Acoustic neuroma causes
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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
Causes
Acoustic neuromas may occur idiopathically (meaning the cause is unknown), however there is a growing body of evidence that sporadic defects in tumor suppressor genes may give rise to these tumors in some individuals.
Although there is an inheritable condition called Neurofibromatosis Type 2 (NF2) which can lead to acoustic neuroma formation in some people, most acoustic neuromas occur spontaneously without any evidence of family history (95%).The cause of acoustic neuromas is largely unknown. No environmental factor (such as cell phones or diet) has been scientifically proven to cause these tumors. They can be sporadic or caused by an inherited condition called neurofibromatosis type 2 (NF-2). Sporadic tumors occur 95% of the time, while 5% of acoustic tumors are caused by NF-2.
Neurofibromatosis is a rare disease that occurs in two forms. Type 1 causes tumors to grow on nerves throughout the body, especially the skin. Type 2 can cause acoustic tumors on both left and right sides, creating the possibility of complete deafness if the tumors grow unchecked. The presence of bilateral acoustic tumors affects the choice of treatment, as hearing preservation is a prime objective.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. Acoustic neuroma occurs in two forms: a sporadic form and a form associated with an inherited syndrome. About 95% of all cases are sporadic. The cause of the sporadic form is unclear. Some small studies have found an association of acoustic neuromas with cellular phone use or prolonged exposure to loud noises, but other studies do not find this link (Christensen et al 2004, Edwards et al 2006, Edwards et al 2007, Hardell et al 2003, Lonn et al 2004, Myung et al 2009, Schlehofer et al 2007, Schoemaker et al 2007). There is not hard evidence supporting a link between environmental factors and acoustic neuromas.
The inherited syndrome called neurofibromatosis type II (NF2). NF2 is rare; there are only several thousand affected individuals in the entire United States, corresponding to about 1 in 40,000 individuals. Roughly 5% of patients with acoustic neuroma have NF2.
The exact cause of an acoustic neuroma is unknown. Most cases seem to arise for no apparent reason (spontaneously). No specific risk factors for the development of these tumors have been identified.
A variety of potential risk factors for acoustic neuroma have been studied including prior exposure to radiation to the head and neck area (as is done to treat certain cancers) or prolonged or sustained exposure to loud noises (as in an occupational setting). Research is under way to determine the specific cause and risk factors associated with an acoustic neuroma.
In a small subset of cases, acoustic neuromas occur as part of a rare disorder known as neurofibromatosis type II. This rare genetic disorder is usually associated with acoustic neuromas affecting both ears at once (bilateral). (For more information on this disorder, choose “neurofibromatosis” as your search term in NORD’s Rare Disease Database.)
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. The eighth cranial nerve is separated into two branches the cochlear branch, which transmits sound to the brain and the vestibular branch, which transmits balance information to the brain. Most acoustic neuromas occur on the vestibular portion of the eighth cranial nerve. Because these tumors are made up of Schwann cells and usually occur on the vestibular portion of the eighth cranial nerve, many physicians prefer the use of the term vestibular schwannoma. However, the term acoustic neuroma is still used more often in the medical literature There are two types of acoustic neuroma: a sporadic form and a form associated with a syndrome called neurofibromatosis type II (NF2). NF2 is an inherited disorder characterized by the growth of noncancerous tumors in the nervous system. Acoustic neuromas are the most common of these tumors and often occur in both ears by age 30.
NF2 is a rare disorder. It accounts for only 5% of acoustic neuromas. This means the vast majority are the sporadic form. Doctors aren't certain what causes the sporadic form. One known risk factor for acoustic neuroma is exposure to high doses of radiation, especially to the head and neck. 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 is a growing body of evidence that sporadic defects in tumor suppressor genes may give rise to these tumors in some individuals. Other studies have hinted at exposure to loud noise on a consistent basis. One study has shown a relationship of acoustic neuromas to prior exposure to head and neck radiation, and a concomitant history of having had a parathyroid adenoma (tumor found in proximity to the thyroid gland controlling calcium metabolism). There are even controversies on hand-held cell phones. It remains to be seen whether or not the radiofrequency radiation has anything to do with acoustic neuroma formation. To date, no environmental factor (such as cell phones and diet) has been scientifically proven to cause these tumors. ANA does recommend that frequent cell phone users use a hands-free device to enable separation of the device from the head. Please refer to cell phone statement on our website
Acoustic neuroma occurs in two forms: a sporadic form and a form associated with an inherited syndrome called neurofibromatosis type II (NF2). About 95 percent of all cases are sporadic.
NF2 is rare; there are only several thousand affected individuals in the entire United States, corresponding to about 1 in 40,000 individuals (see MRI image below). Roughly 5% of patients with acoustic neuroma have type II neurofibromatosis. A recent study suggested that the mechanism of hearing loss in NF2 is elevated intralabyrinthine protein rather than compression of the 8th nerve (Asthaqiri et al, 2012). While we doubt that this is always the case, we have also invoked this cause in patients with tiny tumors such as intralabyrinthine schwannomas, driven by MRI findings. More information about NF2 can be found at http://ghr.nlm.nih.gov/gene=nf2.
There is no credible evidence that radiation from cellular phones causes acoustic neuroma (Muscat et al, 2002).
Most patients diagnosed with an acoustic neuroma have no apparent risk factors. Exposure to high-dose ionizing radiation is the only definite environmental risk factor associated with an increased risk of developing an acoustic neuroma. Multiple studies have determined cell phone use is not associated with an increased risk of developing an acoustic neuroma, although data on the effects of long-term cell phone use are still pending.
Neurofibromatosis type II occurs in individuals who have defective tumor suppressor gene located on chromosome 22q12.2. The defective protein produced by the gene is called merlin or schwannomin. Bilateral acoustic tumors are a principle clinical feature of neurofibromatosis type II, although other manifestations, including peripheral neurofibromata, meningioma, glioma, and juvenile posterior subcapsular lenticular opacities, are often present as well. Peripheral neurofibromatoma and cafe au lait spots, however, are much less frequently observed than is typical in neurofibromatosis type I. Many patients with neurofibromatosis type II present in late adolescence or early adulthood but occasionally may present later in the fifth to seventh decade with slowly growing tumors.
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.