------------------------------------------------------------------------------ TITLE: SKULL BASE SURGERY SOURCE: Dept. of Otolaryngology, UTMB, Grand Rounds DATE: November 3, 1993 RESIDENT PHYSICIAN: Daniel P. Slaughter, M.D. FACULTY: Christopher P. Rassekh, M.D. DATABASE ADMINISTRATOR: Melinda McCracken, M.S. ------------------------------------------------------------------------------ "This material was prepared by resident physicians in partial fulfillment of educational requirements established for the Postgraduate Training Program of the UTMB Department of Otolaryngology/Head and Neck Surgery and was not intended for clinical use in its present form. It was prepared for the purpose of stimulating group discussion in a conference setting. No warranties, either express or implied, are made with respect to its accuracy, completeness, or timeliness. The material does not necessarily reflect the current or past opinions of members of the UTMB faculty and should not be used for purposes of diagnosis or treatment without consulting appropriate literature sources and informed professional opinion." INTRODUCTION: Cranial base surgery is now an area of renewed interest and increasing importance in otolaryngology/ head and neck surgery. New methods of accurate imaging, reliable long term anesthetic techniques, new dynamic studies that evaluate cerebral blood flow and help predict the safety of carotid artery resection, and the development of vascularized flaps for reconstruction have all made this advancement possible. In this discussion I will focus on surgical anatomy and various surgical techniques involved in cranial base surgery of the anterior and middle cranial fossas. Posterior cranial base surgery relies heavily on otologic approaches and will not be discussed. RADIOLOGICAL ASSESSMENT: In patients with cranial base tumors, CT scan and gadolinium enhanced MRI are invaluable in determining the extent, resectability, differential diagnosis, and guiding biopsy. CT scan should include axial and coronal soft tissue and bone window cuts. CT is most useful to detect bone erosion, intracranial extension, and tumor vascularity. MRI is inferior in bone erosion but more useful to detect soft tissue invasion, great vessel distortion, and dural involvement. MRI is particularly useful in its ability to differentiate types of tissue and tumors based on signal response. MRI can also generate sagittal images. MRI angiography can give useful vascular information in a noninvasive fashion. MRI and CT three dimensional reconstructions are very useful to understand the three dimensional relationship of the lesion and the complex anatomy of the skull base. Interventional radiology has also become commonplace in the preoperative workup. Cerebrovascular evaluation in cases were the carotid artery may have to be resected can be performed. Temporary occlusion of the carotid artery with a balloon catheter during angiography can be performed. Neurologic signs precludes carotid resection. Because 10-15% of those patients who pass the balloon test still go on to develop cerebral ischemia after carotid resection, the xenon enhanced CT scan is gaining popularity. The patient breathes xenon gas during occlusion of the carotid which serves as a marker for cerebral perfusion. The amount of blood flow before and after occlusion is then compared. This evaluation can accurately predict ( 2% failure ) the relative risk for ischemia. SURGICAL ANATOMY: The cranial base can be divided in to three distinct regions, the anterior, middle, and posterior skull base. These three regions are distinct in their anatomy, pathological lesions, and surgical approaches. ANTERIOR CRANIAL BASE: The intracranial surface of the anterior cranial base extends from the frontal bone, over the orbital roofs, to the anterior edge of the greater wing of the sphenoid. The frontal sinus is located in the frontal bone and is extremely variable in size and must be dealt with in most anterior cranial base surgery. The supraorbital foramina, which may be incomplete, transmit the supraorbital nerves and vessels. These vessels contribute a major portion of the blood supply to the galea and the pericranium. A pericranium or combination galeal and pericranial flap is often used in the reconstruction after anterior cranial base surgery. Superiorly, the anterior cranial base is formed by the frontal, ethmoid, and sphenoid bones. That portion of the ethmoid sinus that contributes to the cranial base floor is called the fovea ethmoidalis. The fovea ethmoidalis is the most common location for an iatrogenic CSF leak. From an intracranial standpoint several landmarks are important. The most anterior of