Cavernous Malformations: What They Have Taught Us.

Abstract

ABBREVIATIONS: BSCM brainstem cavernous malformation CC corpus callosum CCM cerebral cavernous malformation CM cavernous malformation LD lateral dorsal LP lateral posterior SSS superior sagittal sinus VA ventral anterior VI ventral intermediate VL ventral lateral VPL ventral posterior lateral. THE SURGICAL CHALLENGE Surgical treatment of cavernous malformations (CMs) of the central nervous system requires mastery of both neurosurgical anatomy and the operative technique. CMs can develop anywhere along the neural axis, and their effect on critical neural functions is specific to each lesion (Figure 1). Although resection of CMs can be straightforward, the surgical strategy and the surgeon's experience in balancing gross total resection against harmful resection are keys to determining successful patient outcomes. As in most complex neurosurgical entities, the decision to not operate on a patient with a disease that can be cured surgically, such as a CM, is a difficult one, and the emotional toll of deciding not to operate has been one of the main drivers for neurosurgical operative innovation.FIGURE 1.: Illustration of the various anatomic locations that may harbor cerebral and spinal CMs, which may arise from any anatomic location in the brain, cranial nerves, or spinal cord. The CM locations represented in this illustration are only a sample of the complex locations we have encountered in our experience. CMs, cavernous malformations. Used with permission from Barrow Neurological Institute, Phoenix, Arizona.Deep cerebral cavernous malformations (CCMs), which were once considered inoperable, can be now resected safely because of the rapid advancements in the operative technique, neuronavigation, and intraoperative neuromonitoring, among other factors. Curative treatment of deep CCMs is now possible, but it requires a rare—and exciting—combination of both the creative mind of a master skull base neurosurgeon and the finesse of a cerebrovascular neurosurgeon. Access to a deep CCM requires tailoring a cranial or skull base approach so that the surgical access aligns with the longest axis of the CM (ie, the 2-point method).1,2 For example, a contralateral orbitozygomatic approach may be used to access an anterior medial midbrain CCM. Experience has also taught us that the odds of leaving a remnant of a deep CCM increase if the surgical trajectory requires steep angles to reach the lesion (such as with the right-angle method),3 which demands the further refinement of the surgical strategy. Although resection of CCMs is often a straightforward part of the operation, deep CCMs present the unique challenge of a constant judgment between curative gross-total resection and the risk of devastating surgical sequelae. This constant intraoperative equipoise between curative and harmful resection is a complex task that depends heavily on exquisite knowledge of neurosurgical anatomy, mastery of operative technique, and years of clinical experience. We aimed to identify the most valuable lessons learned over decades of passionate dedication to neurosurgical innovation and caring for more than 1000 patients with CMs, including more than 500 patients with brainstem CMs (BSCMs). We present the expert and personal reflections of the senior author (R.F.S.) regarding the management of CMs and our vision for the future of neurosurgical training. MANAGEMENT OF DEEP CCMs: INDICATIONS AND CONTRAINDICATIONS The most difficult decision for neurosurgeons treating a patient with a deep CCM is to not operate. Many factors influence our judgment toward aggressive surgical treatment. Because gross-total resection of isolated CMs is curative, the decision on whether to resect them profoundly affects the lives of our patients. Some patients may not cope well with the uncertainty of future bleeding and sudden new neurological deficits, which can result in severe disability or impairment. Management of both BSCMs and spinal CMs remains very patient-specific and is largely guided by the neurosurgeon's personal experience and consensus among colleagues.4-10 The eagerness to operate on an anxious patient or the personal quest for incremental technical challenge should be tamed by the collective experience of surgical harm.11 Among the lesions treated in our cerebrovascular discipline, those that require the most abundant surgical acumen, careful patient selection, and personalized longitudinal care are BSCMs and spinal CMs. The decision not to operate on a patient with a BSCM should be weighed within the context of one's professional progression. As with mastering other complex neurosurgical entities, the learning curve should include mastering surgical anatomy, familiarization with the tenets of CM dissection, and mastering the haptic feedback and visual clues that confirm a safe resection. A safe and carefully designed learning curve requires insight and self-monitoring, for which mentoring is essential. Tackling increasingly challenging surgical locations will eventually lead to surgical excellence and the ability to tackle BSCMs once deemed inoperable when surgery is