Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

COLON AND RECTUM

disease; manometry helps distinguish between these two.

Allergic Colitis in Infancy

Allergic colitis, such as an allergy to cow milk proteins, tends to mimic the barium enema appearance of Hirschsprung’s disease. A barium enema reveals an irregular rectal narrowing and a transition zone (34); rectal biopsies identify ganglion cells and a lamina propria inflammatory infiltrate. Symptoms resolve after diet change.

Cystic Fibrosis

Cystic fibrosis is discussed in more detail in Chapter 4. Mentioned here are findings pertinent to the large bowel. Some authors use the term fibrosing colonopathy or severe indeterminate colitis to describe colonic changes.

A child with cystic fibrosis developed ascending colon diverticulitis (35).

Patients with cystic fibrosis receiving large doses of pancreatic enzymes develop colonic strictures. Histologically, these strictures consist of chronic inflammation, extensive collagen deposition, submucosa fibrosis, and ulcerations. These strictures are probably different from those seen in Crohn’s disease. Confusing the issue is that several of these patients have developed an inflammatory bowel disease. Whether this indeed represents classic inflammatory bowel disease or is a sequela of cystic fibrosis and thus a separate entity is conjecture.

Abnormal barium enema findings are common in children with severe cystic fibrosis and suspected distal bowel obstruction unresponsive to medical management and consist of strictures, loss of haustra, and longitudinal foreshortening. Computed tomography shows bowel wall thickening, mesenteric infiltration, increased pericolonic fat, and mural striation, findings more prominent in the proximal colon.

Patients with cystic fibrosis developing

Clostridium difficile colitis do not develop watery diarrhea. Therapy for impaction and related conditions is unsatisfactory, and CT detection of a pancolitis in such a clinical setting should suggest antibiotic-associated (C. difficile) colitis (36).

Short Colon Syndrome

In the congenital short colon syndrome the colon is either entirely or partially replaced by a dilated pouch. In the partial type some normal-appearing colon is evident between the ileum and a dilated pouch; in the complete variety the ileum inserts directly into a pouch. Some of these infants have associated anorectal malformations and colourinary fistulas. This condition appears to be more common in northern India than in other regions.

Trauma

Perforation

Most colon perforations manifest as a pneumoperitoneum and are readily detected. A retroperitoneal perforation, on the other hand, is more difficult to detect; at times no perforation is evident initially, but an abscess or hematoma develop later.

Penetrating injuries to the colon are usually managed by a diverting colostomy, although the trend is toward primary repair.

Obstetrical Trauma

Endorectal and transvaginal US are useful to evaluate sphincter injury after childbirth trauma, although endorectal MRI appears superior in establishing the site and extent of a tear; MRI also evaluates the integrity of individual anal sphincter muscles.

Wall Thickening

An uncommon cause of colonic wall thickening, generally involving mostly right colon in otherwise asymptomatic cirrhotic patients, is congestion secondary to portal hypertension.

Inflammation/Infection

Detection of Colitis

Focal or diffuse colonic wall thickening is the primary CT finding of colitis. Although such thickening is abnormal, it is nonspecific and occurs not only with various colitides but also

with some tumors and other infiltrative conditions. Similarly, the target sign (discussed in Chapter 4) is abnormal but nonspecific; it is, however, a marker for colitis and some infiltrative conditions but is not found with neoplasms. The term target sign is also used by ultrasonographers when describing an intussusception. Differentiating patients by clinical presentation, some investigators use the term colitis synonymously with inflammatory bowel disease, although in a broader sense these are not the same entities.

Using criteria of mucosal thickness >1.5mm, bowel wall thickness >4mm, mucosal irregularity, absence of haustra, and terminal ileal thickness >4mm, hydrocolonic US in ulcerative colitis and Crohn’s patients achieved 100% sensitivity in detecting active inflammatory bowel disease and 87% in identifying disease extension (37).

