Acoustic Neuroma or Vestibular Schwannoma
Acoustic Neuroma is a benign (not malignant) tumor located between the inner ear and the brainstem. It is more accurately called vestibular schwannoma, since it is a tumor of the Schwann cell component of the nerve of balance (vestibular nerve). The tumor usually begins in the boney canal in the skull base, which contains the hearing (auditory/acoustic) nerve, vestibular nerve, and facial nerve. Less often, it can develop in the cerebellopontine angle region, adjacent to the inner ear canal, closer to the brainstem. Before the days of microsurgical removal, these tumors were called acoustic neuroma because they initially presented with clinical symptoms of hearing loss or ear noises (tinnitus) in the involved ear by compression of the auditory nerve and/or its blood supply by the tumor of the vestibular nerve. The loss of vestibular or balance function was usually not noticed because there was adaptation or compensation by the nervous system to the gradual loss of balance function from these very slowly growing tumors.
The tumor grows at an overall rate of about one to two millimeters per year, slowly enlarging before clinical symptoms of hearing loss or ear sounds (tinnitus) are noticed. The growth rate may also fluctuate to include periods of no growth as well as phases of more rapid growth rates.
The incidence rate of acoustic neuroma (vestibular schwannoma) is about 10 per million individuals per year in the United States. It is most frequently diagnosed in the third to sixth decades of life, and very rarely in children. It may occur slightly more often in women, and there is no racial preponderance.
Vestibular schwannoma appear in two major groups: as sporadic non-familial type or part of familial disorder neurofibromatosis type 2 (NF-2). 95% of the clinical acoustic neuroma cases are the sporadic non-familial type, which present with an isolated single tumor of the vestibular nerve. The familial neurofibromatosis type 2 (NF-2) occurs in 5% of the acoustic neuromas, but its tumors are bilateral, and involve multiple sites in the central nervous system (brain and spinal cord). Presumably one half of the NF-2 patients inherit the defective gene from a parent and the other half have a new genetic mutation for the defective gene. Unlike the sporadic vestibular schwannoma, NF-2 frequently presents in childhood or early teens.
Symptoms from vestibular schwannoma derive from structures under compression by the growing tumor. As noted earlier, these are the auditory, vestibular, and facial nerves. Usually the first symptoms are auditory with unilateral (one-sided) tinnitus (ear noises) or decreased hearing. Less frequently, dizziness is encountered, due to clinical compensation to very gradual loss of balance in one inner ear. One sided facial nerve weakness or paralysis are usually uncommon unless the tumor is much larger. As the tumor enlarges, it can compress other lower cranial nerves of swallowing, tongue movement, vocal cord movement and facial sensation. Continued tumor compression of the brainstem and other brain regions will impair central nervous system function, eventually threatening life.
Initially, most small vestibular schwannomas are asymptomatic, so it is not readily diagnosed. After clinical symptoms such as one sided tinnitus, unilateral hearing loss, dizziness or vertigo, or facial weakness appear, a thorough evaluation by a neurotologist or otologist (ear doctor) is suggested. Such an examination will include a complete physical examination with hearing and balance (vestibular) testing. Ultimately, imaging studies with an MRI with contrast dye (gadolinium or similar) to differentiate tumor from normal brain structure is required to confirm or disprove the diagnosis of vestibular schwannoma. Current MRI imaging is capable of demonstrating very small tumors as small as one to two mm. If a patient is not able to have an MRI, a high resolution CT with contrast dye of the brain can be utilized. The MRI does not involve radiation and visualizes nerve/brain structures better than the CT scan.
Treatment of Acoustic Neuroma
Acoustic neuroma treatment options depend upon tumor size, status of hearing in ear with the tumor, patient’s medical health, age, and patient’s wishes. These choices are surgical, radiation, and observation ("watch and wait") for tumor growth.
Surgical removal of acoustic neuroma classically involves one of three approaches, depending on hearing function and tumor size. Surgery is the only way to achieve complete cure with tumor removal. It is usually considered for younger patients, and patients with larger tumors. Surgery for acoustic neuroma utilizes general anesthesia, microsurgical techniques with the operating microscope, and sophisticated intra-operative monitoring of the facial nerve.
If there is no functional hearing, the tumor can be removed with the translabyrinthine approach, which is performed behind the ear through the mastoid and inner ear behind the ear. Since the inner ear is removed, any residual (usually too poor to be of benefit) hearing is lost in this non-hearing preservation operative approach. This approach is notable for minimal brain retraction compared to other surgical techniques. Size of the tumor is generally not a limitation with this approach. The incidence of postoperative headaches may be less with translabyrinthine when compared to the classical neurosurgical craniotomy approach from the back of the head.
If there is functional hearing, then a hearing preservation approach may be preferred with either a retrosigmoid (further behind the ear, towards the back of the head) approach or a middle fossa (above the ear) operative procedure. The retrosigmoid approach is for all size tumors, whereas the middle fossa approach is usually for tumors smaller than 1 cm. Hearing preservation is more successful with the smaller size tumors, generally under 1.0-1.5 cm. At best, the success rate for hearing preservation is about 50-60%, with contemporary microsurgical technique. Facial nerve preservation is also best with smaller tumors. These surgeries are performed with a combined team consisting of a neurotologist and neurosurgeon.
Nonsurgical Management: Stereotaxic (Gamma Knife) radiation therapy is utilized in older and less healthy patients who cannot undergo surgery. It is based on precise MRI imaging and a highly focused one-session radiation treatment (one day). Tumors are not eliminated but their growth is controlled with radiation therapy. A small number of tumors have been found to shrink after radiation therapy. Alternative forms of radiation may include fractionated radiation therapy, administered over several sessions. Radiation therapy is for tumors less than 2.5 cm. Since the long-term outcome of radiation therapy for a benign tumor is not well known with regards to potential conversion to a malignant (cancerous) lesion, such radiation therapy is generally not advised in the younger population. Nevertheless, more middle-aged and some younger people are opting for Gamma Knife stereotaxic radiation treatment to avoid potential complications associated with surgery. Residual hearing maybe lost but facial nerve preservation is usually maintained following radiation therapy. Sometimes patients develop other symptoms such as facial pain, but this side affect has been reduced with lower radiation levels, which been successful in treating the tumor. Recent reports suggest that hearing preservation has been more successful with lower radiation dosage. Patients will require annual MRIs to monitor tumor response to therapy. A very small number of patients, less than 2%, have recurrent growth following radiation therapy. Such tumor resistance to Gamma Knife radiation therapy is managed with surgical removal of the tumor. There have been some reports of increased difficulty is surgically removing these tumors due to "scarring" in the operative field from radiation therapy.
At NYU Medical Center, we have a very close well established relationship between Otology/Neurotology and Neurosurgery for the management of acoustic neuroma, whether it be surgical or with non surgical stereotaxic Gamma Knife therapy.
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