An acoustic neuroma is a benign tumor that grows from the 8th cranial nerve (the nerve of hearing and balance) as it passes through the base of the brain. This tumor attached to the 8th nerve, is the 7th nerve, the facial nerve, which controls movements of the face such as blinking and smiling. The cells involved in the tumor are Schwann cells, which create the insulating sheath (myelin) of the nerve. The growth begins at the bony canal at the base of skull where all these nerves travel through and extends outwards into the base of the skull, forming a distinctive pear-shaped tumor. The growth of the tumor is generally slow, as this tumor is non-cancerous. While it does not typically infiltrate and invade the brain, it may push on other nerves or on the brainstem, resulting in symptoms.
Cross section of a large acoustic neuroma
Recent reports state that acoustic neuromas are becoming more common, but this phenomenon may be due to advances in imaging which allow physicians to see more of the tumors. Current estimates indicate that acoustic neuromas occur in about 2 people per 100,000. They are most commonly diagnosed in patients who are between the ages of 30 and 60.
There is no well-defined cause of acoustic neuromas. Some studies performed in the past have linked the tumor with prolonged exposure to loud noise, but this is not confirmed. There may also be a link to exposure to radiation over one’s lifetime. There is no hereditary basis for acoustic neuromas - most cases are spontaneous.
What are the symptoms of an acoustic neuroma?
Often acoustic neuromas cause no symptoms at all – this is especially true of smaller tumors. Larger tumors, however, may compress structures around them and cause symptoms as a result. The earliest symptom seen in 90% of cases is a reduction in hearing on the side with the tumor. This may be especially noticeable when speaking on the phone.
One may also notice tinnitus or “ringing” in the ear as an early sign. Both of these symptoms are a result of the disturbance of the hearing nerve by the tumor. Another potential symptom is unsteadiness or balance problems. This is caused by the tumor disturbing the nerve to the balance system in the ear – this nerve is the same as that for hearing and is the part of the nerve where the tumor begins its growth.
Larger tumors may cause even more symptoms. If the facial nerve, which runs alongside the auditory nerve, becomes involved, there may be weakness of the muscles of the face. Additionally, the increase in pressure inside the head can lead to headache, weakness, and confusion. In extreme cases, large tumors may compress the trigeminal nerve, which provides sensation to the face, resulting in facial numbness. Such large tumors may also impinge upon the brainstem, where vital functions, such as breathing and heart rate, are coordinated. Large tumors may cause difficulty in walking or unsteadiness.
How are acoustic neuromas diagnosed?
The current gold standard for diagnosing acoustic neuromas is an MRI scan of the brain. This is often performed with the contrast material, gadolinium, which helps to define the tumor precisely. An audiogram should be performed along with the MRI to test hearing function in both ears. Finally, some patients may undergo an auditory brainstem response test. This test measures the conduction of electrical impulses along the nerve to the brain. A defect in conduction through this nerve may suggest the presence of a tumor.
What are the treatment options?
There are currently three main treatment options that patients and physicians can choose from:
This the most conservative approach to the acoustic neuroma. Since the tumor is not cancerous and grows very slowly, many patients choose to simply follow the tumor over a period of time. This option is especially favorable for older patients and patients with very small tumors which were discovered incidentally. If this option is chosen, the patient must have MRIs on a yearly basis for at least 5 years to make sure the tumor is not growing.
Some patients choose to proceed with surgery. This option is good for patients with larger tumors that are pushing on the brainstem. This method can either remove the tumor completely or partially, depending on the decision reached by the surgeon and the patient. Partial removal will decrease the tumor’s size and reduce or eliminate any symptoms associated with it. This does, however, require regular MRI follow-up to ensure the tumor is not growing. Complete removal of the tumor is possible in many cases. In attempting complete removal, the goal of the surgeon is to preserve both the facial and hearing nerves. Both nerves are electrically monitored during the entire surgery, and the surgeon is alerted immediately if any defect is detected. After surgery, most patients are in the hospital for 3-5 days and have 4-6 weeks of at-home recovery. Many patients do not need this much time to reach the same level of wellness as before the surgery.
