Patient presented with headaches and facial pain with paresthesias.
There is evidence of a well-marginated, lobulated, approximately 1.9 x 2.9 x 1.7 cms sized extra-axial, intermediate signal intensity mass lesion on the T1W images in the prepontine cistern on the right extending into the Meckel’s cave on the right, along the course of the right trigeminal nerve. This lesion appears hyperintense on the T2W and STIR images. Mild scalloping of the tip of the right petrous temporal bone is noted as also is mild indentation along the antero-lateral margin of the pons on the right. Also seen is slight extension of this lesion into the proximal right cavernous sinus. The right seventh and eighth cranial nerve complex is located just inferior and posterior to the lesion.
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 the trigeminal ganglion and trigeminal nerve usually cause progressive facial numbness, pain and paresthesias. Some may attain considerable size in the absence of facial pain or numbness because they arise from the nerve sheaths and secondarily compress the nerve fibers. Schwannomas involving the trigeminal nerve are divided into three anatomical divisions - preganglionic, ganglionic and postganglionic. Lesions that originate from the trigeminal ganglion typically straddle the incisura, with a component extending into the cavernous sinus (preganglionic) and another portion extending into the prepontine cistern (postganglionic).
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.
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