Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

Occasionally a malrotation is suspected with radionuclide gastric emptying studies performed for other reasons.

detects some. An occasional one contains ectopic pancreatic tissue.

Duplication

A small bowel duplication is lined with intestinal mucosa and is usually intramural in location. It may or may not communicate with the true lumen. Ileal duplications are more common than jejunal. Multiple duplications are rare.

Some patients with a duplication are asymptomatic, whereas others present with a palpable mass or even obstruction. In general, most larger duplications manifest early in life. A communicating ileal duplication is an occasional cause of massive hemorrhage.

Imaging typically shows a duplication as an oval or elongated cystic structure adjacent to bowel lumen. It generally has a thick wall, thus distinguishing it from a mesenteric cyst, which is thin-walled. Ultrasonography reveals the cystic nature of a duplication; in addition, US shows a characteristic inner hyperechoic mucosa and an outer hypoechoic muscle layer.

Many duplications contain ectopic gastric mucosa, and thus scintigraphy (Meckel’s scan)

Obstruction

Atresia

Small bowel atresia, believed to be secondary to intrauterine ischemia, ranges from complete to partial and involves the jejunum, the ileum, or both. It is associated with midgut malrotation and is more prevalent in a setting of cystic fibrosis. Autosomal dominance is evident in some families with bowel atresia. Not all atresias are detected in early life; a rare infant develops an ileocolonic fistula to bypass an atretic segment.

Some authors subdivide jejunoileal atresia into four types:

Type I: a short, web-like narrowing

Type II: blind-ending proximal and distal tracts with a fibrous connection

Type III: a short, atretic segment in the proximal jejunum

Type IV: multiple atretic segments

In the rare apple-peel type of atresia the distal duodenum or proximal jejunum ends blindly, the distal superior mesenteric artery is absent, and the distal small bowel is foreshortened and spirals around its rudimentary blood supply, appearing similar to an apple peel. This condition presumably is due to a distal superior mesenteric artery occlusion or failure to develop.

Meconium Ileus

Meconium ileus is in the differential diagnosis with a suspected low intestinal obstruction. Almost all neonates developing meconium ileus have cystic fibrosis. Some have an underlying atresia, stenosis, or volvulus. Untreated, meconium ileus may progress to bowel necrosis and perforation (meconium peritonitis is discussed in Chapter 14).

Conventional radiographs reveal a low obstruction. Other findings, such as a bubbly appearance in the right lower quadrant, lack sufficient sensitivity and specificity to be diagnostic.

A water-soluble contrast enema is not only diagnostic but also often therapeutic. It identifies a small-caliber colon (microcolon), but keep in mind that both a meconium ileus and distal small bowel atresia result in a small caliber colon. With a distal ileal obstruction, however, the entire colon has a small caliber; otherwise a colonic abnormality should be sought Meconium ileus results in intraluminal content at the site of obstruction.

Ultrasonography in six neonates with a meconium ileus revealed multiple loops of bowel filled with hyperechoic material (13); this is in distinction to neonates with ileal atresia who have dilated loops of bowel filled with fluid and gas but not hyperechoic content.

A distinction between a meconium ileus and ileal atresia is more than academic, because the former is usually relieved by a contrast enema but the latter requires surgical intervention.

Cystic Fibrosis

Cystic fibrosis, with a recessive-autosomal inheritance pattern, involves multiple organs,

ADVANCED IMAGING OF THE ABDOMEN

with the lungs and pancreas most often affected. A number of affected patients eventually develop chronic liver disease. The sweat test should initially be performed when cystic diagnosis is suspected.

In patients eventually shown to have cystic fibrosis, newborn meconium ileus occurs as an initial presentation only in a minority. On the other hand, those with a meconium ileus almost all have cystic fibrosis.

The bowel wall becomes thickened in these children; histology reveals extensive intramural fibrosis and fatty infiltration, but unlike Crohn’s disease, acute inflammation is not a prominent feature.

Distal ileal obstruction due to inspissated intestinal content developing after the neonatal period is called meconium ileus equivalent. The prevalence of such obstruction increases with age. Especially during the first such obstructive episode, acute appendicitis is often in the differential diagnosis. A contrast enema is generally diagnostic. Often contrast does not reflux into the terminal ileum. Computed tomography is occasionally used to monitor these patients. The diagnosis can be suspected with US.

Other causes of bowel obstruction in these patients include intussusception and colonic strictures. One child with cystic fibrosis developed partial ascending colon obstruction secondary to diverticulitis (14), a very rare complication indeed.

