J Neurosurg Volume 124 • January 2016 248 laboratory iNvestigatioN J Neurosurg 124:248–263, 2016 R ECENT advances in microsurgical and endoscopic techniques, imaging, and electrophysiological mon- itoring have facilitated safe resection of brainstem lesions that were previously considered inoperable. 3–5,9,11,12, 14–16,19,21,23–26,37,38,40,44,62,64,65,70,73,75,79,86 The brainstem surface, when near to or accessed by the lesion, is the shortest and most direct path for surgical treatment. 3,5,9,14,21,64,75,79 Several safe entry zones have been proposed and used for lesions inside the brainstem. 3,5,9,12,13,23,24,40,65,86 To maximize the chances of safe and precise removal of these lesions, suff- cient exposure of the brainstem surface is critical, as is se- lection of an appropriate entry corridor into the brainstem. Cerebral fssures, such as the sylvian fssure, are rou- tinely opened in the supratentorial region to access deeply situated pathology without dividing any neural tissue. Fis- sure dissection has also been used in the infratentorial re- gion. 22,49,53,60,61 Opening the arachnoid membranes and tra- beculae along the cerebellar-brainstem fssures, as in the telovelar or transcerebellomedullary fssure approaches, was originally proposed to access the pineal region, cra- nial nerve (CN) V, and the fourth ventricle. 22,49,60 However, brainstem surgery frequently requires the opening of the 3 cerebellar-brainstem fssures and/or adjacent cerebellar fssures to expose the cerebellar peduncles and brainstem surface hidden by the parts of the cerebellum forming the walls of the 3 cerebellar-brainstem fssures. 61,71 Most of the major cerebellar arteries, veins, and vital neural structures, including a majority of the CNs and all 3 cer - ebellar peduncles, are located inside or close to these fs- sures. 27,43,45,56,66,68,85 Detailed knowledge of these fssures is abbreviatioNs AICA = anterior inferior cerebellar artery; CN = cranial nerve; PCA = posterior cerebral artery; PICA = posterior inferior cerebellar artery; SCA = superior cerebellar artery. submitted November 26, 2014. accepted February 3, 2015. iNclude wheN citiNg Published online August 14, 2015; DOI: 10.3171/2015.2.JNS142707. Anatomy and approaches along the cerebellar-brainstem fssures Ken matsushima, md, 1,2 Kaan yagmurlu, md, 1 michihiro Kohno, md, phd, 2 and albert l. rhoton, Jr., md 1 1 Department of Neurological Surgery, University of Florida, Gainesville, Florida; and 2 Department of Neurosurgery, Tokyo Medical University, Tokyo, Japan obJective Fissure dissection is routinely used in the supratentorial region to access deeply situated pathology while minimizing division of neural tissue. Use of fssure dissection is also practical in the posterior fossa. In this study, the microsurgical anatomy of the 3 cerebellar-brainstem fssures (cerebellomesencephalic, cerebellopontine, and cerebel- lomedullary) and the various procedures exposing these fssures in brainstem surgery were examined. methods Seven cadaveric heads were examined with a microsurgical technique and 3 with fber dissection to clarify the anatomy of the cerebellar-brainstem and adjacent cerebellar fssures, in which the major vessels and neural struc- tures are located. Several approaches directed along the cerebellar surfaces and fssures, including the supracerebellar infratentorial, occipital transtentorial, retrosigmoid, and midline suboccipital approaches, were examined. The 3 heads examined using fber dissection defned the anatomy of the cerebellar peduncles coursing in the depths of these fs- sures. results Dissections directed along the cerebellar-brainstem and cerebellar fssures provided access to the posterior and posterolateral midbrain and upper pons, lateral pons, foor and lateral wall of the fourth ventricle, and dorsal and lateral medulla. coNclusioNs Opening the cerebellar-brainstem and adjacent cerebellar fssures provided access to the brainstem surface hidden by the cerebellum, while minimizing division of neural tissue. Most of the major cerebellar arteries, veins, and vital neural structures are located in or near these fssures and can be accessed through them. http://thejns.org/doi/abs/10.3171/2015.2.JNS142707 Key words brainstem; cerebellum; cerebellomedullary fissure; cerebellomesencephalic fissure; cerebellopontine fissure; microsurgical anatomy ©AANS, 2016