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The transbasal approach offers extradural exposure of the anterior midline skull base transcranially. It can be used to treat a variety of conditions, including trauma, craniofacial deformity, and tumors. This approach has been modified to enhance basal access. This article reviews the principle differences among modifications to the transbasal approach and introduces a new classification scheme. The rationale is to offer a uniform nomenclature to facilitate discussion of these approaches, their indications, and related issues.
Since the introduction of the transbasal approach, which consisted of a low bifrontal craniotomy with extradural exposure, numerous modifications have been made. Numerous terms (n=60) for these modifications followed (Table 1). Beals and Joganic1 tried to incorporate transbasal approaches in a much broader classification of anterior midline skull base approaches. However, their classification did not prove applicable to the existing literature on such approaches. Moreover, it did not focus on the transbasal approach with extensions. Rather, all major anterior skull base approaches, whether intra- or extracranial, were grouped together. Raso and Gusmäo2 recently suggested a classification system. However, they failed to apply their system to the existing literature to validate its global applicability.
The absence of a uniformly accepted terminology for the various modifications of the transbasal approach impedes a common understanding of these approaches and inhibits communication and discussion. The many names used to refer to transbasal approaches underscore the potential benefit that a clear and concise classification could offer. The general differences in the modifications consist of various osteotomies of the upper facial structures (orbital bony frame and nasal bone) as well as the status of the canthal ligaments. We therefore highlight the basic differences among these osteotomies added to the original transbasal approach to help unify the existing terminology. We also discuss the unique features of each modification to improve understanding of the modifications and their clinical significance.
By definition, all transbasal approaches must be performed via a bicoronal scalp incision, which separates them from anterolateral skull base approaches. Approaches performed through a forehead or extended forehead/facial incision not requiring a bicoronal incision are not considered to be transbasal procedures.
Transbasal approaches consist of an anterior transcranial exposure of the skull base, primarily for extradural pathology. We do not consider frontobasal approaches for intradural lesions to be transbasal procedures (including entirely intradural resection of olfactory groove meningiomas), unless extradural surgery was performed at the same time.
The intracranial entry of a transbasal approach can range from an enlarged bur hole to a wide bifrontal craniotomy. It can be placed unilaterally. However, all these cranial openings should have a frontal location that excludes entry into the temporal fossa. These openings do not include facial osteotomies of the orbital bar or nasion (Fig. 1).
This modification of the transbasal approach consists of the addition of osteotomies along the orbital bar, nasion, or nasal bone to the frontal bone flap (Fig. 2). These osteotomies may be cut as separate fragments from the frontal bone flap, or they can be cut in one piece. The extent of the osteotomy of the orbital bar can include only the nasion (Fig. 2), or it can include the entire orbital bar (Fig. 3). Again, the bone flaps can be unilateral or they can include the orbital roof. The medial canthal ligaments are left intact (Fig. 4). The key to this modification is the presence of an orbital and/or nasal osteotomy without the detachment of the medial canthal ligaments. In particular, osteotomy of the nasal bone without ligamentous detachment is considered here as level I. This definition is a major modification to the classification of Beals and Joganic.1
A level II transbasal approach combines orbital bar osteotomies along with the uni- or bilateral detachment of the medial canthal ligaments. The orbital bar osteotomies typically include the nasal bone, medial orbital wall, and orbital roof. The lateral orbital wall is not included to a significant extent (Fig. 5). This means that the infraorbital fissure is not incorporated in the osteotomy of the orbital bar fragment. The lateral canthus may or may not be detached. Since, by definition, the medial canthal ligaments are detached, a medial canthopexy is performed at time of reassembly (Fig. 6). Osteotomy of portions of the medial orbital wall places the lacrimal system at risk for violation. Therefore, epiphora can be a postoperative complication associated with this modification of the transbasal approach. Avoiding damage to the lacrimal sac during osteotomy is key to avoiding this problem. If, however, injury occurs, dacryocystorhinostomy or lacrimal stenting may help relieve the symptoms of epiphora. Because the nasal bone is incorporated into the orbital bar, valvular collapse or saddle-nose deformity can occur.