A number of cystic fibrosis patients now reach adulthood and they are developing new complications, such as cancer, which is not common in the pediatric age group.

A barium enema reveals an irregular and spiculated colon outline, nodules, and loss of haustra. Irreversible and progressive colonic strictures tend to develop, at times to the point of almost complete obstruction.

Thickened small bowel folds and nodularity are familiar to most radiologists. Conventional abdominal radiographs reveal calcifications only in a minority ranging from small specks of calcium to extensive curvilinear calcifications; most calcifications are intramural in location, with a minority being intraluminal or serosal. An occasional newborn with cystic fibrosis has multiple atresias and calcified intraluminal meconium.

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Celiac Disease (Sprue)

Clinical

Celiac disease, or gluten-sensitive enteropathy, is a familial genetically determined disease associated with human leukocyte antigen (HLA)-B8-DR3 and manifesting by a digestive tract cytotoxic T-lymphocyte reaction. Affected individuals have a lifelong intolerance to dietary gluten. Certain foods, such as wheat, barley, rye, and others, exacerbate the symptoms of this protein-losing enteropathy. A strict gluten-free diet protects against known complications.

Considerable variability exists in presentation. A latent form of celiac disease appears to exist; affected individuals have subtle small intestine abnormalities consisting of increased intraepithelial lymphocyte levels, abnormal mucosal permeability, and elevated levels of secretory immunoglobulin A (IgA) and IgM antibody to gliadin. Occasionally celiac disease is first diagnosed even in elderly patients, presumably as an expression of silent disease that has been present since childhood. At times unrelated abdominal surgery unmasks latent celiac disease. It does present in the puerperium. It is a complex disease; variants such as celiac disease with immunologic activation of normal small bowel mucosa have been suggested. Some patients have liver disease, detected as a hypertransaminasemia and often labeled a nonspecific reactive hepatitis, which reverts to normal on a gluten-free diet. In an occasional patient an immunologic basis is suggested by finding associated primary biliary cirrhosis, primary sclerosing cholangitis, or autoimmune hepatitis.

Worldwide prevalence of celiac disease varies considerably. An interesting postulate is that the introduction of a high gluten content diet at an early age increases the risk and affects the symptomatology of celiac disease in a specific population. A study involving roughly half of the French pediatric population found an annual incidence of one per 2419 (15); the diagnosis was made before age 2 years in 77% of affected individuals, with the symptoms being failure to thrive, diarrhea, anorexia, abdominal distention, weight under two standard deviations, and short stature.

Orocecal transit time is delayed, and small bowel villous atrophy develops in untreated patients.

Highly sensitive and specific noninvasive blood tests for celiac disease are available, and imaging is relegated to a study of complications. A small bowel biopsy appears superfluous to confirming the diagnosis in symptomatic adults with antibodies to IgA antiendomysial.

Imaging

Patients with celiac disease have fasting or postprandial bowel motor abnormalities, which in most affected individuals are similar to those found in neuropathic disorders (Fig. 4.2). Dilated distal small bowel containing excessive fluid is a late but characteristic finding, at times mimicking partial distal small bowel obstruction. These abnormalities decrease on a glutenfree diet.

Enteroclysis achieves an almost 100% specificity but only about 80% sensitivity in detecting celiac disease in adults (16).

Ultrasonography of infants with known celiac disease detects abdominal fluid and hyperperistalsis in most. The US findings in adults include fluid-filled, dilated small bowel. Disordered motility is common. Bowel wall thickening is mild; marked thickening suggests hypoproteinemia, and asymmetric thickening raises suspicion for a neoplasm. Abdominal fluid is found in a minority. None of these findings are pathognomonic, but detection of several findings should raise suspicion for this disease.

Mild-to-moderate mesenteric adenopathy is common. If an untreated patient is placed on a gluten-free diet, serial CT should show a reduction in node size. Mesenteric lymph node necrosis develops in a rare patient.

Superior mesenteric artery Doppler US in untreated patients reveals significantly higher fasting peak systolic velocity, end diastolic velocity, mean velocity, and flow volume than in controls, and a resistive index that is significantly lower (17). Of interest is that the superior mesenteric artery and portal vein tend to dilate. The effect of therapy on the superior mesenteric artery resistive index is illustrated in Table 4.1.

Protein-losing enteropathy can be studied with several scintigraphic agents, including Tc99m human serum albumin (HSA), Tc-99m human immunoglobulin, indium 111 transferrin, and Tc-99m-dextran.

