12 Safe Entry Zones to the Brainstem

10.1055/b-0039-173903

12 Safe Entry Zones to the Brainstem

Daniel D. Cavalcanti

Abstract

In this chapter on safe entry zones to the brainstem, readers will gain detailed knowledge of the main skull base exposures, enhanced with images from cadaveric dissections, which allow neurosurgeons to reach lesions in the brainstem. Despite its minimal volume, the brainstem contains a rich concentration of nuclei and fibers in a small sectional area, resulting in an increased likelihood of morbidity after manipulation. Thus, whenever lesions do not rise to the pial or ependymal surface of the brainstem, it is essential to have a fundamental understanding of the concept of safe entry zones. Such zones represent entry points and trajectories where eloquent structures and perforators are sparse. Moreover, when manipulation is performed by experienced neurosurgeons through these corridors, deficits are minimized. Using the right combination of surgical approach and safe entry zone is key to reducing morbidity for any lesion that does not emerge to the pial or ependymal surface. In this chapter, we detail seven safe entry zones that have been described to manage mesencephalic lesions, seven zones for management of pontine pathology, and six for operating on medullary lesions.

Introduction

Numerous approaches can lead the neurosurgeon to the different structures of the brainstem and various locations on the brainstem. Notably, similar lesions may demand different approaches, depending on the long axis of the lesion and its proximity to the surface and surrounding at-risk structures. ¹ ^, ² ^, ³ However, when lesions do not reach a pial or ependymal surface, known safe entry zones should be selected to avoid or reduce surgical morbidity. These zones, which have been described in surgical series, anatomical reports, and electrophysiological reports, consist of entry points or trajectories where arterial perforators, nuclei, and tracts are sparse. ² ^, ⁴ ^, ⁵ ^, ⁶ ^, ⁷ ^, ⁸ ^, ⁹ ^, ¹⁰ ^, ¹¹ ^, ¹²

Baghai et al ¹⁰ were pioneers in elucidating safe entry zones. In 1982, they described a safe entry zone between the emergence of the trigeminal nerve (cranial nerve [CN] V) and the facial nerve (CN VII) via a retrosigmoid approach as a rational alternative to transgression of the fourth ventricular floor. Since then, other specialty centers introduced additional safe entry zones for managing brainstem tumors and vascular malformations. ⁹ ^, ¹³ ^, ¹⁴ ^, ¹⁵ ^, ¹⁶

Comprehensive knowledge of different skull base exposures, gained through laboratory dissections and training, allows neurosurgeons to select the correct corridor during careful imaging analyses. Moreover, combining safe entry zones with the two-point method enhances approach selection. ¹ ^, ¹⁷ Combining the right window with the optimal trajectory increases surgical freedom and odds of gross total resection while reducing undesired deficits. Image guidance and intraoperative monitoring are crucial adjuncts for optimizing safety during brainstem surgery. This chapter focuses on the safe entry zones commonly used for resection of brainstem pathologies ( Fig. 12.1 ).

**Fig. 12.1** Main safe entry zones to the brain-stem. The colored ovals and dashed lines represent points where small neurotomies are possible to avoid small perforators, main nerve tracts, and nuclei. **(a)** Anterolateral view of brainstem illustrating some anterior and anterolateral safe zones. **(b)** Posterior view of brain-stem demonstrating the safe entry zones on the surface of the quadrigeminal plate (*green dashed lines*), floor of the fourth ventricle (*blue dashed lines and colored ovals*), and lower medulla (*red dashed lines*). Abbreviations: ALS, anterolateral sulcus; AMZ, anterior mesencephalic zone; ant., anterior; CN, cranial nerve; IBTZ, inferior brachium triangular zone; ICZ, infracollicular zone; ICR, intercollicular region; LMS, lateral mesencephalic sulcus; LMZ, lateral medullary zone; LPZ, lateral pontine zone; med., median; MS, median sulcus of fourth ventricle; OZ, olivary zone; ped., peduncle; PIC, paramedian infracollicular; PIS, posterior intermediate sulcus; PLS, posterior lateral sulcus; PMS, posterior median sulcus; PSC, paramedian supracollicular; PTZ, peritrigeminal zone; SCZ, supracollicular zone; SFT, superior fovea triangle; STZ, supratrigeminal zone. Used with permission from Barrow Neurological Institute, Phoenix, Arizona.

