Acoustic Neuroma

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Acoustic neuroma (or Vestibular Schwannoma) is a benign primary intracranial tumor of the myelin forming cells called "Schwann cells" (Schwannoma) of the 8th cranial nerve --- also known as the acoustic nerve, (or more properly the vestibulocochlear nerve). Approximately 3000 cases are diagnosed each year in the United States with an incidence of about 1 in 100,000. Incidence peaks in the fifth and sixth decades and both sexes are affected equally.

Pathogenesis

Acoustic neuromas may occur sporadically, or in some cases occur as part of von Recklinhausen neurofibromatosis, in which case it may take on one of two forms.

  • In Neurofibromatosis type I, a schwannoma may sporadically involve the 8th nerve, usually in adult life, but may involve any other cranial nerve or the spinal root. Bilateral acoustic neuromas are rare in this type.
  • In Neurofibromatosis type II, bilateral acoustic neuromas are the hallmark and typically present before the age of 21. These tumors tend to involve the entire extend of the nerve and show a strong autosomal dominant inheritance. Incidence is about 5 to 10%.

The usual tumor in the adult presents as a solitary tumor, originating in the nerve. It usually arises from the vestibular portion of the 8th nerve, just within the internal auditory canal. As the tumor grows, it usually extends into the posterior fossa to occupy the angle between the cerebellum and the pons (cerebellopontine angle). Because of its position, it may also compress the 5th, 7th, and less often, the 9th and 10th cranial nerves. Later, it may compress the pons and lateral medulla, causing obstruction of the CSF and increased intracranial pressure.

Clinical manifestations

Earliest symptoms of acoustic neuromas include unilateral sensorineural hearing loss/deafness, disturbed sense of balance and altered gait, vertigo with associated nausea and vomiting, and pressure in the ear, all of which can be attributed to the disruption of normal vestibulocochlear nerve function. Additionally more than 80% of patients have reported tinnitus (most often a uniteral high-pitched ringing, sometimes a machinery-like roaring or hissing sound, like a steam kettle).

Large tumors that compresses the adjacent brainstem may affect other local cranial nerves. Involvement of the 7th cranial nerve (facial nerve) may lead to facial weakness, sensory impairment, and impairment of glandular secretions; involvement of the 5th cranial nerve (trigeminal nerve) may lead to loss of taste and loss of sensation in the face and mouth. Involvement of the 9th and 10th cranial nerves are uncommon, but may lead to altered gag or swallowing reflexes.

Even larger tumors may lead to increased intracranial pressure, with its associated symptoms such as headache, vomiting, and altered consciousness.

Diagnosis

Contrast-enhanced CT will detect almost all acoustic neuromas > 2.0 cm in diameter and project further than 1.5 cm into the cerebellopontine angle. Those tumors that are smaller may be detected by MRI with gadolinium enhancement. Audiology and vestibular tests should be concurrently evaluated using the Weber's and Rinne's test to assess for sensorineural versus conduction hearing loss.

Treatment

Indicated treatments for acoustic neuroma include surgical removal and radiotherapy.

Conservative treatment

Because these neuromata grow so slowly, a physician may opt for conservative treatment beginning with an observation period. In such a case, the tumor is monitored by annual MRI to monitor growth. Records suggest that about 45% of acoustic neuromata do not grow detectably over the 3-5 years of observation. In rare cases, acoustical neuromata have been known to shrink spontaneously. Often people with acoustic neruromata die of other causes before the neuroma becomes life-threatening. (This is especially true of elderly people possessing a small neuroma.)

Since the growth rate of an acoustic neuroma rarely accelerates, annual observation is essential.

Acoustic neuromata may cause either gradual or—less commonly—sudden hearing loss and tinnitus. However, the surgical and radiotherapy treatments are even more dangerous to the hearing in the affected ear.

