Abstract
Intracranial pressure (ICP) has been proposed to play an important role in the sensitivity to intraocular pressure (IOP) and susceptibility to glaucoma. However, the in vivo effects of simultaneous, controlled, acute variations in ICP and IOP have not been directly measured. We quantified the deformations of the anterior lamina cribrosa (ALC) and scleral canal at Bruch’s membrane opening (BMO) under acute elevation of IOP and/or ICP.
Four eyes of three monkeys were imaged in vivo with OCT under four pressure conditions: IOP and ICP either at baseline or elevated. The BMO and ALC were reconstructed from manual delineations. From these, we determined canal area at the BMO (BMO area), BMO aspect ratio and planarity, and ALC median depth relative to the BMO plane. To better account for the pressure effects on the imaging, we also measured ALC visibility as a percent of the BMO area. Further, ALC depths were analyzed only in regions where the ALC was visible in all pressure conditions. Bootstrap sampling was used to obtain mean estimates and confidence intervals, which were then used to test for significant effects of IOP and ICP, independently and in interaction.
Response to pressure manipulation was highly individualized between eyes, with significant changes detected in a majority of the parameters. Significant interactions between ICP and IOP occurred in all measures, except ALC visibility. On average, ICP elevation expanded BMO area by 0.17mm2 at baseline IOP, and contracted BMO area by 0.02 mm2 at high IOP. ICP elevation decreased ALC depth by 10μm at baseline IOP, but increased depth by 7 μm at high IOP. ALC visibility decreased as ICP increased, both at baseline (−10%) and high IOP (−17%). IOP elevation expanded BMO area by 0.04 mm2 at baseline ICP, and contracted BMO area by 0.09 mm2 at high ICP. On average, IOP elevation caused the ALC to displace 3.3 μm anteriorly at baseline ICP, and 22 μm posteriorly at high ICP. ALC visibility improved as IOP increased, both at baseline (5%) and high ICP (8%).
In summary, changing IOP or ICP significantly deformed both the scleral canal and the lamina of the monkey ONH, regardless of the other pressure level. There were significant interactions between the effects of IOP and those of ICP on LC depth, BMO area, aspect ratio and planarity. On most eyes, elevating both pressures by the same amount did not cancel out the effects. Altogether our results show that ICP affects sensitivity to IOP, and thus that it can potentially also affect susceptibility to glaucoma.
Research Highlights
- In vivo ONH deformations caused by acute, controlled, simultaneous changes in IOP and/or ICP can be directly visualized and measured in the monkey eye using OCT.
- Acute changes of either IOP or ICP significantly deformed both the scleral canal and the lamina cribrosa, regardless of the other pressure level.
- Pressures interacted, meaning that the effects of one pressure depended significantly on the level of the other pressure.
- Elevating both pressures did not cancel out the effects of one of them being elevated.
- Our results show that ICP affects sensitivity to IOP, and thus that it can potentially also affect susceptibility to glaucoma.
Competing Interest Statement
Proprietary Interest: J.S. Schuman receives royalties for intellectual property licensed by Massachusetts Institute of Technology to Zeiss. All other authors: Nothing to disclose
Footnotes
↵† Should be considered co-first authors.
Proprietary Interest: J.S. Schuman receives royalties for intellectual property licensed by Massachusetts Institute of Technology to Zeiss. All other authors: Nothing to disclose
Financial support: National Institutes of Health grants R01EY025011, R01EY013178, R01EY023966, R01EY022928, R01EY028662, T32-EY017271 and P30EY008098; Glaucoma Research Foundation Shaffer Grant; Eye and Ear Foundation of Pittsburgh, PA; Research to Prevent Blindness (Support to the Departments of Ophthalmology at the University of Pittsburgh, and at NYU).
Comprehensive revision from abstract to discussion. New figures.