8 Surgical Approaches to the Ventral Brainstem and Thalamus
Abstract
Surgical approaches to the ventral brainstem and thalamus have been designed on the basis of the risks and complexities of a region densely populated with nuclear tissue and fiber tracts that are unforgiving if transgressed. With so few cases available and with limited cumulative experience for any one surgeon, each case often requires surgical innovation in addition to extensive anatomical knowledge and personal confidence. Therefore, this chapter includes two principles that can be applied to guide the design of surgical approaches to lesions of the ventral brain-stem and thalamus. The first principle is to “let the lesion do the talking” because the lesion will determine the timing and goals of surgery and will guide the operative approach. The second principle is to “build the approach from the inside out.” Using the second principle, one is to start with the target, proceed to the safe entry zone, continue the trajectory, and conclude with the craniotomy. Five basic cases are described in which these two principles were applied. Standard surgical approaches are described for these common scenarios to serve as guidance for early-career surgeons operating on the brainstem and thalamus. The chapter also includes an overview of the basic knowledge and skills required to enter the practice of brainstem and thalamus surgery (e.g., anatomy, skull base approaches, transfacial approaches, endoscopic skills).
Designing the Surgical Approach
Only a small subset of neurosurgeons is willing to tackle the complexities and risks associated with surgery of the ventral brain-stem and thalamus. With so few cases available and with limited cumulative experience for any one surgeon, one must often be innovative to meet the demands of each case, while also applying anatomical knowledge and drawing on personal confidence. Surgical approaches for the ventral brainstem and thalamus are considered complex and often require experience in skull base surgery. Additionally, the target “real estate” is densely populated with nuclear tissue and fiber tracts that are unforgiving if transgressed. To address these issues, this chapter emphasizes two principles in the design of surgical approaches to lesions of the ventral brainstem and thalamus. The first principle is to “let the lesion do the talking” because the lesion will determine the timing of surgery, the goals of surgery, and the operative approach. The second principle is to “build the approach from the inside out,” meaning that one should start with the target, proceed to the safe entry zone, continue to the trajectory, and conclude with the craniotomy.
Let the Lesion Do the Talking
First and foremost, you must develop an understanding of what the lesion can tell you. Surgical planning begins by understanding the characteristics and allowances of the target lesion. For example, cavernous malformations reside in a hematoma cavity that, once entered, can be explored with little risk to surrounding tissues. Gliomas in the brainstem are diffusely infiltrative and thus allow only biopsy or resection of exophytic components. Arteriovenous malformations in the thalamus become operative when nonfatal hemorrhage creates a period of neurologic deficit, a hematoma cavity that helps to define the approach, and a compelling indication for intervention that makes surgery palatable both to the surgeon and to the patient. Once the lesion is understood and the goals of surgery (e.g., biopsy, partial resection, radical resection) are defined, the safe entry zone should be determined to minimize operative morbidity. These zones represent small areas that are devoid of critical fibers, nuclei, or perforating vessels. Anatomical assessments of the safe entry zones for the anterolateral brainstem have been reported. 1 , 2 , 3 However, no true safe entry zones exist within the thalamus itself. Instead, for the thalamus, some semblance of a safe entry zone can be maintained by avoiding transgression of the basal ganglia as well as the genu and posterior limb of the internal capsule. For the ventral brainstem, the “no-go zone” means avoiding transgression of the nuclei and the pyramidal tracts.
Once the safe entry zone is defined, a trajectory should be plotted to it that minimizes the amount of brain tissue traversed and maximizes the use of natural tissue planes for dissection (e.g., sulci, cisterns, ventricles). After the approach trajectory is defined, so is the entry point on the skull. Therefore, surgeons should be familiar with all variations of incisions and craniotomies, which include the orbitozygomatic osteotomies, endoscopic clivectomies, and anterior and posterior petrosectomies. Competency in these craniotomies and osteotomies often is best achieved with a multidisciplinary team rather than an individual.
Build the Approach from the Inside Out
The second principle of designing surgical approaches to lesions of the ventral brainstem and thalamus—“build the approach from the inside out”—conveys the idea that surgical planning should start with the target lesion, plot a path outward to reach the skull, and conclude with a craniotomy that takes into consideration the cosmesis of the incision. Because of the high risk associated with resection of the target lesion, the indications for surgery must be rigidly defined. For lesions deep below the brainstem surface, aggressive interventions should be avoided, and other treatment modalities should be considered instead.
