Frontobasal Approaches to Clival Chordomas

The frontobasal approach is designed to expose strictly extradural lesions of the anterior fossa as well as middle clivus and inferior clivus, with little or no lateral extension. For such inferior-midline tumors, the approach provides a very direct reach. Lesions located as low as the anterior arch of C1 (atlas) can be addressed. When compared with other anterior midline approaches, such as the transsphenoidal approach, the frontobasal technique also provides ample material for reconstruction and therefore a safer closure is possible. The deep and narrow surgical corridor provided by the frontobasal approach is limited in its lateral extension by rigid and vital anatomical structures such as the optic nerves, internal carotid arteries, and the lower cranial nerves. Also, a direct view of the superior-third of the clivus is obstructed by the sella turcica and therefore this approach is not suited for tumors having a large superior clival component, although the pituitary gland is pushed rostrally by large and invasive tumors. For tumors with significant lateral extension into the petrous bone or the cavernous sinus, there is the possibility of same-stage combinations of the frontobasal approach with anterolateral approaches such as cavernous sinus explorations or the subtemporal approach to address single-sided lateral extensions. In conclusion, this is a well-defined and very specific approach for very well-defined indications, which has withstood the test of time since its first application for clival-tumor resection in 1972.

16.2 History

The group of frontobasal approaches are modifications of the basic subfrontal approach. The subfrontal approach was first used by Dandy in 1936 to resect a large anterior fossa meningioma. ¹ In 1958, Unterberger utilized the approach for repair of traumatic anterior skull base fractures. ² Tessier et al ³ refined the approach for reconstruction of craniofacial anomalies. Ketcham et al ⁴ combined the procedure with transfacial approaches to resect maxillofacial malignancies. Derome ⁵ first reported the use of the approach for sphenoethmoidal tumors in 1972. The first systematic description of the approach under the name “transbasal approach” was reported by Derome and Guiot and colleagues in 1979. ⁶ This extensive monograph describing the procedure in details in 33 skull base chordoma patients remains a cornerstone. Derome et al acknowledged that the approach was based on Tessier’s technique. ⁶ Further studies defined modifications in handling of various anatomical units, including the supraorbital bar, nasion, cribriform plate, and superior and medial orbital walls, to extend/optimize the surgical exposure as well as the safety/morbidity of the surgical technique. The removal of the of each or bilateral orbital bar(s) as well as the nasion further increased the field of view for upper clival pathologies, for which the standard transbasal approach was very restricted. ⁷^,⁸ Sekhar et al ⁷ combined radical ethmoidectomy with the subfrontal approach and named this the extended subfrontal approach. This was followed by minor or major modifications and with an arborescent catalog of names describing these approaches. Apart from these modifications, various combinations with transfacial or lateral approaches (in the same setting or as staged procedures) have also been described. The initial description of the “transbasal approach” involved removal of the cribriform plate with bilateral sacrifice of olfactory nerve fibers with resultant permanent anosmia. To prevent this complication, a major refinement of the technique with a cribriform plate osteotomy was described by Spetzler et al in 1993, enabling preservation of smell reportedly in 90% of the cases. ⁹^,¹⁰

16.3 Definition

The basic approach was first popularized under the name “transbasal approach” by Derome in 1972, which was followed by the landmark paper in 1979 by Derome et al. ⁶ Since this first description, the approach has been modified by numerous authors, resulting in countless variations that are not necessarily very different from each other. A guideline regarding the nomenclature was provided by Feiz-Erfan et al from the Barrow Neurological Institute. ¹¹ In summary, “frontobasal approaches” are extensions of the subfrontal approach and one can in essence speak of a basic “transbasal approach and of two modifications.” These modifications change the angle of view and extend the anatomical boundaries and therefore alter the indications. The basic “transbasal approach,” as described by Derome, involves varying amount of frontal craniotomies (flush with the anterior fossa) combined with ethmoidal, sphenoidal, and clival bone resections to provide an exposure from middle clivus down to arch of C1 at the midline between the glossopharyngeal nerves. The first major modification is the removal of the supraorbital bar partially (nasion only, nasion + unilateral supraorbital bar) or the supraorbital bar in toto to gain a more extensive exposure of the upper one third of the clivus. Cadaveric anatomical studies revealed that the transbasal approach doubles the viewing angle, whereas the removal of the supraorbital bar increases the viewing angle 5 times when compared with the simple subfrontal approach. ¹²^,¹³ The second group of major modifications, which are very seldom indicated in the management of clival chordomas, include various facial osteotomies to address lesions extending into sinonasal structures. Other than these two major modifications, there is a major refinement devised by Spetzler et al ¹⁰ that involves a “cribriform plate osteotomy” to preserve olfactory nerve fibers. The frontobasal approaches are midline approaches and are virtually blind to tumor extensions into lateral structures such as the cavernous sinus or the petrous bone. In such cases, combinations of frontobasal approaches with lateral approaches such as the subtemporal approach or cavernous sinus explorations in the same setting or in staged fashion have been described. ¹⁴^,¹⁵^,¹⁶^,¹⁷^,¹⁸

