The Third ventricle is a median cleft between the two thalami which communicates caudally with the fourth ventricle through the cerebral aqueduct, and rostrally with the lateral ventricles through the interventricular foramen. It is surrounded by critical structures. The hypothalamus lies in its floor and thalami in its lateral walls. It is closely related to important endocrine glands: the pituitary and pineal gland.
The sagittal plane identifies the key components of the anterior contour and floor of the third ventricle. The superior limit is the anterior commissure (AC) while the lamina terminalis forms the anterior wall descending to the optic chiasm. The anterior ‘fish mouth’ shape of the third ventricle is created by two recesses, the optic recess superiorly and the infundibular recess inferiorly.
The infundibular recess is formed by the funnel-shaped proximal hypothalamic infundibulum (pituitary stalk). Around the base of the infundibulum is the median eminence, a raised portion of hypothalamic gray matter. The median eminence and the tuber cinereum, which is also part of the hypothalamus, form most of the floor of the third ventricle. The Mammillary bodies (MB) and midbrain posterior to the tuber cinereum form the posterior aspect of the floor of the third ventricle. The roof of the third ventricle has five layers: the body of the fornix, the superior layer of tela choroidea, the vascular layer, the inferior layer of tela choroidea, and the choroid plexus of the third ventricle.
The interventricular Foramen of Monro allows the two lateral ventricles (white arrowheads) to communicate with each other and with the third ventricle (yellow arrowhead). The foramen is located posterior to the anterior columns of the fornix. Some anatomists describe it as one Y-shaped foramen; others describe it as right and left foramina that communicate with the third ventricle. The egress of cerebrospinal fluid (CSF) from the third ventricle occurs through the cerebral Aqueduct of Sylvius (purple arrow), the opening of which is at the posterior aspect of the third ventricle, below the posterior commissure (PC).
Third ventricle masses can be classified as arising in or immediately adjacent to one of five locations: anterior, posterior, inferior, foramen of Monro, and intraventricular.
Anterior Third ventricle
- Structures involved: Optic and infundibular recesses
- Manifestations: Dysfunction of the hypothalamic–pituitary gland axis
- Origin: Sellar-suprasellar masses/ Hypothalamus / Chiasm
- Pathology: Pediatric: Germinoma, Pilocytic astrocytoma, Craniopharyngioma, Langerehans cell histiocytosis. Adult: Pituitary adenoma, Craniopharyngioma, Lymphoma, Metastases, Sarcoidosis
Posterior Third ventricle
- Structures involved: Posterior commissure and pineal gland
- Manifestations: Hydrocephalus and headaches
- Origin: Pineal/ Tectal / Inbferior Thalamus
- Pathology: Germinoma, Pineocytomas, Pineoblastomas, Other germ cell tumor
Inferior Third ventricle
- Structures involved: Third Ventricle floor
- Manifestations: Central precocious puberty, Gelastic (laughter) seizures
- Origin: Hypothalamus / Basilar artery
- Pathology: Hypothalamic hamartoma, Basilar artery aneurysm
Foramen of Monro
- Structures involved: Anterior superior aspect of the Third ventricle roof
- Manifestations: Intermittent hydrocephalus
- Pathology: Colloid cyst, Subependymoma, SEGA
Intraventricular masses in Third ventricle
- Structures involved: Choroid plexus
- Manifestations: Hydrocephalus and headaches
- Pathology: Chordoid glioma, Ependymoma, Meningioma, Craniopharyngioma
Magnetic Resonance Imaging is the gold standard for delineating the entire extent of a lesion involving the Third ventricle and to further characterize it. The sagittal MR image is often the most useful in determining from which direction a mass involves the Third ventricle.
As detailed above the most commonly found pathologic processes are related to the anterior recesses of the third ventricle and are of sellar-suprasellar or hypothalamic-chiasmatic origin. Lesions deforming the posterior aspect of the third ventricle most commonly arise from the pineal gland, although tectal plate and inferior thalamic masses may obstruct the aqueduct, resulting in dilatation of the third ventricle and hydrocephalus. Lesions may arise in the floor of the third ventricle, such as hypothalamic hamartoma, or be extrinsic masses elevating the floor. The most common mass of the foramen of Monro is a benign colloid cyst. Most intraventricular masses are lesions arising in or metastatic to the choroid plexus.
The main objectives in the surgical treatment of tumors originated from or expanding within the third ventricle include finding the histology of the underlying lesion, achieving maximum lesion excision and normalizing CSF dynamics. The traditional neurosurgical approaches are dictated by the site of origin, the direction of involvement and the presence of hydrocephalus. High resolution Endoscopic procedures are very useful in select cases for tumor extirpation and allow for Endoscopic Third Ventriculostomy (ETV) to relieve the Obstructive hydrocephalus.
Endoscopic extirpation of a Colloid cyst of the Third Ventricle. Preoperative MRI Brain presented under ‘Diagnosis.’
Transcortical extirpation of a Colloid cyst through the Middle Frontal gyrus. The dilated Foramen of Monro is seen (black arrowheads)
Transcallosal approach remains the best microsurgical method of all third ventricle tumors approaches particularly in the absence of hydrocephalus. This route provides for superior visualization of the entire cavity of the third ventricle through different corridors. Using this approach, with care during midline entry in relation to the cortical venous anatomy, and minimal midline retraction, permanent neuropsychological and neurological deficits are not frequent. Transcortical-transventricular approaches to the third ventricle risk injury on the site of corticotomy with postoperative seizures in up to a quarter of patients.
A biopsy may be dispensable if the imaging morphology is typical, in Tectal gliomas, in Optic pathway tumors in patients with Neurofibromatosis and if Serum and CSF markers are positive, the last particularly in Pineal region tumors.
It should be emphasized that, although several corridors to the third ventricle exist, they all ultimately demand the incision of the neural tissue which may be relatively free of the underlying diseases of this approach. Dissection of a tumor within the third ventricle could lead to hemiparesis, memory loss, increased endocrinopathy, visual loss and other signs of diencephalic injury.