What is an acoustic neuroma?

An acoustic neuroma (AN) is a brain tumour with (at least) three names: Acoustic neuroma – Vestibular schwannoma – Cerebellopontine angle tumour.

An acoustic neuroma is a primary tumour – it forms no secondary growths (metastases) – it does not degenerate into cancer – it is not contagious – it is not hereditary – it generally grows very slowly.

However, these traits are the only partially comforting ones, as an acoustic neuroma is nevertheless a tumour in your brain, where it doesn't belong, and it can (if it goes undetected and untreated) lead initially to aggravating and then to life threatening medical conditions, over the short or long term.

Where and how does an acoustic neuroma develop?

In many cases, it is believed that a brain tumour is a growth of the brain, in other words, the brain matter. At the same time, this tumour is then associated with cancer. That is the horror scenario that also prevails for diagnosis of acoustic neuroma tumours amongst those affected. Luckily this is not the case!

Acoustic neuroma - the vertical view

Acoustic neuroma
The vertical view
(Source: website MH Hannover)

Acoustic neuroma - the horizontal view

Acoustic neuroma
The horizontal view
(Source: website MH Hannover)

There are approximately 20 groups of brain tumours, which are partly subdivided into subgroups. The three most important ones are glioma, meningioma and neurinoma or schwannoma. All three are primary tumours, in other words they are not offshoots of tumours in other parts of the body. Glioma are growths in the supporting cells of the brain, which make up around 90% of brain matter. They are often «malignant» (a term medical professionals use when they form metastases in other parts of the body via blood and lymph vessels). Meningioma are growths in the meninges, the shell around the brain matter. They are mostly «benign». Neurinoma (also called schwannomas) develop from and in the sheath of the cranial nerves and form no metastases, and as such are also categorised medically as benign.

Cranial nerves comprise of very, very many thin fibres, which are coated by a sheath of connective tissue. This sheath comprises of the so-called Schwann cells and is therefore called the Schwann sheath. The comparison with a copper strand made from many thin strands and an insulating sleeve is certainly not the worst.

Acoustic neuroma cells
Acoustic neuroma cells

An acoustic neuroma consists of uncontrolled growing Schwann cells in the nerve sheath of the 8th cranial nerve. This vestibulocochlear nerve is a twin nerve, which has two parts – the auditory nerve (nervus cochlearis) and the balance nerve (nervus vestibularis). Both nerve parts from the 8th cranial nerves come, lying close together, from the brainstem and pass through the cerebellopontine angle and the bony auditory canal to the inner ear. An acoustic neuroma almost always grows in the vestibular part of the 8th cranial nerve, which gives rise to its frequently used name Vestibular Schwannoma. The tumour can almost grow sideways in the vestibular nerve, and may also slide between both branches of this nerve and split it. The way it spreads influences, amongst other things, the decisive effort and prospects of success of the treatments (see here).

The auditory nerve transmits the auditory information in the form of bioelectric signals from the inner ear to the centre of the brain. Through conversions, the auditory sense is created. The balance nerve transmits the information from the balance organ of the inner ear to the brainstem, from where it reaches the cerebellum and where, through connections with further information, e.g. the eyes and muscle sensors, the sense of position of the body originates and coordinated movements are controlled.
The 7th cranial nerve (nervus facialis) also passes through the ear canal, the facial nerve, which controls the facial muscles and the sense of taste to two thirds of the tongue, as well as the secretion of tear fluid. Given that the acoustic neuroma begins to grow, in most cases, in the bony ear canal, these three nerves are the first to have their functions impaired.

Generally, the tumour grows slowly, to very slowly, and can stay in the bony ear canal for decades. If it is (still) completely inside its position it is called intrameatal. If there is not enough space in the ear canal after pushing and suppressing the nerves and vessels passing through, the acoustic neuroma grows out of the ear canal and into the cerebellopontine angle, in one of the divided spaces left and right of the extended spinal cord, into the brainstem. This fact gives rise to the term «cerebellopontine angle tumour». The tumour, in this case, is in an extrameatal position. The position of acoustic neuromas determines the operative path.