Mesencephalic Safe Entry Zones

The midbrain connects the diencephalon to the pons and consists of two main parts, the tectum and the cerebral peduncles. The cerebral peduncles are divided into the crus cerebri and the tegmentum. Lesions within the midbrain are approached using seven safe entry zones accessed via anterior, posterolateral, and posterior approaches ( Table 12.1, Fig. 12.2 ). ¹ ^, ²

**Table 12.1** Mesencephalic safe entry zones by approach
Approach	Safe entry zones
Orbitozygomatic, pterional, minisupraorbital, transciliary	Anterior mesencephalic, interpeduncular
Subtemporal	Anterior mesencephalic, lateral mesencephalic sulcus
Subtemporal transtentorial	Anterior mesencephalic, lateral mesencephalic sulcus
Median supracerebellar infratentorial	Lateral mesencephalic sulcus, inferior brachial triangular zone, intercollicular, supracollicular, infracollicular
Extreme lateral supracerebellar infratentorial	Lateral mesencephalic sulcus, inferior brachial triangular zone, intercollicular, supracollicular, infracollicular

Two safe entry zones are available for resection of ventral midbrain pathology: the anterior mesencephalic zone (or the perioculomotor safe entry zone) and the interpeduncular safe entry zone. ¹⁸ Both of these safe entry zones may be approached using a modified orbitozygomatic approach or one of two minimally invasive approaches, the minisupraorbital or the transciliary supraorbital approach ( Fig. 12.3 ). ¹⁹ ^, ²⁰ With a larger craniocaudal extension of a deep lesion, a wider craniotomy is desirable, increasing the need to add an orbital osteotomy to the procedure. Alternatively, a third safe entry zone is available for lesions located laterally on the midbrain: the lateral route via the lateral mesencephalic sulcus provides access by either a subtemporal or an extreme lateral supracerebellar infratentorial (SCIT) approach ( Fig. 12.4, Fig. 12.5 ). ²¹ The routine use of subtemporal dissection has been avoided because this dissection necessitates temporal lobe retraction and risks injury to the vein of Labbé. The SCIT approach, using the lateral mesencephalic sulcus, is the preferred approach for managing lateral and dorsolateral midbrain pathologies with minimal risk to neurovascular structures, tracts, and nuclei.

**Fig. 12.3** The anterior mesencephalic zone (*dashed line*). **(a)** Anterior view of a cadaveric specimen showing the main neurovascular structures bounding the anterior mesencephalic zone (AMZ), namely the posterior cerebral artery (PCA) superiorly, the superior cerebellar artery (SCA) inferiorly, the oculomotor nerve (cranial nerve [CN] III) medially, and the projection of the main fibers of the corticospinal tract on the intermediate three-fifths on the peduncle laterally. Alternatively, to remove centromedian lesions in the mesencephalon, a small neurotomy may be placed between the mammillary bodies and the basilar apex, between the basilar perforators (the interpeduncular zone [IPZ]). **(b)** Surgical view provided by a right modified orbitozygomatic approach. After opening the sylvian fissure, the chiasmatic and carotid cisterns are opened wide; the dissection is taken laterally to the internal carotid artery. The oculomotor nerve is then followed through the interpeduncular cistern until it emerges at the brainstem surface. Abbreviations: A1, A1 segment of the anterior cerebral artery; AICA, anterior inferior cerebellar artery; BA, basilar artery; ICA, internal carotid artery; M1, M1 segment of the middle cerebral artery; P1, P1 segment of the PCA; P2A, anterior part of the P2 segment of the PCA; PCoA, posterior communicating artery. Reproduced with permission from Cavalcanti DD, Preul MC, Kalani MYS, Spetzler RF. Microsurgical anatomy of safe entry zones to the brainstem. J Neurosurg 2016;124:1359-1376.

