Acoustic neuroma is a rare tumor that originates from the Schwann cells of the eighth cranial nerve, also known as vestibulocochlear nerve. This condition is characterized by unilateral hearing loss but has other sequelae as well.
Acoustic neuroma, also referred to as vestibular schwannoma, is a benign tumor that derives from the Schwann cells of the vestibulocochlear nerve. Specifically, the vestibular branch of the nerve accounts for almost all cases of this condition. Most tumors are slow growing and some can expand from the internal auditory canal to the cerebellopontine angle. If the lesion enlarges, it may encompass the brainstem, cerebellum, and compress other cranial nerves. The etiology is unknown but there are potential risk factors.
The chief symptom at initial presentation is the unilateral sensorineural hearing loss. Other symptoms that develop as the tumor progresses include tinnitus, disequilibrium, headache, and other abnormalities that reflect affected anatomic structures.
Unilateral hearing impairment should warrant a high index of suspicion for acoustic neuroma. The workup consists of a detailed assessment of the patient's clinical picture and history, a physical examination with a hearing evaluation, and imaging tests. Due to the availability of gadolinium-enhanced magnetic resonance imaging (MRI), the detection of acoustic neuromas is on the rise.
Patients with acoustic neuroma can be managed by observation, radiation therapy, or microsurgery. The appropriate treatment depends on variables such as the size, location, and growth rate of the tumor, as well as the patient's age, comorbid medical issues, and patient preference.
The major known risk factor for developing an acoustic neuroma is high-dose ionizing radiation, although the majority of affected individuals do not have apparent risk factors. Some studies suggest that exposure to loud noise   and nonmedullary thyroid cancer . may be risk factors for developing this tumor.
Neurofibromatosis type 2
Acoustic neurofibroma is the major clinical feature of the genetic disorder, neurofibromatosis type 2, which is the result of a mutation of a tumor suppressor gene that is found on chromosome 22.
The incidence of acoustic neuroma is 1 per population of 100,000 annually. The incidence is rising which may be explained by the widespread utilization of various imaging modalities such as MRI and computed tomography (CT), leading to a higher number of incidental diagnoses  . The prevalence in the population is likely higher according to autopsy studies  .
With regards to patient demographics, the median age at diagnosis of the condition is about 50 years old . This tumor comprises almost 8% of adult intracranial tumors and at least 80% of cerebellopontine tumors.
Acoustic neuromas are benign, slow growing, and noninvasive. They rarely undergo malignant transformation. Additionally, there are three distinct patterns of growth as some will remain dormant, while others may grow slowly or rapidly. While most are slow growers, tumors may erratically alternate between these patterns.
Location of tumor and symptomatology
In addition to its size, the tumor is also characterized by its location. It may develop in the auditory canal and expand into the cerebellopontine angle. The symptoms result from the compression of nerves, vessels, spinal fluid spaces, as well as the displacement of the brain stem.
Most of acoustic neuromas arise from Schwann cells located on the vestibular segment of the vestibulocochlear nerve. When these tumors compress the vestibular fibers, the resultant damage ensues in a gradual and slow manner. Therefore, vertigo is not a prominent feature.
Note that lesions emerging in the internal auditory canal may compress and impinge upon the vestibular nerve, cochlear nerve, or labyrinthine artery. These mechanisms result in hearing loss and vestibular disturbance.
When these tumors grow, they may extend into the cerebellopontine angle, which is an empty space that contains spinal fluid. Larger lesions can compress other cranial nerves, impinge on neighboring structures, and shift the position of the brain stem. If greater than 4cm, they cause hydrocephalus.
The prognosis can be evaluated by the outcome of each symptom. For example, cases with hearing preservation have been noted to increase during the last decade. It is likely that post-radiation and post-surgical patients have similar rates of conservation. However, hearing impairment has been observed in patients who underwent surgery.
Although tinnitus may improve after surgery, it can actually worsen in a minority of patients. Furthermore, it may develop postoperatively as well. The post-surgical recurrence rate of acoustic neuroma is less than 5%. Note that patients may have residual tumor since intraoperative visualization is quite challenging in many cases.
Hearing loss following surgical resection may require rehabilitation such as the bone-anchored hearing aid (BAHA) or contralateral routing of signals (CROS) hearing aid. Additionally, patients with post-surgical facial weakness warrant artificial tears and other eye care products until the nerve recovers. Generally, facial nerve recovery is associated with tumors that are smaller than 1.5cm. Moreover, the utilization of intraoperative facial nerve monitoring may lead to the improvement of function.
Acoustic neuromas are unilateral in greater than 90% of patients . The bilateral cases are confined to neurofibromatosis type 2 .
The most common presenting feature in acoustic neuroma is the unilateral sensorineural hearing loss, which is slow and gradual in the majority of patients. This occurs secondary to either vascular insufficiency or direct injury to the cochlear nerve. The hearing deficit can be sudden or it can fluctuate depending on cochlear perfusion and its interruption. Some patients may recover their hearing spontaneously. Note that hearing impairment is not related to the size of the acoustic neuroma.
