A Giant Tumefactive Perivascular Space: A Rare Cause of Obstructive Hydrocephalus and Monoparesis.

Cerebral perivascular spaces (PVSs), otherwise known as Virchow-Robin spaces, are interstitial fluid-filled channels, <2 mm in diameter that form around arterial perforators as they course from the cortex into the brain parenchyma. In contrast, a giant tumefactive PVS is a rare entity comprising of clusters of such channels larger than 15mm resembling a neoplastic process as the name suggests. We report a 55-year-old male who presented with unsteady gait, cognitive decline, and left lower limb weakness for 6 months. Magnetic resonance imaging revealed a noncontrast enhancing multicystic intraaxial lesion of the right mesencephalon-diencephalon junction extending into the anterior third ventricle causing obstructive hydrocephalus. A ventriculoperitoneal shunt was inserted with a complete reversal of his neurological symptoms. Such PVSs can easily be misidentified for a cystic tumor, and their unique radiological features are discussed to prevent unnecessary surgery. We also demonstrate that when they cause hydrocephalus and midbrain compression symptoms cerebrospinal fluid shunting alone can result in excellent outcomes.