Technetium-99m (Tc-99m)–hexamethyl- propyleneamine oxime (HMPAO) leukocyte scintigraphy is a noninvasive test requiring no bowel preparation and is readily performed even in ill patients. It is a sensitive test for inflammation, being positive even in a setting of normal barium studies in patients later shown to have Crohn’s disease or ulcerative colitis. In patients with acute inflammatory and infectious colitis, Tc-99m-HMPAO leukocyte scintigraphy achieves sensitivities and specificities >80% in detecting involved segments. Tc-99m–2,3-dimercaptosuccinic acid (DMSA) scintigraphy reaches an overall sensitivity and specificity of >90% in detecting bowel inflammation (38).

18F-fluoro-deoxy-D-glucose (FDG) PET is primarily a neoplasm imaging agent, yet increased uptake is also evident in other hypermetabolic states such as acute enterocolitis.

Inflammatory Bowel Disease

General Findings

The term inflammatory bowel disease is reserved by most authors for Crohn’s disease and ulcerative colitis. The term is also used by some authors to describe a similar-appearing colitis when a specific underlying disorder is in doubt, at times prefacing the term with nonspecific or indeterminate.

ADVANCED IMAGING OF THE ABDOMEN

Some of the infective, drug-related and ischemic colitides mimic inflammatory bowel disease in their appearance; they are classified separately once a specific etiology is established. In particular, an etiology other than Crohn’s disease or ulcerative colitis should be sought if a previous colitis resolves. Occasionally described is a nongranulomatous ulcerative enterocolitis manifesting with severe chronic diarrhea; even after an extensive investigation, no specific diagnosis is made.

Most patients with inflammatory bowel disease have a chronic, indolent presentation. An occasional patient, however, is first seen with an acute massive bleed or acute episode of severe colitis. Even toxic megacolon has manifested during an initial presentation.

A number of indices measure disease activity, but most have had limited clinical adoption. An interesting one involves use of the water-soluble radiographic contrast agent iohexol. Urinary excretion of ingested iohexol is significantly higher in patients with active inflammatory bowel disease than in those with quiescent disease or in controls (39).

Several studies have documented a lower seroprevalence of Helicobacter pylori infection in patients with Crohn’s disease and ulcerative colitis than in controls.

Inflammatory bowel disease is not limited to the bowel, but also involves hepatobiliary, musculoskeletal, ocular, dermatologic, and other organ systems. A possible association with glycogen storage disease type IB exists. An association exists between inflammatory bowel disease and sclerosing cholangitis and cholangiocarcinoma, although a relationship between colonic and bile duct disease activity is not straightforward; after liver transplantation for sclerosing cholangitis, inflammatory bowel disease can be reactivated despite maintenance immunosuppression. Likewise, colon carcinoma has developed after liver transplantation. Thus serial colon cancer screening is necessary after liver transplantation.

Pathologic Findings

The presence of crypt abnormalities, basal plasmacytosis with chronic inflammation, and distal Paneth cell metaplasia point toward inflammatory bowel disease rather than other colitides. Thr histology of a colorectal biopsy

COLON AND RECTUM

suggests Crohn’s disease if epithelial granulomas, microgranulomas, or isolated giant cells are detected; these findings, however, are often lacking from biopsies of patients with obvious Crohn’s disease. In some patients a diagnosis of indeterminate colitis is appropriate.

Biopsy in ulcerative colitis reveals an irregular or villous surface, a decrease in mucous content, and crypt atrophy. Initially diffuse small ulcers develop surrounded by inflammation and hyperemia. With disease progression the bowel wall thickens and lumen narrows. The muscularis mucosa thickens markedly and foreshortens, leading to shortening of colonic length. In distinction to Crohn’s disease, a vasculitis is uncommon in ulcerative colitis. A transmural lymphocytic phlebitis, however, is detected in some patients.

Postinflammatory polyps, also called pseudopolyps, are a common finding in ulcerative colitis, although they also develop in a number of other colitides, including some infections. With partial healing the overlying mucosa becomes hyperplastic. In some patients these polyps become sufficiently large and extensive to obstruct bowel lumen. These polyps can cause a protein-losing enteropathy.