Comparing pretreatment and follow-up enteroclysis in adults with celiac disease, clinical response to a gluten-free diet correlated better with enteroclysis findings than with repeat biopsy (19).

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Associated Conditions/Complications

Lymphocytic gastritis develops in a minority of patients with celiac disease. Of interest is that gastric mucosa-associated lymphoid tissue (MALT) regresses in Helicobacter pylori– negative celiac patients when treated with a gluten-free diet.

Patients with long-standing celiac disease are at increased risk for malignancies, with small bowel high-grade T-cell lymphoma being most common and tending to involve the proximal small bowel. This is in distinction to lymphoma in normal small bowel, which is usually B cell in origin. Celiac disease and lymphoma have been diagnosed during the same presentation. A common appearance is that of small, discrete nodules varying in size or localized fold thickening. Nodules are not found in uncomplicated celiac disease, and such a finding should suggest superimposed lymphoma. A CT finding of marked adenopathy also suggests lymphoma.At times a barium study outlines a shaggy, ulcerated bowel segment, mimicking inflammatory bowel disease. Complicating the issue is that these lymphomas are often associated with considerable inflammation, and a biopsy may suggest inflammatory bowel disease. Enteropathyassociated lymphoma has a poor prognosis, with malignant ulcers not uncommonly resulting in bowel perforation.

Small bowel adenocarcinomas also occur in these patients. In fact, an occasional patient presents with small bowel adenocarcinoma, and celiac disease is diagnosed only after cancer resection. The prevalence of pharyngeal and esophageal carcinomas is also increased in these patients.

An occasional patient develops intestinal ulcerations, loss of valvulae conniventes, and a thickened, tube-like bowel wall; the overall imaging appearance mimics chronic ischemia.

Type 1 diabetes mellitus is found in an occasional patient with celiac disease diagnosed in adulthood. An anecdotal association exists with primary sclerosing cholangitis, lupus, and antiphospholipid syndrome. Some evidence suggests an increased prevalence of celiac disease in patients with primary biliary cirrhosis. An autoimmune linkage is suggested with idiopathic thrombocytopenic purpura and hepatic granulomatous disease.

Splenic atrophy is common in these individuals. Whether splenic infarction and splenic

venous thrombosis in one patient was fortuitous is conjecture (20).

Bone scintigraphy is positive for sacroiliitis in some adults with celiac disease.

Some patients with unresponsive celiac disease are treated with immunosuppressive therapy. A lack of response to therapy should raise the possibility of lymphoma, a setting contraindicating immunosuppressive therapy.

Agammaglobulinemia

An arrest in B-lymphocyte development leads to agammaglobulinemia. The primary defect involves mutations consisting of missense, nonsense, and splice mutations as well as deletion and insertion mutations in the gene encoding Btk (Bruton tyrosine kinase) (21).

Because maternal IgG passes through the placenta, affected newborns initially have normal serum IgG levels, but then these levels decrease and hypogammaglobulinemia ensues. Affected patients are prone to mostly bacterial infections. They have normal resistance to viral infections, except for enteroviral infections, leading to vaccine-related paralytic poliomyelitis and a dermatomyositismeningoencephalitis syndrome (21). Intestinal giardiasis is common in these patients.

A barium small bowel study often is diagnostic in affected patients; marked lymphonodular hyperplasia is evident throughout the small bowel, especially the jejunum, a site where even in pediatric patients lymphonodular hyperplasia is uncommon (Fig. 4.3).

Figure 4.3. Agammaglobulinemia. Small nodules are scattered in the jejunum (arrows).

Omphalomesenteric Anomalies

Failure of the omphalomesenteric duct to regress results in a residual umbilical sinus, omphalomesenteric cyst, enterocutaneous fistula, or a Meckel’s diverticulum.

Cysts and Fistulas

An omphalocele is the presence of midgut structures in the umbilical cord remnant (Fig. 4.4). A variety of intestinal content may be present within the defect. The midgut is malrotated. Some patients have other anomalies.

An omphalomesenteric cyst is formed when both omphalomesenteric duct ends involute, leaving a persistent central cavity. Most are incidental findings, with an occasional one becoming infected or compressing and obstructing an adjacent structure. A persistent fibrous omphalomesenteric remnant between the umbilicus and midgut is a potential source for small bowel volvulus around the fibrous band.

