Vestibular schwannoma:
- Background
- Vestibular schwannomas (also known as acoustic
schwannomas) are benign intracranial extra-axial tumors
that arise from Schwann cell sheaths that invest the
vestibular or cochlear nerve in cerebellopontine
angle-internal auditory canal (CPA-IAC). Acoustic neuromas
account for approximately 80% of tumors found within the
CPA. The remaining 20% are principally meningiomas. In rare
cases, a facial nerve neuroma, vascular tumor, lipoma, or
metastatic lesion is found within the cerebellopontine
angle.
- Acoustic tumors, like other space-occupying lesions,
produce symptoms by any of 4 recognizable mechanisms: (1)
compression or distortion of the spinal fluid spaces, (2)
displacement of the brain stem, (3) compression of vessels
producing venous or arterial infarction, or (4) compression
and/or attenuation of nerves.
- Because the CPA is relatively empty, tumors can continue
to grow until they reach 3-4 cm in size before they come in
contact with important structures. Growth is often
sufficiently slow that the facial nerve can accommodate to
the stretching imposed by tumor growth without clinically
apparent deterioration of function. Tumors that arise within
the IAC may produce relatively early symptomatology in the
form of unilateral sensorineural hearing loss or vestibular
disturbance by compressing the cochlear nerve, vestibular
nerve, or labyrinthine artery against the bony walls of the
internal auditory canal.
- Imaging findings
- MRI
- Gold standard for diagnosis is gadolinium enhanced
T2 MRI of the brain and CPA-IAC.
- MRI can detect masses as small at 2 mm.
- T1WI: lesion will appear as an intermediate signal
most commonly; a high signal foci will be present if
there is a hemorrhagic lesion (which is rare).
- T2WI: lesion will appear as a "filling defect" in
high signal CSF of CPA-IAC cistern. Small lesions will
appear as ovoid filling defect, where as larger lesions
have an "ice cream on cone" shaped filling defect.
- T1 C+: Focal, enhancing mass of CPA-IAC cistern.
- Other MR findings: 0.5% associated arachnoid cyst.
- CT with contrast
- Well-delineated, enhancing mass of CPA-IAC cistern
with no calcification.
- Smaller intracanalicular lesions < 6 mm may be
missed.
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