Why a One-Way Ticket to Mars May Result in One-Way Directional Glymphatic Flow to the Eye: Response

Letters to the Editor the eye. However, in long-duration space flight-induced increased ICP, paravenous outflow may fail to match ocular paraarterial inflow due to the reduction of the normal TLCPD. Ocular paravenous outflow may be completely impeded if there is reversal of the TLCPD. This may result in glymphatic stasis predominantly within the prelaminar region of the optic nerve head, which could contribute to the optic disc edema seen in astronauts. The accumulation of toxic metabolites due to glymphatic stasis then may cause further disc swelling. Although this mechanism is speculative, it is important to note that Denniston et al (6) provided support for the importance of the ocular glymphatic system in the pathogenesis of papilledema secondary to idiopathic intracranial hypertension (IIH). Using spectral domain optical coherence tomography in a cohort of patients with IIH, they demonstrated a number of structural differences not seen in healthy controls, including the presence of perivascular "black holes." This was observed both in the optic nerve head and within the retinal nerve fiber layer, and the authors hypothesized that this represents dilated ocular perivascular glymphatic channels. There seemed to be a relationship between the degree of papilledema and the extent of these changes. In healthy controls, the authors did not detect these features. Given these findings, we believe that our proposed theory deserves further study and may ultimately contribute to the field of space health research. Peter Wostyn, MD Department of Psychiatry, PC Sint-Amandus, Beernem, Belgium Hanspeter Esriel Killer, MD Department of Ophthalmology, Kantonsspital Aarau, Aarau, Switzerland Why a One-Way Ticket to Mars May Result in One-Way Directional Glymphatic Flow to the Eye: Response W e greatly appreciate the opportunity to respond to the comments of Dr. Wostyn et al. Their theory focuses on a glymphatic flow imbalance mechanism at the optic nerve head that may, at least partially, explain the development of optic disc swelling in astronauts during long-duration space flight. This hypothesis would still be dependent on a space-flight induced rise in cerebrospinal fluid (CSF) pressure by some mechanism within the subarachnoid space (SAS) surrounding the optic nerve. Perhaps the effects of increased intracranial pressure, Letters to the Editor: J Neuro-Ophthalmol 2017; 37: 458-465 Peter Paul De Deyn, MD, PhD Laboratory of Neurochemistry and Behavior, Institute Born-Bunge, University of Antwerp, Department of Biomedical Sciences, Antwerp, Belgium Department of Neurology and Alzheimer Research Center, University of Groningen and University Medical Center Groningen, Groningen, the Netherlands Department of Neurology and Memory Clinic, Middelheim General Hospital (ZNA), Antwerp, Belgium The authors report no conflicts of interest. REFERENCES 1. Mader TH, Gibson CR, Otto CA, Sargsyan AE, Miller NR, Subramanian PS, Hart SF, Lipsky W, Patel NB, Lee AG. Persistent asymmetric optic disc swelling after long-duration space flight: implications for pathogenesis. J Neuroophthalmol. 2017;37:133-139. 2. Iliff JJ, Wang M, Liao Y, Plogg BA, Peng W, Gundersen GA, Benveniste H, Vates GE, Deane R, Goldman SA, Nagelhus EA, Nedergaard M. A paravascular pathway facilitates CSF flow through the brain parenchyma and the clearance of interstitial solutes, including amyloid b. Sci Transl Med. 2012;4:147ra111. 3. Hu P, Arfuso F, Madigan MC, Adamson S, Shaw LC, Boulton ME, Grant MB, Chan-Ling T. Evidence for a glymphatic system in human, primate, rat and mouse retina. Invest Ophthalmol Vis Sci. 2016;57. ARVO E-Abstract 996. 4. Wostyn P, Killer HE, De Deyn PP. Glymphatic stasis at the site of the lamina cribrosa as a potential mechanism underlying open-angle glaucoma. Clin Exp Ophthalmol. 2017;45:539-547. 5. Berdahl JP, Allingham RR. Intracranial pressure and glaucoma. Curr Opin Ophthalmol. 2010;21:106-111. 6. Denniston AK, Keane PA, Aojula A, Sinclair AJ, Mollan SP. The ocular glymphatic system and idiopathic intracranial hypertension: author response to "Hypodense holes and the ocular glymphatic system". Invest Ophthalmol Vis Sci. 2017;58:1134-1136. sequestration of CSF within the SAS of the optic nerve or a combination of the 2, acting in conjunction with the authors' 2 proposed mechanisms, may produce varying degrees of optic disc swelling. It also is possible that the contribution of this mechanism to disc swelling is a relatively constant low magnitude finding that occurs with even a small change in the translaminar pressure difference. Continued examination of all space flight crew members, including those without clinically apparent optic disc swelling, may provide additional insight regarding the relative contribution of glymphatic stasis. More measurements of opening pressure on lumbar puncture are needed in astronauts completing longduration space flights. These data, in conjunction with 463 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Letters to the Editor future preflight, in flight and postflight analysis of the optic discs and choroid by optical coherence tomography as well as ultrasound and MRI examination of the globe and optic nerve sheaths, may provide important information for analysis. Thomas H. Mader, MD, COL(R) US Army, Cooper Landing, Alaska C. Robert Gibson, OD Coastal Eye Associates, Webster, Texas Christian A. Otto, MD Weill-Cornell Medical College, New York, New York Ashot E. Sargsyan, MD KBRWyle, Houston, Texas Neil R. Miller, MD Wilmer Eye Institute, Baltimore, Maryland Contiguous Silicone Oil Migration From the Vitreous Cavity to the Optic Tract W e would like to applaud Boren et al (1) for their article "Retrolaminar migration of silicone oil." We evaluated a patient who adds to the spectrum of neuroimaging findings of this disorder. A 92-year-old man with a longstanding history of glaucoma underwent vitrectomy with silicone oil endotamponade in his right eye for a retinal detachment 3 years previously. Because of transient right hand weakness and gait ataxia, brain MRI was performed. This showed hyperdense material involving the right optic nerve, optic chiasm, and right optic tract (Fig. 1). Automated visual fields showed changes consistent with advanced glaucoma, but a homonymous defect was not detected. We are unaware of previous reports of optic tract involvement with silicone oil. Perimetry showed that it did not cause homonymous visual field loss, supportive of the 2 asymptomatic cases reported by Boren et al. Given that the mechanism of intracranial silicone oil migration is not yet fully understood, future postmortem histopathological examination of the eyes and 464 Prem S. Subramanian, MD, PhD Department of Ophthalmology, University of Colorado, Aurora, Colorado Stephen F. Hart, MD Life Sciences Division, NASA, Houston, Texas William Lipsky, MD Coastal Eye Associates, Webster, Texas Nimesh B. Patel, OD, PhD Department of Vision Science, University of Houston, Houston, Texas Andrew G. Lee, MD Department of Ophthalmology, Methodist Hospital, Houston, Texas The authors report no conflicts of interest. brain will be essential in better understanding this disorder. Samantha Bobba, MD Prince of Wales Hospital, Sydney, Australia Olivia J.K. Fox, MBBS Mitchell B. Lee, MBBS Nepean Hospital, Sydney, Australia Richard Parker, MBBS Department of Ophthalmology, Save Sight Institute, Sydney Eye Hospital, Sydney, Australia Neera S. Jain, MD Prince of Wales Hospital, Sydney Alison K. Semmonds, BMed FRACP St Vincent's Private Hospital, Lismore, Australia Letters to the Editor: J Neuro-Ophthalmol 2017; 37: 458-465 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited.