31 The optic nerve is unique among cranial nerves, in that it is a projection of the midbrain, myelinated by oligodendroglial cells. Its orbital and intracanalicular components are invested by a nerve sheath continuous with the intracranial meninges that delineates it from the intraconal fat. As such, the unit of the optic nerve and sheath share common pathologic processes, which can be related to intraconal or intracranial pathologies. Most meningiomas occurring in the orbit are primary optic meningiomas, which develop along the intraorbital or intracanalicular segments of the optic nerve, whereas secondary lesions originate along the intracranial dura, and spread into the orbit along the sheath. There are three patterns of growth: uniform tubular enlargement of the nerve sheath complex, fusiform spindle-shaped expansion, and excrescent outgrowth (Table 31.1). Although lacking specificity, the “tram track sign,” where a hypointense/hypodense, and often atrophic optic nerve is demonstrated within a hyperintense/hyperdense/enhancing sheath, is the classic way to establish pathology that primarily involves the optic nerve sheath. It has been described for unenhanced computed tomography (CT), where the sheath may calcify, as well as enhanced CT and magnetic resonance imaging (MRI), where the sheath enhances preferentially. Mild enhancement around the periphery of a non-enlarged nerve sheath complex can be normal. The most common cause of the “tram track sign” is meningioma. Very rarely, schwannomas can occur involving the optic nerve sheath complex.
Optic Nerve-Sheath Complex Lesions
Meningiomas
Optic Gliomas
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