Meckel’s Diverticulum

Clinical

A Meckel’s diverticulum is the most common congenital gastrointestinal tract abnormality (Fig. 4.5). It results from persistence of the embryonic yolk sac. Most Meckel’s diverticula are asymptomatic but they can be involved in two clinical scenarios: More common is the patient presenting with bleeding suspected

from gastric mucosa in a Meckel’s diverticulum, with bleeding usually being gradual rather than massive. Less often encountered is the patient with sequelae to inflammation or intussusception. Obstruction is more common in patients under the age of 10 years, and perforation is more common in young adults. A rare Meckel’s diverticulum first manifests in the elderly.

Figure 4.5. Meckel’s diverticulum (arrow), detected on a retrograde small bowel examination.

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Meckel’s diverticula vary in size and shape, with an occasional one being very large. A rare one even has a second outpouching, or daughter diverticulum, connected to the main diverticulum. Ultrasonography reveals an uncomplicated, fluid-filled Meckel’s diverticulum as a cyst-like structure. Some of these diverticula mimic an appendiceal mucocele, mesenteric cyst, enteric duplication, or even a cystic ovarian tumor if a separate ovary cannot be identified by imaging.

Complications

Ectopic gastric mucosa is found in 20% to 50% of patients with Meckel’s diverticula and “gastritis” is common in those with gastric mucosa; only an occasional one contains H. pylori. Similar to the stomach, gastric secretions by this ectopic mucosa are stimulated by hormonal factors. As expected, hemorrhage and perforation develop in a setting of ectopic gastric mucosal tissue.

Incidentally, ectopic gastric mucosa is also found at other sites. As already mentioned, ectopic gastric mucosa is not uncommon in gastrointestinal duplications. In some patients, islands of heterotopic gastric mucosa in the jejunum and ileum account for recurrent small bowel bleeding and accumulation of Tc-99m- pertechnetate at these sites. The Tc-99m- pertechnetate concentrates in gastric mucosal mucin secreting cells, and scintigraphy reveals persistent focal activity only if such gastric mucosa is present within the diverticulum. Cimetidine or ranitidine premedication inhibits gastric secretion and helps diminish background activity. Still, both false-positive and false-negative results occur. At times abnormal Tc-99m-pertechnetate uptake is shown with single photon emission computed tomography

scan detects only the presence of gastric mucosa, rather than hemorrhage or other complications.

Gastric mucosa enhances more than any other bowel mucosa on postcontrast MRI, and any gastric mucosa in a Meckel’s diverticulum reveals marked enhancement.

Calculi are not common in a Meckel’s diverticulum. An enterolith consists mostly of calcified food residue (phytobezoar). Some

stones are multiple, calcified in their periphery, or even faceted. Rarely, a milk of calcium suspension forms in a Meckel’s diverticulum (23). Most stones contain enough calcium to be visible with conventional radiography. Computed tomography and US also detect most of these stones.

A stone (or bezoar) passing into the ileal lumen can cause bowel obstruction and mimic gallstone ileus. A rare enterolith within a Meckel’s diverticulum obstructs the diverticular communication with bowel lumen and leads to diverticulitis. Imaging should identify such a calcified enterolith; superficially an appendicolith is in the differential.

Perforation of a Meckel’s diverticulum is commonly ascribed to ulcerated heterotopic gastric mucosa. An 8-day-old boy perforated a Meckel’s diverticulum (24), a highly unusual age for this to occur; no ectopic gastric tissue was found in the resected specimen, and a narrow diverticular neck was believed to have led to poor emptying, subsequent inflammation, and perforation.

Inflammation does result in Meckel’s diverticulitis. A CT scan showing a fluid-filled cavity surrounded by a thickened, contrast-enhancing wall suggests diverticulitis.

There is no reason why a Meckel’s diverticulum should not be involved by Crohn’s disease. Differentiating terminal ileal Crohn’s disease (and possibly involving a Meckel’s diverticulum) from an inflamed and ulcerated Meckel’s diverticulum is another matter; resultant distortion is similar with both.

A Meckel’s diverticulum is a potential point for small bowel fixation. Torsion initially involves only the diverticulum, but a sufficient twist leads to ileal volvulus. Adhesions secondary to an inflamed diverticulum form a site for an internal hernia.

An inverted Meckel’s diverticulum is a potential lead point for a small bowel intussusception, and in the distal ileum most intussusceptions are due to either a polyp or a Meckel’s diverticulum. An inverted Meckel’s diverticulum appears as an intraluminal polyp. A barium study reveals an inverted diverticulum as a solitary, elongated, smoothly marginated, often club-shaped intraluminal tumor. This inverted diverticulum often contains fat along its serosal surface, detected by CT as an intraluminal elongated mass containing central fat surrounded by