Acoustic neuroma overview
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Acoustic neuromas (also known as vestibular schwannomas) are benign tumors (WHO grade 1), Schwann cell-derived tumors that commonly arise from the vestibular portion of the vestibulocochlear nerve (CN VIII), and it are responsible for approximately 80% of cerebellopontine angle tumors. While the majority of cases of acoustic neuroma are idiopathic, common risk factors in the development of acoustic neuroma are neurofibromatosis type 2, radiation, exposure to loud noise, history of parathyroid adenoma. One the most knowable causes of acoustic neuroma is neurofibromatosis type 2 (NF2). NF2 gene on chromosome 22q12.2 that encodes a protein named merlin or schwannomin a cell membrane-related protein that acts as a tumor suppressor Dephosphorylated Merlin proteins are active and roll in the normal cell growth, phosphorylated Merlin ( synthesized due to a mutation in NF2 gene) can not play normal roll in cell growth and causes increased cell growth. Acoustic neuroma may be classified into several subtypes based on MRI scan, microscopic histopathology, and whether or not they are associated with neurofibromatosis type 2. Acoustic neuroma may be classified based on MRI scan findings into three subtypes: entirely canalicular, intracranial extension without brainstem distortion, and intracranial extension with brain stem distortion. Based on microscopic histopathology, acoustic neuroma may be classified into four subtypes, conventional schwannoma, cellular schwannoma, plexiform schwannoma, and melanotic schwannoma. Symptoms of acoustic neuroma include hearing impairment, tinnitus, disequilibrium, headaches, facial weakness, facial numbness and tingling, dizziness, taste changes, difficulty swallowing and hoarseness. Gadolinium-enhanced MRI scan is definitive diagnostic test of of acoutic neuroma. On brain MRI, acoustic neuroma is characterized by hypointense mass on T1-weighted MRI and hyperintense mass on T2-weighted MRI. Head CT scan may be diagnostic of acoustic neuroma. Findings on CT scan diagnostic of acoustic neuroma include erosion and widening of the internal acoustic canal. Other diagnostic studies for acoustic neuroma include audiometry, auditory brainstem response test, and electronystagmography. The predominant therapy for acoustic neuroma is surgical resection. Adjunctive radiation and radiosurgery may be required. Since acoustic neuromas tend to be slow-growing and are benign tumors, careful observation over a period of time may be appropriate for some patients.
Acoustic neuroma was first described by Eduard Sandifort , a professor of anatomy in the Netherlands, in 1777. Bilateral acoustic neuroma was first described by Wishart in 1822. He described a patient who became progressively deaf, blind, with uncontrollable vomiting and headaches, and facial jerking, and died at 21 years of age. Sir Charles Bell provided the first known report of a case of Meckel cave neuroma in 1833, demonstrating the relationship of the tumor to the cerebellopontine angle. Sir Charles Ballance successfully removed an acoustic neuroma in 1894, although the patient had right side facial paralysis and trigeminal anesthesia. Since then, tremendous efforts of many surgeons have been continuing to provide surgical approaches to improve outcomes of treatments and decrease side effects of interventions.
Acoustic neuroma may be classified according to findings on magnetic resonance imaging (MRI) or classified based on microscopic histopathology, and whether or not they are associated with neurofibromatosis type 2. Koos grading scale provides four stage based on extrameatal extension and compression of the brain stem , a reliable method for tumor classification which is used in practice. Based on microscopic histopathology, acoustic neuroma may be classified into four subtypes: conventional schwannoma, cellular schwannoma, plexiform schwannoma, and melanotic schwannoma.
Acoustic neuroma arises from Schwann cells, which are the cells that are normally involved in the conduction of nervous impulses along axons, nerve development and regeneration. On microscopic histopathological analysis, acoustic neuroma may display two types of growth patterns Antoni type A and Antoni type B. Antoni type A growth pattern is composed of elongated cells with cytoplasmic process arranged in fascicles, little stromal matrix and verocay bodies. Antoni type B growth pattern is composed of loose meshwork of cells, less densely cellular matrix, microcysts and myxoid change.
