Imaging Appearances, Diagnosis and Treatment of Atypical Brain Abscesses: Review of the Literature

Introduction: Atypical brain abscesses mostly occur in immuno-compromised patients especially in various endemic regions of the world. The atypical imaging appearances as well as other diagnostic difficulties cause delays in making diagnoses and hence prognosis is generally very poor. Outcomes can be improved if the clinician has a high index of suspicion in a patient with positive risk factors and suggestive radiological appearances. This will enable early institution of appropriate therapy to improve outcomes.

Methods: A review of existing English literature was done by performing a PubMed search. The various imaging appearances of atypical brain abscesses are described and recommendations made to aid early diagnosis and treatment of atypical brain abscesses.

Results and Discussion: The clinical features of atypical brain abscesses are mostly insidious and non- specific and occur frequently with a medical background of obvious or latent immunodeficiency. The imaging appearances of atypical brain abscesses including Brain CT and MRI scans can be very atypical and non-specific but with the application of modalities like Diffusion Weighted Imaging (DWI) and MR Spectroscopy, atypical brain abscesses can be differentiated from pyogenic bacterial brain abscesses, granulomas and brain tumours.

Microbiological identification has also progressed with advances in molecular microbiology, nuclear medicine and immunology, making differentiation of the various causative organisms of atypical brain abscesses possible and more readily.

Clinical management relies upon early surgical drainage or excision and early use of intravenous antimicrobial agents adapted to the strains identified. Most fungal species are susceptible to Amphotericin B, Voriconazole, Caspofungin, Itraconazole and to a lesser extent Fluconazole. Nocardia species are treated with Cotrimoxazole, Amikacin and Linezolid whilst Toxoplasmosis can be successfully treated with Pyrimethamine and Sulphadiazine or Clindamycin. Actinomyces abscess can be treated with Penicillins and Mycobacterium abscesses are treated with antituberculous agents- Isoniazid, Pyrizinamide, Rifampicin and Ethambutol. Adjuncts to therapy include CSF diversion, corticosteroid and antiseizure medications.

Conclusion: A high index of suspicion, careful reviews of radiological images, early pus/abscess wall samples obtained by drainage, biopsy or surgical resection are needed to establish a definitive microbiological diagnosis and prompt administration of appropriate antimicrobial agents will improve outcomes of atypical brain abscesses.