The Procedure of an acoustic neuroma surgery

An acoustic neuroma operation can be performed both by neurosurgeons and by ENT surgeons, or a team of both. The operating team may also consist of anaesthetists and doctors for intraoperative neuromonitoring.

Access to the tumour happens in different ways. Accordingly, the position of the patient is different. You can find out more about this at the end of this page. The operation always takes place under a general anaesthetic. A small area of hair shall be shaved if absolutely necessary where the skull will be opened for the operation. Before the brain is exposed for the surgeon, the thick, blood rich (outer) scalp, the cranial bone and the thick, hard (inner) meninges shall be opened. Nowadays, this is done with modern technology and with minimum dimensions. The times of large openings have passed. The miniaturisation of the technology and the operating experience make this possible. The cut through the scalp is around the length of a normal sized ear and a diamond milling cutter makes an incision through the cranial bone that is as small as a two Euro coin.

According to the operation approach, a part of the brain must be carefully pushed to the side in order to gain access either to the bony ear canal or to the tumour itself. The bony ear canal, in which generally the point of origin of the acoustic neuroma lies, is drilled.

The removal (resection) of the acoustic neuroma begins with an inspection of the tumour area and the course of vessels and nerves in the operation area. Predominantly this is what the operating microscope is used for. Due to the better view of structures found "further behind", endoscopes are additionally used more and more. This way even the smallest tumour remains can be detected and removed.

If the surgeon gets close to or touches the nerves, this will make a sound and be recorded by the intraoperative neuromonitoring and displayed on a monitor. For one thing this provides the hoped for information about the presence of nerves and their functions, as well as letting the surgeon know that he/she is dangerously close to the nerve. In this way, the surgeon can optimise the procedure and achieve maximum protection of the nerve functions.

An acoustic neuroma is generally surrounded by a tissue capsule. This is then opened in a place where no nerve vessels are found. The numerous small blood vessels are constantly washed away and suctioned in order to have a clear view.

Only when the acoustic neuroma is still small can it be removed in one piece. In most cases, and imperatively with large tumours, it is carefully removed and suctioned piece by piece. This is essential because the abrasion of a large, heavy tumour mass would stretch the thin nerve fibres and rip them. For histological examinations small specimens are by all means taken.

Removing an acoustic neuroma from the nerve sheath is the most difficult part of the operation. This is when the individual experiences of the surgeon come into play.

An acoustic neuroma can be located on one side of the nerve wall, it can carry individual nerve fibres into the tumour capsule, however the tumour may also encase an entire nerve. It often presses both strands of the balance nerve far apart from each other, and the facial nerve is often pressed completely flat and fanned out into numerous small strands. The balance nerve, in whose sheath the acoustic neuroma begins to grown, is generally severed in order to be able to remove the tumour at all. The deficiencies are minor and can be later compensated for by the patients, just as the vertigo is already partially compensated for long before the operation.

One particular challenge for the surgeon is if, due to its size, the acoustic neuroma has already made contact with the cerebullum and/or the brainstem in the cranial fossa. Removing a tumour from these vital parts of the brain is particularly complex and dangerous.

As acoustic neuromas fundamentally begin growing from the balance nerves outwards, and due to their priority, first the facial nerves and then the auditory nerve are preserved anatomically and in terms of their function, the balance nerve is generally severed in order to be able to remove the tumour completely. With damage to the facial nerves and to prevent a later facial palsy (facial paralysis), in special cases the tumour may not be completely removed. That needs to be agreed between the patient and the doctor prior to the operation.

The space that has become free in the bony ear canal is filled with autologous fat. Subsequently, it is monitored whether the visible ways, via the cerebral ventricle fluid (liquor) from the intracranium outwards can be penetrated, or if they are too dense; if necessary they are closed with fat, muscle fibres or an "adhesive".

The operation ends with the closing of the opening in the skull. Like the opening of the skull, this is often carried out by experienced senior physicians. It is absolutely sufficient, if the opening in the bones is protected by the re-sewn, strong inner meninges and the exterior scalp. The drilled hole in the skull is closed by several surgeons with bone cement. Some surgeons reset the piece of bone and a few even screw it together.

The length of the operation is dependent on the size and location of the tumour and the skills of the surgeon. An average sized acoustic neuroma can be removed by an experienced surgeon in 30 to 45 minutes. However, the length of the whole operation, including the skull opening and closing, can extend from five to eight hours.

Transtemporal, translabyrinthal (or transmastoid) and suboccipital (also retrosigmoidal) are the anxiety provoking names that characterise which way the surgeon will approach the tumour in the intracranium.

Transtemporal = through the temporal bone area.
Translabyrinthal = through the labyrinth of the inner ear.
Suboccipital = via the posterior cranial fossa.

