Vestibular Schwannoma

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.

Schwannomas (also known as neuromas, neurinomas “of Verocay” and neurilemmomas) are benign, well-encapsulated, slow-growing nerve sheath tumors composed exclusively of Schwann cells derived from the neural crest.[rx][rx] The tumor can originate from any myelinated central or peripheral nerve with Schwann cells. The World Health Organization classifies schwannoma as a grade I benign tumor. Schwannomas are solitary in 90% of the cases. Multiple tumors in the same patient should bring attention to syndromic associations (neurofibromatosis type 2, schwannomatosis, and Carney complex).[rx][rx][rx][rx]

Types

Classic Schwannoma: This is an encapsulated tumor with two distinct histological regions. Antoni A tissue shows hypercellular spindle cells, sometimes palisade around eosinophilic areas (Verocay bodies). Immunostain is positive for S100 protein staining.[rx] Antoni B tissue shows a hypocellular myxomatous pattern with a background of loose connective tissue. Cysts, hemorrhage, and fatty degeneration may be present. Calcifications and mitotic figures are rare.

Cellular schwannoma: It is a relatively uncommon, but significant variant of schwannoma. It is located principally in the paravertebral region. It shows compact hypercellular areas composed entirely of Antoni A areas and devoid of Verocay bodies. Cellular schwannoma prompts consideration of malignancy due to its high cellularity, increased mitotic activity, fascicular growth pattern, and occasional locally destructive character. Clues that aid in the diagnosis include the presence of histiocyte aggregates and high expression of pericellular collagen IV. The diffuse expression of the S100 protein is uncommon in spindled malignant peripheral nerve sheath tumors (MPNST), which should point toward cellular schwannoma if present. Cytokeratin immunoreactivity may manifest in some cellular schwannomas and may indicate cross-reactivity with glial fibrillary acid protein. It is important to note that cellular schwannoma is characterized by weak expression of desmin, smooth muscle actin, CD117, and DOG1.

Despite their high cellularity, cellular schwannomas do not have malignant potential and do not metastasize. Local recurrence varies and may be higher compared to classic schwannomas. This recurrence relates partially to the location considering the propensity for deep anatomic regions not always amenable to gross total resection. However, recurrent lesions have slow growth. Mitotic activity, most of the time, does not exceed 5 per 10 high-power fields. Mitotic activity, above 10 per 10 high power fields, may occur in rare instances. If other diagnostic features of cellular schwannoma are present, the proliferative action is still compatible with a benign diagnosis.

Plexiform schwannoma: This variant occurs in subcutaneous or cutaneous superficial locations and is defined by a pattern of growth that may be considered plexiform. Plexiform schwannoma has an association with schwannomatosis, NF2, and other schwannoma-predisposing syndromes. These tumors may not be as circumscribed as a classic schwannoma, and a capsule might even be absent. They involve multiple fascicles. The tumors have an Antoni A pattern. Neurofilament protein immunoreactive axons are present within the lesion.

Plexiform schwannomas that originate in deep anatomic lesions are more problematic. The anatomic locations may be within soft tissues or principal peripheral nerves. They may demonstrate high mitotic activity and cellularity and may not be easily distinguishable from MPNST. Although their potential for malignancy is negligible, these tumors may have a relatively high local recurrence, occurring in approximately 50% of the cases.

Melanotic schwannoma: This subtype of schwannoma may be described as a distinctive, rare, potentially malignant neoplasm that presents with epithelioid cells and melanin accumulated in neoplastic cells and melanophages. Usually arises around the spinal nerve roots. It is positive for Melanie, and sometimes for glial fibrillary acidic protein and neurofilament protein.

Causes

Schwannomas occurring in specific syndromes (neurofibromatosis type 2, schwannomatosis, and Carney complex) may have a possible genetic etiology, not only in the syndromic schwannoma but in sporadic cases. Neurofibromatosis type 2 accounts for 3% of the syndromic schwannomas, while schwannomatosis for 2%, and meningiomatosis with or without neurofibromatosis type 2 in 5%.

Genetic studies show that the NF2 gene on chromosome 22 plays an essential role in sporadic and syndromic schwannoma development. The NF2 gene encodes for the merlin protein (schwannomin). Specific gene mutations in the NF2 gene cause the inactivation of the gene, thus preventing the formation of the merlin protein. Inactivation of both alleles of the NF2 gene is observed in most schwannomas. Carney complex may have a loss of PRKAR1A expression.

