Epidural Middle Fossa Approaches

Chordomas are rare, slow-growing, infiltrative tumors that are most often identified in the fourth to sixth decades of life. ¹ They account for 1 to 4% of all primary bone tumors and 0.1 to 0.2% of all intracranial neoplasms. ²^,³ Chordoma is widely recognized to originate from vestigial remnants of the intraosseous primitive notochord, involving the axial skeleton and most frequently occurs in the cranial base, mobile spine, and sacrococcygeal region. ⁴ Although chordomas are locally invasive and has ill-defined borders in the bone, surgery is the most effective treatment for patients. The extent of resection is correlated with lower risk of recurrence. Specialized skull base approaches achieve better prognosis and longer survival for chordomas. ⁵ The prognosis for chordoma is poor and is very much influenced by the extent of tumor resection. ⁶

18.2 Indication of the Epidural Middle Fossa Approach

Clival chordomas mainly originate from the clival bone and sometimes extend into the middle fossa lateral to the carotid artery, petrous bone, and infratemporal fossa. Currently, epidural middle fossa approaches (EMFAs) are performed in 14.8% of all surgery for clival chordomas ( ▶ Table 18.1), and the rate is decreasing recently due to development of endoscopic transsphenoidal surgery (ETS). However, the EMFA is indicated in the following two cases: (1) In patients showing lateral extension beyond the carotid artery. It may be difficult to remove completely by a single surgical approach of ETS, and the EMFA may be a safe approach not to cross the carotid artery. (2) In patients showing marked subdural extension. They have a risk of vascular involvement and a large dural defect, and two-step operations may be safer to spare the risk of vascular injury and cerebrospinal fluid (CSF) leakage: ETS as the first surgery for removal of the epidural part and dural reconstruction, and the EMFA as the second surgery to remove the subdural part.

**Table 18.1** Surgical approaches for clival chordomas
Approach	Number of Surgeries
Frontobasal	57 (11.8%)
Middle fossa	71 (14.8%)
Suboccipital	34 (7.1%)
Transsphenoidal	129 (26.8%)
Transfacial, transoral	80 (16.8%)
Others, unknown	111 (21.7%)
Total	482 (100%)
Source: Co-operative study in Japan, 2005, unpublished data.

18.3 Surgical Technique

Three types of EMFAs are presented according to the location of the tumor: zygomatic, petrosal, and zygomatic–petrosal approaches.

18.3.1 The Zygomatic Approach (Case 1)

This approach is indicated for parasellar-type tumors showing marked extension lateral to the carotid artery ( ▶ Fig. 18.1 a) or extending down to the infratemporal fossa.

The patient is placed in supine position with a shoulder pad, rotating the head laterally and the axis down. The upper body is lifted 20 degrees to reduce venous bleeding. The lateral head position is important for identifying the anatomy of the temporal bone. Before draping, the auricle is folded forward with sutures to cover the ear. A preauricular question mark skin incision is made close to the auricle, to protect the facial nerve. The skin flap is reflected to touch the zygomatic arch, and the temporal fascia on it is detached to expose the zygoma, which is cut by surgical saw thereafter. The superior margin of temporal muscle is detached along the temporal line, and reflected inferiorly ( ▶ Fig. 18.1 b).

A basal craniotomy above the mandibular joint is made flush to the base of the middle fossa with three burr holes; dura on the middle fossa is elevated from the skull, until the foramen ovale and rotundum are identified. Epidural venous bleeding can be controlled with Surgicel or oxy-cellulose cotton balls. The middle meningeal artery (MMA) is coagulated and detached from the foramen spinosum. The greater superficial petrosal nerve (GSPN) is localized posterior to the foramen spinosum, from the point adhesive to outer layer of the dura (hiatus facialis). Care must be taken not to stretch the greater petrosal nerve excessively because it is connected to the facial nerve in the pyramid. On the surface of the pyramid, two bone landmarks can be observed: arcuate eminence and trigeminal impression. The geniculate ganglion is located on the extension line of the greater petrosal nerve, at the intersection of the line between the external and internal auditory meatuses. The outer periosteal layer of the dura is then incised along GSPN and foramen ovale, to expose the maxillary and mandibular branches of the trigeminal nerve ( ▶ Fig. 18.1 c).