The availability of advanced technology is critical. For example, certain cases are better suited to the use of stereotactic frameless guidance to locate the lesion, fiber tracking to avoid critical pathways, neurophysiologic monitoring to determine a safe entry zone, or intraoperative imaging (e.g., intraoperative computed tomography, intraoperative magnetic resonance imaging, intraoperative angiography) to assess the progress or completion of surgery. The surgeon must maintain competency in a variety of approaches, techniques, and instruments, and must have the courage to innovate.
In addition, a detailed understanding of the anatomy of the lesion, coupled with experience with neuromonitoring and image guidance, is paramount. Furthermore, knowledge of brainstem safe entry zones is critically important to minimize operative morbidity. Specifically, these zones represent small areas that are devoid of critical fibers, nuclei, or perforating vessels. Although there are no true safety zones within the thalamus itself, those for the anterolateral brainstem have recently been more intimately defined. 1 , 2 , 3 For example, Rangel-Castilla and Spetzler 4 reclassified the thalamus into six different anatomical regions on the basis of the optimal surgical approach. Nonetheless, surgical experience and knowledge of the optimal surgical approach to thalamic lesions remain limited.
Surgery by Design: Five Easy Targets
Unlike other areas of neurosurgery, brainstem and thalamus surgery does not follow a “cookie cutter” approach. In fact, a neurosurgeon must possess a diverse armamentarium and skill set. This section includes five safe approaches to five easy target lesions in the ventral brainstem and thalamus. These cases are neither exotic nor unique but instead are typical of their type of lesion, and the surgical approaches to these lesions are neither complex nor innovative, making them suitable for early-career surgeons of the brainstem and thalamus. Each case demonstrates the basic knowledge and skills required to meet the goals of surgery (e.g., knowledge of the anatomy of the cranium and brain, skull base approaches, transfacial approaches, and endoscopic skills). With mastery of these basics, one can begin to innovate. Truly innovative approaches can be found in case reports and short case series in the peer-reviewed surgical literature, and they can be found in operating rooms around the world where master surgeons are at work.
Target 1: Pontine Cavernous Malformation
Case 1
A 25-year-old woman with a pontine cavernous malformation was initially placed under observation after presenting with transient neurologic symptoms and deficits. Later, after further hemorrhage and when the hematoma cavity had reached the surface of the brainstem, surgery was performed ( Fig. 8.1a-c ).
Strategy
Using the concept of “let the lesion do the talking” to define the timing, goals, and approach for this patient, we opted to delay surgery until the lesion demonstrated its own natural history of repetitive hemorrhagic episodes. With repeat hemorrhage, a lesion will expand to the surface of the pons, which will then determine its own safe entry zone. At this point, we then use the strategy to “build the approach from the inside out” by designing the trajectory directly from the safe entry zone outward to the cranium (i.e., the area where the hematoma cavity presents itself at the surface of the pons with the least overlying normal tissue). In this patient, that area was within the peritrigeminal zone, a well-known and well-defined safe entry zone to the pons. To reach that zone safely, we used a subtemporal approach, which was expanded caudally using an anterior petrosectomy (Kawase approach) that necessitated a temporal craniotomy ( Fig. 8.1d, e ).
Surgery Overview
Before the patient is positioned, a lumbar drain is inserted and cerebrospinal fluid is drained to decrease retraction injury to the temporal lobe. The patient is placed supine with a wedge under the shoulder, and the patient’s head is rotated until the sagittal suture is parallel to the floor. The head is then tilted down about 15° so that the zygoma is the highest point in the surgical field. A straight vertical incision is begun in the preauricular crease at the level of the tragus and then is extended superiorly to the level of the superior temporal line. Underlying muscle and fascia are opened along the same line of incision. Two bur holes are placed (one over the root of the zygoma, one at the superior aspect of the exposure), and a 6 × 6-cm craniotomy is performed. Using rongeurs or a drill with a cutting bur, the surgeon extends the inferior edge of the craniotomy down to be flush with the floor of the middle cranial fossa. In the early years of performing this surgery, we would “down fracture” the zygomatic root and mobilize the temporalis muscle to be below the base of the skull. We no longer do this when approaching lesions in this posterior middle fossa location. That is, we came to realize that the zygomatic root and middle fossa floor are at the same level, unlike the anterior middle fossa, where the floor is much lower and the zygomatic osteotomy still offers some benefit.
