Gross Pathology

An angle is defined geometrically as the intersection of two lines. This definition inadequately depicts the three-dimensional characteristics of the physical union of the cerebellum with the pons. Alternatively, the dictionary provides the definition, "a secluded place, a nook." Although this description appropriately depicts the cloistered nature of this anatomic region, it does not truly portray its dimensional configuration, for the ventral aspect of the cerebellum adjoins the pons with the conformation of an elongated cleavage, a furrow, bounded by two hemispheric structures. Cranial nerves V and VII to XI cross this furrow as strands of a necklace.

The native structures of the cerebellopontine angle are the source of two expressions of neoplasia, schwannoma and meningioma, as well as one space-occupying lesion, the arachnoid cyst. The extraneous products of embryonic development contribute alien tissue to the cerebello­pontine angle, from whence the origin of dermoid and epidermoid cysts. Anatomic structures that are contiguous to the cerebellopontine angle also spawn a variety of neoplasms that impinge upon, disfigure and occasionally occupy the angle. Notable among these neoplasms are the pontine and cerebellar gliomas, cerebellar hemangioblastoma, papilloma and carcinoma of the choroid plexus, ependymoma and a variety of lesions nascent in the base of the skull. Among the latter are chemodectoma or glomus jugulare tumor, chordoma, metastasis and nasopharyngeal carcinoma (Schmincke's tumor).

The present discussion concerns a common lesion of the eighth cranial nerve, an entity with many names: acoustic neuroma, neurinoma, schwannoma, neurilemoma and perineural fibroblastoma. The designations acoustic neuroma or neurinoma and vestibular schwannoma enjoy common clinical usage. The penchant of this schwann cell lesion for the acoustic nerve makes it appropriate to individualize the entity in its anatomic relation to the cerebellopontine angle.

Certain structural peculiarities of the acoustic nerve ostensibly predispose to neoplastic transformation, for unlike other cranial and spinal nerves, glia rather than schwann cells frequent its proximal 8- to 12-mm segment. Only upon entry through the porus acusticus does the nerve acquire a vestment of Schwann cells and assume the true characteristics of a peripheral nerve. It is principally at this interface, between a stroma of oligodendroglia and Schwann cells, that neoplastic transformation occurs. Interestingly, the propensity for neoplasia is also evidenced at a comparable location in fetal rats exposed transplacentally to nitrosourea.

In view of this unique structural composition, a review of embryonic development seems in order. The eighth nerves are derived bilaterally from the lateral margins of the primitive rhombencephalon, from paired groups of cells known as the acousticofacial ganglia, which lie medial and ventral to the auditory vesicles. The cells of the vestibular ganglia are the first to extend fibers toward the auditory vesicles. Shortly thereafter there is an outgrowth of fibers from the cochlear ganglion. This egress of fibers becomes associated with primitive Schwann cells: however, the fibers grow rapidly and glia are drawn from the rhombencephalon into the proximal segment of the nerve. The human adult acoustic nerve is approximately 18 mm in length. Its proximal 8 to 12 mm is endowed with neuroglia, while the distal segment is sustained by Schwann cells. Within this distal portion, Schwann cells and fibroblasts elaborate an endoneurium, epineurium and perineurium after the manner of a true peripheral nerve. The interface of neuroglia and Schwann cells is usually positioned more distally in the vestibular than in the cochlear division and the transition zone in the former is generally marked by a greater degree of intermingling of the two cell types, usually with regional overproduction of Schwann cells.

Characteristically, acoustic schwannomas arise in the vestibular division, predictably in that short segment of passage through the porus acusticus or the internal auditory meatus. The youthful neoplasm compresses the cochlear division and may also obstruct the labyrinthine blood vessels that supply the organ of Corti and the vestibular end organs. Although the origin of the tumor is typically within the vestibular division, the earliest symptoms generally do not reflect this localization but rather the compression of the auditory component. Tinnitus is the most common initial symptom and in approximately 25 percent of cases, is forthwith accompanied by vertigo.

As the neoplasm expands. it erodes the porus acusticus. Escaping confinement, the fleshy tissue protrudes from the bony canal and enters the cerebellopontine angle. This compartment is bounded superiorly and ventrally by the tentorium cerebelli and the petrous pyramid, dorsally by the cerebellum and medially by the brain stem. Initially, the pliable tissue remains closely applied to the petrous pyramid and its ventral surface acquires the mosaic irregularities of this bony ridge. By contrast, the vertex of the mass encounters only the soft tissues of the cerebellum and pons and is therefore characteristically dome shaped, smoothly contoured, or slightly bosselated. The color is variegated with gray, yellow, and red, in accordance with the areas of dense cellularity, regions of xanthomatous degeneration and degrees of vascularity. The yellow areas are generally soft and reflect a rich content of lipid. Their color and consistency contrast with the firmer gray tissues, which histologically are dense with Schwann cells.

