The retrosigmoid (RS) approach is the paradigmatic approach to the cerebellopontine angle for not only vestibular schwannomas but also a wide range of pathological processes. Indeed, it would be considered the standard and most common neurosurgical approach to this region. As shown more recently, auditory brainstem implants (ABIs) can be placed safely and effectively in adults via the RS approach with excellent results both for patients with neurofibromatosis type 2 (NF2) undergoing tumor resection at the same setting and for patients without tumors. For pediatric patients, the RS approach is the standard approach for ABI placement in all cases. The technique of ABI placement via the RS approach is discussed in this chapter, including use of the semi-sitting position (SSP) in select cases.
Key wordsauditory brainstem implant – retrosigmoid – semi-sitting position – vestibular schwannoma – neurofibromatosis type 2
6 The Retrosigmoid Approach in Auditory Brainstem Implantation
6.1 Historical Remarks
The retrosigmoid approach was one of the earliest approaches in acoustic neuroma surgery. The earliest description of a vestibular schwannoma was probably given by Sandifort in 1777 (Fig. 6.1).
It is widely known that Bell gave the first clinical description in 1830. However, the first comprehensive description of clinical symptoms and pathology was given by Jean Cruveilhier in 1835. 2
The first successful surgical resection was accomplished by Sir Charles Balance in London in 1894. 3 He performed a two-step surgery and used the unilateral suboccipital approach and an osteoclastic trepanation. Also, Fedor Krause from Berlin propagated the unilateral suboccipital approach for these lesions as early as 1904. 4
In the early 20th century, Cushing entered the field and performed a bilateral suboccipital approach for decompression and debulking of the tumor, which reduced the mortality and morbidity significantly. 5 Dandy, his pupil, went a step further and removed his first acoustic neuroma completely in 1917. He described his technique in 1925 using a unilateral suboccipital approach, debulking of the tumor, and painstaking resection of the capsule. 6
Of course, also other approaches were used. Panse, for example, proposed the translabyrinthine access in 1904. 7 This technique was later modified and improved by House and his group. 8 He and Kurze and Rand also introduced the operative microscope in the acoustic neurinoma surgery in the mid-1960s. 9
6.2 The Invention and Evolution of the ABI
It is very interesting and remarkable that only a bit more of a decade after the introduction of the operative microscope in vestibular schwannoma surgery, the first auditory brainstem implant (ABI) was successfully placed by House and Hitselberger in 1979. 10 , 11
However, there wasn’t much interest in ABI surgery at that time and therefore it took another decade until the interest rose again. This was mainly due to the technological development of the cochlear implant (CI) systems. At the beginning of the 1990s several CI providers (Cochlear, MED-EL, Advanced Bionics) produced ABIs and the first clinical experiences with multichannel ABI systems were obtained. 12 , 13 , 14
6.3 Surgical Approaches to the Foramen of Luschka
From that time on, there was always a discussion about the most appropriate approach and positioning of the patient.
In the pediatric patient population, there is in principle no discussion about the access to the foramen of Luschka, the implantation site. The retrosigmoid approach and the lying position are the gold standard. In adults however, especially in NF2 patients, in principle two different approaches are possible. Next to the retrosigmoid, the translabyrinthine approach is widely used in acoustic neurinoma removal and ABI implantation. Contrary to the translabyrinthine approach, the retrosigmoid approach can be used in lying and in semi-sitting position (SSP), which has some advantages. In addition, the retrosigmoid approach in ssp seems to facilitate tumor resection, preservation of anatomical structures, and is related to improved hearing results with ABI. 15 Another option to reach the cochlear nucleus is the midline approach and subtonsillar dissection, which can also be used for bilateral implantation. 16
6.4 Surgical Procedure for Retrosigmoid Approach
6.4.1 Positioning of the Patient
In the lying position (Fig. 6.2), the patient is placed on the contralateral side to the tumor and the head is turned opposite to the tumor side and inclinated. It is fixed with a pin head holder like Mayfield clamp. The patient must be fixed and secured on the table since during operation the table should be turned deliberately around its length axis to gain a direct access to the cerebellopontine angle (CPA).
On the contrary, in the ssp the patient’s head is turned to the tumor side (Fig. 6.3) and inclinated. In both cases the large cervical veins should not be compressed.
In the ssp, either a precordial doppler device or a trans-esophageal ultrasound probe should be applied for detection of air embolism. In addition, an end-tidal CO2 measurement is necessary.
It is advisable to check with magnetic resonance imaging (MRI) before the operation if there are additional schwannomas at the large peripheral nerves, for example, brachial plexus and lumbosacral plexus, large brachial nerves, and sciatic nerve, to prevent damage to these vulnerable structures by positioning. For example, a large tumor of the proximal sciatic nerve is dangerous in terms of compression lesion during ssp. It is also advisable to measure the somatosensory evoked potentials (SSEP) and motor evoked potentials (MEP) just before positioning of the patient and to repeat these examinations after positioning. Especially in NF2 patients, who may have tumors at cervical spine or have had operations and adhesions there, rotation and inclination of the head may cause impairment of blood circulation or direct compression of the spinal cord. This can be detected before skin incision by the abovementioned monitoring procedures.
6.4.2 Skin Incision and Trepanation
We usually prefer a question mark-like skin incision to create a double layer flap for the housing of the implant (Fig. 6.2 and Fig. 6.4). The base of the flap is always anterior to preserve a good blood supply from the temporal skin vessels. The occipital artery is always in danger of being sacrificed during the suboccipital and retrosigmoid dissection. So on the left side the incision is like a question mark, and on the right side, it is like a reversed question mark.
