What every neuropathologist needs to know: Update on neuro infectious disease workups and consultation resources

Efficient histopathological diagnosis of central nervous system infections can be challenging but is critical for therapeutic decision making in cases for which less invasive blood or cerebrospinal fluid testing has been unrevealing. A wide variety of bacteria, fungi, viruses, and parasites can cause infections, particularly in immunocompromised individuals. Histological findings may be nonspecific or overlap with noninfectious inflammatory conditions. To minimize wasted tissue and time, a systematic approach is recommended in which: (1) relevant patient history (eg, comorbidities, travel and other exposures, and immune status) and radiological findings are reviewed, (2) a preliminary differential diagnosis based on this information and on inflammatory patterns and visualization of potential microorganisms on hematoxylin and eosin stains is generated, (3) special stains, immunohistochemistry, in situ hybridization, or molecular testing (pathogen-specific or broad-spectrum) are used for confirmation and further classification, and (4) correlation with culture results and other laboratory testing is performed to arrive at a final integrated diagnosis. Discrepancies between molecular and histological findings are often due to contamination and require careful evaluation to avoid treatment of false positives. Consultation with infectious disease pathologists or public health reference laboratories may be needed to confirm diagnoses of unusual organisms or when specialized testing is required.