The pseudocapsule of a pituitary adenoma is a lining of normal pituitary gland cells formed due to gradual compression of the gland by the tumor [6]. Before this pseudocapsule was explored as a viable surgical plane, resection of pituitary adenomas was performed using intracapsular dissection. The pseudocapsule was immediately opened along with the dura, and the tumor was removed in a piecemeal fashion using ring curettes. However, this form of resection relied solely on the surgeon’s ability to differentiate the tumor from the normal pituitary gland by recognizing the texture, color and consistency of the tumor [7]. Problems with leaving a large amount of residual tumor behind or inadvertently damaging the normal gland structure were reported, as even the most experienced surgeons sometimes had difficulty differentiating the tumor from the normal gland.

To overcome the above problems, surgeons started to implement extracapsular dissection. After a wide visualization of the sella, the dura was opened carefully while maintaining the integrity of the compressed paper-thin pituitary gland and its pseudocapsule. A plane between the pituitary gland and pseudocapsule was dissected and followed in order to achieve extracapsular dissection of the tumor. Instead of removing the tumor in a piecemeal fashion, surgeons were thus able to “peel off” the tumor from the compressed pituitary gland or the dura (Fig. 1). Since this dissection was performed under direct endoscopic visualization of the tumor and its periphery, extracapsular dissection had advantages over the intracapsular approach. Most importantly, the tumor could be reliably distinguished from the surrounding structure. Furthermore, this technique allowed the exact site of CS invasion, if any, to be identified. This allowed surgeons to achieve complete removal of the tumor with confidence [8]. Theoretically, due to its aggressive nature, extracapsular dissection may increase the risk of surgical damage to the surrounding structure. However, direct visualization of these critical structures seems to offset such risks. To date, numerous studies have reported excellent surgical results for both nonfunctioning and functioning pituitary adenomas, with no significant increase in postoperative complications [9-12].

For both nonfunctioning and functioning pituitary adenomas, gross total resection is the most important prognostic factor for good tumor control and endocrinological cure [13]. However, large tumors, especially those with CS invasion, have often been considered as obstacles to achieving such results [14]. CSs are complex venous dural sinuses housing important neurovascular structures, including the internal carotid artery (ICA) and cranial nerves, making tumor resection in these spaces particularly challenging [15]. A CS is defined by four walls: medial, lateral, posterior and superior [16]. Resection of these walls, especially the medial CS wall, to achieve access to the CS has been explored over the years and has now been widely adopted. This technique has provided an effective and a safe way for improved visualization of the tumor, which has led to a greater degree of tumor removal and has significantly increased the chance of endocrinological remission [15,17-22].

Truong et al. [23] published a detailed article on surgical techniques for medial CS wall resection. They distinguished the sellar medial wall, which covers the pituitary gland, from the anterior wall, which corresponds to the sphenoidal part of the medial wall. The concept of medial CS wall removal proceeds from the fact that both the medial and the anterior wall are made of a single dural layer with distinct origins. The single-layered anterior wall can usually be directly exposed at the lowest aspect, which is considered the safest location. After the anterior wall is exposed, the inferior petrosal ligament anchoring the medial wall to the anterior CS wall can be cut to mobilize the medial wall away from the ICA. A subsequent cut through the sellar floor and the sellar roof is then performed. Finally, the caroticoclinoid ligament (CCL), which anchors the medial CS wall superiorly, can be cut, releasing the medial CS wall (Fig. 2A, B). An alternative method of entering the CS can be achieved by dissecting the inner and outer dura of inferior intercavernous sinus (Fig. 2C, D) [24], which make up the medial and anterior CS walls, respectively. This allows access to the CS without violating the anterior wall of CS overlying the ICA.

