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. Acoustic neuroma is a noncancerous tumor. It grows slowly from an overproduction of Schwann cells and is additionally called a vestibular schwannoma. The tumor then presses on the hearing and balance nerves within the internal ear . Schwann cells normally encircle and support nerve fibers. An outsized tumor can continue the facial , which controls facial muscles and sensation. Or it can continue brain structures.
Historical Perspective
- In 1777, Eduard Sandifort first described Acoustic neuroma.
- In 1822, Wishart described Bilateral acoustic neuroma for the first time.
- In 1833, Charles Bell provided the first known report of a case of neuroma, demonstrating the relationship of the tumor to the cerebellopontine angle.
- In 1894, Charles Ballance successfully removed an acoustic neuroma surgically. 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:
| microscopic histopathology |
|---|
| Conventional schwannoma, |
| Cellular schwannoma, |
| Plexiform schwannoma, |
| Melanotic schwannoma |
- 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 now a days.
- In addition, several disease variants, including macrocytic and hemorrhagic vestibular schwannomas, may have a more aggressive course.
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.
- 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 obstructive hydrocephalus leading to severe heading and vomiting.
- 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.
- While acoustic neuromas are benign tumors, there is no established system for the staging of acoustic neuromas.
Causes
- Numerous studies show the correlation between Neurofibromatosis type 2 (NF2) and acoustic neuroma. Other causes can include Constant or continuous exposure to loud noise (such as music or work-related noise) Neck or face radiation can lead to acoustic neuroma many years later and cellular telephone use. Bilateral acoustic neuromas affects both ears and is inherited. It is caused by a genetic problem called neurofibromatosis-2 (NF2).
Differential Diagnosis
- Vestibular schwannoma must be differentiated from 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. Acoustic neuroma must also be differentiated from intracranial epidermoid cyst ( Epidermoid are the third most common tumor of the cerebellopontine angle and are isointense to surrounding cerebrospinal fluid on T1 and T2 weighted imaging), facial nerve schwannoma, trigeminal schwannoma, ependymoma, leiomyoma, intranodal palisaded myo-fibro-blastoma, 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.
- The incidence of acoustic neuroma ranges from 0.3 to 1 per 100, 000 individuals 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.
- 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.
- Denmark’s national registry showed the average age at diagnosis increased from 49 to 60 years, the mean tumor size decreased from 2.8 cm to 0.7 cm.
- Developed countries with widespread access to MRI, population-based data suggest that up to 25% of all new cases are diagnosed incidentally during imaging that was obtained for unrelated indications like severe headache or imbalance problem.
Risk Factors
Common risk factors in the development of acoustic neuroma are
- neurofibromatosis type 2
- radiation exposure
- Less common risk factors include sporadic defects in tumor suppressor genes
- Exposure to loud noise
- History of parathyroid adenoma
- 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.
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.
- Indications 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.
- The average age at diagnosis increased from 49 to 60 years, the mean tumor size decreased from 2.8 cm to 0.7 cm because of early detection of tumor via advance technology.
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.
- Large tumors associated with symptomatic brain-stem compression, hydrocephalus, trigeminal neuralgia or neuropathy, or a combination of these complications.
- Although many of the stigmata of facial-nerve injury can be electively managed, incomplete eye closure must be aggressively treated to reduce the risk of exposure keratopathy, commonly manifested as blurred vision, ocular pain, and redness.
- 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.
Prognosis:
- Population-based data showed that 334 of 636 patients had useful hearing at diagnosis, with a speech discrimination score of more than 70% (indicating that 70% percent of words were repeated back correctly by the patient), but after 10 years of observation, only 31% retained hearing above this threshold. Notably, 88% of patients who started with a speech discrimination score of 100% still had a score of more than 70% at 10 years, suggesting that excellent speech comprehension at diagnosis portends favorable long-term hearing outcomes.
- Unfortunately, since symptom progression is not strongly correlated with tumor growth and since the growth rate is highly variable, patients who are lost to follow-up are at increased risk for the development of a large tumor, with an associated increase in the risk of a poor outcome with eventual treatment.
- The risk of a secondary cancer from radiosurgery approaches 0.02%.
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 hypo-intense mass on T1-weighted MRI, and hyperintense mass on T2-weighted MRI.
History and Symptoms
Chronic gradual unilateral hearing impairment is that the commonest complaint resent in 95% of the patients. Common symptoms include chronic gradual unilateral hearing loss, ringing iwnthi 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 usually subtle initially and should first become apparent when the patient is employing a telephone or lying in bed with the contralateral ear covered.