right for the patient. However, treating BSCMs has taught us that crossing certain fine lines carries an almost certain risk of incapacitating sequelae or unsuccessful resection, even in the hands of experts.3,11-17 When presented with these circumstances, such as those for the case illustrated in Figure 2, one should give the patient realistic counsel regarding possible outcomes and proceed to surgery only after careful and thoughtful consideration.FIGURE 2.: Example of a case that should be managed conservatively. A woman in her 20s with chronic headaches and no previous significant health problems was found to have a brainstem CM. A, Axial fluid-attenuated inversion recovery MRI without contrast demonstrating a CM in the anterior aspect of the inferior half of the fourth ventricle protruding into the pons, surrounded by a thin layer of edema. B, T1-weighted MRI without contrast showing the same CM in the sagittal plane. The CM reaches the pontomedullary junction anteriorly and expands posteriorly and superiorly toward the anterior wall of the fourth ventricle. CM, cavernous malformation. Used with permission from Barrow Neurological Institute, Phoenix, Arizona.Our experience treating patients with CMs has taught us that surgical treatment is generally successful when there is documented rehemorrhage, a progression of neurological deficits, or significant mass effect or when the lesion can be safely reached by an intra-axial route (ie, when the lesion reaches the pial surface or is reachable through a safe entry zone).5,16-18 Although there is strong evidence supporting the role of surgical resection for CM-related epilepsy, there is less scientific evidence supporting management strategies for BSCMs and spinal CMs. Dammann et al6 led an international Delphi consensus on the management of BSCMs. This exceptional work provides an academic framework to approach patient selection and an expert consensus for surgical indications, based on the cumulative experience of those who have reached peak performance in our field. The features carrying strong consensus toward surgical treatment align with our experience. Interestingly, an agreement was not reached on the exact definition of a deep-seated lesion or what is considered a difficult BSCM location, both of which are subjective in nature and for which perceptions evolve with experience. A foreseeable future solution to provide objective and accurate guidelines for the management of BSCMs and spinal CMs may be the development of an international registry in which radiological, clinical, and outcome data could be analyzed through big-data computing systems and artificial intelligence. SURGICAL PLANNING: NEUROSURGICAL ANATOMY IS EVERYTHING Deep CMs require mastery of both surgical and functional neuroanatomy. The surgical strategy for deep CCMs involves designing a surgical approach that aligns the surgical corridor with the lesion according to the 2-point and right-angle methods and that maximizes surgical freedom (eg, by the use of gravity, short surgical corridors, and maximizing the target exposure).1-3 The anatomic factors that determine which surgical approach is optimal include extracranial structures, subarachnoid space, pia mater, and white matter tracts, as well as the available safe entry zones and the craniotomy chosen. The aggregate of these neurosurgical anatomy factors combined through an operative perspective determines the single best approach for each complex CCM. The epitome of the effect of neurosurgical anatomy on operative choice is found in the thalamus, where CMs located a few millimeters apart may require very different approaches (Figure 3).19 The treatment of thalamic cavernous malformations is the epitome of applying neurosurgical anatomy to optimize surgical planning. A, Operative regions of the thalamus. B, The orbitozygomatic approach offers the best surgical access to lesions in the anteroinferior region. C, The anterior interhemispheric transcallosal approach allows access to the medial region. D, The anterior contralateral interhemispheric transcallosal approach is best for accessing the lateral region. E, The posterior interhemispheric transcallosal approach allows for direct access to the posterosuperior region. F, The parieto-occipital transventricular approach is used for accessing the lateral posteroinferior region. G, The supracerebellar infratentorial approach provides access to the medial posteroinferior region. Ant, anterior; LD, lateral dorsal; LP, lateral posterior; Pulv, pulvinar; VA, ventral anterior; VI, ventral intermediate; VL, ventral lateral, VPL, ventral posterior lateral. Used with permission from Barrow Neurological Institute, Phoenix, Arizona. Our cumulative experience in treating deep CCMs has taught us valuable surgical tenets that affect the choice of the operative technique and improve outcomes. This knowledge is the result of integrating neurosurgical anatomy with the relentless quest for surgical innovation. The following are principles that we apply routinely when evaluating and operating on deep CCMs. The Power of Gravity Because there is a law such as gravity, the universe can and will