Transcavernous Approach

23 Transcavernous Approach

Carlos Eduardo Prata Fernandes Ferrarez, Sebastião Nataniel Silva Gusmão, and Roberto Leal da Silveira

Abstract

The region of the anterior upper third of the posterior fossa represents an area of difficult access due to its narrow corridors and being surrounded by noble, vascular, and nervous structures present in the medial superior portion of the interpeduncular cistern, in the prepontine cistern, and in the cavum region of Meckel, medially.

The transcavernous approach consists of exposing these structures in order to provide a wider access to lesions in these regions. For this, it is necessary to bear in mind that access has not only stages common to almost all cases, such as pretemporal craniotomy and the peeling of the middle fossa, but also steps that should be performed according to the need, on a case-by-case basis, such as resection of the anterior and posterior clinoid processes, access by the Meckel’s cave, or even resection of the petrous apex at Kawase’s triangle.

Next, we will describe each of these step by step, followed by a brief discussion about the process of choosing each one of them according to the different types of pathologies of the skull base.

Keywords: Keywords: transcavernous approach, pretemporal approach, anterior clinoidectomy, posterior clinoidectomy, petrosectomy, schwannoma, basilar aneurysm, meningioma, chordoma

23.1 Introduction

23.1.1 Anatomy of the Cavernous Sinus and Temporal Fossa 1

The cavernous sinus is defined as the dural envelope that contains the cavernous segment of the internal carotid artery (ICA) and represents the venous confluence that receives the terminal branches of several veins that drain the orbit, the lateral fissure, and the anterior and middle cranial fossae, besides presenting free communication with the basilar, superior, and inferior petrosus and intercavernous sinuses (Fig. 23.1).

Fig. 23.1 Schematic representation of the transcavernous access routes in a pretemporal right access surgical photo.

The cavernous sinus is shaped like a boat, with the narrow keel along the superior orbital fissure. It presents four limits: one ceiling and three walls—lateral, medial, and posterior.

The roof of the cavernous sinus faces upward, while its lower border, formed by the union of the medial and lateral walls, just below the cavernous segment of the ICA, gives the sinus a triangular aspect in the axial cuts. The roof is formed anteriorly by the dura mater that covers the lower margin of the anterior clinoid process and posteriorly by the dura mater of the oculomotor triangle. The medial border of the oculomotor triangle is formed by the interclinoid dural fold (which extends from the clinoid process anterior to the posterior clinoid process). The lateral border is formed by the dural fold extending from the clinoid process anterior to the petrous apex; and the posterior border is formed by the posterior petroclinoid fold, which extends from the clinoid process posterior to the petrous apex (Fig. 23.2).

Fig. 23.2 Dissection in anatomical piece demonstrating the arrangement of the cavernous sinus and middle fossa triangles.

23.1.2 Triangles of the Cavernosus Sinus²

●Cinoidal Triangle

This triangle lies between the optic nerve (CN II) and the oculomotor nerve (CN III) and inferior to the clinoid, which constitutes its ceiling. The optic strut is located in the anterior portion and inside it; the clinoid portion of the ICA is identified. The roof of the cavernous sinus forms the back of this triangle.

●Oculomotor Triangle

This triangle is formed by the portion of the ceiling where CN III penetrates the cavernous sinus. The anterior and posterior clinoid processes and the petrous apex constitute the vertices of this triangle. Connecting the clinoid processes, we have the interclinoid ligament, and the petrous apex departs the anterior petroclinoid ligaments, the anterior clinoid, and the posterior petroclinoid ligaments, to the posterior clinoid.

●Supratrochlear Triangle

This triangle lies inferiorly to the entrance of CN III and superiorly to the point of entry of CN IV in the cavernous sinus.

●Infratrochlear Triangle (Parkinson’s)

This triangle is located inferiorly to CN IV and superiorly to the ophthalmic nerve (CN V1). The third margin consists of a line connecting the points of entry of CN IV to the entry point of the trigeminal nerve (CN V) in the Meckel’s cave. The posterior fold of the cavernous portion of the ICA, together with the origin of the meningohypophyseal trunk, is located in this triangle.

