Brainstem surgery has some important disadvantages compared with surgeries performed on other parts of the brain: 1) The surgical corridor is longer, 2) the surgical field of view is cone shaped and gradually narrows, 3) the incision must necessarily be minimal, and 4) there is no or limited possibility of using a static retractor in surgery. The most important disadvantage is the presence of the high density of critical white matter tracts and cranial nerve nuclei in this region. For all these reasons, the brainstem has been surgically considered as "terra incognita" (no man's land) throughout decades. After the 1990s, brainstem surgery began to be performed relatively safely with advances in neurosurgical techniques, modern neuroimaging methods, and a somewhat better understanding of the intrinsic structure of the brainstem.1, 2, 3, 4, 5

Complications in brainstem surgery mostly occur during resection of the lesion (cavernoma or tumor) and may result in severe cranial nerve dysfunctions, motor/somatosensory symptoms, cerebellar symptoms, and even death. For this reason, the neurosurgeon must know well the intrinsic structures of the brainstem in detail. For a better understanding of this complex anatomy, we schematically drew the axial sections of the brainstem showing the intrinsic structures at the level of 9 safe entry zones (SEZs) that we used, taking into account basic neuroanatomy books and atlases.6, 7, 8, 9, 10, 11, 12 Some illustrations are supported with intraoperative pictures to provide better surgical orientation. These illustrations are quite rare in the neurosurgical literature and may be helpful young neurosurgeons. In addition, clinical symptoms that may occur due to surgical injury to delicate structures in SEZs were also reviewed in detail, considering basic neurology books.13, 14, 15, 16, 17, 18, 19, 20, 21

In brainstem surgery, the brainstem surface is sometimes entirely normal with no discoloration and no dark-blue area indicating the bulging cavernoma/tumor or hematoma. For these cases, some SEZs into the brainstem have been described (Figure 1A and B). These zones represent entry points and trajectories where tracts and cranial nerve nuclei are sparse.22, 23, 24 If brainstem surgery is performed through these corridors, the possibility of neurologic deficit is not eliminated but is minimized. However, it should be noted that there are no completely SEZs to the brainstem. In addition, the described SEZs may “no longer be safe” secondary to distortion of the normal anatomy by the lesion itself or hematoma.

Therefore despite all care and attention during surgery, some neural structures may be damaged. Possible clinical symptoms due to surgical injury for each SEZ are presented in Table 1.

To our knowledge, 21 SEZs have been described so far to access brainstem lesions.26 To date, we have used 9 of these SEZs in nearly 50 brainstem surgeries (mostly cavernoma) and would like to emphasize the importance of their intrinsic structures in light of current literature information.