Overview

An acoustic schwannoma (also called vestibular schwannoma, acoustic neuroma or neurinoma) is a non-cancerous growth that arises from the eighth cranial nerve.

The eighth cranial nerve has two divisions:

1. The cochlear division - associated with transmitting hearing and
2. The vestibular division - associated with sending balance information from the inner ear to the brain.

These two parts pass through a bony canal called the internal auditory canal to reach the brain. The seventh cranial nerve (also called the Facial Nerve as it controls facial movements) and important blood vessels also pass through this canal. It is here that acoustic schwannomas usually begin to grow from the tissue surrounding the vestibular division of the eighth cranial nerve. 

Causes

What causes Acoustic Schwannoma?

The cause is not known, except for a small percentage of individuals in whom both sides of the eighth cranial nerve are involved. In these instances there is often a hereditary factor.

What is its growth pattern?

Acoustic schwannomas are slow growing tumours, and usually enlarge by 1mm to 3mm per year, while some may grow by more than 5mm per year. In some 9-21% of patients, the growth can remain inert for several years, and in a small proportion of patients (6-15%), its growth can even regress with time.

Symptoms

Over 90% experienced one-sided deafness, often accompanied by noise in the affected ear (tinnitus). The deafness may be gradual or sudden. A common pattern is the lessening of speech discrimination ("I can hear sounds but I cannot understand what is being said") when listening on the telephone. Unfortunately many people merely shift the telephone to the other ear to compensate for the one-sided hearing loss, and seek medical attention only when the hearing loss is almost complete or when other symptoms appear.

Unsteadiness and balance problems may occur early in the growth of the tumour, and worsen as the balance function is destroyed on the affected side. The remainder of the balance function sometimes compensate for this loss, and thus balance problems may be forgotten after some time. Pressure by large tumours on other cranial nerves causes facial numbness, weakness of the facial muscles or swallowing problems. Unsteady gait may be caused by pressure on the cerebellum. Very large tumours can also cause headaches.

Risk Factors

Specific risk factors known for increasing a person’s risk of developing acoustic neuroma include age and family history of neurofibromatosis type 2.

Most people develop acoustic neuroma between the ages of 30 to 60 years.

Neurofibromatosis type 2 is a rare inherited disorder characterised by the development of malignant central nervous system tumours. This disorder can be passed on by only one parent, thus if either parent has it, their children have a 50 per cent chance of inheriting it.

Prevention

There is no known prevention for acoustic neuromas.

Treatment Options

Surgical treatment of Acoustic Schwannoma

The only cure for the patient with acoustic schwannoma is complete removal of the tumour by using microsurgery techniques. The surgery is performed by the neurosurgeon and sometimes, in conjunction with an ear, nose and throat (ENT) surgeon.

There are basically three surgical approaches: Retrosigmoid, Tranlabyrinthine and Middle Fossa. Combination of these approaches may be used for very large tumours. The choice of the approach will depend on the size and location of the tumour, whether the patient still has good hearing and the medical fitness of the patient.

Fat or muscle may also be harvested from the abdomen or thigh during surgery to assist with the closure.

Post-operatively, the patient will usually spend one to several days in the intensive care unit for close monitoring and treatment.

Risks and Complications of surgery

Possible complications from surgery are similar for all brain surgeries. These include infection, bleeding, death, coma, stroke and seizures. The likelihood of serious complications is fairly low in modern neurosurgical centres.

Other complications specific to acoustic neuroma surgery includes:

• Total hearing loss in the affected ear.
• Tinnitus (ear noise) usually remains after surgery even despite total hearing loss.
• Facial Weakness or Paralysis. The surgeon has to manipulate and dissect the tumour off the nerve, or sometimes to remove a portion of this nerve. Nerve damage or swelling may occur and cause weakness or paralysis of the facial muscles. This results in incomplete eye closure and sagging of the face. The exposed cornea is dry and easily injured by dust particles. To protect it, the eyelids have to be taped together after instillation of eyedrops. A small surgical procedure called tarsorrhaphy (stitching together of the edges of the outer third of the eyelids) may be advised later to protect the eye. Nerve regeneration may occur if the nerve is still anatomically intact after surgery. However, this is a slow process and it may take up to a year for some recovery of facial movement to be seen. If no recovery is visible after one year, a second operation may be considered to connect the healthy portion of the Facial Nerve to the Hypoglossal Nerve leading to that side of the tongue. This procedure is called Hypoglossal -Facial Nerve anastomosis and restores some, but not perfect, facial functions.
• Facial Numbness can develop after removal of a large tumour pressing on the Trigeminal Nerve.
• Swallowing, Throat, Voice Problems, Taste Disturbance, Mouth Dryness may occur after surgery to remove a large tumour pressing on the IX and X cranial nerves.
• Balance Problems. The vestibular portion of the eighth nerve is removed during surgery and dizziness is common following surgery. As the balance system in the normal ear compensates for this loss, unsteadiness should improve but may never be perfect. Unsteadiness may therefore occur when the person is tired.
• Gamma Knife Radiosurgery. The Gamma knife uses highly focused Gamma Ray beams produced by 201 independent Cobalt 60 sources to produce its biological effects on tissues inside the intact skull. The treatment is done in a single sitting and despite its name, no cutting is involved. It has been used to treat acoustic schwannomas. The tumour is not removed, but is prevented from growing further.

The results of microsurgery in the best hands are still superior to the current results obtained with the Gamma Knife. However when a comparison is made to average available results of microsurgery for acoustic neuromas, Gamma Knife Radiosurgery emerges as a viable alternative for the management of these lesions. It is definitely indicated in patients with residual tumors following microsurgery and in cases where age or illness preclude open surgery.