23.1.3 Triangles of the Middle Fossa

●Anteromedial Triangle

This triangle lies between the inferior margin of the ophthalmic nerve (CN V1) and the upper margin of the maxillary nerve (CN V2). The third margin is formed by a line connecting the point where the CN V1 penetrates the upper orbital fissure and the entrance of the CN V2 in the round foramen. Removal of the bone between V1 and V2 allows access to the sphenoid sinus.

●Anterolateral Triangle

This triangle lies between the lower margin of V2 and the maxillary nerve margin (CN V3) and a line between the entry point of V2 in the round foramen and the entry point of V3 in the foramen ovale represents the third margin of this triangle. The opening of this triangle exposes the lateral wall of the sphenoid sinus.

●Posteromedial Triangle (Glasscock’s)

This triangle is formed medially by the lateral portion of V3, distally to the point of encounter of this with the greater superficial petrosal nerve (GSPN), the inferior border by the own (GSPN), and the lateral limit defined by a line that connects the spinous foramen and the geniculate ganglion (GG).

●Posterolateral Triangle (Kawase’s)

The Kawase’s triangle is delimited medially by the lateral border of V3 in its posterior segment to the point of encounter with GSPN, previously by the major superficial petrosal nerve itself and laterally by a line joining the Gasser’s ganglia at the point of union between V3 and the ganglion and geniculate.

23.2 The Pretemporal Craniotomy 3

The objective of the pretemporal craniotomy is to provide an extended exposition of the temporal lobe and Sylvian fissure, evidencing the inferior and middle frontal gyri (inferior portion), as well as the superior, middle, and inferior temporal gyri, allowing the transsylvian, temporopolar, and subtemporal.

23.2.1 Positioning

With the patient in the supine horizontal position and supported with cranial fixation in three points (Sugita or Mayfield). The distribution of the pins arranged with one pin in the ipsilateral mastoid and two pins in the contralateral upper temporal line, above the temporal muscle.

The positioning of the head follows four sequential movements: elevation, extension, rotation, and lateralization.

Elevation causes the head to be above the level of the cardiac atrium, favoring venous drainage, thereby reducing venous turgidity and consequently, intracranial pressure.

The amount of extension and rotation varies according to the pathology and its location, as discussed below. Excessive rotations should be avoided so that there is no compression of the internal jugular veins, which would impair the cranial/cerebral venous return. To do this, a minimum distance of about two fingers (3–4 cm) should be preserved between the mandible and the clavicle on the contralateral side.

Lateralization promotes an increase in the distance between the shoulder and the head, thus increasing the angle between these structures. During this stage, the cervical region should be inspected. Increased tonus/stiffness of the extended ipsilateral musculature may indicate stress in the brachial plexus, and therefore, should be avoided.

23.2.2 Trichotomy

A 2- to 3-cm tricotomy is performed along the planned incision. This step should be performed in the operating room, using an appropriate tricotomy machine, avoiding the use of blades. Hair scraping allows better fixation of sterile, curative surgical fields and decreases the frequency of operative wound infections.

23.2.3 Incision

After antisepsis (performed with chlorhexidine or polyvinylpyrrolidone), placement of sterile surgical fields, and infiltration of the subcutaneous with 2% lidocaine with vasoconstrictor, we make the incision demarcation.

The incision should be initiated at the level of the zygomatic arch, 1 cm anterior to the ipsilateral tragus, following in the form of an arch, with posterior extension to the level of the pina of the auditory pavilion, terminating posteriorly to the capillary implantation line, toward the hemi pupillary line, contralaterally. During this process, injury to the superficial temporal artery (ATS) should be avoided, which should be cautiously dissected and bent anteriorly. The patency of the frontal branch of this artery is of fundamental importance for the preservation of the adequate tissue perfusion of the planned flap. The parietal branch is invariably intercepted by the incision at a variable location, and must be coagulated and divided. Another important care is to avoid subcutaneous dissection anterior to the superficial temporal artery, especially near the zygomatic arch, since the frontal branch of the facial nerve is located anteriorly to the ATS.

