The middle fossa approach to the skull base is one of the most versatile approaches and follows the paramount principles of skull base surgery: avoiding brain retraction, shortening the distance, and widening the field through increased bone removal. The approach to the internal auditory meatus through a subtemporal craniectomy has been used since the beginning of the 20th century. 1 It was eventually discredited, however, until its reintroduction when House and Cabtree 2,3,4 introduced the middle fossa translabyrinthine approach to reach the petrous pyramid and for exposure of the internal carotid artery (ICA) to treat intracanalicular acoustic neuromas, with the goal of hearing preservation, facial nerve exploration, and vestibular nerve sectioning. The expertise and low mortality and morbidity achieved through an extradural route roused interest in its use for different pathologic processes, such as in the infratemporal, sphenopalatine, and temporal fossae, and in the orbit and cavernous sinus. 5,6,7,8,9,10,11,12,13 Subsequent contributions from Kawase, MacDonald, Liu, Cho, and Al Mefty and their colleagues, especially with techniques such as anterior petrosectomy, zygomatic osteotomy, anterior mobilization of V3, superior petrosal sinus scission, and combined petrosectomy, further expanded its indications, making access to this “no mans land” of the cavernous sinus and clivus not only possible, but also safe. Several variants of the middle fossa approach have been described, each tailoring the degree of bone removal to widen the field.
19.2 Advantages of the Middle Fossa Approach
Versatility in approaching the middle fossa, posterior fossa, infratemporal fossa, pterygopalatine fossa, and pharyngeal region.
An extradural route.
The means to avoid excessive retraction of the temporal lobe.
The means to avoid transposing major arterial structures or cranial nerve.
Increased exposure through further dissection of the temporal bone or the infratemporal fossa.
Reconstruction with vascular tissue.
19.3 Anatomical Background
In order to have a better understanding of the region to be studied, some anatomical landmarks must be observed ( ▶ Fig. 19.1, ▶ Fig. 19.2). The middle meningeal artery is the most constant landmark in the middle fossa. The foramen ovale, through which the mandibular nerve passes, is located anteromedial to the foramen spinosum, and both foramina are anterior to the petrous carotid canal. 14 Paullus et al 15 reported in detail the anatomy of the temporal bone and its contents in the middle fossa. The foramen spinosum is separated from the foramen ovale by an average of 3.2 mm and from the carotid canal by an average of 4.7 mm. The foramen ovale lies an average of 4.4 mm anterolateral to the carotid canal. The greater superficial petrosal nerve (GSPN) provides a reliable landmark for localization of the ICA in the middle fossa. It originates in the geniculate ganglion, exits through the bone at the hiatus fallopian, and runs in the sphenopetrosal groove, to join with the deep petrosal nerve from the sympathetic carotid plexus and form the vidian nerve. It then courses through the pterygoid canal to the pterygopalatine ganglion. It generally courses directly above the anterior margin of the horizontal segment of the ICA. Embedded in the external layer of the dura mater, the lesser superficial petrosal nerve, also a branch originating in the geniculate ganglion, is located lateral to the GSPN and courses to the optic ganglion. The geniculate ganglion is generally separated from the floor of the middle fossa by a layer of bone with an average thickness of 1.2 mm; however, in 16% of the cases, there is no bone over the geniculate ganglion. The geniculate ganglion is located posterolateral to the lateral loop of the ICA in 58% of cases, posterior in 26%, and lateral in 16%. The average distance between the posterior loop of the ICA and the geniculate ganglion is 6.5 mm. The cochlea is a very dense bone structure located below the floor of the middle fossa. It lies an average of 2.1 mm posterosuperior to the lateral loop of the ICA. The eustachian tube and tensor tympani muscle are located anterior and parallel to the petrous segment of the ICA. The muscle lies superior to the eustachian tube in 72%, anterior in 20%, and posterior in 8%. A thin bony septum (average 1.3 mm) separates the tensor tympani muscle from the ICA. The middle ear cavity lays posterolateral to the posterior loop of the ICA in 63% of cases, lateral in 20%, and posterior in 17%. The arcuate eminence is considered to be a landmark of the superior semicircular canal (SSC), although, unfortunately, the precise position of the SSC can be difficult to identify. Lying perpendicular to the petrous bone, the SSC is located about 120 degrees from the course of the greater superficial petrosal nerve. It is located on average 4.6 mm posteromedial to the geniculate ganglion. The petrous segment of the ICA under the lateral border of the trigeminal nerve is not covered by bone in 84% of cases, and in 38% this defect extends laterally. We use this as the most remarkable landmark to identify the ICA. This superior dehiscence of the carotid canal is covered by connective tissue called the petrosphenoid ligament. After its identification, the drilling procedure can be extended posteriorly, exposing the horizontal segment of the ICA for an average length of 10.2 mm.
