Microsurgical anatomy of safe entry zones to the brainstem

Abstract

H istorically, the surgical management of intrinsic brainstem lesions has been controversial. The surgical extirpation of focal gliomas, cavernous malformations, or hemangioblastomas within the brainstem has caused heated discussions in scientific meetings and the literature. In 1939, Bailey et al. 3 declared this subject to be a pessimistic chapter in neurosurgery; 30 years later, Matson and Ingraham 26 would still claim such lesions were inoperable. However, in 1971, Lassiter et al. 25 were among the first to advocate surgical intervention. By 1986, Epstein and McCleary reported that surgery was feasible with reasonable morbidity and mortality.15 Concurrent with Epstein and McCleary's report, Raimondi would rationally state that to have the child merely survive (i.e., with severe neurological deficits) is no justification for surgery. 33 The development and improvement of complex skull base surgical approaches and incremental advances in neuroimaging, parallel to image-guided surgery, allowed a few authors to safely and effectively resect lesions in the brainstem. 5,23,32Knowledge of different skull base exposures, gained through laboratory dissections, allows neurosurgeons to approach lesions in the brainstem. Nevertheless, the brainstem, roughly the size of the human thumb, contains a rich concentration of nuclei and fibers in a small sectional area, resulting in a high likelihood of morbidity after manipulation. Awareness of the main safe entry zones on the brainstem is key to reducing morbidity for any lesion that does not emerge to the pial or ependymal surface. Such zones represent entry points and trajectories where eloquent structures and perforators are sparse and where a neurotomy would cause the least possible damage.abbreviatioNs CN = cranial nerve; mini-OZ = mini-modified orbitozygomatic approach; PCA = posterior cerebral artery; P 2 P = posterior P 2 ; SCA = superior cerebellar artery; TAPS = transanterior perforating substance. obJective The aim of this study was to enhance the planning and use of microsurgical resection techniques for intrinsic brainstem lesions by better defining anatomical safe entry zones. methods Five cadaveric heads were dissected using 10 surgical approaches per head. Stepwise dissections focused on the actual areas of brainstem surface that were exposed through each approach and an analysis of the structures found, as well as which safe entry zones were accessible via each of the 10 surgical windows. results Thirteen safe entry zones have been reported and validated for approaching lesions in the brainstem, including the anterior mesencephalic zone, lateral mesencephalic sulcus, intercollicular region, peritrigeminal zone, supratrigeminal zone, lateral pontine zone, supracollicular zone, infracollicular zone, median sulcus of the fourth ventricle, anterolateral and posterior median sulci of the medulla, olivary zone, and lateral medullary zone. A discussion of the approaches, anatomy, and limitations of these entry zones is included. coNclusioNs A detailed understanding...

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