Origin of the acoustic neuroma

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.

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.

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.

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.