Acoustic neuroma overview
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]Associate Editor(s)-in-Chief: Mohsen Basiri M.D. Simrat Sarai, M.D. [2]
Overview
The most common neoplasm of the cerebellopontine angle in adults is vestibular schwannoma (8% of all intracranial tumors). The term “vestibular schwannoma” is preferred over acoustic neuroma which is a misnomer.
Historical Perspective
Acoustic neuroma was first described by Eduard Sandifort in 1777.
Bilateral acoustic neuroma was first described by Wishart in 1822.
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.
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.
Classification
Acoustic neuroma may be classified according to the findings on
- microscopic histopathology, Based on microscopic histopathology, acoustic neuroma may be classified into four subtypes: conventional schwannoma, cellular schwannoma, plexiform schwannoma, and melanotic schwannoma. While acoustic neuromas are benign tumors, there is no established system for the staging of acoustic neuromas.
- Associated with neurofibromatosis type 2 or not.
- Koos grading scale provides four grades based on extra meatal extension and compression of the brain stem , a reliable method for tumor classification which is used in practice.
Pathophysiology
Acoustic neuroma arises from Schwann cells of the vestibulocochlear (eighth cranial) nerve, which are the cells 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 processes arranged in fascicles, little stromal matrix and verocay bodies. Antoni type B growth pattern is composed of loose meshwork of cells, less dense cellular matrix, microcysts and myxoid change. the mean tumor size decreased from 2.8 cm to 0.7 cm
Depending on size, tumors may be fully confined to the internal auditory canal or may extend to varying degrees into the cerebellopontine angle. As a tumor grows, it expands within the confines of the internal auditory canal and exerts pressure on adjacent nerves before growing medially into the cerebellopontine angle. Larger tumors that extend into the cerebellopontine angle may compress the trigeminal nerve located cranially, the lower cranial nerves located caudally, and the brain stem and cerebellum medially. Progressive medial effacement of the pons may result in obstruction of the fourth ventricle and subsequent hydrocephalus
Neurofibromatosis type 2 is a rare autosomal dominant disorder caused by pathogenic variants within the NF2 gene; nearly half of affected people have a positive family history, and the remaining cases result from new variants
Causes
Numerous studies show the correlation between Neurofibromatosis type 2 (NF2) and acoustic neuroma. Other causes can include exposure to occupational noise and cellular telephone use.
Differential Diagnosis
schwannomas originating from other nerves (e.g., facial-nerve schwannoma), meningiomas(In contrast to vestibular schwannomas, posterior fossa meningiomas involve the internal auditory canal less frequently and typically grow eccentrically to the medial canal opening) , metastases from primary tumors at other sites, or malignant peripheral nerve sheath tumors that develop new or secondarily within preexisting schwannomas, either spontaneously or after radiation treatment. In addition, several disease variants, including macrocystic and hemorrhagic vestibular schwannomas, may have a more aggressive course. Acoustic neuroma must be differentiated from meningioma, intracranial epidermoid cyst (Epidermoids are the third most common tumor of the cerebellopontine angle and are isointense to surrounding cerebrospinal fluid on T1 and T2weighted imaging), facial nerve schwannoma, trigeminal schwannoma, ependymoma, leiomyoma, intranodal palisaded myofibroblastoma, malignant peripheral nerve sheath tumor (MPNST), gastrointestinal stromal tumor, neurofibroma, Meniere's disease, and Bell's palsy.
Epidemiology and Demographics
Enhanced diagnostics leading to increase detection has lead to rise in incidence of vestibular schwannoma. From the 1900s the incidence of vestibular schwannomas remained static, since patients presented with large tumors causing symptoms that had grown over a period of years without being detected. 1 case per 100,000 person-years in 1970, current incidence rates range from 3 to 5 cases per 100,000 person-years. 20 cases per 100,000 person-years in patient aged 70 years. The incidence of acoustic neuroma ranges from 0.3 to 1 per 100, 000 individuals. A lifetime prevalence is exceeding 1 case among 500 persons. Sporadic unilateral vestibular schwannomas, which account for more than 95% of cases. Women are more commonly affected by acoustic neuroma than men. Most cases of acoustic neuroma develop in individuals between 30 and 60 years of age.
