Vestibular Schwannoma

= ACOUSTIC NEUROMA = ACOUSTIC SCHWANNOMA = NEURILEMMOMA

[Theodor Ambrose Hubert Schwann (1810–1882), German anatomist and physiologist, professor of physiology and comparative anatomy in Louvain and at the State University of Liège, Belgium]

• Most common neoplasm of internal auditory canal / cerebellopontine angle!

Prevalence: 6–10% of all intracranial tumors; 85% of all intracranial neuromas; 60–90% of all cerebellopontine angle tumors

Age:

1. sporadic tumor: 35–60 years; M÷F = 1÷2
2. type 2 neurofibromatosis: 2^nd decade

Histo:

encapsulated neoplasm composed of proliferating fusiform Schwann cells with

1. highly cellular dense regions (Antoni A) with reticulin + collagen, and
2. loose areas with widely separated cells (Antoni B) in a reticulated myxoid matrix; common degenerative changes with cyst formation, vascular features, lipid-laden foam cells

May be associated with: central neurofibromatosis (NF2)

• Solitary intracranial schwannoma is associated with type 2 neurofibromatosis in 5–25%!
• Bilateral acoustic schwannomas allow a presumptive diagnosis of type 2 neurofibromatosis!
• long history of slowly progressive unilateral sensorineural hearing loss mostly for high-frequency sounds (in 95%)
• tinnitus, pain; diminished corneal reflex
• unsteadiness, vertigo, ataxia, dizziness (<10%)

Doubling time: 2 years

Location:

1. arises from within internal auditory canal (IAC) in 80% / cochlea
2. may arise in cerebellopontine angle cistern at opening of IAC (= porus acusticus) with intracanalicular extension in 5%

Site:

1. in 85% from the vestibular portion of 8^th nerve (around vestibular ganglion of Scarpa / at the glial-Schwann cell junction) posterior to cochlear portion
   [Antonio Scarpa (1752–1832), professor of anatomy at the Università di Pavia, Lombardia, Italy]
2. in 15% from the cochlear portion

• round mass centered on long axis of porus acusticus forming acute angles with dural surface of petrous bone
• funnel-shaped component extending into IAC
• IAC enlargement / erosion (70–90%)
• widening / obliteration of ipsilateral cerebellopontine angle cistern
• shift / asymmetry of 4^th ventricle with hydrocephalus
• degenerative changes (cystic areas and hemorrhage) with tumors >2–3 cm

Plain film:

• flaring porus acusticus
• erosion of IAC: a difference in canal heights of >2 mm is abnormal + indicates a schwannoma in 93%

CT:

• small isodense / large hypodense solid tumor
• cyst formation in tumor (= central necrosis in 15% of large tumors) / coexistent extramural arachnoid cyst adjacent to tumor
• usually uniformly dense tumor enhancement with small tumors (50% may be missed without CECT) / ring enhancement with large tumors
• NO calcification
• intrathecal contrast / carbon dioxide insufflation (for tumors <5 mm)

MR (most sensitive test with Gd-DTPA enhancement):

• iso- / slightly hypointense relative to gray matter on T1WI
• intensely enhancing homogeneous mass / ringlike enhancement (if cystic) after Gd-DTPA
• hyperintense on T2WI relative to gray matter (DDx: meningioma remains hypo- / isointense)

Angio:

• elevation + posterior displacement of anterior inferior cerebellar artery (AICA) on basal view
• elevation of the superior cerebellar artery (large tumors)
• displacement of basilar artery anteriorly / posteriorly and to contralateral side
• compression / posterior and lateral displacement of petrosal vein
• posterior displacement of choroid point of PICA
• vascular supply frequently from ECA branches
• rarely hypervascular tumor with tumor blush

DDx: ossifying hemangioma (bony spiculations)