create itself from nothing. —Stephen Hawking Gravity is a timeless and constant physical force that has guided human interaction with the planet since our existence began. Therefore, it is natural to use it as our powerful ally during deep CCM resections. Deep CCMs requiring long intracranial trajectories often involve a degree of interaction with neural tissue (eg, brain or cerebellum) to allow surgical freedom. However, static retraction over a prolonged surgical time is harmful to neurovascular tissue and must be avoided.20,21 Alternatively, there are safe surgical maneuvers that allow dynamic surgical freedom targeted to real-time dissection. Patient positioning is key to maximizing the use of gravity. The lateral head position for the interhemispheric approach is a good example of a recent modification of the classic straight-up positioning (Figure 4). In addition, applying neurosurgical anatomy to this concept further improves both surgical access and surgical freedom. In the instance of the lateral head position of the interhemispheric approach, a contralateral trajectory uses the falx cerebri as a natural retractor to the contralateral hemisphere as a brake against gravity while the dependent hemisphere is subject to both the uniform distribution of gravity-powered retraction and dynamic intermittent surgical retraction (Figure 5).22,23 Finally, early dissection of the arachnoid cisterns enhances gravity retraction and allows for the natural unfolding of the subarachnoid cerebral surface.FIGURE 4.: Case illustration of the use of gravity for accessing a deep CCM. A woman in her 20s presented with progressive headaches and an otherwise grossly intact neurological examination with excellent functional memory. A, Axial T1-weighted MRI with gadolinium contrast demonstrating a CCM within the lateral wall of the left lateral ventricle extending into the third ventricle. The CCM has mixed density with a fluid level. B, Sagittal T1-weighted MRI with gadolinium contrast revealing the caudal extent of the lesion within the third ventricle. C, Axial FLAIR MRI demonstrating the fluid level of the CCM and its position relative to the frontal horn of the left lateral ventricle. After discussion with the patient, a lateral interhemispheric transcallosal approach (left side down) was performed. D, Intraoperative photographs demonstrating patient positioning (upper left and upper right), craniotomy (lower left), and surgical incision (lower right). E, Axial and F, sagittal postoperative FLAIR-weighted MRI demonstrating gross-total resection without surgical damage to the surrounding parenchyma. The patient tolerated the procedure well and had an uneventful recovery. CCM, cerebral cavernous malformation; FLAIR, fluid-attenuated inversion recovery. Used with permission from Barrow Neurological Institute, Phoenix, Arizona.FIGURE 5-1.: Case illustration of the use of a contralateral interhemispheric transfalciform approach demonstrating the integration of ergonomics and the 2-point and right-angle methods. A young patient was referred for treatment of a right frontal horn CCM with radiological evidence of recurrent hemorrhages. A, Axial T1-weighted MRI without contrast demonstrating a lesion in the right frontal horn (white oval). B, Coronal T2-weighted MRI revealing a hemosiderin ring. The arrow illustrates the optimal surgical trajectory to the lesion (white oval), respecting the right-angle method and optimizing ergonomics. Other trajectories were considered but not chosen. C, Sagittal FLAIR-weighted MRI demonstrating a CCM in the anterior part of the right frontal horn of the lateral ventricle (white oval). The right frontal transcortical, ipsilateral interhemispheric transcallosal, and contralateral interhemispheric transcingulate (left side down) approaches were considered. After much consideration and discussion with the patient, the latter option was chosen. D, Illustration of the surgical corridor for the lateral interhemispheric transfalciform transcingulate approach. E, Intraoperative photograph of the incision of the falx cerebri. F, Intraoperative photograph and corresponding neuronavigation screen of the transcingulate pial entrance. G, Intraoperative photograph and corresponding neuronavigation screen of the exposure of the CCM. H, Postoperative sagittal (left) and coronal (right) FLAIR MRIs demonstrating gross-total resection without surrounding tissue damage. The patient tolerated the procedure well and recovered without neurological deficits. CC, corpus callosum; CCM, cerebral cavernous malformation; CmaA, callosomarginal artery; FLAIR, fluid-attenuated inversion recovery; L, left; PcaA, pericallosal artery; R, right; SSS, superior sagittal sinus. Figures5A-C and5E-H are used with permission from Barrow Neurological Institute, Phoenix, Arizona. Figure 5 D is used with permission from the Congress of Neurological Surgeons, from Davies J, Tawk RG, Lawton MT. The contralateral transcingulate approach: operative technique and results with vascular lesions. Neurosurgery. 