Colon: Non Neoplastic Disease: Inflammatory Disease of the Colon

Pathologic Conditions

Inflammatory Conditions

Appendicitis

Acute appendicitis is a common cause of right lower quadrant pain. It occurs when the appendiceal lumen becomes occluded, resulting in an accumulation of fluid, appendiceal dilation, inflammation, ischemia and eventually perforation with possible abscess formation. CT has a 96% positive predictive value for appendicitis.

The normal appendix can often be identified on routine CT scan of the abdomen. It appears as a small thin walled tubular structure arising from the posteromedial aspect of the cecum between the ileocecal valve and the cecal tip. The appendix can be variable in length, measuring up to 20cm. Usually the appendix lies anterior to the cecum although a retrocecal appendix occurs in up to 25% of patients (4). The normal appendix is surrounded by homogeneous low density fat.

On CT, an abnormal appendix will appear dilated (>6mm) with a thickened wall which may homogeneously enhance after the administration of intravenous contrast. The appendix may be filled with fluid or debris. An appendicolith can be detected in up to 25%-40% of cases. (2-3) The presence of an appendicolith along with pericecal inflammation or mass is considered diagnostic for appendicitis. A hallmark of acute appendicitis is the presence of varying degrees of inflammatory thickening in the fat surrounding the diseased appendix. However, the presence of a dilated thickened appendix, even in the absence of pericolonic inflammation, is suggestive of possible appendicitis. (1) In addition, the presence of pericecal inflammatory changes , without definite identification of an abnormal appendix is suspicious for acute appendicitis, but not diagnostic, as many other conditions such as Crohn’s disease or cecal diverticulitis may have a similar appearance.

Perforation is a potential complication of appendicitis and appears as small pockets of extraluminal air or pneumoperitoneum. Appendiceal abscesses appear as a soft tissue mass containing air or necrotic debris and surrounded by inflammatory changes.. Less common complications include hepatic abscess or small bowel obstruction.

(1) Balthazar Ej, Birnbaum BA, Yee J, et al. Acute appendicitis: CT and US Correlation in 100 Patients. Radiology 1994; 190:31-35.

(2) Balthazar EJ, Megibow AJ, Hulnick D, et al. CT of appendicitis. AJR 1986; 1 47 (4):705-10.

(3) Scatarige JC, Yoseum DM, Fishman EK, et al. CT abnormalities in right lower quadrant inflammatory disease: review of findings in 26 adults. Gastrointestinal radiology. 1987; 12 (2): 156-62

-Diverticulosis/Diverticulitis

Diverticulosis is a common condition in the U.S.A.. Diverticula can occur anywhere throughout the colon but are most common in the sigmoid. They represent small outpouchings of the colonic mucosa and submucosa through the muscular layers of the wall. Diverticulae usually range in size from 2-3 mm up to 2 cm. A giant sigmoid diverticulum is a distinct entity which can range up to 25cm in size. This may result from a ball-valve mechanism or could represent a walled off cyst secondary to inflammation. The exact etiology is not known. (1)

The most common complications of diverticular disease include bleeding, due to erosion of a feeding nutrient artery, and infection. Acute diverticulitis occurs when the neck of a diverticulum is occluded by stool, inflammation or food particles resulting in a microperforation of the diverticula and surrounding pericolonic inflammation. Left sided diverticula more commonly lead to infection, while right sided diverticula more frequently bleed.

CT is well suited for the evaluation of diverticular disease, as it is able to image the wall of the colon as well as the surrounding pericolonic fat.

On CT, diverticulosis appears as small air filled outpouchings of the wall of the colon, most abundant in the sigmoid colon. The wall of the colon may appear thickened due to muscular hypertrophy. Diverticulitis appears as segmental wall thickening, hyperemia and inflammatory changes in the pericolonic fat.A frank abscess can be seen in up to 30% of cases (2). A diverticular abscess appears as a soft tissue mass with surrounding inflammatory changes. The center of the collection may contain air or air fluid levels or have low attenuation representing necrotic debris. CT also allows detection of other complications of diverticulitis such as colovesical fistula or perforation. Colovesical fistula is suspected when air is seen in the bladder and there is thickening of the bladder wall adjacent to a diseased segment of bowel (usually sigmoid.) . Focal contained perforations appear as small extraluminal pockets of air or extravasation oral contrast material.

CT also provides guidance for percutaneous drainage of diverticular abscesses, which can eliminate the need for emergent surgery, thus permitting a single elective resection after proper patient preparation. (3)

(2) Hulnick DH, Megibow AJ, Balthazar EJet al Computed tomography in the evaluation of diverticulitis. Radiology 1984;152:491-495.

(3) Hachigian MP, Honickman s, Eisenstat TE, et al. Computed tomography in the initial management of acute left sided diverticulitis. Dis Colon Rectum 1992 35(12)::1123-9.

