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Acoustic Neuroma
The most common type of brain tumor is a non-cancerous tumor that grows on a tiny nerve between the inner ear and the brainstem. (Source: Komatsuzaki and Tsunoda, 2001).
This happens on the eighth cranial nerve and is made up of several nerves that connect the ear drum to the brain. The cochlear nerve, carries hearing information. There are two others, left and right, that carry information to the brain which controls a person’s balance.
These nerves are protected by what are called Schwann cells, and with Acoustic Neuroma a tumor begins to grow on these cells. If the tumor continues unchecked, it will sooner or later begin to compress the brainstem and will be life threatening.
Acoustic neuromas are rarely inherited. Acoustic neuroma caused by neurofibromatosis type II (NF 2) should be suspected in young patients and those with a family history of neural tumors. There are several other tumors that can occur in the same region of the brain.
Frequency
Acoustic neuromas comprise about 6% of all intracranial tumors, about 30% of brainstem tumors, and about 85% of tumors in the region of the cerebellopontine angle -- another 10% are meningiomas. Only about 10 tumors are newly diagnosed each year per million persons in the United States, corresponding to between 2,000 and 3,000 new cases each year. In patients with hearing asymmetry, it is believed that only about 1 in 1,000 has acoustic neuroma (Source: National Institute of Health http://www.nih.gov/ ).
The treatment approach to facial paralysis in this patient population depends on the intraoperative surgical findings. Conservative approach is prudent f the surgeon who performs the acoustic neuroma feels that the nerve was saved during the operation. Patients will often require eye care and temporary measures such as suture suspensions to prevent unwanted complications and improve oral issues. The risk of facial paralysis with acoustic neuroma surgery is between 4-15%. Tumor size, surgeon experience and approach for surgery are important factors.
Symptoms
Hearing loss is the most frequent symptom, occurring in more than 95% of patients. About 90% present with a one-sided, slowly progressive hearing impairment (see Figure 2). A high-frequency sensorineural pattern is the most common type, occurring in approximately two-thirds of patients. In the remaining third, the next most common observation is hearing loss at low frequency, which is more typical of Meniere's disease. Even less commonly, some have the "cookie bite" pattern, which is suggestive of congenital hearing loss. A sudden hearing loss occurs in about 25% of patients with acoustic neuroma. However, because acoustic neuroma is a rare condition, sudden hearing loss attributable to an acoustic tumor occurs in only 1 to 5% of patients with sudden hearing loss as there are many more common causes (Source: Daniels et al, 2000). Hearing can be completely normal in about 11% of patients (Source: Morrison and Sterkers, 1996). Tinnitus is very common in acoustic neuroma, and is usually unilateral and confined to the affected ear.
Because of the origin of acoustics in the vestibular nerve (Source: Komatsuzaki and Tsunoda, 2001), only about twenty percent of the patients will experience vertigo. Vertigo is more common with smaller tumors than larger ones. Lack of balance is common with about seventy per cent of patients experiencing it. Most of these patients will have large tumors. Loss of use of one or more of the arms and legs may occur but are unusual.
Although there may be facial nerve damage these have only occurred where a large tumor is present. Prior to operating this damage may be controlled with carbamazepine medication for neuralgia, at least in some cases. Before surgery a patient may experience headaches if there is a large tumor present.
If the nerve was cut intraoperatively or if the facial palsy does not resolve, then a more active approach must be taken. Definitive eyelid reconstruction with gold weight or palpebral spring must be considered. Facial reanimation as discussed in other sections of this web site must then be utilized. The following section is a more detailed discussion about acoustic neuroma.
The risk of facial paralysis with acoustic neuroma surgery is between 4-15%. Tumor size, surgeon experience and approach for surgery are important factors. The treatment approach to facial paralysis in this patient population depends on the intraoperative surgical findings. Conservative approach is prudent if the surgeon who performs the acoustic neuroma feels that the nerve was saved during the operation. Patients will often require eye care and temporary measures such as suture suspensions to prevent unwanted complications and improve oral issues.
