43 years-old woman with hyperthyroidism and long data of exopthalmus. The woman presents progressive visual loss in the left eye and proptosis, without diplopia or eye movements’ limitation. A CT is performed.
The CT shows an isoattenuated intraconal orbital mass surrounding the left optic nerve with punctuate calcification (Fig. 1a). Following administration of contrast the mass has homogeneous enhancement (Fig. 1b), surrounding the non-enhancing optic nerve, results “tram-track sign” (Fig. 1b, arrow) on axial images or “dot sign” on coronal (Fig. 1c). Bone window (Fig. 1d) does not reveal hyperostosis of the adjacent bone or other pathological findings.
MRI is better to delineate posterior extension. T1 weighted sequence shows an isointense to hypointense mass compared to the optic nerve (Fig. 2a), T2 weighted image shows isointense to somewhat hyperintense compared to the optic nerve (Fig. 2b). Fat –supressed T2 shows a hyperintense mass with “perioptic cysts” (Fig. 2c arrow). Diffusion weighted image shows a moderate diffusion restriction (Fig. 2d). On fat-supressed T1 and after contrast administration, the mass has homogeneous enhancement (Fig. 3a, 3b, 3c) and we can see an extension of the mass through the optic canal (Fig. 3b, arrow).
Primary orbital meningioma is a rare tumour of the anterior visual pathway and constitutes approximately 2% of all orbital tumours and 1-2% of all meningiomas. The differentiation from secondary orbital meningioma of intracranial origin is sometimes difficult on image, but there are some keys imaging findings to suspect them (1).
Optic nerve sheath meningioma (ONSM) is typically unilateral but can be bilateral, particularly in the setting of type II neurofibromatosis (2).
The typical clinical presentation is painless, gradual vision loss and proptosis in a woman between ages 30 and 50 (3).
Radiology is essential for a differential diagnosis of the intraconal orbital mass and to determine the extent and tissue involved.
The key imaging finding of ONSM is a homogeneously enhancing mass that surrounds the optic nerve. MR is preferred imaging modality for tumor assessment, but CT is useful for looking for calcifications. CT images show a mass with uniform and moderately enhancement and punctate calcifications (4). “Tram track sign” is characteristic of ONSM. It refers to the parallel thickening and enhancement around the optic nerve, but is not pathognomonic because pseudotumor, lymphoma or sarcoid may show this feature (5). However, this sign is essential for the differential diagnosis with optic pathway glioma which enlarged optic nerve (Fig.4a, b, c)
Tumour extending into the optic canal may lead to canal widening, or alternatively hyperostosis or erosion of the adjacent bone (6).
MRI is preferred imaging modality for tumour assessment and defining better the involve of orbital apex, optic canal and intracranial structures. T1 weighted images show an isointense mass to other soft tissue. T2- weighted images show a hyperintense mass with “perioptic cysts”,as in the case on point. This feature is specific and is defined as CSF increase within nerve sheath surrounding distal optic nerve between tumor and globel (7,8). This sign is not seen in other entities such as lymphoma (Fig.5a,b, c,d,e,f,g) or sarcoidosis. ADC value could be useful to distinguish meningioma from benign processes where the ADC value is higher. Nevertheless, in other entities such as lymphoma, there is an overlap in the ADC value of ONSM and lymphoma, so it is not useful for differentiate these pathologies. ONSM often shows restricted diffusion because their hypercellular nature, as well as lymphoma (9, 10). T1 weighted sequence after contrast shows homogeneous enhancement of the mass and is useful for delineating the extent of the tumour and for evaluating for intracranial extension.
Treatment of these lesions depends on the degree of visual impairment and proptosis. In patients with preserved vision and no proptosis, conservative management with frequent ophthalmologic and radiological follow up is usually preferred.
Fractionates stereotactic radiotherapy is considered the first line therapy for patients with preservable vision but progressive impairment (11). When the patient has severe proptosis or evidence of posterior growth, surgery is required, but almost invariably results in loss of vision (12).
Although rare, ONSM is the second-most common primary optic nerve tumour and could cause vision loss so early diagnosis is important.
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