Acoustic neuroma is a benign, slow-growing intracranial tumor. It is also called “vestibular Schwannoma” due to its origin of the Schwann cells of the vestibular nerve. As the tumor enlarges, it can cause also cerebellar findings and obstructive hydrocephalus, becoming a life-threatening situation.

According to the American Hearing Research Foundation, acoustic neuromas in 2012 are about 6% of all intracranial neoplasms, the majority of which are sporadic (95%) and about 5% of them are genetic—part of the chromosome 22 abnormality, causing neurofibromatosis type 2. It is a familial autosomal dominant form in which patients have bilateral acoustic neuromas in addition to other intracranial tumors.

Clinically it is divided into 2 forms—medial and lateral. Medial neoplasm is located in the cerebello-pontine angle arising from the intracranial part of the vestibulocochlear nerve. Lateral neoplasm is located in the internal auditory canal.

The tumor is consisting of Schwann cells and grows on the vestibulocochlear nerve. Normally, a gene on chromosome 22 helps to control the growth of Schwann cells, which covers the vestibulocochlear nerve. The cause of gene malfunction isn't clear. In the case of neurofibromatosis type 2, the gene is inherited and acoustic neuromas are present in both sides of the head.

The only known risk factor for acoustic neuroma is a parent with genetic disorder neurofibromatosis type 2. Each child of an affected parent has a 50-50 chance of inheriting the gene mutation.

Acoustic neuroma almost always develops in the vestibular part of the vestibulocochlear nerve, from surrounding Schwann cells. The tumor develops in the nerve sheath and mainly compress it, not invade. The neuroma is not cancer and does not spread to other areas of the body. When they grow, they are situated intracranially, so they can compress the brain and cranial nerves resulting in relevant symptoms. The growth rate can be divided into three patterns:

1. No growth or very slow growth
2. Slow growth (i.e. 0.2 cm in a year)
3. Rapid growth (i.e. >1 cm in a year)

Although schwannomas usually grow slowly, it can grow very fast and double in volume within 6 months. If it becomes too large, it can exert the brainstem and cerebellum, affecting vital functions.

Many acoustic neuromas never become clinically apparent as the brain compensates for slow-growing tumors. Because it arises from the vestibulocochlear nerve, the symptoms are usually connected with vestibular or hearing impairments.

Most frequent symptoms are:

• Asymmetric progressive sensorineural hearing loss (rarely sudden hearing loss) causes the difficulties to localize the sound
• Tinnitus (ringing in the ear), usually high-pitched
• Balance disturbances

Less common and later-onset symptoms include:

• Vertigo (dizziness) may indicate rapid growth rate or vascular insufficiency

• Trigeminal dysfunction which takes forms such as the feeling of thickening in the cheek area and trigeminal neuralgia (mild stimulation to the face may trigger intolerable pain)
• Paresis of the facial nerve
• Reduced corneal reflex (eyelid closing as the the cornea is stimulated)
• Diplopia (double vision)
• Headache
• Nausea and/or vomiting
• Ataxia (muscle movement coordination disorders)
• Altered taste and tearing
• Dysphagia
• Hoarseness
• Hitzelberger's sign (reduced sensation of the posterior external meatus

In patients with neurofibromatosis type-2, physical examination reveals different findings, such as café au lait spots throughout the body.

1. Audiometry is done to determine the severity of sensorineural hearing loss.
2. In patients with balance disorders, the practitioner may perform some vestibular testing.
3. Gadolinium-enhanced magnetic resonance imagining (MRI) is considered as a gold standard for acoustic neuroma diagnosis. The method is sensitive if the size or the tumor is greater than 2 mm.
4. Computer tomography (CT) scan can be used as well, but it is hard to diagnose tumors in size less than 10 mm.
5. Auditory brainstem response tests give information about the inner ear and brain pathways for hearing. It is performed by pasting electrodes on the head and recording brain wave activity in response to sound. The patient rests quietly or sleeps when the procedure is performed. It is sensitive in most of the cases if the tumor is greater than 1.5 cm.

There are 3 options for treatment:

1. Observation is recommended if the neuroma is small and is not clinically apparent or symptoms are mild. Approximately half of small tumors (<1 cm) show little or no growth in 1 to 3 years. Follow-up MRI should be made in 6 months and once a year for the following years if the neuroma is solid. If it is greater than 1 cm, repeated MRI is recommended every 4 - 6 months.
2. Surgical resection has three different approaches. There are various criteria for which approach to use, but the foremost are tumor size and hearing, tumor localization, patient age, vestibular function.
3. Stereotactic radiosurgery is a non-surgical therapy to halt the growth of the neuroma. Radiotherapy changes the DNA of the tumor cells and prevents them from dividing and increasing. The method can be recommended if the neuroma is smaller than 3 cm. For larger neuromas, surgical resection is the preferred option.

There are different complications for each treatment method.

For surgery, it can be hearing loss, paresis of the facial nerve (mostly temporary in a year period), continuous headache, cerebrospinal fluid leak (otorrhea, rhinorrhea, drainage through the skin incision), meningitis, recurrence of the tumor, stroke, intracranial hemorrhage.

Stereotactic radiosurgery: trigeminal neuropathy, facial nerve neuropathy, hydrocephalus, tinnitus, ataxia, vertigo, the formation of a peritumoral cyst, vestibular disorders, malignant transformation in 5—18 years.

The most frequent symptoms can become permanent: hearing loss, central vestibular disorders, tinnitus, and facial numbness. The most dangerous complication in the neuroma growing is the compression of the brain (resulting in vomiting, headache, visual disturbances), which can result in cessation of vital functions.

Avoiding conception for a patient with neurofibromatosis type 2 is the only prevention related to future generations of this individual. Early detection of the tumor may protect from large forms, although screening is not recommended in asymptomatic patients. A hearing test may be useful for diagnostic unilateral hearing loss.

Early diagnosis of this tumor improves patient morbidity and mortality.

Scientists, Kutz et al, from the University of Texas Southwestern Medical Centre reported about the clinical outcome in patients after the treatment as follows:

• Depending on the criteria for successful hearing, hearing can be maintained in 30—80% of patients. Rehabilitation options for patients with a hearing deficit after acoustic neuroma removal include a contralateral routing of signals hearing aid or bone anchored hearing aid. In patients with neurofibromatosis type 2, all patients eventually lose hearing in both ears, so the auditory brainstem implant can be the treatment of choice.

• Tinnitus becomes worse in 6—20% of patients after tumor removal. In about 25—60%, tinnitus is eliminated or improved. Although 30—50% of patients who had no preoperative tinnitus develop in the postoperative period.

• Tumor recurrence rate is less than 5%. It can be suspected by recurrent headache, altered sensation to the face or dysarthria. A small amount of tumor can be left in the lateral end of the internal auditory canal where it is hard to visualize.

Facial nerve paralysis may be delayed and may develop within a few hours to a week or more after tumor removal. It varies between 10—30%. Most of the patients make complete and total recoveries.

Acoustic neuromas are rare, an annual incidence in the US Surveillance, Epidemiology, and End Results database of between 0.2 and 1.7 per 100,000 per year. The incidence may be rising as a result of widespread use of MRI. A study by the University of California, San Francisco in 2005 suggests the prevalence of incidental acoustic neuromas to be in 2 people per 10,000. Acoustic neuroma is seen with a slight female predominance.