10 Skull Base Approaches to the Lateral Brainstem and Cranial Nerves
Abstract
Various microsurgical approaches to the lateral brainstem are described, including their concepts, application, microanatomy, and surgical techniques. Beginning with the basic retrosigmoid approach, the “cerebellopontine angle rule of three” is defined to provide a better understanding of microvascular decompression surgeries. The technique of internal auditory canal unroofing is described for resection of intracanalicular tumors. The retrolabyrinthine transsigmoid approach and translabyrinthine approach, two variations of a mastoidectomy, are specified to show how to obtain an anterior corridor to the lateral brainstem. Two types of transcondylar approaches are defined that can be used with pathologies situated in the jugular foramen, with and without extension to the high cervical area. For lesions situated rostrally or with more extension into the supratentorial area, the middle fossa rhomboid anterior petrosectomy or combined petrosal approach can be applied. Finally, optimum approaches to brainstem cavernous malformations are described.
Introduction
Surgery of parabrainstem lesions and cranial nerve (CN) disorders is one of the most exciting and challenging subjects for neurosurgeons, particularly for skull base specialists. The senior author (T.F.) of this chapter has experience with more than 10,000 cases of parabrainstem lesions ( Table 10.1 ). In many of these cases, microvascular decompressions were performed through a retrosigmoid approach. However, for radical resection of parabrainstem tumors, 731 extended middle fossa approaches (including the anterior petrosectomy approach), 271 far lateral transcondylar approaches, and 254 combined petrosal approaches were used.
Lesion type | No. of patients |
Tumors | 3,763 |
| 2,332 |
| 1,431 |
| 588 |
| 130 |
| 77 |
| 361 |
| 131 |
| 56 |
| 31 |
| 29 |
| 15 |
| 13 |
Cavernomas | 108 |
| 102 |
| 6 |
Microvascular decompressions | 6,560 |
| 3,605 |
| 2,867 |
| 88 |
Total | 10,431 |
Abbreviation: CN, cranial nerve. | |
Since 1980, revolutionary changes have been made in skull base surgery. From conventional intradural microsurgery to extradural microsurgery, skull base techniques and skull base approaches have become a definitive subspecialty in the neurosurgical field. The field has seen developments of many new skull base operative approaches, improved dissection techniques, and development of special skull base microsurgical instruments, as well as production of innovative equipment, such as electric power drills and ultrasonic aspirators. Innovative extradural skull base approaches have been established, such as the combined transpetrosal approach, middle fossa rhomboid anterior petrosectomy approach, and extreme lateral infrajugular transcondylar exposure (ELITE) approach ( Fig. 10.1 ). These approaches were mostly developed in the 1980s and include the revolutionary development of the extradural anteromedial transcavernous approach by Dolenc 1 and also others by Japanese neurosurgeons such as Hakuba, Kawase, 2 and Fukushima. After the development of such skull base approaches and extradural dissection techniques in the 1980s, a boom of skull base surgery occurred in the 1990s all around the world. Along with the expansion in the number and types of approaches, tremendous advances were made in neuroradiology imaging, monitoring, and neurosurgical instruments. Magnetic resonance imaging (MRI) technology was invented in 1982, and high-resolution 1.5- and 3-tesla MRI equipment was developed in the late 1990s and in 2000. Ultrasound technology has also significantly advanced with high-resolution intraoperative imaging. Nowadays, we have three-dimensional fused computed tomography and MRI images, with functional MRI as well as fiber tracking images of the brain and brainstem, which can be used intraoperatively. Development of highly accurate three-dimensional computerized volumetric navigational systems enabled us to precisely indicate the important cranial base neurovascular structures, tumor size, and extensions preoperatively and intraoperatively. Extradural bone shaving, drilling, and removal facilitate closer and less invasive access to the lesion with minimum brain manipulation.
What makes surgery to the lateral brainstem so difficult? Is it the temporal bone covering and protecting the brainstem with vital structures, such as the internal carotid artery (ICA), venous sinuses, vestibular and auditory system, or other CNs tunneling through the area? Is it the narrow corridor between the cerebellum and the medial wall of the petrosal bone? Or is it the CNs that arise from the brainstem and run through this narrow cistern? All of these are obstacles that surgeons encounter in reaching the lateral brainstem. That is why a neurosurgeon inevitably needs to understand the precise microanatomy, pathologic process, and surgical approach thoroughly before sufficiently and safely performing lateral brainstem surgery. In this chapter, approaches to the lateral brainstem are divided into posterior fossa intradural lateral approaches, combined suprainfratentorial approaches, and the combined petrosal approach and are discussed in detail, whereas approaches to cavernous malformations in the lateral brainstem are briefly described.
