Acoustic neuroma (vestibular schwannoma)

An acoustic neuroma is a non-cancerous tumour on the vestibulocochlear nerve, which helps control hearing and balance

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What is an acoustic neuroma/Vestibular Schwannoma?

An acoustic neuroma is a non-cancerous tumour on the vestibulocochlear nerve, which helps control hearing and balance.

The nervous system consists of the brain and the spinal cord, and all the connections these have with the various tissues of the body.

Most nerves in the body are connected first to the spinal cord and then to the brain.

However, there are 12 pairs of nerves which arise directly from the brain and pass through openings in the skull to reach their locations. These are called the cranial nerves.

The eighth cranial nerve, the 'vestibulocochlear' nerve, is concerned with hearing and balance

It is really in two parts: The cochlear nerve is associated with transmitting sound information from the hearing part of the inner ear, the cochlea, to the brain. The Vestibular nerve sends balance information to the brain from the inner ear.

An acoustic neuroma/vestibular schwannoma is a benign (non-cancerous) growth that arises from the nerve sheath which covers the vestibular nerve.

Nerves are like electric cables and have insulating cells around them, the Schwann cells. It is from the Schwann cell that the tumour arises.

Acoustic neuromas are a relatively rare type of tumour. They are diagnosed in between 1 in 50000 to 1 in a 100000 people per year. However post mortem studies have indicated that acoustic neuromas which never cause symptoms may occur with a frequency of 1-2%. This, as we shall see below, may create a dilemma as to what to do if an acoustic neuroma is discovered.

Acoustic neuromas are usually slow growing tumours. Subtle symptoms may go back many years. The vast majority of symptoms pertain to the nerves of hearing and balance when the tumours are small. However, larger tumours can start to press on surrounding structures such as other nerves in the same area. 

The Fifth cranial nerve, which supplies sensation to the face, may be affected leading to facial numbness. The Seventh cranial nerve which supplies movement to the muscles of the face is very close to the Vestibulocochlear nerve and may be affected although this is usually a late sign with very big tumours. The nerves of swallowing may be affected and large tumours may press on the brain causing unsteadiness and inco-ordination. They may also affect the circulation of brain fluid leading to a condition called hydrocephalus which causes headaches and visual loss.

What causes an acoustic neuroma?

There is a very rare inherited condition called Neurofibromatosis 2 which is caused by a defect on Chromosome 22. In this condition people develop acoustic neuromas in both ears as well as other tumours. For the much commoner sporadic acoustic neuroma there is no known trigger.

What are the symptoms of acoustic neuroma?

  • The main symptom of acoustic neuroma is a reduction in hearing in one ear. Most commonly this is initially subtle and then gradually worsens. However the hearing may drop suddenly. There may also be difficulty understanding speech in the one ear despite being able to hear noise
  • Ear noise or 'tinnitus' in one ear.
  • Dizziness and balance problems (since the balance portion of the eighth nerve is where the tumour arises). Again these may be subtle.

    The following symptoms are rare and occur with larger tumours:

    • Facial numbness and tingling may occur if the tumour is large enough to press on the fifth cranial nerve.
    • Progressive facial weakness usually only with very large tumours.
    • Swallowing difficulties.
    • Headaches, clumsy gait, mental confusion and visual loss may be experienced if the tumour is large enough to cause hydrocephalus where the pressure on the brain rises.

      The pattern of symptoms depends mainly on the exact place along the nerve that the tumour arises, which in turn determines which of the local structures (nerves and brain tissue) are pressed on first. If a tumour starts growing closer to the ear, this is in a narrow bony canal and will usually start to cause symptoms when the tumour is small. If a tumour starts growing on the nerve closer to the brain it may grow considerably larger before causing symptoms.

      How is acoustic neuroma diagnosed?

      • A routine hearing test, the Pure Tone Audiogram, may reveal a difference in hearing. If there is sufficient asymmetry then this will suggest the need to perform further investigations.
      • Magnetic resonance imaging (MRI) is the most commonly performed type of scan to reveal an acoustic neuroma.
      • Sometimes a Vestibular Schwannoma is picked up as an incidental finding on an MRI scan performed for another reason (most usually headache.) It is very rare for a Vestibuar Scnwannoma to be the cause of headache.
      • For patients who are unable to have an MRI scan, a CT scan with a dye injection will usually diagnose all but a small acoustic neuroma

        How is acoustic neuroma treated?

        There is no medication known to have a substantial effect on growth or to shrink acoustic neuroma tumours. There are three options:

        • Conservative management or 'watch, wait and rescan':
        • Stereotactic Radiosurgery (gamma knife or cyberknife)
        • Microsurgical removal

          Because an acoustic neuroma is a slow growing tumour in the majority of cases, there is usually no need to urgently intervene unless the tumour is very large. Indeed a number of these tumours do not seem to grow after discovery and this has led to the concept of simply re-scanning the patient at an interval after discovery.

          Usually the first follow-up scan would take place 6 months after the initial scan. If there is no growth then scans are performed at yearly intervals. The major risk with this strategy is that the hearing in the affected ear may worsen even with apparently no growth. However, all management strategies carry this risk. Because of the now widespread availability of MRI scanning increasing numbers of tumours are diagnosed when they are small

          If it is decided to treat the acoustic neuroma, then the two major options are gamma knife stereotactic radiosurgery, or surgical treatment(with usually the aim of total removal.) Treatment will be considered if growth has been seen, or if the tumour is quite large at presentation (over perhaps 2 cm)

          Gamma knife radiosurgery

          Gamma knife radiosurgery has become the preferred method of treatment in the last decade. It is a less invasive treatment which aims to 'control' the growth of the tumour, i.e. not to remove it but to stop it growing or shrink it. Control rates are in excess of 95%. 

          Side effects include hearing loss, balance disturbance, facial nerve symptoms. The rates of each of these side effects is lower than with surgery. It is performed as a day case surgery and recovery time is usually very short. There is a maximum size of tumour that can be treated due to the risks to adjacent structures. This is approximately 3 cm although each case is variable and requires careful multidisciplinary assessment.

          Microsurgical removal

          Microsurgical removal is the only option for tumours that are too large for stereotactic radiosurgery. These are therefore the tumours with usually the greatest risk that may present with complications such as increased intracranial pressure. As the facial nerve, which controls the muscles of the side of the face, usually runs very close to the tumour there can be a high risk of facial nerve damage from surgery. This can lead to partial or total paralysis of one side of the face. 

          Removal of the tumour might interfere with balance, leading to 'vertigo' – an unpleasant feeling of unsteadiness. Because the tumour is in an awkward place with other important brain structures immediately around it there are risks of damage for example to the nerves controlling eye movement and swallowing. The risk to a person's hearing varies slightly with the surgical approach and the technical problems in an individual person's case. For the majority of people undergoing surgery, the hearing may already have disappeared on the affected side, but if not, complete loss of hearing is the usual outcome of surgery.

          The risks of surgery diminish when the surgical team has a high level of experience in operating on acoustic neuroma, but they should be fully discussed as part of the consent process. Surgery will usually involve a 7-10 day stay in hospital and the convalescent period may last up to 3 months.

          Last updated 01.04.2016

          References

          Wright A, Bradford R: Management of acoustic neuroma. British Medical Journal 1995;311:1141-1144 http://bmj.com/cgi/content/full/311/7013/1141#F1



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