Diagnosis
Conventional audiometry is the most useful diagnostic test for acoustic neuroma. The most common abnormality is an asymmetrical high-frequency sensorineural hearing loss. It has been estimated that 5% of persons with sensorineural hearing loss have acoustics (Source: Daniels et al, 2000).
Testing such as auditory gadolinium enhanced magnetic resonance imaging, can establish the diagnosis. A new technique called “Auditory Brainstem Response” testing is less sensitive than enhanced magnetic resonance imaging, but it is considerably less expensive. (Figure 3)
Medical Treatment
There are four distinct treatment options for an acoustic neuroma:
• Medical treatment or "wait and see" (conservative management)
• Surgery
• Gamma-knife procedure
• Cochlear implantation
About 25% of all acoustic neuromas are treated with medical management. Medical management consists of periodic monitoring of the patient's neurological status, use of hearing aids when appropriate, and periodic imaging studies. It is felt to be an appropriate method of management in some patients (Source: Hoistad et al, 2001).
There are no medications known to effect the growth of acoustic neuroma tumors. The tumors may grow very slowly. A patient may want to follow a tumor with serial audiometry and/or MRI scans (Shin et al, 2000). In individuals of advanced age, a serious threat to life or bodily function from tumor growth may be judged unlikely in the remainder of a patient's expected lifespan, and for this reason, medications may be elected. Once a tumor is diagnosed, a repeat scan is obtained at six months and then at yearly intervals (Source: Perry et al, 2001).
This procedure has its own risks. Even when the tumor is not growing on MRI, there is a risk of losing useful hearing in this situation, making the individual no longer a candidate for hearing preservation type surgery. Somewhere between 10 and 43% of patients followed for about two years lose "useful" hearing (Warrick et al, 1999; Shin et al, 2000). A reasonable estimate is that over a year, about 75% of tumors will have visible enlargement, averaging 1.5 mm, and about 25% will not. Some variants grow much faster than others.
Surgery
About half of all acoustic neuromas are presently treated with surgery. In most instances, surgical removal of the tumor is the preferred option because it prevents potentially fatal complications of tumor growth. Surgery may enable preservation of hearing. Usually the surgery is done at an academic center by a team of surgeons including a neurotologist (a specialized otolaryngologist) and a neurosurgeon. There are several operations, as follows:
• Retrosigmoid or suboccipital (through the skull, more posterior approach)
• Translabyrinthine (through the inner ear -- hearing loss is expected)
• Middle fossa (through the skull -- hearing preservation is the goal)
Each of these approaches has advantages and disadvantages that must be considered in selecting an optimal approach. Most patients are admitted to the hospital a day before the operation. After surgery, they spend a night in a monitored unit. Most are discharged from the hospital within four to six days after surgery, and return to work is usually possible in six weeks. MRIs are usually obtained at 1 and 5 years to detect residual or recurrent tumor. (Reference: http://www.american-hearing.org )
Conventional audiometry is the most useful diagnostic test for acoustic neuroma. The most common abnormality is an asymmetrical high-frequency sensorineural hearing loss. However, recall that only about 1 in 1,000 patients with hearing asymmetry have acoustics. It has been estimated that 5% of persons with sensorineural hearing loss have acoustics (Daniels et al, 2000), but this estimate is suspect as it would imply a much higher prevalence of acoustic neuromas than are commonly accepted. When abnormal with a progressively worsening pattern, audiometry usually leads to further testing such as ABR (auditory brainstem response) and gadolinium enhanced MRI (magnetic resonance imaging), which establishes the diagnosis. ABR testing is less sensitive than MRI, but it is considerably less expensive. A new technique, called summated ABR, which is essentially several ABRs compared over time, may provide better sensitivity.
Electronystagmography (ENG testing) is frequently abnormal, and about 50% of all tumors are associated with unilateral loss of calorics. Nevertheless, ENG is not a reasonable diagnostic test because it is not specific. Rotatory chair testing is less sensitive than caloric testing. Posturography is insensitive to acoustic neuroma.
