Acoustic neuromas are also called vestibular schwannoma, acoustic neurinoma, vestibular neuroma, and acoustic neurofibroma. These are tumors that evolve from Schwann cell sheath and can be either intracranial or extra-axial. They usually occur adjacent to the cochlear or vestibular nerve. Anatomically, acoustic neuromas tend to occupy the cerebellopontine angle. About 20% of internal carotid artery (ICA) tumors are meningiomas and may occur elsewhere in the brain. Bilateral acoustic neuromas also tend to be exclusively in individuals with type 2 neurofibromatosis.
Acoustic neuroma is allied to neurofibromatosis type 2 (defect on Chromosome 22) bilateral disease. Studies have shown that acoustic neuroma had a causative predisposition mutation. Radiation exposure may predispose a patient to the development of that condition as well.
Approximately 8% of all intracranial tumors which manifest clinically are attributed to schwannomas. The majority of acoustic neuromas are unilateral and sporadic. Bilateral acoustic neuromas are in fact genetic and constitute a bit less than 5% of all schwannomas. Acoustic neuromas in general, tend to present between the fourth to sixth decades of life. Acoustic neuromas developing in individuals with neurofibromatosis type 2 (NF II) are likely to present earlier, with a peak incidence around the third decade of life. Although rare, acoustic schwannomas can occur in children. There is a small female preponderance with an aggravation of problems during pregnancy. Hereditary acoustic neuroma occurs in NF II much more often compared to neurofibromatosis type 1 (NF I), although the latter is much more common. Unilateral acoustic neuroma has been reported exclusively in 24% of cases with NF I, while bilateral acoustic schwannoma is a hallmark of NF II. Both these conditions are autosomal dominant. The defective genetic locus has been localized to chromosome 17 in NF I and chromosome 22 in NF II.
Grossly, the tumor is rubbery-firm with a pale, gray color. It shows different degrees of vascularity and has a well-defined capsule, which could be discoursed by the displaced and stretched nerve fibers. The cut section is usually pale gray and firm with a finely trabeculated appearance. Often, evidence of cystic degeneration, hemorrhage, xanthomatous changes and points of calcification may be present and are encountered in large tumors. These changes give a variegated appearance in consistency and color to the giant tumors. The blood supply of the tumor comes mainly from the internal auditory artery, which forks the surface of the tumor with several tiny branches. For larger tumors, there might be blood supply through small branches of the neighboring cerebellar and pontine arteries.
On light microscopy, the tumor is made up of spindle cells that have elongated nuclei and fibrillary cytoplasm. Those cells are arranged in two specific ways: Antoni A and Antoni B. Antoni A tissue is small, with a prominent, organized, interwoven course of elongated bipolar cells. Occasionally, the arrangement of the nuclei and fibers results in a spiral framework simulating that seen in meningiomas. Antoni B is represented by a random gathering of cells grouped around foci of cystic change, necrosis, old hemorrhage, and blood vessels. There is a variable amount of lymphocytic infiltration of this tissue. The Antoni B tissue type, generally seen in large tumors, is thought to be the outcome of ischemia. The consistency of the tumor is determined by the distant relative quantities of these two types. Although these are benign changes, as malignant transformation almost never happens, nuclear pleomorphism is typical in schwannomas. Mitotic figures are relatively rare. Necrosis, if found, is more due to a poor blood supply rather than fast development. Edema, development of micro or possibly macrocysts, xanthomatous alteration, and areas of calcification are attributed to the degenerative changes in the tumor tissue. Electron microscopy (EM) reveals the characteristic basement membrane of the Schwann cells as well as the presence of wide-spaced collagen.
The signs and symptoms of acoustic neuroma are attributed to the involvement of the cranial nerve VIII, compressing surrounding cranial nerves, cerebellum, brainstem, as well as raised intracranial pressure (ICP). The majority of acoustic neuromas present with unilateral hearing loss due to cochlear nerve interruption or impairment of blood supply to the nerve. Other clinical features include tinnitus, decreased word understanding, vertigo, headaches, and facial numbness. With enough growth, the mass of CP angle will eventually compress the brainstem and cause gait abnormalities. The following is a summary of the detailed clinical features.
Due to the involvement of Cranial Nerve VIII
Due to compression of other Cranial Nerves
Due to Cerebellar Compression
Due to Brainstem Compression/Torsion
Due to Raised ICP
These include, but are not limited to, headache, nausea, vomiting, increased blood pressure, decreased mental abilities, confusion, disturbance of level of consciousness, and papilledema.
The diagnosis of an acoustic neuroma is made with a contrast MRI or a CT scan. Contrast is essential; otherwise, the non-enhanced scan can miss small tumors. If hearing impairment is present, audiometric tests are needed. Auditory brainstem evoked response is not used frequently to screen for acoustic tumors as it can miss small malignancies.
Acoustic neuroma may be treated by
Acoustic neuroma accounts for around 80% to 90% of cerebellopontine angle (CPA) lesions. Differential diagnosis of an acoustic neuroma include:
Recurrence is not common after removal, but tinnitus may worsen in some patients. Hearing loss and facial paralysis may improve with time.
The pre-operative distinction between a meningioma and an acoustic schwannoma is important for technical and prognostic reasons.