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Acoustic neuroma
ICD9 = 225.1| ICDO = M9560/0| Image = | Caption = | OMIM = | MedlinePlus = 000778 | eMedicineSubj = ent | eMedicineTopic = 239 | DiseasesDB = 100 | }} Acoustic neuroma (or Vestibular Schwannoma) is a benigntumorof the the myelinforming cells called "Schwann cells" of the 8th cranial nerve, known as the acoustic nerve, (or more properly the vestibulocochlear nerve) just after it has left the brainstem, in the pontine angle; also at the point where the peripheral part of the nerve meets the brain part of the nerve called "Hensen's node". Multiple names used for this type of tumor include acoustic neuroma, acoustic neuronoma and vestibular schwannoma because it attacks the 8th cranial nerve which branches in to the vestibular and acoustic; and under a microscope the tumor resembles schwann cells.
AssessmentAssociated symptomsare unilateral sensorineural hearing loss/deafnessand vertigo. Additionally more than 80% of patients having acoustic neuromas have reported tinnitus. Larger tumors can compress local structuressuch as the facial nerve, and lead to local symptoms such as hydrocephalus. If the tumor grows next to the brainstem and grows large enough, the brainstem may become compressed. Also associated with these nerves is the 7th cranial nerve; this nerve controls the muscles of the face, salivation, tearing, and taste. If a significantly large tumor develops it can involve the 5th cranial nervecontrolling the sensation face and eyes. While most cases occur sporadically, acoustic neuroma may be attributable to neurofibromatosis(type 2) in about 5% to 10% of the cases. If the tumor is caused by Neurofibromatosis a slightly different type of tumor grows, then the tumor often involves the whole nerve rather than particular sections of the nerve as does the schwannoma which can make treatment more a greater challenge. Also the tumor is classically bilateral with this syndrome. TreatmentIndicated treatments for acoustic neuroma include surgical removal and radiotherapy. Conservative treatmentBecause 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 MRIto 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. Oftentimes, peoplewith acoustic neruromata dieof other causes before the neuroma becomes life-threatening. (This is especially true of elderlypeople possessing a small neuroma.) Since the growth rate of an acoustic neuroma rarely accelerates, annualobservation is essential. Acoustic neuromata may cause either gradual or—less commonly—sudden hearing lossand tinnitus. However, the surgical and radiotherapy treatments are even more dangerous to the hearing in the affected ear. SurgeryThe surgeryis 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 vestibularnerve is usually removed on the operated side, resulting in severe imbalance, vertigoand 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 anesthesiatime) and allows the most complete removal of the tumor (less chance of recurrence). The two other approaches (suboccipitaland 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 TherapyRadiation therapyis done in a variety of ways, but mainly by two methods: gamma kniferadiosurgeryor fractionatedstereotacticradiotherapy. In the gamma knife approach, 201 beams of gamma radiationare 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 carcinogenicprogression of the acoustic neuroma (conversion from benignto malignant) or inductionof 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 tumorigenicprogression 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/), were 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 paralysisimmediately 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 cognitiveeffects) on a scale of 10-20 years are not yet established for gamma knife surgery. Fractionated stereotactic therapy involves a beam of ionizingradiation 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. External links
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