Signal change of cerebrospinal fluid with eye drops of O-17-labeled saline

Abstract Purpose Aqueous humor drains fluid from the eye not only via the conventional pathway through the trabecular meshwork and Schlemm’s canal, but also within the eye is known to occur via pathways through the posterior chamber and optic nerve to the cerebrospinal fluid (CSF) surrounding the optic nerve. The mechanism is poorly understood, and non-invasive method for evaluation in living humans has not been established. We previously showed that eye drops containing O-17-labeled water (H 2 17 O) distribute in the anterior chamber but not the vitreous. This study aimed to evaluate the distribution of H 2 17 O in the CSF along the optic nerve. Methods Five ophthalmologically normal participants (20-31 years, all females) were selected from a previous prospective study based on 1 H MR images of the eyes that included the optic nerve. They received eye drops of 10 mol% H 2 17 O in their right eye. Dynamic image time series was created by normalizing the signal of each 1 H-T2WI by the pre-drop average signal. Region-of-interest analyses were performed for signal changes in the anterior chamber, vitreous, and CSF. Results In the quantitative evaluation, the normalized intensity in the anterior chamber and CSF was significantly lower than that in the pre-drop signal (anterior chamber: 0.78 ± 0.07, p < 0.005; CSF: 0.89 ± 0.07, p < 0.05). No distribution was identified in the vitreous. Qualitatively, the distribution of H 2 17 O in the anterior chamber was detected in all five participants and in the CSF of four participants (80%). Conclusion H 2 17 O eye drops were distributed in the anterior chamber and CSF, but not in the vitreous. These findings suggest that the visualization of aqueous humor outflow, not via the Schlemm’s canal, may contribute to ocular fluid homeostasis, including the ocular glymphatic system.