Furthermore, endoscopically accessing the clinoidal venous space dorsal to the ICA, known as the dorsal clinoidal space (DCS), was recently explored [25]. The DCS is formed by a meningeal dural layer, a distal dural ring and a CCL, which make up its medial wall, roof, and floor, respectively. Previously, the DCS was not accessible during transcranial surgeries due to obstruction by the ICA and was a potential site for residual tumors when the distal dural ring was invaded. The DCS can be accessed by first extensively exposing the sellar and parasellar areas. The sellar dura is opened and extended inferolaterally to separate the anterior and medial walls of the CS. Along this plane, the ICA is identified and dissected away from the medial wall of the CS up to the level of the CCL. A blunt dissector is then used to dissect the space below and above the CCL and finally the medial CS wall from the clinoidal ICA to gain access to DCS. Understanding the anatomy of DCS and physically being able to access it have increased the chances of gross total resection of the tumor.

Similar to tumors with CS invasion, tumors with suprasellar extension into the third ventricle have historically posed great challenges due to their deep location and the critical nature of the surrounding structures [26]. An extended endoscopic transsphenoidal approach has now been widely adopted and has been demonstrated in various studies to be an effective method of removing third ventricle tumors [27-30]. Seo et al. [31] described the surgical methods and outcomes of 82 patients with these types of tumors. After the appropriate portion of the skull base was drilled out and the dura was opened, arachnoid dissection was first performed to protect the anterior communicating artery and/or superior hypophyseal artery. The tumor was dissected gently under direct visualization, and one of the two surgical corridors was chosen according to the characteristics of the tumor to gain access to the third ventricle: a suprachiasmatic approach through the lamina terminalis or an infrachiasmatic approach through the tuber cinereum (Fig. 3). The tumor capsule was preferentially dissected in an en bloc fashion following the glial plane to avoid injury to the pia mater. The surgeon carefully dissected the tumor while minimizing injury to the lateral wall of the third ventricle and the hypothalamus. Gross total resection was achieved in 86.5% of the patients, with a visual improvement rate of 81.6%.

The endoscopic transtubercular/transplanar approach is an anterior and superior extension of the transsellar approach where additional tuberculum sellae and planum sphenoidale are removed after opening the sellar floor (Fig. 4). The degree of rostral opening is dependent on the extent of the tumor, whereas the degree of lateral opening is limited by the medial opticocarotid recess [32]. This approach provides excellent access to tumors with major frontal and/or suprasellar extension [33], which otherwise would have required a transcranial approach. These include pituitary adenomas with anterior cerebral artery (ACA) adhesion and sellar tumors invading the third ventricle. When the outer arachnoid membrane is used as a dissection plane, the sellar and frontal extensions of the tumor can be removed under direct visualization while protecting the frontal cortex and the ACA. This technique does not involve excessive brain traction as seen in the transcranial approach; therefore, less damage is inflicted on the diencephalon and the optic nerve area [34]. However, a transcranial approach may be considered when a cuff of the frontal lobe is present between the anterior extent of the tumor and the skull base (Fig. 5).

The endoscopic transclival approach is appropriate for accessing midline clival tumors or brainstem lesions. It can provide a whole range of access to tumors of the upper clivus with dorsum sellae extension to the lower clivus [35]. However, it is limited in its ability to provide access to lateral areas [36], and it is limited by the ICAs superiorly and by the hypoglossal and jugular foramens inferiorly (Fig. 6) [37].

The endoscopic trans-pterygoid palatine fossa approach can be used to access tumors extending into the lateral recess of the sphenoid sinus, as well as invasive sellar tumors (e.g., pituitary macroadenomas) involving the lateral portion of the CS (Fig. 7). The surgical approach starts with removal of the middle turbinate, uncinate process, and bulla ethmoidalis. The sphenoid sinus is opened, and maxillary antrostomy is performed to expose the posterior wall of the maxillary sinus, which is the face of the pterygoid palatine fossa. After the posterior wall is removed, the sphenopalatine artery is cauterized, and the pterygopalatine fossa is opened. The Vidian nerve, foramen rotundum, and maxillary nerve are identified and retracted laterally, allowing continuation of the bone work to ultimately gain access to the CS [38-41]. The advantage of this approach is that the entire lateral sphenoid and CS can be directly visualized. However, to utilize this approach, the tumor must be large enough that the space around the tumor allows sufficient surgical working space within the sinus [35]. Therefore, tumors appropriate for this approach usually have an increased chance of laterally displacing the ICA due to their size, leading to an increased chance of ICA morbidity as the surgeon approaches the tumor.