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Orbital schwannoma


Key points

  • Definition: Usually found in middle-aged adults, orbital schwannomas (OS) are well-defined, encapsulated, slowly growing benign tumors arising from peripheral nerve Schwann cells, and comprise 1% to 2% of orbital tumors.

  • Classic clue: Middle-aged patient presenting with optic neuropathy, proptosis, or diplopia with a well- circumscribed retrobulbar intraorbital mass separate from the optic nerve (ON) which on dynamic magnetic resonance imaging (MRI) shows enhancement progressing from the lesion’s periphery to its center.

  • OSs are rarely seen in children.

  • Because of their occasional occurrence and scarce unique signs, the preoperative diagnosis of OS is somewhat difficult.

Imaging

General imaging features

  • Well-circumscribed, encapsulated mass.

  • The specific shape of an OS is probably not helpful in an individual case. OSs may occur in several shapes (in decreasing frequency): cone, dumbbell, oval, round, polylobular, fusiform, sausage, and irregular shapes.

  • Orbital locations included superior, medial superior, orbital apex, lateral, lateral inferior, and medial, but not lateral superior in one series of 62 cases.

Computed tomography features

  • Most OSs show mild enhancement on contrasted CT (see Figure 16-1 , D ).

    FIGURE 16-1 ■, A, B, Coronal CT shows a large heterogeneous left intraconal mass. C, Axial nonenhanced CT shows a large oval heterogeneous intraconal mass extending from the posterior globe to the orbital apex causing exopthalmos. D, Axial enhanced CT shows a large mildly enhancing heterogeneous oval intraconal mass extending from the posterior globe to the orbital apex. E, F, Axial T1 Gd enhanced CT shows a large isointense mass with areas of avid “ring” enhancement extending from the posterior globe to the orbital apex causing exopthalmos.

  • Enlargement of the superior orbital fissure or invasion of the cavernous sinus may be apparent.

  • See General Imaging Features earlier.

Magnetic resonance imaging features

  • The MRI appearance of OSs varies with histology. ,

  • This probably varies with the mix of Antoni-A and Antoni-B cells.

  • T1 is generally described as low (and is relatively nonspecific showing isointensity with small patches of hypointensity). ,

  • T2 is generally described as high. ,

  • T1 Gd enhancement patterns correlate with T2 patterns.

  • T1 Gd enhancement can cause three general patterns:

    • Heterogeneous enhancement in approximately half of cases. ,

    • Homogeneous enhancement in approximately one-quarter of cases. ,

    • Ring enhancement pattern in approximately one-quarter of cases.

      • Ring enhancement may occur as a ring around the periphery of the tumor or as smaller rings inside the tumor (see Figure 16-1 , E and F ).

  • On dynamic MRI some investigators reported “progressive” enhancement of an OS, which begins peripherally and progresses to the center.

  • Other investigators reported no “progressive” enhancement pattern in OS.

  • The difference between these two studies may be related to technique, as the imaging of the reported cases of progressive enhancement is quite convincing. When present this “progressive” enhancement can be very dramatic.

    • OS may undergo central necrosis and be confused with a cystic orbital tumor.

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