Acoustic Neuroma Microsurgery: An Overview of the Three Main Surgical Approaches

Acoustic neuromas account for the great majority of cerebellopontine angle neoplasms. The gold standard treatment option remains surgical total tumor extirpation. The aim of this article is to describe and give an overview of the three main surgical approaches for acoustic neuroma removal. The translabyrinthine, retrosigmoid and middle cranial fossa approaches and their expansions and adaptations are the three most used craniotomies for acoustic neuroma removal and represent different pathways to the skull base and cerebellopontine angle. The advantages and disadvantages of each of them and their main surgical steps are described by the authors. The treatment options and the most suitable surgical approach to use in each case shall be a very careful discussion between the patient and his physician.


Introduction
Acoustic neuromas (AN), also called vestibular schwannomas, are benign neoplasm's that account for the vast majority of cerebellopontine angle (CPA) tumors, followed by meningiomas of the CPA and other benign lesions occurring in this site [1]. The treatment options are microsurgery (total or subtotal), radiotherapy, radiosurgery and observation (watch and scan), depending on the tumor size, site and growth rate, patient´s situation and agreement of patient and physician of the pros and cons of each sort of therapy. To the best of the authors´ knowledge, total surgical removal remains the gold standard treatment option and the top priorities are, in order of importance: patient´s life maintenance, preservation of facial nerve function and hearing preservation on the ipsilateral ear.
The three main surgical pathways for surgical extirpation of AS are: the translabyrinthine (translab), the retro sigmoid and the middle cranial fossa approaches. There other approaches, such as retro labyrinthine, transonic, transcochlear that are adaptations or extensions of the translabyrinthine craniotomy and can be indicated according to tumor site and size or hearing status of the injured ear. The goal of this review is to give an overview of the three main surgical approaches for a removal, describing their indications, pros, cons and principal surgical steps of each technique.

The Translabyrinthine Approach
The translaband enlarged translabcraniotomiesare lateral skull base approaches used to reach the internal auditory canal (IAC) and (CPA) regions. They offer a good exposure of the structures of those sites, such as the facial, cochlear and vestibular nerves; posterior fossa dura; lateral venous sinus (sigmoid sinus); middle fossa dura; brainstem; superior petrous sinus and jugular bulb. Using the enlarged translab approach, its modifications and the modern skull base surgical techniques, one can remove any size of (AN). The enlarged translab approach is preferred in cases of larger tumors extending to either jugular foramen or middle fossa whereas the indications of the classical translab approach should be limited to smaller tumors and vestibular neurectomy [2]. The main disadvantage of this approach is that the hearing of the injured earis sacrificed. Surgical steps of the translabapproach:

The Middle Cranial Fossa Approach
This approach is not used as frequently as the traditional translabyrinthine and retro sigmoid approaches for accessing ANs [3].

The Retro sigmoid Approach
The retro sigmoid and the enlarged retro sigmoid approaches are also performed to respect ANs. They both give access to the CPA region and posterior fossa. The enlarged retro sigmoid approach includes the skeletonization of the transverse-sigmoid sinus and an optional posterior mastoidectomy to expose the jugular bulb [4]. Retro sigmoid craniotomies are indicated most commonly for CPA tumors (with mostly cisternal component and serviceable or residual hearing), lateral cerebellar, petrous face or lateral lesions up to Meckel´s cave, auditory brainstem implantation, vestibular neurectomy and micro vascular decompression [5]. Using the retro sigmoid approach, as in the enlarged translab approach, the surgeon can reset any size of acoustic neuroma, despite the major difficulty to reach the more lateral part of the IAC (the fundus of the IAC) and remove tumor in this area (Figures 6 & 7). Main surgical steps of the retro sigmoid approach: Figure 6: Dural incision has been performed and the cerebellum is exposed.   q) Face electrodes and Mayfield removal, cleaning and dressing (Figures 8 & 9).
The middle cranial fossa approach is indicated to remove intracanalicular ANs or tumors that extent up to 1.5 cm into the CPA.
The enlarged middle fossa approach allows the resection of even larger neoplasm than the ones mentioned above. This approach is mainly used when there is an attempt to preserve useful hearing on the tumor side [7]. The main disadvantage of this approach is the morbidity that can be caused by the temporal lobe retraction to expose the IAC region and the tumor site. Hence, this approach is not usually indicated for individuals over 65 years or with regular or bad clinical situation. Main surgical steps: a) Patient positioning w) retrosigmoid, translabyrinthine, or middle fossa approaches and their adaptations or extensions, depending on the tumor size, preoperative hearing status, surgical team experience, and patient preference [8].