34 Presigmoid Retrolabyrinthine Approach The principle of the retrolabyrinthine approach (RLA) is to achieve an enlarged mastoidectomy, while sparing the neuro-otologic structures. The RLA has been originally described by otological groups for the treatment of refractory Meniere’s disease but indications have been gradually extended to the neurosurgical field for various cerebellopontine angle (CPA) tumors or vascular disease. The RLA is a demanding technique because it requires an excellent knowledge of the petrous bone anatomy and of its main variations. We present herein the technique, limitations and indications of the RLA. In the neurosurgical field, the indications have substantially decreased over years. • Presigmoid broad based meningeal tumors. • Tumors infiltrating the transverse and sigmoid sinuses, which require extensive surgery (aggressive/recurrent meningiomas). • Endolymphatic sac tumors. • Some surgeons are still operating vestibular schwannomas using the approach in order to preserve hearing. • Exceptional cases of dural arteriovenous fistulas are treated through a RLA in case of failed endovascular treatment. This approach can extensively disconnect the totality of arterial dural feeders. • Intra-axial brain diseases like cerebellar or pontine cavernomas. • In our hands, this approach is mostly used as a first step of combined approaches. We routinely propose the RLA associated with an anterior petrosectomy, the so-called “combined petrosectomy” to resect large petroclival meningiomas. We also combine the RLA with a high cervical dissection for large jugulo-tympanic paragangliomas. • The petrous pyramid makes the junction between the middle and the posterior fossa. • The base of the pyramid corresponds to its outer surface, which main part is occupied by the cortical bone that covers the mastoid process (MP). • The external contours of MP grossly describe a triangle delineated by three points, the mastoid tip at the bottom, the posterior zygomatic point at the anterosuperior corner and the asterion at the postero-superior corner. Fig. 34.1 Bone exposure after soft tissues dissection. • The first step of the surgery is conducted through this triangle with gradual exposure of the posterior surface of the petrous bone and presigmoid dura. • The drilling of the mastoid will give access to the mastoid antrum that is always identified regardless the degree of pneumatization of the petrous bone. Medially to the antrum, the external contour of the posterior labyrinth is seen. • The 3 semicircular canals (SCC) display a constant orientation in between them with a 90° angulation to each other. The average diameter of each canal is around 8 mm. The canals are covered by a shell of compact bone with a density that is usually very distinct from the compact bone, different from the cancellous bone of the mastoid air cells. • Two important landmarks are exposed at the level of the posterior labyrinth. The first one is the loop of the lateral SCC that covers the second portion of the intra-petrous facial nerve that runs into the Fallopian canal. The second is the junction of the posterior SCC to the superior SCC, named the common crus. • The dura that covers the posterior surface of the petrous bone is framed by venous sinuses (superior and inferior petrosal sinuses, sigmoid sinus), which have variable size and diameter. The petrous ridge is a groove where runs the superior petrosal sinus. This sinus corresponds to the upper limit of the drilling depth. The superior petrosal sinus and tentorium collect part of the venous drainage of the temporal lobe through temporo-basal veins and the vein of Labbé. The optimal knowledge of the individual tailored pattern of drainage is worthwhile before deciding to expose and divide a sinus or tentorium if needed during the approach. • Several variations of the key structures need to be mentioned (Fig. 34.2). These variations should be identified before starting the surgery while checking the preoperative images. Their diagnosis may influence the operative technique and be responsible for potential complications if overlooked. • In many cases, the course of the sigmoid sinus is located anteriorly; this anterior location is defined by a distance between the anterior border of the sinus and the posterior wall of the external auditory canal of less than 15 mm (Fig. 34.2A). • The roof of the jugular bulb may be highly seated and reach the level of the posterior SCC or even higher at the level of the posterior