||Histologically, an acoustic neuroma shows the features of a benign neoplastic proliferation of Schwann cells in a collagenous matrix. On the basis of histologic features Antoni (1920) divided acoustic neuromas into two different microscopic types: Antoni type A and Antoni type B tissue.
In Antoni type A, the cellular elements of the tumor are compactly arranged in merging and diverging streams of spindle-shaped cells. Each fusiform Schwann cell has an abundant eosinophilic cytoplasm, although the cell margins cannot be distinguished in H&E-stained sections. The long, oval, spindle-shaped nuclei of adjacent cells are often aligned in columns parallel to one another (palisading of the nuclei). Occasionally there are zones where the cells are arranged in palisades at either end of a bundle of parallel fibers; this arrangement is called a Verocay body.
In Antoni type B tissue there is evidence of degeneration and necrosis with a heterogenous cellular mixture of myxoid elements interspersed with tumor cells and groups of lipid-laden foam cells. The areas of foam cells often impart a yellowish discoloration to the gross specimen. Electron microscopic examination of acoustic neuromas characteristically shows the presence of bundles of broad-banded collagen known as Luse bodies.
In rare cases a malignant schwannoma may be encountered. Malignant schwannomas represent approximately 10% of all malignant nerve sheath tumors. Patients with von Recklinghausen's disease have a 20 to 30% risk of developing a malignant schwannoma, and consequently any rapidly expanding mass in these patients should be suspected.
A malignant schwannoma usually presents as a painless expanding mass spreading along nerves and fascial planes. Hematogenous metastasis can also occur.
Histologically, these tumors contain plump spindle cells which display varying degrees of pleomorphism. The cells are arranged in streams or cords, with some conspicuous mitoses.
Electron microscopy can be used to differentiate a tumor of neuroectodermal origin from one of mesodermal origin by demonstrating the characteristic investing basement membrane of Schwann cells. Immunoperoxidase-staining for the neural crest marker S-100 protein is another valuable technique for the identification of tumors of neural origin.
Treatment consists of surgical removal, usually by the translabyrinthine or suboccipital approach. Earlier diagnosis of smaller tumors combined with meticulous micro, surgical techniques have led to frequent preservation of facial function, and occasional salvage of useful hearing.