Patient presented with headaches, right orbital pain, slight decrease in vision and diplopia,
There is evidence of a well-defined, lesion, measuring approximately 2.1 x 2.1 x 2.2 cms sized, and of intermediate signal intensity on the T1W images in the superior aspect of the right orbit. This lesion appears heterogeneously hyperintense on the T2W and STIR images with hypointense areas within. There is slight inferior displacement of the optic nerve.
Schwannomas are benign tumors that arise from Schwann cells that surround the cranial and peripheral nerves and sympathetic plexus. Histologically a schwannoma contains a cellular component (formed from the Antoni-A cells) and a myxoid component (formed from the Antoni-B cells). The presence of these components and the absence of nerve fibers in the body of the tumor allow differentiation of a schwannoma from a neurofibroma. A neurofibroma contains neural elements with a fibrous core and may arise from cranial, spinal, or peripheral nerves. Most neurofibromas are seen in young adults. Approximately 10% of patients with neurofibromas have neurofibromatosis type I (NF-1 or von Recklinghausen's disease). Two types of neurofibroma are known to be associated with NF-1. Multiple or single solid lesions may be seen along the cranial, spinal, or peripheral nerves. Another type of lesion presents as an infiltrating soft tissue mass with extension along the cranial nerves through the orbits, skull base, or posterior fossa. This so-called plexiform neurofibroma encases the muscles and soft tissues along the course of the nerves.
Schwannomas of cranial nerves III, IV, and VI may be seen along the cisternal, cavernous, or intraorbital course of these nerves. Symptoms include diplopia, ophthalmoplegia, and proptosis, depending on the site of greatest mass effect. These tumors may be visualized on MRI as they course through the cranial canals. Bulbous expansion of the canal at the site of a lesion is considered pathognomonic; however, differentiation of schwannomas from various malignancies extending along nerve roots may be difficult.
MRI:
The MR appearance of all schwannomas is similar. The key differentiating point is the location of the central portion of the tumor within or near the nerve of origin. Schwannomas are fairly fusiform in appearance and well circumscribed because of their capsule. There may be smooth and uniform enlargement of the cranial canal through which the nerve traverses. Smaller tumors usually appear homogeneous because they are predominantly of the Antoni-A cell type, whereas larger tumors demonstrate heterogeneity and variable signal intensities because of the presence of hemorrhage, necrosis and cystic degeneration. Larger tumors typically contain primarily Antoni-B cells. These can have foci of high and low signal intensities on both T1W and T2W images. The presence of intramural cysts characteristically produces foci of high signal intensity on the T2W images, which may be caused by necrotic material, blood, or colloid-rich fluid. Extramural cysts show high signal intensity related to higher protein, colloid contents secreted by the tumor, or both. Extramural cysts are thought to originate from peritumoral adhesions, which lead to a trapping effect of fluid between the leptomeninges and the schwannoma.
Enhancement of schwannomas after gadolinium contrast administration is variable, although it is typically rapid because of extravasation of contrast into the extracellular compartment of the often-vascular schwannoma. There is a sharp demarcation between the margin of the tumor and the adjacent structures. Enhancement of the cranial nerves or their ganglia without an associated soft-tissue mass is consistent with neuritis or ganglionitis rather than a neoplasm. Sarcomas, hemangiomas, or paragangliomas may have similar enhancement characteristics and should be considered in the differential diagnosis of schwannomas.
The MR characteristics of neurofibromas are quite varied and usually simulate schwannomas. Neurofibromas project an intermediate signal intensity on the T1W and proton density images. They sometimes have a "salt and pepper" appearance whenever there is intense vascularity within the stroma of the tumor. The T2W images are variable, depending on whether the tumor is cystic or solid in nature. Variable enhancement is seen after gadolinium administration because of the presence of cystic areas, calcifications, or vascularity. The margins of a neurofibroma appear less distinct than the margins of a schwannoma because neurofibromas lack a capsule. It may be impossible to differentiate a neurofibroma from a schwannoma on the basis of MR characteristics.
Differential Diagnosis - Orbital Tumors:
Meningioma:
Commonly arise from the optic nerve sheath or from the from orbital wall
periosteum. Bilateral perioptic meningiomas may be associated with
neurofibromatosis. These lesions are frequently calcified and usually enhance
well. Secondary bone changes (canal widening, bone disruption, hyperostosis)
from orbital meningiomas may be seen. Variable signal intensity patterns may be
seen.
Optic Nerve Gliomas:
Orbital Metastases:
Cavernous Hemangiomas:
Phleboliths and calcification are seen. They are soft lesions.
Lymphangiomas:
They have a propensity for intermittent hemorrhage and tend to infiltrate
adjacent soft tissues.
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