Skull Base Disease
What is the skull base?
The skull is the bone that surrounds the brain and forms the skeleton of the face. The skull base is the bottom part of the skull that supports the brain. It is a very complicated area of the body with many openings through which important structures pass. This includes the spinal cord, all the nerves that control the head and neck area and the major vessels carrying blood to and from the head.
Diagram of the skull base showing all the openings through which major structures pass
What diseases can affect the skull base?
Diseases of the skull base arise from the tissues that are found within the skull base. These tissues include:
The bone of the skull
The lining of the brain
The nerves of the head and neck region (cranial nerves)
Lining of the nasal cavity
Skin around the skull base
The most common diseases to affect the skull base are benign tumours of the cranial nerves (called schwannomas) or benign tumours of the lining of the brain (called meningiomas). There are, however, many other types of disease that can affect the skull base including:
Lining of the brain:
Acinic cell carcinoma
Basal cell carcinomas
Squamous cell carcinomas
Vestibular schwannomas are also known as acoustic neuromas. They are benign tumours of the lining of the balance. The cells that line the nerve are called Schwann cells, hence the name. They arise from the balance nerve as it passes from the brain to the inner ear, usually within a narrow bony channel in the skull base called the internal auditory canal. They grow from this canal into the brain cavity and if large, can start to press on the brain. The space into which they grow is called the cerebellopontine angle, or CPA. They make up about 8% of all tumours occurring within the skull and are by far the most common type of tumour occurring in the CPA. The image below shows a typical vestibular schwannoma on an MRI scan.
An MRI scan showing a horizontal slice through the head with a medium sized left vestibular schwannoma (Red arrow)
They can occur at any age, but are most likely to occur in 50-60 year olds. They are fairly rare, occurring in about 2 per 100,000 people per year. That means that in the UK there are around 1200 new diagnoses each year.
In most cases there is no specific identifiable reason for developing these tumours, and they are not passed on to the children of the affected individual. There is, however, an inherited form of acoustic neuroma called neurofibromatosis type 2 (see section on neurofibromatosis type 2). In particular, there is no evidence that the use of mobile phones causes the development of these tumours.
The vast majority of patients with acoustic neuromas have hearing or balance symptoms, with 95% of people having hearing loss, 70% having tinnitus, and 50% of patients having imbalance. With larger tumours patients can also develop numbness in the face, and if they become very large and press on the brain stem, they can cause problems with walking and headache. If very advanced, they can potentially become life threatening. Fortunately, it is very rare for acoustic neuromas to grow large.
Most patients that are diagnosed with acoustic neuromas initially present to their doctor with worse hearing loss in the affected ear. This then triggers an MRI scan, which allows a diagnosis to be made.
There has been a great deal of research over the past 20 years looking at the way acoustic neuromas behave, and we now know that only 20%-30% of tumours grow after diagnosis. If they do grow, they generally grow slowly, around 2-3mm per year, although in rare cases they can grow up to 15mm a year. If they are going to grow, they tend to do so in the first few years following diagnosis. If there has been no growth during the first five years, then it becomes increasingly unlikely that they will ever grow.
Because we know that many tumours do not grow after diagnosis, the vast majority of patients with acoustic neuromas are now managed conservatively with serial imaging, and they never need to have any active treatment.
If the tumour is moderately large at presentation, or has shown definite growth on serial scanning, then there are two main treatment options. The first is radiotherapy, and the second is surgery. Occasionally, treatment may be offered to a patient with a stable tumour if there is disabling vertigo or if the patient wishes to have active treatment..
There are several different types of radiotherapy. In the UK, the vast majority of patients that have radiotherapy undergo a single treatment using either gamma knife treatment, or treatment using a machine called a linear accelerator.
A linear accelerator
If it is decided that surgery is needed, there are three main ways by which the tumour can be removed. It can be removed by removing bone around the ear in the so-called translabyrinthine approach. It can also be removed through a small hole in the bone behind the ear called the retrosigmoid (also called suboccipital) approach, and finally, it can be removed through a small window in the bone above the ear, called the middle fossa approach. The vast majority of patients in the Manchester Skull Base Unit undergo translabyrinthine surgery, although the retrosigmoid approach is sometimes used, particularly if the aim is to preserve hearing.
The approaches to removal of a vestibular schwannoma
Meningiomas are the most common of the intracranial benign brain tumours. They can arise anywhere within the skull and the symptoms that they cause usually relate to their site of origin and, therefore, to which neurological structure they compress. This rule holds true for skull base meningiomas. Whilst any meningioma can present with symptoms of raised intracranial pressure, notably headaches, in the skull base it is more common for them to present with features related to the structure they are compressing. Examples would include visual problems related to compression of the optic nerve, hearing loss due to compression of the vestibular cochlear (hearing) nerve and facial numbness due to compression of the trigeminal nerve. Compression of the brain stem and cerebellum by skull base meningiomas can produce unsteadiness and impaired coordination. These are just examples of how skull base meningiomas can present and is not intended to be exhaustive.
Treatment options are very much tailored to the site and size of the tumour and the risk/benefit ratio of the various treatments.
It is not uncommon for the symptoms to be so mild and, in the skull base specifically, the tumour to be sufficiently small that the safest option is to simply monitor it with regular scanning. Strangely, in this situation, it often happens that no treatment is required and the tumour does not progress. The reason why a tumour should appear but then not progress is poorly understood but there is no doubt that the Skull Base Clinic have considerable experience of monitoring skull base meningiomas over many years without the need for intervention.
Should intervention be required, this may be in the form of surgery or radiotherapy. Given that it is likely that the tumour has presented by compressing on a vital structure, the most common therapeutic option to be recommended is surgical removal of that tumour. This surgery is aimed, therefore, at both removal of the tumour and, in doing so, decompression of the relevant neurological structure with the aim to prevent further neurological deterioration and, in many cases to resolve the pre-operative symptoms. The precise nature of the surgery is very specific to the individual tumour. Radiotherapy is primarily designed to prevent further tumour growth. It has the advantage that it does not involve open neurosurgery and can be given in a variety of ways including a single dose - stereotactic radiosurgery – or in multiple doses over a 3 to 5 week period - fractionated radiotherapy.
Neurofibromatosis type 2
Neurofibromatosis Type 2 (NF2) is a genetic condition that results in the formation of benign tumours in the brain and spine. The hallmark tumour is a vestibular schwannoma, and characteristically, patients with NF2, will have vestibular schwannomas on both sides.
The genetic fault that leads to NF2 occurs on chromosome 22. This can be inherited from a parent, although up to 50% of new presentations of NF2 have a new mutation, in other words, they are the first person with this condition in their family.
NF2 can occur in a variety of severities. In its mildest form, it usually only affects a certain proportion of cells in the body, so-called mosaic NF2. The severity of the NF2 also has an impact on the risk of the condition being passed on to children.
Other than the hallmark tumour of NF2 (vestibular schwannoma), there are a variety of other benign tumours that can also occur. These include schwannomas of other nerves inside your brain such as the trigeminal nerve, the glossopharyngeal nerve, or the vagus nerve. Schwannomas can also occur on any of the spinal nerves as they emerge from the spinal cord, or on peripheral nerves in various parts of the body. Meningiomas also occur frequently in NF2 and may occur anywhere within the brain cavity or spinal canal. In addition, a small proportion of patients with NF2 can develop a type of tumour within the spinal cord called an ependymoma.
Other types of skull base tumour
Chordomas and chondrosarcomas