32 Endoscope-Assisted and Fully Endoscopic Microvascular Decompression
32.1 Introduction
The introduction of endoscopes into cranial surgery has revolutionized ventral skull base surgery but not lateral skull base surgery. In the 1980s and 1990s, the major surgical advance was to remove bone to minimize retraction of the brain, thus creating the field we call skull base surgery. Microscopic visualization, however, requires very large openings to illuminate structures in the depths of the field. The introduction of the endoscope into the ventral skull base for pituitary surgery1 and then subsequently for tuberculum sella, suprasellar, planum sphenoidale, and clival lesions2 resulted in a revolution that has sparked a plethora of teaching courses and of minimally invasive techniques. In approaches to the lateral skull base, however, the endoscope has not yet brought about the same revolution.
One of the barriers in the use of endoscopes in the cerebellopontine angle is the delicacy of the neural structures involved, including multiple cranial nerves and the lateral cerebellar surface. In contrast to the pituitary and ventral skull base approaches where surrounding structures consist primarily of cartilage, mucosa, and bone, the cerebellopontine angle approach has surrounding structures, which consist of neural tissue except on the side of the petrous bone and the tentorium. In addition, the sinonasal cavity is a natural air-filled space to which the entire field of rhinology has dedicated its surgical attention for decades. In contrast, the lateral skull base is a bony region without a natural corridor. It is this delicacy and lack of natural air spaces that mandates a different surgical paradigm. We believe that endoscope-assisted microsurgery is a valuable adjunct in lateral skull base surgery, but the one-handed surgeon is fundamentally restricted in dexterity, and thus it is only by placing the endoscope on a holding arm that true bimanual, panoramic cerebellopontine angle surgery can be performed. In this chapter, we discuss the role of endoscopy in microvascular decompression (MVD) and compare endoscope-assisted MVD (EA-MVD) to fully endoscopic MVD (E-MVD).
32.2 Anatomical Considerations
The introduction of the operating microscope into neurosurgery in the 1960s revolutionized intracranial surgery by providing magnification and illumination as well as stereoscopic visualization of intricate anatomy. The major limitation of the microscope, however, is that there must be a straight path of light from the microscope to its target. Hence, visualization of internal structures requires that there be a direct line from the microscope through the bony opening to the target. In contrast, the endoscope has a typical diameter of < 5 mm field of view from a standard 0º endoscope of 90º from the tip of the endoscope. Since the endoscope can be introduced through smaller openings and it can then visualize structures 45º (half of the full 90º) from the axis of entry, the endoscope allows the surgeon literally to see “around corners.” Several authors have compared the microscopic approach to endoscopic approach in an attempt to define the advantages of the endoscopic approach in the cerebellopontine angle (CPA). Takemura et al concluded in their anatomical study that the 0º and 45º endoscope provided superior views of the CPA when compared with a microscope. Specifically, with respect to MVD, authors recognized that the root entry zone of the trigeminal nerve and the distal entrance into Meckel’s cave are better visualized with endoscopy than with microscopy. In addition, for hemifacial spasm, the origin of the seventh cranial nerve at the brainstem and the branches of the anterior inferior cerebellar artery were better visualized with the endoscope (Video 32.1). These same authors, however, highlighted the lack of stereoscopy and the potential danger of navigating the angled endoscope as disadvantages of the endoscope.3 Tang et al compared the visualization and maneuverability of the endoscope versus the microscope in a cadaveric study of the CPA for MVD. They looked at overall qualitative grading between the two approaches and whether sacrifice of the superior petrosal vein (SPV) impacted their grading. They found the maneuverability score to be superior for the endoscope and noted that sacrificing the SPV did not improve maneuverability and therefore was often not necessary with the endoscope.4 Ebner et al in a cadaveric study showed that the endoscope-assisted retrosigmoid intradural suprameatal approach allows exposure to Meckel’s cave and the suprasellar and parasellar areas of the middle fossa even without a tumor distorting the normal anatomy. They hypothesized this approach could be valuable in the resection of epidermoid tumors.5 Hence, anatomical studies demonstrate the superiority of endoscopic visualization within the CPA, and, based on this, multiple surgeons have pioneered endoscopic surgical techniques for microvascular decompression.
32.3 Indications/Contraindications
The indications for endoscope-assisted and/or fully endoscopic MVD are the same as the indications for conventional MVD, and thus we will only briefly review them here. In the United States, most patients are treated with MVD for trigeminal neuralgia (TN), whereas in Asia, most patients are treated for hemifacial spasm. In addition to these two major indications, the same surgical approach can be performed for glossopharyngeal neuralgia, geniculate neuralgia, and vestibular nerve section for Meniere’s disease. We will focus primarily on trigeminal neuralgia, but we will also provide specific examples where endoscopes are useful for seventh and ninth cranial nerve decompression.