Posterior Fossa Intradural Lateral Approaches
Four types of intradural approaches through the posterior fossa that give access to the lateral portion of brainstem are discussed below: (1) retrosigmoid (transtemporal retrolabyrinthine), (2) retrolabyrinthine transsigmoid, (3) translabyrinthine, and (4) far lateral transcondylar exposure, or ELITE.
Retrosigmoid Approach (Transtemporal Retrolabyrinthine)
The transtemporal retrolabyrinthine or retrosigmoid approach is one of the most commonly used skull base approaches to access the cerebellopontine angle (CPA) and the lateral brainstem, particularly for resection of vestibular schwannomas or meningiomas. This approach is essential for preserving the hearing of patients when treating smaller vestibular schwannomas. The advantages of the retrosigmoid approach are that (1) it is simple and familiar, (2) anatomical landmarks are easily identifiable, (3) a wide range of access is created to the lateral brainstem from the tentorial edge down to the foramen magnum, and (4) no temporal lobe retraction or manipulation is necessary. The disadvantages are that (1) occasionally swelling occurs because of cerebellar retraction and edema; (2) it is difficult to visualize the very lateral portion (2–3 mm) and fundus of the internal auditory canal (IAC), when necessary; and (3) access to the supratentorial extension is limited. The retraction of the cerebellum can be minimized with the use of lumbar drains and adequate bone removal. Both insufficient and excessive opening can cause unnecessary problems to the vital structures, such as postoperative swelling or contusion of the cerebellum and thrombosis within the sigmoid or transverse sinuses.
We propose that the approaches to the CPA be divided into three types, depending on the location of the target anatomy. This “CPA rule of three” provides a better understanding of the CPA microanatomy and dissection as described below. Retrosigmoid approaches to the CPA should be divided into three types: upper, middle, and lower ( Fig. 10.1 ). This concept helps surgeons to understand where the keyhole opening should be placed when planning to remove a CPA lesion. The upper CPA approach is used for lesions in the trigeminal area; the middle CPA approach is mostly for lesions at and near the internal acoustic meatus; and the lower CPA approach is for lesions around the jugular foramen, such as glossopharyngeal neuralgias, hemifacial spasms, and tumors. The size of the craniotomy should be customized according to the size and extent of the tumor, but the surgeon should always keep the minimally invasive concept in mind to avoid unnecessarily large openings.
Fukushima Lateral Position and Head Fixation
For any surgery, positioning the patient properly and safely is the first step in a successful surgery. The Fukushima lateral position is used for the retrosigmoid approach, along with many other approaches dealing with the lateral skull base ( Fig. 10.2 ). After general anesthesia, the patient is placed in the lateral position with the backboard elevated approximately 20°. After the patient is placed in the lateral position, the patient’s back is brought close to the edge of the table so that the surgeon does not have to extend his or her arms during surgery, which can cause fatigue. The patient’s shoulders are positioned at the cephalad end of the surgical table, with an axillary roll placed underneath the axilla to prevent any compression of the brachial plexus. The patient’s feet are laid toward the other side so the body lies obliquely across the table. This allows the patient’s back to roll slightly posteriorly in order to expose the abdomen. The patient’s lower leg is flexed 90° at the knee, while the other leg is kept only slightly flexed. Gel pads are placed underneath the trochanter, and pillows are placed between the legs to prevent decubitus ulcers. Both arms are outstretched on arm boards with care taken to pad the ulnar nerve at the medial epicondyle and the radial nerve at the radial groove of the humerus. The dependent arm is positioned 90° to the longitudinal axis of the body while the upper arm is positioned 45° to the body to ensure space is available if abdominal fat is needed. The shoulder of the patient’s nondependent arm must be rolled anteriorly and pulled gently in the caudal direction to create space for the surgeon to “look up” in a caudal to cranial direction if necessary.
The patient’s head is then placed in three-point fixation. The ultimate purpose is simply to make the mastoid the highest point and the surface of the mastoid parallel to the floor. This is achieved by first lifting up the patient’s head to secure space between the shoulder and the neck of the lower side to avoid obstruction of the venous pathways. Then the patient’s cranial vertex should be tilted slightly down to make the nose parallel with the floor, because the backboard has been elevated ( Fig. 10.2a-c ). This head position will provide the surgeon with access to the middle fossa, the CPA, the mastoid process, the petrous bone, and the far lateral skull base extending down to the foramen magnum and upper cervical spine.
