Diverticulitis with fistula formation.

Diverticular disease is the most common colonic disease in western nations occurring in 5% of the population. The high prevalence in industrialized countries is due to the highly refined diet that is deficient in dietary fiber [1].

Diverticula can be either acquired or congenital. Acquired diverticula are more common and occur due to herniation of mucosa and submucosa through the muscularis. Congenital diverticula are outpouching of the entire thickness of the intestinal wall. They usually occur adjacent to the vasa recta therefore diverticula commonly occur on the mesenteric side of the colon. They commonly involve left side of the colon and are multiple. Sigmoid colon is the commonest site when the Diverticulosis is left sided. But they occur in Ceacum in 5% of the cases and usually solitary [1].

Diverticulitis is the most common complication and reported in 10% to 35% of the patients with Diverticulosis. It is primarily cause by obstruction of diverticula by fecal material resulting distention and inflammation of the peridiverticular tissues. In uncomplicated diverticulitis the inflammatory process is confined to the colonic serosa [2]. However if the inflammation persist perforation can occur into pericolic fat resulting fibrinous exudates, abscess formation, local adhesions or peritonitis. The abscess formations usually in diverticulitis are sealed off or contain sinus tracts and fistulae. These fistulae commonly involve adjacent structures such as bowel, urinary bladder, vagina and anterior abdominal wall [2].
Uncommon complications are large and small bowel obstruction, portal and mesenteric vein gas, venous thrombosis and colovenous fistulae to the Inferior mesenteric vein [2]. Rupture of a uninflammed diverticula causing fecal peritonitis carries the highest risk of an adverse outcome [3].

Classical clinical symptoms and signs in sigmoid diverticulitis include left lower quadrant pain, tenderness, fever and leukocytosis [2].

Ultrasonography can be of value in early assessment of diverticulitis. On Grey scale sonography features of acute diverticulitis are segmental and concentric thickening of the bowel wall, inflamed diverticula and inflamed perinephric fat [4]. The thickened bowel wall will be Hypoechoic, and Inflamed diverticula seen as bright echogenic foci with acoustic shadowing within or beyond the bowel wall. The inflammatory changes of the pericolonic fat will appear as poorly defined Hyperechoic zones. Torsion of appendices epiploicae can have appearances similar to diverticulitis [4].
A fistulous tract on Grey scale sonography would appear as a linear tracts that extend from the involved segment of bowel to bladder, vagina or adjacent loops. The achogenicity of the tract would vary according to the content within the tract which is usually gas or fluid. However a negative ultrasound scan with a highly suggestive clinical picture of diverticulitis justifies a CT scan [4].

The imaging modality of choice to diagnose diverticulitis is CT because it depicts intramural and extramural components of the disease unlike in barium enema where only the secondary effects of the extramural inflammation on the barium column can be seen [5].

The most important CT findings of diverticulitis include identification of inflamed diverticula, inflammatory bowel wall thickening, and pericolonic fat stranding [2].
Other helpful signs are arrow head sign where contrast material form an arrow head shape collection adjacent to a focally inflamed bowel wall. And the inflamed diverticulum appears as a thick and enhancing ring within inflamed peridiverticular area [2, 5].
In complicated cases there may be extraluminal air bubbles, plegmon, pericolonic abscess, colonic fistula and sinus tracts.

There can be overlap of CT findings between colonic diverticulitis and colonic cancer in 10% of the cases.
Features that favour diverticulitis are fluid at the root of the mesentery, vascular engorgement. In colonic cancer there is an abrupt zone of transition with the normal bowel, enlarged pericolic lympnodes and mural thickness greater than 1.5cm [2].
CT findings of right sided diverticulitis may be difficult to differentiate from appendicitis unless the normal appendix is seen [2].