Differential Diagnosis

Clinical

Although in most patients differentiation between Crohn’s disease and ulcerative colitis is straightforward, in a small subset radiologic studies, colonoscopy, and biopsy do not differentiate with any degree of certainty between these two entities. An attempt at differentiation using serologic testing has had rather limited success. Intestinal mucosal lymphocytes isolated mostly from lamina propria contain higher levels of CD19, transferrin receptor, T- cell receptors alpha/beta, and T-cell receptors gamma/delta in ulcerative colitis patients than in Crohn’s patients (40); the clinical significance of these findings is not clear.

In a vast majority of patients ulcerative colitis extends from the rectum proximally for varying lengths, with a pancolitis found in a minority. Skip areas are not found, and in a setting of a right-sided colitis with rectal sparing ulcerative colitis should not be in the differential diagno-

sis. Nevertheless, it is difficult to place some patients in the proper perspective. For instance, a clinical, radiographic, endoscopic, and histologic follow-up in a patient with right-sided colitis consisting of multiple shallow ulcers, erosion, and stenoses and sparing of the rectum and left colon appeared to exclude Crohn’s disease, tuberculosis, yersiniosis, Behçet’s disease, and ischemic colitis (41); the authors concluded that the patient was indeed suffering from right-sided ulcerative colitis.

Toxic megacolon is generally associated with ulcerative colitis, on rare occasions even being an initial manifestation of this disease. One should keep in mind, however, that toxic megacolon is also encountered in severe acute Crohn’s disease and some infectious colitides.

Imaging

The role of double-contrast barium enema and colonoscopy in diagnosing both Crohn’s disease and ulcerative colitis is well established. A number of studies have concluded that the double-contrast barium enema and colonoscopy are complementary imaging modalities for optimal detection of all mucosal and structural colonic lesions, with the exception of those that do not distort the mucosa.

The terminal ileum is often involved in Crohn’s disease but is spared in ulcerative colitis (with the exception of backwash ileitis). The barium enema appearances of terminal ileal Crohn’s disease and backwash ileitis are quite different, and radiologists readily differentiate these entities.

In Crohn’s disease CT identifies thickening of diseased bowel in most patients, and in the acute phase such thickening is universal. The bowel wall has a homogeneous appearance in Crohn’s disease, but in ulcerative colitis it ranges from homogeneous to a target sign (discussed in Chapter 4). The outer colonic contour tends to be smooth in severe ulcerative colitis; in Crohn’s disease it can be either smooth or irregular.

Rectal involvement results in presacral space widening and is found both with Crohn’s proctitis and ulcerative colitis. Computed tomography shows this widening to be mostly secondary to infiltration by fat-containing linear and nodular soft tissue densities.

In young adults with chronic abdominal pain and bowel dysfunction, some authors establish an arbitrary upper normal US bowel wall thickness, with inflammatory bowel disease suggested if this thickness is exceeded. Thus in consecutive outpatients, use of a 7-mm upper normal bowel wall thickness achieved a sensitivity of 74% and specificity of 98% in suggesting inflammatory bowel disease (42); these findings could be further subdivided into an 84% sensitivity and 98% specificity for Crohn’s disease and 38% and 98%, respectively, for ulcerative colitis.

After distending the large bowel with water (hydrocolonic US), US normally differentiates five wall layers having different echogenicities. The colonic wall is hypoechoic and thickened, and these five colonic wall layers cannot be visualized in most Crohn’s disease patients, but they tend to be preserved in ulcerative colitis; loss of this layer stratification is due to fibrosis and implies an irreversible change.

Postcontrast MR in colonic Crohn’s disease reveals transmural bowel wall enhancement, a finding distinct from typical ulcerative colitis where enhancement is limited to the mucosa and MRI identifies less bowel wall thickening. Thus after lumen distention with oral and rectal fluid, pre– and post–gadolinium (Gd)–diethyl- enetriamine pentaacetic acid (DTPA) HASTE and dynamic fast low-angle shot (FLASH) axial and coronal images allowed differentiation of Crohn’s disease and ulcerative colitis in 81% of 27 patients with inflammatory bowel disease (43). Low-field MRI (0.1T) tends to overestimate and underestimate disease extension both in Crohn’s patients and in ulcerative colitis patients.

