Introduction

Due to its deep location and strict relationships with vital neurovascular structures, the third ventricle is one of the most complex areas to access. Several surgical transcranial routes to the different portions of the third ventricle, including translamina terminalis, the frontal transcortical transforaminal, the anterior transcallosal, and the infratentorial supracerebellar approaches, have been described.1–7 Most of these approaches enable the entrance to the third ventricle cavity via its natural communication foramina and/or passing through relatively avascular areas of the third ventricle walls, such as the suprapineal recess, the lamina terminalis, and the tuber cinereum—the tuber cinereum represents a gray matter layer of the third ventricle floor that is commonly opened during endoscopic third ventriculostomy.

The endoscopic transsphenoidal approach, initially adopted for sellar and, later on, for suprasellar lesions, has recently expanded its boundaries, providing the possibility to access the third ventricle cavity. Indeed, in 1987 Weiss8 termed and originally described the extended endonasal transsphenoidal approach. He performed a transsphenoidal approach removing additional bone along the tuberculum sellae and the posterior planum sphenoidale, between the optic canals, with subsequent opening of the dura mater above the diaphragma sellae. Initially, such procedures were performed with the microscope8–10; more recently, the endoscope has contributed to broaden the surgical anatomy knowledge of these areas, thus widening the possibilities of the transsphenoidal route11–16 and allowing a safe surgical exploration of the third ventricle cavity.17,18

11.1 Indications

• 
  

  Meningiomas of the planum sphenoidale or tuberculum sellae.

  
  
• 
  

  Suprasellar craniopharyngiomas.

  
  
• 
  

  Giant pituitary macroadenomas.

  
  
• 
  

  Suprasellar Rathke′s cleft cysts.

  
  
• 
  

  Suprasellar and third ventricle arachnoid cysts.

  
  
• 
  

  Intraventricular tumors (colloid cysts, choroid plexus papillomas, and ependymomas).

11.2 Surgical Steps

The endoscopic endonasal approach allows the visualization of the third ventricle chamber from a different perspective as compared with the conventional transcranial one, that is, from below.

With the patient supine, the head is positioned in a slightly extended position to optimize access to the anterior cranial base. The face is turned 5 to 10 degrees toward the surgeon. The endoscope is inserted into the chosen nostril, usually the right, parallel to the nasal floor, and the nasal septum is visualized medially. The inferior turbinate is identified laterally and its tail is followed until the choana, which is limited by the vomer medially and the floor of the sphenoid sinus superiorly. Once the choana is identified, the endoscope is angled upward, along the sphenoethmoid recess for approximately 1 to 1.5 cm above the roof of the choana, and the sphenoid sinus can be opened either through its natural ostium or through the sphenoid prow. Unilateral middle turbinectomy along with bilateral removal of the posterior ethmoidal cells allows an adequate corridor.

The nasoseptal flap can be designed at this point, as described by Hadad et al.19 The flap is then stored in the nasopharynx until tumor resection is completed, taking care to prevent twisting the pedicle to prevent ischemic damage of the flap. On the other hand, the flap can be drawn on the nasal septum at the beginning of the procedure, while it can be raised and rotated on the osteo-dural defect at the end of the surgery; in this way, the ischemia of the flap due to twisting of its pedicle can be prevented.

At this point, the nasal septum is detached from the anterior wall of the sphenoid sinus with a dissector or a high-speed microdrill. Then 1 to 2 cm of the posterior edge of the nasal septum (vomer) is resected with back-biting forceps. It is crucial to widely open the anterior wall of the sphenoid sinus to gain a proper working angle for the instruments. Subsequently, the contralateral middle turbinate is outfractured. All sphenoid intrasinus septae are trimmed. The main anatomic landmarks of the posterior wall of the sphenoid sinus must be recognized ( Fig. 11.1 ). These include the optic nerves and intracavernous carotid artery canals as well the lateral and medial opticocarotid recesses (MOCR, medial optic-carotid recess; LOCR, lateral optic-carotid recess, respectively) and the clival recess.

A complete removal of the tuberculum sellae, that is, the suprasellar notch20 as seen from the endonasal perspective up to both medial opticocarotid recesses, is mandatory to enter the suprasellar area and reach the third ventricle: the bone is thinned with the drill and then removed with a Kerrison rongeur. At this stage, the dura is opened and the entire suprasellar region comes into view ( Fig. 11.2 ). This region can be divided into four areas by two ideal planes, one passing along the inferior surface of the optic chiasm and the mammillary bodies and another passing via the posterior margin of chiasm and the dorsum sellae, thus resulting in the suprachiasmatic region, the subchiasmatic region, the retrosellar area, and the ventricular region17 ( Figs. 11.3 and 11.8 ).