Diagnosis of TN is purely clinical and relies on the presence of sharp, shooting, lightning-bolt pain that lasts less than 2 minutes in the distribution of the V1, V2, and V3 branches of the trigeminal nerve in the absence of gross trigeminal sensory neuropathy.6 Preoperative imaging studies are used to rule out concomitant structural pathology, but the low negative predictive value of the study limits its usefulness as a tool for decision-making.7 The most common etiology of classic TN is vascular compression of the dorsal root entry zone, where the trigeminal nerve exits the brainstem. The pathogenesis is thought to involve pulsations from the offending vessel leading to demyelization in the dorsal root entry portion of the trigeminal nerve, resulting in abnormal afferent conduction. While arterial compression is most commonly identified at the time of surgery, venous compression is also observed and hypothesized to play a causative role. However, in this era of improved imaging and improved surgical visualization, there are patients with trigeminal neuralgia who do not have neurovascular compression,8 and so the exact pathogenesis in all cases remains unknown. In cases where no obvious vascular compression is identified, or where the compression is primarily venous without significant nerve distortion, the senior author (J.Y.K.L.) performs an internal neurolysis with a round knife without nerve section.9
Hemifacial spasm (HS) is a hyperactivity disorder manifested by unilateral facial spasms that usually start with the orbicularis oculi and progress to the lower face. Botulinum toxin injections provide temporary relief but they are not a cure. The preoperative and intraoperative electromyelography (EMG) findings of lateral spread response can be useful to predict postoperative improvement.10 The compression is usually identified between the eighth and ninth cranial nerves at the root entry zone of seventh cranial nerve. Angled endoscopes have proved to be especially useful for this particular disorder (Video 32.1).
Glossopharyngeal neuralgia (GPN) is a rare disorder characterized by transient, severe, sharp, stabbing pain in the distribution of the auricular and pharyngeal branches of the vagus nerve in addition to the glossopharyngeal nerve. Medical treatment of GPN with anticonvulsants is less effective than with TN, which often leads to surgical treatment. Geniculate neuralgia is a rare disorder of brief paroxysmal pain deep in the auditory canal.6
Microvascular decompression for these cranial nerve hyperactivity disorders is performed via retrosigmoid craniectomy and is a highly effective surgical treatment of neurovascular disorders associated with facial pain and spasm.11 This procedure separates the offending vessel(s) from the nerve and provides the patient relief by targeting what is believed to be the underlying cause of TN. However, poor visualization of the dorsal root entry zone or medial vascular compression can make it difficult to visualize the offending vessel(s). Classically, MVDs have been performed with the operative microscope, but more recently endoscope-assisted MVD12,13,14,15,16,17,18,19,20,21 and fully endoscopic MVD22,23,24,25,26,27,28,29,30 has been used for improved visualization and identification of the offending vessel(s). The endoscope is now a standard tool in neurosurgical procedures of the anterior skull base and ventricular system.2,31 It has also proved useful to visualize structures in the CPA through smaller exposures while minimizing cerebellar retraction.
32.4 Operative Techniques
The patient’s positioning and opening are similar to previously described techniques,32 and only key aspects of the procedure will be reviewed. All operations are performed under general anesthesia. Prior to positioning, the patients head is secured in a Mayfield clamp. The patient is positioned fully lateral, and neuronavigation is not necessary. In addition, a lumbar drain or lumbar puncture is not used. Anatomical landmarks are identified both before skin incision and after muscle dissection and include the following: the tip of the mastoid process, the insertion of the posterior belly of the digastric muscle, and the approximate course of the sigmoid and transverse sinus. The digastric notch is a useful bony anatomical landmark, whereas the mastoid emissary vein is an inconsistent landmark. A single bur hole is placed, and this is expanded toward the junction of the sigmoid and transverse sinuses (Video 32.1). For an endoscope-assisted procedure, a 1.5-cm diameter incision is created. For a fully endoscopic procedure, a 1-cm diameter C- or U-shaped dural incision is created (Fig. 32.1).
The first step in the procedure is to dissect the arachnoid and release cerebrospinal fluid (CSF). Careful steady gentle retraction of the cerebellum with a cottonoid patty allows for gradual CSF egress especially once the medial cerebellopontine angle cistern arachnoid, or cerebellomedullary cistern, arachnoid is opened. The cranial nerves of interest (CN V, trigeminal neuralgia; CN VII, hemifacial spasm, CN IX, glossopharyngeal neuralgia; nervus intermedius, geniculate neuralgia) are inspected, and the appropriate vessel is decompressed. Teflon is placed as necessary, and neurolysis is performed as needed. The dura is closed in a watertight fashion, supplemented with muscle autograft as necessary. The bone is closed either with titanium plate or bone cement, and appropriate layers are closed. Patients stay in the hospital on average 2 days, and postoperative imaging is not necessary (Video 32.1).
