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|Classification and external resources|
Bilateral schwannomas in a patient with neurofibromatosis 2
|Classification and external resources|
Bilateral schwannomas in a patient with neurofibromatosis 2
A vestibular schwannoma, often called an acoustic neuroma, is a benign primary intracranial tumor of the myelin-forming cells of the vestibulocochlear nerve (8th cranial nerve). A type of schwannoma, this tumor arises from the Schwann cells responsible for the myelin sheath that helps keep peripheral nerves insulated. Approximately 3,000 cases are diagnosed each year in the United States. Most recent publications suggest that the incidence of acoustic neuromas is rising because of advances in MRI scanning. Studies in Denmark published in 2004 show the incidence is 17.4 per million or close to 2 persons per 100,000. Most acoustic neuromas are diagnosed in patients between the ages of 30 and 60 and men and women appear to be affected equally.
Acoustic neuromas normally develop gradually over a period of years, roughly 1–2 mm each year. They expand in size at their site of origin and when large, can displace normal brain tissue. The brain is not invaded by the tumor, but the tumor pushes the brain as it enlarges. Vital functions to sustain life can be threatened when large tumors cause severe pressure on the brainstem and cerebellum. Tumors are typically described as small (less than 1.5 cm), medium (1.5 cm to 2.5 cm) or large (more than 2.5 cm). Early symptoms are easily overlooked, sometimes mistaken for the normal changes of aging or attributed to noise exposure earlier in life, often delaying diagnosis. The first symptom in 90% of those with an acoustic neuroma is unilateral hearing loss (a reduction in hearing in one ear), often accompanied by ringing in the ear called tinnitus. The loss of hearing is usually subtle and worsens slowly, although occasionally a sudden loss of hearing can occur. There may be a feeling of fullness in the affected ear.
Since the balance portion of the eighth nerve is where the tumor arises, unsteadiness and balance problems or even vertigo (the feeling like the world is spinning), may occur during the growth of the tumor. The remainder of the balance system sometimes compensates for this loss, and, in some cases, no imbalance will be noticed. Larger tumors can press on the trigeminal nerve, causing facial numbness and tingling - constantly or intermittently. Although the facial nerve may be compressed by the tumor, it is unusual for patients to experience weakness or paralysis of the face from an acoustic neuroma, although this may occasionally occur. Tumor related increase of intracranial pressure may cause headaches, clumsy gait and mental confusion. This can be a life-threatening complication requiring urgent treatment.
Large tumors that compress the adjacent brainstem may affect other local cranial nerves. Paradoxically, the 7th cranial nerves are rarely involved pre-operatively; involvement of the trigeminal nerve (CN V) may lead to loss of sensation in the involved side's face and mouth. The glossopharyngeal and vagus nerves are uncommonly involved, but their involvement may lead to altered gag or swallowing reflexes.
Larger tumors may lead to increased intracranial pressure, with its associated symptoms such as headache, vomiting, and altered consciousness.
One of the last great obstacles in the management of acoustic neuromas is hearing preservation and/or rehabilitation after hearing loss. Hearing loss is the most common disability in patients with acoustic neuromas and affects all age groups. Hearing loss is a symptom that affects one’s quality of life regardless of the treatment option chosen and it can vary from no or mild hearing loss to complete deafness (also known as profound hearing loss or single-sided deafness – SSD). Hearing loss can disrupt one’s social and vocational life; it can contribute to depression and a sense of isolation. Hearing rehabilitation, through the use of hearing aids and assistive listening devices, can enhance one’s ability to communicate with others (by phone or in person) and significantly improves a patient’s quality of life.
Acoustic neuromas typically cause sensorineural hearing loss, meaning there is damage to the inner ear (cochlea) or nerve pathways from the inner ear to the brain. It involves a reduction in sound level, speech understanding and hearing clarity.