Acoustic neuroma are the most common tumors of the cerebellopontine angle and responsible for 6-8% of all intracranial tumors, these evidence demonstrate the importance of how acoustic neuroma can develop. Numerous studies show the correlation between Neurofibromatosis type 2 (NF2) and acoustic neuroma and some contraversial causes such as: occupational noise and cellular telephones use.
Acoustic neuroma must be differentiated from meningioma, epidermoid, facial nerve schwannoma, trigeminal schwannoma, ependymoma, leiomymoma, intranodal palisaded myofibroblastoma, malignant peripheral nerve sheath tumour (MPNST), gastrointestinal stromal tumor, neurofibroma, Meniere's disease, and Bell's palsy.
Epidemiology And Demographics
The incidence of acoustic neuroma is approximately 1 per 100,000 individuals worldwide. The prevalence of acoustic neuroma is approximately 2,500 new cases per 100,000 individuals. Women are more commonly affected with acoustic neuroma than men. Most cases of acoustic neuroma develop in individuals between 30 and 60 years of age.
According to the U.S. Preventive Services Task Force (USPTF), screening for acoustic neuroma is not recommended. Evaluation for NF-2should be done in individuals with an apparently sporadic vestibular schwannoma occurring at less than 30 years of age, or a spinal tumor or meningioma occurring at less than 20 years of age.
Natural History, Complications and Prognosis
If left untreated, an acoustic neuroma can block the flow of cerebrospinal fluid and cause hydrocephalus, which may lead to severe vision problems and difficulty breathing and swallowing. Acoustic neuroma patients have a very good prognosis with minimal complications if treated.
History and Symptoms
Symptoms of acoustic neuroma may vary depending on the cranial nerve involvement, cerebellar compression, mass size and tumor progression. hearing loss and tinnitus almost always are seen in cochlear nerve involvement, while unsteadiness is common among patients with involvement of the vestibular nerve. Trigeminal nerve involvement occurs among one-sixth of patients and usually presents with facial paresthesia or hypesthesia. Facial paresis, taste disturbances, xerophthalmia, paroxysmal lacrimation, and xerostomia are less common symptoms which may present due to involvement of the facial nerve.
There are no diagnostic laboratory findings associated with acoustic neuroma.
There are no electrocardiogram findings associated with acoustic neuroma.
Chest X Ray
There are no chest x ray findings associated with acoustic neuroma.
Findings on CT scan diagnostic of acoustic neuroma include erosion and widening of the internal acoustic canal.
Echocardiography or Ultrasound
There are no echocardiography or ultrasound findings associated with acoustic neuroma.
Other Imaging Findings
There are no other imaging findings associated with acoustic neuroma.
Other Diagnostic Studies
Audiometry as the best initial screening laboratory test for the diagnosis of acoutic neuroma, can detect asymmetric sensorineural hearing impairment in about ninety five percent of patients.Pure tone (audiogram) and speech audiometry should be performed, however test results does not essentially correlate with tumor size. Brain stem-evoked response audiometry (ABR, BAER, or BSER) may be done in some cases with unexplained asymmetries in standard audiometric testing as a further screening measure and an abnormal auditory brain stem response test should be followed by an MRI.
The mainstay of therapy for acoustic neuroma is surgery and radiation therapy. Since acoustic neuroma tends to be slow-growing and is benigntumor, careful observation with follow-up MRI scans every 6 to 12 months may be appropriate for elderly patients or patients with small tumors, or among patients with significant medical conditions and patients who refuse treatment.
Surgery is the mainstay of treatment for acoustic neuroma. Patient with age under 65 years and/or medium to large-grade tumors and/or significant hearing loss, and/or higher headache severity scores will have more satisfying outcomes from surgery in comparison with observation. There are three main surgical approaches for the removal of an acoustic neuroma: translabyrinthine, retrosigmoid or sub-occipital, and middle fossa. Selection of a particular approach is based on several factors, including the size and location of the tumor and whether or not preservation of hearing is a goal.
There are no primary preventive measures available for acoustic neuroma.
Secondary prevention strategies following acoustic neuroma treatment include follow-up MRI scans.