Left spheno-orbital en plaque meningioma revealed by progressive exophthalmos: imaging features and a compartment-based approach to spheno-orbital hyperostosis

En plaque meningioma is an uncommon variant of meningioma (2–9% of cases), defined by a thin carpet-like sheet of tumor spreading along the dura together with bone hyperostosis that is disproportionate to the intradural component. The spheno-orbital region is the site of election, and because the dominant abnormality on imaging is a sclerotic bone lesion rather than a mass, the diagnosis is frequently delayed or mistaken for fibrous dysplasia. We report the case of a 75-year-old woman with no history of malignancy, cranio-orbital trauma, or infection, who presented with slowly progressive left orbital swelling, unilateral exophthalmos, and ipsilateral visual loss. Magnetic resonance imaging showed a left spheno-orbital lesion with broad-based dural thickening, isointense to cortex on T1-weighted images, iso- to slightly hyperintense on T2/FLAIR, with intense and relatively homogeneous enhancement after gadolinium, exerting mass effect on the orbital contents without adjacent parenchymal signal abnormality. Computed tomography demonstrated marked hyperostosis along the left greater sphenoid wing with mild narrowing of the ipsilateral optic canal. The association of a broad-based enhancing dural sheet with disproportionate hyperostosis was highly suggestive of an en plaque meningioma of dural origin. Histopathologic examination of the resected specimen confirmed a meningothelial meningioma, CNS WHO grade 1, with bone invasion. This case illustrates a compartment-based reading of spheno-orbital hyperostosis (orbital, osseous, or meningeal) and the value of imaging–pathology correlation: bone invasion was histologically confirmed in this patient, although meningioma-associated hyperostosis does not necessarily reflect direct neoplastic invasion in every case.