23.2.4 The Temporal Muscle

The temporal muscle has two portions, an outer one, which originates from the superior temporal line and is inserted in the coronoide process of the mandible, and an internal portion, which originates along the scales of the temporal bone and is inserted in the temporal crest of the mandible. For adequate exposure of the temporal fossa, it should be folded lower and posteriorly. For this, two types of dissections of the superficial fascia of this muscle are used. The muscle is covered by a superficial fascia and a deep fascia. The superficial fascia is composed of two layers, one internal, covering the facial muscle and inserting in the upper temporal line and border of the zygomatic bone, and an outer layer, which continues with the periosteum of the frontal and zygomatic bones. The frontal branch of the facial nerve walks superficially to this layer. Subfascial dissection (below the two layers of the superior fascia) or interfascial (between the two superficial and deep layers of the upper fascia) may be chosen. For the subfascial dissection, an incision is made in the fascia, perpendicularly to the superior temporal line, 2 cm behind the zygomatic process of the frontal bone, toward the tragus and the dissection is carried out toward the zygomatic arch. Then, with the use of a rugina, the subperiosteal dissection is performed on the zygomatic arch, raising this plane toward the folded flap. After the complete exposure of the zygomatic arch, the temporal muscle is completely dissected from the squamous portion of the temporal bone, preserving the integrity of the periosteum and the deep fascia. Retractors are then positioned, directing the temporal muscle inferiorly.

Another technique is the interfascial dissection described by Yasargil 4 ^, ⁵ in which the outer layer of the superficial fascia of the temporalis muscle is dissected while the inner layer remains attached to the muscle. The incision in the external fascia begins 2 cm from the zygomatic process of the frontal bone, perpendicular to the superior temporal line, toward the tragus. The dissection proceeds toward the zygomatic arch, preserving the integrity of both layers of the superficial fascia. As described previously, the outer layer is continued with the periosteum over the zygomatic arch and over the frontal bone, which must be dissected to elevate the plane toward the cutaneous flap, preserving the frontal branch of the facial nerve. After dissection of the outer layer, the muscle is dissected from its insertions in the temporal line and in the zygomatic arch, proceeding in a similar way to that described previously.

If necessary, one can choose to remove part of the zygomatic bone using a round drill, one of its surfaces, or even perform the osteotomy of the frontal process of the zygomatic bone. After the procedure, this fragment should be fixed again in its anatomical position to prevent aesthetic defects and chewing problems.

23.3 Craniotomy

We must begin the craniotomy with three burr holes. The first should be performed in the keyhole (below the upper temporal line, at the level of the frontozygomatic suture, 1.5 cm posterior to it), allowing access to the frontal and temporal fossae. The second should be made at the Eustachian point (the meeting point between the coronal suture and the upper temporal line) and the third in the lower temporal aspect, next to the dissected temporal muscle, to allow generous exposure of the temporal lobe. The trephination points are then interconnected using a side-cutting saw, always looking for the outer edges of each drill hole.

The orbital ceiling as well as the lesser wing of the sphenoid should be extensively removed with drill and/or rongeurs until the dural plica of the meningorbital artery is exposed. The remainder of the temporal scale must be removed in order to allow complete release of the basal and inferior surfaces of the temporal. The removal of these bone structures will facilitate access to the basal structures with minimal retraction, which can still be optimized after opening of basal cisterns and aspiration of cerebrospinal fluid (CSF).

Copious washing of the exposed dura mater with saline solution to remove bone fragments after craniotomy, hemostasis of bony edges as bone wax, as well as dural anchoring at the edges of craniotomy with prolene 4.0 should be performed before the durotomy.

The patient will follow the specificities appropriate for each case, according to the pathology, always taking into account that the dura mater elevated should adapt to the adjacent bone surface, without forming folds or elevations that could obstruct the microsurgical vision (Fig. 23.3).