This modification is the most extensive form of a transbasal procedure. It resembles a level II transbasal approach with the addition of osteotomies to the lateral orbital wall incorporating the infraorbital fissure (Fig. 7). Both canthal ligaments (medial and lateral) are taken down. This approach also requires a medial canthopexy at the time that the osteotomies are reassembled (Fig. 6). The lacrimal system is at risk and must be protected during osteotomies. Due to the extensive orbital osteotomies and periorbital stripping, the potential risk of enophthalmos is particularly high for this variation.
We reviewed the international literature on transbasal surgery and classified approaches reported in the articles (n=61) based on the above four categories of transbasal approach (Table 2). We found no report of a transbasal procedure that could not be classified according to the preceding criteria. Because all reports were reclassified successfully (Table 2), this classification may provide a system to unify the terminology of transbasal approaches.
By definition, the transbasal approach is a transcranial extradural anterior approach to the midline skull base. On November 21, 1936, Dandy3 used a transbasal approach to resect a large frontal meningioma involved with the ethmoid sinus. In 1958 Unterberger4 used this approach to repair traumatic injuries of the anterior skull base. At the time, the concept was new because such injuries were typically approached extracranially. However, using a transcranial approach, Unterberger increased the safety of the operation by affording protection to the intracranial contents. Tessier and associates5,6,7 then used this procedure to correct craniofacial anomalies.
Earlier, Ketcham and colleagues,8 stimulated by the experience of Smith et al,9 discovered that combining a transbasal approach with a transfacial approach in a craniofacial procedure offered safe and effective removal of sinonasal malignancies. Compared with historical controls of patients undergoing transfacial resection, craniofacial resection of sinonasal malignancies improved length of survival. Ketcham et al8 concluded that the intracranial exposure allowed appropriate staging of the transcranial extent of the malignancy and that it allowed successful en bloc resection of the contents of the anterior fossa along with the sinonasal specimen. The major disadvantage was the potential for frontal bone flap infections. The authors tried to prevent such infections by decreasing the size of the frontal bony opening to the size of an extended bur hole.10,11,12
Finally, Derome13 was the first to name this approach the transbasal approach and proposed it for the surgical treatment of tumors involving the anterior midline skull base. According to Derome's description, the transbasal approach was designed to allow neurosurgeons to resect transcranial tumors that invade the frontal fossa. The benefit was to avoid a transfacial procedure and hence to permit craniofacial resection through a craniotomy-only approach. Derome13 further noted that more centrally located structures, such as the clivus, potentially could be reached through a transbasal approach.
To expose the extradural frontal fossa, the transbasal approach always resulted in complete and permanent anosmia because the olfactory fila were sacrificed (Fig. 8). Subsequently, in selected cases, attempts were made to minimize this disadvantage.5,14 In 1993 Spetzler et al15 used a cribriform plate osteotomy (CPO) to preserve olfaction after transbasal approaches. When applied in selected patients, this procedure preserved olfaction more than 90% of the time.16 Other groups also confirmed the feasibility of CPO.17,18,19 Previous attempts to preserve olfaction during transbasal approaches in a few patients20,21,22 had involved unilateral sacrifice of the fila.
The following indications and rationales for choosing a particular form of transbasal approach represent our opinion based on more than three decades of using different variations of transbasal approaches. Hence, our perspective might not be shared by all experts around the world. However, our recommendations have been crafted especially to help introduce younger colleagues to these approaches and to improve their understanding of the potential differences and options among the various modifications of the transbasal approach.
We believe that the transbasal approach is best applied in patients with a traumatic injury to the frontal fossa and sinus who require repair of a cerebrospinal fluid leak (Fig. 9A, ,B).B). Avoiding facial osteotomies is potentially important when trying to minimize potential osteomyelitis of a precontaminated traumatic wound. Hence, the cosmetically important orbital bar is at less risk of loss to infection (Fig. 9C, ,DD).