The tumor in the cavernous sinus can be removed by this step without exposure of the temporal lobe. ⁷ The carotid artery, mostly deviated superiorly in the cavernous sinus, must be confirmed using a micro-Doppler probe. A tumor in the pyramid can be removed by drilling of the petrous apex. Small amount of the posterior fossa tumor can be removed after removal of epidural tumor, through a breached dura ( ▶ Fig. 18.1 d).

In a case of tumor extension in the infratemporal fossa, resection of the base of middle fossa offers a corridor to the infratemporal fossa. ⁸^,⁹^,¹⁰ After resection of the middle fossa until an oval or round foramen adjacent to the tumor is opened, the extracranial space can be exposed by incision of the periosteal dura, which covers the middle fossa tumors. At closure, the zygomatic arch is replaced and fixed with a small titanium plate.

Fig. 18.1 Case 1, a 58-year-old male complained of double vision (right abducens palsy). (a) Preoperative MRI showed right parasellar mass involving the right cavernous sinus lateral to the carotid artery (arrow) and the petrous apex, and invading subdurally in the posterior fossa. Medial part of the tumor was removed partially by microscopic transsphenoidal approach (arrowhead). (b) This patient was reoperated by the zygomatic EMFA to remove the residual tumor. A drawing shows the craniotomy of the zygomatic EMFA. The temporal muscle is retracted more inferiorly by zygomatic osteotomy than common middle fossa craniotomy, offering easier epidural subtemporal access without significant retraction to the temporal lobe. (c) An operative drawing of microscopic view after epidural exposure of the parasellar space. The tumor in the petrous apex and posterior cavernous sinus is demonstrated. MMA, middle meningeal artery; GPN, greater superficial petrosal nerve; V2, maxillar nerve; V3, mandibular nerve. (d) A CT image with contrast enhancement after the EMFA. The tumor was totally removed, including the subdural part in the posterior fossa. After surgery, the patient had no tumor recurrence, with additional carbon ion radiosurgery targeting his clivus.

18.3.2 The Petrosal Approach (Case 2)

This approach, the same as the anterior petrosal approach described elsewhere, ¹⁰^,¹¹^,¹²^,¹³ is indicated for chordomas invading the petroclival area ( ▶ Fig. 18.2 a). Preoperative lumbar drainage is placed to decrease retraction to the temporal lobe, in young patients who have tight intracranial space. The facial and auditory monitoring are prepared only in case of tumor invasion into the internal auditory meatus (IAM). Patient’s position is the same as in the zygomatic EMFA. A U-shaped skin flap is made above the auricle. The temporalis fascia is dissected off the muscle, leaving a pedicle inferiorly, and the muscle is retracted anteriorly, exposing the zygoma root. A 5-cm-sized craniotomy is centered above the external auditory meatus (EAM), with three burr holes that are placed under the zygomatic arch, anterior to the asterion and on the squamous suture. It is created approximately along the outer margin of the squamous suture. Sigmoid sinus exposure is not necessary. The craniotomy ridge medial to the zygoma must be drilled out to obtain a sufficient surgical field along the base of middle fossa, and if possible, mastoid air cells should not be opened. The dura on the middle fossa is dissected and elevated from temporal bone using a hooked retractor until the petrous rim is confirmed. CSF drainage is essential for the exposure.

