2 Safe Entry Zones

Figure 2.1. (a) Anterior view. The cerebrospinal cisterns provide natural pathways to the brainstem. (b) Axial view. These pathways can be reached using skull base approaches (arrows), judicious subarachnoid dissection, and release of cerebrospinal fluid. Lesions should be approached where they abut a pial or an ependymal surface to minimize injury to critical neurovascular bundles.

Figure 2.1. (c) Axial view. When a lesion does not abut a pial or an ependymal surface, the surgeon should use the path that is least likely to disturb eloquent neural structures. The two-point method can assist with selection of the approach both in cases of deep lesions and in cases where the lesion abuts a pial or an ependymal surface. To perform the two-point method, first place a mark (point A) at the center of the lesion, then place a second mark (point B) where the lesion is closest to a surface of the brainstem or at the least eloquent entry point. Next draw a line from point A to point B and ultimately to the surface of the skull. The approach is dictated by this line (arrows). (d) Anterolateral view. Pathology can displace critical nuclei and tracts, which could nullify safe entry into the surgical zones. For example, the peritrigeminal safe entry zone (blue oval) enables the surgeon to access lesions in the lateral pons, but a hemorrhage or tumor can displace tracts so that entry via this safe entry zone could compromise neurologic function.

Figure 2.2. (a) Anterior view. Ventral midbrain lesions can be accessed using any of the anterior or anterolateral approaches through the cerebral peduncle via the anterior mesencephalic (also known as the perioculomotor) safe entry zone (blue oval). This safe entry zone is located lateral to the oculomotor nerve (CN III) between the posterior cerebral artery and the superior cerebellar artery. An alternative ventral safe entry zone is the interpeduncular safe entry zone (dotted line). (b) Axial view. The anterior mesencephalic safe entry zone (dashed line) is bordered medially by the oculomotor tract and nerve, laterally by the corticospinal tract, superiorly by the posterior cerebral artery, and inferiorly by the superior cerebellar artery. The interpeduncular safe entry zone (dotted line) is bordered laterally by the oculomotor nerves.

Figure 2.2. (c) Axial view. The anterior mesencephalic safe entry zone (dashed line) makes use of the distribution of the corticospinal tract fibers in the middle three-fifths of the cerebral peduncle and the fact that the red nucleus and nigrostriatal circuit are in a deep location. The interpeduncular safe entry zone (dotted line) makes use of the sparse nature of critical fibers in this region, but the proximity to the basilar artery and the variability of distribution of the basilar perforators can make this safe entry zone challenging to use. (d) The anterior mesencephalic safe entry zone (dashed line) is bound superiorly by the posterior cerebral artery and inferiorly by the superior cerebellar artery.

Figure 2.2. (e) The interpeduncular safe entry zone (dotted line) can be used for select lesions that are located ventrally and midline to the oculomotor nerve nucleus.

Figure 2.3. (a) Posterolateral view. Extending from the medial geniculate body to the pontomesencephalic sulcus, the lateral mesencephalic sulcus (blue line) is another safe entry zone into the midbrain. The lateral mesencephalic sulcus is often hidden by the lateral mesencephalic vein. The sulcus separates the peduncular and tegmental surfaces of the midbrain facing the middle incisural space.