Rare large colloid cyst obstructing the posterior third ventricle: illustrative case

BACKGROUND: Colloid cysts of the posterior third ventricle are exceedingly rare. This location is a high-risk zone for colloid cysts because of potential obstruction of the cerebral aqueduct. OBSERVATIONS: The authors report a case of a 57-year-old man who presented with a 6-month history of progressive headache, short-term memory loss, visual blurring, and an episode of double vision. Magnetic resonance imaging (MRI) revealed a colloid cyst, 22 mm in diameter, attached to the roof of the posterior third ventricle. The posterior third ventricle was obstructed, but both foramina of Monro were patent. The entirety of the cyst was resected via a right frontal parasagittal, interhemispheric, transventricular approach through the foramen of Monro. It was not contained within the velum interpositum. The patient did not experience any postoperative deficits. LESSONS: This case supports the theoretical evidence that colloid cysts in the posterior zone of the third ventricle pose a risk for obstructive hydrocephalus. This report of a large cyst in a rare location describes a unique lesion and provides the first published MRI description.