The third ventricle is the midline CSF-filled space between the two thalami, extending from the interventricular foramina of Monro (anterior, connecting to the lateral ventricles) to the cerebral aqueduct (posterior, leading to the fourth ventricle). Its floor is formed by the hypothalamus (infundibular recess, tuber cinereum, mammillary bodies); its roof by the choroid plexus and the internal cerebral veins; its walls by the thalami and hypothalami. The interthalamic adhesion (massa intermedia) crosses it in most individuals.
The third ventricle is the site of colloid cysts — benign mucin-filled cysts arising at the foramen of Monro that intermittently obstruct CSF flow, producing acute obstructive hydrocephalus and potentially sudden death from acute elevation of intracranial pressure. Endoscopic colloid cyst removal through the foramen of Monro (transchoroidal, transforaminal, or transventricular approach) is the standard treatment. Hypothalamic hamartomas (non-neoplastic hypothalamic lesions projecting into the third ventricle floor) cause gelastic epilepsy. Neuroendoscopic third ventriculostomy — creating a hole in the floor of the third ventricle — treats obstructive hydrocephalus by bypassing the aqueductal obstruction.
A colloid cyst at the anterosuperior third ventricle at the foramen of Monro intermittently blocks CSF outflow from the lateral ventricles producing acute obstructive hydrocephalus with severe headache and potentially sudden death; MRI demonstrates the characteristic T1-hyperintense or iso-intense cyst at the foramen of Monro; endoscopic transchoroidal cyst removal provides definitive treatment.
