LAUSR

Dear Editor, We read with great interest the letter by Cebula et al. [2] entitled BSubarachnoidal migration of intraocular silicone oil^ published recently inActa Neurochirurgica. We are grateful to the authors for sharing their findings with the scientific community and would appreciate the opportunity to comment on an issue raised by the authors. In their letter, Cebula et al. [2] describe imaging findings of silicone migration into both the posterior chamber of the eye and the subarachnoid space (including intraventricular spread) after intraocular injection in two patients treated for complex retinal detachment. In both patients, silicone spreading from the eye into the subarachnoid space following the optic nerve and chiasm was visible on imaging [2]. The authors further speculated that this migration was possibly eased by elevated intraocular pressure or by the presence of congenital anatomical abnormalities allowing the passage from the vitreous space into the subarachnoid space [2]. Here, we propose a possible alternative explanation. In 2012, the ‘glymphatic system’ was discovered for the first time in mice by Iliff et al. [4]. Their findings suggested a brain-wide network of paravascular pathways along which a large proportion of subarachnoid cerebrospinal fluid (CSF) recirculates through the brain parenchyma, facilitating the clearance of interstitial solutes, including amyloid-β (Aβ), from the brain [4]. CSF enters the brain along para-arterial channels to exchange with interstitial fluid, which is in turn cleared from the brain along para-venous pathways [4]. One implication of these findings is that glymphatic pathway dysfunction may contribute to the deficient Aβ clearance in Alzheimer’s disease [7]. Until recently, a glymphatic system in the eye and the optic nerve remained a hypothesis only and was based primarily on extrapolation of the findings in the brain and indirect observations [3, 6]. It is only now that studies are providing the first direct evidence for the existence of an ‘ocular glymphatic system’. In a recently published paper [5], we presented supporting preliminary data from a post-mortem study. We examined cross-sections of human optic nerves by light microscopy after injecting India ink into the subarachnoid space of the optic nerve [5]. Intriguingly, the cases studied showed a very striking accumulation of India ink in the paravascular spaces around the central retinal artery and vein, whereas the lumens of these vessels and the surrounding axons remained unlabelled [5]. Furthermore, a number of studies, providing new evidence for the presence of glymphatics in the optic nerve and retina, have been presented at ophthalmology conferences and are underway. If further work confirms the presence of an ‘ocular glymphatic system’, then this could have * Peter Wostyn [email protected]