Primary Angiitis of the Central Nervous System and Its Mimics: A Pattern-Based Approach to Biopsies for Central Nervous System Vasculitis.

CONTEXT.—: Primary angiitis of the central nervous system is a rare vasculitis that affects small parenchymal and leptomeningeal vessels in the brain and spinal cord. As brain biopsy remains the gold standard in diagnosis, the diagnostic approach to brain biopsies for vasculitis is well described. However, a significant minority of brain biopsies for central nervous system (CNS) vasculitis identify alternative diagnoses. Therefore, it is critical to be aware of the spectrum of infectious, inflammatory, and neoplastic diseases that may enter into the differential diagnosis of a biopsy for potential CNS vasculitis and recognize the pathologic patterns that characterize these diseases.

OBJECTIVE.—: To review the broad diagnostic considerations in evaluating a biopsy for potential CNS vasculitis, with an emphasis on a pattern-based approach and diagnostic tools used to evaluate other entities in the differential diagnosis.

DATA SOURCES.—: This review is based on the clinical experience of the authors and a review of the literature.

CONCLUSIONS.—: A diagnosis of CNS vasculitis is dependent on the identification of transmural inflammation and vascular wall damage, preferentially affecting leptomeningeal and superficial cortical vessels. Where a diagnosis of vasculitis is not made, granulomatous, lymphocytic, and necrotizing patterns of inflammation present alternative differential diagnoses. The appropriate application of immunohistochemistry, special stains, and other diagnostic tools is critical in resolving these diagnoses.