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Surgery of Acoustic Neuromas and Tumours of the Skull Base


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What is an acoustic neuroma?

An acoustic neuroma is a benign tumour arising from the Schwann cells or covering of the nerve to hearing and balance and represent about 6 - 10% of all brain tumours. These tumours are not malignant and do not spread to other parts of the body.

The acoustic neuroma arises from the balance portion of the 8th cranial nerve which is located deep inside the skull. The nerves exit from the brain into a small canal (internal auditory canal) connecting to the organ of hearing and balance (Fig. 1).

The first symptoms are a nerve hearing loss usually with tinnitus (head noises). The hearing loss is usually progressive but in approximately 1% of cases may be sudden and dramatic. The hearing symptoms are often associated with imbalance manifested mainly by a tendency to veer or stagger. Episodes of vertigo or spinning attacks are less common.

As the tumours enlarge, they cause compression of a vital area of the brain, the brainstem. Compression of these vital structures of the brain stem can be lethal if not relieved by removing the tumour.

In addition the hearing and balance nerves being involved, the facial nerve is slowly stretched over the tumour. The facial nerve is vital and functions the muscles of the face. Other nerves can also be compressed as the tumour grows, such as the nerve that serves sensation to the face, as well as the voice box and swallowing nerves.

Causes of Acoustic Neuroma Growth

The growth of acoustic neuroma is generally slow and tends to stretch surrounding nerves and structures including the facial nerve. Less frequently, the rate of growth is more rapid but the prediction of the growth pattern of acoustic neuroma is extremely difficult.

The cause of acoustic neuroma is unknown, but the genes controlling the growth of the nerve lining are the major area of research interest presently. Ear Science Institute Australia is involved in research programmes looking at the genes involved in various hearing disorders.

95% of all acoustic neuromas occur without a family history and are one-sided. The other 5% are bilateral (both sides) and are due to a hereditary condition called Neurofibromatosis type II. NF II is a rare disease and occurs in approximately one person per 100,000. All NF II patients require expert care by a team of surgeons, neurologists, radiologists, geneticists, rehabilitation audiologists and speech pathologists.

Diagnosis

An acoustic neuroma is best diagnosed with the use of Magnetic Resonance Imaging (MRI). Some tumours are diagnosed with a CT scan. The presence of a one-sided nerve hearing loss detected at routine hearing testing usually leads to the suspicion of acoustic neuroma but, in fact, most one-sided hearing losses are not due to acoustic neuroma. However, this important diagnosis must be excluded.

There are other tests that may be required on certain occasions including balance testing, central hearing tests and preoperative blood tests.

Treatment

The treatment of acoustic neuroma requires careful consideration by experts in the management of these types of tumours. A patient with a acoustic neuroma should question their clinician about their experience, results and willingness to publish or present these results to their peers. Factors that influence the treatment include tumour size, growth rate (sometimes estimated from serial MRI scans), the patient’s age and general health, as well as the hearing levels in the affected and unaffected ear. The three major treatment strategies are observation, surgery and radiotherapy which are discussed later. Sometimes combinations of these strategies are used. Surgery is usually recommended for patients when the tumour is compressing the brain stem. If the tumour is small and the patient still has useful hearing, then surgery to preserve this hearing may be recommended. Small tumours in older patients may be observed in the long term and growth carefully monitored by serial MRI scans. Other tumours may be treated by radiotherapy. In general, long term observation of tumours in young and middle aged patients is not recommended by our team as the general pattern over many years is that of growth. However, the decision must be individualised. The aims of surgery for acoustic neuroma are to remove the tumour with preservation of all neurological of brain function. As acoustic neuromas are in close proximity to vital brain and nerve structures, this presents a complicated problem.

At The Ear Science Institute Australia (ESIA), the surgeons work in a team comprising ear and skull base surgeons (neuro-otologists), neurosurgeons, audiologists, anaesthetists, specialised nurses and balance physiotherapists. Surgery is carried out at St John of God Hospital Subiaco and Sir Charles Gairdner Hospital on private and public hospital patients. The use of modern surgical, anaesthesia and intensive care techniques including special monitoring of surrounding nerves, especially the facial nerve, minimises damage to these vital structures. In the vast majority of all cases, this team achieves total tumour removal with facial nerve preservation.

The specialised acoustic neuroma unit involving ESIA surgeons has an international reputation attracting patients from South East Asia, South Africa and throughout Australia. In addition, surgeons visit from around the world to adopt new acoustic neuroma surgery techniques performed by our team.

Surgery

A specialised acoustic neuroma surgery unit must be able to utilise three major surgical approaches when removing these tumours. The most commonly used is the translabyrinthine approach which is most suitable for medium or large size tumours or those patients without useful hearing. An attempt to preserve residual hearing in patients with small tumours can be made with two alternative approaches through the middle fossa or the posterior fossa. Hearing may be preserved in up to 60% of patients but the level of hearing may not always be useful to the patient.