There are several types of hearing aids that can aid those dealing with hearing loss depending on the severity of each situation. Conventional hearing aids can be a good choice if some hearing is preserved in the ear affected by the tumor. This type of hearing aid would include Behind-the-ear (BTE), In-the-ear (ITE), In-the canal (ITC) and Completely-in-the-canal (CIC). There are also Specialized hearing devices, which could be helpful in cases of unilateral deafness. These options include the CROS, BiCROS and Phonak CROS systems. Bone Conduction hearing devices are another option, which would include the Baha, the Ponto Pro, the TransEar, the SoundBite and the Sophono Alpha 1. Lastly, there are cochlear implants, which are indicated when the patient has significant hearing loss in both ears and the cochlear nerve is still intact.
Essentially everyone who has been treated for an acoustic neuroma experiences difficulty with balance and/or dizziness to some degree. For some, this instability may be mild and noticeable only in certain circumstances, such as ambulating with head movements, or walking in the dark. For others, there may be difficulty returning to work, or even performing regular daily activities such as driving, shopping, house work and even working on your computer. Perception of stability is the result of a complex brain function that uses three systems to inform the brain how the body is oriented in space and how it is moving in relation to its surroundings. Each system works independently and together, so if one or two systems are being challenged, the remaining one or two will help you maintain your balance. The three major senses that provide balance input to the brain are: (1) Your vestibular system, which is the balance part of the inner ear. It detects movement of your head in space in a straight or turning mode. (2) Your vision orients you to an object you are looking at. (3) Proprioception is the body’s sense of how your joints are aligned during activities using your legs. In other words, are they lined up or are they moving? Information sent to your brain by sensors (proprioceptors) in the joints of your legs assist the brain in contracting muscle around the joints for stability. Most of the time, all three sensors (the vestibular system in the ears, vision and proprioception) work together in a complementary fashion. When one system is challenged (i.e., while standing on foam) the proprioceptors in the joints of your legs will be of no use, so your brain will use information from your visual and vestibular systems to maintain your balance. Sometimes these systems give conflicting information. An example would be looking out the side window of a car that is traveling down a smooth highway at a constant speed. Since the vestibular system in the ears sense acceleration, the ears tell the brain that the body is not moving. Proprioception and pressure sensors tell the brain that the body is not moving on the car seat. But the eyes say that the body is moving at 60 miles per hour. The brain needs to be able to handle that conflict. If it does not, the person feels motion sickness. Because these three senses sometimes give conflicting information, the brain gives priority to some senses over others. Typically, the vestibular system in the inner ears has the highest priority. A healthy vestibular sense is essentially always right. Vision is given second priority under most circumstances. If, however, moving objects fill the entire field of vision, the brain will give vision first priority over the vestibular sense.
Tinnitus is the perception of sound in the ears or head where no external source is present. Some call it "ringing in the ears" or "head noise”. This very common condition affects 1 in 5 people and has no cure. Not all patients with tinnitus have acoustic neuroma and not all AN patients have tinnitus. Most of them do however, both before and after treatment.
Acoustic neuroma patients sometimes complain of a feeling that their ear is plugged or "full".
Head pain is expected in most patients immediately after acoustic neuroma surgery (acute phase) because of the incision, variations in cerebrospinal fluid pressure, muscle pain, or even meningitic pain. It typically responds to appropriate medications and resolves within several weeks. Headache that persists for months or even years after surgery (chronic phase) can be debilitating and may be an under-appreciated complication of acoustic neuroma treatment. In patients who experience chronic headaches, the pain often persists for prolonged periods of time, and does not always respond well to various medical and surgical treatments. The exact prevalence and causes of chronic postoperative headache (POH) are elusive. After surgical treatment of acoustic neuroma, the reported incidence of headache in the 2012 Acoustic Neuroma Association patient survey has ranged from 0% to 35% depending on the type of surgical approach, technique used and reporting interval since surgery. Frequent and severe POHs have been more often associated with the sub-occipital or retrosigmoid approaches than the translabyrinthine or middle fossa approaches.