2012;71(1 suppl operative):4-14. doi:10.1227/NEU.0b013e318246a7f8 .FIGURE 5-2.: Case illustration of the use of a contralateral interhemispheric transfalciform approach demonstrating the integration of ergonomics and the 2-point and right-angle methods. A young patient was referred for treatment of a right frontal horn CCM with radiological evidence of recurrent hemorrhages. A, Axial T1-weighted MRI without contrast demonstrating a lesion in the right frontal horn (white oval). B, Coronal T2-weighted MRI revealing a hemosiderin ring. The arrow illustrates the optimal surgical trajectory to the lesion (white oval), respecting the right-angle method and optimizing ergonomics. Other trajectories were considered but not chosen. C, Sagittal FLAIR-weighted MRI demonstrating a CCM in the anterior part of the right frontal horn of the lateral ventricle (white oval). The right frontal transcortical, ipsilateral interhemispheric transcallosal, and contralateral interhemispheric transcingulate (left side down) approaches were considered. After much consideration and discussion with the patient, the latter option was chosen. D, Illustration of the surgical corridor for the lateral interhemispheric transfalciform transcingulate approach. E, Intraoperative photograph of the incision of the falx cerebri. F, Intraoperative photograph and corresponding neuronavigation screen of the transcingulate pial entrance. G, Intraoperative photograph and corresponding neuronavigation screen of the exposure of the CCM. H, Postoperative sagittal (left) and coronal (right) FLAIR MRIs demonstrating gross-total resection without surrounding tissue damage. The patient tolerated the procedure well and recovered without neurological deficits. CC, corpus callosum; CCM, cerebral cavernous malformation; CmaA, callosomarginal artery; FLAIR, fluid-attenuated inversion recovery; L, left; PcaA, pericallosal artery; R, right; SSS, superior sagittal sinus. Figures5A-C and5E-H are used with permission from Barrow Neurological Institute, Phoenix, Arizona. Figure 5 D is used with permission from the Congress of Neurological Surgeons, from Davies J, Tawk RG, Lawton MT. The contralateral transcingulate approach: operative technique and results with vascular lesions. Neurosurgery. 2012;71(1 suppl operative):4-14. doi:10.1227/NEU.0b013e318246a7f8 .The Importance of Ergonomics Design is not what it looks like and feels like. Design is how it works. —Steve Jobs The human brain sees the world through a horizontal stereoscopic vision and interacts with it best through horizontal dexterity. Surgical dexterity and precision are the cornerstones of successful resection of deep CCMs. The most delicate, precise, and demanding part of operating on a deep CCM is dissecting the lesion from surrounding eloquent tissue at the depths of a long surgical corridor. Therefore, it is desirable that both mind and body align in a rested and peak performance state approaching the last part of the operation, when the patient is most at risk. This state is best accomplished when the position of the surgeon minimizes muscle strain, factors in human dexterity principles, and boosts our senses for peak performance. An example of surgical innovation that includes ergonomic principals is the lateral position for the interhemispheric approach (Figure 5D).24 Aligning the longest surgical axis—the interhemispheric fissure—to the horizontal plane allows a natural horizontal movement of the hands. Horizontal placement of the hands reduces muscle strain by maintaining a neutral hand position, minimizes the pressure exerted by the shafts of the instruments on the cerebral tissue, and increases dexterity because the hands move along the axis we use most. Dissecting deep CCMs is made more difficult by the cumulative layers of critical tissue that limit the approach. These bounding structures not only limit surgical freedom but also block light and impair visualization. Lighted instruments and the seamless operation of the microscope (eg, by using a mouthpiece, joystick mode, or position recall) may provide an advantage for performance that can translate into complication avoidance during critical parts of the operation.24 The Importance of Neuronavigation If one doesn't know to which port one is sailing, no wind is favorable. —Seneca Neuronavigation allows the neurosurgeon to see beyond the pial surface. The ability to know the location of an intra-axial CCM that does not present to the pial surface has enabled us to cross the pia with confidence to perform surgical resections on CCMs that were once deemed inoperable. However, the type of navigation that is necessary to tackle some of the most difficult deep CCMs or BSCMs starts in the mind of the neurosurgeon. Mastering the anatomy that is most neurosurgically relevant remains the safest, most efficient, and most dependable navigation chart. The most valuable resource for navigating the intradural space efficiently is an understanding of the subarachnoid space—specifically, the subarachnoid cisterns. There are many subarachnoid cisterns, and they have been described extensively in the literature.25,26 We firmly believe that the surgical understanding of each cistern gives surgeons the advantage of both confidence and efficiency in navigating the