Skin Incision
After minimal shaving of the retroauricular area hair, all monitoring electrodes are placed. The mastoid body and tip, the root of the zygoma, and the supramastoid crest should be identified before planning the skin incision ( Fig. 10.3 ). As Day et al 3 demonstrated, the line connecting the root of the zygoma to the inion approximates the course of the transverse sinus. A C-shaped postauricular incision measuring 5 cm or a lazy-S incision is made to obtain adequate exposure of the mastoid bone and the suboccipital region. Both incisions start from just above the supramastoid crest, passing 2 cm posterior to the body of the mastoid, terminating at the level of the mastoid tip. The C-shaped incision gives more medial to lateral axis than the lazy-S incision. The lazy-S incision is mainly used for microvascular transposition surgeries. The scalp is elevated with the galea aponeurotica above the fascia of the suboccipital muscles. A fascial graft is harvested for watertight closure at this point. Suboccipital muscle is dissected in the same fashion as the skin incision and reflected anteriorly. Before the craniotomy is performed, the bony landmarks of the suboccipital region are appreciated.
Craniotomy
A bur hole is made with a 5-mm extra-coarse diamond drill at the inferior corner of the digastric groove. A longitudinal groove is made at the posterior border of the mastoid body, safely exposing the sigmoid sinus. The groove is continued to identify the transverse sigmoid sinus junction, then shifts posteriorly to identify the caudal edge of the transverse sinus. Inferiorly, a groove can be drilled downward along the inferior edge of the proposed bone flap; it can go all the way down to open the foramen magnum. A craniotome can be used safely for the inferior and medial portion of the bone flap.
Dural Incision
The dura is cut using either a C-shaped incision or a T-shaped incision ( Fig. 10.4 ), according to where the main focus of manipulation will be. Two key points when cutting and reflecting the dura are to make the incision as close to the sinus as possible and to put a tack-up or stay suture closer to the sinus, not at the edge of the incised dura. These small but imperative techniques will make significant changes in the operative axis, reducing the overhang of the dura and resulting in less retraction of the cerebellum. Furthermore, the dura protects the brain better than any artificial material, so an unnecessarily wide opening of the dura should be avoided.
Cerebrospinal Fluid Aspiration
After the dural incision is made and before the surgeon begins to attack the pathology, cerebrospinal fluid (CSF) should be aspirated to relax the cerebellum and create a corridor to the lateral brainstem, which avoids the need for extensive retraction of the cerebellum. The optimal location for this initial CSF aspiration is by the cerebellomedullary cistern, and it is achieved by incising the arachnoid membrane caudal to the glossopharyngeal and vagus nerve complex (CN IX and CN X) behind the branch of the spinal accessory nerve (CN XII). Combined with other techniques to relax the posterior fossa, such as the use of hypertonic solution, diuretics, and hyperventilation, the release of CSF should relax the cerebellum enough that a tapered 2-mm spatula at the tip of a tubular retractor system will gently “hold” it, providing a wide and safe working space.
Variations of the Retrosigmoid Approach
Fukushima’s CPA rule of three is implied in the transtemporal retrolabyrinthine retrosigmoid approach, depending on the pathology to be treated. Especially for microvascular transposition surgeries, for which a wide opening is not always necessary, a small keyhole craniotomy should be planned according to this rule of three.
Approach to the Upper Cerebellopontine Angle: Trigeminal Area
Approaching the area around the trigeminal nerve (CN V) requires an opening only in the upper corner of the CPA inferoposterior to the transverse sigmoid sinus junction. An approximately 5-cm linear curve or a lazy-S skin incision ( Fig. 10.3a ) is made, starting from the supramastoid crest passing 2 cm posterior to the body of mastoid (within the hairline) and ending at the level superior to the mastoid tip. A 2.5 × 2.5-cm craniotomy is made at the transverse sigmoid sinus junction using a 4- or 5-mm diamond bur. It is essential to drill at least half of the bone covering the sinuses. Removal of the inner plate is crucial to do this efficiently. After the dura is opened, the surgeon should be able to identify the dura covering the medial petrous bone and the tentorium. One must be aware and cautious about the bridging veins that may interfere with the targeted pathology. First, in 5 to 10% of cases, the dorsal cerebellar pacchionian venous plexus may be encountered. This venous plexus exists under the transverse sigmoid junction and complements the draining function with petrosal veins and other draining systems in the posterior fossa. Slightly medial to this venous plexus is a dorsal cerebellar tentorial bridging vein in about 30% of patients. When these veins are damaged by retracting the cerebellum carelessly, intensive venous bleeding follows. Hence, when these veins are identified, they should be physically reinforced with small pieces of fibrin glue–soaked Surgicel (Ethicon) or Gelfoam (Pfizer) to prevent damage before the cerebellum is manipulated. The petrosal vein, which is encountered adjacent to the trigeminal nerve root, often comprises one to three branches that drain into the superior petrosal sinus (SPS). At further depth, the paratrigeminal or infratrigeminal clival vein is found in approximately 5% of patients. In the senior author’s (T.F.) experience, these veins should be preserved and protected at all times. When the veins are sacrificed, significant swelling of the cerebellum, especially the superior portion, may occur postoperatively.