There are currently three surgical approaches, and the patient and physician should discuss which of the three is best to use for a particular patient:
- Translabyrinthine – This approach is used when hearing preservation is not required (i.e. when the patient has already lost hearing in the affected ear). The incision is located behind the ear, and the surgeon drills through the mastoid bone, exposing the tumor from below. This approach allows the surgeon to see the facial nerve early and ensure its preservation. In this procedure, a small amount of fat is taken from the abdomen and used to pack the surgery area to prevent any leak of brain fluid [cerebrospinal fluid (CSF)] after surgery.
- Retrosigmoid – This approach is used when hearing preservation is a major goal. The incision is several centimeters behind the ear, and the tumor is exposed from its back side. This approach also allows identification and preservation of the facial nerve.
- Middle fossa – This approach is also used to preserve hearing. The incision is on the top part of the head, a few centimeters above the ear. This allows exposure of the tumor from the top.
Removal of an acoustic neuroma via the retrosigmoid approach
Radiation is the third treatment option for acoustic neuromas. There are two ways to deliver this treatment – one is radiosurgery, which involves one large dose of radiation delivered to the tumor. The other is fractionated radiotherapy, in which multiple small doses of radiation are delivered over a period of several weeks. This procedure is non-invasive and requires no hospital stay for recovery.
In radiosurgery, hundreds of small, concentrated beams of radiation are aimed at the tumor using one of the three radiosurgical devices – the Gamma Knife, Cyber Knife, or Novalis. This procedure itself takes 30-50 minutes, and the patient is placed in a stereotactic headframe beforehand on the day of the procedure, which stabilizes the patient’s head and allows the treatment team to calibrate the machine to the patient’s head and tumor. A potential complication of radiosurgery is transient facial numbness and hearing reduction/loss. These are usually temporary and will pass within several weeks of the procedure.
The patients who undergo fractionated radiotherapy receive smaller doses of radiation over several visits. Each visit takes only a couple of hours, and the patient is able to go about their business before and after the visit. Fractionated radiotherapy has less risk to the facial and hearing nerves.
It is important to note that radiation therapy does not remove the tumor. It may, in some cases, cause the tumor to shrink. The main goal of radiation is to stop the growth of the tumor. As a result, the patient must return for MRI scans on a yearly basis to check for any new growth.
What are the potential complications of surgery?
As with any surgery, there is the potential for infection following the operation, although this risk is quite small. Also, as with any surgery involving the brain, there is the potential for the leak of CSF (cerebrospinal fluid) out of the nose or the incision site – this is a rare occurrence, but if it does happen, the surgeon should be alerted immediately. Other risks include stroke, hearing loss, temporary paralysis of the face muscles, dizziness/vertigo, and headaches. One may also experience dryness of the eyes, which can be treated with eye drops. Hearing loss and paralysis of the facial muscles may occur because of the manipulation of these nerves during the removal of the tumor. Tinnitus may also result from this manipulation. Depending on the extent of the damage to these nerves and the amount of involvement with the tumor, this may become permanent. The temporary paralysis can be treated by physical therapy.
Most patients experience vertigo, or a feeling of the room spinning around them, for about one week or more after the surgery. Headaches are a common complaint after surgery. These will pass within a few weeks, but some patients may experience them on a chronic basis. These patients can be treated with a variety of medications to relieve their discomfort. The occurrence of these complications is lowest with smaller tumors.
What are the complications of radiation therapy?
As mentioned before, radiotherapy does not remove the tumor, but may shrink it. However, since it is non-invasive, radiation therapy does not carry the risks of infection, CSF leak, and stroke that are associated with surgery. Also unlike surgery, there is usually no vertigo after the procedure. Radiosurgery patients may experience some temporary facial numbness and weakness, as well as hearing loss. Also, with fractionated radiotherapy, it may take up to 18 months for the therapy to completely stop growth of the tumor. Also unlike surgery, there is a very small chance of headache after radiation therapy.
After surgery or radiation, the patient should have regular follow-up MRI scans and appointments with her or his surgeon to ensure that the tumor does not grow back. Most patients who undergo surgical removal experience “cure” or complete removal of the tumor. However, in small portion of people, the tumor can recur, and it is important that the recurrence be closely followed. Any resulting complications should be treated accordingly with medication and physical therapy as needed.
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