Elevation of the temporal lobe dura mater in a posterior-to-anterior direction along the anterior face of the petrous bone and middle fossa floor exposes the arcuate eminence, which is absent in some patients, and the greater superficial petrosal nerve (GSPN) as it exits the facial hiatus. The posterior-to-anterior direction of dissection avoids the possibility of a dissector getting under the GSPN and avulsing it and causing a stretch injury to the geniculate ganglion and facial nerve (cranial nerve [CN] VII). The GSPN can be confirmed by stimulation; it will cause retrograde firing of the facial nerve at sufficient amplitude. The middle meningeal artery is identified, coagulated, and divided at the foramen spinosum. Elevation of the temporal lobe dura along with the dural sleeve of the mandibular nerve (V3) exposes the trigeminal depression. Sharply incising the V3 dural sleeve horizontally will allow mobilization of the dura propria upward with exposure of the inferior inner dural sleeve, which covers the gasserian ganglion, and will further relax the dura in that area. The dura on the anterior face of the temporal bone is dissected medially until the false edge of the petrous ridge is identified. Next, the superior petrosal sinus is elevated to expose the true edge of the petrous bone where retractors can then be hooked to expose the entire meatal plane for drilling. The location of the internal auditory canal is approximated by a line that bisects the angle between the arcuate eminence and GSPN. Some patients do not have a visible arcuate eminence; in these patients, the internal auditory canal can be estimated by dropping an imaginary plumb line down the external auditory canal across the middle fossa floor.
An anterior petrosectomy is then performed using specific anatomical structures as key landmarks for the extent of bony removal. A rhomboid-shaped volume of bone is formed by the petrous ridge medially, the GSPN laterally, the trigeminal nerve (CN V) anteriorly, and the arcuate eminence posteriorly. 5 Additionally, care must be taken while removing bone anteromedial and inferior to the geniculate ganglion to avoid injury to the cochlea. The hard bone of the cochlea can be “blue lined” in this location by the neuro-otologist. A neurosurgeon performing this maneuver tends to be more conservative, leaving a bit more bone in the area to be safe. The dura is opened along the inferior temporal lobe and reflected inferiorly. The dural flap is then split in the midsection down to the superior petrosal sinus. After placement of vascular clips across the superior petrosal sinus, the sinus is sectioned and the adjacent tentorium is incised down to the insertion of the trochlear nerve (CN IV). With the opening of the arachnoid membrane of the basilar cisterns, additional cerebrospinal fluid egress occurs and the anterolateral pons can be well visualized. Alternatively, an intradural anterior transpetrosal approach may be used and tailored to the extent of bony resection required for optimal exposure. 6
At this point, cavernous malformations that extend to the pial surface may be identified by the discoloration of tissue and can be approached directly. If such malformations are not well visualized or are below the pial surface, neuronavigation is used to assist in localization of the lesion and to plan a surgical corridor via the peritrigeminal or supratrigeminal zone, both of which are considered safe entry zones to the lateral pons.
Once the hematoma cavity is entered, microdissection under microscopic magnification is used to determine the plane between the brainstem parenchyma and the outermost layer of the organized hematoma. Dissection in this plane, along with piecemeal removal of the hematoma, will reveal the actual cavernoma as a distinct mass of thin-walled channels. In this area, a minimal amount of bipolar coagulation may be necessary to completely remove the actual malformation. At the conclusion of our case, the dura would not allow a watertight closure, so small fat grafts were placed to obliterate dead space and avoid cerebrospinal fluid leakage. A temporary pseudomeningocele, although not uncommon, is usually self-limiting.
Related posts:
10 Skull Base Approaches to the Lateral Brainstem and Cranial Nerves
12 Safe Entry Zones to the Brainstem
9 Approaches to the Dorsal Brainstem, Thalamus, and Pineal Region
Section IV Surgical Approaches to the Brainstem, Thalamus, and Pineal Region
4 Pathology of the Brainstem
13 Adult Brainstem Gliomas
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