these is the foramen cecum, the site of communication of the veins of the nasal cavity and the origin of the superior sagittal sinus. The next anterior landmark is the crista galli which protrudes upward from the midline to provide attachment for the falx cerebri. On either side of the crista are the many openings of the cribiform plate that transmit olfactory nerves to the nasal cavity. Just posterior to the olfactory foramina is a smooth surfaced area known as the planum sphenoidale. It is the roof of the sphenoid sinus. The anterior clinoid process and lesser wing of sphenoids delineate the most posterior limit of the anterior cranial base. Between and below the clinoids lies the optic canals. Extracranially, the anterior cranial base is topographically related to the nasal cavity, ethmoid sinuses, sphenoid sinuses and the orbits. The floor of the anterior cranial fossa is quite uneven with downward slopping of the orbits, ethmoid roof, and cribiform. The anterior and posterior ethmoid foramina mark the level of the frontoethmoid suture line and serve as a landmark to the level of the ethmoid roof and anterior fossa floor. The superior orbital fissure transmits CN III,IV, V-1, VI, and the opthalmic vein and directly communicates to the middle cranial fossa. The inferior orbital fissure contains the maxillary nerve(V-2) and communicates to the pterygopalatine fossa. The optic canal transmits the optic nerve and opthalmic artery. In most cases the optic canal is 4-7 mm posterior to the posterior ethmoidal foramina. MIDDLE CRANIAL BASE: Forms the floor of the middle cranial fossa. From an intracranial perspective the middle cranial base begins anteriorly at the posterior edge of the lesser wing of sphenoid and posteriorly it ends at the posterior superior edge of the petrous part of the temporal bone. The intracranial surface is formed by the greater wing and body of sphenoid bone as well as the petrous and squamous portions of the temporal bone. As such, it forms the roof of the infratemporal fossa, middle ear, mastoid, and the condylar fossa. The floor of the middle cranial fossa has several important foramina. Anteriorly the superior orbital fissure, optic canal, and foramen rotundum permit communication with the orbit and the pterygopalatine fossa. Next, the foramen ovale delivers the mandibular nerve to the infratemporal fossa below, and the foramen spinosum transmits the middle meningeal artery. Along the superior medial surface of the petrous part of the temporal bone, the roof of the carotid canal is often dehiscent revealing the horizontal petrous ICA and the foramen lacerum. This foramina receives the greater superficial petrosal nerve after it exits from the facial hiatus a few millimeters posteriorlaterally, and conducts it to the pterygopalatine fossa inferiorly. These relationships allow the GSPN to serve as a landmark to the distal petrous carotid. Along the intracranial surface from lateral to medial the temporal lobe, trigeminal ganglion, and optic chiasm are seen. The pituitary in the sella turcica is below the optic chiasm behind the posterior wall of the sphenoid sinus. This area is flanked on both sides by the cavernous sinus. The extra cranial surface of the middle cranial base is quite complex and comes into contact with several distinct regions. The temporal fossa is filled with the temporalis muscle. This muscle based on the deep temporal arteries is commonly used in reconstruction. The temporalis muscle reaches its insertion on the coronoid process by passing underneath the zygomatic arch. It is below this arch that the temporal fossa becomes the infratemporal fossa. The infratemporal fossa is bound anteriorly by the posterior buttress of the malar eminence and maxillary sinus, posteriorly by the glenoid fossa, mandibular neck and condyle and medially by a plane extending from the lateral pterygoid plate to the spine of the sphenoid bone. The muscles of mastication , foramen ovale, and foramen spinosum are all in the infratemporal fossa. The lateral pterygoid plate within the infratemporal fossa is a very useful landmark. It is a palpable guide to orientation, it can easily be exposed by dissecting along the greater wing of the sphenoid, and can be seen easily on CT scan. The root of the lateral pterygoid plate is immediately posterior to foramen rotundum and anterior to foramen ovale. It can therefore be used as a guide to the location of V2 and V3. Once ovale has been identified the foramen spinosum is found just posterior followed by the spine of the sphenoid bone. The pterygopalatine fossa is the space between the posterior wall of the maxillary sinus and the pterygoid plates. This fossa contains