The infratemporal fossa—middle cranial fossa approach gives wide access to all of the subcranial tissues that underlie the middle fossa floor. Ugo Fisch of Zurich, designed the infratemporal fossa approach and first presented it in 1977. It forms the basis for all the accesses to the lateral and inferolateral skull base that have been developed since. The procedure described in this chapter is closely similar to Schramm and Sekhar’s operation, with some modifications. The primary consideration in this chapter is the presence of a tumor, but patients who require lateral skull base surgery present with a wide variety of disease processes. A thorough preoperative evaluation is crucial for a successful operation. The surgical procedure includes the following steps: extensive preparation; incision (anterior or posterior); external auditory canal obliteration; internal carotid artery and internal jugular vein exposure; temporalis muscle elevation; zygomatic ostectomy; mandibular condylectomy; exposure of the subtemporal trapezoid muscle; otologic exposure; craniotomy; carotid dissection and eustachian tube removal; cavernous sinus resection; dura and brain resection; and reconstruction.
31 Surgical Approach to the Lateral Skull Base
Probably the most difficult and complex of all the pathways in skull base surgery is the infratemporal fossa–middle cranial fossa approach. It is also the most versatile, giving access not only to all of the subcranial tissues that underlie the middle fossa floor from the zygoma to the nasopharynx, but also to the contents of the middle cranial fossa from the lesser sphenoid wing to the tentorium, including the petrous ridge and cavernous sinus (▶ Fig. 31.1). The clivus will be widely exposed and cutting the tentorium can expose the upper brain stem.
Considerable credit must be given to the originators of this operation. Ugo Fisch of Zurich designed the infratemporal fossa approach and first presented it in 1977. 1 It forms the basis for all the accesses to the lateral and inferolateral skull base that have been developed since. 2 The first well-coordinated intracranial and extracranial approach to this region was designed by Victor Schramm 3 and Laligam Sekhar. They modified Fisch’s design and created an excellent simultaneous exposure of the intracranial and extracranial aspects of the middle fossa through a small but strategically placed craniotomy. The removal of this bony barrier improved maneuverability in both the intracranial and extracranial compartments. The procedure described in this chapter is closely similar to Schramm and Sekhar’s operation, with some modifications.
Patients who require resection of malignant disease using the middle fossa skull base approach usually have a more extensive lesion than those who need anterior fossa skull base surgery. They often require longer periods of time in the intensive care unit and a more prolonged hospital stay. The keystone structures that are responsible for many of the problems arising from this surgery are the cavernous sinus and the internal carotid artery (ICA). Resection of either or both structures may leave the patient with substantial deficits. In cases of malignancy, the entire sinus often needs to be removed, creating an immobile and sometimes insensate eye with ptosis of the upper lid. The cornea, therefore, is extremely prone to injury, not only from trauma but also from sympathetic dystrophy secondary to autonomic denervation. If the original tumor is an extension from a lesion in the paranasal sinuses, the orbit will usually be exenterated and those sequelae would not be issue. Management of the carotid is still inconclusive, but arterial invasion usually mandates resection in cases of malignancy and grafting rather than sacrifice, even in the presence of a negative balloon test occlusion (BTO) and a favorable single-photon emission computed tomography (SPECT) scan. It should be noted that resection of the cavernous ICA and the cavernous sinus in the setting of malignancy, especially squamous cell carcinoma, remains highly controversial.
31.2 Preoperative Evaluation and Anesthesia
Patients who require lateral skull base surgery present with a wide variety of disease processes. The primary consideration in this chapter is the presence of a tumor. A tumor may be benign or malignant and may originate either intracranially from dura or calvarial bone or extracranially from the numerous soft tissues that occupy the subcranial area. Most common among the intracranial neoplasms that extend extracranially are meningioma, chordoma, chondroma, and chondrosarcoma. Among the extracranial tumors that extend intracranially are schwannoma (often of the trigeminal nerve), parotid tumors (both benign and malignant, especially from the deep lobe), and squamous cell carcinomas from the paranasal sinuses, nasopharynx, and temporal bone. Metastatic deposits in lymph nodes from any head and neck site, but more often from the nasopharynx or paranasal sinuses, may erode through the middle fossa skull base. Rarely, metastases from distant sites may present as a pathologic node at the skull base. A rather curious inflammatory lesion that may present as a skull base neoplasm both in symptomatology and in radiographic appearance is a variant of Tolosa–Hunt syndrome.