Risk Factors
Common risk factors in the development of acoustic neuroma are neurofibromatosis type 2 and radiation exposure. Less common risk factors include sporadic defects in tumor suppressor genes, exposure to loud noise, history of parathyroid adenoma, and the use of cellular phones. Several groups have suggested that environmental exposures, such as cell phone use or long-term noise exposure may increase the risk of tumorigenesis but this relation is still under study. Ionizing radiation are big risk factor
Screening
According to the U.S. Preventive Services Task Force (USPTF), screening for acoustic neuroma is not recommended. Evaluation for NF-2 should be done in individuals with an apparently sporadic vestibular schwannoma occurring before the age of 30, or a spinal tumor or meningioma occurring at less than 20 years of age.
indication for screening MRI study include sudden or asymmetric sensorineural hearing loss detected through pure-tone and speech audiometry
Patients with an isolated, unilateral vestibular schwannoma who do not have other signs of neurofibromatosis type 2 and have no affected relatives generally do not need to undergo genetic testing, nor do their family members
Natural History, Complications and Prognosis
Approximately 50% of all acoustic neuromas grow slowly (1 - 2 mm/year). The growth rate is more rapid (greater than 2 mm/year) in about 20% of the patients. The tumor does not metastasize to other parts of the body. Hearing loss, when occurs, is irreversible. 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. Complications of acoustic neuroma include hearing loss, Hydrocephalus, and recurrence of the tumor. Small, slow-growing tumors may not need treatment. Patients experience similar quality of life whether treatment is observation, radiation, or surgery.
An important ramification of increased disease detection is a potential for overtreatment, which could result in unnecessary complications and health care expenditures. Many patients, who just decades ago would have lived out their lives without having their tumors detected, are now receiving treatment
Diagnostic Study of Choice
Widespread access to sensitive neuro-diagnostic imaging has led to a remarkable rise in the detection of vestibular schwannomas. Gadolinium-enhanced MRI scan is the definitive diagnostic test for acoustic neuroma and can identify tumors as small as 1-2 millimeter in diameter. On brain MRI, acoustic neuroma characterized by hypointense mass on T1-weighted MRI, and hyperintense mass on T2-weighted MRI.
History and Symptoms
Chronic gradual unilateral hearing impairment is the most common complaint present in 95% of the patients. Common symptoms include chronic gradual unilateral hearing loss, ringing in the ear, Disequilibrium, facial numbness, facial pain, and Headache. Less common symptoms include facial muscle weakness, taste disturbances, dryness of the eyes, sudden lacrimation, speech problem, difficulty swallowing, aspiration, hoarseness, and ear pain, ipsilateral sensorineural hearing loss in more than 90% of patients, Hearing loss is often subtle initially and may first become apparent when the patient is using a telephone or lying in bed with the contralateral ear covered. dizziness or imbalance in up to 61%, and asymmetric tinnitus in 55% Tinnitus is thought to result from cochlear deafferentation and cortical maladaptation — a mechanism akin to deafferentation pain, as seen in the phantom limb syndrome, tinnitus may persist even after surgery. increasing difficulty with sound localization and speech comprehension in the presence of background noise, which results from the loss of binaural hearing. symptoms of vertigo and continuous dizziness occur in only about 8% and 3% of cases, respectively.
Patients with large tumors that compress the brain stem and cerebellum may have hypoesthesia in a trigeminal distribution, secondary trigeminal neuralgia, cerebellar dysmetria and ataxia, or slowly progressive hydrocephalus without alteration of consciousness.
symptom progression is not strongly correlated with tumor growth. sensorineural hearing loss and vestibular hypofunction are not reversed with tumor treatment
Physical Examination
Patients with acoustic neuroma usually appear normal. Physical examination of patients with acoustic neuroma is usually remarkable for Sensorineural hearing loss in the affected ear, positive Rinne test, abnormal Weber test, Papilledema, Nystagmus, Diplopia on lateral gaze, decreased or absent ipsilateral corneal reflex, facial twitching or hypesthesia, Drooling, drooping on one side of the face, loss of taste, and ataxia.