Spinal schwannomas can have SMARCB1 mutations and inactivation.[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

 Symptoms

Hearing loss, dizziness, tinnitus, and other symptoms of an acoustic neuroma can be caused by other, more common ear problems, and it is important to consult a doctor for a diagnosis. Because acoustic neuromas often grow on the balance and hearing nerve, the most common symptoms they cause are:

One-sided Hearing Loss

Over 90% of people with acoustic neuromas develop some degree of one-sided (unilateral) hearing loss. People with this type of hearing loss may have difficulty hearing in noisy settings and locating where a sound is coming from. If a person tends to hold a phone to a certain ear or has a hard time following conversation in a crowded room, these may be signs of hearing loss.

The hearing loss usually gets worse over the years and may lead to total deafness in one ear. In some people (about 5% of patients), hearing loss may develop suddenly. The loss may be partial or total, and spontaneous recovery is possible. Sudden hearing loss may be the first event that leads to a diagnosis, or it may occur months or years before the tumor is discovered.

Hearing loss can occur from compression or infiltration of the tumor on the hearing nerve or the secretion of substances toxic to hearing. It can affect the range of sounds heard, as well as the clarity of sound. Hearing loss can occur even if the tumor is not growing.

A small portion of patients with acoustic neuroma may not yet have hearing loss from the tumor. Mild hearing loss due to an acoustic neuroma might not even be noticeable, which can result in a delayed diagnosis.

Ear Fullness

People with an acoustic neuroma might have a sensation of fullness in the ear as if water is in it. This sensation is typically caused by hearing loss from the tumor.

Noise in the Ear (Tinnitus)

Tinnitus is a very common symptom of acoustic neuromas and many other inner ear conditions. People with acoustic neuromas may experience a high-pitched tone in the ear affected by the tumor. In other cases, the tinnitus can sound like hissing, buzzing, or roaring — like when putting a seashell to the ear.

While most patients with acoustic neuromas have both tinnitus and hearing loss in one ear, some may experience tinnitus without losing hearing. Tinnitus can come and go or be constant — with single or multiple tones — and can sound quiet or overwhelmingly loud.

Balance Problems and Vertigo

Because acoustic neuromas arise from the vestibular nerve responsible for balance, unsteadiness or balance problems may be early symptoms of acoustic neuroma. Nearly half of people with acoustic neuromas notice these symptoms, which tend to worsen if the tumor grows. Large acoustic neuromas may compress parts of the cerebellum, which may lead to falls. Patients tend to fall toward the side of the tumor.

The balance system can compensate for the loss of balance, so it may stabilize.

True vertigo (the sensation of spinning or tilting) is not commonly associated with acoustic neuromas, but it can sometimes occur due to tumor growth or bleeding.

Other Signs of Acoustic Neuroma

Acoustic neuromas can also put pressure on other important cranial nerves that are adjacent to where these tumors grow.

• Numbness in the face can result from a tumor pressing on the trigeminal nerve. There may be ongoing or periodic numbness and facial tingling on the side of the acoustic neuroma. Tingling (paresthesia) may be near the corner of the mouth or on the cheek. There may also be eye irritation or redness due to numbness in the eye that prevents appropriate blink reflexes.
• Facial twitching or weakness can result from the tumor pressing on the facial nerve. This can cause twitching (tics or spasms) of the eye, eyebrow, forehead, or mouth muscles. Less often, you might notice weakness in the face. Facial weakness often does not occur until acoustic tumors grow quite large, and it is less common at the time of diagnosis.
• Swallowing problems can occur from the tumor pressing on the vagal and hypoglossal nerves. These nerves control several aspects important to swallowing, including sensation in the throat and movement of vocal cords and the tongue.
• Change in taste and tear production is a less common symptom, but it should be evaluated by a doctor. The facial nerve helps control taste and tear formation. Pressure on the nerve can cause dry eye or even unexpected tears, as well as changes in taste perception.
• Headache and pressure: As the acoustic neuroma grows, it can press on the lining of the inside of the skull (the dura). The dura has sensory fibers that can transmit the sensation of pressure. The headache that results from the acoustic neuroma can be dull or aching, and it is usually on one side of the head. The pain may radiate to the neck or the top or front of the head.

These symptoms can be caused by many other, more common health issues such as cholesteatoma, labyrinthitis and vestibular neuritis, and Meniere’s disease. If you have more than a few of these symptoms (especially if they don’t go away or become worse), your doctor can help you decide whether more testing is necessary.

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.

