27 Cerebellar Convexity Meningiomas
Introduction
The lateral and posterolateral dura of the posterior fossa are relatively uncommon sites of origin for meningiomas, representing ~10% of meningiomas of the posterior cranial fossa. These meningiomas are diverse, and depending on how extensive the lesion is, the complexity of surgical removal can range from a straightforward suboccipital craniectomy and dissection of the tumor’s arachnoidal plane away from the cerebellar convexity, to a complex and extensive removal of the transverse and sigmoid sinuses and adjacent bone. We feel that preoperative planning is especially critical for successful and safe removal of these tumors. In this chapter, we describe our strategy for systematically preparing a surgical plan for addressing these lesions and provide some insight into methods of removing these meningiomas effectively.
Definition
We define cerebellar convexity meningiomas as tumors that possess a dural base lying completely or mostly under the lower occipital and suboccipital bone that over-lies the convexity of the lateral or posterior hemisphere. Superiorly, the tumors are bounded by the transverse sinus, laterally by the sigmoid sinus, medially by the occipital sinus on the midline, and inferiorly by the circular sinus and foramen magnum. It is important that the location of the tumor’s dural base is delineated in its entirety, so that the tumor’s dural and bony attachments can be removed and hyperostotic bone drilled, if possible. Other related meningiomas that are similar in appearance to the untrained eye are the posterior petrous meningioma, the tentorium meningioma, and the foramen magnum meningioma. These tumors are compared in Fig. 27.1 . Differentiating these lesions is important because the best approach for removing them differs between the locations. Often, more extensive tumors can extend into these areas from the cerebellar convexity dura, and in these cases, identifying the likely site of origin preoperatively can greatly facilitate devascularizing tumors early and achieving complete tumor removal.
Important Issues to Determine Preoperatively
Where Is the Tumor Primarily Based?
The exact site of origin on the cerebellar convexity affects the positioning, the skin incision, the ideal placement of the bone and dural flaps, and the reconstruction. For simplicity, these tumors can be divided into three basic types ( Fig. 27.2 ), medial, lateral, and multicompartmental.
Medial
Medial meningiomas occupy the region of the posterior cerebellar convexity dura from the midline to the midway point between the occipital and sigmoid sinuses and are best approached through a conventional suboccipital craniectomy, with the patient positioned prone. The major concern with these tumors is involvement of the torcula because some of these tumors represent inferior extension of posteriorly positioned falcotentorial or peritorcular meningiomas. If questionable, the sinus patency should be determined with magnetic resonance venography or catheter venography. In most cases, the bony reconstruction of these lesions is straightforward because the large nuchal muscle mass makes this area largely cosmetically invisible.
Lateral
Lateral lesions overlie the cerebellar hemisphere from the midpoint of the cerebellar convexity to the sigmoid sinus laterally, and in many cases involve or even occlude the transverse or sigmoid sinuses. These tumors are best approached with the patient in the full lateral or three-quarter prone position and through some form of a modified retrosigmoid approach, depending on the posterior extent of the tumor and the patency of the sinus. Reconstruction is more important than with the medial tumors because the removal of hyperostotic retromastoid and mastoid bone is cosmetically deforming if not appropriately replaced.
Multicompartmental
Multicompartmental lesions extend into other regions, namely over the posterior petrous face, into the tentorium, across the transverse sinus into the supratentorial space, or even invading cranial nerve foramina. They are included based on their primary point of origin on the cerebellar convexity dura and not primarily by the regions they extend into. These complex cases can vary significantly, and preoperative planning is critical. Reconstruction can be complex, especially when surgical removal of hyperostotic occipital or temporal bone is necessary.
Are the Sinuses Involved, and If So Are They Patent?
The patency of the sinus (or its occlusion) should be known if the tumor is at all near the sinuses ( Fig. 27.3 ). A lack of sinus patency makes sacrifice of the involved occluded segment of sinus a possibility; however, complete occlusion should confirmed with catheter venography before sacrificing the sinus.
Where Does the Vein of Labbé Insert on the Sinuses?
The vein of Labbé can drain into the transverse–sigmoid system on the transverse sinus, at the transverse–sigmoid junction, or on the superior petrosal sinus. Knowing the exact location of the insertion of this vein is critical if the transverse or sigmoid sinus is going to be sacrificed in cases of multicompartmental tumors, if an extension of bone removal beyond the suboccipital convexity is going to be used, or if any tumor extension into the supratentorial space is going to be addressed at the same surgery.
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