The mass may attain dimensions of 3 to 6 cm before its clinical detection. As this size is approached, the neoplasm contacts and then displaces cranial nerves. The facial nerve is in the forefront and at the time of surgical excision is generally stretched across the ventral dome of the tumor. In a usual sequence, the fifth nerve is then contacted as it exits from the lateral aspect of the pons. Thereafter, caudal extension of the tumor brings it into contact with the ninth and tenth nerves and occasionally the eleventh. Medial growth disfigures and displaces the lateral aspects of the pons and, to a lesser degree, the medulla. The mass may then incorporate major blood vessels, notably the basilar and vertebral arteries or their branches. This medial extension of the mass may bring it in contact with the sixth cranial nerve. Growth carries the edge of the tumor against the sigmoid sinus and, on occasion, directs it into the jugular foramen.

The expanding lesion within the posterior fossa thrusts the cerebellum downward and imprints the bony ridge of the foramen magnum against the inferior aspect of the cerebellum. Herniation of the cerebellar tonsils is imminent. The increased pressure in the posterior fossa and the disfigurement of the foramina of Luschka and of Magendie dispose to obstruction of cerebrospinal fluid (CSF) flow and the development of hydrocephalus.

As the intruder enters the cerebellopontine angle, it acquires a vestment of arachnoid. This delicate membrane may remain distinct while becoming fibrotic, but more often merges with the fibroblastic tumor and imparts an appearance of encapsulation. On occasion a vestment of arachnoid about the tumor creates a cul-de­sac that fills with clear fluid, presumably CSF. Such cysts may add significantly to the dimensions of the mass and augment compression.

Acoustic schwannomas also seek accommodation within the internal auditory canal and may penetrate into the inner ear. This creates a slender cylindrical extension of tumor tissue that is easily fractured during operation or at postmortem examination. The residual tumor tissue then remains within the bony canal and, having been overlooked by the surgeon, is permitted to regrow or by the pathologist who then underestimates the scope of neoplastic involvement.

Microscopic Histology

Upon microscopic examination, the acoustic neurinoma presents two distinctive architectural patterns, designated Antoni A and Antoni B. Both are created by spindle cells with elongated nuclei and fibrillated cytoplasm, predominantly those of Schwann cells. The two tissue patterns differ in cellular weave and density. Antoni A tissue is compact, with a prominence of interwoven fascicles. Antoni B tissue is porous and less structured. The cells are dispersed randomly about blood vessels, microcysts, collections of xanthomatous cells and sites of previous hemorrhage. Lymphocytes attest to antecedent degenerative events within Antoni B tissue. The degree of nuclear pleomorphism varies considerably among acoustic neurinomas as well as between different areas within the same tumor. This pleomorphism often contributes a random population of large. bizarre nuclei that taunt the pathologist with thoughts of anaplasia: however, fortunately, malignant transformation is of a rarity that permits individual case reports. Mitotic figures are most infrequent. Necrosis is commonly present but most often testifies to the meagerness of native blood vessels and their compression by tumor expansion within a restricted compartment.

The differential diagnosis of a neoplasm within the cerebello­pontine angle embraces the many lesions mentioned above. When histologic examination excludes such distinctive entities as ependymoma, chemodectoma, chordoma and carcinoma and special stains disclose a collagen stroma to distinguish the mesenchymal lesions from exophytic gliomas, the differential diagnosis narrows assuredly to schwannoma versus meningioma. In turn, the whorled architecture of a meningioma, marked also by psammoma bodies, individualizes this lesion. Generally, even in frozen sections, the monotonous, woven appearance of a schwannoma is easily recognized. Although the schwann cells of an acoustic neurinoma, as well as those of all other schwannomas, stain positively with S100, the utility of this histochemical technique is generally not required for diagnosis.

Schwann cells possess a basement membrane that lies external to the plasma membrane. This feature distinguishes Schwann cells from fibroblasts. In addition, the presence of widely spaced collagen validates this identification. The histologic features of an acoustic schwannoma are generally diagnostic, and the assessment of anaplasia or malignancy has already been resolved in favor of benignity by the natural history of this neoplasm.

Acoustic neurinomas occur more often in women than men with an approximate ratio of 3: 2. Interestingly, the same ratio obtains with meningiomas. The peak incidence of this neoplasm has been variably cited between 35 and 55 years of age. These statistics indicate that there is a plateau of high incidence and that this plateau spans the several decades of mid-adult life. Like spinal, peripheral and other cranial nerve schwannomas, the acoustic lesions have a heightened incidence in persons with von Recklinghausen's disease. Bilateral acoustic neurinomas of the eighth nerve distinguish "central neurofibromatosis" (neurofibromatosis type 2) from "peripheral neurofibromatosis" (neurofibromatosis. type 1).  The former condition has been linked with a locus on chromosome 22: the latter, with a locus on chromosome 17.

Cure of this lesion necessitates complete surgical excision. Continued indolent growth follows subtotal resection, much in the manner and tempo of the meningioma. Although the possibility of bilaterality may shadow the patient's prognosis, fortunately, malignant transformation is generally not a worrisome issue.