32.4.1 Endoscope-Assisted Microvascular Decompression
Endoscope-assisted procedures are generally performed by surgeons who have performed standard microsurgical procedures with great alacrity but have also recognized the benefits of the wider angle of view provided by the endoscope. After dissection of the arachnoid and release of CSF, the surgeon identifies the cranial nerves and focuses attention on the cranial nerve of interest. Bimanual surgery is performed with the microscope situated above the operative field in the usual manner. In this setting, the endoscope tower is usually positioned across the patient or above the patient, to the side to facilitate viewing when the microscope is elevated away from the surgical field. In endoscope-assisted surgery, the microscope will block the introduction of the endoscope to the operative field at the usual focusing distances of 375 mm. Hence, to use the endoscope, the surgeon has to either change the focus of the microscope to a focal length much greater than the usual distance, or the surgeon has to pull the microscope dramatically up away from the field. Only in this way can the endoscope be introduced into the operating room corridor.
The endoscope is introduced either one- or two-handed by the operating surgeon. The surgeon performing endoscope-assisted surgery first uses the endoscope to visualize complexities of the neurovascular anatomy. For example, the endoscope is particularly useful in demonstrating where compressive vessels arise and where they end. In addition, the endoscope is useful for demonstrating perforators that may be hidden by arachnoid or neural structures. The surgeon may either direct the endoscope with two hands purely for visualization, or he or she may gradually hold the endoscope in one hand and a second instrument such as the suction in the second hand. This one-handed technique provides excellent illumination and visualization but at the expense of operative dexterity, since the surgeon only has one hand with which to manipulate instruments. Nevertheless, with practice, the surgeon can become quite skilled at using the nondominant hand to drive the endoscope and the dominant hand to manipulate tissue with either a dissector, bipolar cautery, or scissors. Once the anatomy is clearly identified, the surgeon may put the endoscope away and revert back to the microscope to complete the microvascular decompression. Alternatively, the surgeon may continue to drive the endoscope and perform the procedure as a one-handed surgeon.
One of the major advantages of endoscopy is the ability to use angled lenses. The approximate angle of view with a standard endoscope is 90º. Thus, if the shaft of the endoscope is directly pointed at the vestibulocochlear nerve, the 0º endoscope will show on the monitor a view 45º cephalad up to the trigeminal nerve and 45º caudal down to the glossopharyngeal nerve for a total angle of view of 90º. Fixed angled scopes also have 90º of view, but they are biased in their orientation. Hence, if the shaft of the now 45º fixed angle endoscope is directly pointed at the vestibulocochlear nerve, the vestibulocochlear nerve will be just only barely visible on the edge of the monitor. If the endoscope is pointed inferiorly, the vestibulocochlear nerve will be at the top of the monitor, and the glossopharyngeal nerve will be in the center of the monitor, and the vagus and spinal accessory nerve will be within view on the monitor as well. Given this change in orientation and visualization relative to the shaft of the endoscope, more extremely angled endoscopes such as 70º are only rarely used for CPA surgery.
It should be noted that there are a variety of endoscope glass options that have significant implications for resolution. The first variable to consider is the diameter of the endoscope. The larger the diameter of the endoscope, the more image and light can be projected onto the camera. For this chapter written in 2015, we assume that most camera sensors in use in the modern operating room provide video resolution at high definition, or 1920 × 1080 pixels, sometimes called 1080p. It should be noted, however, that it is only larger–outer-diameter endoscopes, such as general surgery 10-mm laparoscopes that can fill the entire camera sensor. In sinonasal endoscopy and ventral skull base endoscopy, a 4-mm outer-diameter scope only covers ~ 60 to 70% of the area of the sensor. Smaller endoscopes such as the 2.7-mm outer-diameter scope cover even less area. Thus, smaller diameter endoscopes represent a compromise between image resolution and delicate dexterity.
One issue unique to endoscope-assisted surgery is how to introduce the endoscope when the microscope is in the field. Leica and Zeiss do offer options for projection of the endoscope image into the oculars of the operating microscope. Hence, if the microscope is pulled back and focused deeper, for example, at a depth of 500 mm rather than 375 mm, the endoscope can be introduced carefully. While staring into the microscope oculars, the endoscopic view can be projected into one of the eyepieces in a monocular fashion. Unfortunately, the pixel quality and resolution remain quite limited, but this option may provide significant value in the future.
A major limitation of the one-handed endoscopic surgical technique is that the neurosurgeon is no longer bimanual. Conventionally, surgeons have used their nondominant hand to navigate a suction device that also serves as a retractor and dissector at different points in the surgery. Frazee et al introduced a technique that possibly addresses this issue by attaching a suction device to the endoscope, thus allowing the nondominant hand to drive both an endoscope and a suction.33 Unfortunately, this technique then introduces problems associated with visualization as the picture on the monitor will move at the slightest movement of the suction, to control bleeding or to retract, for example. Hence, this hardware solution is not widely utilized, and most surgeons who perform the one-handed technique recognize the limitations and perform safe dissection and revert back to the microscope if needed.
Despite the limitations of endoscope-assisted MVD, this technique is easily introduced into an existing microsurgical practice. The visualization is superb, and it can provide important information not easily appreciated with the standard microscope. Further description of benefits and limitations will be described in the discussion section.
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