Although Tc-99m-HMPAO–white blood cell scintigraphy is very sensitive and specific in detecting colonic inflammation (44), it has a limited role in differentiating active colonic Crohn’s disease from ulcerative colitis. Discontinuous uptake suggests Crohn’s disease, but some children with ulcerative colitis also have discontinuous uptake.

Crohn’s Disease

Clinical

ADVANCED IMAGING OF THE ABDOMEN

Crohn’s disease is most often found in the distal small bowel, disease confined to the colon and rectum is not uncommon. A later age of disease onset appears related to an increased prevalence of colorectal rather than small bowel involvement.

In a retrospective cohort study of patients with colorectal Crohn’s disease, distribution was segmental in 40%, total in 31%, and left-sided in 26% (45); perianal or rectal fistulas developed in 37%. In those patients attaining clinical remission, the 5-year cumulative relapse rate was 67%. Half of these patients underwent a resection within the first 10 years, and half of these ultimately required an ileostomy.Analysis of the cause of death of patients with Crohn’s disease during 1973 to 1980 in Rochester, New York, found that Crohn’s disease was the causative factor in 44% of deaths, but during 1981 to 1989 deaths due to Crohn’s disease decreased to 6% (46).

Imaging

Early colonic manifestations of Crohn’s disease consist of lymph follicle prominence and presence of aphthae. These are readily detected with a double-contrast barium enema but are not visualized with CT. The presence of aphthae implies active colitis but is not pathognomonic of Crohn’s disease (Fig. 5.5); lymphoid hyper-

Crohn’s disease is discussed in more detail in Chapter 4. Covered here are those aspects related to the colon (Crohn’s colitis). Although

Figure 5.5. Cecal Behçet’s disease. Aphthae are scattered throughout. Crohn’s disease has a similar appearance.

Figure 5.6. Crohn’s disease. Extensive filiform polyposis is present. (Courtesy of Arunas Gasparaitis, M.D., University of Chicago.)

plasia, on the other hand, exists without disease activity.

Progression of inflammation leads to filiform polyposis and diffuse intramural thickening (Fig. 5.6). With established disease, the presence of homogeneously thickened bowel wall during an arterial phase CT study generally implies the presence of fibrosis.

Magnetic resonance imaging is very accurate in detecting active Crohn’s colitis (Fig. 5.7).After oral bowel opacification with a 2.5% mannitol solution, axial and coronal breath-hold MRI using contrast-enhanced T-1 weighted FLASH and T2-weighted HASTE sequences identified more diseased segments than comparable barium studies (47). Current evidence suggests that a hydro-MRI study is very reliable in both

detecting and evaluating the extent of Crohn’s disease.

Some authors suggest that specific imaging findings can suggest whether a particular stage of Crohn’s disease is reversible or not. Thus presence of a target sign suggests that fibrosis has not yet developed and the underlying abnormalities are reversible with appropriate therapy. Whether this is indeed so remains to be proven.

Complications

The earlier literature rarely mentioned cancer developing in a setting of Crohn’s disease. Some of these publications should be held suspect because a number of these patients were misdiagnosed. More recent publications document an increased incidence of cancer in bowel involved by Crohn’s disease. Typically the interval from initial Crohn’s diagnosis until cancer is detected is up to 20 years. Some cancers are multiple. They have developed in a defunctionalized rectal stump. An occasional one involves a Crohn’s-induced anorectal sinus tract or fistula.

Patients with inflammatory bowel disease (both Crohn’s disease and ulcerative colitis) tend to develop a diffusely infiltrating and metastasizing carcinoma, called linitis plastica of the colon. Most of these are adenocarcinomas. Radiologically, these cancers tend to mimic a benign stricture, and at times even a biopsy is nondiagnostic.

