Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

degrees; a similar appearance is seen with other acute bowel infections and with obstruction to venous drainage.

The diagnosis is established by identifying duodenal or colonic S. stercoralis larvae.

Ascariasis

Ascaris lumbricoides infests a large proportion of the world’s population, mostly in developing countries where the condition is encountered mainly in children. During its life cycle the worm travels through the respiratory tract and portal and hepatic veins, and eventually pulmonary alveoli. After further growth, larvae migrate to the larynx, are swallowed, and mature in the small bowel.

Infected patients range from relatively asymptomatic, having nonspecific abdominal

ADVANCED IMAGING OF THE ABDOMEN

symptoms, to bowel obstruction, and the occasional patient develops a perforation and peritonitis. Pancreaticobiliary manifestations develop with worm migration into the biliary or pancreatic ducts. This is not an innocuous condition and patients developing an acute abdomen have a high mortality.

Classically, ascariasis is diagnosed with barium studies, where barium in the worm intestinal tract is identified on delayed views. Computed tomography reveals not only contrast within the worm gut but also the worm coursing through opacified bowel. The worm produces a long, thin, tubular soft tissue structure coiled in small bowel. Computed tomography often reveals the worm only in cross section. In endemic regions, bowel obstruction by worms is recognized on conventional radiographs (Fig. 4.11).

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Ultrasonography is the most common imaging modality available in some parts of the tropics and is suited to detect ascariasis. Intestinal ascariasis is suspected with US by finding tubular structures within the small bowel lumen. Among 22 individuals with A. lumbricoides eggs in their stools, US visualized a worm in six (65); the parasite appears as a large, curved hyperechoic strip, 4 to 6mm in diameter, and contains an inner, anechoic, longitudinal canal. The US appearance has been described as a winding highway (65).

Whipple’s Disease

Whipple’s disease is a rare, systemic disorder, often limited to the gastrointestinal tract and lymph nodes, although more generalized manifestations are evident in some. Intestinal symptoms are thought to be secondary to invasion by

Table 4.3. Clinical data on 52 French Whipple’s disease patients

* Based on 51 patients.

** Seven of 41 patients initially treated successfully. Source: Adapted from Durand et al. (66).

Figure 4.12. Whipple’s disease. Jejunal wall and valvulae conniventes are slightly thickened. Giardiasis has a similar appearance.

a rod-shaped gram positive aerobic bacillus, called Tropheryma whippelii, found both intraand extracellularly.

Table 4.3 summarizes a retrospective clinical Whipple’s disease study by the Société Nationale Française de Médecine Interne (SNFMI). An antecedent migratory polyarthritis often precedes gastrointestinal symptoms; the arthralgia is seronegative, symmetric, and migratory, and no joint destruction is evident. Some patients develop sacroiliitis. It is associated with renal amyloidosis and extraintestinal adenopathy.

Small bowel valvulae conniventes and bowel wall are thickened (Fig. 4.12). Intraand extraperitoneal lymphadenopathy is common. A characteristic feature of Whipple’s disease is that involved lymph nodes have a lower CT density than normal. Biopsy reveals foamy macrophages and surrounding lipid, accounting for the CT appearance. Presence of hypodense nodes is not pathognomonic for Whipple’s disease, however, and occasionally other infections (especially tuberculosis) or a diffuse neoplasm also exhibit hypodense nodes.

The diagnosis is established by finding periodic acid-Schiff (PAS)-positive material in involved histiocytes. Ultrastructural study identifies bacterial fragments within histiocytes.

Antibacterial therapy is often prolonged but is curative in most.

Cytomegalovirus Infection

Most clinically evident cytomegalovirus infection occurs in immunocompromised patients, and it is a common cause of massive lower gastrointestinal bleeding. Most lesions consist of ulcers and granulation tissue. These findings appear to be due to endothelial infiltration by macrophages and intranuclear inclusion bodies and a resultant cytomegalic vasculitis leading to ischemia and secondary ulceration. The endoscopic appearance of some lesions suggests a malignancy.

