Acoustic Neuromas

An acoustic neuroma (or vestibular schwannoma) is a brain tumour which is benign and in most (but not all) cases is usually slow growing. The cells that form an acoustic neuroma are called ‘schwann’ cells, and make up the lining of the eighth cranial nerve as it passes through a tiny canal which connects the inner ear to the brain. Unknown events lead to an overproduction of schwann cells and as they multiply, they form a small tumour which fills the canal. As the tumour expands, it extends into the brain, assuming a pear shape and putting pressure on the nerves and brain.

Diagnosis

Acoustic neuroma is usually discovered after the patient complains of symptoms such as hearing loss, balance difficulty or tinnitus. In a few cases the acoustic neuroma has been detected during the investigation of another, possibly unrelated problem, such as may occur if the patient has been involved in an accident.

In rare instances acoustic neuroma may show none of the above symptoms and the patient may only notice some minor symptoms such as facial numbness.

The presence of an acoustic neuroma can be confirmed by the use of sophisticated scanning and imaging techniques.

Most large acoustic schwannomas (neuroma) can be detected by CT, although smaller tumours will not usually be seen. The CT scan has been replaced by and large with the MRI scan which improves the accuracy of diagnosis, treatment and follow-up of patients with acoustic neuroma. However, due to limited access to MRI, the CT scan is still widely used.

MRI is used for general imaging of acoustic neuromas, and for deafness problems in general. If a patient has sensory neural deafness, an MRI scan is done to exclude, or confirm, the presence of an acoustic neuroma.

MRI Scanner

The MRI scan can show abnormalities of the inner ear and other causes of deafness such as Meniere’s disease. Acoustic neuroma regrowth and fat can be differentiated on the scan, as surgically implanted subcutaneous fat will show up clearly. Cerebrospinal fluid, facial and vestibular nerves, the cochlear and fluid within the cochlear can be seen on the scan and a cross section of nerves can also be seen as they pass through the acoustic canal.

MRI produces detailed images, and has none of the radiation risks associated with traditional imaging. A scan can be ‘enhanced’ by a substance called Gadolinium. This substance is known as a ‘contrast medium’ which is injected into the patient and the tumour, or any other mass, along with blood vessels, are enhanced on the scan. Gadolinium is considered to be very safe, although some people with certain allergies can be affected. The substance will eventually pass through the body and be disposed of through urine a couple of hours following the injection.

The MRI scanner itself is a huge magnet that uses strong magnetic fields. The scanner encircles the patient’s head, which needs to be kept very still whilst the scan takes place, and takes different views of the brain. Any movement of the head can degrade the end result of the pictures.

Although there are usually no hazardous effects with the strength of the magnet that is used, patients who have metal parts in their bodies e.g. cerebral aneurysm clips, and other metal implants such as pacemakers, or metal foreign bodies in the eye, would either have to have the object removed or alternatively be referred for CT scan.

The scan itself is painless and simple although some patients can find the limited space and noise a problem.

Surgical Treatment

There are several surgical approaches that may be used to remove an acoustic neuroma. The route depends upon the surgeons preferences, the position and size of the tumour, as well as the presence of residual hearing. The Professor presented a drawing of the head, detailing various parts of the skull and showing the different incisional approaches that can be taken. These approaches are known as transmastoid retrolabyrinthine, suboccipital, anterior craniofacial and translabyrinthine.

Each of these approaches has advantages and disadvantages The preferred standard approach would normally be the ‘retrosigmoid’ approach. The Professor no longer takes the ‘translabyrinthine’ approach as this involves removing the inner ear structures and destroys hearing completely. The surgeon will always try to preserve the hearing and facial nerves.

It is easier to save the patient’s hearing with smaller tumours that lie in the ear canal. When a tumour is discovered in an older patient, a ‘wait and see’approach may be taken. The tumour would merely be observed and MRI scans performed periodically to study growth rate of the tumour. If the tumour starts to grow then treatment may become necessary. If the patient is younger, say in their 20’s, the surgeon would suggest removal of the tumour, especially if they had a family history of Nf2 (neurofibromatosis type 2), where patients have bilateral acoustic neuromas.

Before the tumour can be removed, a small section of the skull is cut away to expose the brain material. A slide was shown of a patient with a circular disc of bone cut away from the part of the skull where the operation takes place. The bone that is removed from the skull is usually replaced at the end of the operation. The skin, muscles and membrane that covers the brain are divided and a slide was shown of what is seen through the microscope when the skin and muscle is pulled back.

The surface of the tumour is tested with a facial nerve stimulator to make sure the facial nerve is not on top of the tumour. The centre or bulk of the tumour is removed by an ultrasonic aspirator which ‘sucks out’ the core of the tumour. The outside of the tumour is left and as it is de-bulked, it becomes soft and ‘floppy’. The surgeon will always try to preserve the facial nerve and facial nerve function is routinely monitored during surgery. This careful monitoring has reduced frequency and severity of facial nerve injury. If the tumour is very close to thefacial nerve, parts of the tumour may be left in order not to damage the facial nerve.

Risks

The ‘downsides’ of acoustic neuroma surgery includes the possibility of death, asceptic meningitis (bad headaches), CSF leak resulting in a further operation in order to seal the leak, infarction, and haemorrhage.

Radiotherapy

In some cases radiotherapy can be offered as an alternative to surgery to treat acoustic neuroma. There are different types of radiotherapy available.

• Stereotactic Radiotherapy. A radiotherapy machine moves around the patient in big arcs whilst the patient’s head is in a frame. The beam is directed onto the patients head and burns the tumour.

• Fractionated Stereotactic Radiotherapy. (FSR) Radiotherapy which is given over a longer period of time and in smaller doses.

Within the last few decades, microsurgical techniques have been refined so that the risks associated with total tumour removal have been greatly reduced. Microsurgical instruments and the operating microscope are now routinely used. Damage to surrounding nerve tissue has been markedly decreased and the mortality rate is extremely low.

Wait and See

As the tumour is slow growing a wait and see approach may be adopted.