Surgery

The surgery is done by several approaches and is associated with high incidence of complications and quality of life issues - but it often removes the tumor without recurrence. The vestibular nerve is usually removed on the operated side, resulting in severe imbalance, vertigo and dizziness. However, vestibular function improves rapidly due to compensation by the other ear and other balance mechanisms. Steadiness may never be 100% of the pre-surgical level, but patients are usually walking in the first week after surgery. Surgery also has a risk to the facial nerve which is "monitored" during the surgery. Best results (normal or near normal facial function) are most likely with small acoustic neuromas. The larger the tumor, the higher the risks associated with removing it. Three surgical approaches are commonly used. The first is the translabyrinthine, which destroys hearing in the affected ear. Of the surgical approaches, it tends to be the fastest (less anesthesia time) and allows the most complete removal of the tumor (less chance of recurrence). The two other approaches (suboccipital and middle fossa) are hearing preservation approaches, which have a chance of preserving some or all of the hearing in the affected ear. However, all or most of the hearing in the operated ear is lost approximately 50% of the time. In addition, a number of medical reports indicate that surgically preserved hearing in the operated ear is often not stable, but may deteriorate significantly over a period of years. The hearing preservation approaches tend to require longer surgery, have a higher risk of recurrence and both require brain retraction, which carries a low risk of brain damage. Acoustic neuroma surgery is highly technically demanding, and patients are advised to seek out surgical teams with extensive experience.

Radiation Therapy

Radiation therapy is done in a variety of ways, but mainly by two methods: gamma knife radiosurgery or fractionated stereotactic radiotherapy. In the gamma knife approach, 201 beams of gamma radiation are focused on the tumor in a single session. The damage to the tumor at the convergence point often causes it to stop growing or even shrink. The damage may be to the tumor cells and/or to the tumor vasculature. This "controls" the tumor, preventing growth. It is not clear what percentage of tumors are controlled by this method for long periods. In earlier times when higher radiation doses were used, the failure rate was about 12% (which then required surgery). Some surgeons feel that these tumors are much more difficult to remove after radiation treatment, but not all surgeons agree. Radiation does not remove the tumor, and when irradiated tumors are surgically removed, it is often found that they have growing tumor cells in them. Two risks of radiation treatment are carcinogenic progression of the acoustic neuroma (conversion from benign to malignant) or induction of other tumors (such as glioblastoma) in the nearby irradiated brain tissue. The incidence of these events appears to be low, and it is often said to be one in one thousand or less. This calculation is done by dividing the number of obvious cases of tumorigenic progression or secondary tumor reported in the medical literature by the estimated number of gamma knife procedures done in the world to date. This is not a scientifically valid method of estimating the carcinogenic risk of medical radiation exposures, and involves a list of very questionable assumptions.

The proper and scientifically valid way to estimate such risks can be found at the web site of the Health Physics Society (http://www.hps.org/), where estimates of the risks of CT scans and other procedures can be found. These calculations have never been made for gamma knife radiosurgery. The attraction of radiosurgery is that no one dies from it (not immediately anyway), it is almost painless, and there is no vertigo or facial paralysis immediately after. Facial nerve damage is usually mild, slow (months) and transient. Patients often return to work the next day. Since the tumor is not removed or destroyed by the treatment, there is a possibility of regrowth, which might be rapid. Due to the possibility of regrowth and the possibility of tumorigenic progression or secondary tumors, it is essential that radiation treatments for acoustic neuromas be followed by yearly MRI for the rest of the patient's life. MRI at this time (2005) cost about $3,000. Long-term secondary effects (for instance cognitive effects) on a scale of 10-20 years are not yet established for gamma knife surgery.

Fractionated stereotactic therapy involves a beam of ionizing radiation focused on the tumor from a moving gantry. The beam is wider and less accurate than that of the gamma knife. The total dose is also much higher than that used in gamma knife radiosurgery, but the fractionation of the dose (done on many different days) spares normal tissue. This method has not been done on as many patients as gamma knife procedures and there have not been as many years of follow-up study. This means that the tumor control by this method is not yet established, and the incidence of secondary effects of the radiation are not yet known.

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