Surgical Anatomy of Endoscope-Assisted Approaches to Common Aneurysm Sites BACKGROUND: The endoscope is being introduced as an adjuvant to improve visu- alization of certain areas in open cranial surgery. OBJECTIVE: To describe the endoscopic anatomy of common aneurysm sites and to compare it with the microsurgical anatomy. METHODS: Pterional, anterior interhemispheric, and subtemporal approaches to the most common aneurysm sites were examined in cadaveric heads under the surgical microscope and with the endoscope. RESULTS: The endoscopic view, particularly with the angled endoscopes, provides a sig- nificant improvement compared with the microscopic view, especially for poorly visualized sites such as the medial aspect of the supraclinoid carotid artery and its branches, the area below the anterior perforated substance and optic tract, and the carotid and basilar bifurcations. The endoscope aided in the early visualization of perforating branches at each aneurysm site except the middle cerebral artery. Small-diameter optics (2.7 mm) provided greater space for dissection and less potential for tissue damage in narrow places, whereas the larger 4-mm diameter optics provided better visualization and less panoramic distor- tion. The positioning of the endoscope for each aneurysm site is reviewed. CONCLUSION: The endoscope provides views that complement or improve the microscopic view at each aneurysm site except the middle cerebral artery. Endoscopy training and a thorough knowledge of endoscopic vascular anatomy are essential to safely introduce endoscopic assistance in vascular surgery. KEY WORDS: Aneurysm, Endoscope, Endoscopic anatomy, Endoscopic assistance, Microsurgical anatomy. Operative Neurosurgery 10:121–144, 2014 DOI: 10.1227/NEU.0000000000000205 I mprovements in endoscopic techniques in neurosurgery have led to their being used not only for endonasal approaches to the cranial base but also as an adjuvant in open cranial surgery, 1-10 where they bring added light to and visualization of areas poorly visualized with a standard microscopic view. 7,8 The endoscope has been used to complement the microscope for vascular surgery. 9,11-14 The addition of the endoscope has led to smaller craniotomies, 15 reduced brain retraction, reduced manipulation of the aneurysm before clipping, improved visualization of perforating arteries, and improved assessment of the completeness of clipping and patency and flow after clip- ping. 9,11-14 However, the different perspective and view of vascular anatomy provided by straight and angled endoscopes require further study. The objectives of this study were to describe and compare the endoscopic and microsurgical anat- omy of the most common aneurysm sites, define areas and structures better visualized with the endoscope, aid in selecting the optimal position- ing and angulation of the endoscope for each aneurysm site, and examine the advantages and disadvantages of endoscopic assistance at each site. PATIENTS AND METHODS Sixteen sides of 8 formalin-fixed cadaveric heads in which the vessels were perfused with colored silicone were used for the dissections. A pterional craniotomy was performed and the dura, sylvian fissure, and basal cisterns were opened under the microscope. After microscopic and endoscopic examination via the pterional approach, the craniotomy was extended posteriorly to study the subtemporal approach. A bifrontal craniotomy was then completed to examine the anterior interhemispheric approach. The aneurysm sites examined included those on the internal carotid artery at its branches, the anterior Maria Peris-Celda, MD, PhD Leila Da Roz, MD Alejandro Monroy-Sosa, MD Takashi Morishita, MD Albert L. Rhoton, Jr, MD Department of Neurosurgery, University of Florida, Gainesville, Florida Correspondence: Albert L. Rhoton, Jr, MD, Department of Neurosurgery, University of Florida, College of Medicine, PO Box 100265, Gainesville, FL. E-mail: rhoton@neurosurgery.ufl.edu Received, May 21, 2013. Accepted, October 7, 2013. Published Online, November 13, 2013. Copyright © 2013 by the Congress of Neurological Surgeons. Surgical Anatomy and Technique OPERATIVE NEUROSURGERY VOLUME 10 | NUMBER 1 | MARCH 2014 | 121 Copyright © Congress of Neurological Surgeons. Unauthorized reproduction of this article is prohibited.