Answer to Unknown Case of the Month #113

Findings:
The CT scan shows thickening and induration of the sigmoid colon with a fistulous track extending to the bladder on the right. A small amount of barium is seen in the base of the bladder, along with air in the non-dependent portion.

Diagnosis: Colovevsicle fistula secondary to diverticulitis
Colonic diverticuli are herniations of mucosa and submucosa through the hypertrophied muscularis of the colonic wall between the mesentery and lateral tenia at sites of vessel and nerve entry secondary to increased muscular thickening (myotactic contractures) and chronic increased intraluminal pressure. Between 5-10% of the US population over the age of 45 years have diverticulosis, a prevalence that increases to roughly 80% by the age of 85 years. Approximately 10-35% of these patients will develop symptoms and 25% of those will require surgery. Diverticulitis occurs when the narrow neck of the diverticulum becomes occluded, leading to inflammation and localized perforation. 95% of cases of diverticulitis involve the left colon, with less than 5% occurring on the right. Patients present with left lower quadrant pain, fever and elevated WBC. Bleeding may supervene and is more common on the right.

CT has become the diagnostic modality of choice. The most common findings are bowel wall thickening, disproportionate fat stranding and the presence of diverticuli. Other signs of diverticulitis include: fascial thickening, free fluid, inflamed diverticula, engorged mesenteric vessels, pneumoperitoneum (rare), arrowhead signs, phlegmons, and intramural air. Abscess formation (30%) and the presence of mesenteric air pockets >5mm are poor prognostic signs for non-operative treatment. Intramural sinus tracts and fistula to adjacent organs (most commonly involving the urinary bladder – 65% of all diverticular fistulas) may be suggested on CT scans due to intravesical air, focal bladder wall thickening, or extraluminal masses. Presenting symptoms include: recurrent cystitis, fever, abdominal pain, pneumaturia, and fecaluria. If a colovesicle fistula is suspected, the CT scan should be performed with bowel contrast only (no IV contrast) and prior to bladder instrumentation in order to maximize the chance of observing intravesicle air or contrast (urine centrifugation may be helpful). Barium enemas have a sensitivity of only 35%.

Other causes of colovesicle fistulas include: Chron’s disease (usually younger patients with ileovesical fistulas), radiation therapy, pelvic surgery, foreign bodies and colon cancer. Approximately 10% of sigmoid colon carcinomas will have associated diverticular disease and the two may be occasionally difficult to differentiate. Carcinoma causes a shorter irregular or eccentric stricture with shouldered (apple core) ends, mucosal destruction, a relative paucity of surrounding mesenteric fat stranding (vs. diverticulitis) and lymphadenopathy. 

References:

Horton KM, Corl FM, Fishman EK. CT evaluation of the colon: inflammatory disease. RadioGraphics 2000; 20:399-418.

Joseph RC, Amendola MA, Artze ME, et al. Genitourinary tract gas: imaging evaluation. RadioGraphics 1996; 16:295-308.

Yu NC, Raman SS, Patel M, et al. Fistulas of the genitourinary tract: a radiologic review. RadopGraphics 2004; 24:1331-1352.

Dr. Lawrence C. Swayne, Attending
Diagnostic Radiology
Morristown Memorial Hospital