subarachnoid space. However, anyone devoted to the surgical treatment of deep CCMs needs to master knowledge of certain major subarachnoid cisterns, including the sylvian cistern, cerebellopontine and cerebellomedullary cisterns, and interpeduncular cistern. The first of these important subarachnoid cisterns, the sylvian cistern, is the main avenue to most lesions located in the insula, basal surface of the brain, and midbrain. This cistern is limited externally by the outer arachnoid layer and at its deepest point by the proximal membrane. This cistern and its microvasculature become important when splitting the sylvian fissure. Mastery of splitting the sylvian fissure requires understanding that the superficial sylvian veins run in a tight space between the outer arachnoid membrane and the lateral sylvian membrane, and these veins need to be released carefully. When present, the intermediate sylvian membrane serves as a depth indicator for the sylvian fissure because it marks the transition between the M2 and M3 segments of the middle cerebral artery. In addition, when approaching a superior insular CCM in this region, one should be aware that the medial membrane may harbor the superior (frontal) trunk of the M2. The cerebellopontine and cerebellomedullary cisterns are key to reaching most anterolateral CCMs within the midbrain, pons, and medulla. The petrosal fissure must be split to ensure a medial and posterior entry to the middle cerebellar peduncle. Next, the middle cerebellar peduncle should be entered at the safe entry zone (ie, posterior and medial) to allow a trajectory that is most observant of the white matter fascicle and therefore allows safer access to anterior pontine lesions. The interpeduncular cistern is another subarachnoid cistern that must be thoroughly understood when considering surgical resection of BSCMs in the anterior part of the midbrain. When accessing the interpeduncular cistern, understanding the anatomy of the Liliequist membrane is of utmost importance. The mesencephalic leaflet of the Liliequist membrane is typically constant and thick, serving as a dam separating the supratentorial and infratentorial circulation of the cerebrospinal fluid (CSF). Dissecting through the Liliequist membrane serves 2 important surgical goals: entering the interpeduncular region (eg, anterior mesencephalic entry zone) and improving CSF communication, which may help intracranial CSF diversion in conjunction with fenestration of the lamina terminalis. The importance of other subarachnoid cisterns—the carotid, lamina terminalis, chiasmatic, pericallosal, crural, ambiens, and quadrigeminal and prepontine cisterns, and cisterna magna—are discussed in detail in the literature.27 Beyond the subarachnoid cisterns, other neurosurgical landmarks are important for navigating the neural anatomy. One example of navigation by landmarks is using the relationship between the lateral mesencephalic vein and the mesencephalic sulcus. The mesencephalic sulcus corresponds to the lateral midbrain safe entry zone, which is an optimal venue for resection of lateral midbrain BSCMs posterior to the corticospinal tracts (Figure 6). However, CCMs can occur virtually anywhere along the neural axis, and sometimes neuronavigation is the only reliable guidance to intra-axial CCMs. The spatial registration (stereotactic guidance) of the focal point of the microscope has substantially improved the dynamic efficiency of neuronavigation, which is used routinely during BSCM resections. For spinal CMs, a useful navigation tool is ultrasound. Along with radio-opaque seeds, intraoperative ultrasound gives full reassurance, in real time, that we are in the precisely correct location before we incise the spinal cord itself.FIGURE 6.: Case illustration of the resection of a posterior lateral midbrain CM and the use of the tentorium to guide surgical planning. A man in his 50s with moderate diplopia was referred to our institution after 3 documented hemorrhages of a midbrain cerebral CM. A, Coronal T1-weighted MRI with gadolinium contrast demonstrates a CM within the midbrain paramedial to the left side. B, Sagittal T1-weighted MRI without gadolinium contrast showing a midbrain CM along the height of the midbrain and abutting the aqueduct of sylvius, with mild obstructive hydrocephalus. C, Axial T1-weighted MRI without gadolinium contrast shows a midbrain CM paramedian to the left side. The lesion shared the same axial altitude as the tentorium as both the cerebellum and the temporal lobe could be seen at the same cut. After discussion of all management alternatives, the patient decided to undergo surgical treatment. A left supracerebellar infratentorial approach with the patient in the park-bench position was performed. D, Illustration of the brainstem showing the lateral midbrain safe entry zone (arrow). E, Intraoperative photograph of the removal of the brainstem CM with the corresponding neuronavigation screen showing the trajectory below the tentorium and above the cerebellum. F, Axial T2-weighted MRI demonstrating gross-total resection of the CM. The patient recovered well and was discharged at his neurological baseline