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The skull base is an extremely complex anatomical region in which an enormous variety of pathological entities may be encountered. These include congenital, inflammatory, and vascular lesions, as well as benign and malignant neoplasms. Surgery of the skull base is technically challenging, and many lesions were previously deemed inoperable due to the close proximity of many vital neural and vascular structures.
The multitude of surgical approaches to the cranial base continues to evolve. Advancements in the techniques of surgical resection, as well as the fields of reconstructive surgery, anesthesiology, diagnostic imaging, and interventional radiology have all combined to improve the feasibility and effectiveness of skull base surgery as a treatment modality. A concomitant improvement in the morbidity and mortality associated with these procedures has also become evident. A multidisciplinary team is required to provide optimum diagnosis and therapy for the patient with a lesion of the skull base. Resection often requires the close cooperation of an otolaryngologist-head and neck surgeon and neurosurgeon.
The surgical approaches employed to obtain adequate exposure of the skull base may be extracranial, intracranial, or, in many instances, a combination of these. The numerous approaches which have been developed have arisen as a function of the previously mentioned complex regional anatomy and diverse pathology of the skull base. However, all of the approaches have in common the same four primary goals as follows:
The cranial base is classically divided into anterior, middle, and posterior segments. The middle cranial base may be further subdivided into a single central and two lateral compartments. These compartments may be distinguished when viewing the skull base from extracranially if two imaginary parasagittal straight lines are drawn, one on each side, through the medial pterygoid plate anteriorly and the occipital condyle posteriorly. The central compartment lies medial to the two lines and the lateral compartments lateral to them. Structures contained within the central compartment include the sella turcica, lower sphenoid sinus and rostrum, nasopharynx, pterygopalatine fossa, and lower clivus. Structures within the lateral compartments include the infratemporal fossa, parapharyngeal space, and petrous portion of the temporal bone. For the purposes of this discussion, the surgical approaches mentioned will be limited to those most appropriate for lesions contained within the lateral compartment of the middle cranial base.
Lateral skull base approaches are a diverse group of surgical techniques, and the terminology used to describe them is often confusing. Despite this complexity, there are four main routes by which lateral skull base lesions are typically resected, and it will be noted that there is considerable overlap among them. These include the following approaches:
Extensive lesions may require a combination of these approaches to gain adequate exposure, and each approach has it own advantages and limitations. This discussion will include the anatomy of the middle cranial base followed by an overview of the four groups of approaches listed above. A sound conceptual framework of the available lateral skull base approaches is requisite for optimal surgical management of lesions in this complex area, and it is hoped that this review will provide a general background upon which such a framework may be built.
Cranial base pathology, diagnostic and preoperative evaluation, perioperative management, and complications all constitute complete discussions unto themselves and will not be mentioned further here.
Many bony foramina are located within the middle cranial base which transmit important neural and vascular structures. These include the superior orbital fissure (CNs III, IV, V1), optic canal (CN II), foramen rotundum (CN V2), foramen ovale (CN V3), and foramen spinosum (middle meningeal artery).
The greater superficial petrosal nerve (GSPN) leaves the facial nerve at the geniculate ganglion to exit its hiatus in the middle cranial fossa. The GSPN then courses along the superomedial aspect of the petrous portion of the temporal bone to enter the foramen lacerum. In this manner, the GSPN serves as an important landmark in the middle cranial base and may guide the surgeon to the distal petrous internal carotid artery (ICA). The GSPN carries its parasympathetic nerve fibers to the lacrimal gland via the pterygoid canal and pterygopalatine ganglion. The contents of the middle cranial fossa include the temporal lobe, trigeminal (gasserian ganglion), pituitary gland, cavernous carotid, and other contents of the cavernous sinus (CNs III, IV, VI, V1, and V2).
As many lateral skull base approaches involve the infratemporal fossa, this anatomical region deserves special mention. The boundaries of the infratemporal fossa have been described as follows: 1) anterior-posterior wall of maxillary sinus 2) posteroinferior-parapharyngeal space 3) medial-lateral pterygoid plate 4) lateral-ramus and condyle of mandible 5) superior-greater wing of sphenoid
The infratemporal fossa contains the muscles of mastication and the internal maxillary artery and many of its branches. Two important bony anatomical landmarks may be identified within this area. The first is the lateral pterygoid plate. The foramen rotundum lies anterior to the root of this structure and the foramen ovale posterior to it. Posterior to the foramen ovale lies the foramen spinosum. From there, the second important bony landmark, the spine of the sphenoid, may be identified. This structure lies medial to the condylar fossa and lateral to the carotid canal. The spine of the sphenoid thus serves to guide the surgeon to the most important arterial structure of the cranial base, the ICA.
