What is Acoustic Neuroma?
Acoustic neuroma (vestibular schwannoma) is a benign tumor of the myelin-forming cells of Cranial Nerve VIII, the vestibulocochlear nerve. The vestibulocochlear nerve connects the ears to the brain and is the cranial nerve responsible for the sensory function of the ears. Vestibular schwannoma was derived from the vestibular portion of the vestibulocochlear nerve and develops from the Schwann cells which form the myelin sheath of the peripheral nervous system.
About one in every 100,000 population worldwide is affected by acoustic neuroma. It comprises 10% of all neoplasms in the cranium in adults. Individuals above 50 years old are more at risks for developing it. Both the male and female gender may be affected.
The tumor usually grows slowly and may reach up to one inch in size. Acoustic neuroma is not a form of cancer, but as it grows, it compresses the vestibulocochlear nerves resulting in hearing and balance problems. It can also extend into the angle between the pons and cerebellum and may compress the 5th, 7th, 9th and 10th cranial nerves. Compressing the other cranial nerves will result in problems related to the function of the compressed nerves. Growing tumors may even compress the medulla resulting in cerebrospinal fluid obstruction eventually leading to increase in intracranial pressure.
Acoustic neuroma may also be called as acoustic neurilemmoma, auditory tumor, acoustic tumor, acoustic neurinoma or cerebellopontine angle tumor.
Acoustic Neuroma Location
Acoustic Neuroma Symptoms
Symptoms of acoustic neuroma are associated with the compression of the vestibulocochlear nerve and other adjacent nerves and structures. Symptoms include:
- Ipsilateral Sensorineural hearing Loss – deafness is the earliest symptom of acoustic neuroma. It occurs on the same side where the tumor developed and occurs gradually. Before hearing loss occurs, there may be associated background noise and localizing sounds.
- Disturbed balance – balance is affected because of affectation of the conchlear nerve responsible for stability
- Unstable gait
- Vertigo or dizziness
- Nausea and Vomiting
- Pressure in the ear
- Tinnitus (ringing of the ears)
- Loss of sensation and weakness in the face and mouth on the same side of the affected ear (because of affectation of the trigeminal nerve)
- Increased intracranial pressure leading to headache, projectile vomiting and altered level of consciousness
- Difficulty understanding speech because of hearing loss
- Headache upon awakening, worsens during lying down, standing up, vomiting, coughing, sneezing, lifting or straining on stools
- Vision problems
Affectation of other structures such as the brain stem (medulla and the pons) and cerebellum may lead to life-threatening conditions because it may cause impairment in the respiratory function as manifested by shortness of breath and difficulty breathing.
Acoustic Neuroma Causes
Causes and risk factors involve any affectations in the vestibulocochlear nerve such as:
- Prolonged exposure to loud noise which disrupts the nerves in the ears.
- Prior Head and Neck radiation exposing the vestibulocochlear nerve to radioactive frequency.
- History of parathyroid adenoma
- Use of Hand held cellular phones- exposure to radiofrequency radiation found in cell phones have been associated with some cases of acoustic neuroma
- Defects in tumor suppressor genes which allow tumor growth.
- History of neurofibromatosis
Neurofibromatosis is a genetic disorder that involves tumor growth on the neurologic system. It usually results in tumor growth on the cranial nerves including the vestibulocochlear nerve. Presence of family history of neurofibromatosis may also lead to acoustic neuromas. Neurofibromas are inherited through autosomal dominance of genes.
Neurofibromatosis occurs in two types
- Neurofibromatosis type 1 - may sporadically involve the 8th cranial nerve and occurs during adulthood. Bilateral neuromas are usually rare.
- Neurofibromatosis type 2- Typically causes bilateral acoustic neuromas and may develop before age 21.