Anal fissures and low anal fistulas are common in Crohn’s disease and tend to be associated with an adjacent phlegmon or abscess. Most sinus tracts and fistulas in a setting of Crohn’s colitis end blindly or are enteroenteric; an occasional fistula extends to an unusual

location. A high or complex anal fistula or a rectovaginal fistula is uncommon, with only a minority of these patients having obvious Crohn’s proctitis (the imaging approach to fistulas is discussed later in this chapter; see Fistula).

Most urinary tract fistulas involve the bladder, with only a rare one extending into the ureter, urethra, or Bartholin’s gland. A rare colonic fistula crosses the diaphragm; even developing a colobronchial fistula.

Superimposed infections are common and often account for acute flare-ups.

Similar to the small bowel, only a rare colocolic intussusception has been reported in Crohn’s colitis.

Ulcerative Colitis

Clinical

In large parts of Europe the incidence of ulcerative colitis in school-age children has been about 1.5 to 2.0 per 100,000 children per year for the last several decades (48). In some parts of the world the reverse is true, namely, the incidence of ulcerative colitis is decreasing while Crohn’s disease is increasing. Prevalence is greater in some families and ethnic groups, and an inheritance pattern is evident in some.

The etiology and pathogenesis of ulcerative colitis are unknown. An abnormal immune system response appears to be involved, although whether the immune system itself is abnormal or whether it is responding to some other abnormality is conjecture. Significantly increased secretion of mast cell tryptase, a highly mast cell specific protease, suggests mast cell involvement (49). These patients have a higher prevalence of allergic symptoms, and skin tests show increased rates of immediate and delayed hypersensitivity than controls. The concordance rate of ulcerative colitis is higher in monozygotic twins than in dizygotic twins, although not to the extent seen in Crohn’s disease. Thus in a Danish study of nearly 30,000 twins the proband concordance rate among monozygotic twins was 58% for Crohn’s disease and 18% for ulcerative colitis and among dizygotic twins the rates were 0% and 4%, respectively (50).

One interesting sidelight is that a previous appendectomy appears to protect somewhat against future ulcerative colitis. Also, in women

ADVANCED IMAGING OF THE ABDOMEN

with known ulcerative colitis, the number of relapses/year after pregnancy is lower compared to a similar period before pregnancy.

Ulcerative colitis usually manifests after puberty, with only a minority detected earlier. Most patients have a chronic, relapsing course. A common clinical finding is bloody diarrhea, and its absence should suggest another diagnosis. Experienced clinicians can probably recall an occasional patient first presenting with fulminant toxic colitis.

Past teaching held that once ulcerative colitis was established, it changed little in extent, a finding no longer believed to be true. Over a several year period idiopathic ulcerative proctocolitis in most patients remains similar in extent, in a minority spreads proximally and, less often, decreases in extent.

Associated Conditions

An association between ulcerative colitis and primary sclerosing cholangitis or pericholangitis is well known. Less well known is an occasional association between pancreatitis and both ulcerative colitis and Crohn’s disease. Occasionally pancreatitis manifests before onset of ulcerative colitis. Complicating the picture is an association of acute pancreatitis and azathioprine therapy.

An occasional association exists between ulcerative colitis and pyoderma gangrenosum, glomerulonephritis, systemic lupus erythematosus, sarcoidosis, and Ménétrier’s disease. Whether similar genetic or immunologic pathways are involved is speculation.

One patient developed ankylosing spondylitis at age 28 years, ulcerative colitis at age 49 years, and chronic granulocytic leukemia at age 59 years (51). An uncommon association appears to exist between ulcerative colitis and rheumatoid arthritis.

Imaging

The traditional imaging study to diagnose and quantitate the extent of involvement was a double-contrast barium enema. In parts of Europe a single-contrast enema, using air as a contrast agent, is performed. Gastroenterologists prefer colonoscopy and biopsy.

Presacral space widening due to fatty infiltration is common once the disease is established