Fig. 19.1 Anatomical landmarks. (a) V1, first division of trigeminal nerve; V2, second division of trigeminal nerve; V3, third division of trigeminal nerve; GG, gasserian ganglion; III, oculomotor nerve; IV, trochlear nerve; V, trigeminal nerve; ICA, internal carotid artery. (b) MMA, middle meningeal artery; GSPN, great superficial petrosal nerve; PA, petrous apex.
Fig. 19.2 Anatomical landmarks. (a) V2, second division of trigeminal nerve; V3, third division of trigeminal nerve; GG, gasserian ganglion; IV, trochlear nerve; ICA, internal carotid artery; MMA, middle meningeal artery; PA, petrous apex. (b) ET, eustachian tube; GenG, geniculate ganglion; VII, facial nerve; TC, tympanic cavity; TegT, Tegmen tympani. (c) CO, cochlea; SPS, superior petrosal sinus; SSC, superior semicircular canal.
19.4 Surgical Technique
19.4.1 Patient Positioning
To alleviate the effects of gravity when lifting the temporal lobe and to achieve the best angle of view, the floor of the middle fossa should be vertical and the zygoma almost horizontal. Thus, the patient is placed supine and the head is turned to the opposite side. To avoid stretching the neck, the ipsilateral shoulder is elevated. One of the patient’s legs is also prepared in case grafts of fascia lata, subcutaneous fat, saphenous vein, or sural nerve are needed for reconstruction. The abdomen is also prepared for the rare instance in which a free abdominal rectus flap is needed for reconstruction.
Somatosensory evoked potentials, electroencephalography, brainstem auditory evoked potentials, and cranial nerves are all monitored as necessary. The cranial nerves are monitored by inserting needle electrodes into the masseter muscle (V nerve), the orbicularis oris and oculi (VII nerve), and the medial rectus, superior oblique, and lateral rectus muscles (III, IV, and VI nerves), respectively.
19.4.3 Skin Incision and Soft Tissue Dissection
The skin incision can be tailored according to the compartment to be reached ( ▶ Fig. 19.3). A preauricular incision is begun above the superior temporal line, moves behind the hairline, extends to the tragus, and, if necessary, is made in a transverse skin crest along the mandible. If the skin incision is higher in the temporal area and advances downward, the superficial temporal artery can be identified, dissected from the subcutaneous tissue, and preserved attached to the muscle. After sharp dissection against the galea, the scalp flap is reflected anteriorly, leaving thick areolar tissue with a pericranial layer adhering to the calvarium. The anterior branch of the superficial temporal artery can be cut distally and reflected with the scalp flap. If the incision advances inferiorly, subcutaneous dissection of the skin flap is carried out in a plane superficial to the parotid gland. To preserve the frontotemporal branches of the facial nerve, an incision 1 to 2 cm in length is made through the superficial and deep layers of the temporal fascia. The blades of the scissors are positioned under the deep temporal fascia and over the superficial temporal fascia. Subfascial dissection is done in a course parallel to the zygomatic arch and directed to the posterior root of the zygoma. The temporal fascia is dissected along the zygomatic arch and lateral orbital rim in subperiosteal fashion. Proximal control of the internal carotid artery can be achieved through cervical extension of the incision.
Fig. 19.3 Surgical technique. (a) Head position and skin incision. (b) Subfascial dissection. (c) Exposing the zygoma. (d) Cutting the zygoma and detaching the temporalis muscle. (e) Deflecting the temporalis muscle and frontotemporal craniotomy. (f) Starting the middle fossa peeling by identifying the middle meningeal artery.