Treatment - The Patient Perspective

The natural behaviour and treatment strategies are discussed, including surgery, carefully and clearly with the patient and their family. Some patients will be encouraged to undergo a consultation with a radiotherapist to consider radiotherapy. Before surgery, blood tests, xrays, hearing tests and other special investigations may be undertaken. The case is reviewed by the co-surgeon when treatment is again discussed with the patient.

Most patients are admitted on the day of surgery or sometimes the evening before. The time taken in the operating theatre varies according to the size and complexity of the tumour. All patients are transferred to the Intensive Care Unit (ICU) immediately following surgery. Patients are usually drowsy but awake and close family is allowed into the ICU.

Nausea and sometimes vomiting is not uncommon because the tumour arises from the balance nerve but these symptoms are suppressed with appropriate medications.

Patients are usually transferred to the ward on the first day after surgery and helped out of bed on the second post-operative morning. Balance rehabilitation is supervised by the balance physiotherapist and the patient is usually walking independently by the third or fourth day following surgery.

Risks and Complications of Treatment

The risks and complications described here are the most important and particularly relate to acoustic neuroma surgery, but are not an exhaustive account of risks during surgery. In general, the smaller the tumour at the time of surgery, the less chance of complications. Evidence has shown that the most experienced units have the least complications.

Hearing Loss

Hearing loss is frequently present before surgery in patients with acoustic neuromas. Hearing tests evaluate the degree of hearing loss and this helps to determine the likelihood of saving hearing. If the hearing level is not useful to the patient, the translabynthine approach is used sacrificing hearing in the affected ear and utilise the translabynthine approach.

Some patients with good levels of hearing before surgery have small tumours confined to internal auditory canal. The middle fossa surgical operation or retrosigmoid approach allows the opportunity to save useful hearing in up to 60% of cases. A recent review of hearing preservation at ESIA showed very useful hearing preserved in nearly 60% of cases (similar to results in other major centres in the world).

Tinnitus

Tinnitus (ear noise) is frequently present before surgery, which may remain the same following surgery but rarely represents a significant problem. However, in up to 10% of cases, the tinnitus may be more noticeable.

Taste Disturbance and Mouth Dryness

Taste disturbance and mouth dryness are common for a few weeks following surgery. In about 5% of patients this disturbance is prolonged.

Dizziness and Balance Disturbance

Because the balance nerve may have been damaged by the tumour, surgery may result in improvement in unsteadiness. Vertigo and dizziness occurs frequently in the first few days post-operatively but most patients compensate quickly and return to their normal social and work activities. Some, especially older, patients do not achieve complete balance compensation following surgery and have imbalance. Balance rehabilitation treatment is extremely valuable and the balance physiotherapist is an important part of the team.

Facial Nerve Function

The facial nerve is in direct contact with the acoustic neuroma. This nerve controls the facial muscles and any disruption to it during surgery leads to loss of muscular tone and movement of the face on the side of the tumour (facial palsy). Special surgical techniques and electro monitoring systems are utilised by the ESIA team of surgeons to preserve the facial nerve. Surgeons in the ESIA team are leaders in this area and have published papers showing most, but not all, of their patients have normal or near normal function following removal of acoustic neuroma in small and medium sized tumours.

Major Risks

Stroke, brain injury and death are all rare following surgery in dedicated units (0.5 - 2%). Brain fluid leaks occur in around 10% of cases, usually requiring a temporary spinal drain and less commonly, further surgery. Meningitis occurs in about 2% of cases.

Surgery Techniques

There are three major approaches to acoustic tumour surgery: translabyrinthine, middle fossa and retrosigmoid. The tumour size, location and the level of hearing determine the type of approach used. The surgery is carried out under general anaesthesia.

1. Translabyrinthine approach

A large number of patients with acoustic neuroma have poor hearing at the time of diagnosis.

The translabyrinthine approach involves a C-shaped incision made behind the ear (some hair is shaved) followed by removal of the mastoid bone and the balance canal structures of the inner ear. No hearing is present after surgery. This approach involves minimal pressure on of the brain but excellent exposure allowing early detection of the facial nerve. Complete tumour removal is achieved in nearly all cases.

The patients are usually discharged from hospital between the fourth and seventh day following surgery.

2. Middle fossa approach

An increased number of patients are now being seen at ESIA with small tumours associated with good levels of hearing. The middle fossa approach is utilised when hearing is to be conserved. An incision is made above the ear (hair shaved above ear) and bone removed in this region. The brain is raised from above the ear and dura is elevated and the bone of the internal auditory canal removed. The tumour is removed with preservation of the facial and hearing nerves. Hearing preservation may be achieved in up to 60% of cases.

3. Retrosigmoid approach

This approach exposes the tumour from behind the ear bone and also provides the opportunity to preserve inner ear structures and hearing. An incision is made behind the ear and an opening in the skull created to allow exposure of the tumour. Hearing preservation may be possible if the nerve is not significantly affected. This approach is suited to tumours that do not extend far into the internal auditory canal and are not large.

