting symptoms can include abdominal pain, upper intestinal or rectal bleeding, intussusception, weight loss, or a palpable abdominal mass.
DIAGNOSTIC EVALUATION
Laboratory studies should include plasma and urine analysis to evaluate for elevated levels of plasma serotonin and urinary 5-hydroxyindoleacetic acid. Barium-contrast studies are also useful for diagnosing carcinoid tumors. Barium enemas can demonstrate lesions of the rectum and large bowel, whereas small bowel enteroclysis may show a discrete lesion or a stricture secondary to fibrosis. Because primary tumors are usually small, CT is usually helpful only for detecting hepatic metastases. Colonoscopy can show tumors from the terminal ileum to the rectum.
Because neuroendocrine tumors often express functional receptors, radiolabeled octreotide imaging can be useful in detecting occult disease. Octreotide scanning is based on physiologic function, rather than on detectable anatomic alterations, and may have better diagnostic sensitivity than conventional imaging modalities.
TREATMENT
Surgical resection of the primary tumor is always undertaken, even in cases of metastatic disease. If the tumor is left in situ, bowel obstruction and intussusception ultimately result. At laparotomy, adequate bowel and mesenteric margins must be obtained, as with any cancer-related operation. Depending on tumor size and the degree of spread, lesions can be treated with simple local excision for small primaries to wide en bloc resection for metastatic disease.
Patients who have carcinoid syndrome can achieve symptomatic relief with subcutaneous injections of somatostatin analogs (e.g., octreotide). Induction with general anesthesia may provoke a life-threatening carcinoid crisis characterized by hypotension, flushing, tachycardia, and arrhythmias. Intravenous somatostatin or octreotide rapidly reverses the crisis.
PROGNOSIS
Carcinoid tumors are relatively indolent, slow-growing neoplasms. Prognosis for patients with carcinoid tumors is directly related to the size of the primary tumor and to the presence of metastasis.
For noninvasive lesions of the appendix and rectum <2 cm in size, the 5-year survival rate nears 100%. As the tumor size increases, the survival rate decreases. The presence of muscle wall invasion and positive lymph nodes are poor prognostic signs.
Patients with hepatic metastases have an average survival of approximately 3 years. Liver lesions are usually multiple. Because incapacitating symptoms of the carcinoid syndrome are proportional to tumor bulk, cytoreductive surgery can ameliorate symptoms, as well as prolong survival. Nonsurgical palliation is achieved with somatostatin analog therapy or chemoembolization of the tumor.
KEY POINTS
P.37
involved. Surgical treatment is reserved mainly for complications.