11 Transclival (Midclivus) Approach

10.1055/b-0039-172573

11 Transclival (Midclivus) Approach

Vittorio Rampinelli, Davide Mattavelli, Marco Ravanelli, Davide Lancini, Alberto Schreiber

The transclival route can be classified into upper, middle, and lower approaches according to two landmarks: the sellar floor and the sphenoidal floor (or alternatively the axial plane passing through the vidian nerves in poorly pneumatized sphenoid sinuses). Approaches through the upper (corresponding to the dorsum sellae) and lower portions of the clivus are described in Chapters 10 and 12, respectively. The approach through the middle portion of the clivus (also called the midclivus) takes advantage of the space between the paraclival internal carotid arteries laterally, sphenoidal floor inferiorly, and sellar floor superiorly to access the prepontine cistern and related structures. In view of the optimal exposure and adequate working volume provided, ¹ during the last decades the transclival approach has been employed to manage lesions of the posterior cranial fossa with increasing grades of complexity, ² – ⁵ ranging from spontaneous cerebrospinal fluid leak (e.g., ecchordosis physaliphora) ⁶ to skull-arising tumors (e.g., chordomas, ⁷ ^– ¹⁰ chondrosarcomas), ¹¹ meningiomas, ¹² ^– ¹⁴ aneurysms of the posterior circulation, ¹⁵ ^– ¹⁹ and cisternal (e.g., meningiomas without dural attachment, ²⁰ ^, ²¹ neuroenteric, ²² ^, ²³ endodermal, ²⁴ epidermoid cysts), ²⁵ and brainstem lesions (e.g., cavernous malformations, ²⁶ ^– ³² ependymomas, ³³ gliomas). ³⁴

When harvesting the transsphenoidal transclival corridor, several anatomical structures must be identified to maintain the orientation and to avoid neurovascular injuries. The vidian nerves and the axial plane passing through them serve as landmarks of the lateral and inferior boundaries of the transsphenoidal corridor. The convex vertical shape of the carotid sulcus as seen from the sphenoid sinus (when sufficiently pneumatized) can be used to trace the course of the paraclival tract of the internal carotid artery. The junction between the carotid sulcus and vidian nerve corresponds to the area of the foramen lacerum, which is filled by the fibrocartilago basalis and serves as a bed for the anterior carotid genu (between the horizontal petrous and paraclival tracts). Of note, variability in terms of pneumatization of the sphenoid sinus and course of the internal carotid artery can remarkably affect the shape and size of the window through the midclivus. ³⁵ ^, ³⁶ When dissecting the deep lateral boundary of the corridor through the middle third of the clivus, particular attention should be paid to the abducens nerve. This nerve runs into the basilar plexus with an ascending trajectory, superior, posterior, and lateral to the petrous process of the sphenoid bone, which is a lateral prolongation of the sphenoid body and has variable anatomy. ³⁷ The petrous process of the sphenoid bone lies posterior to the anterior carotid genu and must be removed carefully to prevent injury to the abducens nerve. Notably, the cisternal tract of the abducens nerve can be better exposed with a lower transclival approach rather than through the midclivus.

It is worth mentioning that the transclival corridor encroaches the basilar plexus, which is a venous dural plexus enclosed between the clival periosteum and the dura. The basilar plexus is the crossroad of a number of dural sinuses, thus causing high-flow bleeding when opened intraoperatively. Moreover, some small veins connected with the basilar plexus can be found within the midclivus, especially in the inferior portion, where the spheno-occipital synchondrosis is located. ³⁸ Injection of cadaver veins for laboratory training provides the reader with a direct perception of the position and density of such plexus. At the same time, vein injection may contribute to obscure other anatomical details of the area. Ideally, multiple sessions with differently injected specimens are recommended.

**Fig. 11.1 Structure of the clivus and relationship with respect to sella turcica, sphenoid sinus, and nasopharynx.** This illustration shows the architecture of the clivus, which is subdivided in upper, middle, and lower segment, as shown by the *black dashed lines*. From an anterior-to-posterior perspective, the upper, middle, and lower portions are related to the sella turcica, sphenoid sinus, and nasopharynx, respectively.

**Fig. 11.2 Intracranial neurovascular structures with respect to clival segments.** Intracranial view of the neurovascular structures in relation to the upper, middle, and lower clival segments, which are delineated by the *white dashed lines*. III, oculomotor nerve; IV, trochlear nerve; V, trigeminal stem; VI, abducens nerve; VII, facial nerve; XII, hypoglossal nerve; AICA, anterior inferior cerebellar artery; ASA, anterior spinal artery; BA, basilar artery; LA, labyrinthine artery; LCN, lower cranial nerves; P1, precommunicating tract of the posterior cerebral artery; PICA, posterior inferior cerebellar artery; SCA, superior cerebellar artery; VA, vertebral artery.

Within the intradural space, an in-depth analysis of the basilar artery (whose course varies from straight and median to exceedingly tortuous) and its collateral and terminal branches is of utmost importance. For this purpose, delicate and complete dissection of the anterior pontine arachnoid membrane is required to identify the neurovascular structures of the prepontine area.

As a final exercise after harvesting the approach through the midclivus, the reader is asked to accomplish an extradural hypophysiopexy by removing the sellar floor and dorsum sellae without opening the sellar periosteum. Though less extended compared with the interdural and intradural variants (described in Chapter 10), this corridor provides substantial exposure of the interpeduncular fossa and related structures with minimal manipulation of the pituitary gland.