Horizontal Fissure Approach
In contrast to the approach of the CPA, a stronger retraction of the cerebellum is required to reach and obtain direct visualization of the upper pons. Dissection of the horizontal fissure of the cerebellum between the superior semilunar lobule and flocculus reduces the need for this excessive cerebellar retraction. The horizontal fissure approach will expose the root entry zone of the trigeminal nerve and the middle cerebellar peduncle by holding back the superior semilunar lobule. The lateral surface of the pons is easily visualized around the root entry zone. This approach is very useful not only for vascular compression causing trigeminal neuralgia but also for treating brainstem cavernous malformations. 4
Approach to the Middle Cerebellopontine Angle: Internal Auditory Canal
The middle CPA is mainly used as an approach to the IAC, most often to treat small intracanalicular vestibular schwannomas. The craniotomy is made in the middle between the transverse sigmoid sinus junction to the inferior point of the sigmoid sinus. Anteriorly, the sigmoid sinus should be exposed to obtain a straight-down view to the IAC.
Unroofing the Internal Auditory Canal
Tumors originating from, situated only within, or extending into the IAC are most commonly vestibular schwannomas. For these tumors, the IAC needs to be opened for safe and complete resection. The IAC can be drilled either intradurally or extradurally. Intradural IAC drilling is performed after a retrosigmoid craniotomy, and extradural drilling is accomplished after using a translabyrinthine approach.
Intradural Shaving of the Internal Auditory Canal
IAC drilling should be performed before the arachnoid is dissected to prevent the spread of bone dust around the brain-stem. The dura is elevated over the inner wall of the IAC by making a U-shaped incision using a No. 11 or No. 15 blade scalpel. The incision should begin 2 to 3 mm on either side of the porus acusticus and should extend superolaterally for approximately 10 mm, avoiding the endolymphatic sac. The dural flap is elevated from the bone using a sharp dissector.
In recent years, an ultrasonic bone aspirator (SONOPET; Stryker), which provides extremely safe and effective IAC unroofing, is typically used ( Fig. 10.5a ). The ultrasonic aspirator is compatible with cottonoids and other materials in the surgical field. The tool will not snag cotton patties as a conventional high-speed drill will. If a high-speed drill is used, the cerebellum surface should be covered and protected with flattened bone wax instead of cottonoids.
The position of the endolymphatic sac and vestibule are the lateral limits of the exposure ( Fig. 10.5b ). Keep in mind the possibility of a high jugular bulb when drilling inferior to the IAC ( Fig. 10.5c ). During removal of the posterior wall of the IAC, a relatively uniform depth should be maintained throughout the exposure until the dura of the canal is exposed ( Fig. 10.5d ). Because of the angle of approach, the length of exposure of the IAC is less than the lateral extent of bone removed from the petrous bone surface. Exposing the fundus of the IAC would require blind drilling around the corner, likely resulting in violation of the labyrinth or the vestibule. Thus, the length of IAC drilling from the porus should be kept to a maximum of 7 mm. Keep in mind that the morphology of the temporal bone varies substantially among patients. After the IAC is sufficiently exposed, the dura over the tumor can be excised.
Approach to the Lower Cerebellopontine Angle: Hemifacial Spasms and Glossopharyngeal Neuralgias
The approach to the lower CPA focuses on the part of the CPA leading to the lower cranial nerve complex area. The cranial opening is made from the midpoint of the sigmoid sinus to the inferior sigmoid point ( Fig. 10.1 ). This opening includes removing the bone formulating the condylar fossa. Hemifacial spasms and glossopharyngeal neuralgias are best treated with this approach. Hemifacial spasms are caused by compression of the root exit zone of the facial nerve (CN VII) and not by compression of the nerve fibers in the cistern. Therefore, there is no need to even see the facial and vestibulocochlear (CN VIII) nerve complex (CN VII-VIII) because the compression point is just medial to the lower CNs. Transposition of the compressing vessel should be done between the choroid plexus and rostral side of the glossopharyngeal nerve. Glossopharyngeal neuralgia is caused by direct compression of the nerve, which can also be treated through this approach.
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