In cerebellopontine angle tumours the acoustic neuroma first of all has space again to increase its volume, to expand to other nerves in the posterior fossa, to touch the base of the skull and to press and bruise it, and therefore to show other typical symptoms. It is particularly critical if the brainstem is laterally suppressed. As the growing tumour affects other cranial nerves and impairs their functions, this is reflected gradually by typical symptoms. A solid sheath is typical for acoustic neuromas (the tumour «does not fit in the surrounding tissue»). This sheath offers small tumours the chance to be removed in a single piece.
A very rare form of acoustic neuroma is a cyst in one part formed with fluid in the cavity.

There is no standard as to how an acoustic neuroma triggers complaints in patients. One person already notices an AN when it is 2 x 2 x 2 mm in size, another lives for a long time without complaints with an average 2 cm diameter AN. One reason for this is its slow growth, which gives the body the chance for the nerves and vessels to partially adjust to this altered situation for a certain amount of time.

Which cranial nerves run to the skull base?

Both images show the exit openings of the cranial nerves at the skull base.
Left hand image: schematic view. Source: Medizin, Mensch, Gesundheit, 2005.
Right hand image: View through an operation microscope.
Source: website University Hospital Würzburg

The functions of the cranial nerves:

  1. Smell
  2. Sight
  3. Eye movement
  4. Eye rolling
  5. Innervations of facial skin, eye socket, teeth, muscles from upper and lower jaw
  6. Eye movement
  7. Facial movement (mimic), tear fluid secretion, mandibular glands under the tongue, small palate and nasal glands, taste in the front two thirds of the tongue
  8. Hearing and balance
  9. Throat muscles (swallowing), Mucosa from throat and ear tympanic cavity, taste in the rear third of the tongue
  10. Voice box movement (as well as the vocal chords), throat muscles, sensitivity in the outer ear canal, supply from heart, lungs, stomach and intestine!
  11. Head turning, supply from the trapezium muscle in the back
  12. Tongue movements
Acoustic neuroma – cranial nerves

How frequently does an acoustic neuroma occur?

Eight to ten percent of all brain tumours and 80 to 90% of tumours at the skull base are acoustic neuromas.
Figures from different countries show that around 8 to 10 people out of 1 million will be diagnosed with an acoustic neuroma. It can be assumed that many will remain undetected, because they remain small and don't cause any noticeable complaints.

Why do acoustic neuromas occur?

There are many thoughts, theories and speculations about what causes acoustic neuromas. In order not to feed any speculations here, we have foregone all explanations.
The fact is that to date there is no scientific proof as to why cells suddenly grow uncontrollably, specifically in that place, and why they grow either quickly or slowly. Today it is taken as certain, however, that an acoustic neuroma does not form any metastases and that it is not hereditary!

Here are few remarks on «Recklinghausen disease» which are pertinent:
This disease, also called Neurofibromatosis Type 2 (NF2), is genetic, however it is not necessary passed from every generation to the next. With this disease, tumours form on the nerve tissue at different places in the body, particularly the spinal cord and in the skull. These tumours can remain relatively harmless, but can also lead to severe damage. If an NF2 patient has a tumour in the auditory or balance nerve, then mostly it happens on both sides. Whether someone has this disease or not can be determined through a genetic test. Chromosome 22 is looked for through a blood test. The so-called Merlin gene is responsible for NF2.

Is there an acoustic neuroma prophylaxis?

As the causes of occurrence are unknown, there is also no prophylaxis. As with all diseases, it also applies to acoustic neuroma brain tumours that a healthy lifestyle is beneficial for strengthening the body's defences, the immune system, and reducing the general risk of illness. However, the onset of the disease cannot be prevented this way. Along with a healthy lifestyle, nutrition, exercise and good mental health all contribute. A strong immune system is also important for the rehabilitation phase after any illness.

The facts that the causes of the occurrence of acoustic neuroma are (still) unknown, and that consequently there is (still) no prophylaxis, needs to be accepted by those affected and their relatives. This way time and energy can be spared that would be wasted in the idle search for a miracle cure. Those with acoustic neuromas can be sure that the members of IGAN's Medical Advisory Board would be some of the first to find out if there was an answer to what causes an acoustic neuroma (if they are indeed not involved in answering this question themselves). In this case, IGAN would immediately publish this welcome news.
