Retrosigmoid Approach

35 Retrosigmoid Approach

Guilherme Henrique Weiler Ceccato, Marcio S. Rassi, Jean G. de Oliveira, and Luis A. B. Borba

Abstract

Retrosigmoid approach is a versatile option to reach the cerebellopontine angle (CPA), and its many variations widely cover the lateral posterior fossa from tentorium to foramen magnum. This approach can be initiated by a straight incision behind the pinna in a myocutaneous fashion and then subperiosteal dissection of the surrounding muscles to expose the underlying bone. A single burr hole is placed over asterium and then the osteotomy runs posteriorly and inferiorly, leaving the last anterior cut over the sigmoid sinus to be performed with the drill. The dura mater exposed is bordered anteriorly by sigmoid sinus and superiorly by the transverse sinus. It is important to initially release cerebrospinal fluid (CSF) from the cerebellomedullary cistern in order to provide posterior fossa relaxation and avoid retraction of cerebellum or other neurovascular structures. Meticulous microsurgical techniques and in-depth anatomical knowledge is mandatory to avoid postoperative complications and achieve favorable outcomes.

Keywords: retrosigmoid approach, skull base, posterior fossa, cerebellum, brain tumor, vestibular schwannoma, sigmoid sinus

35.1 Introduction

Retrosigmoid approach is one of the most important skull base approaches, considering its versatility to expose the posterior fossa, and it is particularly useful regarding lesions of cerebellopontine angle (CPA). The first removal of an acoustic tumor is credited to Sir Charles Balance in 1894, using finger dissection through a suboccipital approach.1 ^, ² However, Fedor Krause is credited to be the first to perform the predecessor of the current retrosigmoid approach in 1898. He employed a unilateral suboccipital osteoplastic flap to transect the VIII nerve in a patient with intractable tinnitus and in 1903 used this approach to remove an acoustic tumor.¹ ^, ² ^, ³ ^, ⁴ During the past century the technique significantly evolved, initially thanks to efforts of Cushing, Dandy, and their groups.¹ ^, ² The mortality related to the resection of CPA tumors was initially extremely high, ranging around 80%.⁴ ^, ⁵ However, for example, in 1917 Cushing already published a case series with a mortality between 10 and 20%, due to his surgical skills and the fact that he had only performed the tumor debulking leaving the capsule behind.² ^, ⁵ Thanks to the scientific evolution since the first reports, nowadays the mortality is as low as 0 to 0.4%, with the efforts turned to reduce the postoperative complications.⁶

The retrosigmoid craniotomy is performed at the transverse-sigmoid sinuses junction, and provides a bone window bordered superiorly by the transverse sinus and anteriorly by the sigmoid sinus. This approach can vary in size according to the target pathology, being possible to reach the foramen magnum or even combine a transtentorial extension.⁷ ^, ⁸ Traditionally, the retrosigmoid approach provides to surgeon a wide exposure of CPA, from the V nerve to the XI cranial nerves (CNs).⁷ ^, ⁸

35.1.1 Anatomical Background

Posterior fossa is a complex space located between the tentorial incisura and foramen magnum.⁹ CPA is a V-shape cisternal space located between the superior and inferior limbs of cerebellopontine fissure, formed by petrosal surface of cerebellum folding around the pons and middle cerebellar peduncle⁹ ^, ¹⁰ (Fig. 35.1).

Fig. 35.1 (a–c) Anatomy of cerebellopontine angle related with the retrosigmoid approach. AICA, anterior inferior cerebellar artery; CN, cranial nerve; PICA, posterior inferior cerebellar artery; PV, petrosal vein; Sig. Sin., sigmoid sinus; VA, vertebral artery.

Lateral wall of CPA comprises the occipital and temporal bones, including the Meckel’s cave, internal acoustic meatus, jugular foramen, and hypoglossal canal. Other structures located laterally are the inferior petrosal and sigmoid sinuses connected with the jugular bulb, venous plexus of hypoglossal canal, and condylar emissary veins, as well the superior petrosal, basilar, and marginal sinuses connected with cavernous sinus.¹¹

The medial wall of CPA comprises the brainstem and roots of cranial nerves (CNs), as well the petrosal cerebellar surface. Cerebellopontine fissure can be divided into two limbs, one superior and continuous with cerebellomesencephalic fissure and one inferior in continuity with cerebellomedullary fissure; and in its middle portion it is continuous with petrosal fissure. Flocculus is located at the level or slightly inferior to the petrosal fissure, and choroid plexus protruding from the foramen of Luschka is located inferomedially to flocculus.⁹ ^, ¹¹

The IV nerve leaves the brainstem from dorsal midbrain, coursing close to tentorial incisura until it penetrates the roof of cavernous sinus.⁹ ^, ¹¹ The V nerve is connected with pons by a large sensorial branch and one or two small motor roots medial to sensory one; all of them running toward the Meckel’s cave. The VI nerve leaves the brainstem from the medial portion of pontomedullary sulcus, at base of the cerebellomedullary fissure.⁹ ^, ¹¹

The VII and VIII CNs arise from lateral end of pontomedullary sulcus immediately rostral to foramen of Luschka, slightly separated, but enter the internal acoustic meatus as a single nerve complex.⁹ Inside the internal auditory canal (IAC) the vestibular nerve runs posteriorly and branches into the superior and inferior vestibular nerves. The facial nerve courses anterior to superior vestibular nerve and cochlear nerve runs anterior to inferior vestibular nerve. In the fundus of IAC the transverse crest separates superior vestibular nerve and facial nerve from the other ones, and the transverse crest separates superior vestibular nerve from the facial.¹⁰

