When the Horse Has Stripes: Central Nervous System Blastomycosis Presenting as a Ring-Enhancing Lesion in an Immunocompetent Patient

Abstract A new, single, ring-enhancing lesion in the cortex of an immunocompetent individual has a broad differential diagnosis, although primary or metastatic malignancies are usually highest on the list. The presence of inflammation without overt malignancy, however, is cause for considerations that may require ancillary testing with fresh tissue and obtaining multiple frozen samples. Central nervous system (CNS) infection with blastomycosis is a rare but potentially fatal complication, with a mortality rate approaching 20% secondary to multiple factors including delay in presentation and diagnosis (Clin Infect Dis 2010;50:797–804). What makes CNS blastomycosis so difficult to diagnose, along with other rare fungal infections, is that it can masquerade as a primary neural tumor. Infectious etiologies should always be considered in ring-enhancing lesions that show inflammation, necrosis, and no significant glial atypia on frozen section. Here we describe a case in an otherwise healthy woman, who presented with seizures and was found to have a peripherally enhancing intraparenchymal lesion on imaging. Differential diagnosis, frozen-section management, histology, and microbiology in the workup of CNS blastomycosis are discussed.