At the time most people learn they have an acoustic neuroma, they are also told that this tumor may involve the nerve that controls facial movement. The functioning of facial muscles is taken for granted, and a patient may find it difficult to grasp the connection between a benign growth on the nerve of hearing and any compromise of the facial nerve. In the 2012 Acoustic Neuroma Association patient survey, 29% of the respondents reported facial weakness or paralysis, some of which were pre- and some were post-treatment. This represents a significant improvement from the 1998 Acoustic Neuroma Association patient survey of post-treatment acoustic neuroma patients, which revealed that at the time they completed the survey, only 59% were satisfied with the appearance of their face. People scarcely appreciate the exquisite musculature of the face that allows them to express themselves through speech and emotional expression—from wide-mouthed laughter to scowling disapproval. The eyes blink and are precisely moistened, or tear voluntarily after receiving a message from the brain that more tears are needed. Taste, a sensation that reflects accurately sweet, sour, bitter and bland, is also a function of the facial nerve. Treatment for an acoustic neuroma may damage the facial nerve – either with surgery or radiation. It is usually possible, however, to preserve some degree of facial function even in cases where the nerve is extensively involved. For those with partial nerve regeneration, in whom some facial weakness remains, non-surgical facial rehabilitation therapies also may be beneficial.
Acoustic neuromas may occur idiopathically (meaning the cause is unknown), however there is a growing body of evidence that sporadic defects in tumor suppressor genes may give rise to these tumors in some individuals. Other studies have hinted at exposure to loud noise on a consistent basis. One study has shown a relationship between acoustic neuromas and prior exposure to head and neck radiation, and a concomitant history of having had a parathyroid adenoma (tumor found in proximity to the thyroid gland controlling calcium metabolism). There are even controversies on hand held cellular phones. Whether or not the radiofrequency radiation has anything to do with acoustic neuroma formation, remains to be seen. To date, no environmental factor (such as cell phones or diet) has been scientifically proven to cause these tumors. The Acoustic Neuroma Association (ANA) does recommend that frequent cellular phone users use a hands free device to enable separation of the device from the head.
Although there is an inheritable condition called Neurofibromatosis Type 2 (NF2) which can lead to acoustic neuroma formation in some people, most acoustic neuromas occur spontaneously without any evidence of family history (95%). NF2 occurs with a frequency of 1 in 30,000 to 1 in 50,000 births. The hallmark of this disorder is bilateral acoustic neuromas (an acoustic neuroma on both sides). This creates the possibility of complete deafness if the tumors are left to grow unchecked. Preventing or treating the complete deafness that may befall individuals with NF2 requires complex decision making. The trend at most academic U.S. medical centers is to recommend treatment for the smallest tumor which has the best chance of preserving hearing. If this goal is successful, then treatment can also be offered for the remaining tumor. If hearing is not preserved at the initial treatment, then usually the second tumor, in the only-hearing ear, is just observed. If it shows continued growth and becomes life-threatening, or if the hearing is lost over time as the tumor grows, then treatment is undertaken. This strategy has the highest chance of preserving hearing for the longest time possible.
Advances in medicine have made possible the identification of small acoustic neuromas (those still confined to the internal auditory canal). Routine auditory tests may reveal a loss of hearing and speech discrimination (the patient may hear sounds in that ear, but cannot comprehend what is being said). An audiogram should be performed to effectively evaluate hearing in both ears. A loss in one ear should prompt an MRI. Magnetic resonance imaging (MRI) using Gadolinium as an enhancing contrast material is the preferred diagnostic test for identifying acoustic neuromas. The image formed clearly defines an acoustic neuroma if it is present and this technique can identify tumors measuring only a few millimeters in diameter. An auditory brainstem response test (a.k.a. ABR, BAER, or BSER) may be done in some cases. This test provides information on the passage of an electrical impulse along the circuit from the inner ear to the brainstem pathways. An acoustic neuroma can interfere with the passage of this electrical impulse through the hearing nerve at the site of tumor growth in the internal auditory canal, even when the hearing is still essentially normal. This implies the possible diagnosis of an acoustic neuroma when the test result is abnormal. An abnormal auditory brainstem response test should be followed by an MRI. When an MRI is not available or cannot be performed, a computerized tomography scan (CT scan) with contrast is suggested for patients in whom an acoustic neuroma is suspected. The combination of CT scan and audiogram approach the reliability of MRI in making the diagnosis of acoustic neuroma.
