Acoustic neuroma, also called a schwannoma, is a benign tumor that arises from Schwann cells covering the eighth cranial nerve (vestibulocochlear nerve) which is responsible for
transmitting sensory information of hearing and balance to the brain. Schwann cells provide a protective sheath to the vestibulocochlear nerve. Most of the schwannomas are sporadic and only 5% are genetic. Acoustic neuroma grows very slowly or not at all and can remain confined to the bony ear canal for decades. The symptoms appear to be due to direct damage to the cranial nerve or increased intracranial pressure due to the growing tumor. The main complication arises when the tumor compresses the surrounding soft tissue parts of the brain. This limits the flow of fluid to nearby areas such as surrounding other cranial nerves, cerebellum, and brainstem, and can result in hydrocephalus (buildup of fluid in the cavities of the brain).
Types
Acoustic neuroma can be divided into 2 types; unilateral acoustic neuroma and bilateral acoustic neuroma based on whether the hearing is affected in one or both the ears.
Unilateral acoustic neuroma, also called sporadic type, affects hearing in only one ear. The sporadic cases are usually diagnosed during the 5th or 6th decade of life.
Bilateral acoustic neuroma also called genetic type, affects hearing in both ears. Genetic schwannomas are closely related to neurofibromatosis type II (NF2) and are caused by a mutation on chromosome 22. It manifests, earlier, during the 3rd decade of life.
Global perspective
Schwannoma constitutes about 8% of all brain tumors that develop inside the brain. In rare cases, acoustic neuromas can develop outside the brain parenchyma. Unlike other brain tumors, it does not spread to other parts of the brain and affects women more often than men. ​ Most cases of acoustic neuroma develop in individuals between the ages of 30 and 60 years. Although quite rare, they can develop in children as well. Every year, around 2 in 100,000 people are diagnosed with an acoustic neuroma.
Indian perspective
The symptoms are dependent on the location, size, and growth of the tumor. The most common symptoms of acoustic neuroma are hearing loss, tinnitus, and imbalance. Sporadic schwannomas have unilateral hearing loss whereas genetic schwannomas have bilateral hearing loss. Tinnitus is described as hissing, ringing, buzzing, or roaring in the ear. It occurs in almost 80% of acoustic neuroma patients. Other symptoms include decreased word understanding, vertigo, headaches, and facial numbness. Initially, when the tumor is limited in size and is located on the bony canal, it only causes unilateral hearing loss, tinnitus, and dizziness. But if the tumor grows and interacts with nearby parts, balance problems and involuntary eye movements are encountered. Various other symptoms appear depending upon the part of the brain with which the tumor is interfering (Table 1).
Part of the brain compressed | Associated symptom |
The facial nerve (7th cranial nerve) | Twitching of facial muscles increased lacrimation and facial weakness. In severe cases, secretions from the eyes, palate, and nose, and loss of ability to taste on some parts of the tongue. |
Trigeminal nerve (5th cranial nerve) | Sensation problems, numbness, or facial pain. These symptoms are rare due to the distance of the trigeminal nerve from the tumor. |
Cerebellum | Incoordination and ataxia |
Brainstem | Hydrocephalus |
Glossopharyngeal (9th cranial nerve) and vagus nerve (10th cranial nerve) | Difficulties in swallowing, inability to taste on some parts of the tongue, and hoarseness of voice. |
Diagnosis
Diagnosis of acoustic neuroma starts with a physical examination when the patient presents with unilateral hearing loss which occurs in 90% of acoustic neuroma cases. This hearing loss is detected by a device called pure tone audiometry. The most common finding under pure tone audiometry is high-frequency hearing loss. Gadolinium-enhanced magnetic resonance imaging (MRI) provides a definite diagnosis of acoustic neuroma.
Treatment
As in other types of brain tumors, there is no single ‘best’ treatment for acoustic neuroma. The type of treatment depends on the size and location of the tumor, rate of growth, and the general condition of the patient. The common options of treatment are observation, surgery, and radiotherapy. The wait and watch strategy of observation is adopted in case of slow tumors. Moreover, the surgery itself may cause hearing loss so the risks are weighed before proceeding to surgery.
The retrosigmoid (RS) surgical approach to acoustic neuromas is currently the most common and is associated with post-surgical complications.
The translabyrinthine (TL) approach does not attempt to preserve hearing and does not open as much of the skull. This approach is generally a good one in terms of little collateral damage, but of course, there is no expectation that hearing will be preserved. It is not a good approach for large tumors as the visibility is poor.
The middle cranial fossa (MCF) approach, can theoretically preserve hearing, but like the rectosigmoid, the approach is intrinsically more dangerous as it involves retraction of the brain, and is also more prone to complications.
Radiotherapy provides a non-invasive option for dismantling the tumor. With radiotherapy, the tumor can recur at any time after the treatment. Stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT) have established that these techniques as safe, efficient, and minimally invasive alternatives.
In SRS, high-intensity beams of radiation are delivered all at once and are focused on a single spot in the brain i.e., a tumor.
SRT, on the other hand, delivers the radiation in small doses, over a longer period.
Quality of life
In addition to the primary treatment, interventions are aimed only at improving the quality of life of the patient as an individual. These strategies include physiotherapy, vestibular rehabilitation, and long-term monitoring.
Physiotherapy helps manage muscle problems related either directly to the disease or the postsurgical complications. Exercise can improve facial palsy and coordination problems.
In addition to lip-reading and sign language training, the patient can benefit from an auditory brainstem implant. Customized vestibular rehabilitation incorporating adaptation, habituation, balance, and mobility is helpful for patients suffering from incoordination and vertigo after the surgery.
The patient has been regularly monitored for recurrence of the tumor or any postsurgical complication.
​​These interventions help the patient return to normal life.