the foramen rotundum (V-2) and the vidian nerve supplying parasympathetic and sympathetic to the nasal cavity and lacrimal glands. The post styloid space , located behind a plane connecting the medial pterygoid plate to the styloid process, contains the entrance to the carotid canal and jugular foramen. An important group of structures is centered around the spine of the sphenoid. The spine is positioned medial to the glenoid fossa, just anterior to the carotid canal, immediately behind and slightly lateral to the foramen spinosum and ovale, and lateral to the cartilaginous insertion of the eustachian tube in to the skull base. The spine of the sphenoid is the most consistent and reliable extracranial landmark for locating the the ICA as it enters the petrous portion of the temporal bone. ANTERIOR CRANIAL BASE APPROACHES: Surgical approaches to the anterior cranial base include methods that are purely extracranial and those that are combined extracranial and intracranial. The extracranial techniques - external ethmoidectomy, frontal sinusotomy, and intranasal ethmoidectomy - are suitable for discrete limited lesions such as CSF leaks or small benign tumors The remaining majority of anterior cranial base lesions are best managed by a combined approach of which there are two basic techniques: the anterior craniofacial resection and the basal subfrontal approach. The ANTERIOR CRANIOFACIAL APPROACH combines a bifrontal craniotomy through a coronal approach with transfacial exposure of the nasal cavity, ethmoid, maxillary and orbital areas usually by modifications of lateral rhinotomy, midfacial degloving, or other transfacial approaches. The facial incisions can be modified to allow for orbital exenteration. This approach is most often used for neoplasms that originate in the sinonasal tract and invade the anterior cranial fossa floor, such as SCCA, esthesioneuroblastoma, and adenocarcinoma. The operation begins with the transfacial exposure followed by the bifrontal craniotomy. If the patient has a large frontal sinus an osteoplastic flap followed by removal of the posterior table can be used. If the sinus is small a frontal bone flap is performed with a guarded osteotome via burr holes created above the hairline. The lower frontal bone cut should be kept low to avoid brain retraction. The frontal lobes are elevated from the anterior cranial fossa by incising the dura and severing the olfactory nerves. This will result in a CSF leak that will require dural patch as well as permanent anosmia. The extent of resection is dictated by tumor involvement. If the tumor involves the planum sphenoidale, the bony optic canal will need to be unroofed to protect the optic nerves. The transfacial exposure usually uses modifications of the lateral rhinotomy which may or may not transect the lip depending upon whether total maxillectomy will be performed. In most cases a complete en bloc ethmoidectomy will need to be performed necessitating a contralateral Lynch incision. The frontal lobes are retracted and a reciprocating saw is placed through the nasal and ethmoid exposures. Under direct vision osteotomies are created from the planum sphenoidale, along the ethmoid roof, forward to the front of the cribiform plate. If the specimen extends anteriorly the supraorbital bar can be removed with these osteotomies and replaced as a free graft if not involved. Reconstruction involves a water tight dural closure with a patch of pericranium, temporalis fascia or fascia lata. The vascularized pericranial flap will fill the defect in the anterior cranial floor. The nasofrontal ducts are obliterated and the sinus obliterated or cranialized. The facial incisions are closed with attention to the medial canthal tendon an lacrimal apparatus. Approaches that avoid facial incisions such as the midfacial degloving and transpalatal give more limited exposure to the upper nasal vault and ethmoid sinuses. BASAL SUBFRONTAL APPROACH- The basal subfrontal approach or transbasal approach is similar to the anterior cranial facial resection except that the transfacial exposure is much more limited. The target area for this approach is more posterior at the sphenoid and clivus rather than cribiform and ethmoids as in the anterior craniofacial. The craniotomy is larger and the orbital bone cuts broader in this approach. It is used for lesions that primarily or secondarily involve the posterior aspect of the anterior cranial base such as meningiomas, chordomas, chondrosarcomas, and ossifying fibromas. The entire procedure can be performed through a coronal incision with dissection down and over the orbital roofs and zygoma. A bifrontal craniotomy is performed as described