after an initial transient worsening of his diplopia. CM, cavernous malformation. Used with permission from Barrow Neurological Institute, Phoenix, Arizona.The Tentorium: A Beacon for Surgical Planning Expose every belief to the light of reason, discourse, facts, scientific observations; question everything, be skeptical because this is the only chance at life you will ever get. —James Randi We have challenged the dogma that supratentorial lesions must be approached through a supratentorial approach. As a form of respect and honor to our patients, we must constantly seek innovation, especially to challenge contemporary surgical dogmas that either carry high risk or have suboptimal results. Over the past decades, we have maintained a constant quest to improve access to deep CCMs located within the posterior lateral midbrain, medial posterior thalamus, medial temporal lobe, and basal occipital lobe. These deep CCMs are considered supratentorial, yet all supratentorial surgical options either carry significant surgical risk or provide suboptimal exposure. The novel neurosurgical look at the tentorium as a beacon for using infratentorial surgical trajectories is not the one that comes with common sense, but with empirical rationale. It is an example that accepting challenges, respectfully rejecting norms, and pushing against the boundaries of neurosurgical convention is a proven professional creed that, when combined with an exquisite knowledge of anatomy and a calculated dose of risk, may change operative dogma. An important principle is that lesions that share the same axial plane as the tentorium as seen on axial imaging that are either near or in contact with the tentorium should be considered for a supracerebellar infratentorial or supracerebellar transtentorial approach (Figures 6 and 7).28 A simple proxy for the location of the tentorium on an axial magnetic resonance image is to find the images where the temporal lobe and cerebellum are both seen at the same time. We have treated patients with CCMs in the posterior lateral midbrain, medial posterior thalamus, medial temporal lobe, and basal occipital lobe through an infratentorial or transtentorial trajectory with consistently successful results. The supracerebellar infratentorial and supracerebellar transtentorial approaches can be performed with the patient in either a park-bench or sitting position. A sense of balance between treatment that is too risky on one hand and too ineffectual on the other hand is critical to prevent crossing the fine line of risk in the quest for surgical innovation. Small midbrain lesions that contact the pial surface medial to the cerebral peduncle are best accessed using a contralateral orbitozygomatic approach, even if they are at the tentorial level (Figure 8).29,30 Although using a contralateral orbitozygomatic approach to the anterior-medial midbrain may seem counterintuitive and rather out of reach at first, careful examination reveals the opposite. The main surgical trajectory of the contralateral orbitozygomatic approach follows the 2-point and right-angle methods, uses bone deconstruction instead of brain transgression, and offers a shorter intracranial reach than the supracerebellar infratentorial approach. However, this approach requires dissection within the thalamoperforators and through the optic-carotid triangle and is safest in the hands of those at the peak of their careers.FIGURE 7.: Case illustration of a supracerebellar transtentorial approach for resection of a medial temporal lobe CM. A woman in her 50s with headaches and seizures that were nonresponsive to medication was referred to our institution for resection of the CM. A, Axial T1-weighted MRI without gadolinium contrast showing a medial temporal lobe cerebral cavernous malformation protruding within the medial incisural space. The lesion was at the tentorial level as both the cerebellum and the temporal lobe share the same cut. B, Coronal T1-weighted MRI with gadolinium contrast demonstrating a CM within the medial temporal lobe and resting on the tentorial surface. The patient elected to undergo surgical treatment. A supracerebellar transtentorial approach with the patient in the park-bench position was performed. C, Intraoperative photograph of the tentorial incision through the supracerebellar trajectory. D, Axial and E, coronal fluid-attenuated inversion recovery–weighted MRI demonstrating gross-total resection of the temporal CM without surgical sequelae to the surrounding parenchyma. The patient tolerated the procedure well and was able to discontinue antiseizure medication and return to work. CM, cavernous malformation. Used with permission from Barrow Neurological Institute, Phoenix, Arizona.FIGURE 8.: Case illustration of a contralateral orbitozygomatic approach for an anterior medial midbrain BSCM. A woman in her 20s presented with 3 episodes of diplopia and right hemiparesis. A, Axial T2-weighted (left) and T1-weighted (right) MRI without contrast showing an anterior medial midbrain BSCM in contact with the pial margin in the interpeduncular cistern. After counseling, the patient elected to undergo surgical treatment. A