The patient cannot choose between these. These three access ways are selected by the surgeons performing the operation depending on the size and position of the acoustic neuroma. Only if they decide for the translabyrinthal way is an agreement between the doctor and patient required, as this approach leads to a forced total loss of hearing on one side.

Transtemporal operation approach
This approach via the temporal region is preferred by ENT surgeons. The acoustic neuroma should not be too big and show be positioned exclusively or mainly in the bony ear canal. Both the facial nerve and the auditory nerve can be preserved in this case, therefore this approach is recommended for patients with hearing ability on the side affected by the tumour. The operation is performed with the patient lying down. The head is stretched far back and underneath.

At the front, above the ear an incision is made into the scalp, a piece of bone is drilled from the temporal bone, the meninges and the so-called temporal lobe of the brain is lifted with a special instrument to gain access to the petrous bone.

Then the upper bone of the inner ear canal in the petrous bone is drilled so that the upper balance archway and the inner ear canal are open. This presents a good overview of the facial and auditory nerves. At this point the facial nerve can be freely dissected, the meninges opened and the tumour cut and made smaller, one piece at a time. This is essential to be able to remove the shell completely from the nerve skin by the completion of the operation.

The balance nerve is severed during this procedure. Lastly, the remaining hole is filled with tissue from the patient's body, such as body fat or muscles from the temples and bone flap is reset. Then the wound is sewn.

Translabyrinthal operation approach
This approach is a typical path chosen by ENT surgeons and for acoustic neuroma situated further back in the ear canal.

It is a very safe way through the ear (labyrinth) and is chosen unanimously if the hearing ability has already been destroyed by the tumour. The patient is not weighed down, which for older patients can be significant. During the operation almost no cranial structures are compromised. Another advantage lies in the fact that the facial nerve can be exposed early and almost always, therefore almost no facial paralysis is caused by the operation. However, the hearing on the affected side is gone in every case and any remaining hearing ability is lost.

With this approach the an incision is made behind the ear, a hole drilled in the so-called mastoid and then the whole inner ear labyrinth is removed. The facial nerve can already be seen in the middle ear and can easily be tracked to the inner ear. Subsequently, the tumour can be removed step by step. In this case the remaining holes will also be filled with body tissue.

Suboccipital operation approach
This approach is typical for neurosurgeons and is preferred for medium and large-sized acoustic neuroma. For tumours that have grown far into the posterior cranial fossa and cerebullum and/or press hard on the brainstem, it is compulsory. Even here the preservation of hearing ability is possible, along with preventing facial paralysis.

Hereby, an incision is made in the skin at the back of the head, behind the ear. The bone cover is removed from the so-called squama occipitalis and the hard meninges is cut. With the patient lying on his/her side, the cerebullum moves far enough back that the ear canal is exposed. If the patient is perpendicular, the cerebullum must be carefully moved to the side, to expose and identify the acoustic neuroma in the posterior cranial fossa. The operation mainly occurs in a (half) sitting position, which allows for a particularly good view of the sensitive nerve structure and cleaning of the operation area.
Blood vessels, that supply parts of the brain, may lie nearby the tumour and can be preserved.

First of all, the tumour is by all means made smaller from within, as generally they are dealing with larger tumours. The tumour parts in the cranial fossa are the first to be removed. After that the inner ear canal is bored and the parts within are removed. Through teamwork of neuro and ENT surgeons, the ENT surgeons take the lead on the boring of the ear canal. The bored out bone piece is reset by many surgeons, although many close the opening "only" with bone adhesive. The hard, inner meninges and the thick external, muscular scalp are themselves sufficient protection for the intracranium.

Intraoperative electrophysiological monitoring comprises the monitoring of important functions of the nervous system during a neurosurgical procedure.

Intraoperative = during the operation
Electrophysiological = electric display of the processes in the nerves
Monitoring = monitoring on the screen

It has two goals: firstly, it can help localise important structures in the operating area during the initial phase of the operation. Secondly, it will alert the doctor monitoring of any irritation to the cranial nerves, which can happen if the surgeon is dangerously near to a nerve. The surgeon then has the chance to interrupt the work briefly, undo the possible changes that have occurred and at the same time change the operation strategy.

It works under the principle of "stimulation and impulse response". A desired stimulation, to establish the existence and functionability of a nerve and an undesired contact when removing the tumour activates an impulse, which is recorded, signally visually and acoustically.

Die Kurven zeigen die Funktion des Hörnerven vor (links) und nach (rechts) der Tumorentfernung.
Es konnte ein funktionell nützliches Hörvermögen erhalten werden.
(Quelle: Webseite des UK Greifswald)