Meningiomas occur at sites of dural folds, most commonly the frontoparietal parasagittal convexities, falx, tentorium cerebelli, sphenoid wings, olfactory groove, and tuberculum sellae. Ninety percent of meningiomas are supratentorial, and the majority of infratentorial meningiomas occur at the cerebellopontine angle. Some meningiomas—such as those of the olfactory groove, sphenoid wing, and tuberculum sellae—express themselves by highly distinctive syndromes that are almost diagnostic; these are described further on in this chapter. Rarely, the tumors are multiple. Inasmuch as the meningioma extends from the dural surface, it commonly incites hyperostosis of adjacent bone and can, in more malignant cases, invade and erode the cranial bones or excite an osteoblastic reaction, giving rise to an exostosis on the external surface of the skull. Most of the following remarks apply to meningiomas of the parasagittal, sylvian, and other surface areas of the cerebrum.
Small meningiomas, less than 2.0 cm in diameter, are often found at autopsy in middle-aged and elderly persons without having caused symptoms. Only when they exceed a certain size and indent the brain or cause a seizure do they alter function. The size that must be reached before symptoms appear varies with the size of the space in which the tumor grows and the surrounding anatomic arrangements. Focal seizures are an early sign of meningiomas that overlie the cerebrum. The parasagittal frontoparietal meningioma may cause a slowly progressive spastic weakness or numbness of one leg and later of both legs, and incontinence in the late stages. The larger sylvian tumors are manifest by a variety of motor, sensory, and aphasic disturbances in accord with their location, and by seizures.
Before brain imaging became widely available, a meningioma often gave rise to neurologic signs for many years before the diagnosis was established, attesting to its slow rate of growth. Even now some tumors reach enormous size, to the point of causing papilledema, before the patient comes to medical attention. Many are detected on CT in individuals with unrelated neurologic diseases. The diagnosis of meningioma is greatly facilitated by their ready visualization with contrast-enhanced CT and MRI (Figs. 30-7 and 30-8), which reveal their tendency to calcify as well as their prominent vascularity. These changes are reflected by homogeneous contrast enhancement and by “tumor blush” on angiography. Typically the tumor takes the form of a smoothly contoured mass sometimes lobulated, with one edge abutting the inner surface of the skull, along the dura. On CT performed without contrast they are isodense or slightly hyperdense; calcification at the outer surface or heterogeneously throughout the mass is common. The amount of edema surrounding the tumor is highly variable and may relate to the extent of local brain symptoms. The CSF protein is usually elevated.
Treatment Surgical excision should afford long-term or permanent cure in most symptomatic and accessible tumors. Recurrence is likely if removal is incomplete, as is often the case, but for some the growth rate is so slow that there may be a latency of many years or decades. A few tumors show malignant qualities; that is, a high mitotic index, nuclear atypia, marked nuclear and cellular pleomorphism, and invasiveness of brain. Their regrowth is then rapid if they are not completely excised. Tumors that lie beneath the hypothalamus, along the medial part of the sphenoid bone and parasellar region, or anterior to the brainstem are the most difficult to remove surgically. By invading adjacent bone, they may become impossible to remove totally. Carefully planned radiation therapy, including various forms of stereotactic treatment, is beneficial in cases that are inoperable and when the tumor is incompletely removed or shows malignant characteristics.