Laboratory Findings
There are no diagnostic laboratory findings associated with acoustic neuroma.
Electrocardiogram
There are no electrocardiogram findings associated with acoustic neuroma.
X Ray
There are no x-ray findings associated with acoustic neuroma.
Echocardiography/Ultrasound
There are no echocardiography or ultrasound findings associated with acoustic neuroma.
CT
CT scan of the head may be diagnostic of acoustic neuroma. Findings on CT scan diagnostic of acoustic neuroma include erosion and widening of the internal acoustic canal.
MRI
25% of all new cases are diagnosed incidentally during imaging that was obtained for unrelated indications (e.g., headache). Gadolinium-enhanced MRI scan is the definitive diagnostic test for acoustic neuroma and can identify tumors as small as 1-2 millimeter in diameter highly sensitive and specific accurate radiologic diagnosis in most cases, without the need for a confirmatory biopsy. On brain MRI, acoustic neuroma characterized by hypo intense mass on T1-weighted MRI, and hyperintense mass on T2-weighted MRI.
Other Imaging Findings
There are no other imaging findings associated with acoustic neuroma.
Other Diagnostic Studies
Audiometry as the best initial screening test for the diagnosis of acoustic neuroma. It can detect asymmetric sensorineural hearing impairment in about 95% of the patients. 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.
Medical Therapy
Treatment strategies can be divided into an observational wait-and-scan approach, irradiation, microsurgery, and a combination of these methods. each strategy has a set of advantages and limitations. Tumor size chiefly drives treatment recommendations; however, decision making is also guided by subtle patient- and provider-related factors. Several new drug therapies that aim to halt tumor growth, including aspirin and monoclonal antibodies, have recently been explored but remain investigational. The most consistent predictor of future growth during an observational strategy is larger tumor size at diagnosis
Wait-and-Scan Approach
Typically, tumors that have a maximal diameter of less than 1.5 cm in the cerebellopontine angle are considered for a wait-and-scan approach.. The wait-and-scan approach has gained popularity for at least two reasons: many tumors are now discovered as small masses in older people with mild symptoms; furthermore, reports over the past 15 years have documented radiographically that only 22 to 48% of tumors have shown growth (most commonly defined as an increase of ≥2 mm in diameter) The mainstay of therapy for acoustic neuroma is surgery and radiation therapy. Since acoustic neuroma tends to be slow-growing and is a benign tumor, careful observation with follow-up MRI scans every 6 to 12 months may be appropriate for elderly patients, patients with small tumors, patients with significant medical conditions, and patients who refuse treatment.
Surgery
Surgery is the mainstay of treatment for acoustic neuroma. If growth is definitively confirmed, most patients receive a recommendation to undergo radiosurgery or microsurgery. Patients with age under 65 years, medium to large-grade tumors, significant hearing loss, 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.
active monitoring of the tumor with serial imaging, signifying a transition in clinical care from up-front microsurgical resection, which epitomized treatment in earlier eras, to management of chronic disease.
Primary Prevention
There are no established measures for the primary prevention of acoustic neuroma.
Secondary Prevention
Secondary prevention strategies following acoustic neuroma treatment include follow-up MRI scans. Imaging and audiologic evaluation are commonly performed 6 months after the diagnostic MRI in order to identify a fast-growing tumor or a more aggressive process mimicking a vestibular schwannoma. If there is no growth at 6 months, imaging and hearing assessments are performed annually thereafter until year 5, when many specialists advocate every-other-year assessments. lifelong follow-up is recommended, To minimize the cost of ongoing tumor surveillance and the risk of adverse events related to contrast material, several groups have transitioned to the use of thin-slice, heavily T2-weighted magnetic resonance cisternography without contrast material, which has a high degree of accuracy and interrater reliability.
A research showed that excellent speech comprehension at diagnosis portends favorable long-term hearing outcomes.