This approach provides a more basal trajectory to the frontal fossa than the transbasal approach and hence minimizes brain retraction. It improves access to the central skull base, including the planum sphenoidale, sphenoid sinus, and clivus. Therefore, we recommend this procedure for primary extradural tumor surgery or for large midline meningiomas, where early devascularization of the tumor by cauterization of the ethmoidal arteries aids resection. It can be used in combination with a transfacial approach for en bloc craniofacial resection or as a craniotomy-only form of en bloc craniofacial resection (Fig. 10) if the extracranial tumor is limited. Optimally, extradural clival tumors, including chordomas, can be resected piecemeal via this approach. Cadaveric studies indicate superior exposure of the central skull base compared with the transbasal approach.23 The lateral extent of resection is the internal carotid arteries, optic nerves, and hypoglossal nerves.
Cosmetic deformities of the orbital bar associated with fibrous dysplasia (Fig. 11), encephaloceles (Fig. 12), or synostotic craniofacial suture repair requiring fronto-orbital advancements can also be repaired through this approach.
By including the nasal bone and medial orbital wall into the osteotomy of the orbital bar, this modification of the transbasal approach enhances direct exposure of the nasal cavity and paranasal sinus. Therefore, this approach is particularly suited for en bloc craniofacial resection of tumors with limited sinonasal extension, especially if the goal is to avoid a separate transfacial approach. This modification is also suitable for accessing the clivus (Fig. 13). We believe that this level offers slightly more exposure of the clivus compared with the level I transbasal approach.
A second indication for this modification is the application of a CPO to preserve olfaction during transbasal surgeries. To preserve olfaction, at least 1 to 2 cm of olfactory mucosa must be attached to the CPO. Adequate nasal access is needed for the surgeon to maneuver heavy, curved scissors low enough for the mucosal division (Fig. 14). This maneuver can be achieved reliably using the level II modification of the transbasal approach. The procedure includes a nasal bony osteotomy and enhanced access to the nasal cavity.
This approach is best for extensive tumors with significant sinonasal involvement (Fig. 15) and for tumors that reach or penetrate one or both medial orbital walls. The addition of a lateral orbital wall osteotomy facilitates retraction of the globes, an otherwise potentially hazardous maneuver that can be associated with visual decline. This modification of the transbasal approach potentially decreases retraction pressure on the globes (Fig. 16), thereby likely protecting the visual apparatus. By maximizing transcranial access to the sinonasal compartment and, in particular, by maximizing transcranial microscopic exposure of the maxillary sinus (Fig. 17), this approach is best indicated for craniotomy-only craniofacial resections, in particular, for benign tumors.24 It also is well suited for avoiding extensive facial skin incisions during a combined craniofacial resection when it is paired with sublabial transfacial approaches for sinonasal pathologies at or below the level of the middle and inferior turbinate.25 Because the nasal bone is incorporated in the orbital bar osteotomy, this variation of the transbasal approach is suitable when a CPO is planned. In such cases, the osteotomized cribriform plate is retracted along with the frontal fossa dura (Fig. 18). The globes are retracted laterally, and tumors of the cranionasal region are widely exposed from a transcranial perspective (Fig. 19).
The transbasal approach provides anterior transcranial access to the extradural midline skull base. By including various osteotomies of the orbital bar and nasal bone, the access to sinonasal contents or the central skull base can be increased. We propose a new classification system, which represents a revision and modification of the broader classification scheme of anterior skull base approaches initially reported by Beals and Joganic in 1992.1 The proposed system is focused solely on the transbasal approach and was validated by its retrospective application to the published world literature. This classification outlines clear steps that characterize transbasal procedures (Fig. 20). The goal is to unify the terminology used to describe all transbasal procedures. We hope that this classification will facilitate interinstitutional communication and understanding of these procedures, clarify basic differences and similarities among these modifications, and clarify why and when to use a particular version of the transbasal approach.