Introduction

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

The eighth (VIII) cranial nerve has two divisions:-

• the cochlear division associated with transmitting hearing and,
• 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 (VII) 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 neuromas usually begin to grow from the tissue surrounding the vestibular division of the eighth cranial nerve.

What Causes Acoustic Neuroma?

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.

Acoustic Neuroma

What Is Its Growth Pattern?

Acoustic neuromas are slow growing tumors, and usually enlarge by 1mm to 3 mm 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.

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 tumor, and worsen as the balance function is destroyed on the affected side. The remainder of the balance function sometimes compensates for this loss, and thus balance problems may be forgotten after some time. Pressure by large tumor on the other cranial nerves cause facial numbness, weakness of the factual muscles or swallowing problems. Unsteady gait may be caused by pressure on the cerebellum. Very large tumors can also cause headaches.

Confirmation of Presence of Acoustic Neuroma

Auditory tests can reveal loss of speech discrimination and hearing loss.

Magnetic resonance imaging (MR) scans done after injecting a contract medium into the patient will show the presence of acoustic neuromas, even those that are still confined to the internal auditory canal.

Brain scan of a patient with a Large Acoustic Neuroma

Surgical Treatment

The only cure for patients with acoustic neuroma is complete removal using microsurgical techniques. The surgery is performed by the neurosurgeon depending on approval.

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

RISK & COMPLICATIONS

Possible complications for surgery are similar for all brain surgeries. The likeliheed of serious complications is fairly low in modern neurosurgical centres.

Other complications specific to acoustic neuroma surgery include:

• 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 swelling may occur and cause weakness or paralysis of the facial muscles. The results in incomplete eye closure and sagging of the face. The exposed cornea is dry and easily injured by dust particles. The protect it , the eyelids have to be taped together after instillation of eye drops. 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.

• Double vision may occur after removal of a large tumour pressing on one of the cranial nerves (usually the sixth) which control the muscles that move the eye.
• 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 the elimination of a large tumour pressing of the ninth and tenth 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 product 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 neuromas. 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 tumours following microsurgery and in cases where age or illness preclude open surgery.