Although it is relatively costly compared to audiometry or ABR, the optimal test for excluding an acoustic neuroma is a gadolinium enhanced T1 MRI (Figure 3). On MRI, acoustic neuromas are frequently uniformly enhanced, dense, and expand the internal auditory meatus. A fast spin-echo T2 variant of MRI is very sensitive to acoustics, and in some clinical settings, can be done fairly inexpensively. If an MRI cannot be done, an air-CT scan should be obtained in high-risk individuals, particularly if the ABR is suggestive of an acoustic neuroma.
Acoustic neuromas range in size up to 4 cm. The smallest, the intracanalicular acoustic, is measured in millimeters. A "small" acoustic is less than 1.5 cm, a "moderate" acoustic is 1.5 to 3 cm, and a "large" acoustic is 3 cm or greater. Tumors are staged by a combination of their location and size: An intracanalicular tumor is small and in the internal auditory canal (IAC). A cisternal tumor has extended outside the IAC. A compressive tumor is touching the cerebellum or brainstem. Some tumors cause hydrocephalus by obstructing cerebrospinal fluid (CSF) drainage pathways in the 4th ventricle.
Rarely, acoustic neuromas are inherited. Acoustic neuroma caused by neurofibromatosis type II (NF 2) should be suspected in young patients and those with a family history of neural tumors. There are several other tumors that can occur in the same region of the brain [the cerebellopontine angle (CPA)] as acoustic neuromas. Of all lesions in the CPA, acoustic neuromas account for 70 to 90%. Meningiomas are second most common (10%), followed by epidermoids, and then lipomas.
There are four distinct treatment options for an acoustic neuroma:
• Medical treatment or "wait and see" (conservative management)
• Surgery
• Gamma-knife procedure
• Cochlear implantation
Medical Treatment
About 25% of all acoustic neuromas are treated with medical management. Medical management consists of periodic monitoring of the patient's neurological status, use of hearing aids when appropriate, and periodic imaging studies. It is felt to be an appropriate method of management in some patients (Hoistad et al, 2001).There is no medication known to have a substantial effect on the growth of acoustic neuroma tumors. The tumors may grow very slowly, about 1- 1/2 mm per year, and one may elect to follow a tumor with serial audiometry and/or MRI scans (Shin et al, 2000). In individuals of advanced age, a serious threat to life or bodily function from tumor growth may be judged unlikely in the remainder of a patient's expected lifespan, and for this reason, medical management may be elected (Perry et al, 2001). Once a tumor is diagnosed, a repeat scan is obtained at six months and then at yearly intervals (Perry et al, 2001).
This procedure has its own risks. Even when the tumor is not growing on MRI, there is a risk of losing useful hearing in this situation, making the individual no longer a candidate for hearing preservation type surgery. Somewhere between 10 and 43% of patients followed for about two years lose "useful" hearing (Warrick et al, 1999; Shin et al, 2000). A reasonable estimate is that over a year, about 75% of tumors will have visible enlargement, averaging 1.5 mm, and about 25% will not. Some variants grow much faster than others.
Surgery
About half of all acoustic neuromas are presently treated with surgery. In most instances, surgical removal of the tumor is the preferred option because it prevents potentially fatal complications of tumor growth. Surgery may enable preservation of hearing. Usually the surgery is done at an academic center by a team of surgeons including a neurotologist (a specialized otolaryngologist) and a neurosurgeon. There are several operations, as follows:
• Retrosigmoid or suboccipital (through the skull, more posterior approach)
• Translabyrinthine (through the inner ear -- hearing loss is expected)
• Middle fossa (through the skull -- hearing preservation is the goal)
Each of these approaches has advantages and disadvantages that must be considered in selecting an optimal approach. Most patients are admitted to the hospital a day before the operation. After surgery, they spend a night in a monitored unit. Most are discharged from the hospital within four to six days after surgery, and return to work is usually possible in six weeks. MRIs are usually obtained at 1 and 5 years to detect residual or recurrent tumor.
1 Excerpts from American-hearing.org web site |
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