13 The Cerebellopontine Angle

10.1055/b-0035-104225

13 The Cerebellopontine Angle

Michael E. Sughrue and Charles Teo

13.1 Introduction

In some ways this is the most straightforward chapter in the book. We operate on almost all tumors, large and small, in the cerebellopontine angle (CPA), the pons, the cisternal portion of the jugular foramen, the posterior opening of the Meckel cave, the lateral cerebellar hemisphere, the transverse–sigmoid sinus junction, and the petrous face through a retrosigmoid craniectomy. A keyhole retrosigmoid craniectomy is difficult to beat for this area. It is simple and quick, usually taking less than 15 min to open and about the same time to close. The incision and opening are small. It provides outstanding access to the entire ventral posterior fossa. The approach-related morbidity is low, and generally lower than with other more complex and invasive approaches. It is also easy to learn. Cerebellar retraction and even minor cerebellar resection is well tolerated in most patients. Thus, while pathology entirely within the temporal bone needs more complex otological approaches, we feel that most pathology in this region is best addressed with the retrosigmoid approach. This is demonstrated vividly in Video 13.1.

The main focus of this chapter is how to maximize the use of the retrosigmoid approach for these patients. We discuss modifications for common conditions and how to expand this approach for more complex disease as needed. We also discuss the use of the endoscope in these patients.

13.2 The Retrosigmoid Approach to the Ventral Posterior Fossa

13.2.1 Positioning of the Patient

In no other cranial operation is positioning more important than in the retrosigmoid approach to the CPA. There are many variations, all with specific advantages and disadvantages. To discuss these in any detail defies the scope of this book. However, we would like to underscore our technique and offer some justification for its desirability. Patients are placed supine with the head turned maximally to the contralateral side (Fig. 13.1). The head is elevated in the horizontal plane in order to minimize any obstruction to the surgeon’s view from large-breasted or barrel-chested patients. The head is not turned far enough to obstruct venous drainage. To achieve the rest of the necessary angle of view, the patient is secured thoroughly to the bed so that the bed may be tilted away from the surgeon when a view of the face of the petrous bone is required. If limited by an inflexible neck, it is not forced, but rather the bed is tilted instead. The extent of reverse Trendelenburg positioning varies according to the fullness of the posterior fossa contents after opening the dura. Typically, we do not use lumbar drainage, even with large tumors.

**Fig. 13.1** **a, b (a)** Images depicting the standard opening and positioning for the retrosigmoid approach. The position is supine with the head turned as far to the side as neck flexibility will allow. Keeping the patient supine prevents the shoulder from interfering with the angles needed to work within a keyhole craniotomy, which is an issue with lateral and semilateral positions. If the neck is inflexible, the appropriate head position can be obtained by rotating the table. **(b)** To allow for maximum rotation toward the contralateral side, we attach the head of the bed to the side of the bed using a modified side attachment so that the patient can be maximally rotated away from the operative side, as required, to obtain the necessary working angles.

13.2.2 Placement of the Opening

The basic details of this approach have been described in see Chapter 6. As with any keyhole approach, it is important to put the opening in the correct spot, and some variations in this regard are worth mentioning. The keyhole approach is not good for “looking behind the door,” and thus surface tumor needs to be exposed, which may require a larger incision (Fig. 13.2). This is especially salient when using this approach to remove a lateral cerebellar tumor. We typically use linear incisions, but larger cranial openings may need an S-shaped incision to increase the surface area of posterior fossa bone that is exposed. Any lesion that is more than 2 cm below the surface does not require the surface to be exposed, as moving the microscope will provide these angles of visualization, as predicted by the keyhole principle.

**Fig. 13.2** **a–f** A keyhole approach to an extensive posterior fossa meningioma. **(a, b)** Preoperative images demonstrating a posterior fossa meningioma arising from the jugular foramen and involving the transverse and sigmoid sinuses, which were occluded on imaging. The goal was to remove the involved sinuses and cisternal tumor portions, and to leave the tumor in the jugular foramen alone. **(c)** The steps involved in this operation meant that we needed to expose the involved sinuses, and because these are surface structures the keyhole principle dictates that a larger craniotomy is in order. The S-shape of this incision parallels the nerves, yet provides the necessary surface area without the areas of tenuous blood supply inherent with a horseshoe-shaped flap. This is the minimal exposure we can perform for a tumor of this kind. **(d)** A craniectomy was performed to expose the posterior fossa, and a bone flap was utilized to expose the temporal lobe and sinuses so the latter could be divided and removed with the tumor mass. **(e)** Following sacrifice of the sinuses, the tumor resection proceeds as with any cerebellopontine angle tumor. **(f)** Tumor was left in the jugular foramen as planned preoperatively.

While the need for exposure all the way up to the sinuses is familiar to most neurosurgeons and easy to understand, other site-specific nuances in the drilling are worth mentioning. Even small retrosigmoid keyhole craniotomies require some consideration of the correct placement for the opening such that instruments can enter the field at a relatively flat trajectory (Fig. 13.3). This means the opening should be somewhat wider opposite the direction you are planning on working in. This is because the first maneuvers in this approach are to gently follow the curve of the cerebellum to the angle where the posterior fossa curves medially, either at the tentorium (for supracerebellar approaches to the fifth nerve, the Meckel cave, and the high petroclival junction), or at the petrous face (for approaches to the CPA and jugular foramen). If the bone opening is insufficiently inferior (for supracerebellar approaches) or insufficiently open posterior (for all approaches), then the instruments will not be able to lie flat enough along the cerebellum to turn the corner and retract the cerebellum to access the cisterns. For this reason, we place craniectomies to access higher structures, such as the Meckel cave, slightly lower along the sinus to allow these maneuvers (Fig. 13.3). They are not tucked so far into the transverse–sigmoid sinus junction in these patients. Similarly, all of these craniectomies need some posterior space to allow for instruments to be used in a posterior-to-anterior (i.e., flat) angle (Fig. 13.3). This involves only a few extra millimeters of bone work; however it is important to make a conscious effort to ensure this exposure is obtained.