above. The frontal lobes are retracted and under direct vision the supraorbital bar and orbital roofs are removed as a unit as far back as the posterior ethmoidal foramina. The neurosurgeon will the use the operating microscope to unroof the optic nerves, sphenoid sinus, and superior orbital fissure as required for tumor removal. Reconstruction follows as in anterior craniofacial approach. MIDDLE CRANIAL BASE APPROACHES: In contrast to anterior cranial base, middle cranial base approaches are numerous and varied. A multitude of transoral, transfacial, and transtemporal approaches have been described. The selection of a particular approach depends upon the exact location and histological type of the tumor. When considering surgical approaches, it is useful to divide the middle cranial base into one central compartment and two lateral compartments. The central compartment is defined by that area between two parasagittal lines drawn from the medial pterygoid plate and the occipital condyle. This roughly corresponds to the pathway of the horizontal carotid artery. The central compartment contains the sella turcica, sphenoid rostrum and lower sphenoid sinus, the nasopharynx, the pterygopalatine fossa and lower portion of the clivus. The lateral compartment encompasses the infratemporal fossa, the parapharyngeal space, and the petrous portion of the temporal bone. No other area has such a high density of neural and vascular structures. APPROACHES TO CENTRAL COMPARTMENT- A multitude of approaches to the central skull base with varying degrees of exposure to key anatomic areas have been described. These are used primarily for management of extradural lesions along the skull base. When lesions are limited to the sphenoid sinus or sella turcica the transeptal or transethmoid sphenoidotomy are safe and effective. The transseptal is performed through a sublabial incision with subperichondrial and periostial elevation combined with a swinging quadralateral cartilage flap held in place with a self retaining retractor. The perpendicular plate is taken back to the sphenoid rostrum where the sphenoid is entered centrally. The transethmoidal sphenoidotomy is made through and external ethmoidectomy approach working postereromedially until the sphenoid is entered. The frontoethmoid suture line serves as the marker to prevent intracranial penetration. The lateral rhinotomy and transantral approaches afford a wider exposure to the anterior sphenoid and adjacent nasopharynx, ptergopalatine fossa, maxilla and ethmoid regions. However, they are usually not satisfactory for any but the smallest lesion in the sphenoidal-clival area. The midfacial degloving approach is more suitable for dealing with larger central compartment lesions because it allows improved midline access through the nose and both maxillary sinuses. Medial maxillectomy and resection of ascending process of the palatine bone are included if necessary for exposure. The Lefort I osteotomie approach can be used alone or in combination with midfacial degloving to add more exposure to the oronasal pharynx and clivus by displacing the hard and soft palates inferiorly. For lesions of the clivus that also involve the upper cervical vertebrae the mandibulotomy approach will afford a paramedian route to reach the lower areas of extension. Transoral and transpalatal approaches are also useful for craniovertebral junction lesions. The extended maxillotomy and subtotal maxillectomy approaches were also designed for wide local exposure to the clivas and cranioveterbral junction by unilaterally displacing or resecting the hemimaxilla. For very extensive clival lesions, the midfacial spliting approach can be employed. This approach uses craniofacial disassembly to completely displace the midfacial skeleton. APPROACHES TO THE LATERAL COMPARTMENT- four basic routes are used either alone or in combination. The transtemporal routes include the transcochlear and translabrynthine. These are extradural techniques that traverse the temporal bone to provide access to the petrous apex, clivus, and CPA. These are not primary routes to treat middle cranial fossa lesions but can be used as an adjunct. Infratemporal approaches to the lateral compartment have three distinct variations described by Fisch and all involve mastoidectomy, facial nerve transposition and obliteration of the middle ear. These approaches are better discussed in the context of posterior cranial base surgery. The transparotid approach is the most useful for dealing with tumors of the parapharyngeal space and infratemporal fossa that originate in or secondarily involve the parotid gland. This procedure is begun with a superficial parotidectomy