**Fig. 12.4** Surgical windows provided by the sub-temporal approach to the brainstem. **(a)** Surgical demonstration of a left subtemporal craniotomy on a cadaver. A linear vertical incision is placed in front of the tragus. A bur hole is made just above the root of the zygomatic arch, and the craniotomy is tailored according to a preoperative plan targeting a lesion on the surface or one of two safe zones. Different angles of approach and areas of exposures are provided according to both the size and location of the craniotomy over the anteroposterior axis. The lower edge of the craniotomy should be flush with the middle fossa floor. **(b)** The dura is opened with its base positioned over the caudal edge of the craniotomy. **(c)** Microsurgical dissection is carried out between the temporal lobe and tentorium: below the temporal lobe and above the tentorium to the tentorial edge. Lumbar drainage of cerebrospinal fluid and extensive opening of the arachnoid membrane reduces the need for brain retraction. **(d)** The subtemporal approach provides a view of the anterior and the entire lateral incisural space, allowing inspection of the lateral midbrain. **(e)** This approach also provides a lateral view of the anterior mesencephalic zone (AMZ); orthogonal manipulation may cause injury to the tract of the oculomotor nerve (cranial nerve [CN] III). **(f)** Dividing the tentorium significantly enhances the exposure of the pontomesencephalic junction and the lateral upper pons. Tentorial division allows the surgeon to view the superior cerebellar artery (SCA) and the trochlear nerve (CN IV). **(g)** Progressive increases to a traditional subtemporal approach, in both area and length of exposure, after the addition of a transtentorial extension, and then after an anterior petrosectomy. Abbreviations: a., artery; ICA, internal carotid artery; LMS, lateral mesencephalic sulcus; P2A, anterior part of the P2 segment of the posterior cerebral artery; P2P, posterior part of the P2 segment of the posterior cerebral artery; PCoA, posterior communicating artery; PMJ, pontomesencephalic junction; post., posterior; quad., quadrangular; SCA, superior cerebellar artery; tent., tentorial. Reproduced with permission from Cavalcanti DD, Preul MC, Kalani MYS, Spetzler RF. Microsurgical anatomy of safe entry zones to the brainstem. J Neurosurg 2016;124:1359-1376.

**Fig. 12.5** The extreme lateral supracerebellar infratentorial (ELSCIT) approach is the main route to the posterolateral mesencephalic surface and its safe zones. Two surgical positions used to perform an ELSCIT, the semi-sitting **(a)** and the lateral **(b)** positions. The retroauricular incision is extended slightly cranially, allowing exposure of the transverse sinus; with slight retraction, this incision thus increases exposure of the cerebellomesencephalic fissure. **(c)** The dissection is carried out along the tentorial surface of the cerebellum to the limits of the posterior incisural space. Small bridging veins can be coagulated and divided close to the cerebellar surface to avoid avulsion from the tentorium. **(d)** The ELSCIT offers an oblique view of the quadrigeminal plate. A neurotomy over the inferior brachium triangular zone (IBTZ, dashed line) is depicted in this dissection. **(e)** A straightforward corridor to the lateral mesencephalic sulcus (LMS, *dashed line*) is possible with such an approach, retracting the superior cerebellar artery (SCA) and the trochlear nerve (cranial nerve [CN] IV). **(f)** Area of exposure provided by the ELSCIT approach, providing a wide view of the posterolateral mid-brain as well as the safe zones cited above (dashed line represents the LMS safe entry zone). **(g-n)** A 44-year-old woman presented with left hemiparesis and a contralateral oculomotor nerve (CN III) deficit. Axial **(g)** and coronal **(h)** T2-weighted magnetic resonance images show a large right lateral mesencephalic cavernous malformation. **(i)** Screenshot of image-guidance demonstrates the corridor through the ELSCIT used for resection of this lesion. **(j)** Surgical view of the cerebellomesencephalic fissure, after the quadrigeminal and ambient cisterns were opened. **(k)** The SCA and the trochlear nerve (CN IV) are dissected free and kept away from the resection field. **(l)** Final view of the microsurgical site through the LMS, depicting complete resection of the lesion. **(m)** Axial and **(n)** coronal 3-month follow-up magnetic resonance images reveal complete resection. Abbreviations: IC, inferior colliculus; SS, sigmoid sinus; TS, transverse sinus. **Fig. 12.5f** is reproduced with permission from Cavalcanti DD, Preul MC, Kalani MYS, Spetzler RF. Microsurgical anatomy of safe entry zones to the brainstem. J Neurosurg 2016;124:1359-1376.

Finally, the last four mesencephalic safe zones are accessible via the median, paramedian, or lateral SCIT approach, with selection being dictated by the long axis of the lesion. ²² The routine employment of the two-point method is strongly recommended to identify the optimal corridor to deep-seated lesions. ¹⁷

Anterior Mesencephalic Zone

Pathology involving the anterolateral midbrain can be accessed through an intricate area on the cerebral peduncle bounded medially by the intramesencephalic segment of the oculomotor nerve (CN III) and laterally by the corticospinal tract ( Fig. 12.3, Fig. 12.4g ). ³ This narrow corridor, also known as the perioculomotor zone, has an advantageous distribution of corticospinal tract fibers, which are mainly in the intermediate three-fifths of the peduncle. An additional benefit is that the red nucleus and the nigrostriatal circuit are located in a deep medial area. Inside the interpeduncular cistern, the superior limit of the entry point is the posterior cerebral artery and the inferior limit is the main trunk of the superior cerebellar artery.

