Petroclival Meningiomas: Suboccipital Retrosigmoid Approach

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CHAPTER 39 Petroclival Meningiomas

Suboccipital Retrosigmoid Approach

Madjid Samii, Venelin M. Gerganov

INTRODUCTION

Petroclival meningiomas are defined as arising at the petroclival junction from the upper two thirds of the clivus, medial to the trigeminal nerve. The more extensive sphenopetroclival additionally extend to the anterior cavernous sinus, sella turcica, and/or the sphenoid sinus. Their central location at the center of the cranial base, adjacent to important neurovascular structures, makes their surgical removal with acceptable mortality and morbidity rates extremely difficult. However, the natural history of petroclival meningiomas is characterized by progressive growth and neurologic deterioration, and an attempt at surgical removal is therefore justified.

Complete visualization of meningiomas and all adjacent vascular structures was once a must for making surgery more feasible and safer, and the literature was concerned with extensive cranial base approaches and their management. Malis, Mayberg, and Symon described the combined supra- and infratentorial approach, which involved the ligation of the transverse–sigmoid sinus. A combined retrosigmoid–subtemporal transtentorial approach with or without ligation of the sigmoid sinus was later introduced. As the main anatomic obstacle to a good exposure of the petroclival area is the temporal bone, approaches with partial or complete resection of the pyramid were later described. We introduced the intradural subtemporal approach with resection of the petrous apex. Kawase described an epidural subtemporal approach to the petrous apex. All of these lateral cranial base approaches provided a shorter distance to the tumor and surrounding neurovascular structures, improved visualization, and minimized brain retraction. Multiple modifications were later described differing mainly in the amount of temporal bone resection. However, the initial enthusiasm of performing extended approaches with wide bone resection was counteracted by the unacceptably high mortality and morbidity related to the surgical approach including a high rate of facial nerve palsy, hearing loss, and cerebrospinal fluid (CSF) leaks. Impingement or interruption of the basal temporal venous drainage also resulted in devastating consequences. Although these extensive approaches have contributed significantly to improve tumor respectability and patient outcome, growing experience and long-term follow-up directed us toward simpler and safer approaches to improve outcome and lessen the morbidity. The senior author’s concept has evolved over more than 25 years, through operation on 184 petroclival meningiomas. We currently manage large tumors with extension into the suprasellar and parasellar areas with involvement of the cavernous sinus, in stages using two simple approaches. Decompression of the brain stem is initially achieved by removal of the infratentorial tumor with a retrosigmoid suprameatal approach. The remaining supratentorial component is removed at a second stage with a frontotemporal approach.

The suboccipital retrosigmoid approach (SRA) is a safe and relatively simple approach to the cerebellopontine angle (CPA) and the petroclival area. It is related to a very low procedure-related morbidity rate ¹^–³ and therefore is among the most frequently utilized operative routes in neurosurgery.⁴^–⁶ The advantages of the approach are: it offers an excellent panoramic visualization to the whole CPA; it is a hearing-preserving technique that allows for a wide exposure of the tumor, whatever its size and pathologic type; and it offers increased safety when working in the vicinity of the brain stem. The dissection of the tumor at all stages is performed under direct visual control and the cranial nerves can be identified early both in their proximal and distal ends, thus increasing the chances for their preservation. If the SRA is combined with a resection of the petrous apex and incision of the tentorium, any petroclival lesions can be adequately exposed.^5,⁷^–⁹ This retrosigmoid–suprameatal approach (RSMA), which was developed and introduced by the senior author in 1982, provides access to meningiomas of Meckel’s cave, petroclival area, and even the posterior cavernous sinus.^8,¹⁰ It avoids the risks related to alternative approaches, such as extensive petrous bone resection or retraction of the temporal lobe with the associated risks of damage to neural and vascular structures. Meningiomas extending to the foramen magnum level can be removed via a similar retrosigmoid craniectomy and a C1 hemilaminectomy/laminectomy.⁷

Potential drawbacks of the approach are the need of cerebellar retraction, as well as the relatively higher rate of postoperative headache. With current neuroanesthesia and some modifications of the original retrosigmoid approach, described in the chapter, these disadvantages are rather theoretical.³