right contralateral orbitozygomatic approach was performed. B, Intraoperative photograph showing the opticocarotid triangle through a right orbitozygomatic approach. The arrow indicates the right internal carotid artery. C, Intraoperative photograph of the removal of the BSCM. D, Axial fluid-attenuated inversion recovery (left) and coronal T2-weighted (right) postoperative MRIs demonstrating gross-total resection of the midbrain BSCM without surgical damage to the surrounding parenchyma. The patient tolerated the procedure well and was discharged neurologically intact. BSCM, brainstem cavernous malformation; CN, cranial nerve. Used with permission from Barrow Neurological Institute, Phoenix, Arizona.Safe Entry Zones to the Brainstem: Inoperable is Obsolete Impossible is just a small word that is thrown around by small men who find it easier to live in a world they've been given than to explore the power they have to change it. Impossible is not a fact. It's an opinion. Impossible is not a declaration. It's a dare. Impossible is potential. Impossible is temporary. Impossible is nothing. —Muhammad Ali Most BSCMs that we operate on today were until recently considered inoperable. Although there are regions in the neural axis where access still comes at great risk, we have been able to resect many BSCMs that were once deemed inoperable. Inoperable is a term typically used to convey a surgeon's opinion in which he or she sees a clear negative risk-benefit balance for the patient relative to removing a lesion—generally one within the core of the neural axis. As the evolution of the surgical treatment of BSCMs has taught us, this balance is greatly dependent on neurosurgical innovation, which in many cases has been able to turn the needle toward surgical treatment. Therefore, the term inoperable speaks more about the need for neurosurgical innovation than about sentencing our patients—and our field—to an academic surrender. Our institution's and others' quest for safe entry zones to the brainstem is a testament to a necessary academic attitude—to push the boundaries and challenge norms—practiced through reliable and calculated surgical research.31-33 Our experience treating BSCMs has taught us the regions of the brainstem surface that can be traversed with relative safety in carefully selected cases; these regions are called safe entry zones.31-33 The Safe Entry Zones The midbrain can be divided into 3 main areas: anterior, lateral, and posterior. The anterior mesencephalic zone can be entered lateral to the oculomotor nerve (perioculomotor) and the midline interpedunclular zones. The perioculomotor zone is bounded medially by the oculomotor nerve and tract and laterally by the corticospinal tract within the cerebral peduncle. The lateral mesencephalic sulcus, which correlates with the vein that shares the same name, allows lateral entry to the midbrain, limited anteriorly by the cerebral peduncle and posteriorly by the tegmental area. The midbrain can also be accessed posteriorly through the line between the left and right paired colliculi, the pericollicular line. The pons may be accessed through 3 main zones: the supratrigeminal zone above the trigeminal nerve and below the pontomesencephalic sulcus; the middle cerebellar peduncle zone along the axis of the middle cerebellar peduncle and through an entry point within the depths of the petrosal fissure; and the peritrigeminal zone just below the sensory root of the trigeminal nerve. The medulla oblongata may be entered through 3 zones: the anterolateral sulcus, the inferior olivary nucleus (ie, the olive), and the lateral medullary (through the inferior cerebellar peduncle). The anterolateral sulcus safe zone allows an angled trajectory lateral to the corticospinal tracts for lesion within the anterior aspect of the medulla and upper cervical cord. The olive can be traversed with relative safety to reach BSCMs that are anterior and lateral or that reach the pial surface at that level. The lateral medullary zone resembles the entrance within the middle cerebellar peduncle with a parafascicular trajectory aligning with the inferior cerebellar peduncle itself. BSCMs that are near the anterior wall of the fourth ventricle may be accessed through safe entry zones at the final steps of the telovelar approach and approaches using the Magendie and Luschka foramina. The median sulcus of the fourth ventricle may be used to enter the brainstem dorsally with care not to deviate laterally, where the medial longitudinal fascicle runs. The suprafacial triangle is bound by the facial colliculus inferiorly, the cerebellar peduncles laterally, and the medial longitudinal fasciculus medially. The infrafacial triangle, located below the facial colliculus, is limited medially by the medial longitudinal fasciculus, inferiorly by the hypoglossal trigone, and laterally by the corticospinal tract.29-38 THE FUTURE: GLOBAL NEUROSURGICAL EDUCATION Surgical management of CMs is an art. Mastering an art is a long process that includes learning tangible and intangible lessons, the aggregate of which results in a refined wisdom that is unique. For the surgical resection of