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and is readily apparent with most imaging modalities. Loss of rectal valves of Houston is also common.
Early in the course of disease a CT study is normal and thus for early disease detection CT is not warranted. With more extensive bowel involvement CT identifies a colonic wall target sign. The hypertrophied mucosa is seen as a soft tissue density surrounded by a lower density ring representing fatty submucosal infiltration, this in turn is surrounded by a soft tissue density muscularis propria. As already mentioned, such a target sign is not specific for ulcerative colitis, but is also found in a number of other colitides. Bowel wall thickening and lumen narrowing evolve with chronicity.
Ultrasonography assesses both the extent and activity of ulcerative colitis and follows the response to medical treatment. Using a thickened bowel wall and loss of haustra as evidence of disease in patients with active ulcerative colitis, US correctly identified diseased segments in 74% of patients (52); also, US revealed bowel wall thickening decreasing after medical therapy in patients showing clinical improvement.
Doppler US reveals increased portal and mesenteric blood flow and a lower resistance index in the superior mesenteric artery in those with active disease but not in those with quiescent disease. Active Crohn’s disease or ulcerative colitis involving the left colon lead to a marked increase in inferior mesenteric artery blood flow (53); compared with controls, Doppler US reveals increased velocity, flow volume, and a decreased pulsatility index. These findings should be balanced against laser Doppler flowmetry of rectal blood flow; flowmetry detected significantly reduced rectal perfusion in those with ulcerative colitis but not Crohn’s colitis (54); suggesting that impaired local blood flow plays a pathogenetic role in ulcerative colitis.
Postcontrast MR reveals marked mucosal enhancement of diseased segments and relative submucosal and muscle sparing. Because of this limited transmural involvement, MRI tends to underestimate the extent of disease. A fatsuppression technique is helpful in outlining diseased segments.
Technetium-99m-HMPAO leukocyte scintigraphy is useful in evaluating the extent and sites of disease activity. This noninvasive study pre-
dicts and localizes acute inflammation, but a negative study does not exclude acute inflammation. Also, sites of positive leukocyte scintigraphy become negative after leukocyte apheresis or glucocorticoid therapy, and thus leukocyte scintigraphy appears useful in evaluating treatment response. Also, Tc-99m-HMPAO scintigraphy appears to predict proximal disease extension better than CT.
Complications
Perforation in a setting of toxic megacolon is a well-recognized complication of ulcerative colitis. Instead of frank perforation, an occasional patient with severe ulcerative colitis develops retroperitoneal emphysema.
Dysplasia is believed to be a precursor in the pathway to cancer in ulcerative colitis. Detecting dysplasia is generally in the province of a pathologist, although at times the colonoscopic or barium enema appearance suggests this condition by finding nodular protrusions, irregular mucosa, and minute spiculations. If detected incidentally, such a finding warrants a biopsy, yet for cancer surveillance and dysplasia detection most physicians rely on histopathology for these notoriously difficult-to-detect tumors. Cancers developing in a setting of ulcerative colitis typically are flat or plaque-like rather than polypoid or ulcerated as seen in nondiseased bowel. Surrounding diseased mucosa makes early cancer detection even more difficult.An underlying cancer in a colonic stricture is readily missed both radiologically and colonoscopically. In some, previous endoscopy did not reveal either dysplasia or cancer, with an advanced adenocarcinoma being later discovered.
Prevalence of colon cancer increases in a setting of pancolitis. Also, the risk of colorectal dysplasia and carcinoma appears increased by the presence of primary sclerosing cholangitis.
Surveillance colonoscopic biopsies are useful to detect dysplasia, with high-grade dysplasia being considered a precursor to cancer. Nevertheless, in spite of the extensive literature on surveillance colonoscopic biopsies, no consensus exists on its clinical value in patients with ulcerative colitis. In general, surgery should be considered in patients with an underlying stricture even if a biopsy is negative for dysplasia or malignancy.
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A number of reports describe primary colonic lymphoma developing in a setting of ulcerative colitis. Complicating the picture are reports of colonic lymphoma misdiagnosed as ulcerative colitis; superficial colonic biopsies in these patients are believed to be compatible with ulcerative colitis, with lymphoma manifesting only several months later. In distinction to ulcerative colitis, colonic mucosal abnormalities resolve after therapy for lymphoma.
Thromboembolic disease is a known complication of ulcerative colitis. Massive pulmonary emboli and even dural sinus thrombosis have occurred.
Pulmonary interstitial fibrosis and fibrosing alveolitis have developed in a setting of ulcerative colitis.
Infections develop readily, and some acute flare-ups are secondary to superimposed infection. Infection should be suspected in a patient being treated with steroids who becomes fulminant. In some countries, ulcerative colitis patients receiving steroids have developed pulmonary or intestinal tuberculosis. Severe infection can result in pseudomembranous colitis developing even without antibiotic use.
