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: 1) the cochlear division – associated with transmitting hearing 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 (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.
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.
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. 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.
Confirmation of Presence of Acoustic Neuroma
Auditory tests can reveal loss of speech discrimination and hearing loss. Magnetic Resonance Imaging (MRI) scans done after injecting a contract medium (or coloured dye) into the patient will show the presence of acoustic neuromas, even those that are still confined to the internal auditory canal.
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 neuromas 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.
Very large tumours causing severe brain compression require surgical treatment. Smaller tumours can be treated by radiation.
This is carried out by the neurosurgeon, and sometimes in conjunction with the ear, nose and throat (ENT) surgeon using microsurgical technique.
There are basically three surgical approaches: Retrosigmoid, Tranlabyrinthine, and Middle Fossa. Combinations 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.
Radiation treatment is carried out using the Novalis Shaped-Beam machine located at the NNI-Khoo Teck Puat Radiosurgery Suite at Singapore General Hospital (SGH) Level B1, Block 2. This delivers narrow beams of strong radiation aimed precisely at the tumour from many different directions. Normal brain tissue therefore receives only a fraction of the total radiation dose received by the tumour. Exact knowledge of the tumour location is necessary, and this is achieved by securing the head firmly but painless in a custom made mask system and doing a CT scan of the head with the mask system in place. For treatment planning a MRI scan of the head is also required.
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 include:
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. Area inside the inner ear canal where acoustic neuroma usually grows from. 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.
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