Vestibular schwannoma, Acoustic neurinoma, Acoustic neurilemoma

A vestibular schwannoma (also known as acoustic neuroma, acoustic neurinoma, or acoustic neurilemoma) is a benign, usually slow-growing tumor that develops from the balance and hearing nerves supplying the inner ear. The tumor comes from an overproduction of Schwann cells—the cells that normally wrap around nerve fibers like onion skin to help support and insulate nerves. As the vestibular schwannoma grows, it affects the hearing and balance nerves, usually causing unilateral (one-sided) or asymmetric hearing loss, tinnitus (ringing in the ear), and dizziness/loss of balance. As the tumor grows, it can interfere with the face sensation nerve (the trigeminal nerve), causing facial numbness. Vestibular schwannomas can also affect the facial nerve (for the muscles of the face) causing facial weakness or paralysis on the side of the tumor. If the tumor becomes large, it will eventually press against nearby brain structures (such as the brainstem and the cerebellum), becoming life-threatening.

Causes

NF2 is inherited as an autosomal dominant trait in some patients. The abnormal gene can be inherited from either of the parents, and the risk of passing the gene to offspring from a parent is 50%. In some patients with NF2, there is no family history; and the disease is caused by a de novo mutation in the NF2 gene. NF2 is caused by mutations in the NF2 gene located in the long arm of chromosome number 22 (22q12.2). The NF2 gene encodes for the protein known as merlin, which acts as a tumor suppressor gene.[rx] Merlin is found in the Schwann cells in the nervous system.[rx]

Bilateral acoustic neuromas may be a part of neurofibromatosis type 2 occurring due to a defect on chromosome 22q12.2 at the location of the neurofibromin 2 gene which encodes merlin protein. Studies have shown that acoustic neuroma had a causative predisposition mutation. Radiation exposure may predispose a patient to the development of that condition as well.[rx] Even though mobile phone radiation has been of concern, several studies have failed to prove its causative effect on vestibular schwannomas.[rx]

• Meningioma
• Neurofibroma
• Malignant peripheral nerve sheath tumor
• Carcinomatous meningitis
• Plexiform neurofibroma
• Metastatic melanoma
• Malignant melanoma
• Pigmented neurofibroma
• Leiomyoma/leiomyosarcoma
• Chordomas
• Chondroblastomas
• Giant cell tumors
• Traumatic neuroma
• Pleomorphic hyalinizing telangiectatic tumor
• Palisaded encapsulated neuroma

• Ectodermal inclusion tumors

1. Epidermoid (5% to 7% of CPA lesions)
2. Dermoid

• Metastases
• Neuroma from cranial nerves other than cranial nerve VIII:

1. Trigeminal neuroma.
2. Facial nerve neuroma
3. Neurinoma of the lowest four cranial nerves (IX, X, XI, XII)

• Arachnoid Cyst
• Neurenteric Cyst
• Cholesterol granuloma (distinct from epidermoid)
• Aneurysm
• Dolichobasilar ectasia
• Extensions of nearby lesions in the CPA:

1. Brainstem or cerebellar glioma
2. Pituitary adenoma
3. Craniopharyngioma
4. Chordoma and tumors of skull base
5. Fourth ventricle tumors (ependymoma, medulloblastoma)
6. Choroids plexus papilloma from the fourth ventricle through foramen of Luschka
7. Glomus jugulare tumor
8. Tumors of the temporal bone

Symptoms of an acoustic neuroma

An acoustic neuroma may not cause any obvious symptoms at first.

Any symptoms tend to develop gradually and often include:

• hearing loss that usually only affects 1 ear
• hearing sounds that come from inside the body (tinnitus)
• the sensation that you’re moving or spinning (vertigo)
• persistent headaches
• temporary blurred or double vision
• numbness, pain, or weakness on 1 side of the face
• problems with limb co-ordination (ataxia) on 1 side of the body
• a hoarse voice or difficulty swallowing

How is a vestibular schwannoma diagnosed?

Unilateral/asymmetric hearing loss and/or tinnitus and loss of balance/dizziness are early signs of a vestibular schwannoma. Unfortunately, early detection of the tumor is sometimes difficult because the symptoms may be subtle and may not appear in the beginning stages of growth. Also, hearing loss, dizziness, and tinnitus are common symptoms of many middle and inner ear problems (the important point here is that unilateral or asymmetric symptoms are worrisome ones). Once the symptoms appear, a thorough ear examination and hearing and balance testing (audiogram, electronystagmography, auditory brainstem responses) are essential for proper diagnosis. Magnetic resonance imaging (MRI) scans are critical in the early detection of a vestibular schwannoma and help determine the location and size of a tumor and in planning its microsurgical removal.