Surgery for Colonic Inflammatory Bowel Disease
Surgical approaches to colonic involvement by inflammatory bowel disease consist of (1) total proctocolectomy with an ileostomy, (2) total colectomy with an ileorectal anastomosis, and
(3) colectomy with a mucosal proctectomy and ileoanal anastomosis (ileal pouch).
The choice of surgery is influenced by the type of inflammatory bowel disease. A retrospective study of 86 patients with colonic Crohn’s disease who underwent a single-stage proctocolectomy and 65 who underwent total colectomy and ileorectal anastomosis found that 29% of proctocolectomy patients and 68% of ileorectal anastomosis patients developed symptomatic recurrence (55); after proctocolectomy the 5-, 10-, and 15-year cumulative reoperation rates for recurrence were 13%, 16%, and 26%, compared to 29%, 46%, and 48%, respectively, after ileorectal anastomosis.
Ileostomy
A total proctocolectomy is curative for bowel manifestations of ulcerative colitis but not
ADVANCED IMAGING OF THE ABDOMEN
Crohn’s disease. Some of the extraintestinal complications of ulcerative colitis have a clinical course independent of bowel disease and are not affected by a colectomy. Currently total proctocolectomy is rarely performed on an elective basis for ulcerative colitis because of associated complications, including sexual dysfunction.
Among patients with Crohn’s disease who underwent total colectomy, end ileostomy, and an oversewn rectal stump, over half later required a proctectomy (56); of note is that 23% of these patients later developed small bowel recurrence requiring surgery.
Some patients with ileostomies performed for ulcerative colitis develop polyps at their ileostomy stomas; most of these are inflammatory polyps, but an occasional neoplastic polyp also develops.
Ileorectal Anastomosis
A subtotal colectomy leaves behind a diseased rectum with its associated complications, including the risk of cancer.
An interesting retrospective study found that 21% of patients undergoing a subtotal or total colectomy for ulcerative colitis required reoperation for postoperative acute cholecystitis; none of the patients undergoing colectomies for other reasons (mostly cancer) developed acute cholecystitis (57).
Ileal Pouch
An ileoanal anastomosis and creation of an ideal pouch eliminates all diseased rectal mucosa and is the current therapy for both familial polyposis syndrome and ulcerative colitis. An ileoanal anastomosis is performed in two stages: resection, anastomosis, and a diverting loop ileostomy initially, followed by closure of loop ileostomy several months later.
A special ileoanal barium study has been proposed to predict the frequency of bowel movements and measure pouch spasticity (58); with the patient standing, barium sulfate is instilled into the pouch until reflux into more proximal small bowel occurs. The total volume infused and volume voided are measured. The clinical usefulness of such a quantitative test remains to be determined.
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A leak, adjacent inflammation, abscesses, and fistula formation are immediate complications related to creation of an ileoanal pouch. A contrast enema is commonly performed to check for stricture, leak, or other complication prior to ileostomy closure. Strictures developing on a more chronic basis are evaluated by endoscopy, pouchography, CT, and endoluminal US. Coronal CT visualizes most anastomoses and adjacent structures. A high accuracy is claimed for endoluminal transpouch US, but this study has not achieved popularity.
Postoperatively, functioning ileal pouch mucosa undergoes colonic metaplasia. Pouch inflammation (pouchitis) is a late complication and develops in about 10% to 30% of patients. Although the underlying cause of pouchitis is not clear, in some patients pouchitis presumably represents recurrent ulcerative colitis. Patients with primary sclerosing cholangitis develop significantly more severe chronic pouch inflammation than those without cholangitis (59). Liver transplantation does not alter the course of pouchitis in most patients. Pouchitis appears to be related to ulcerative colitis because it is uncommon in a pouch created for familial polyposis.
Pouchitis is detected both by a barium study and endoscopy. The final diagnosis is based on histologic findings.
A long-term complication of an ileal pouch is fistula formation. The pouch and any related fistulas are readily evaluated with a barium enema, although MRI is also useful in evaluating pelvic fistulas (60). For poorly understood reasons, cholelithiasis is relatively common in patients after an ileoanal anastomosis; most stones are composed primarily of cholesterol.
One late complication after an ileoanal anastomosis is small bowel volvulus, believed to be related to surgical manipulation and the resultant mesenteric tension.
Diverticulitis
Clinical
Symptomatic diverticular disease tends to manifest in one of two ways: either as an infection evolving into diverticulitis or as a lower gastrointestinal hemorrhage, often massive but typically self-limiting. The latter condition is
discussed later in this chapter; see Vascular Lesions (Bleeding).
Acute right colonic diverticulitis is an uncommon diagnosis in the Western world, although the actual prevalence is not known because some of these patients are treated conservatively. A higher prevalence of right colonic diverticulitis is found in the Orient.Among consecutive patients admitted to Singapore General Hospital with diverticular disease, 42% had right-sided diverticula, 34% had left-sided, and 24% had bilateral (61); of these patients, 47% had rectal bleeding, 36% diverticulitis, 12% obstruction, and the rest presented with fistulas. In the West, a correct preoperative diagnosis of right colic diverticulitis is usually not made. A not uncommon scenario is a preoperative diagnosis of acute appendicitis in an adult, found to represent right colonic diverticulitis at surgery. A necrotic cecal carcinoma is also in the clinical differential diagnosis.
