Actinomycosis and Nocardiosis

Actinomycosis is most commonly caused by Actinomyces israelii, an atypical Gram-positive anaerobic bacterium that exists as part of the normal oral and intestinal flora in man. It is not an opportunistic pathogen, but needs devitalized tissue for the anaerobic milieu required to support its growth and, to this end, often coexists with a commensal bacterial infection [24]. Actinomycosis occurs following a break in the mucosal barrier and produces a chronic suppu-rative granulomatous infection, characterized by draining sinuses, intense fibrosis and purulent abscesses containing the characteristic multilobulated "sulphur granules".

CNS infection occurs in 2-5% cases. The organism reaches the brain through hematoge-nous dissemination or, less commonly, through direct extension from local cervicofacial disease, and most commonly results in a cerebral abscess [24]. Actinomycotic abscesses are usually solitary, multilobulated lesions, with a thick capsule. The clinical picture is that of a space-occupying lesion, causing focal neurological deficit and signs of increased ICP. Occasionally, an SDE or EDA may develop, following penetrating trauma or calvarial invasion from cervicofacial disease.

Actinomycosis is best diagnosed by microscopic examination of pus and infected tissue. The organism has long branching filaments that stain positively with Gram's stain, while hematoxylin and eosin (H&E) staining demonstrates basophilic filaments, terminating in eosino-philic "clubs". The characteristic "sulphur granules" consist of compact clumps of organisms. Culture of the organism is difficult and only possible under anaerobic conditions. There are no serologic or skin tests for Actinomyces.

Cranial imaging usually demonstrates a solitary, ring-enhancing lesion, with a thick capsule and surrounding edema. Management entails drainage and/or excision of the mass, followed by high-dose penicillin therapy (10-20 million units per day) for 3-4 months [23]. Luckily, with early diagnosis and optimum therapy, the prognosis is good.

Nocardia is another "fungus-like" atypical bacterium. However, Nocardia has little in common with Actinomyces, aside from their classification and tendency to produce brain abscess. Nocardiosis is most often due to Nocardia asteroides, a Gram-positive aerobe which is not part of the normal bacteriological flora, but is usually found in the soil and in decaying vegetable matter. Seventy-five percent of patients who develop Nocardiosis harbor an underlying disabling medical disorder (TB, COPD, carcinoma, diabetes, alcoholism, collagen vascular disease) and some form of immunocompromise is identified in 44% of patients [24]. Infection usually begins in the lung following inhalation of the airborne organisms. CNS disease in the form of single or multiple cerebral abscesses occurs from hematogenous dissemination in about 25% of cases. Nocardia abscesses tend to be mul-tiloculated and poorly encapsulated due to the weak inflammatory response invoked by the organism.

The diagnosis of Nocardiosis may prove difficult. The organism is rarely recovered from CSF. Cranial CT is fairly sensitive but non-specific; it typically demonstrates a hypodense, enhancing lesion, with surrounding edema. The diagnosis is made by histological examination, which demonstrates beading, branching Grampositive, acid-fast filaments; the organism is not seen using H&E or PAS techniques.

Mortality rates as high as 80% have been reported with CNS Nocardiosis [24]. However, in the absence of significant concomitant debilitating disease, prompt diagnosis and treatment are associated with an improved outlook. Treatment should consist of abscess drainage and/or excision along with intravenous sul-fisoxazole. Relapse rates are high and prolonged antibiotic therapy (sometimes lasting 6-12 months) is required.

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