Ulcerations presumably account for the massive intestinal hemorrhage developing in some patients. Either jejunum or ileum can be involved. Deep ulcers occasionally result in perforation and peritonitis.

Although angiography does detect a bleeding site, the tangle of vessels induced by this infection can suggest angiodysplasia.

Anisakiasis

Human anisakiasis most often involves the stomach and duodenum. A minority of patients present with findings suggesting an acute abdomen, at times mimicking appendicitis. Laparotomy in two patients revealed an ileitis, with an edematous ileal wall infiltrated by eosinophils and containing Anisakis larvae (67).

Ultrasonography detects ascites, small bowel dilation and thickened valvulae conniventes. These are nonspecific abnormalities found in a number of other inflammatory conditions.

Protein-Losing Gastroenteropathy

Protein-losing gastroenteropathy is not a single entity but a description of a type of bowel reaction to a variety of insults. Some of the conditions leading to clinical, imaging, and pathologic findings of protein-losing gastroenteropathy are listed in Table 4.4. These conditions are discussed in their respective sections.

Some imaging tests are useful to detect protein-losing gastroenteropathy in its broad sense rather than being specific to a particular disease. A Tc-99m-HSA scan is considered

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Table 4.4. Conditions associated with protein-losing gastroenteropathy

Eosinophilic gastroenteritis

Ménétrier’s disease

Radiation enteritis

Celiac disease

Intestinal lymphangiectasia

Extensive amyloidosis

Certain infections

Whipple’s disease

Tropical sprue

Bacterial overgrowth

Intestinal lymphoma

positive for protein-losing gastroenteropathy if tracer exudate is identified in the gut. Serial scanning up to 24 hours is necessary to detect bowel activity in some patients. Among patients with suspected protein-losing gastroenteropathy, 96% revealed Tc-99m-human serum albumin (HAS) in bowel, but among these 28% were positive only on 24 hours images (68); no bowel activity or only slight activity is identified in controls.

Drug Related

Some drugs are known to induce small bowel ulcerations and stenoses. Among others, potassium salts and nonsteroidal antiinflammatory drugs are implicated (Fig. 4.13). Some patients develop multiple ulcerations.

Enteroclysis aids in detecting superficial ulcers and subtle stenoses. Thus among patients with drug-induced stenoses, enteroclysis reveals one of three findings: a web-like stricture; a short, smooth stricture; or an ulcerated, circumferential stenosis (69). At times CT detects a thickened, irregular bowel wall.

Ischemia

Ischemia due to major vessel abnormalities and vasculitides are covered in Chapter 17. Isolated colon ischemia is more common than small bowel.

Bowel ischemia ranges from acute to chronic. Early or mild ischemia results in intramural edema or hemorrhage (hematoma), findings similar to those seen as sequelae from a bleeding diathesis, such as anticoagulation therapy

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Figure 4.13. Jejunal webs secondary to nonsteroidal antiinflammatory drug use. (Courtesy of Arunas Gasparaitis, M.D., University of Chicago).

(small bowel bleeding is discussed later, see Vascular Lesions). A partial or gradual decrease in intestinal blood flow opens collateral vessels and bowel stenoses develop.

Computed tomography findings of ischemia vary depending on the severity of the ischemia and whether an infarct has developed (Table 4.5). Aside from reduced wall enhancement, the other signs are relatively nonspecific.

Ultrasonography findings suggesting small bowel ischemia consist of thickened bowel wall and absent blood flow, identified by color Doppler US; bowel wall thickening with readily apparent color Doppler flow suggests inflammation.

Ultrasonography reveals a hematoma as focal bowel wall thickening, at times consisting of a

Table 4.5. Computed tomography findings of ischemia in a setting of small bowel obstruction*

* Based on CT findings in 142 patients with acute bowel obstruction and suspected intestinal ischemia.

Source: Adapted from Zalcman et al. (70).

thick, hyperechoic inner layer and a thin, hypoechoic outer layer; the bowel lumen tends to be narrowed, decreased peristalsis is evident, and often fluid is identified between involved bowel loops.