How is a vestibular schwannoma treated?

Early diagnosis of a vestibular schwannoma is key to preventing its serious consequences. There are three options for managing a vestibular schwannoma: (1) surgical removal, (2) radiation, and (3) observation. Sometimes, the tumor is surgically removed (excised). The exact type of operation done depends on the size of the tumor and the level of hearing in the affected ear. If the tumor is small, the hearing may be saved, and accompanying symptoms may improve by removing it to prevent its eventual effect on the hearing nerve. As the tumor grows larger, surgical removal is more complicated because the tumor may have damaged the nerves that control facial movement, hearing, and balance and may also have affected other nerves and structures of the brain.

Acoustic neuroma may be treated by:

1. Observation of small and elderly patients with numerous comorbidities
2. Stereotactic radiotherapy or
3. Open craniotomy. The type of treatment depends on the surgeon’s experience and the patient preferences

The main surgical approaches are:

1. Retrosigmoid: This is the workhorse approaches in skull base surgery. It provides good access to the cranial nerves located in this region as well. Tumors of any size can be approached with the possibility of hearing preservation via this route.

2. Middle cranial fossa: This is also a hearing-preserving approach that is most suitable for tumors with a dominant intracanalicular component and a small cisternal component. The main disadvantage is the need for temporal lobe retraction, which may produce postoperative seizures and venous infarction if the vein of the Labbe is damaged.

3. Translabyrinthine: Suited for patients with large tumors and no serviceable hearing, The advantage is that the facial nerve can be exposed early and protected and the cerebellum need not be retracted. But the access to the contents of the jugular foramen and lower parts of the foramen magnum is limited.

Surgical Therapy

Tumor biopsy should be a consideration after confirming a nerve tumor on relevant imaging tests or nerve biopsy in case of aggressive peripheral neuropathy mimicking large nerve tumors. Schwannomas respond positively to local resection. Schwannomas do not infiltrate the nerve of origin; therefore, they usually are separated from it. Marginal excision of the lesion is done, though the nerves are not affected. If the surgeon suspects that complete resection would cause a permanent neurologic deficit, they might call for an intralesional resection or stereotactic radiosurgery.[rx][rx] Trigeminal schwannomas in the pterygopalatine fossa or infratemporal fossa benefit more from endoscopic endonasal approaches.[rx] Surgery on vestibular schwannomas uses the retrosigmoid, middle fossa, or translabyrinthine approach depending on the size of the tumor, the location, and the experience of the surgeon.

Radiation Therapy

Stereotactic radiosurgery becomes necessary if the growing tumor is near vital blood vessels or nerves. Hearing preservation and tumor control show excellent results with treatment. More than 75% of the patients retain serviceable hearing. The radiation dose to the central cochlea should be less than 4.2 Gy.[rx]

Stereotactic radiosurgery. Your doctor may recommend a type of radiation therapy known as stereotactic radiosurgery. It’s often used if your tumor is small (less than 2.5 centimeters in diameter), you are an older adult or you cannot tolerate surgery for health reasons.

Stereotactic radiosurgery, such as Gamma Knife radiosurgery, uses many tiny gamma rays to deliver a precisely targeted dose of radiation to a tumor without damaging the surrounding tissue or making an incision.

The goal of stereotactic radiosurgery is to stop the growth of a tumor, preserve the facial nerve’s function and possibly preserve hearing.

It may take weeks, months, or years before you notice the effects of radiosurgery. Your doctor will monitor your progress with follow-up imaging studies and hearing tests.

Stereotactic radiotherapy. Fractionated stereotactic radiotherapy (SRT) delivers a small dose of radiation to the tumor over several sessions. SRT is done to curb the growth of the tumor without damaging surrounding brain tissue.

Proton beam therapy. This type of radiation therapy uses high-energy beams of positively charged particles called protons. Protons are delivered to the affected area in targeted doses to treat tumors and minimize radiation exposure to the surrounding area.

After surgery, an MRI is required within 6-12 months to document the extent of tumor removal and serve as a baseline. The most common complications of surgery include injury to the anterior inferior cerebral artery, hemorrhage, cerebellar trauma, facial paralysis, hearing loss (the most common), and hydrocephalus.

The removal of tumors affecting the hearing, balance, or facial nerves can sometimes make the patient’s symptoms worse because these nerves may be injured during tumor removal.

