CLINICAL ARTICLE T emporary or permanent perioperative visual loss (POVL) or visual disturbance is an infrequent but serious adverse event primarily reported in patients undergoing spine surgery in the prone position. The rate of POVL ranges between 0.03% and 0.28% depending on the surgical procedure. 18,32,37 Currently, the pathomechanism of POVL is a matter of debate, but 3 causative connec- tions are favored by most studies, namely, ischemic optic neuropathy (ION), central retinal artery occlusion, or cor- tical blindness. 3,18,20 ION may occur as either anterior ION, defned as ischemic damage of the optic nerve before it enters the lamina cribrosa sclerae, or posterior ION, de- fned as an ischemic event of the optic nerve behind the lamina cribrosa sclerae. 22 ION has been observed to be ABBREVIATIONS BP = blood pressure; ION = ischemic optic neuropathy; IOP = intraocular pressure; POVL = perioperative visual loss. SUBMITTED December 11, 2017. ACCEPTED March 19, 2018. INCLUDE WHEN CITING Published online August 24, 2018; DOI: 10.3171/2018.3.JNS173098. * P.C. and T.K. contributed equally to this work. Intraocular pressure during neurosurgical procedures in context of head position and loss of cerebrospinal fuid *Patrick Czorlich, MD, 1 Theresa Krätzig, MD, 1 Nikolas Kluge, 1 Christos Skevas, MD, 2 Volker Knospe, MD, 2 Martin Stephan Spitzer, MD, 2 Marc Dreimann, MD, 3 Klaus Christian Mende, MD, 1 Manfred Westphal, MD, 1 and Sven Oliver Eicker, MD 1 Departments of 1 Neurosurgery, 2 Ophthalmology, and 3 Trauma, Hand and Reconstructive Surgery, University Medical Center Hamburg-Eppendorf, Hamburg, Germany OBJECTIVE Perioperative visual loss (POVL) is a rare but serious complication in surgical disciplines, especially in spine surgery. The exact pathophysiology of POVL remains unclear, but elevated intraocular pressure (IOP) is known to be part of it. As POVL is rarely described in patients undergoing intracranial or intradural surgery, the aim of this study was to investigate the course of IOP during neurosurgical procedures with opening of the dura mater and loss of CSF. METHODS In this prospective, controlled trial, 64 patients fell into one of 4 groups of 16 patients each. Group A in- cluded patients undergoing spine surgery in the prone position, group B patients had intracranial procedures in the prone position, and group C patients were treated for intracranial pathologies in a modifed lateral position with the head rotated. In groups A–C, the dura was opened during surgery. Group D patients underwent spine surgeries in the prone position with an intact dura. IOP was measured continuously pre-, peri-, and postoperatively. RESULTS In all groups, IOP decreased after induction of anesthesia and increased time dependently after fnal po- sitioning for the operation. The maximum IOP in group A prior to opening of the dura was 28.6 ± 6.2 mm Hg and de- creased to 23.44 ± 4.9 mm Hg directly after dura opening (p < 0.0007). This effect lasted for 30 minutes (23.5 ± 5.6 mm Hg, p = 0.0028); after 60 minutes IOP slowly increased again (24.5 ± 6.3 mm Hg, p = 0.15). In group B, the last mea- sured IOP before CSF loss was 28.1 ± 5.0 mm Hg and decreased to 23.5 ± 6.1 mm Hg (p = 0.0039) after dura opening. A signifcant IOP decrease in group B lasted at 30 minutes (23.6 ± 6.0 mm Hg, p = 0.0039) and 60 minutes (23.7 ± 6.0 mm Hg, p = 0.0189). In group C, only the lower eye showed a decrease in IOP up to 60 minutes after loss of CSF (open- ing of dura, p = 0.0007; 30 minutes, p = 0.0477; 60 minutes, p = 0.0243). In group D (control group), IOP remained stable throughout the operation after the patient was prone. CONCLUSIONS This study is the frst to demonstrate that opening of the dura with loss of CSF during neurosurgical procedures results in a decrease in IOP. This might explain why POVL predominantly occurs in spinal but rarely in intra- cranial procedures, offers new insight to the pathophysiology of POVL, and provides the basis for further research and treatment of POVL. German Clinical Trials Register (DRKS) no.: DRKS00007590 (drks.de) https://thejns.org/doi/abs/10.3171/2018.3.JNS173098 KEYWORDS intraocular pressure; neurological surgery; prone position; supine position; perioperative visual loss; spine surgery J Neurosurg August 24, 2018 1 ©AANS 2018, except where prohibited by US copyright law