SURGICAL APPROACH

Patient Positioning and Anesthesia Considerations

The position of the patient on the operating table—semi-sitting, lateral, or supine with the head rotated 90 degrees to the contralateral side ^1,^2,^6,¹¹—is a matter of surgeon’s preference and institutional tradition. We prefer the semi-sitting position,^3,¹² which has several major advantages: (1) the surgeon can work bimanually because there is no need for constant suction and (2) the continuous irrigation of the operative field performed by the assistant obviates the need for frequent coagulation during tumor removal. The head is held with a three-point head fixation frame; then it is flexed and rotated approximately 30 degrees to the involved side, avoiding occlusion of venous jugular outflow or hyperflexion of the cervical spine (Fig. 39-1).

FIGURE 39-1 The head of the patient is fixed with the three-point head fixation frame, flexed, and rotated 30 degrees to the left side. The course of the venous sinuses (dark line); the parietomastoid, occipitomastoid, and lambdoid sutures; and the slightly curved skin incision (dotted line) are shown.

A drawback of this patient position is the risk of venous air embolism, paradoxic air embolism, tension pneumocephalus, or circulatory instability. However, in experienced hands these are not related to any lasting morbidity.^13,¹⁴ Transesophageal echocardiography is the most specific and sensitive method in detection of air embolism. However, the combined monitoring of end-tidal carbon dioxide and precordial Doppler echocardiography yield similar results.¹³ If immediate measures are carried out at the first sign of venous air embolism, the related morbidity is insignificant. Further, some studies have shown that the morbidity due to venous air embolism is similar in both the semi-sitting and supine positions.

Neurophysiology

Continuous neurophysiologic monitoring should be performed throughout the surgery: from the positioning of the patient to the skin closure.^3,^15,¹⁶ Monitoring of somatosensory evoked potentials is especially important during patient positioning in order to prevent spinal cord compression. Preoperative functional radiographs of the cervical spine could determine the patients at risk, such as those with severe degenerative spine disease or spinal instability.

The functional integrity of the facial nerve is monitored continuously by electromyography transferred by loudspeakers. Bipolar recording needle electrodes are fixed at the eyebrow for the orbicularis oculi muscle and at the mouth angle for the orbicularis oris muscle. Electrical activation by 1 to 4 mA is applied in the course of surgery in case of difficult nerve identification or for testing the reactivity of the nerve to mechanical stimulation. Monitoring of the brain stem auditory evoked potentials allows for control and prediction of auditory function during tumor resections. It provides instant feedback information on the functional status of the cochlear nerve. Waves I, III, and V are functional correlates of the cochlea, the nucleus cochlearis, and the colliculus inferior; the main parameters that are observed are the waves’ latencies, amplitudes, and losses. The loss of a wave V is the parameter most definitely associated with deafness.¹⁵ The loss of any wave may be prevented by early recognition of its deterioration, and the surgeon’s actions should be modified accordingly. Monitoring of the abducent and of the caudal cranial nerves is performed if needed according to the particular tumor extension and clinical presentation.

Skin Incision and Craniotomy

A slightly curved skin incision 1.5 to 2 cm medial to the mastoid process is performed (see Fig. 39-1) and the underlying muscles are incised in line with the skin. Traditionally the position of the burr hole was determined in relation to the asterion.¹⁷ However, some recent studies have shown that the asterion is not an absolutely reliable anatomic landmark and is variable both in the cranial–caudal plane and in the anterior–posterior plane.¹⁸^–²⁰ The superior nuchal line is more reliable and overlies the transverse sinus in all cases.²¹ The course of the sigmoid sinus is less variable. It descends along an axis defined by the mastoid tip and the squamosal–parietomastoid suture junction or over the mastoid groove.²¹ We prefer to place the burr hole 2 to 2.5 cm below the superior nuchal line, two thirds behind and one third in front of the occipitomastoid suture.

