Optimizing the Retrosigmoid approach for vestibular schwannoma (VS) will improve hearing and facial nerve outcomes. This is accomplished by adequate exposure of the contents of the internal auditory canal (IAC).
Surgical approaches for Vestibular Schwannoma
There are basically 3 surgical approaches that may be utilised for the removal of VS. The choice between the translabyrinthine, middle fossa and retrosigmoid (RS) approaches depend on several factors such as the age of the patient, this size of the tumour, the tumour location, the hearing status and the preference of the surgeon. It is advisable that an opinion on surgical treatment is obtained from a unit that is able and experienced to offer all 3 surgical approaches.
How safe is surgery for Vestibular Schwannoma?
Nowadays, surgery is very safe and mortality rates have fallen to far less than 1%. Similar facial nerve preservation is virtually 100% with hearing preservation, depending on the size and preoperative hearing excellent. The rates of hearing preservation and facial nerve injury (with lower dose stereotactic radiation) are like conventional VS surgery.
The indications for a Retrosigmoid Approach
The RS approach can be utilised for tumours of all sizes, especially if hearing preservation is the aim. It provides good exposure of the cranial nerves and other vital neurovascular structures of the brain and cerebellopontine (CP) angle. In some rare cases of Neurofibromatosis type 2 (NF2) it can be used to combine tumour removal with auditory brainstem implantation (ABI).
Opening of the internal auditory canal
The internal auditory canal (IAC) is opened from the posterior (back) aspect with a micro drill and small diamond burrs. The drill is operated at low speed with constant fluid irrigation to reduce friction and heat as these may injure the nerves and other soft tissue. Opening of the IAC is a vital step to expose and grant access to the intracannalicular part of the tumour as well as the other neurovascular structures inside the IAC. As demonstrated by the white dotted line on the computed tomography (CT) scan (Picture 1), removal of this bone and exposing the contents of the IAC will provide optimal visualisation of the facial nerve at the fundus (lateral aspect of the IAC). The bone that is removed (*) can be compared with the intact part on the other side which obscures the contents of the IAC.
Advantage of adequate exposure of the internal auditory canal
The advantage of adequate exposure of the internal auditory canal up to the fundus is the increased visualisation of the facial nerve. Visualising the facial nerve at this constant anatomical landmark markedly decreases the risk of injury during tumour removal. The facial nerve is monitored by a neurophysiologist with every case. Facial nerve monitoring during surgical removal of the tumour is vital to ensure facial nerve preservation and combining monitoring with adequate visualisation assures the best results. In those cases where hearing preservation is the aim care should be taken not to enter the vestibule of the inner ear at this point as this may lead to hearing loss. In Picture 2 the preserved facial nerve (f) and cochlear nerve fibers (c) can be seen after tumour removal. This result can only be safely achieved after adequate exposure of the contents of the IAC.
Restoration of hearing after Retrosigmoid removal of a Vestibular Schwannoma
If hearing is lost during surgery different options of rehabilitation exist. If the hearing is adequate in the non-operated ear a CROS hearing aid, a BONEBRIDGE implant or a PONTO bone anchored hearing device may be an option. If cochlear nerve fibers (c) can be preserved as in the photo during tumour removal, a cochlear implantation (CI) may be an excellent choice.