Unfortunately, the presenting symptoms of lesions in this area are often subtle. Many patients complain of pain only in the region of the infratemporal fossa, ear, or behind the eye. This symptom is often passed off as myofascial dysfunction or “temporomandibular joint” (TMJ) syndrome. Trismus secondary to pterygoid muscle invasion or direct involvement of the mandibular branch of the trigeminal nerve may likewise be misdiagnosed. Any patient with a history of carcinoma of the nasopharynx or oropharynx must be considered with a high index of suspicion when presenting with pain in the infratemporal fossa.
Special attention should be focused on the patient with oropharyngeal carcinoma who, after the standard therapy of composite resection and postoperative radiation therapy, presents with pain deep in the infratemporal fossa, especially if the pain is new. There is often little to find on physical examination other than an occasional increase in trismus. Because the inferior alveolar nerve had been sacrificed in the initial resection, the only remaining sensory nerve from V3 is the auriculotemporal, which may also have been injured at surgery. Such patients should undergo magnetic resonance imaging (MRI). Because of the anatomic distortions created by past surgery and radiation, it is often difficult to distinguish fibrosis and edema from tumor, even with gadolinium contrast. Serial MRIs at 6-week to 3-month intervals are often the only means by which a diagnosis can be made. Unfortunately, valuable time may be lost as the tumor continues to grow and is constantly at risk of metastasizing. Analysis by positron emission tomography (PET) scanning may be of considerable assistance. Symptoms of carcinoma of the temporal bone may range from subtle to obvious. Painful otorrhea unresponsive to vigorous local therapy is highly suspicious of carcinoma.
As in temporal bone carcinoma, malignancies in the infratemporal fossa, especially parotid gland carcinomas, may present with a facial nerve paralysis. Recurrent carcinoma or tumors of the deep lobe often involve the facial nerve. Paresis or complete paralysis of the entire nerve, indicating main trunk invasion, is usually the rule.
Extension of tumor through the foramen ovale along the third division of the trigeminal nerve can produce numbness over its sensory distribution and, by spreading through the gasserian ganglion, hypesthesia of both the first and second divisions of the trigeminal nerve. This manifests itself initially as numbness over the chin and lower face and then advances to numbness over the midface, especially the area innervated by the infraorbital nerve with V2 involvement and the forehead and cornea with V1 involvement. Extension into the cavernous sinus is imminent once the tumor has reached the gasserian ganglion in Meckel’s cave, and paralysis of the oculomotor, trochlear, and abducens nerves often follows, resulting in ptosis and ophthalmoplegia.
Tumor extension directly from the nasopharynx usually follows the foramen lacerum. The subsequent invasion of the cavernous sinus commonly produces abducens nerve paralysis with a lateral rectus palsy. Invasion of the lateral wall of the sinus results in affliction of cranial nerves III and IV, and eventually involvement of V2, V3, and the optic nerve with extension superiorly.
Invasion of the carotid artery is usually asymptomatic. The tumor is rarely so advanced as to completely occlude the vessel but, were that to occur, a stroke may ensue. This is possible even with reduced flow in a patient with already compromised cerebral circulation.
Involvement of temporal dura commonly produces unremitting, intense headache. Because much of the temporal lobe is “silent,” brain invasion provides little in the way of differentiating signs and symptoms.
Lower cranial nerves may be affected when there is metastatic spread to high internal jugular, parapharyngeal, or retropharyngeal lymph nodes. Extracapsular tumor spread from those nodes may erode skull base bone and invade the jugular foramen, producing symptoms secondary to affliction of cranial nerves IX, X, XI, and XII (the latter with spread to the adjacent hypoglossal canal). The cervical sympathetic may be involved, producing Horner’s syndrome. Clinical investigation of these patients, after a thorough head and neck and neurologic examination, includes a thorough general medical assessment. If a tumor is deemed operable, two other medical conditions preclude an attempt at infratemporal fossa–middle cranial fossa excision. The first is the presence of distant metastasis and the second is general lack of medical fitness, such as that from extensive physiologic aging, severe cardiovascular disease, poorly controlled diabetes, or incipient renal or hepatic failure, all of which are contraindications to that surgery. A final contraindication to surgery is lack of patient commitment. Some natural reluctance to undergo such an extensive procedure is anticipated, but a tendency toward resistance on the part of the patient, coupled with overzealous enthusiasm on the part of family or friends, should be carefully assessed as well.
MRI with gadolinium contrast and, if indicated, fat suppression should be complemented by fine-cut computed tomography scanning through the skull base. The coronal plane is most helpful in clearly outlining carotid involvement and cavernous sinus invasion. Bony detail is best delineated with computed tomography, and soft tissue invasion by MRI. Gadolinium contrast is extremely helpful in differentiating tumor from adjacent soft tissue, especially in recurrent or persistent disease. A word of warning regarding these studies: there are false-negative and false-positive scans, and those studies do not always accurately delineate the extent of tumor, tending to minimize or exaggerate the true dimensions.
