The tumors are rare, accounting for only five to seven percent of all brain tumors. However, for the part of the brain where they are located, called the cerebellopontine angle, it is the most common tumor type.
The annual incidence of being diagnosed with a vestibular schwannoma is one per 100,000 people, with approximately 3,000 newly diagnosed tumors each year in the United States. This number may be rising as our ability to detect smaller tumors has improved. The tumor grows on the eighth cranial nerve, which travels from the inner ear to the brainstem to communicate hearing and balance information to the brain. The nerve has three distinct parts that connect to the inner ear: the superior and inferior vestibular (balance) nerves and the cochlear (hearing) nerve, which come together at the brainstem.
Vestibular schwannomas are also referred to as acoustic neuromas. It was once believed that the tumor originated on the cochlear portion of the eighth cranial nerve because hearing loss is often the first symptom. We now know that the tumor most often arises from one of the vestibular portions of the eighth cranial nerve, therefore the more accurate name is vestibular schwannoma. Both names are still commonly used and refer to the same tumor.
Vestibular schwannomas come in different sizes and cause a variety of problems. This tumor does not spread (metastasize) nor does it invade the brain, but if large enough, it can push on and squeeze the brain.
The symptoms of a vestibular schwannoma may include:
The exact cause of most vestibular schwannomas is unknown. Ninety-five percent of tumors occur spontaneously and are found on one side (unilateral) only. A small subset of vestibular schwannomas is associated with a genetic condition called Neurofibromatosis Type 2 (NF2). NF2 is rare and results in bilateral vestibular schwannomas. Regardless of the cause, the tumor originates from Schwann cells, which insulate nerves allowing them to transmit their neural signal quickly. When a tumor develops, the Schwann cells grow too quickly and can damage the nerve. In general, vestibular schwannomas grow slowly with an average growth rate of one to two millimeters per year. However, some tumors do not grow for several years and others grow rapidly.
Researchers continue to look for potential causes of vestibular schwannomas. High-dose therapeutic radiation to the head may increase the risk. Overall, there is no clear evidence that environmental factors, such as cell phones, cause these tumors. The rising incidence of vestibular schwannomas likely relates to improvements in magnetic resonance imaging (MRI) and increased screening for concerning symptoms.
Management of vestibular schwannomas centers around three different options: observation with repeated imaging, radiation, and surgery. The decision is complex and must account for tumor size, hearing status, symptoms, patient health, patient preference, and physician preference. Weighing these factors requires a thorough discussion with your ENT (ear, nose, and throat) specialist, or otolaryngologist, and often referral to additional specialists in neurotology, neurosurgery, and/or radiation-oncology.
Tumors can be classified as small, medium, or large. In general, small tumors are less than 1.5 cm, medium tumors are between 1.5 – 2.5 cm, and large tumors are greater than 2.5 cm. As tumors grow, they tend to cause more problems including hearing loss and compression on the brainstem.
Observation—The “wait and scan” approach may be an option for small tumors or patients for whom treatment carries increased risk. Repeat imaging can help determine the tumor’s growth rate and whether symptoms are changing over time, both of which may influence eventual treatment decisions. In some patients, the tumor may grow at such a slow rate that treatment within their life expectancy is unlikely. With this option, continuing to monitor the tumor with MRIs is critical.
Radiation—Stereotactic radiosurgery (SRS) can be delivered in a single dose or the radiation may be divided into three to five treatment sessions. Radiation is often reserved for growing small- to medium-sized tumors, but other factors should be discussed with your treating physician(s). Different technologies can deliver SRS, such as Gamma Knife®, CyberKnife®, and Triology®. Research has not shown that one radiation technology is better than another. So far, proton beam technology has not been recommended for vestibular schwannoma treatment. In general, the goal of radiation is to stop tumor growth. For some patients, after radiation the tumor may enlarge or swell before it stabilizes or shrinks. Also, long-term hearing often gets worse after radiation. With this option, continuing to monitor the tumor with MRIs is critical.
Surgery—The goal of microsurgery is to remove the tumor while preserving the function of nearby nerves and the brainstem. This option may be selected for a variety of reasons, including large, symptomatic tumors or small tumors with a chance of hearing preservation. There are three primary approaches—behind (retrosigmoid/suboccipital), above (middle cranial fossa), or through (translabyrinthine) the ear—all of which require opening the skull to reach the tumor. The specific approach depends on tumor size, hearing function, and surgeon preference. After surgery, patients can expect a three- to five-day hospital stay. Further details about hospitalization and the risks of surgery should be discussed with the surgeon.
If removing the tumor jeopardizes facial function or brainstem health, a portion of the tumor may be left behind. If the portion of tumor left behind is very small, it can be monitored and sometimes does not grow. If it does grow, radiation or revision surgery may be recommended. If a larger portion of the tumor is left behind, there is an increased likelihood of needing additional treatment.
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