-Typhlitis

Typhlitis, also known as neutropenic enterocolitis, is a condition which occurs in neutropenic patients undergoing treatment for a malignancy, most frequently in patients with acute leukemia on chemotherapy. Typhlitis is characterized by edema and inflammation of the cecum, ascending colon and sometimes terminal ileum. The inflammation can be so severe that transmural necrosis, perforation and death can result. The etiology of the condition is not known but probably represents a combination of ischemia, infection, mucosal hemorrhage and perhaps neoplastic infiltration. Treatment consists of antibiotic and aggressive fluid and electrolyte replacements.

CT is the study of choice for the diagnosis of typhlitis, due to the risk of perforation with colonoscopy or contrast enema. CT demonstrates cecal distention and circumferential thickening of the wall, which may have low attenuation secondary to the edema. Inflammatory stranding of the adjacent mesenteric fat is a common finding. Complications such as pneumatosis, pneumoperitoneum, or pericolonic fluid collections can also be present. Due to its involvement of the cecum, differentiation of typhlitis from appendicitis can be difficult, although the clinical presentation and history are frequently distinct.

-Infectious Colitis

There are many causes of infectious colitis. Bacterial infections include Shigella, Salmonella, Yersinia, Campylobacter, Staphylococcus., and Chlamydia trachomatis. Fungal infection such as histoplasmosis, mucormycosis, and actinomycosis can involve the colon. Viral causes of colitis include herpes, CMV, and Rotavirus. Amebiasis, a protozoan causes a colitis which can resemble inflammatory bowel disease. TB is another cause of colitis. Overall, the infectious colidities are typically diagnosed clinically and do not rely on CT for detection or differential diagnosis. They may, however, be identified on CT incidentally or in cases where the patients clinical symptoms are not straight forward.

On CT, patients with infectious colitis, of any etiology, demonstrate wall thickening, which usually displays homogenous enhancement.Sometimes low attenuation is detected within the wall, representing edema (2) Ascites or inflammation of the pericolic fat can may be present. Increased fluid within the colon results in multiple air fluid levels.

Typically, most infectious colitis is limited to involvement of the right colon, although diffuse involvement also occurs. In contrast, Gonorrhea, herpes and Chlamydia Trachomatis ( lymphogranuloma venereum) (image) typically involve the rectosigmoid colon. In addition, schistosomiasis, involvement is usually confined to the descending and sigmoid colon, as the adult worm have a tendency to enter the inferior mesenteric vein (1).

In summary, there is considerable overlap of the appearances of infectious colitis on CT, and usually a specific diagnosis cannot be suggested without laboratory studies.

-Radiation Colitis

Radiation therapy can result in injury to colon. More than half of patients undergoing radiation therapy to the pelvis will experience a self-limited acute proctitis. Some patients may experience a severe radiation induced proctitis which may lead to imaging.

Radiation colitis tends to be left sided, as radiation therapy is often given for pelvis pathology. (5) CT findings include nonspecific wall thickening, typically in the rectum, increased pelvic fat accumulation and thickening of the perirectal fibrous tissue are also seen. (4) Stricturing and fistula are possible complications.

Philpotts LE, Heiken JP, Wescott MA et 1l. Colitis: Use of CT findings in Differential diagnosis. Radiology 1994: 190:445-449.

Frommhold W., Hubener KH. The role of computerized tomography in the aftercare of patients suffering from carcinoma of the rectum. Comput Tomogr 1981;5:161-168.

-Pseudomembranous Colitis

Pseudomembranous colitis results from an overgrowth of the organism Clostridium dificile. Although first described as a complication of antibiotic therapy, pseudomembranous colitis has also been described with hypotensive episodes, chemotherapeutic agents, following abdominal surgery, and proximal to a large bowel obstruction.

Patients typically present with a profuse watery diarrhea with abdominal pain and fever. Histologically the condition is characterized by pseudomembranes which represent exudate of necrotic cells from the denuded mucosa. The diagnosis is typically made with stool assay for the C.difficile toxin or by stool culture. Treatment with metronidazole and vancomycin is usually effective.

CT findings include marked low attenuation wall thickening, which can be circumferential or eccentric. In one series the average wall thickness was 14.7mm, significantly greater than wall thickening seen in other inflammatory conditions.(1) Haustral folds are thickened and can appear as broad transverse bands, referred to as "accordion pattern.(1) The colon wall may enhance secondary to the hyperemia. Classically PMC is a pancolitis although there are reports of PMC sparing the rectum (2) Ascites can occasionally be present.

1. Fishman EK, Kavuru M, Jones B, et al. Pseudomembranous colitis: CT evaluation of 26 cases. Radiology 1991;180:57-60.

2. Rubesin SE, Levine MS, Glick SN, et al. Pseudomembranous colitis with rectosigmoid sparing on barium studies. Radiology 1989;170:811-813.

-Inflammatory Bowel Disease

Although contrast studies remain the principle tools for the diagnosis and evaluation of suspected inflammatory bowel disease, CT can sometimes aid in differentiating Crohn’s disease and UC when barium studies are equivocal. In addition, CT plays an important role in detection of complications of inflammatory bowel disease.