Acoustic neuroma diagnosis includes:

• A hearing test (audiometry). This is a test of hearing function that measures how well you hear sounds and speech. It is usually the first test performed to diagnose acoustic neuroma. A doctor asks you to listen to sounds and speech while you are wearing earphones attached to a machine that records responses and measures hearing function. If you have an acoustic neuroma, your audiogram may show the following.
• Increased pure tone average (PTA). This metric evaluates how loud a sound frequency needs to be before you hear it.
• Increased speech reception threshold (SRT). This metric evaluates how loud speech needs to be before you hear it. Similar to the pure tone average, the higher the score, the worse the hearing.
• Decreased speech discrimination (SD). This metric evaluates how many words you can detect, one ear at a time. The lower the score, the worse the hearing.
• Imaging scans of the head. If other tests point to a possibility of acoustic neuroma, MRI can confirm the diagnosis. MRI with a contrast dye can help pinpoint the tumor. If an acoustic neuroma is present, it will soak up more dye than normal brain tissue and appear clearly on the scan. MRI commonly shows a densely “enhancing” (bright) tumor in the internal auditory canal.

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.

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.

Acoustic Neuroma Observation

Acoustic neuromas are noncancerous tumors, so they can often be closely monitored without treatment. After the initial diagnosis, unless the tumor is already very large or is causing significant symptoms, doctors may recommend getting an additional MRI after six to twelve months. This allows you and your doctor to determine if the tumor is growing. Small tumors, that cause minor or no symptoms, and that are not growing can often continue to be observed with regular MRI. If the imaging shows that a tumor is growing, or if the tumor begins to cause significant symptoms, it might be time for treatment.

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]

Acoustic Neuroma Surgery

Modern microsurgical advancements have made acoustic neuroma surgery procedures safer, more effective, and easier to recover from.

The goals of surgery are to remove the tumor while preserving the facial nerve’s function. Hearing preservation after surgery for acoustic neuroma is possible but depends on several factors, including how well you hear before the surgery and how big the tumor is. Roughly half of the patients with the smallest tumors who have useful hearing before surgery will maintain useful hearing after surgery. Hearing preservation is less likely for larger tumors. The risk of hearing and facial nerve complications after the surgery increases with a larger acoustic neuroma size.

Surgeons have developed different approaches to removing acoustic neuromas — the best depends on tumor size and location, patient characteristics, and the goals of the surgery. The most commonly used approaches are suboccipital, translabyrinthine, and middle fossa craniotomy.

• Suboccipital or retrosigmoid craniotomy. During this procedure, the surgeon accesses the acoustic neuroma from the back of the head. It offers the best view of the brainstem, particularly for the nerves involved in swallowing, which can be affected if a tumor is large. This approach is often recommended for patients with larger tumors. It also might preserve hearing for patients with smaller tumors, because the inner ear structures are preserved.
• Translabyrinthine craniotomy. During this procedure, the surgeon removes the bone behind the ear to access the tumor through the inner ear. This provides the best view of the entire length of the facial nerve and can require less retraction of the brain. However, it requires going through the structures of the inner ear and does not allow the preservation of hearing. This approach is generally considered for patients who have no functional hearing.
• Middle fossa craniotomy. This is an option for patients with smaller acoustic neuromas and intact hearing. It is generally considered to provide the greatest chance of hearing preservation, but there is a slightly greater risk of facial nerve weakness after the surgery, and it cannot be performed for medium or large-size tumors.

Image Guidance and Monitoring During Acoustic Neuroma Surgery

Monitoring of brain and nerve function is a critical part of acoustic neuroma surgery. A team of neurologists and electrophysiologists watches for any changes in facial and hearing nerve activity, as well as for changes in the brain. Being aware of such changes can help the surgeon avoid neurologic complications.

Stereotactic Radiosurgery for Acoustic Neuroma

Radiosurgery, also called stereotactic radiosurgery, is a noninvasive procedure that uses precisely focused, narrow beams of radiation to treat the acoustic neuroma while limiting the amount of radiation that affects surrounding structures, including the hearing, balance, and facial nerves. This form of radiation therapy can reduce the growth of an acoustic neuroma. Doctors may recommend radiosurgery for older patients with acoustic neuromas who might be too fragile to endure more invasive treatment.  Radiosurgery may also be used in combination with surgery for large tumors that cannot be removed completely without permanently damaging the facial nerve or other structures.

Some studies report cancers developing within the field of radiation treatment for acoustic neuroma.

What if an acoustic neuroma returns after radiosurgery?

Radiation treatment requires ongoing follow-up and annual scans to watch for tumor regrowth. Parts of the tumor unaffected by the radiation may give rise to new growth. Signs of an acoustic neuroma coming back could include facial muscle weakness and spasms that slowly worsen, and new growth can often be seen on an MRI scan. Few studies have documented the effects of radiation beyond five years.

Repeated radiation for an acoustic neuroma that comes back after radiosurgery is typically unsafe, and the doctor may recommend surgery. Acoustic neuroma surgery after radiotherapy treatment can be complicated by scar tissue (fibrosis) that can make it difficult to separate the tumor from adjacent nerves.

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 develops 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.

References

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