The craniectomy is performed using bone rongeurs, exposing the borders of the sigmoid and transverse sinuses (Fig. 39-2). More extensive exposure of these sinuses is unnecessary and might lead to their laceration or desiccation, with the risk of subsequent thrombosis. The issue of whether to make a craniectomy or craniotomy is a matter of individual preference.^22,²³ Our experience, however, showed that the craniotomy might be more dangerous procedure. Therefore, we avoid making one-piece craniotomy, although we performed it earlier, due to the related high risk of injury to the underlying sinuses and tearing the dura by the craniotome.

FIGURE 39-2 Extent of bone removal. The inferior border of the transverse sinus (arrow) and the medial border of the sigmoid sinus (asterisks) are exposed.

Special care should be directed toward preservation of the mastoid emissary vein/veins. Its location should be studied on the thin sliced bone window CT of the pyramid. Excessive traction to the vein could lead to sinus laceration and increases the risk of venous air embolism. The vein should be skeletonized with a diamond drill until it is free of any bony encasement and can be safely coagulated.

The bone opening should extend to the floor of the posterior fossa, thus allowing access to the lateral cerebellomedullary cistern. In case of caudally extending or craniocervical meningiomas, it might be necessary to open the foramen magnum. The dura is incised in a curvilinear manner just 1.5 to 2 mm medial to the sigmoid and inferior to the transverse sinus (Fig. 39-3). This allows for a primary watertight dural closure and avoids the need of using dural substitute in almost all cases. The lateral cerebellomedullary cistern is then opened and CSF is drained. Thus, the cerebellum relaxes away from the petrous bone and the self- retaining retractor supports and protects the cerebellar hemisphere, instead of compressing it.

FIGURE 39-3 The dura is opened along the sinuses and held with two sutures. The posterior surface of the pyramid (P), the tumor (Tu) between the trigeminal (V) and the facial nerve is seen. The cerebellar hemisphere is protected with a retractor (R). In this case the meningioma could be removed via the pure retrosigmoid approach (caudal cranial nerves [asterisk]).

Petroclival meningiomas are located medially to the Vth, VIIth, and VIIIth cranial nerves and their removal should be performed through one of the CPA levels: the upper level (defined by the tentorium and the trigeminal nerve), the second level (between the trigeminal and the VIIth–VIIIth nerve complex), the third level (between the VIIth and VIIIth nerves and the caudal nerves), or the lowest level (between the caudal nerves and foramen magnum).^8,⁹ The anatomic space between the cranial nerves may be relatively narrow; however, the tumor usually widens it sufficiently to allow safe manipulations. The removal should start at the level that is most widely expanded by the meningioma (see Fig. 39-3). Initial internal decompression is performed using an ultrasonic surgical aspirator at the appropriate levels. Importantly, the dissection of the tumor capsule should be performed only after sufficient internal decompression has been achieved. The extent of tumor removal is determined by its relation to neural structures: in the 10% to 20% of cases in which the pia mater of brain stem is infiltrated, a small part of the tumor capsule may be left.

In case of large meningiomas, the brain stem is displaced so that adequate access even to the contralateral side or supratentorially is provided. If necessary, however, the petrous apex may be resected, as soon as the tumor portion in the CPA is removed. The apex of the pyramid is resected by intradural drilling of the suprameatal tubercle, which is a bone eminence located above and anterior to the internal auditory meatus (Figs. 39-4 and 39-5). The extent of bone removal is determined by the individual anatomic characteristics and tumor expansion. Some studies have shown that the retrosigmoid approach may thus be extended anterior as far as 13.0 mm.¹⁰ The opening of Meckel’s cave allows for mobilization of the trigeminal nerve, which further increases the working space (Fig. 39-6). The structures at risk in the area are the internal carotid artery anterolaterally, the petrosal sinus superiorly, and the superior and posterior semicircular canals laterally.⁹ The abducens nerve may be identified early at its brain stem exit zone and followed during tumor removal up to Dorello’s canal, which allows for its preservation. The exposure of the rostral tumor portion can be further enlarged by resection of the tentorium. Thus, the RSMA provides access to Meckel’s cave, the petroclival area, and the middle fossa including the posterior cavernous sinus.