Unilateral tinnitus very often coexists with hearing loss. Although if present alone, the presence of tinnitus should prompt suspicion for acoustic neuroma.
The majority of patients will have low speech discrimination scores as displayed on audiological tests. According to one study, almost 65% of individuals demonstrated a link between hearing loss severity and reduced speech discrimination .
Approximately half of acoustic neuroma patients will experience balance disturbance although vertigo and disequilibrium are not present initially.
At least half of the patients exhibit headaches especially in cases with growing tumors and those with obstructive hydrocephalus.
Another manifestation is facial numbness, which is exhibited in approximately 25% of patients. Up to 70% of those with large tumors exhibit hypoesthesia on exam but are not aware of it.
Individuals presenting with unilateral sensorineural hearing deficits should be presumed to have an acoustic neuroma. The diagnosis should be confirmed through evaluation of the clinical picture, history, a physical exam with focus on hearing and neurologic assessment, and imaging studies.
The confirmatory imaging modality is the gadolinium-enhanced MRI, which detects small lesions measuring 1 to 2mm in diameter. This is contraindicated in patients with ferromagnetic implants. Adding a fast-spin echo feature to MRI imaging is rapid and inexpensive but is ineffective in diagnosing differentials such as demyelinating disease. Fine-cut CT with contrast can identify medium or large tumors in the internal auditory canal but fail to discover lesions less than 1 to 1.5cm.
Histologic analysis of tissue samples of portrays characteristic histologic findings. There are two types of tissues: Antoni A and Antoni B. The former exhibits a pattern of spindle cells with palisading nuclei referred to as Verocay bodies while the latter displays loose stroma with fewer cells.
While previously common, audiometric studies are not used for diagnostic purposes anymore.
The are three main therapeutic approaches in the treatment of acoustic neuroma which include observation, stereotactic radiation therapy, and surgical removal of the lesion.
When deciding the best course of management, the medical team should assess numerous factors . such the parent's hearing status, the size and growth rate of the tumor, the patient's life expectancy, the probability of developing facial nerve paralysis, and the risks of surgery. The latter includes outcomes such as hearing loss and other postsurgical complications. Another variable for consideration is whether the patient has NF2.
These tumors are typically benign and slow growing. Therefore, careful observation is appropriate for those with small tumors, particularly in the elderly. Groups who are good candidates for observation are the elderly, those with comorbid medical conditions that render them risky for surgery, and patients with the tumor presenting on the side of the unaffected ear. Furthermore, patient preference may favor observation.
Studies have indicated that up to 40% of patients under observation will eventually warrant intervention and many will become ineligible for surgery during this time period.
As an alternative to microsurgery, this treatment modality applies radiation to target tissues hence minimizing the potential exposure to nearby tissues. The goals of radiation are to inhibit the growth of the tumor cells and to obliterate the lesion's vascular perfusion. One study reported that Gamma Knife radiation is effective in tumor control . although another investigation observed that these tumors are resistant to low dose radiation .
Surgical excision is the preferred treatment for the elimination of acoustic neuroma. There are numerous techniques that may be utilized depending on the surgeon's technical skills, size and location of the tumor, and the desire for hearing preservation.
When choosing between radiation and microsurgery, there are many variables to be considered such as postsrgical morbidity and mortality, hospital length of stay and cost, etc.
The patient is monitored with periodic imaging studies and hearing tests.
There are no preventative measures. However, it is important for patients with hearing loss to seek medical care in a timely manner.
What is an acoustic neuroma?
This is a rare tumor that grows on the cranial nerve called vestibulocochlear nerve which travels from the brainstem to the ear canal. Specifically, this nerve is responsible for hearing and balance maintenance. The exact cause of this condition is unknown.
This benign tumor rarely becomes cancerous. Also, most are slow growing and non-invasive.
What are the risk factors for developing this condition?
Exposure to loud noise may increase the probability of developing acoustic neuroma. Also, having a personal or family history of neurofibromatosis type 2 (NF2) may also be a risk factor.
What are the signs and symptoms?
Although this is a benign tumor, it can cause serious problems for the patient. Early signs and symptoms are as follows:
The growing neuroma can cause:
How is it diagnosed?
The clinician will assess the patient's symptoms, obtain the medical history, perform a physical exam and a detailed hearing evaluation. Also, imaging tests such as the following will be ordered:
How is it treated?
The medical team will choose the most appropriate treatment by considering factors such as the size, location, and growth of the acoustic tumor. Also, the patient's age, overall health, and preference will also be important factors. There are 3 main options for treatment:
The patients will follow-up with their doctor and undergo periodic imaging and hearing tests.
What is the prognosis?
The probability of the tumor recurring after surgery is less than 5%.
Hearing may be preserved in patients who have undergone surgery as well as those who received radiation. However, hearing loss may occur after surgery as well.