Detachment of the MMA and confirmation of the GSPN are the same as in the zygomatic EMFA. One of the landmarks of the lateral margin of the anterior petrous bone is the major petrosal groove. The GSPN courses on it between the hiatus facialis and sphenopetrosal fissure. It is wrapped with a band of soft tissue. In order to spare the facial nerve from injury during dural dissection, occasionally it has to be cut. Another surgical landmark, the arcuate eminence, is recognized medial to the external EAM and must not be opened if hearing is to be preserved. Firstly, the foramen spinosum, which is located in the bottom of the middle fossa, is identified, and then the MMA is coagulated and cut. After that, the periosteal dura, adhesive to the GSPN, is cut to preserve the nerve. The interdural dissection is extended on the mandibular nerve to reduce tension of the dura, and trigeminal impression is observed on the petrous apex.

The axis of the microscope is shifted anteriorly to overlook the petrous apex. Location of the internal auditory canal (IAC) is imagined anatomically medial to the external auditory meatus, between the GSPN and the arcuate eminence. It is usually located slightly anterior to the arcuate eminence, at a depth of 7 mm from the bone surface. The height of the arcuate eminence also varies in each case and must be checked by bone-targeted computed tomography before surgery. In normal bone, the anterior pyramidal bone is usually of softer consistency than the middle ear bone, and this can provide an important orientation during pyramid resection. Extent of pyramid resection is dependent on the tumor invasion in the pyramid. To avoid facial nerve injury, the bone above the fundus of the IAM should not be drilled off, because the facial nerve courses superficially under the bone surface. Removal of the lateral part of the trigeminal impression is necessary, and ultrasonic bone curette may be useful for removal of the deep bone spur. In a case of tumor extension around the petrous carotid artery, the GSPN is sacrificed and the petrous carotid artery is exposed along with the tumor resection. ¹¹^,¹⁴

The dura of the middle fossa base is incised 2 cm inward toward the superior petrosal sinus (SPS). The incision is extended in a T shape along the SPS, and the tentorium is exposed. After incision of the dura in the posterior fossa, double ligation with sutures is performed on the SPS at the most posterior part of the bone removal. The SPS and the tentorium are detached by scissors toward the tentorial notch. The junction of the petrosal vein should be included in the posterior portion so that normal venous flow can be preserved. Care must be taken not to injure the trochlear nerve around the free edge of the tentorium.

At the time of tumor resection in the clivus, location of the abducens nerve must be cared, for it locates several millimeters medial to the trigeminal impression. Clival bone invasion medial to the abducens nerve is better to be treated by ETS or radiosurgery. In case of marked subdural invasion in the posterior fossa, the middle fossa dura and the tentorium must be cut by the same manner as in the anterior petro- sal approach, to remove the subdural tumor ( ▶ Fig. 18.2 b). In this case, the subdural space must be closed completely using abdominal fat and fascial flap sutured with dura and coated with fibrin glue. The drilled petrous apex and opened mastoid air cells are covered with pieces of abdominal fat and coated with fibrin glue. The temporalis fascia flap is used to cover the fat and the skull base, suturing with the dura to preserve CSF accumulation under the scalp. The cranial window is fixed with titanium plates, and artificial bone is not necessary. The spinal drainage tube is kept for a few days without drainage. If CSF rhinorrhea occurs, spinal CSF drainage is performed for 1 or 2 weeks.

Fig. 18.2 Case 2, a 14-year-old male complained of headache. No neurologic deficit was seen. (a) A sagittal section MRI demonstrated that a bulky tumor invaded subdurally, toward the brainstem. (b) An operative drawing of microscopic view after petrosectomy and tentorial incision. A dashed line indicates the clival line. The tumor is removed after internal decompression, to protect the marginal cranial nerves III to VI (arrows). MC, Meckel’s cave; PCP, posterior clinoid process. (c) After the anterior petrosal approach, the subdural tumor was totally removed. The clival bone invaded by the tumor was irradiated by proton beam, and the tumor was completely gone. This patient is surviving without neurologic deficit, for more than 12 years without tumor regrowth.

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