Symptoms of dizziness or imbalance in up to 61% of patients, and complaint of asymmetric tinnitus in 55%. Tinnitus is assumed 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.
There is an increasing difficulty with sound localization and speech comprehension in the presence of background noise, which results from the loss of binaural hearing. Similarly, 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 during a trigeminal distribution, secondary tic douloureux, cerebellar dysmetria, and ataxia, or slowly progressive hydrocephalus without alteration of consciousness.
Symptom progression isn't strongly correlated with tumor growth. The sensorineural deafness and vestibular hypofunction aren't reversed even 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 are often divided into an observational wait-and-scan approach, irradiation, microsurgery, and a mixture of those methods. each strategy features a set of benefits and limitations. As such, patient preference plays a serious role in shared decision-making. Tumor size chiefly drives treatment recommendations; however, deciding is additionally guided by the 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 foremost 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 but 1.5 cm within the cerebellopontine angle are considered for a wait-and-scan approach. The wait-and-scan approach has gained popularity for a minimum of 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 radiotherapy. Since acoustic neuroma tends to be slow-growing and should be a benign tumor, careful observation with follow-up MRI scans every 6 to 12 months could even be appropriate for elderly patients, patients with small tumors, patients with significant medical conditions, and patients who refuse treatment.
Surgery
Surgery is that 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 deafness , or higher headache severity scores will have more satisfying outcomes from surgery as compared with observation. There are three main surgical approaches for the removal of an acoustic neuroma: translabyrinthine, retro sigmoid or sub-occipital, and middle fossa approach. The choice of a specific approach is predicated on several factors including the dimensions and site of the tumor and whether or not the preservation of hearing may be 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 the chronic disease.
Radiosurgery
It is performed in an outpatient setting, with no activity restrictions for the patient after radiosurgery. It prevents tumor growth but doesn't cure. The diameter of but 3.0 cm within the cerebellopontine angle is typically considered to be candidates for radiosurgery. the utilization of highly conformal radiation, defined as radiation delivered in 1 to five fractions to an image-defined target, with maximal sparing of the encompassing tissue. Gamma-knife radiosurgery is one sort of conformal radiation. Gamma knife treatment consists of 192 cobalt-60 sources arranged concentrically to deliver an ovoid isocenter of radiation performed under local anesthesia the dose prescribed is typically 12 to 14 Gy at the five hundred isodose lines, delivered during a single fraction. Treatment typically incorporates a stereotactic head frame and thin-slice, non–contrast-enhanced computerized tomography, and contrast-enhanced axial MRI to stereotactically target the tumor in three-dimensional space. Linear accelerator–based platforms also are employed by many centers. Most of those systems involve one, collimated radiation beam with a gantry that rotates around the patient, creating a focused arc of radiation that stereotactically targets the lesion of interest. Transient tumor enlargement within the primary 3 years after radiosurgery is common, although variable tumor shrinkage eventually occurs in additional than half treated cases. tumor control is reported in additional than 90% of cases of vestibular schwannoma at 10 years of follow-up. Radiosurgical treatment failure is usually defined by tumor growth that persists for quite 3 years, the event of signs or symptoms related to progressive mass effect, and rapid tumor enlargement. Salvage microsurgery is usually recommended after failure.
Cyber-knife radiotherapy
Cyber knife involves frameless, LINAC-based radiation delivered by means of a highly maneuverable robotic arm with 6 df for movement, with real-time image guidance. The treatment plan prescribes hypo fractionated radiation, at a dose of 25 Gy delivered in 5 fractions to the 80% isodose line.
Delivery of high-energy photon radiation using LINAC systems.
linear accelerator (LINAC)–based systems (Panel C) use a single, collimated radiation beam with a mobile gantry to create a focused arc of radiation, with frameless stereotaxis; the patient is typically immobilized with the use of a customized, soft, plastic face mask. . The treatment plan (right) prescribes a dose of 12.5 Gy delivered to the 80% isodose line in a single fraction
Risk of radiation
radiation-induced brainstem edema, trigeminal neuropathy or neuralgia, and hydrocephalus, as well as diminished long-term tumor control. The risk of a secondary cancer from radiosurgery approaches 0.02%
Microsurgery
Microsurgery is generally preferred for the treatment of tumors that are larger than 3 cm in diameter. the treatment of choice for large tumors associated with symptomatic brain-stem compression, hydrocephalus, trigeminal neuralgia or neuropathy, or a combination of these complication and large tumor size. All procedures are performed while the patient is under general anesthesia and require the use of an operating microscope with intraoperative neural monitoring.three primary microsurgical approaches used to remove vestibular schwannomas are the middle fossa, translabyrinthine, and retrosigmoid approaches .