As an alternative to conventional surgical techniques, radiosurgery (that is, radiation therapy—the “gamma knife” or LINAC) may be used to reduce the size or limit the growth of the tumor. Radiation therapy is sometimes the preferred option for elderly patients, patients in poor medical health, patients with bilateral vestibular schwannoma (tumor affecting both ears), or patients whose tumor is affecting their only hearing ear. When the tumor is small and not growing, it may be reasonable to “watch” the tumor for growth. MRI scans are used to carefully monitor the tumor for any growth.

What is the difference between unilateral and bilateral vestibular schwannomas?

Unilateral vestibular schwannomas affect only one ear. They account for approximately 8 percent of all tumors inside the skull; approximately one out of every 100,000 individuals per year develops a vestibular schwannoma. Symptoms may develop at any age but usually occur between the ages of 30 and 60 years. Most unilateral vestibular schwannomas are not hereditary and occur sporadically. Approximately one out of every 100,000 individuals per year develop a vestibular schwannoma.

Bilateral vestibular schwannomas affect both hearing nerves and are usually associated with a genetic disorder called neurofibromatosis type 2 (NF2). Half of the affected individuals have inherited the disorder from an affected parent and half seem to have a mutation for the first time in their family. Each child of an affected parent has a 50 percent chance of inheriting the disorder. Unlike those with unilateral vestibular schwannoma, individuals with NF2 usually develop symptoms in their teens or early adulthood. In addition, patients with NF2 usually develop multiple brain and spinal cord-related tumors. They also can develop tumors of the nerves important for swallowing, speech, eye and facial movement, and facial sensation. Determining the best management of the vestibular schwannomas as well as the additional nerve, brain, and spinal cord tumors is more complicated than deciding how to treat a unilateral vestibular schwannoma. Further research is needed to determine the best treatment for individuals with NF2.

Scientists believe that both unilateral and bilateral vestibular schwannomas form following the loss of the function of a gene on chromosome 22. (A gene is a small section of DNA responsible for a particular characteristic like hair color or skin tone). Scientists believe that this particular gene on chromosome 22 produces a protein that controls the growth of Schwann cells. When this gene malfunctions, Schwann’s cell growth is uncontrolled, resulting in a tumor. Scientists also think that this gene may help control the growth of other types of tumors. In NF2 patients, the faulty gene on chromosome 22 is inherited. For individuals with unilateral vestibular schwannoma, however, some scientists hypothesize that this gene somehow loses its ability to function properly.

What is being done about vestibular schwannomas

Scientists continue studying the molecular pathways that control normal Schwann cell development to better identify gene mutations that result in vestibular schwannomas. Scientists are working to better understand how the gene works so they can begin to develop new therapies to control the overproduction of Schwann cells in individuals with vestibular schwannoma. Learning more about the way genes help control Schwann cell growth may help prevent other brain tumors. In addition, scientists are developing robotic technology to assist physicians with acoustic neuroma surgery.

Is an acoustic neuroma cancer?

An acoustic neuroma is a benign tumor (not cancer). Benign tumors do not spread to other parts of the body the way cancerous tumors do. They cause problems by growing and pressing on important structures and nerves. These tumors tend to grow slowly, sometimes over many years.

How does an acoustic neuroma affect people?

The tumor can cause symptoms most commonly by affecting hearing or balance functions. More than 90% of acoustic neuromas affect the hearing nerve in one ear, which is called unilateral hearing loss.

An acoustic neuroma can also press on nerves that control your facial movement, sensation, and expression. Large tumors that press on brain structures that control the flow of spinal fluid out of the brain can be life-threatening.

What is the difference between an acoustic neuroma and a vestibular schwannoma?

An acoustic neuroma and a vestibular schwannoma are the same conditions. Vestibular schwannoma is the technically proper term because it more accurately describes the type of tumor (schwannoma) and the nerve it originates from (vestibular nerve).

How common is an acoustic neuroma?

Acoustic neuromas are considered rare. Every year, about one out of every 100,000 people develops an acoustic neuroma. Unilateral (one-sided) acoustic neuromas represent about 8% of all skull tumors.

Who might get an acoustic neuroma?

While anyone can develop an acoustic neuroma, some populations are at higher risk. They occur more commonly with increasing age, peaking in those aged between 65 and 74. (Acoustic neuromas are very rare in children.) They occur equally among men and women. Asian Americans have the highest incidence, followed by White, and then Black Americans.

Typically, acoustic neuromas affect the hearing nerve in only one ear. Acoustic neuromas might affect both ears in people with neurofibromatosis type 2 (NF2), a genetic disorder. People with NF2 represent about 5% of people with acoustic neuromas.