Background: Third ventricle cavernous malformations (CMs) associated with ventriculomegaly and obstructive hydrocephalus are quite rare in patients. Preoperative surgical planning can be challenging due to the lesion's non-specific appearance on CT and MR imaging that can mimic other intraventricular pathologies, such as a colloid cyst. Management of these lesions can be varied in the setting of obstructive hydrocephalus.

Case description: The patient is a 78-year-old female who first presented to her primary care provider with balance difficulties and unable to ambulate on her own. She also had bladder incontinence and progressive, severe headaches. Imaging of the brain demonstrated entrapment of the right lateral ventricle and obstructive hydrocephalus due to a lesion in the third ventricle obstructing the right foramen of Monro, thought to be a colloid cyst. A right frontal neuroendoscopic approach with direct visualization, however, confirmed a third ventricle cavernous malformation (CM). A septal pellucidum fenestration was performed to restore CSF communication and no resection of the lesion was performed. The patient recovered well after the operation and at clinical follow-up reported no headaches and was walking well without a walker and with no neurological deficits.

Conclusions: A third ventricle CM was discovered after a neuroendoscopic approach for resection of a presumed colloid cyst in a patient with obstructive hydrocephalus. A neuro-endoscopic septostomy was performed to treat the obstructive hydrocephalus and no resection was attempted. The patient suffered no complications and is at her neurologic baseline with no deficits.

Keywords: case report; cavernous malformation; colloid cyst; neuroendoscopic approach; third ventricle.