Atypical meningiomas comprise about 4% of all meningiomas [1
] and show increased mitotic activity, hypercellularity, large nuclei with prominent nucleoli, and necrosis. According to the classification scheme proposed by the World Health Organization [1
], they are grade II tumors. Their etiology is uncertain, but typical meningiomas may undergo gene losses and gains, transforming them into atypical ones. Radiation-induced meningiomas are also more likely to be atypical. Despite gross total resection, approximately 29-40% of atypical meningiomas recur. We present an atypical meningioma showing extension into the perivascular spaces.
A 45-year-old man underwent partial resection of an atypical right frontal meningioma. Two years later the patient developed an increasing visual deficit and was evaluated on MR imaging. The study revealed a large and diffuse dural-based plaquelike mass extending throughout both middle and anterior cranial fossa and along the anterior falx cerebri (Fig. 1A
). Extension along the perivascular (Virchow-Robin) spaces at the level of the right basal ganglia was present. Because of the extensive nature of the disease, the patient decided to receive only supportive therapy.
Atypical meningiomas are more aggressive and invasive than typical meningiomas. Atypical meningiomas may be plaquelike in configuration and extend through the dura to the skull and scalp. The term “meningiomatosis” refers to a completely different disorder. Meningiomatosis is an entity in which a primary sarcoma (similar to a fibrosarcoma) diffusely involves the meninges [2
]. In meningiomatosis, no dominant mass is found. The imaging findings of this disease are nonspecific, and the differential diagnosis includes diffuse meningioma, lymphoma, leukemia, sarcoidosis, metastases, and primary meningeal gliomatosis (and oliodendrogliomatosis). The perivascular spaces are cuffs of arachnoid accompanying blood vessels entering or leaving the brain [3
]. These spaces are separated from the brain parenchyma by the pia-glia that is considered the external limiting membrane. The perivascular spaces thus contain subarachnoid space (and cerebrospinal fluid), a central blood vessel, and the external limiting membrane. As a blood vessel travels deeper, the pia, intima, and arachnoid fuse and become one membrane without any potential space. At this level, astrocytes surround the basement membrane of the capillary endothelium. Although the function of the perivascular spaces is not certain, they allow cerebrospinal fluid to travel into the brain and may play a role in its absorption. The perivascular spaces may serve as a conduit for the spread of infectious (particularly cryptococcus), inflammatory (sarcoid), and neoplastic (glioblastoma multiforme metastases) processes. Although this is the first case of a meningioma spreading into the perivascular spaces that we have witnessed, this experience is not surprising because these tumors arise from arachnoidal cells and tend to extend along meningeal surfaces. Atypical meningiomas are also known to invade the brain, and perhaps it is via the perivascular spaces that this invasion first occurs as seen on our patient.