16.4 Preoperative Work-up

Needless to say, skull base chordomas deserve the most detailed and exhaustive preoperative work-up, as they are a locally aggressive, widely osteoinvasive tumors. Magnetic resonance imaging (MRI) establishes the diagnosis and defines the extent of the tumor, whereas computed tomography (CT) provides valuable information on the status of bony invasion and destruction. As noted earlier, the frontobasal approaches are midline approaches and are restricted by important and immobile anatomical structures. Anatomical variations of these also strongly influence the decision of the surgical approach. Measurements of the distance between two medial orbital walls, two optic canals, and two carotid siphons (at the anterior knee and the horizontal segment) can all be obtained from preoperative imaging studies. Both optic canals are angulated by roughly 30 degrees to the midsagittal plane. Variations in the paranasal sinus anatomy, which are not an exception but the rule, are also defined by CT imaging. Digital subtraction angiography (DSA), once an indispensable part of preoperative imaging, has largely been replaced by MR angiography. Variations of the carotid siphon that narrow the intercarotid distance may even preclude the use of the frontobasal approach altogether. Unlike meningiomas, chordomas do not parasitize local vascularity, but definition of smaller but relevant branches (other than the main trunks) in preoperative angiography aids in differential diagnosis. This region is supplied by the anterior and posterior ethmodial arteries, and in 40% of the cases, there may be a third ethmoidal artery. ¹⁹ The posterior ethmoidal artery leaves the ophthalmic artery 5 mm anterior to the optic canal. The lateral portion of the lesser wing of the sphenoid bone is supplied by the middle meningeal artery. A third, small, direct branch of the intracranial internal carotid artery supplies the anterior clinoid process.

16.5 Surgical Technique

The patient is positioned supine with the head slightly extended to promote gravity retraction of the frontal lobes. No routine neurophysiologic monitoring is used. A wide bicoronal skin incision is the first indispensable step for all frontobasal approaches. The skin incision is placed well behind the hairline, far more posterior than the standard bicoronal incision for the subfrontal approach ( ▶ Fig. 16.1 a). This is meant to harvest the widest possible, pedicled, vascularized galeal–periosteal flap, which will serve as the main barrier for the reconstruction of the skull base at closure.

Fig. 16.1 All frontobasal approaches are best performed with a wide bicoronal skin flap to harvest the largest possible galeal graft (a). The standard craniotomy for the transbasal approach is done flush with the anterior fossa using three burr holes and by exenterating the frontal sinus (b). Resection of the midline basal structures down to the frontonasal suture widens the rostral viewing angle behind the sella turcica (c). A supraorbital bar osteotomy may be used for the same purpose (d).