wall of the IAC in rare cases (Fig. 34.2B); this configuration may hamper the drilling process. • Another variation involves the degree of aeration of the petrous bone. Indeed, pneumatization may change from very compact petrous bone (Fig. 34.2C), where the air cells are almost absent excepting the constant antrum, to the highly aerated petrous bone that makes the approach easier and faster (Fig. 34.2D). • Position: The patient is positioned supine with the head fixed in a Mayfield 3-pin holder. • Head: The head is rotated 80° toward the opposite side. Care is taken not to occlude the contralateral jugular vein by excessive rotation. • The surgeon stands behind the head and the pinna. • The facial nerve monitoring is compulsory. • Neuronavigation may be helpful to find the mastoid antrum and the SCCs in case of very compact mastoid. • Peri-Auricular skin incision (See Chapter 7) ◦ Starting point: Incision starts at preauricular temporal region superiorly. ◦ Course: It runs 1 cm away from the external circumference of the pinna. ◦ Ending point: It ends at the level of the mastoid tip inferiorly. • Myofascial Level ◦ The skin incision spans the galea and the underlying pericranium. • Muscles ◦ The muscles and deep fascia are elevated from the bone with a monopolar section, and retracted anteriorly. ◦ The sternocleidomastoid muscle is detached from the mastoid and mobilized downward. • Bone Exposure ◦ The base of the petrous pyramid is now clearly exposed with an anterior limit that corresponds to the posterior wall of the external auditory canal, that is marked by the spine of Henle (Fig. 34.1). ◦ Occipital artery, which needs to be ligated. ◦ Emissary vein (bone wax hemostasis). ◦ The skin of the external auditory canal in case of incision proceeding excessively in front of the spine of Henle. • The surgeon is equipped with a 6 to 8 mm cutting burr and will gradually shave the mastoid under copious irrigation. The drill is held like a pen and oriented tangentially to the structures that must be shaved. • During this step, the key point is to skeletonize the sigmoid sinus and its junction with the SPS, which is named sinodural angle (or angle of Citelli) (Fig. 34.3). It is strongly recommended to leave a thin shell of compact bone over the sinuses to avoid any tear. This shell will be subsequently elevated with a sharp dissector. • Depending on the course of the sigmoid sinus the opening of the angle may be a matter of variation. For instance, in case where the sigmoid sinus (SS) is anteriorly displaced as shown in Fig. 34.2, the angle is very narrow which hampers the access to the mastoid cells. In order to increase his working space, the surgeon will extend the drilling process behind the SS and above the superior petrosal sinus (SPS) (floor of the temporal fossa); proceeding in this way, the sinus will be mobilized downward and the angle will become wider. • The opening of the mastoid antrum that is located at the postero-superior corner behind the external auditory canal (EAC) allows the identification of the posterior labyrinth (Figs. 34.4, 34.5). The surgeon keeps drilling with a diamond burr under microscope and continuous irrigation. • The semicircular canals are covered by a shell of compact yellowish bone which texture is in sharp contrast with the loosely aerated bone around. ◦ The lateral SCC is seen first and care is taken to avoid any drilling under its surface due to the close vicinity of the second portion of the facial nerve. ◦ The posterior SCC is then skeletonized. At its posterior border, the notch of the endolymphatic canal is seen under microscope and communicates with a triangular shape thickening of the presigmoid dura that correspond to the endolymphatic sac. ◦ The last canal to be exposed is the superior SCC lying in the depth and connected posteriorly to the lateral SCC by the common crus. The surgeon should avoid any excessive anterior drilling at the level of the ampulla of the superior and lateral SCCs because the facial nerve runs (junction first and second portion) very nearby.
34.1 Introduction
34.2 Indications
34.3 Surgical Anatomy (Figs. 34.1, 34.2)
Abbreviations: A = asterion; EAC = external auditory canal; HS = spine of Henle; M = mastoid; PRZ = posterior root of the zygoma; TS = temporal squama.
34.4 Variations in Surgical Anatomy
34.5 Patient Positioning
34.6 Skin Incision
34.7 Soft Tissue Dissection
34.8 Critical Structures
34.9 Craniectomy (Figs. 34.3–34.9)
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