There are three treatment options available to a patient. These options are Observation, Microsurgical Removal and Radiation (radiosurgery or radiotherapy). Determining which treatment to choose involves consideration of many factors including the size of the tumor, its location, the patient's age, physical health and current symptoms. About 25% of all acoustic neuromas are treated with medical management consisting of a periodic monitoring of the patient's neurological status, serial imaging studies, and the use of hearing aids when appropriate.
Since acoustic neuromas tend to be slow-growing and are benign tumors, careful observation over a period of time may be appropriate for some patients. When a small tumor is discovered in an older patient, observation to determine the growth rate of the tumor may be indicated if serious symptoms are not present. There is now good evidence from large observational studies that suggest many small tumors in older individuals do not grow, thus allowing tumors with no growth to be observed successfully. If the tumor grows, treatment may become necessary. Another example of a group of patients for whom observation may be indicated includes patients with a tumor in their only hearing or better hearing ear, particularly when the tumor is of a size that hearing preservation with treatment would be unlikely. In this group of patients, MRI is used to follow the growth pattern. Treatment is recommended if either the hearing is lost or the tumor size becomes life-threatening, thus allowing the patient to retain hearing for as long as possible.
Microsurgical tumor removal can be done at one of three levels: subtotal removal, near total removal or total tumor removal. Subtotal removal is indicated when anything further risks life or neurological function. In these cases the residual tumor should be followed for risk of growth (approximately 35%). If the residual grows further, treatment will likely be required. Periodic MRI studies are important to follow the potential growth rate of any tumor. Near total tumor removal is used by experienced centers when small areas of the tumor are so adherent to the facial nerve that total removal would result in facial weakness. The piece left is generally less than 1% of the original and poses a risk of regrowth of approximately 3%. Periodic MRI studies are important to follow the potential growth rate of any tumor. Many tumors can be entirely removed by surgery. Microsurgical techniques and instruments, along with the operating microscope, have greatly reduced the surgical risks of total tumor removal. Preservation of the facial nerve to prevent permanent facial paralysis is the primary task for the experienced acoustic neuroma surgeon. Preservation of hearing is an important goal for patients who present with functional hearing.
There are three main surgical approaches for the removal of an acoustic neuroma: translabyrinthine, retrosigmoid/sub-occipital and middle fossa. The approach used for each individual patient is based on several factors such as tumor size, location, skill and experience of the surgeon, and whether hearing preservation is a goal. The surgeon and the patient should thoroughly discuss the reasons for a selected approach. Each of the surgical approaches has advantages and disadvantages, and excellent results have been achieved using all three of the techniques.
The translabyrinthine approach may be preferred by the surgical team when the patient has no useful hearing, or when an attempt to preserve hearing would be impractical. The incision for this approach is located behind the ear and allows excellent exposure of the internal auditory canal and tumor. This also results in permanent, and complete hearing loss in that ear, but the surgeon has the advantage of knowing the location of the facial nerve prior to tumor dissection and removal. Any size tumor can be removed with this approach and this approach affords the least likelihood of long-term postoperative headaches.
The incision for this approach is located in a slightly different location. This approach creates an opening in the skull behind the mastoid part of the ear, near the back of the head on the side of the tumor. The surgeon exposes the tumor from its posterior (back) surface, thereby getting a very good view of the tumor in relation to the brainstem. When removing large tumors through this approach, the facial nerve can be exposed by early opening of the internal auditory canal. Any size tumor can be removed with this approach. One of the main advantages of the retrosigmoid approach is the possibility of preserving hearing. For small tumors, a disadvantage lies in the risk of long-term postoperative headache.
This approach is in a slightly different incision location and is utilized primarily for the purpose of hearing preservation in patients with small tumors, typically confined to the internal auditory canal. A small window of bone is removed above the ear canal to allow exposure of the tumor from the upper surface of the internal auditory canal, preserving the inner ear structures.
