Acoustic neuromas

ALSO KNOWN AS: Vestibular schwannomas

RELATED CONDITIONS: Neurofibromatosis type 2

DEFINITION: Acoustic neuromas or vestibular schwannomas are benign (or nonmalignant) tumors that originate from Schwann cells surrounding the vestibular nerveeighth cranial nervein the internal auditory canal. The term neuroma is somewhat misleading, as the tumors are not neuromas, nor do they arise from the acoustic or cochlear nerve. Acoustic neuroma typically occurs as unilateralone-sidedsporadic tumors in 95 percent of all cases, but in rare cases, tumors can be bilateraltwo-sidedand are associated with an inherited syndrome called neurofibromatosis type 2 (NF2). Approximately 5 percent of patients diagnosed with acoustic neuromas have NF2 type, which has an estimated incidence of one in thirty-three thousand people worldwide.

Risk factors: Although high-dose ionizing radiation, such as from computed tomography scans in childhood or adolescence, is a known risk factor of acoustic neuroma, environmental factors including long-term loud noise exposure, radio frequency electromagnetic fields, and allergens have been reported as potential sources that may contribute to the formation of acoustic neuroma. Cell phone use has been proposed as a potential risk factor for acoustic neuroma, but the association between cell phone use and acoustic neuroma has inconsistent evidence in case-control studies.

Etiology and the disease process: Acoustic neuroma is caused by an abnormal proliferation of Schwann cells, although the most common typeunilateraloccurs sporadically. Currently, the etiology of acoustic neuroma is not known. As an anomaly, it is rarely inherited. Nonetheless, neurofibromatosis type 2 should be suspected in young patients and those with family history. Neurofibromatosis is an autosomal dominant disease; thus patients who inherit a defective copy of the NF2 tumor-suppressor gene have a 95 percent chance of developing bilateral tumors; however, half of the cases have no family history of NF2, which could indicate mutations in the germline that were not inherited.

Incidence: Sporadic acoustic tumors, the most common form of manifestation, occur in approximately 1 per 1 million persons per year in other words the chance of an average person developing an acoustic neuroma in his or her lifetime is about 1 in 100,000. Neurofibromatosis is rarer, with only several thousand affected persons in the entire United States, corresponding to 1 in 33,000 individuals. However, a study has highlighted that the true incidence of acoustic neuroma may be higher than what has previously been estimated, as 7 unsuspected schwannomas per 10,000 brain magnetic resonance imaging studies were identified, an equivalent of 0.07 percent. Acoustic neuromas, or schwannomas, occur largely in adults, typically in the fourth and fifth decades, with a mean presentation age of fifty years. They are uncommon in children.

Symptoms: Acoustic neuromas are histologically benignhowever, if large, they can cause hydrocephalus, brain stem compression, herniation, and eventually death. Hearing loss is the most prevalent symptom, occurring in approximately 94 percent of patients, and the duration of hearing loss may extend to three or four years before clinical diagnosis is made a majority of the patients experience one-sided, slowly progressing hearing impairment associated with high-frequency sounds. Alternative complaints or accompanying symptoms include tinnitus, dizziness, vertigo, and a sensation of fullness in the ear. With the progression of the tumor, patients may experience facial numbness, headaches, loss of coordination, and difficulty in swallowing. Vertigo is prevalent with smaller tumors, while unsteadiness, headache, and facial sensory disturbance are associated with large tumors.

Screening and diagnosis: Acoustic neuroma can be diagnosed by a number of screening methods. These include conventional audiometry, auditory brain-stem response (ABR), and gadolinium-enhanced magnetic resonance imaging (MRI). Among these, gadolinium-enhanced MRI is the optimal diagnostic test. Typically, on MRI scans acoustic neuromas appear as dense and uniformly enhanced. Acoustic neuromas are staged according to their location and size. Small tumors are less than 1.5 centimeters (cm), moderate tumors are between 1.5 and 3 cm, and large tumors are greater than 3 cm in size. Based on the location, they are staged as the following:

• intracanalicular: located in the internal auditory canal
• cisternal: extending outside the internal auditory canal
• compressive: having progressed to touch the cerebellum or brain stem
• hydrocephalus: having progressed to obstruct the drainage of cerebrospinal fluid in the fourth ventricle

Treatment and therapy: The treatment options for acoustic neuroma include observation, microsurgery, vestibular rehabilitation, stereotactic radiosurgery, and radiotherapy. Patients with advanced age or those deemed unfit for surgical intervention with small tumors at diagnosis are observed; treatment is withheld while tumor progression is monitored in serial imaging studies. However, treatment by observation has its own risks, as there is a greater risk of losing useful hearing.

If microsurgery is the choice for treatment, many factors need to be assessed when evaluating its primary and secondary outcomes. First, there are three standard surgical approaches, each with its own advantages and disadvantages: suboccipital, middle fossa, and translabyrinthine. In the suboccipital approach, the tumor is reached through the skull behind the ear. As the procedure involves the retraction of the cerebellum, the approach is intrinsically dangerous and prone to complications. Although the middle fossa method can preserve hearing in theory, this approach is also dangerous as it too requires the retraction of part of the brain. In the translabyrinthine approach, the tumor is accessed through the inner ear, and thus hearing loss is expected and inevitable. However, this method is unsuitable for large tumors. Second, microsurgery is technically challenging; therefore the rate of success is lower with a less experienced surgeon. Third, large tumorsgreater than 3 cmare difficult to resect without concomitant morbidity, such as facial palsy.

Some of the complications that may arise from microsurgery include:

• cerebrospinal fluid leak: 12 percent of all cases
• meningitis: 5 percent
• intracranial hemorrhage: 2 percent
• facial weakness with complete paralysis: 31 percent
• delayed or partial paresis: 50 percent)

Radiosurgery refers to the administration of a single fraction of radiotherapy using stereotactic techniques to localize the tumor and align the fields. Radiosurgery may be performed either with a Gamma Knife or a linear acceleratorlinacbased system. Gamma Knife is an emerging treatment option for those who are at high risk during microsurgery. High-dose Gamma Knife procedures are less favored due to the possibility of radiation complications; thus low-dose radiationfor example, 13 gray, or Gytherapies are advised because of safety and lower risk of facial weakness.

Radiotherapy refers to the administration of fractionated radiotherapy and includes stereotactic radiotherapyradiation with other than gamma raysand conventional radiotherapy techniques.

Complications of Gamma Knife radiosurgery include injury to facial and trigeminal nerves. However, with the current dosing regimen of 12.5 Gy, the risk of trigeminal or facial nerve injury has decreased significantly. Although the potential for complications is higher with microsurgical procedures than with radiotherapy or radiosurgery, an important issue that should be considered with irradiation therapies is the low risk of inducing malignancies within the radiation area. Current recommendations are to offer microsurgery and radiosurgery options for patients with definite treatment indications. While microsurgery is the treatment of choice for large tumors because of the low risk of radiation-induced malignancies, microsurgery is also considered for younger patients.

Prognosis, prevention, and outcomes: The microsurgical techniques for acoustic neuroma have improved the anatomical and functional preservation of the facial and cochlear nerves. These techniques, accompanied by continuous electrophysiological monitoring, have resulted in marked changes in the primary goals of management. In the past, the primary goal of acoustic neuroma management was to preserve the patient’s life, whereas the objective today is to preserve the neurological function. Long-term follow-ups show negligible recurrence rates, suggesting that the preservation of neurological function does not restrict the tumor removal. Despite these advances, loss of nerve function and even deafness may occur postoperatively in some cases.

In May 2024, researchers at the University of California-San Diego published results indicating a new medical procedure to remove an acoustic neuroma. This procedure was done by exposing the internal auditory canal and the area around the inner ear.  The reported outcomes were impressive. Preservation of hearing in patients occurred at a rate of 68 percent as well as high facial nerve outcomes in 94 percent of those who had received the procedure.

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