Acoustic Neuroma Diagnosis
Imaging studies are the typical diagnostic tests done to detect tumors inside the cranium. These include:
Computed Tomography Scan - Contrast-enhanced CT-scan involves the use of contrast medium, usually an iodine based dye to detect tumor growth. CT scans are able to detect tumors that are 1.5 to 2.0 cm in size.
Magnetic Resonance Imaging - MRIs best detect tumors that are smaller than 1.5 cm in size.
Other tests include:
- Audiology - Patients are examined to determine sensorineural or conductive hearing loss. The patient wears headphones and sounds are played at one ear at a time. The examiner will let you indicate when you hear the sound. The sounds are also played at various volumes to identify the faintest sound you can hear.
- Rhine’s Test - This involves the use of tuning fork to evaluate bone conduction and air conduction threshold.
- Brainstem Auditory Evoked Response - Electrodes are placed on the scalp and earlobes which capture the brain’s activity as sounds are heard over the earphones.
- Electronystagmography - This test determines the vestibular function by assessing the eye movements during various balance tests.
Acoustic Neuroma Treatment
Treatment for acoustic neuroma involves radiation and surgery. More conservative treatments are also employed. Various managements include:
Physicians usually institute conservative measures such as observing the tumor using imaging studies. Some of the tumors shrink and may not need advanced treatments. Tumors that appear to grow and compress structures may need radiation and surgery.
Radiation therapy is done to suppress or slow down the growth of acoustic neuroma tumors.
Radiation therapy does not remove tumors, but the tumors are surgically removed after the therapy. Radiation involves various types such as proton therapy, Cyberknife, gamma knife radiosurgery or fractionated stereotactic radiotherapy.
Radiation therapy poses some risks such as progression of the acoustic neuroma to a malignant tumor and development of other tumors on adjacent structure. However, the incidence of such is low. To evaluate the result of the radiation therapy, yearly imaging tests should be done to determine any progression or re-growth of the tumor.
Surgery has been the only management to completely eradicate the tumor. Surgery is also employed to large tumors, tumors that grow fast, and tumors that develop signs and symptoms. There are several approaches that can be performed during surgery. It requires a highly technical procedure involving neurosurgeons and otorhinolaryngologists.
Microsurgery involves the removal of the tumor together with the inferior and superior vestibular nerves. Because of this, vestibular function on the affected side may be lost. However, the nerve on the other ear can still perform the function that’s why the patient is still able to hear.
Removing the tumor may involve potential damage to the facial nerve. Evaluating the facial nerve before, during and after the procedure is essential.
Three types of surgery can be employed such as:
Translabyrinthine Approach - This is the safest approach because it does not employ brain retraction however; it often leads to complete destruction of hearing on the operative side. Because of this, people who have already poor speech discrimination on the affected ear are the ones undergoing this approach.
Suboccipital Retrosigmoid Approach - This procedure is one of the hearing preservation approaches and is preferred by neurosurgeons because they are more familiar with the surgery.
Middle Fossa Approach - This is also one of the two approaches that preserve the hearing. This procedure is more advantageous than the retrosigmoid approach because of more direct access to the internal auditory canal.
Endoscopic Surgery - Endoscopic surgery is a minimally invasive surgery which involves the use of scope. This is usually done in specialized centers.
Undergoing surgery also poses risks especially when other structures are affected during the procedure. Complications include leakage of cerebrospinal fluid, tinnitus, persistent headache and balance and gait problems.
Acoustic Neuroma Prognosis
Acoustic neuroma has good prognosis because it is not malignant. Small tumors may not even require any surgery. It does not spread to other parts of the body. But because the tumor increases in size, it can compress other structures that may lead to serious conditions such as:
- Permanent deafness
- Permanent facial nerve affectation leading to paralysis of the facial nerves
- Hydrocephalus as a result of obstruction of cerebrospinal fluid
- Increase in intracranial pressure
Once a patient had hearing loss, it cannot be returned even after surgery. One should contact a medical provider when symptoms occur such as gradual hearing loss and gait problems for early detection and management of acoustic neuroma.
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