In the suprachiasmatic region, the chiasmatic and the lamina terminalis cisterns with relative contents are accessible. The anterior margin of the chiasm, the medial portion of the optic nerves, the anterior cerebral arteries, the anterior communicating artery, and the recurrent Heubner′s arteries, with the most posterior portion of the straight gyrus, can be visualized ( Figs. 11.4 and 11.5 ).

In the subchiasmatic space, the pituitary stalk is encountered below the chiasm, with the superior hypophyseal arteries and its perforating branches supplying the inferior surface of the chiasm and the optic nerves. The superior aspect of the pituitary gland and the dorsum sellae, posteriorly, are also visible. The superior hypophyseal arteries supply the optic chiasm, the floor of the hypothalamus, and the median eminence. Each of the superior and inferior hypophyseal arteries anastomoses with the corresponding vessels of the opposite side, forming an arterial ring around the hypophysis; this is an important anatomic note ( Fig. 11.3 ).

The retrosellar area, explored passing with the endoscope between the pituitary stalk and the internal carotid artery above the dorsum sellae, encloses the upper third of the basilar artery, the pons, the superior cerebellar arteries, the oculomotor nerves, the posterior cerebral arteries, and lastly the mammillary bodies and the floor of the third ventricle at the level of the tuber cinereum ( Fig. 11.6 and 11.7 ).

As seen from the endonasal perspective, the third ventricle cavity can be divided into four areas by means of two ideal planes, one passing through the optic chiasm and the interthalamic commissure, and one passing through the posterior edge of the foramen of Monro and the interthalamic commissure. Accordingly, two anterior (infundibular and foraminal) and two posterior (mesencephalic and tectal) areas can be defined ( Fig. 11.8 ).

Through the endoscopic endonasal approach, two different corridors can identified, namely the suprachiasmatic and the subchiasmatic.

• 
  

  Through the suprachiasmatic pathway, the lamina terminalis cistern is entered passing above the optic chiasm. Once the lamina terminalis is opened, the infundibular area of the third ventricle can be accessed ( Fig. 11.9 ). As soon as the third ventricle chamber is entered, endoscopic inspection with 0-degree endoscope permits the visualization of the thalami laterally and the interthalamic commissure. The use of angled endoscopes allows a better view, especially of the foraminal area.

  
  
• 
  

  The subchiasmatic route allows the entry into the third ventricle cavity through its floor, that is, the tuber cinereum, which is localized on the floor of the third ventricle between the pituitary stalk and the mammillary bodies. Removal of the dorsum sellae, preferably coupled with the anterior transposition of the pituitary gland, is a preliminary step during the endoscopic endonasal approach to the tuber cinereum. As a matter of fact, after complete removal of the sellar floor, the dorsum sellae, together with the posterior clinoids, has to be exposed extradurally and carefully removed. To obtain a wider visualization of the tuber cinereum, an anterior transposition of the pituitary gland should be performed as described by Kassam et al.21 To simplify the anterior pituitary gland transposition procedure, two cuts can be made on each lateral aspect of the gland to leave in place only a thin glandular cuff on both cavernous sinuses, thus avoiding any dissection on its medial wall. After that, the gland can be displaced superiorly. Once the transposition has been completed, a wide view of the retrosellar space and of the floor of the third ventricle can be obtained (see Fig. 11.6 ). The endoscope can be advanced in an inferosuperior trajectory inside the ventricular cavity, passing through the tuber cinereum. Once inside the third ventricle, the thalami and the interthalamic commissure, the foramen of Monro, and the bulging of mammillary bodies can be seen.

The endoscopic endonasal exploration of the foraminal area permits to show the inner surface of the foramen of Monro, that is, the portion that faces the third ventricle ( Fig. 11.10 ). As seen from this perspective, the body of the fornix is located on the middle of the field and it continues upwards and laterally with its columns; on the other hand, the inferolateral surface of each foramen of Monro, as seen from below, is formed by the ipsilateral thalamus. The choroid plexus extends within each foramen of Monro and surrounds the body of the fornix like a collar before entering the lateral ventricle through the choroidal fissure. The anterior commissure is visualized anteriorly to the foramen of Monro.

Finally, passing under the interthalamic commissure, the posterior portion of the third ventricle can be reached, that is, the mesencephalic area ( Fig. 11.11 ). Accordingly, it is possible to show the pineal and suprapineal recesses, the posterior commissure, the habenular commissure, the stria medullaris, the tela choroidea, and the beginning of the cerebral aqueduct. The pineal gland and the internal cerebral veins lateral to the pineal gland can be seen as well.