Left-sided diverticulitis in younger adults tends to be more virulent than in the elderly. These patients have a higher rate of emergency surgery, and an erroneous preoperative diagnosis, such as appendicitis, is more often entertained.
Imaging
Quite often a patient with typical clinical findings of acute sigmoid diverticulitis does not undergo any imaging but is treated medically. Only in patients with an atypical presentation or those not responding to medical management is CT obtained to evaluate for possible complications. Although CT readily detects diverticulitis, the diagnosis is generally already suspected clinically and a surgeon is more interested in presence of complications. In some European centers, on the other hand, a water-soluble contrast enema and, more recently, CT are obtained during the acute attack.
Ultrasonography has been proposed in a setting of acute diverticulitis; it is of particular value in women for whom a gynecologic abnormality is in the differential diagnosis. A contrast enema should be approached with caution in these generally acutely ill patients; first, a barium enema is contraindicated because of a possible perforation into the peritoneal cavity (keeping in mind that a free perforation is a known complication of acute diverticulitis), and
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if a contrast enema is needed a water-soluble contrast agent is employed; second, these sick patients tolerate an enema poorly, spasm is common, and, due to the inherent low radiographic contrast of water-soluble contrast agents, details are difficult to evaluate. Also, although water-soluble contrast agents are innocuous in the peritoneal cavity, with a perforation invariably infected colonic matter is also spilled.
Imaging studies are also requested if a patient does not improve with medical therapy and surgery is contemplated. For this latter indication imaging is performed primarily to exclude complications of diverticulitis, namely abscesses, fistulas, or other abnormalities such as a necrotic tumor.
Further imaging is generally requested once an acute attack has subsided and if an elective resection is contemplated. At this point an abscess is generally not a consideration, as the surgeon is interested in the presence of any other disease, namely, a necrotic, infected cancer. Clinically, such a cancer can mimic diverticulitis. The controversy here revolves around the best modality with which to detect a necrotic colon cancer. In this setting I prefer a double-contrast barium enema over CT, although gastroenterologists argue for colonoscopy. A differentiation between a necrotic cancer and diverticulitis is usually straightforward with a barium enema but can be rather subtle with CT. To illustrate this dilemma, a retrospective study of patients with proved diverticulitis and colon cancer, with readers blinded to diagnosis, found that pericolic inflammation and colonic involvement >10cm in length were the most significant findings for diverticulitis, whereas enlarged pericolic lymph nodes and intraluminal tumor were the most significant findings for colon cancer (62); using these criteria, a prospective CT study achieved a correct unequivocal diagnosis in only 40% of patients with diverticulitis and 66% of patients with colon cancer (62), not a very satisfactory result. Also, some patients with diverticulitis have enlarged lymph nodes, thus further confusing the differential between diverticulitis and a necrotic cancer.
Table 5.2 outlines typical CT findings in acute sigmoid diverticulitis. Not all findings are, of course identified in any one patient. Most commonly detected is pericolic inflammation.
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Table 5.2. Computed tomography findings in acute sigmoid diverticulitis
Focal sigmoid wall thickening
Arrowhead sign (see text)
Inflammation
Pericolic
Sigmoid mesentery
Phlegmon
Abscess
Intramural
Pericolic
Fistula
Obstruction
Sigmoid obstruction
Small bowel obstruction
Portal and mesenteric vein gas
Peritonitis
Although diverticula are often identified, the presence of diverticula is not a sign of diverticulitis. Focal mural thickening is often found in a setting of chronic diverticular disease without evidence of acute inflammation. Some radiologists subdivide CT findings of diverticulitis into mild and severe, with the latter including a mesenteric abscess, fistula, and peritonitis, but such differentiation is generally obvious clinically.
An arrowhead sign, consisting of an arrow- head-shaped collection of contrast located within a thickened colon wall, was identified by postcontrast enema CT imaging in 27% of patients with a final clinical diagnosis of colon diverticulitis (63); an inflamed diverticulum, consisting of a rounded, paracolic outpouching with surrounding fat stranding, was found in 33% of these patients. Neither the arrowhead sign nor an inflamed diverticulum was found in any other condition in the authors’ study of 150 consecutive patients suspected of diverticulitis, and thus both achieved 100% specificity; of interest is that almost half of the inflamed diverticula identified contained high attenuation material.
The results of US in detecting and evaluating sigmoid diverticulitis are mixed. Comparison studies of CT and US in patients suspected of having acute colonic diverticulitis reveal similar sensitivities and specificities for CT and US; more pericolic diverticular abscesses are identified with CT than US. Either a concomitant adynamic ileus or an obstruction tends to limit adequate US visualization of the region in