Abdominopelvic Actinomycosis: Radiographic Features

Etiology & Demographics

Actinomycosis is a rare chronic granulomatous infection, which can mimic other inflammatory diseases. Actinomyces israelii is a gram-positive anaerobic organism which is considered to be a normal endogenous flora of the oral cavity, gastrointestinal and genital tracts [1]. The abdominopelvic region is involved in 20% of actinomycosis cases, third in rank following the cervicofacial (50%-65%) and thoracic regions (15%- 30%) [2]. It is approximately three times more common in males than in females, even though it is linked to several cases in association with the use of intrauterine contraceptive devices (IUCDs) [3].

$$ \| $$ Abdominal actinomycosis usually happens after disruption of the mucosal barrier following appendicitis, diverticulitis, surgical procedure or penetrating trauma [3]. Many abdominal organs can be involved including the gastrointestinal tract, ovaries, and liver, yet the ileocecal region and its surrounding structures are most commonly affected by this infection (Table 1) [4, 5].

Clinical & Imaging Findings

Both clinical presentation and imaging findings of this entity are nonspecific and frequently misdiagnosed by many clinicians. Most patients present with nonspecific constitutional symptoms such as chronic low-grade fever, weight loss, and night sweats, which can remain undetected for months to years [6]. The imaging findings of abdominal actinomycosis are also nonspecific, yet the important radiologic feature of abdominopelvic actinomycosis is the aggressive infiltrative nature and its tendency to cross natural anatomic barriers.

The barium study is not frequently used in diagnosing these cases due to its inability to asses for extraluminal involvement. The radiographic findings include mural invasion with stricture formation, mass effect with luminal narrowing, and thickened mucosal folds. Ultrasound might be used in the initial assessment as in our case which demonstrates mixed echogenicity complex mass. On the other hand, CT scan is the most useful modality for determining the anatomic location and the extent of disease. The most frequently reported features are concentric enhancing bowel wall thickening, peritoneal masses, and inflammatory fat infiltration that has a tendency to cross the soft tissue planes associated with abscesses. Regional lymphadenopathy and ascites are minimal or absent [7, 8]. In a recent retrospective study, Lee, et al. analyzed CT scans in 18 patients with pathologically proven abdominopelvic actinomycosis, the majority of them were diagnosed postoperatively. The sigmoid colon was most commonly affected (50%), bowel wall thickening was seen in all patients and (17/18) patients showed pelvic or peritoneal masses [9]. Unfortunately, in our study and other studies from literature most actinomycosis cases are diagnosed postoperatively. The most important CT clue for the correct diagnosis in our study was a large inflammatory infiltrative mass at the ileocecal junction. The definite diagnosis is based on the histopathological identification of the actinomycotic granule or culture of the Actinomyces or both. However, the success rate of recovery of bacteria by culture is less than 50% and the granules are often scarce in the specimens [7].

Treatment & Prognosis

The mainstay treatment for abdominal actinomycosis is a high dose of intravenous penicillin G followed by oral antibiotics. The prognosis is good, though some cases may require treatment for up to 18 months. Surgery is required only in complicated cases [10, 11].

Differential Diagnoses

The main differential considerations towards a large inflammatory infiltrative mass at the right iliac fossa are intestinal TB, Crohn disease, and malignant tumors (Table 2).

Table 2: Differential table. Intestinal Tuberculosis: Abdominal tuberculosis accounts for 11-16% of extrapulmonary tuberculosis. It can involve the intestine, peritoneum, lymph nodes, or solid abdominal organs. Terminal ileum and ileocecal junction are the most commonly involved sites. Strictures are the most common finding in intestinal TB and are typically short, concentric and smooth in outline with significant pre-stenotic dilatation. At CT scans mural thickening with contiguous ileocecal involvement is the most frequent finding. The presence of hypodense lymph nodes with peripheral enhancement in the mesentery and retroperitoneum are helpful to reach a diagnosis [12, 13]. Crohn disease: As with intestinal TB, the ileocecal region is the most commonly affected site, yet isolated involvement of ileocecal junction not seen in Crohn disease. Strictures are usually long, eccentric with sacculations at the anti-mesenteric border and without significant pre-stenotic dilatation. At barium study aphthous ulcers and skip lesions are pathognomonic. At computed tomography mural thickening with stratification and vascular engorgement of the mesentery are seen with active inflammation. Mesenteric fibrofatty proliferation, fistulae, and anorectal disease are frequently encountered findings [12, 13]. Complicated colonic cancer: A perforation complicates colonic CA in 2.5-10% of patients. If perforation or abscess is present in patients with colon cancer, CT finding of severe pericolic fat stranding is seen similar to any other benign inflammatory conditions. The presence of marked asymmetric wall thickening with loss of stratified enhancement that involves a short segment of the colon with an abrupt transition from a normal to an abnormal bowel wall favors the diagnosis of an underlying malignancy. The presence of pericolic lymphadenopathy more commonly detected in colon cancer [14]. Carcinoid: The small intestine is the most common location for gastrointestinal carcinoids followed by the rectum, and the appendix. Small solitary carcinoid is typically not detected on CT scans instead, it may manifest as asymmetric or concentric mural thickening of small intestine secondary to an infiltrating tumor and a desmoplastic submucosal fibrosis. Calcification is detected in 70% of mesenteric lymph node metastases. Fluoroscopic barium evaluation of the small bowel reveals solitary or multifocal, smooth, rounded nodules or mucosal elevations in the distal ileum. A fixed, rigid, curved segment of small intestine might be seen. The appendiceal carcinoid may manifest as appendicitis or a focal soft-tissue mass within the appendix on CT scans [15].

Teaching Point

Abdominal actinomycosis is a rare infection, yet to avoid unnecessary surgical intervention in these patients it should be included in the differential diagnosis of a patient who presents with certain radiographic features. The most commonly reported CT findings are concentric enhancing bowel wall thickening, peritoneal masses, and an inflammatory fat infiltration that has a tendency to cross the soft tissue planes associated with abscesses.