Diagnosis: Retroperitoneal and renal actinomycosis.

Discussion: Actinomycosis is an uncommon disease. Unless specifically suspected, diagnosed, and with specific treatment started, this destructive disease may cause suppurative necrosis or granulomatous destruction of the adjacent structures.¹ The disease is caused by Actinomyces israelii, which exist as an anaerobic or microaerophilic, filamentous gram-positive saprophyte of the mouth and gastrointestinal tract. Since the organism cannot penetrate healthy epithelium, invasive infection is rare.^1,2 However, when tissue is disrupted as a result of trauma, surgery or other factors, the organism can enter the tissue and remain there silently for months or even years in protected colonies known as sulphur granules. Once the bacteria are able to proliferate, they cause a subacute or chronic granulomatous inflammatory reaction that is followed by necrosis and fibrosis. Spread of infection follows connective tissue planes toward the outer skin surfaces, with invasion into adjacent tissues and eventual sinus tract formation, while frank pus is scant.¹ In rare instances, actinomycosis spreads by hematogenous or lymphatic routes.³ The site of the entry of the micro-organism usually remains obscure.^2-4

The most common sites of primary actinomycosis are cervicofacial, abdominal and thoracic.⁵ Genitourinary involvement is rare,^1,6 and renal involvement (reported in around 50 cases) can be the result of hematogenous spread, direct extension from intra-abdominal process, or transdiaphragmatic extension from a thoracic focus.¹ In the present case, the right kidney was involved due to local extension from the retroperitoneum. The lesion may mimic extension from the retroperitoneum. The lesion may mimic pyelonephritis, a renal mass or perinephric abscess.^7,8

In the retroperitoneum, the most common tissue reaction is the formation of a firm, fibrous avascular mass which may be mistaken for carcinoma or other type of malignancy.¹ In children, retroperitoneal actinomycosis may mimic inflammatory pseudotumor.⁹ Thus retroperitoneal actino-mycosis should be considered in the differential diagnosis of any obscure retroperitoneal mass, as it is likely to be misdiagnosed, because it may present itself within the abdomen in so many different ways and at such unlikely sites.

There is no reliable serological or skin test for the detection of actinomycosis. The diagnosis is made on the basis of the histological examination of the tissue sample when the identification of the so-called sulphur granules in the infected tissue is characteristic of actinomycosis,^1,10 or on the result of anaerobic culture of the offending microorganism.¹ The sulphur granules are round or lobulated white-to-yellow particles 1 to 2 mm in size, with a radiating fringe of eosinophilic projections. They are composed of a matrix of calcium phosphate and contain colonies of Actinomyces, cellular debris and associated organisms.¹ In the present case, histological examination of the mass showed typical sulphur granules (Figure 2) surrounded by neutrophils along with lymphocytes, plasma cells and histiocytes. Increased fibrosis was seen. In addition, a microscopic section of the involved kidney showed micro-organismal colonies dispersed in the capsule and subcapsular areas which were associated with dense fibrosis and chronic inflammatory exudate.

Treatment of actinomycosis requires: 1) use of antibiotics, and 2) the removal or debridement of infected tissue.¹⁰ Penicillin remains to this day the drug of choice, although the dose of the drug and duration of treatment has not been standardized.¹ Review of the literature suggests that prolonged treatment of several months is essential.^1,11 Crosse et al.³ believe that therapy should be continued for three months after all traces of infections have disappeared. In patients who are allergic to penicillin, clindamycin, erythromycin, or tetracyline can be used.^11,12 Use of ciprofloxacin has also been suggested.¹³ Because adequate concentration of the antibiotics is not obtained within the densely scarred avascular areas where organisms flourish, surgical excision of the involved tissue is essential.³ In the present case, the patient received oral ciprofloxacin for four months after three weeks of intravenous benzylpenicillin and was doing fine at 11 months' follow-up.