An acoustic neuroma (more correctly known as vestibular schwannoma) is a benign (nonmalignant) tumor originating from Schwann cells, which surround the vestibular nerves, the nerves that carry balance signals from the inner ear to the brain.  A patient with an acoustic neuroma usually notices hearing loss and ringing in only one ear and sometimes dizziness.  It is the “asymmetry” in the hearing that usually leads to the diagnosis of an acoustic neuroma, as the hearing in the opposite ear will not be affected. The hearing, balance, and facial nerve, as well as blood vessels that supply the inner ear, pass through the internal auditory canal, a narrow bony canal between the brainstem and the inner ear structures. As the acoustic neuroma enlarges, it puts pressure on the hearing nerve, leading to the loss of hearing.  As it enlarges further, it may also put pressure on the brain stem, compromising vital functions if it becomes very large.  In most patients, acoustic neuromas grow very slowly over a period of years.  They may occur at any age, are usually in one ear only, and are not hereditary. 

Diagnosis

An acoustic neuroma is diagnosed with an MRI scan of the brain with gadolinium (contrast dye).  This is generally ordered when a patient’s hearing test demonstrates loss that is significantly poorer in one ear than the other or when the patient has had a sudden decrease in the hearing in only one ear.  Sometimes acoustic neuromas are discovered incidentally when an MRI of the brain is ordered for some other reason.  If you have been diagnosed with an acoustic neuroma, your doctor will likely refer you to a neurotologist (surgeon specializing in disorders of hearing and balance) like Dr. Paul F. Shea at the Shea Clinic, who will discuss the options for treatment with you. 

Tumor size

Acoustics neuromas are classified as small, medium, or large.

Small (less than 1 centimeter) tumor – A small acoustic neuroma is still confined to the internal auditory canal (narrow bony canal that extends from the inner ear to the brain).

Medium (1 centimeter to 2.5 centimeter) tumor – A medium-sized acoustic neuroma extends from the internal auditory canal into the cerebellopontine angle (area adjacent to the brain stem) but is usually not yet putting pressure on the brain stem.

Large (greater than 2.5 centimeter) tumor – A large acoustic neuroma extends out of the internal auditory canal into the cerebellopontine angle and is large enough to put pressure on the brain stem.  

Options for treatment

There are several options for treatment for an acoustic neuroma.  If you are diagnosed with one, you and your surgeon will make the decision of which treatment is best for you.  This decision will be based on several factors, including your age, general health, and how much hearing you still have on the affected side.  There are three main types of treatment for acoustic neuromas:

1. Observation   As stated before, acoustic neuromas are slow growing, benign tumors.  They may go through periods where they do not grow at all.  For many patients, once diagnosed, a period of observation may be appropriate to observe the tumor for signs of growth before any decision about the necessity for treatment is made.  During this time the surgeon will generally order follow-up MRI scans every six to twelve months to monitor the size of the tumor.  The patient should have an audiogram (hearing test) once a year to monitor the hearing as well. 
2. Radiation therapy (also known as stereotactic radiation) Acoustic neuromas may also be treated with a form of radiation.  The common name for this is “gamma knife”.  The patient’s head is placed into a machine that aims beams of radiation at the tumor to kill the tumor cells and arrest the growth of the tumor.  The treatment is painless and is done as an outpatient procedure in a single session.  Following treatment, MRI scans are performed on a periodic basis to watch for signs that the tumor has started growing again.  Although low-risk, gamma knife does not result in disappearance of the tumor.  In rare cases, acoustic neuromas can begin to grow again, necessitating surgical removal.  Gamma knife is well-suited for older patients or patients who cannot or do not wish to undergo a lengthy surgical procedure to remove the tumor.
3. Surgical resection Surgical removal is a procedure in which all or part of the acoustic neuroma is removed via a surgical procedure. The decision about when this is appropriate is made during a consultation between patient and the surgeon. In general, surgical removal is recommended when the tumor is enlarging or is already large enough to put pressure on the brainstem, or when it is causing some other complication.  Some surgeons might recommend surgical removal to a younger patient when there is a desire to preserve the patient’s hearing on the affected side.  There are three surgical approaches, listed below, which can be used to remove an acoustic neuroma and the decision of which is best is based on tumor size, the amount of residual hearing, and sometimes the preference of the surgeon.  In many cases it is not possible to save the hearing.  In general, the larger the tumor, the less the chance of saving the hearing, and if hearing is saved, it will be no better than it was preoperatively.  Surgical resection is a major procedure and involves risk, although the risks are low with a team approach, including a neurotologist like Dr. Paul Shea, a neurosurgeon, an anesthesiologist, a surgical nurse, a circulating nurse, and an audiologist monitoring function of the facial nerve during the operation.
4. Translabyrinthine approach – A curved incision is made behind the ear and the mastoid and inner ear structures are removed to expose the tumor.  After tumor removal, fat from the abdomen is used to seal the opening and the wound is closed.  This method sacrifices any remaining hearing in the affected ear but reduces the possibility of brain swelling and other complications and offers an easier recovery in most cases.  It is the most frequently used approach and is suitable for any size tumor.    
5. Middle cranial fossa approach – A vertical incision is made above the ear and a “bone window” is made in the side of the skull to expose the skull base.  Bone is then removed to expose the tumor, which is then resected.  This method is used for smaller and more laterally located tumors and when there is a desire to preserve residual hearing on the affected side.  It is the most technically challenging for the surgeon and carries the risk of additional complications.  
6. Retrosigmoid Approach – A curved incision is made behind the ear, bone is removed behind the mastoid to enter the posterior fossa (rear compartment of the skull), and the brain is retracted slightly toward the midline to expose the tumor, which is then resected.  This method is suitable for all size tumors and in cases where there is a desire to preserve residual hearing.  It carries a significant risk of headaches postoperatively.  

Complications

All treatments for acoustic neuroma involve the possibility of further loss of hearing, loss of balance, and weakness or paralysis of the face.  Surgical procedures also include the possibility of complications from anesthesia, headaches, spinal fluid leakage, bleeding, infection, and even death, although this is extremely unlikely.   

Goals of treatment

The primary objective in treatment for acoustic neuroma is to balance the effect of the tumor on the patient with the effect of treatment in order to minimize the impact it has on the patient’s functional status and to preserve to best quality of life.  Dr. Paul Shea at the Shea Clinic is fellowship-trained in neurotology and partners with a team of highly trained specialists to offer patients with acoustic neuroma state of the art care and management of their condition.  

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