| primary microsurgical approaches | Description | Benefits | Risks | |
|---|---|---|---|---|
| Translabyrinthine approach | This surgical approach encompasses a postauricular incision, removal of bone between the ear canal and sigmoid sinus, and removal of the semicircular canals to reach the IAC and cerebellopontine angle (CPA) | Complete exposure of the IAC and fundus
early identification of CN VII Clear view of lateral brain stem facing tumor |
the only approach that inherently sacrifices hearing function, since it involves drilling through the inner ear. So hearing sacrifice unavoidable
Autologous fat graft required |
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| Retrosigmoid approach | The surgery entails a curvilinear, vertically oriented occipital incision and a craniotomy positioned just posterior and inferior to the sigmoid and transverse sinuses. Once the dura is opened, the posterior lip of the IAC is removed to expose the extension of the tumor into this bony canal. Drilling is typically limited by the posterior semicircular canal and vestibule, which cannot be breached if the hearing is to be preserved | Wide-field visualization of posterior fossa
Possible hearing conservation or preservation |
May be difficult to visualize the lateral-most aspect of the IAC
May require cerebellar retraction |
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| Middle cranial fossa approach | used only for small tumors confined to the IAC or those with less than 1 cm of medial extension into the CPA, when hearing preservation is a primary goal.
The procedure includes a temporal incision and a craniotomy centered just above the root of the zygoma. Extradural dissection is then performed under the temporal dura, and the bone covering the IAC is removed to provide access to the tumor. |
Usually offers access to full length of IAC
Possible hearing conservation or preservation |
Requires temporal lobe retraction
Limited CPA access for larger tumors |
The goal is : maximal tumor removal with preservation of neurologic function. Intraoperative facial-nerve monitoring with electromyography is routinely used. Cochlear-nerve monitoring is frequently used when hearing preservation is attempted, and monitoring of other regional cranial nerves may be incorporated for large tumors. hospitalized for 2 to 4 days after the procedure and are ambulatory at the time of discharge. The risk of tumor recurrence after gross total resection is 0 to 2%.
Fortunately, the risk of other major neurovascular complications, such as permanent injury to other regional cranial nerves or perioperative stroke, is rare, even with large tumors. The prevalence of postoperative cerebrospinal fluid leak is 9 to 13%, aseptic meningitis 2 to 4%, and culture-positive bacterial meningitis 1%. eye dryness. incomplete eye closure must be aggressively treated to reduce the risk of exposure keratopathy, commonly manifested as blurred vision, ocular pain, and redness.
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.
After radiosurgery, patients undergo audiometric evaluation and MRI studies annually for the first 3 years, then every other year until 10 years, then every 5 years indefinitely
Rehabilitation
Those with long-term facial-nerve paralysis, bilateral hearing loss, or chronic dizziness or imbalance may require rehabilitative intervention.
Bilateral hearing loss:
Patients in whom serviceable hearing is maintained in the ipsilateral ear, observation (i.e., no additional hearing rehabilitation) or use of a conventional hearing aid is generally adequate. surgical means (i.e., bone-conduction implants) or nonsurgical means (e.g., contralateral routing of signals [CROS] hearing aids.
Facial-nerve paralysis:
t facial-nerve paralysis is uncommon overall, but the risk approaches 50% among patients with large tumors. Flaccid paralysis and eye dryness are primary concerns in the early postoperative period. Eye lubricants and moisture chambers generally provide adequate protection. referral to an ophthalmologist for uppereyelid weight placement, punctal plugs, or tarsorrhaphy should be considered if longer-term paralysis is anticipated or ophthalmologic complications appear. . Improvement is generally greatest within 6 months after the onset of paralysis, but continued recovery can be seen for up to 18 months
Dizziness or imbalance:
people who report substantial dizziness or imbalance should undergo a comprehensive balance assessment to accurately identify any coexisting disorders and to assess and mitigate the risk of falling. Common conditions that may exacerbate dizziness include peripheral neuropathy, age-related loss of contralateral vestibular function, vision loss, and vestibular migraine. mammalian peripheral vestibular system has limited regenerative capacity. Thus, balance therapy is the therapeutic mainstay for people who have troublesome symptoms related to chronic vestibular hypofunction