Radiotherapy

Acoustic neuromas are benign tumours but radiotherapy has been increasingly proposed and considered as a treatment. Recent results indicate that radiotherapy is very effective in stopping tumour growth but long term results, after 10 years, are lacking. Radiotherapy does not result in disappearance of the tumour but aims to stop growth of acoustic neuromas. As with surgery, hearing loss, facial paralysis and other serious complications have all occurred following radiotherapy for acoustic neuroma. However, these are less common with radiotherapy. In a small number of patients, the acoustic neuroma continues to grow after radiotherapy but surgical treatment is made much more difficult following radiation treatment.

Stereotactic radiotherapy is the most common form of radiotherapy and refers to a method of delivering a radiation dose designed to minimise the effect of the irradiation on surrounding tissues. Stereotactic radiotherapy is also called the Gamma-knife, fractionated radiosurgery and proton beam therapy.

Initial publications detailing the results following radiotherapy indicated that complications could occur.

These include facial paralysis, hearing loss, damage to the brain stem, hydrocephalus (raised brain pressure), balance problems and numbness affecting the face. These complications have led to a decrease in the radiation dose delivered in most centres and correspondingly lower complications.

Patients should request detailed long-term results and complications following radiotherapy in the unit they are consulting.

The reduced radiation may lead to a lowering of the effectiveness of this form of treatment in benign tumours in the long term. Therefore, the long term effectiveness of radiotherapy for acoustic neuroma is still unknown and will not be a fully established treatment until these results are known in future years.

Radiotherapy is ineffective in larger tumours.

So, the take home messages (at the time of writing) are:

  • Around 55% of acoustic neuromas grow within 3 years.
  • 90-95% of small acoustic neuromas can be controlled by radiotherapy up to 10 years. Longer term results utilising modern dosages are unknown. 30% of larger tumours regrow but surgery followed by radiotherapy may be very effective.
  • Hearing is preserved in many cases initially but progressively worsens over time compared to hearing preservation surgery.
  • Facial paralysis is uncommon.
  • Surgery after unsuccessful radiotherapy (“salvage”) results in higher complications
  • Malignant (cancer) change following radiotherapy has occurred but appears to be rare.

Selected peer reviewed publications relating to skull base surgery

PROFESSOR M. ATLAS and other co-authors

  1. Tumours of the temporal bone. Scott Brown Textbook of Otolaryngology 7th Edition, 2003.
  2. Reanimation of the Lower Lid in Facial Paralysis, Aust. J. Otolaryngology., 4: 111-114, 2001.
  3. Evaluating the role of Magnetic Resonance Imaging Scans in the Surgical Management of Acoustic Neuromas. Laryngoscope, 110(7): 1194-1197, 2000.
  4. Multichannel Auditory Brainstem Implantation: The Australian Experience. Journal of Laryngology and Otology, 114, Sup 27, 46-49. 2000.
  5. Giant Medial Acoustic Tumours - The Question of Hearing Preservation. Aust. J. Otolaryngol., 3:2: 136-138; 1998.
  6. High Resolution MRI and Asymmetric Sensorineural Hearing Loss. Aust. J. Otolaryngol., 3:2: 182-185, 1998.
  7. The Dilemma of Imaging Destructive Lesions of the Petrous Apex. Laryngoscope 108:599-604, 1998.
  8. Aggressive Papillary Tumour of the Temporal Bone-New Immunohistochemical Evidence for Endolymphatic Sac Origin. Aust J. Otolaryngol., 3:1: 50-58, 1998.
  9. The Management of Dysphagia in Jugular Foramen Surgery. J. Laryngol. & Otol. 110: 144-147, 1996.
  10. Endoscopic Transsphenoidal Approach to the Pituitary Fossa. J. Clin. Neuroscience. 3:1:1-4, 1996.
  11. Results of Surgery of Tumours of the Cerebellopontine Angle. Aust. J. Otolaryngol. 2:4: 429-436, 1996.
  12. Insulated Fisch Dissector in Acoustic Neuroma Surgery. Otolaryngol. Head Neck Surg. 114: 494-495, 1996.
  13. The Evolution of the Management of Hydrocephalus Associated with Acoustic Neuroma. Laryngoscope 106: 204-206, 1996.
  14. Goldweight Implants in Facial Paralysis. Aust. J. Otolaryng., 2: 193-195, 1995.
  15. Facial Nerve rehabilitation. A Colour Atlas and Textbook of the Salivary Glands - Diseases, Disorders, Surgery. Hasby-Wolfe, 308-314; 1995.
  16. The Value of the Facial Nerve Monitor in Cerebello/pontine Angle Surgery, Aust. J. Otolaryng. 1: 415-417, 1994.
  17. The Value of Hearing Preservation in Acoustic Neuroma Surgery. Aust. J. Otolaryng., 1: 223-225, 1993.
  18. Calcified Internal Auditory Canal Tumours. Annals Otol. Rhinol. Laryngol., 101: 620-622, 1992.
  19. Hearing Preservation in Acoustic Neuroma Surgery: A Continuing Study, Laryngoscope, 102: 779-783, 1992.