| Discussion: |
Colonic diverticulitis is a condition resulting from the perforation of a colonic diverticulum which leads to inflammatory changes occurring mainly in the pericolic structures. The majority of patients with diverticulosis will not present with complications but in 10-25% diverticulitis will develop at some time. The presumed mechanism is infection of the mucosa of the dome of the diverticulum leading to erosion of its thin wall. The site of the perforation may still be in the colonic wall itself but in the majority of cases will occur in the surrounding pericolic tissue structures. In the former case the inflammatory process remains intramurally located but in the latter case the inflammation will spread more or less rapidly outside the colon itself, causing fibrinous exudate in the pericolic fatty tissue and the mesentery. The infectious focus may be walled-off or the inflammatory process may spread and lead to a paracolic abscess. If the inflammation spreads longitudinally along the colon wall other diverticula may become involved, creating an intramural sinus tract. A fistulous tract may also develop towards adjacent structures such as the bladder, vagina, small bowel and skin. Because of the marked pericolonic fibrosis, free perforation in the peritoneal cavity is rare. Clinical symptoms of diverticulitis are fever, abdominal pain, vomiting and nausea, localized tenderness and/or palpable mass and leucocytosis.
Radiological features on plain films of the abdomen include a soft tissue or fluid density mass within the left pelvis and signs of bowel obstruction or even of bowel perforation.
Ultrasound provides a useful noninvasive method for visualizing the extracolic changes, for instance a fluid collection may be visible as a circumscribed anechoic mass adjacent to the colon wall. The thickening of the bowel wall itself is depicted as an abnormally thick hypoechoic band (broader than 5 mm), surrounding an echogenic mucosa. Ultrasound is also useful for image guidance of direct percutaneous drainage of superficially located pelvic or paracolic abscess formations.
Contrast enema with barium will not be performed as the initial imaging procedure in patients suspected of diverticulitis because of the inconvenience for the patient, the risk of exacerbating the condition or provoking barium peritonitis. If performed, water-soluble contrast agents and antispasmodic drugs should be used and the examination should be conducted carefully and with restraint. The features seen on contrast enema are as follows:
eccentric luminal narrowing due to adjacent intramural or pericolic collection which is constant even after antispasmodic administration and displacement of the normal course of the colon;
altered mucosal pattern characterized by thickened, distorted but not destroyed mucosal folds ("saw toothing"); and
deformation of the diverticula.
It is important to note that in a minority of patients with diverticulitis no diverticula are depicted on the contrast enema.
Direct evidence for peridiverticulitis is provided on enema when extravasation of contrast medium outside the lumen is demonstrated, visible either as a thin streak or as a larger fistulous tract into the wall of the colon or into the pericolic structures or into neighbouring organs such as the bladder. However, extraluminal collection of contrast material on contrast enema is noted in a minority of patients with diverticulitis only. Contrast enema has a sensitivity of between 70-80% for diagnosing diverticulitis. Due to the existing narrowing, preventing appropriate distension of the colon lumen and to myochosis, a coexisting carcinoma of the colon, particularly in the sigmoid, is not uncommonly missed on contrast enema performed in patients with peridiverticulitis.
As compared with contrast enema, CT is able to depict optimally the extra mucosal and extracolonic changes noted in this condition. Moreover, it causes less discomfort for th deformed secondary to the adjacent inflammatory process. A further typical pericolic change is the thickening of the mesocolon of the sigmoid. Other changes are the presence of small fluid collections with a diameter of 1–2 cm and located in the wall of the colon in the case of intramural abscesses.
Intramural or pericolic fistulas are visible on CT as linear or tubular branching structures with fluid attenuation values. They are situated either within the colonic wall or running in the pericolic fatty tissue. Fistulas to the bladder are diagnosed on the base of the presence of air or of contrast medium in the bladder in the absence of instrumentation or gas-forming organisms and are associated with bladder wall thickening adjacent to the pericolic soft tissue changes. The differential diagnosis of CT changes in the pericolic fatty tissue associated with bladder wall thickening include Crohns disease, perforated carcinoma, infarction of appendices epiploicae and appendicitis. In some patients with perforated acute appendicitis the periappendicular inflammatory changes in the mesentery spread mainly downwards and to the left side closely simulating diverticulitis of the sigmoid colon.
Because contrast enema and CT provide complementary information for patients with diverticulitis both methods are frequently needed for the optimal management of these patients. For ultrasound- and CT-guided drainage the anterior approach is used most frequently. If the lesion is deeply seated a transrectal approach is preferred.