The ICA enters the petrous portion of the temporal bone at the carotid canal and continues its vertical course prior to turning anteromedially to form the horizontal segment. The artery then turns vertically once again to run along the lateral wall of the sphenoid sinus and enter the cavernous sinus.
The transparotid approach allows access to the parapharyngeal space and begins with mobilization of the posterior aspect of the parotid gland followed by identification and dissection of the facial nerve. Parotidectomy with facial nerve preservation is then performed. The posterior belly of the digastric muscle and styloid complex may be divided along with anterior displacement of the mandible to enhance exposure. Larger lesions may require mandibulotomy, and neck dissection may be necessary to allow proximal control and preservation of major neurovascular structures. This approach may be combined with a transtemporal approach to improve exposure to lesions which extend posteriorly to involve the temporal bone.
Extended Rhytidectomy (Facial Biflap Procedure) involves elevation of an anteriorly based skin flap followed by elevation of the mimetic muscles, parotid gland, and facial nerve as a unit in a posteriorly based flap. This exposes the facial skeleton and provides access to the infratemporal fossa while avoiding actual dissection of the facial nerve. The transparotid and extended rhytidectomy approaches are most appropriate for lesions of the lower infratemporal fossa and parapharyngeal space.
The lateral facial and lateral temporal sphenoid approaches allow for a more superior orientation owing to inferior displacement of the zygomatic arch and temporalis muscle. This is followed by dissection along the greater wing of the sphenoid bone with identification of the lateral pterygoid plate. Subtemporal craniectomy with extradural dissection is performed to provide both intra- and extracranial access. No facial nerve dissection is necessary. The lateral facial approach provides limited access for smaller lesions, while the lateral temporal sphenoid approach allows wider exposure of the parasellar region, clivus, infratemporal fossa, and nasopharynx.
The subtemporal-preauricular infratemporal fossa approach affords wide exposure to the structures of the middle cranial base via a combination of transparotid, lateral transtemporal sphenoid, and temporal craniotomy approaches. Access to the greater wing of the sphenoid, petrous apex, and middle and lower clivus is provided with improved intracranial access. There is considerable latitude with this technique for modification to suit the specific lesion. The same landmarks, including the spine of the sphenoid and lateral pterygoid plate, are used for identification of other important middle cranial base structures. The intrapetrous ICA may be mobilized and controlled, and neck dissection may be added if proximal control of vital neurovascular structures is warranted. Following tumor removal, meticulous reconstruction and closure is performed with attention toward restoring an adequate barrier between the intracranial space and upper aerodigestive tract, as well as adequate function and cosmesis.
Hemicoronal and lateral rhinotomy skin incisions are connected via a horizontal limb, and the frontal branches of the facial nerve and infraorbital nerve are identified, tagged, and divided. Soft tissue flaps are raised in a subperiosteal plane from the maxilla and zygoma, and osteotomies are performed with removal of the orbitozygomaticomaxillary complex. The temporalis muscle is then displaced inferiorly to permit exposure of the skull base. Subtemporal craniectomy, or frontal or temporal craniotomy may be performed if intracranial access is necessary. Following tumor removal, bone segments are replace and neurorrhaphy is performed along with meticulous reconstruction and closure.
Temporal craniotomy allows direct access to lesions of the lateral compartment of the middle cranial base, especially those which are intracranial in this region. It may sometimes be used alone for limited lesions. Lesions which are located further medially may require a significant amount of temporal lobe retraction, thereby leading to possible adverse neurologic sequelae including cerebral edema, seizure disorder, or cognitive deficits. Optimizing direct access can be achieved by positioning the craniotomy as low as possible. If necessary, approaches which afford wider exposure should be employed if it is anticipated the the need for retraction of the brain will be excessive.
The middle fossa approach employs temporal craniotomy and removal of bone from the superior aspect of the petrous temporal bone to reach lesions of the internal auditory canal. These are often tumors of CNs VII and VII located in the posterior fossa.
Subtemporal craniectomy is valuable for limited lesions of the cranial base which are principally extracranial and may be necessary to obtain adequate margins of resection.
It is evident fRom the preceding discussion that the cranial base surgeon has a wide variety of approaches from which to chose when addressing lesions of the lateral skull base. A considerable degree of overlap exists among the procedures, and the approaches may be individualized to suit a particular lesion. The primary goals of cranial base surgery should remain at the forefront of the clinical decision making process, and it is hoped that this overview has provided a foundation for developing a sound conceptual framework for planning a surgical approach to lesions of the lateral skull base.
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