followed by dividing the digastric muscle and styloid complex and retracting the mandible forward to access the infratemporal space. For larger tumors a mandibulotomy can be performed along with a vessel finding neck to increase safety and exposure. The extended rhytidectomy approach is an alternative method that avoids direct facial nerve dissection and instead exposes the infratemporal fossa by first elevating the skin flap based anteriorly followed by a posteriorly based flap of mimetic musculature, facial nerve and parotid gland to expose the facial skeleton. The infratemporal approach can the be accessed by way of mandibulotomy and retromaxillary dissection. The transparotid and extended rhytidectomy approaches are better for lesions in the more inferior portions of the infratemporal fossa. For lesions that do not involve the lower portions of the middle cranial base, the lateral facial approach and the lateral transtemporal sphenoid approach have been described. The lateral facial approach reaches the the infratemporal fossa by displacing the zygoma and inferiorly reflecting the temporalis muscle. Dissection along the greater wing of sphenoid is performed to the lateral pterygoid plate. This can be combined with a craniectomy by partial removal of the greater wing of sphenoid to further outline the neurovascular structures as they exit the skull base. . The lateral transtemporal sphenoid approach combines techniques of transtemporal approaches with a transfacial approach to afford greater exposure. This does not require mastoidectomy or facial nerve rerouting but instead involves drilling of the petrous ICA combined with craniotomy to further evaluate the foramina. The subtemporal- preauricular infratemporal approach combines techniques of the transparotid with the lateral transtemporal sphenoid approach with temporal craniotomy to allow excellent exposure to much of the middle cranial base. Transfacial approaches allow excellent exposure to the middle cranial base through craniofacial disassembly techniques and vascularized reconstructive flaps. The facial translocation approach is performed by incising the face with a horizontal incision across the lower conjunctival sulcus to connect a lateral rhinotomy to a hemicoronal incision to create superior and inferior soft tissue flaps.(frontal branches are divided and tagged). After extensive subperiostial elevation a orbitozygomaticomaxillary skeletal segment is temporarily removed. The temporalis muscle is then reflected inferiorly to view the infratemporal fossa. Intracranial exposure via craniotomy may be added. The pterygoid plates may be removed along with the pterygoid muscle to reveals the nasopharynx sphenoid sinus and clivus. This approach provides access to both the central and lateral compartments of the middle cranial base. Closure involves use of the temporalis muscle for reconstruction, replacement of the remove facial skeleton, and stinting the nasolacrimal system. The use of a maxillary osteoplastic flap instead of soft tissue flaps has also been described. Intracranial approaches to the middle cranial base are used primarily by neurosurgeons and neurootolgists and will not be discussed. RESULTS AND PROGNOSIS: A multi-institutional retrospective study evaluated 155 malignant craniofacial tumors. One quarter were primary and three quarters were recurrent. Of 68 tumors operated on within the anterior cranial fossa with an average 6.5 year follow up 59% were alive without disease, 10% alive with disease, and 31% dead. Of 43 tumors within the middle cranial fossa with an average of 6 year followup 42% were alive without disease, 28% alive with disease and 30% were dead. ----------------------------------------------------------------------------- BIBLIOGRAPHY 1. Cummings, Charles W. Otolaryngology- Head and Neck Surgery 2nd edition 1993 Mosby 2. Bailey, Byron J. Head and Neck Surgery- Otolaryngology 1993 J.B. Lippincott 3. Janeka, Ivo P. Surgery of Cranial Base Tumors 1993 Raven Press 4. Ivo P Janeka. Facial translocation: a new approach to the cranial base. Otolaryngology- Head and Neck Surgery Vol. 103 Sept. 1990 5. Biller, Hugh F. Extended Osteoplastic Maxillotomy, A versatile new procedure for wide access to the central skull base and infratemporal fossa. Arch Otolaryngol Head and Neck Surg- Vol 119, April 1993 6. Jackson, Ian T. Results and prognosis following surgery for malignant tumors of the skull base. Head and Neck Mar/Apr 1991 7. Frodel, J. L. The coronal approach, anatomic and technical considerations and morbidity. Arch Otolaryngol Head an Neck Surg Vol 119, Feb 1993 ----------------------------------------------------------------------------