Interpeduncular Zone

As an alternative to the anterior mesencephalic safe entry zone, the surgeon may make use of the sparse density of motor fibers in the middlemost one-fifth of the cerebral peduncle to enter the brainstem. ¹ In this approach, known as the interpeduncular zone, the oculomotor nerve (CN III) is again used to trace the path back to the brainstem, but instead of disconnecting the lateral arachnoid adhesions to the temporal lobe and tentorium to mobilize the oculomotor nerve (CN III), the surgeon should dissect the medial arachnoid adhesions of the oculomotor nerve (CN III) to allow it to be attached laterally. The surgeon then develops the narrow corridor between the internal carotid artery and the optic nerve to arrive between the mammillary bodies and the perforators from the top of the basilar artery. The brainstem is incised in the interpeduncular safe entry zone for resection of centromedian lesions. The choice of the approach is dependent on the relationship of the brainstem to the clivus and posterior clinoid and on where the lesion is closest to the surface of the brainstem.

Lateral Mesencephalic Sulcus

Beginning at the medial geniculate body, the lateral mesencephalic sulcus extends downward in a concave fashion to the pontomesencephalic sulcus, separating the peduncular and tegmental surfaces of the midbrain facing the middle incisural space ( Fig. 12.4, Fig. 12.5 ). ²³ The lateral mesencephalic vein is a helpful landmark, usually running along the sulcus. One study by Recalde et al ⁵ reported the mean total length of the sulcus as 9.6 mm (range 7.4–13.3 mm). Several arteries and nerves cross the sulcus: superiorly, surgeons encounter the posterior P2 segment (P2P); centrally, one encounters the medial posterior choroidal artery; and inferiorly, one encounters the cerebellomesencephalic segments of the superior cerebellar artery, trochlear nerve (CN IV), and tentorial edge. The entry zone is located between the substantia nigra anterolaterally and the medial lemniscus posteriorly. The mean working-channel length at this point is 8.0 mm (range 4.9–11.7 mm). The fibers of the oculomotor nerve (CN III) that cross from the red nucleus to the substantia nigra limit dissection anteromedially.

Intercollicular Region

Bricolo and Turazzi first suggested the use of the intercollicular region for resection of dorsal midbrain pathology. ³ ^, ⁹ The quadrigeminal plate or tectum comprises two superior rounded eminences (superior colliculi) and two inferior rounded eminences (inferior colliculi); these eminences represent the dorsal surface of the midbrain ( Fig. 12.1b ). The most appropriate area for a small neurotomy on the posterior surface of midbrain has been described as the intercollicular region, because of its sparseness of fibers ( Fig. 12.2c, Fig. 12.6 ).

**Fig. 12.6** Median supracerebellar infratentorial approach for posterior mesencephalic pathology. **(a)** Using the prone position with the head flexed, a cadaveric dissection demonstrates the linear medial incision from just above the external occipital protuberance down to the spinous process of the axis. The craniotomy should carefully expose the torcula and both transverse sinuses, allowing a wide operative view between the cerebellum and tentorium. **(b,c)** The vein of Galen, internal cerebral veins, and basal veins of Rosenthal occupy most of the field between the tentorium and anterior vermis, making the safe dissection and caudal exposure of the quadrigeminal plate challenging. **(d)** This microdissection reveals the safe entry zones on the tectum, namely the intercollicular region (ICR, *dashed line*), supracollicular zone (SCZ), infracollicular zone (ICZ), and the inferior brachium triangle zone (IBTZ). Abbreviations: IC, inferior colliculus; ICR, intercollicular region; mesenceph., mesencephalic; PCA, posterior cerebral artery; PG, pineal gland; SC, superior colliculus; v., vein. Reproduced with permission from Cavalcanti DD, Preul MC, Kalani MYS, Spetzler RF. Microsurgical anatomy of safe entry zones to the brainstem. J Neurosurg 2016;124:1359-1376.

The superior colliculi are part of a network of areas responsible for spatial attention. They play a major role in initiation and execution of saccadic eye movements and visual fixation. ²⁴ ^, ²⁵ A superior brachium connects them to each lateral geniculate body; retinotectal fibers run in this path. ²⁶ Spinotectal and corticotectal fibers lead to the superior colliculi, while tectospinal, tectothalamic, and tectocortical tracts leave these structures. The inferior colliculi are part of the auditory system. They receive fibers from the contralateral cochlear nucleus, dorsal and ventral nuclei of the lateral lemniscus, contralateral and ipsilateral superior olive, ipsilateral medial superior olive, and descending projections from sensory areas through the corticollicular neurons. The inferior colliculi are connected by commissural fibers; these colliculi extend laterally through the inferior brachium to the medial geniculate body of the thalamus, which projects to the primary auditory cortex.

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