Arachnoid cysts are benign congenital lesions of the central nervous system accounting for 1% of all intracranial masses. Intraventricular location of arachnoid cyst especially in the fourth ventricle is rare. Fenestration or marsupialization of these "innocent-looking" benign cysts can lead to dreadful life-threatening complications. We report a rare case of development of malignant irreversible global cerebral edema in a 21-year-old lady following surgical decompression of fourth ventricular arachnoid cyst.

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