After dissection of the skin flap which is gently retracted, a periosteal flap is created with its base opposite to the skin flap base. By these means a double-layered covering of the implant housing is possible.
Thereafter, the muscle layers over the posterior fossa are divided, for example, with monopolar cautery, and the periosteum is strapped medially and laterally to expose the bone. One landmark is the retromastoid incisure and the other the asterion. This is the area where three bony sutures join: the lambda suture, the occipitomastoid suture, and the parieto-mastoid suture. Once these landmarks are identified, a burrhole is placed just below the asterion and a second behind the mastoid incisure as much low to the base of the posterior fossa as you can get (Fig. 6.5). An optional third burr hole may be placed more medially and perpendicular to a connecting line between the first two burrholes, forming a triangle. After dissecting the dura off the bone, it is cut out using a high-speed saw. The last cut should be the most lateral one, close to the expected location of the sigmoid sinus. In case the sinus is injured it can be accessed very quickly if there are no bone cuts left. In some cases, the trepanation must be enlarged osteoclasticly to the base of the posterior fossa; no bony rim should be left in order to come as close as possible to the sigmoid sinus. Cranially, the junction between the transverse and sigmoid sinus should be visible; then you know that the trepanation is close to the tentorium (Fig. 6.6). The bony rims of the trepanation should be thoroughly inspected if mastoid cells are opened. They have to be closed properly, mostly using bone wax or muscle with fibrin glue. Before opening the dura, the bony bed for the implant housing should be drilled. The main reason is that no bone dust should enter the intradural space. Therefore, a template is available for this purpose. The bed should be centered below the skin flap and above the transverse sinus. In adults it should be drilled deep enough to ensure a secure fixation with nonresorbable sutures. These can be inserted through a bony canal drilled with a small, 1 mm, diamond drill at the rims of the bony bed, or they can be fixed with micro titanium plates which are normally used for bone flap fixation. Especially in children who have thin bone, this is a good alternative.
As a second option for implantation of the housing, a subperiosteal, subcutaneous pocket, can be created and the device housing is slipped into this pocket without a fixation by a suture. Some colleagues drill a small well into the bone just to fit in a little bit of the electronic part of the housing, which is in general below the transmitter coil and the magnet. This method does not need a skin flap to cover the housing. Some implants are available with small pins which enhances the fixation by inserting into the bone. By this method the size of the wound and operating time are reduced. However, a disadvantage is that in case of a subcutaneous cerebrospinal fluid (CSF) leak, which is not seldom in children because they are not as compliant as adults, the housing starts floating in its pocket which may affect the lead by tension and torsion and cause dysfunction of the implant. Therefore, a secure fixation should be achieved.
6.4.3 Dura Opening and Tumor Approach
After cleaning the operative field by water irrigation, the dura can be opened. It is advised to palpate the tension of the dura beforehand and if it is quite strong, talk to the anesthetist to take some measures to lower intracranial pressure (ICP) and elevate the head of the patient by moving the table. Especially in the lying position and in children, the tension can be very strong which will cause herniation of the cerebellum during dural opening. It is recommended to open the dura by only 1 cm at the base of the trepanation and fix the dura with a suture to the muscles or to the bone. This will keep the opening patent during suction of CSF. Under the use of the microscope for better illumination a small spatula is inserted and the cerebellar hemisphere is gently retracted. Once the arachnoid membrane is reached in the depth, mostly behind the lower cranial nerves, it is opened either by suction or with a pair of microscissors. Immediately after the opening of the arachnoid and draining of CSF, the cerebellum will relax and start pulsations. Now the dura can easily be opened in a semicircular fashion, with its two ends at the inferior sigmoid sinus and the junction of the transverse and sigmoid sinuses. The base of the dura flap is lateral, along the sigmoid sinus. The dura is fixed under gentle retraction and tension with two microsutures to the muscles. It should be protected with a sponge and kept wet throughout the whole procedure. The wound which is outside the center of interest should also be covered with wet patties as usual. With the use of a self-holding retractor device, the cerebellum is gently mobilized laterally. After draining enough CSF, this is normally possible without pressure, especially in the ssp. Now the arachnoid membranes can be dissected and the tumor visualized and resected step by step. Typically, this is done under monitoring of the facial and auditory nerves, if they are still functioning. The main steps are tumor debulking and then dissection of the tumor borders off the cranial nerves and the brainstem. This is facilitated in ssp since there is less need for suction of blood, irrigation fluid, and CSF, which are flowing out of the wound due to gravity. The tumor borders can be grabbed with a forceps, mobilized a little laterally, and with the second hand and a dissector the arachnoid membrane can be stripped over the tumor keeping the correct dissection plane. With this technique, the tumor can be resected in piecemeal with utmost protection of the neural and vascular structures. After completion of tumor resection outside the internal auditory canal (IAC), its medial bony wall is drilled and the tumor inside the IAC is removed. In our institution this part of the surgery is done by the ENT colleagues. 18
In some situations, it may be necessary to open the IAC earlier, mainly if it is very difficult to localize the facial and/or auditory nerve, for example, in recurrences, after previous operation or radiation. Once visualized in the IAC, it may be easier to dissect the nerves also, medially to the IAC.
After drilling the IAC and tumor removal, it is highly important to inspect the bony walls of the IAC for opened cells. An endoscope with 30 degrees optic to look into the IAC is helpful. Opened cells have to be occluded properly to prevent transmastoid CSF leak.
When tumor removal is finished it is recommended to perform a bilateral jugular vein compression, especially in ssp, to identify small bleeding veins. In case of a dry field, the ABI implantation can be initiated, which however is not the topic of this chapter.