There may considerable overlap of the CT findings in Crohn’s disease and Ulcerative colitis. However, there are often certain features present which may help distinguish the two. Extensive involvement of right colon and small bowel more common in Crohn’s, although involvement of the left colon and rectosigmoid does occur. In contrast, UC is typically left sided or diffuse, and only rarely involves the right colon exclusively. (1) On CT scan , the most frequent finding in both Crohn’s disease and ulcerative colitis is wall thickening. The mean wall thickness in Crohn’s colitis( 11-13mm) is usually greater than in UC 7.8mm (1 -2) Wall thickening in UC may be diffuse and symmetric, while wall thickening in Crohn’s may be eccentric and segmental with skip regions.(1)

The halo sign, a low attenuation ring in the bowel wall due to deposition of submucosal fat, seen more commonly in UC than Crohn’s colitis. In Crohn’s disease, the bowel wall tends to enhance homogeneously, although edema within the wall may result in low attenuation (1)

Proliferation of mesenteric fat is seen almost exclusively in Crohn’s, while the proliferation of perirectal fat is nonspecific and can be present in Crohn’s, UC, Pseudomembranous colitis, or radiation colitis. (1).

The presence of mesenteric lymphadenopathy suggests Crohn’s rather than UC, although is certainly not specific for inflammatory bowel disease..

Complications of IBD can be imaged with CT , and, in the case of Crohn’s disease has been shown to effect patient management in 28% of cases (2).

Abscess formation is detected almost exclusively in Crohn’s, not UC (1,3).Abscesses can be confined to the bowel wall and pericolonic fat, or can involve adjacent structures such as the bladder, psoas muscle, or pelvic sidewall. Fistula can also be reliably detected. Enterovesical, enterocutaneous, perianal, rectovaginal have all bee detected with CT.

Toxic megacolon is a severe, life threatening fulminant transmural colitis most commonly associated with UC. Toxic megacolon, however has also been reported to occur infrequently with Crohn’s, amebiasis, salmonella, pseudomembranous colitis and ischemic colitis. The patient typically presents with profuse bloody diarrhea, abdominal pain , fever and leukocytosis.. Histiologically there is mucosal sloughing as well as thinning and necrosis of the muscular layers of the bowel wall. On CT, there is distention of the colon, most commonly involving the transverse colon, containing large amounts of fluid and air. The haustra appear edematous and distorted, or may be absent. The presence of pneumatosis signifies ischemia and necrosis. The major complication of toxic megacolon is perforation with resulting in sever sepsis, shock and possibly death.

Philpotts LE, Heiken JP, Wescott MA et 1l. Colitis: Use of CT findings in Differential diagnosis. Radiology 1994: 190:445-449.

(2) Fishman EK, Wolf EJ, Jones B et al. CT evaluation in Crohns Disease, effect on patient management. AJR 1987;148:537-549.

(3) Gore RM Marn CS, Kirby DR, Vogelzang Rl et al. Ct findings in ulcerative, granulomatous and indeterminate colitis. AJR 1984;143:279-284.

-Ischemic colitis

Ischemic colitis is a common cause of abdominl pain in the elderly and results when blood flow to the colon is compromised, usually as a result of hypoperfusion in the inferior mesenteric artery distribution. Watershed areas of the colon,( splenic flexure and rectosigmoid) are most commonly effected. Most patients are over 50 years of age and many have a history of heart disease. The extent and severity of the effected colon varies with etiology ( hypoperfusion vs. thrombus vs. trauma). Most cases of transient ischemia result in full resolution. Strictures are common complications of more serous episodes. If severe, ischemic bowel may become infarcted, which is a life threatening condition requiring immediate surgical resection.

CT demonstrates circumferential segmental wall thickening with fold enlargement. inflammatory changes in the pericolonic fat may also be present. (1)Ischemic colitis can be diffuse or left sided, rarely will it be localized to the right colon. (2) Pneumatosis with or without air in the mesenteric vessels or portal vein are ominous findings suggesting necrosis.Although often not pathognomonic, CT findings can suggest the diagnosis of ischemic colitis in the appropriate clinical setting.

(1) Jones B, Fishman EK, Siegelman SS. Ischemic colitis demonstrated by computed tomography. Journal of Computer Assisted Tomography 1982; 6(6) 1120-1123.

(2)Philpotts LE, Heiken JP, Wescott MA et 1l. Colitis: Use of CT findings in Differential diagnosis. Radiology 1994: 190:445-449.

-Inflammation of Appendage Epiploicae

Appendices epiploicae are fat containing peritoneal outpouchings which are attached to the serosal surface of the colon. . They are most abundant in the transverse colon and sigmoid.

These structures can become torsed resulting in ischemia and infraction. The clinical presentation and imaging findings are often nonspecific, and the diagnosis is usually made at surgery. On CT, the diagnosis of inflammation of the appendices epiploicae should be suspected when there is focal localized inflammatory changes in the pericolonic fat.(1)However, this finding is very nonspecific and the distinction of this entity from diverticulitis is usually not possible with CT.

(1) Torres GM, Bbbitt PL, Weeks M. CT Manifestations of Infarcted Epiploic Appendages of the Colon. 1994 Abdominal Imaging. 19:449-450.