The IX to XI CNs leave brainstem from retro-olivary sulcus, usually the IX nerve as a large nerve rootlet, X as multiple small rootlets, and XI also comprising a branch from the upper cervical spine. The flocculus usually hides the VII and VIII nerves from the perspective of a retrosigmoid approach, and also the rootlets of IX to XI CNs running anteriorly to the flocculus and choroid plexus.⁹ The XII CN exits from the pre-olivary sulcus and is composed of multiple rootlets entering the hypoglossal canal as several nerve bundles.⁹ ^, ¹¹

Anatomical structures in CPA can be classified into three groups in order to facilitate understanding of complex anatomy of this region, each group comprising a related bony foramina, CN, and artery.¹⁰ ^, ¹¹ From top to bottom the upper group refers to Meckel’s cave, trigeminal nerve, and superior cerebellar artery (SCA); the middle to internal acoustic canal (IAC), VI to VIII CNs, and anterior inferior cerebellar artery (AICA); the third to jugular foramen, IX to XII CNs, and posterior inferior cerebellar artery (PICA).

The SCA emerges from distal segment of basilar artery and runs around the mesencephalon between the IV and V CNs forming a downward convex loop close to the V nerve. The AICA arises from proximal basilar artery (BA) making close contact with the VI CN in its exit point, and runs around the lower pons and flocculus laterally, close to internal acoustic meatus. PICA branches from the vertebral artery (VA) making contact with rootlets of the XII CN immediately after leaving the VA, and when running toward the cerebellomedullary fissure it goes among the IX to XI CNs medially to jugular foramen.9 ^, ¹⁰ ^, ¹¹

Posterior fossa veins can be divided into three draining groups: galenic group, draining into vein of Galen; petrosal group that drains to superior petrosal sinus; and tentorial group, comprising veins draining in the sinuses close to the torcula.¹² ^, ¹³ Several bridging veins exist, connecting the veins over neural structures toward these drainage systems; however, the most often involved during retrosigmoid approach are superior petrosal veins.¹⁴ Their tributaries are classified into four groups: posterior mesencephalic, anterior pontomesencephalic, tentorial cerebellar surface, and a petrosal group, from the lateral aspect of pons and medulla around the petrosal fissure and fourth ventricle.¹⁴ These groups can vary in size; however, usually the largest tributary of them are the petrosal goup, especially the vein of cerebellopontine fissure, receiving the vein of cerebellomedullary fissure that drains the fourth ventricle.¹⁴

35.1.2 Indications

The retrosigmoid approach can be used in cases of neoplastic or vascular lesions of lateral posterior fossa from the level of tentorium to foramen magnum (Fig. 35.2 and Fig. 35.3).¹⁵ Neoplastic or vascular lesions centered in the CPA, as well as the V to XI CNs pathologies, lesions arising from the lower side of tentorium, lateral cerebellar hemisphere disorders, and also pathologies of lateral surface of brainstem can be approached.¹⁶

Lesions medial to the IAC as well as medial to the V, VII, VIII, IX, X, and XI CNs are challenging to approach through this route, because the surgeon must work between CNs.¹⁵ Despite the possibility of extending the approach to supratentorial compartment, it is important that the predominant part of tumors in this region be located at the level of posterior fossa.¹⁷

This approach allows removal of tumors of different sizes, and it is a good option in cases where hearing preservation is the goal. Other aspect is that the facial and cochlear nerves are behind the tumor, giving the surgeon quick and safe access to the mass. Retrosigmoid approach provides a wide access to the VII and VIII CN roots’ entry zones in brainstem, cisternal component of tumor, and also exposes posterior part of IAC. Lesions extending toward the lateral third of IAC may be more challenging to remove, requiring the assistance of endoscope or other approach, for example.18 Due to this reason the retrosigmoid approach is a better option for hearing preservation in cases of tumors not extending to the lateral third of IAC and not exerting great compression over the brainstem.¹⁸

35.2 Surgical Technique

35.2.1 Patient Positioning

The patient should be placed in lateral decubitus position, with contralateral arm well supported outside and below the surgical table with slight flexion. The head should be positioned parallel to the ground and fixed on Mayfield device. Ipsilateral shoulder should be displaced slightly inferiorly, in order to not obstruct the surgeon’s working space. The patient must be well padded, with cushions under the contralateral armpit, between the knees, beyond other pressure points. The ipsilateral thigh should be flexed and fixed to be ready in case of fat or fascia harvesting 8 (Fig. 35.4).

Fig. 35.2Illustrative case of a 53-year-old man presenting with right side deafness. (a, b) Magnetic resonance imaging (MRI) suggestive of a right-side vestibular schwannoma presenting with mass effect over the brainstem. (c) Postoperative axial fluid-attenuated inversion recovery (FLAIR) demonstrating normal position of brainstem and no signs of ischemia as well as complete tumor removal. (d) Postoperative axial computed tomography (CT) scan showing complete radiological removal of the tumor.

Neurophysiological monitoring must be employed, especially somatosensory evoked potentials, facial nerve electromyography, and auditory brainstem response.