Another treatment option for an acoustic neuroma is radiation. Stereotactic radiation can be delivered as single fraction stereotactic radiosurgery (SRS) or as multi-session fractionated stereotactic radiotherapy (FSR). Both techniques are performed in the outpatient setting, not requiring general anesthesia or a hospital stay. The purpose of these techniques is to arrest the growth of the tumor causing the tumor to die, which is called necrosis.
In single dose treatments, many hundreds of small beams of radiation are aimed at the tumor. This results in a high dose of radiation to the tumor and very little to any surrounding brain structures. Many patients have been treated this way with high success rates. Facial weakness or numbness, in the hands of experienced radiation physicians, occurs in only a small percent of cases. Hearing can be preserved in some cases with a slightly greater opportunity with FSR.
The multi-dose treatment, FSR, delivers smaller doses of radiation over a period of time, requiring the patient to return to the treatment location on a daily basis, from 3 to 30 times, generally over several weeks. Each visit lasts a few minutes and most patients are free to go about their daily business before and after each treatment session. Early data indicates that FSR may result in better hearing preservation when compared to single-session SRS.
The treatment team should consist of a neurosurgeon ad/or a neurotologist (ear and skull base surgeon), a radiation oncologist and a physicist. Follow-up after SRS and FSR typically involves a MRI scan and audiogram at six months, one year, then yearly for several years, then every second or third year indefinitely to make sure the tumor does not start to grow again. Patients should understand that all types of radiation therapy for acoustic neuromas may result in “tumor control" in which the tumor cells die and necrosis occurs. Tumor control means that the tumor growth may slow or stop and, in some cases, the tumor may shrink in size. In almost no cases have acoustic neuroma tumors been completely eliminated by radiation treatments. In other words, radiation does not remove the tumor like microsurgery can. Furthermore, radiated patients require lifetime follow-up with MRI scans. Tumors under 2.5 - 3.0 cm, without significant involvement of the brainstem, are more favorable for radiation treatment. Side effects can occur when the brainstem is irradiated and in some cases of large tumors, radiation is contraindicated. Patients should understand there have been rare reports of malignant degeneration (a benign tumor becoming malignant) after radiotherapy. In some cases the tumor does not die and continues to grow. In those instances, another treatment is necessary - either microsurgery or sometimes another dose of radiation. Retreatment must be done as always, in the hands of experienced physicians.
Several types of machines deliver focused radiation treatment suitable for treating acoustic neuromas, such as Gamma Knife® and linear accelerator (LINAC), such as CyberKnife®, Novalis® and Trilogy®. The underlying premise is to treat the tumor with a high dose of radiation while sparing the nerves and brain tissues. Much of the long term data comes from the Gamma Knife literature since this was one of the earliest techniques used to radiate acoustic neuromas on a large scale.
The Gamma Knife uses 195-201 fixed Cobalt-60 radiation sources that are “collimated” to intersect at the site of the tumor and is a single dose treatment. In this way, each individual beam of radiation has very little effect, but where they all intersect produces a maximum effect on the tumor. Very similar results can be obtained using a linear accelerator (LINAC) as the radiation source, such as with the Novalis or CyberKnife with multi-dose treatment.
Studies are beginning to appear for the other modalities. All of the techniques use computers to create three dimensional models of the tumor and surrounding neural structures. Radiation physicists then create dosimetry maps showing the level of radiation to be received by the tumor and the normal tissues. Surgeons, radiation therapists and physicists then modify the dosimetry to maximize tumor doses and minimize radiation toxicity to surrounding normal tissues. The head is stabilized with a metal frame pinned to the head (Gamma Knife) or a fitted mask shield (CyberKnife, linear accelerator, fractionated XRT). Treatments generally last 30–60 minutes. Just like for surgery, the experience of the team in treating acoustic neuromas with all modalities (surgery and radiation) can affect outcomes.
There are a multitude of studies supporting short-term (<5 yrs.) and longer-term (over 10 yrs.) tumor control with radiation. Unfortunately, as is the case with microsurgical studies, most have inconsistent follow-up to draw definitive conclusions.
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