Unilateral Primary Angiitis of the Central Nervous System: A Rare and Under-Recognized Entity.

Primary angiitis of the central nervous system (PACNS) is an uncommon inflammatory vasculitis restricted to the brain, spinal cord, and/or the leptomeninges. While typically bilateral, a rare subset presents with strictly unilateral involvement. This variant remains underrecognized and is often misdiagnosed as glioma, demyelination, or chronic encephalitis due to overlapping clinical and imaging features. Unilateral PACNS (U-PACNS) may manifest as tumor-like solitary or multifocal lesions, infarcts, or hemorrhages. Imaging, including vessel wall imaging, perfusion, and spectroscopy may demonstrate underlying abnormalities but is often nonspecific. Histopathologically, most cases show small- to medium-vessel lymphocytic vasculitis, in contrast to granulomatous inflammation, which is more common in bilateral disease. Diagnosis is challenging, particularly in angiography-negative presentations, and often relies on excluding systemic, infectious, and neoplastic mimics. Recognition of U-PACNS is critical, as timely immunosuppressive therapy can avoid misdiagnosis, alter disease course, and prevent irreversible deficits. The authors found 49 reported cases of U-PACNS in the English language literature. Herein, we present a review of this rare and underrecognized entity, outlining key imaging features and pertinent differentials that may help with a more accurate and timely noninvasive diagnosis.