CMs, tangible lessons include an exquisite knowledge of anatomy,31,39 nuanced radiological interpretation,16 comprehension of operative techniques,1,2,8,19,23 understanding the natural history of the different types of CMs (eg, familial vs nonfamilial genetic profile),4,5,40 knowing the outcomes of the different management options (eg, CM-related epilepsy, quality of life, hemorrhage-related disability),6,11,17,18,41 and understanding the modifiable risk factors for hemorrhage and rehemorrhage.40,42 These factors have been—and continue to be—well described in the literature. These are tangible facts that are common ground for the surgical treatment of CMs. However, the key to mastering the art of CM treatment resides in the intangible factors, which are largely based on experience and cumulative wisdom, which are a complex integration of the routine interpretation of tangible factors. Experience and cumulative wisdom are gained as a result of repeated application of factual knowledge to CM cases. Transferring intangible knowledge (ie, teaching at a large scale) is inherently complex. Although classic educational methods (eg, books, scientific publications) are an excellent source of tangible knowledge, they are not designed for—and do not deliver—intangible wisdom. The rapid and massive production of webinars (internet-based virtual seminars) has opened the international stand to anyone with a microphone and a camera. Webinars are a true form of democratization of knowledge and have many advantages over classic forms of communication. However, they lack the pedagogic structure, peer-review accreditation, and ability to be pondered of the written word. The recent development of professional website-based educational resources is an optimal combination of the written word and democratization of knowledge at a global scale.43,44 The strongest benefit of the virtual platform is its capability to be both interactive and constantly updated. These forms of education may help bridge the gap of global neurosurgical training inequality and are a necessary step forward toward the unmet need for quality neurosurgical care in the most vulnerable parts of the world.45 The Interactive Surgical Atlas: A Concept for Patient-Specific Operative Guidance We are caught in an inescapable network of mutuality, tied in a single garment of destiny. —Martin Luther King, Jr. The responsibility of leaders is to pass on their artful mastery of their craft to the next generations. Our team is creating an operative guidance platform that integrates the tangible and intangible lessons learned over decades of experience in the surgical treatment of CCMs. This interactive surgical atlas will be a virtual operative guidance platform that will allow neurosurgeons from anywhere in the world to have immediate access to highly curated information that is specific to their patient. This virtual tool will combine the best of classical teaching methods (eg, peer-reviewed, factual, and scientific) with the best of contemporary teaching technology (eg, democratization of knowledge, interactive, adaptable, and up-to-date) and uses the principles of neuronavigation to apply this aggregate wisdom to guide surgical treatment of any CCM. Our goal is to provide information regarding surgical approach options, relevant neurosurgical anatomy, pitfalls, pearls, surgical tenets, and video examples that are pertinent to the patient that the user is treating. This interactive surgical atlas would serve as a guiding voice of collective wisdom for treating CCMs. The user would first indicate the epicenter of the patient's lesion in the software's brain model. This model contains a phantom map of all possible surgical targets within the brain. Using a computational probabilistic method, the input patient's CCM epicenter is matched with the best surgical target in the phantom map. This match allows the software to display a comprehensive operative guideline specific to the lesion that includes a menu with all possible surgical approaches (ie, approaches with proven success) to the CCM that has been inputted. Based on previous surgical research and collective expert wisdom, a best approach option is offered. At this point, the neurosurgeon may choose to register the patient with the surgical atlas in the operating room. If so, an automated process will feed the stereotactic coordinates to the surgical microscope, which then aligns with the proposed surgical trajectory in real time. This platform may also be used preoperatively to visualize the operation and prepare for a challenging case. Selecting the proposed approach will display a menu with a comprehensive operative rationale. The operative menu includes alternative positioning, key neurosurgical anatomy review, surgical tenets, pearls, operative video examples, risk avoidance warnings, and an option to input feedback. The latter option is key to the ongoing growth of collective wisdom within this platform. Through big data and artificial intelligence processing of such collective feedback, this platform has the potential to grow to become a guideline for best practices made by and for our specialty.