Radiation

The amount of small bowel included in a radiation treatment field is a factor in subsequent radiation enteropathy. Two pretherapy methods are used to calculate bowel volume: a grid method using orthogonal radiographs, and algorithms based on CT data using either polyhedral or cylindric approximations.

Irradiation results in bowel wall thickening and lumen narrowing. During the acute phase edema and hemorrhage predominate, with eventual bowel wall fibrosis developing. Lumen narrowing can be either on an acute basis due to edema or chronic due to stenosis. Some obstructions develop only months or years later.

Radiation therapy induces mesenteric lymph node fibrosis and lymphatic occlusion resulting in lymphatic congestion and is one of the causes of protein-losing enteropathy.

Early vascular-phase CT in patients with irradiated intestine revealed bowel wall thickening in a three-layered pattern, believed to represent early contrast enhancement of the mucosal and muscular-serosal layers and decreased enhancement of submucosa (71); this three-layered appearance gradually faded into homogeneous enhancement during delayed equilibrium.

Angioneurotic Edema

Hereditary angioneurotic edema, a rare condition, is most often encountered in a setting of hereditary deficiency of the complement system. Acquired angioneurotic edema is even less common, although the clinical presentation is similar to the hereditary form. An acute attack, often initiated by mild trauma, involves skin,larynx,or abdominal structures and can be fatal. Abdominal involvement often mimics an acute abdomen.

Computed tomography findings in three patients with visceral angioneurotic edema revealed thickened small-bowel wall, including mucosa, increased wall contrast enhancement,

prominent mesenteric vessels, ascites, and fluid within bowel lumen (72).

Small bowel barium studies show diffuse thickening of valvulae conniventes in addition to bowel wall infiltration. One patient with hereditary angioedema developed focal small bowel involvement, mimicking a tumor on CT (73); if in doubt, a follow-up study should clarify whether these changes are permanent.

Mastocytosis

An uncommon disease, mastocytosis consists of a proliferation of mast cells that originate from CD34+ hemopoietic stem cells in bone marrow. In the past it was believed to represent a reactive or hyperplastic process, but more recent evidence suggests that mastocytosis is a neoplastic condition. Although urinary histamine metabolite levels are suggestive of the diagnosis, bone marrow biopsy is often required.

Clinical manifestations of mastocytosis result from mast cell degranulation and release of mediators inducing local paroxysmal congestion. Bone involvement is most common, followed by digestive tract and hematologic structures.

Thickening of valvulae conniventes and bowel wall develop during the acute attack, reverting to normal between attacks.

Lymphangiectasia

Intestinal lymphangiectasia is one of the protein-losing enteropathies, and it leads to malabsorption, hypoproteinemia, ascites, and edema. It is either congenital or acquired and presumably is secondary to obstruction of lymphatic flow and resultant lymphatic engorgement. Most imaging modalities simply show bowel wall and valvulae conniventes thickening, representing nonspecific findings (Fig. 4.14). A CT target sign (halo sign), consisting of an inner hypodense ring surrounded by a higher density outer ring, has been described in a patient with intestinal lymphangiectasia (74).

Technetium-99m-dextran lymphoscintigraphy detects chylous ascites, but this test has a limited role in evaluating intestinal lymphangiectasia. An occasional patient with primary intestinal lymphangiectasia has abnor-

Figure 4.14. Intestinal lymphangiectasia. Both bowel wall and valvulae conniventes are diffusely thickened.

mal intestinal accumulation of tracer during Tc- 99m–methylene diphosphonate (MDP) skeletal scintigraphy.

Amyloidosis

Primary gastrointestinal tract amyloidosis is rare. More often it is associated with multiple myeloma or chronic infection or inflammation such as Crohn’s disease. Gastrointestinal amyloidosis occasionally develops in patients on chronic hemodialysis.

If extensive enough, amyloidosis presents with ulcerations, bleeding, ischemia, or even a protein-losing enteropathy. A rare patient develops pneumatosis cystoides intestinalis.