There are many methods of evaluating the patency of the circle of Willis but, more importantly, the surgeon must assess the amount of blood flow from the collateral circulation if the ICA on the tumor side is removed. The gold standard has been BTO of the ICA using radioactive xenon to provide a calculation of cerebral blood flow before and after temporary occlusion. Xenon studies are notoriously difficult to perform because of frequent fluctuations in gas concentrations and problems with the measuring equipment. The determination of cerebral blood flow SPECT with technetium-99m hexamethylpropyleneamine oxime contrast has been proven to be much more practical.
The PET scan is the investigative tool most recently incorporated into the armamentarium. Because of the avidity of tumor cells for glucose, the patient is given a non-metabolizing, radioactive, fluoridated analogue after which the PET scans are carried out. As with all skull base tumor patients, there must be extensive discussion between the patient and the head and neck surgeon, the neurosurgeon, and the plastic surgeon. It is important for the patient to understand the arduousness of the procedure, what functions will be lost, what cosmetic deformities will result, and what other treatment options exist. Skull base surgery often remains the only chance for cure for malignant tumors. The surgeon must be realistic—not overly enthusiastic, but encouraging—when discussing options with these often desperate patients.
The usage of paralytic agents during anesthesia should enable proper monitoring of the cranial nerves. Perioperative antibiotics should be administered.
31.3 Surgical Technique
A Mayfield horseshoe head rest is used for ease of surgical access unless the neurosurgeon insists upon fixation pins. Central venous pressure and arterial lines are usually placed, and cranial nerve monitoring devices are inserted. Electroencephalography electrodes are sutured or stapled to the scalp to monitor cerebral electrical activity in cases of possible ICA sacrifice.
An extensive surgical preparation is usually necessary, including a scrub of the entire face, head, neck, and anterior chest. 3 , 4 The abdomen is also prepared if a rectus abdominis flap is to be used as a vascularized free tissue transfer for reconstruction. Both legs are often scrubbed in anticipation of the possible harvest of a lateral thigh-free flap, split-thickness skin, fascia lata, or saphenous vein grafts.
There are two standard types of skin incision, 5 –, 8 depending upon the location of the lesion (▶ Fig. 31.2). The incision for parotidectomy extends in a curvilinear fashion from the vertex of the skull, in front of the ear in a preauricular crease, and under the lobule of the ear, then curves forward in the upper neck, resembling a modified Blair incision (▶ Fig. 31.2a). The posterior incision for more anteriorly located lesions, such as deep-lobe parotid tumors, lesions invading the foramen ovale, or nasopharyngeal carcinoma, starts 2 to 4 cm behind the ear, similar to the incision described by Fisch, and extends into the neck toward the hyoid (▶ Fig. 31.2b). Posterior incisions are used for lesions such as temporal bone carcinomas, extensive glomus jugulare tumors, and clival chordomas A lazy-S incision is subtended into the neck if a neck dissection is planned to accompany the procedure.
The scalp incision extends to the level of the pericranium 5 (▶ Fig. 31.3 and ▶ Fig. 31.4). The temporalis muscle is identified as the flap is formed. The superficial and deep temporalis fascia are incised in a semilunar manner to include the frontal branch of the facial nerve, which emerges approximately 1.5 cm in front of the auricle and arches into the frontalis muscle approximately 2 to 2.5 cm above the brow. 6 This maneuver protects the nerve from injury.
The cervicofacial portion of the incision proceeds superficially to the platysma and superficial muscular aponeurotic system fascia in a plane similar to that in a superficial plane face lift. It continues forward anteriorly from the angle of the mandible to the lateral orbital rim. When using this anterior incision, a decision whether to cut across the external auditory canal (EAC) is made, usually predicated on the necessity of exposing the ICA. In contrast, the posterior incision always cuts across the canal because carotid exposure is usually necessary.
31.3.3 External Auditory Canal Obliteration
Exposure of the petrous portion of the ICA will be required in most instances of both anterior and posterior incisions. This is best facilitated by exposing and later obliterating the eustachian tube. A decision regarding the elimination of the middle ear cleft and the EAC or the preservation of a non-ventilated middle ear cleft and the permanent instillation of a ventilating tube is often difficult. For ease of management and fewer complications, there is little question that obliteration is the treatment of choice. However, maintenance of the integrity of the middle ear and EAC and the use of a ventilating tube is preferable in patients for whom the preservation of hearing is a consideration as well as in young patients.
If the EAC is to be obliterated, it is best done at this juncture. The obliteration technique described by Fisch is easy to perform and has predictable results. This is certainly the technique of choice in patients with nasopharyngeal carcinoma or in those with failed radiation therapy.