The transbasal approach utilizes a bifrontal craniotomy flush with the floor of the anterior cranial fossa. The craniotomy is performed with three burr holes placed bilaterally at the pterion and the midline. Alternatively the frontal free bone flap can be elevated in two pieces. There is considerable interpersonal variation in the extent of aeration of the frontal sinuses, but regardless of the anatomical variation, the frontal sinuses are exenterated and cranialized to assure that the lower limit of the craniotomy is flush with the anterior fossa. The posterior–superior limit, hence the size of the bifrontal craniotomy flap, can be tailored to the extent of exposure needed ( ▶ Fig. 16.1). Simple eye brow incisions followed by subfrontal keyhole approach has also been described. ²⁰ The standard bifrontal free bone flap will in all but the most variative cases provide the necessary exposure down to the arch of C1 (atlas) and the dens process of C2 (axis). More extensive frontobasal approaches involve removal of the supraorbital bony bar ( ▶ Fig. 16.1 b–d). These approaches are classified as “level 1” by Feiz-Erfan et al ¹¹ from Barrow Neurological Institute and are indicated to gain a wider viewing angle to the rostral blind spot behind the sella, but the caudal anatomical limit of the approach does not change ( ▶ Fig. 16.2). The anterior–inferior margin of the supraorbital bar resection is defined by the frontonasal suture, and the osteotomy follows at this location. During removal of the supraorbital bars, the periosteum of the bar is elevated in continuum with the periorbita. The supraorbital branch of the frontal nerve along with the artery and nerve are identified, protected, and elevated from the supraorbital notch using blunt dissection. Variation in the form of a supraorbital foramen is managed with a wedge-shaped osteotomy surrounding the foramen and safely elevating the contents with a small bony sheath. After elevation of the frontal dura, osteotomies at the zygomatic sutures, frontonasal sutures, as well as the orbital roof (anterior to the olfactory groove) are carried out using chisels or power instruments. Even more extensive approaches have been described and are used to address tumor extension into the paranasal sinuses, but these are most commonly used for pathologies other than chordomas. These combined subfrontal–transfacial approaches all involve the resection and therefore require the reconstruction of the medial canthal ligament, which attaches to the frontal process of the maxillary bone. Such an osteotomy lower than the nasofrontal suture will also put the nasociliary duct under risk for surgical injury. In the standard transbasal or various modifications of the frontobasal approach, the initial craniotomy is followed by the mid–skull base osteotomy. Following the craniotomy, the frontal dura is detached from the anterior skull base creates the base.

The frontobasal craniotomy for lower clival chordomas is essentially an epidural procedure, and the dura is not opened. However, this phase is routinely complicated by lacerations at the olfactory groove due to olfactory nerve fibers perforating the dura. In the standard procedure, the bilateral olfactory nerves are sacrificed to prevent possible cerebrospinal fluid (CSF) fistulas. The lacerations are either primarily sutured or obliterated with a piece of galeal graft. This in turn results in permanent and total anosmia, which is not a vital but a very debilitating complication of the procedure. A refinement in the procedure was devised by Spetzler et al that involves a horizontal osteotomy at the base of the cribriform plate to mobilize a rostral portion of it together with the basal frontal lobe dura. ¹⁰ For this maneuver, the medial orbital walls are osteotomized perpendicular to the coronal and axial planes and the cribriform plate is detached by cutting the base with curved heavy scissors. Caudally, the anterior ethmoidal arteries mark the floor of the cribriform plate for the cribriform plate osteotomy. Anterior ethmoidal arteries are routinely coagulated and divided. Although the cribriform plate osteotomy does not alter the viewing angle of the approach, it is a major refinement, as prevention of permanent anosmia is a major contribution to the comfort of life of the patient. For the standard approach, both optic canals are unroofed, and at the end of this stage, both optic nerves as well as both orbits remain as the sole lateral limits of the midline frontobasal approach. A second refinement was also devised, again by Feiz-Erfan et al, ²¹ that includes bilateral removal of the lateral orbital wall to create extra room to retract the orbits without exerting excess pressure on orbital contents. For the mid–skull base osteotomy, the planum sphenoidale is drilled to expose first the sphenoid sinus and then the clivus. Lateral borders of this second stage of the operation are formed by the medial walls of both cavernous sinuses, which are followed down to both petrous apices. At times, the hemostasis at the medial cavernous sinus wall may have to be done at awkward angles. Bone resection at the middle and lower clivus is done along with tumor resection. The anterior arch of C1 (atlas) can be exposed by dissection of the pharyngeal mucosa anteriorly, but in essence the mucosal planes of the nasal and oral cavities are preserved intact.

Fig. 16.2 Frontobasal approaches are strict corridors to the deep central skull base and the lower clivus, but most boundaries are fixed and vital anatomic structures. On the rostrocaudal axis, the approach is well suited for lesions in the lower two thirds of the clivus. The frontal craniotomy with or without osteotomy of the supraorbital bar provides this wide viewing angle (a). The most rostral portion of the clivus shadowed by the sella turcica (outlined with a red triangle and marked with a star in a). Lateral boundaries at different levels (b) are defined as follows. Most anteriorly, the orbits define the lateral boundaries (c). At the level of the anterior clinoid process, the lateral margins are defined by both cavernous sinuses containing the caroitd siphons (d). At this level, the ethmoid and sphenoid sinuses are drilled away at the midline, medial to both cavernous sinuses. At the level of the posterior clinoid process, the mass of the occipial bone, which forms the clivus, can be drilled at the midline until lower cranial nerve foramina (e). Using this approach, a view down to the anterior arch of C1 can be gained (f).