Acoustic neuroma pathophysiology

Acoustic neuroma arises from Schwann cells, which are the cells involved in the conduction of nervous impulses along axons, nerve development and regeneration. On microscopic histopathological analysis, acoustic neuroma may display two types of growth patterns: Antoni type A and Antoni type B. Antoni type A growth pattern is composed of elongated cells with cytoplasmic processes arranged in fascicles, little stromal matrix and verocay bodies. Antoni type B growth pattern is composed of loose meshwork of cells, less dense cellular matrix, microcysts and myxoid change.

• Acoustic neuromas are benign tumors (WHO grade 1), usually arising from the intracanalicular segment of the vestibular portion of the vestibulocochlear nerve (CN VIII), near the transition point between glial and Schwann cells (Obersteiner-Redlich zone).
• An acoustic neuroma arises from a type of cell known as the Schwann cell. These cells form an insulating layer over all nerves of the peripheral nervous system (i.e., nerves outside of the central nervous system) including the eighth cranial nerve.
• Most acoustic neuromas are found along the vestibular portion of the eighth cranial nerve.
• As these tumors are made up of Schwann cells, and usually located along the vestibular portion of the eighth cranial nerve, many physicians prefer to use the term, “vestibular schwannoma“. However, the term acoustic neuroma is still used more often in the medical literature.^[1]
• Acoustic neuromas are well circumscribed encapsulated masses, which unlike neuromas, arise from but are separate from nerve fibers.

• One the most common causes of acoustic neuroma is neurofibromatosis type 2 (NF2), an autosomal dominant disease caused by loss of function mutation.
• Genetic studies have linked both sporadic and NF2-associated acoustic neuromas to a single gene, the NF2 gene, located on chromosome 22 band q11–13.1.

• Acoustic neuroma is strongly associated with neurofibromatosis type 2 (NF2).^[2]

On gross pathology, following are the characteristic findings of acoustic neuroma:

• Rubbery-firm with a pale, gray color^[3]
• Well-defined capsule
• Different degrees of vascularity

• Pale gray and firm^[4]
• Fine trabeculated appearance
• Cystic degeneration
• Hemorrhage
• Calcification
• Xanthomatous changes

• The tumor is made up of spindle cells with elongated nuclei and fibrillary cytoplasm.^[5]
• The spindle cells are arranged in two ways:

1. Antoni A

• Antoni A tissue is small with organized and interwoven course of elongated bipolar cells.^[6]
• The spiral framework, formed by the arrangement of the nuclei and fibers, can resemble a meningioma.
• Verocay bodies can also be seen.^[7]

2. Antoni B

• It is represented by a random grouping of cells around foci of necrosis, cystic change, hemorrhage, and blood vessels.^[8]
• This tissue can also have a variable amount of lymphocytic infiltration.

The following findings on electron microscopy are characteristic of an acoustic neuroma:

• Characteristic basement membrane of the schwann cells.^[9][10]
• Wide-spaced collagen.

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