Acoustic neuroma classification
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]Associate Editor(s)-in-Chief: Simrat Sarai, M.D. [2]
Overview
Calssification
Unilateral vestibular schwannomas affect only one ear. They account for approximately 8 percent of all tumors inside the skull; one out of every 100,000 individuals per year develops a vestibular schwannoma. Symptoms may develop at any age but usually occur between the ages of 30 and 60 years. Unilateral vestibular schwannomas are not hereditary.
Bilateral vestibular schwannomas affect both hearing nerves and are usually associated with a genetic disorder called neurofibromatosis type 2 (NF 2). Half of affected individuals have inherited the disorder from an affected parent and half seem to have a mutation for the first time in their family. Each child of an affected parent has a 50 percent chance of inheriting the disorder. Unlike those with a unilateral vestibular schwannoma, individuals with NF2 usually develop symptoms in their teens or early adulthood. In addition, patients with NF2 usually develop multiple brain and spinal cord related tumors. They also can develop tumors of the nerves important for swallowing, speech, eye and facial movement, and facial sensation. Determining the best management of the vestibular schwannomas as well as the additional nerve, brain, and spinal cord tumors is more complicated than deciding how to treat a unilateral vestibular schwannoma. Further research is needed to determine the best treatment for individuals with NF2.
Scientists believe that both unilateral and bilateral vestibular schwannomas form following the loss of the function of a gene on chromosome 22. (A gene is a small section of DNA responsible for a particular characteristic like hair color or skin tone). Scientists believe that this particular gene on chromosome 22 produces a protein that controls the growth of Schwann cells. When this gene malfunctions, Schwann cell growth is uncontrolled, resulting in a tumor. Scientists also think that this gene may help control the growth of other types of tumors. In NF2 patients, the faulty gene on chromosome 22 is inherited. For individuals with unilateral vestibular schwannoma, however, some scientists hypothesize that this gene somehow loses its ability to function properly.
There are two types of acoustic neuroma: unilateral and bilateral. Unilateral acoustic neuromas affect only one ear. They account for approximately 8 percent of all tumors inside the skull. Symptoms may develop at any age but usually occur between the ages of 30 and 60 years.
Bilateral acoustic neuromas, which affect both ears, are hereditary. Inherited from one's parents, this tumor results from a genetic disorder known as neurofibromatosis-2 (NF2). Affected individuals have a 50 percent chance of passing this disorder on to their children. Unlike those with a unilateral acoustic neuroma, individuals with NF2 usually develop symptoms in their teens or early adulthood. Because NF2 patients usually have multiple tumors, the surgical procedure is more complicated than the removal of a unilateral acoustic neuroma. Further research is needed to determine the best approach in these circumstances.
In addition to tumors arising from the hearing and balance nerves, NF2 patients may develop tumors on other cranial nerves associated with swallowing, speech, eye and facial movement, and facial sensation. NF2 patients may also develop tumors within the spinal cord and on the brain's thin covering.
Both types of acoustic neuroma occur following a loss of the function of a gene on chromosome 22. A gene is a small section of DNA responsible for a particular trait like hair color or skin tone. This particular gene on chromosome 22 suppresses the growth of Schwann cells. When this gene malfunctions, Schwann cells can grow out of control. This gene may help suppress other types of tumor growth. In NF2 patients, the faulty gene on chromosome 22 is inherited.
Acoustic neuroma is also called an acoustic neurinoma or a vestibular schwannoma.
Cytologically, no differences have been found between the vestibular schwannomas of NF2 and those found in sporadic cases. Histologically, however, the tumors in NF2 often appear as grape-like clusters that can infiltrate the fibers of individual nerves and may adumbrate a polyclonal origin. Both unilateral and bilateral tumors vary in their precise location along the vestibular nerve, tending to arise at the border between the central and peripheral segments of the nerve. Why tumors arise at this transition zone is not known, but variation in the site at which a tumor is located can have a major influence on the symptoms it produces.
For clinical management, the most useful classification of vestibular schwannomas is by size, location, and growth rate. However, tumors tend to enlarge unpredictably. Some do not change in size for many years, while others may grow at a rate of up to 20mm in diameter per year. Currently, the best method to monitor tumor growth is with gadolinium-enhanced MRI. To facilitate the interpretation of clinical studies, both the greatest diameter of the tumor within the posterior fossa and the extent of penetration into the intracanalicular space should be documented.
A second important classification is between familial and sporadic cases. All cases of vestibular schwannomas are thought to result from the functional loss of a tumor-suppressor gene that has been localized to the long arm of chromosome 22. In at least 95 percent of patients, however, the disease is unilateral and the majority of these cases are sporadic, resulting from somatic mutations that are not associated with an increased risk for other tumors either in the individual or in close relatives. About 5 percent of patients exhibit bilateral disease or other features that define NF2. These patients are thought to carry a single germline mutation of the chromosome 22 linked gene and sustain the loss of the remaining normal allele as a somatic event in those cells that give rise to the tumor. Thus, the trait is recessive at the cellular level but exhibits a dominant pattern of genetic transmission in families. Even when a thorough family history is obtained, in about one half of all recognized cases of NF2, no evidence of other affected family members can be found. These may patients represent new germline mutations and are at risk of transmitting the disease to their offspring.
Patients with NF2 who carry new mutations tend to be more severely affected than familial cases, and some recent studies have raised the possibility that in familial cases the onset of symptoms may be earlier and the severity greater when the disease is inherited from the mother. Such effects can arise from genomic imprinting, and although the precise genetic mechanism for this phenomenon is unknown, a growing number of examples of such parental origin effects now have been documented. If confirmed, these findings could have practical implications for the management of families with NF2.
Molecular studies on NF2 and on unilateral tumors are at an exciting juncture. The gene for NF2 should soon be identified and may provide molecular explanations for clinical differences among families with NF2 as well as differences in the growth rate among tumors. Further studies on the molecular biology of the gene may suggest treatments for vestibular schwannomas, both in NF2 and in patients with unilateral diseases.
Patients with NF2 may have associated meningiomas and spinal root schwannomas as well as cafe-au-lait spots and peripheral Schwann cell tumors and often develop posterior subcapsular cataracts at an early age. The prevalence of these findings varies greatly among families.