S.BELABBES, N.ELYOUSFI, S.CHAOUIR, T.AMIL, A.HANINE, H.EN-NOUALI Department of Radiology, Military Teaching Hospital Mohammed V of Rabat. Morocco NR31

 rare, representing 1,8 to 3, 2 ​​ % of intracranial meningiomas.  Among the meningiomas of the posterior fossa, foramen magnum (FM) meningiomas deserve special consideration because of their characteristics in symptomatology, and complications  They are causing a high risk of spinal cord compression.  Several classifications, with a surgical interest, have tried to categorize them according to dural attachment, which underscores the value of MRI

 A 33-year-old female presented with mild headache lasting for a year, neurological examination revealed paresthesia in upper limbs. A CT brain scan and an MRI were performed

 CT showed a process in the level of the foramen magnum spontaneously isodense that enhances after injection of contrast. CEREBRAL CT C+: large tumor occupies slightly more than half of the transverse diameter of the foramen magnum. the rostral spinal cord is compressed

 MRI objectified a process with broad-base dural implantation at the expense of the clivus, in isosignal T1 and hypersignal T2, enhanced after injection of contrast. This process drove back the spinal cord behind, coming in contact with the vertebral artery which is not narrowed. The patient was operated and anatomopathological examination found a meningioma of the foramen magnum

MRI SAGITAL SECTION MRI T1:a large anterior foramen magnum meningioma isointense to surrounding brain severely compresses the neuraxis MRI T2: pocess hyperintense to surrounding brain

homogeneously enhancing tumor arises predominantly in an anterior location

 Meningiomas are common neoplasms representing 14.3 to 19% of all intracranial tumors.  Slowgrowing benign tumors arising at any location where arachnoid cells reside.  Among all the meningiomas, only 1.8 to 3.2% arises at the foramen magnum (FM).

Definition

 The indolent development at the craniospinal junction makes clinical diagnosis complex and often leads to a long interval between onset of symptoms and diagnosis.  Clinical presentation of the FM lesions may be in form of neck pain, dysasthesiasis in the upper limbs, quadriparesis or quadriplegia, cruciate hemiparesis, impaired pain and touch sensations and occasionally pseudoathetoid movements of the hands.  Classic foramen magnum syndrome is defined by development of unilateral arm sensory and motor deficits, which progress to the ipsilateral leg, then the contralateral leg, and finally contralateral upper extremity. Clinical diagnosis

 FM meningiomas arise from arachnoid at the craniospinal junction.  The borders of this zone, range anteriorly from the lower third of the clivus, to upper margin of the body of C-2, laterally from the jugular tubercle to the upper margin of the C-2 laminae, and posteriorly from the anterior edge of the squamous occipital bone to the C-2 spinous process. Classification

 the classification of these lesions is based on their size relative to that of the foramen magnum:  small, lower than one third the transverse dimension of the foramen magnum  medium, one third to one half its dimension  large, superior with one half  Most lesions arise anterolaterally  Posterolateral origin is the second most frequent,  Purely posterior lesions the third  The least common are entirely anterior. Classification

 Neuroimaging confirms the clinical diagnosis and allows the planning of a surgical approach.  The diagnosis of FMM is essentially based on morphological criteria.  It is extra-axial with a large insertion base and obtuse angle connection.  wider than thick.  The reaction in the vicinity of bone insertion area is less than Supratentorial findings, but has a high diagnostic value when it exists in the form of bone erosion or hyperostosis Neuroimaging

 the decreasing Thickening of the dura and the contrast enhancement "comet tail" adjacent to the meningioma is highly suggestive of meningioma, but not specific, it is met in 59% to 71% of cases.  Magnetic resonance imaging:  Modality of choice for defining tumors of the foramen magnum.  provides high-resolution images of soft-tissue anatomy that is not susceptible to degradation by the surrounding skull base, a pitfall of CT scanning.  On T1-weighted image: meningiomas may appear isointense, mildly hypointense, or hyperintense to surrounding brain. Neuroimaging

 On T2-weighted image: isointense to slightly hyperintense compared with brain  The T1-weighted enhanced contrast imaging shows the dural attachment site of the tumor and it provides ready discrimination between tumor and brainstem  Once the diagnosis of meningioma evoked, MRI should focus on:  locate the tumor in the axial plane at the foramen magnum: anterior, lateral or posterior  define the compartment where it develops: intra dural or extradural or both (in most cases is intradural)  clarify its relation to the adjacent vertebral artery which can be invaded by the meningioma Neuroimaging

 Other tumors such as neuromas and metastasis  vascular lesions such as vertebro basilar aneurysm  benign tumors of the clivus especially meningiomas, and tumors of the jugular foramen extending to the foramen magnum  Retro clival meningioma that is not always easy to differentiate of the foramen magnum meningioma Diagnosis differential

 the Foramen magnum meningioma is an uncommon intracranial tumor that presents a particular gravity because of the risk of bulbo-medullary compression.  CT and MRI comprise the essential of current meningioma imaging, and the diagnostic information provided by these modalities is complementary.  MRI provides excellent soft tissue resolution, while CT far superiorly demonstrates the bone changes.  The relationship of the FMM with vertebral artery and the lower cranial nerves is important, and must be mentioned.

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