Causes: Colloid cyst; suprasellar tumors (especially craniopharyngioma); intraventricular tumors; arachnoid cysts of the suprasellar cistern; intraventricular hemorrhage (trauma, arteriovenous malformation, hemophilia). Unilateral tumors, such as those arising in the hypothalamus, basal ganglia, or cerebral parenchyma, may obstruct only one side and cause dilatation of the opposite ventricle and mass compression of the ipsilateral ventricle.
Level of the foramen of Monro. (A) Bilateral enlargement of the frontal horns with a normal-sized third ventricle in a patient with a hyperdense colloid cyst (c). (B) Unilateral enlargement of the left frontal horn caused by a tiny hypodense unilateral tumor (arrow).