INTRODUCTION

Approximately 10% of all meningiomas arise in the posterior fossa. Six different subtypes of posterior fossa meningiomas have been described, which differ in their localization, natural history, required surgical technique, and outcome. These subtypes include cerebellar convexity, cerebellopontine angle, petroclival, tentorial, foramen magnum, and jugular foramen meningiomas. Cerebellar convexity meningiomas have a very important distinction from other posterior fossa meningiomas: The tumor arises dorsal to the cerebellum, which results in the interposition of the cerebellar hemispheres between the tumor and the cranial nerves; therefore, no cranial nerves are encountered during the surgical exposure. Cerebellar convexity meningiomas are easily and accurately diagnosed via magnetic resonance imaging (MRI) or computed tomography (CT), and the surgical outcome is quite favorable.

HISTORY

In 1938, Cushing reported his own experience with 313 cases of meningioma operated in the early 20th century. Fifteen of these cases were cerebellar convexity meningiomas.¹ He classified these meningiomas as cerebellar chamber, subtentorial and recess tumors. He operated all those meningiomas during the 29 years from 1903 to 1932 and described the clinical and surgical record of each case precisely. Among his cases was a 27-year-old woman with a cerebellar convexity meningioma. She had complained of right facial numbness and tinnitus for 1 year, had stiffness of her neck for 3 months, and showed ataxic unsteady gait. She also had disturbance of vision, and on funduscopy choked discs were observed. Cushing operated this patient with a diagnosis of cerebellar tumor in two stages. The first operation was performed on February 11, 1913 and he made bilateral cerebellar explorations an atlas laminectomy to prevent tonsilar herniation. The second operation was performed 11 days later, on February 22, 1913 and the tumor was completely removed. The weight of the tumor was 56 grams. Cushing diagnosed this tumor as a fibroblastic meningioma with no psammomatous depositions and only slight tendency to the formation of whorls. The patient was in excellent condition with no neurological deficits in 1937, 24 years after the tumor removal. Cushing stated that generally speaking, these cases have proved to be surgically favorable, even those operated upon at a time when there had been little experience with what were still called “dural endotheliomas,” all of which supposedly had a good ultimate prognosis if the bulk of the lesion was removed. Among 15 cases of cerebellar convexity meningiomas operated by Cushing, 11 cases demonstrated long survival time after the surgical removal of the tumor and were in good clinical condition.

In the later literature, Castellano and Ruggiero² classified posterior fossa meningiomas into five subgroups: cerebellar convexity, tentorium, posterior petrous, clivus, and foramen magnum meningiomas. In 1953, Russel and Bucy³ mentioned the difficulties on diagnosis of posterior fossa meningiomas. Huang and Wolf ⁴ published their detailed description of the venous phase of the vertebral arteriography for diagnosis of posterior fossa tumors. It is obvious that the invention of CT and MRI ameliorated the diagnosis for cerebellar convexity meningiomas.⁵ In their detailed description of basal posterior fossa meningiomas, Yasargil and colleagues⁶ classified cerebellar meningiomas as the fourth group of dorsal meningiomas and divided them into median, paramedian, and lateral lesions. Since the 1970s, with the introduction of the CT scan and the development of microsurgical techniques and cranial base approaches, the surgical management of cerebellar convexity meningiomas have been evolving, leading to better outcome and a marked fall in surgical morbidity and mortality. The operative mortality rate was 22% in 1938 that published by Cushing.¹ In 1979 it was improved to 4% with the series of Yasargil.⁶ Two recent reports indicated no operational mortality.^7,8 Gross total resection must be the treatment of choice in all types of cerebellar convexity meningiomas. Roberti and colleagues⁸ reported 77% gross total resection with no mortality and morbidity. The most common cause for an incomplete resection was adherence of the tumor to cranial nerves or vascular structures, particularly venous sinuses that could not be sacrificed. If a complete resection presents an unacceptable risk of neurologic deficit, then a subtotal removal should be considered, and adjuvant treatment contemplated.

INCIDENCE

The location of meningiomas reported in the Japan Brain Tumor Registry⁹ is shown in Table 43-1. In total 13,838 meningiomas registered, 324 cases (2.3% of all meningiomas) were cerebellar convexity meningiomas. The ratio of supra- to infratentorial convexity meningiomas was 11:1. In the literature, cerebellar convexity meningiomas account for 8% to 18% of all posterior fossa meningiomas and 1% to 2% of all meningiomas.^2,10–12 According to the Japan Brain Tumor Registry, they account 12.3% (324/2,629 all posterior fossa meningiomas). The number of meningiomas in posterior fossa other than cerebellar convexity were 877 cases (6.3%) in the cerebellopontine angle (CPA), 1041 (7.5%) in the tentorium, 226 (1.6%) in the clivus, 12 (0.1%) in fourth ventricle, and 149 (1.1%) in the foramen magnum. Some of the cerebellar convexity meningiomas were closely located or attached to the tentorium, CPA, or foramen magnum.

TABLE 43-1 Location of meningioma*

* Report of the Brain Tumor Registry of Japan.⁹

CLINICAL PRESENTATION