Clinically and with imaging, amyloidosis mimics a number of other conditions. Hypoperistalsis and superimposed bacterial overgrowth are common. Some patients develop a pseudo-obstruction, thickened valvulae conniventes and small nodules scattered throughout the small bowel—findings secondary to mostly submucosal amyloid deposition.

In a patient with secondary amyloidosis and a protein-losing enteropathy, Tc-99m– diethylenetriamine pentaacetic acid (DTPA) HSA scintigraphy identified the protein-losing site (75).

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Tumors

Enteroclysis and capsule endoscopy are the current imaging gold standards in detecting primary small bowel tumors, yet for a number of reasons enteroclysis is performed mostly in select centers. In spite of several enthusiastic reports, CT and MR have had a limited role, except for tumors with diffuse involvement such as lymphomas. Undoubtedly the role of MRI will expand in the future.

Nonneoplastic

Splenic Rests

Small bowel splenic rests are not common. Typically they tend to present as submucosal tumors. A number of these patients have had a prior splenectomy, thus these foci presumably represent splenosis. At times in postsplenectomy patients Tc-99m–red blood cell scintigraphy suggests the correct diagnosis and thus needless surgery can be avoided.

Lymphoid Nodular Hyperplasia

Common variable hypogammaglobulinemia syndrome with intestinal lymphoid nodular hyperplasia is one of the hypogammaglobulinemic enteropathies, manifesting by decreased serum immunoglobulins, recurrent respiratory tract infections, and chronic diarrhea. From an imaging viewpoint the primary interest in hypogammaglobulinemia is the commonly associated intestinal lymphoid nodular hyperplasia.

Prominent lymphoid follicles are often detected in children and young adults and are believed to be a normal finding. A similar nodular pattern throughout the small bowel in more elderly patients should suggest hypoor agammaglobulinemia. Many of these patients also have superimposed giardiasis, leading to thickened valvulae conniventes throughout the proximal small bowel.

Occasionally a lymphoid hyperplasia pattern is detected in an elderly immunocompetent patient with no known hematologic abnormality, leading to a diagnostic dilemma. Ileoscopy in one woman showed multiple soft, pliable, ileal polyps, and biopsies revealed mucosal inflammation consisting of numerous

enlarged follicles containing reactive germinal centers and a mixed infiltrate of lymphocytes, plasma cells, and eosinophils (76); enlarged adjacent lymph nodes were detected by CT, and the patient underwent a right ileocolectomy. The final diagnosis was simply lymphoid hyperplasia.

Heterotopic Pancreas

Pancreatic rest tissue (heterotopic pancreas) is occasionally found in the small bowel. Some of these pancreatic rests measure up to several centimeters in size, are vascular, and bleed, generally on a chronic basis. Less common is pancreatitis developing in this heterotopic pancreatic tissue. Presumably some idiopathic small bowel strictures represent fibrosis from prior pancreatitis within a pancreatic rest.

Arteriography of larger small bowel pancreatic rests simply reveals a vascular tumor.

Inflammatory Polyp

Small bowel inflammatory polyps consist of connective tissue infiltrated by inflammatory cells, usually eosinophils. They are not neoplastic in origin. Most inflammatory polyps are located in the ileum and most are solitary. Occasionally one acts as a lead point for an intussusception.

Hemangioma

Jejunal and ileal hemangiomas are not common. They occur in Maffucci’s syndrome. Most are discovered incidentally. Gastrointestinal hemorrhage is the most common presentation, with some patients simply becoming anemic. In a rare patient bleeding is intraperitoneal.

A 3-week-old boy with a cutaneous hemangioma and gastrointestinal bleeding also had gastrointestinal hemangiomatosis (77); contrast-enhanced CT revealed diffuse small bowel wall enhancement.

Small bowel hemangiomas vary in size from very small to a large tumor filling the bowel lumen. These are soft tumors and thus some are missed by barium studies. Unless they are actively bleeding, angiography also misses some of these lesions.