The Impact of Choroidal Swelling on Optic Nerve Head Deformation

Abstract

Purpose : Previous work has shown that finite element (FE) models provide vital insight for understanding the impact of intraocular pressure (IOP) and intracranial pressure (ICP) on optic nerve head (ONH) deformation. Here, we expand on these FE models to investigate the biomechanical effects of choroidal swelling on the ONH, as such swelling may influence ONH mechanical strain.

Methods : We extended a recent model of the posterior eye (Feola et al., IOVS, 2016) to include Bruch’s membrane and the choroid. The choroid was represented as a mixture material with a linear-elastic solid matrix and a Donnan equilibrium component to simulate swelling. The sclera, peripapillary sclera, annular ring, pia and dura were modeled as a neo-Hookean matrices with embedded collagen fibers, while the neural tissues and lamina cribrosa (LC) were modeled as linear-elastic. We assumed an IOP=15 mmHg, ICP=10 mmHg, and a mean arterial pressure=86 mmHg as our baseline condition, and imposed a 5 uL increase in choroidal volume, estimated to occur over a cardiac cycle. The computed strain field were decomposed into three principal strains, with the 1^st and 3^rd components representing tension and compression, respectively. Our outcome measures were the peak 1^st and 3^rd principal strains in the LC and prelaminar neural tissue (PLNT), since cells are sensitive to mechanical strain.

Results : Choroidal swelling altered the strain distributions in the LC and PLNT (Fig), causing a small increase in the peak 1^st (0.91% to 1.1%) and 3^rd (-0.89% to -1.14%) principal strain magnitudes within the LC. The effect of choroidal swelling on strain magnitudes in the PLNT was more pronounced: peak 1^st principal strain increased from 0.67% to 2.77%, and peak 3^rd principal strain changed from -1.69 to -2.71%.

Conclusions : Choroidal swelling had an impact on strain magnitudes in the LC, although not to the same degree as elevating IOP to 30 mmHg which led to peak 1^st and 3^rd principal strains of 1.45% and -1.89%. However, choroidal swelling considerably increased peak strain magnitudes in the PLNT. Therefore, prolonged or higher degrees of choroidal swelling may initiate a mechanobiological response, highlighting the potential impact of choroidal swelling on biomechanical strains in the ONH.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.

 

Contour plots of the 1^st (left column) and 3^rd (right column) principal strains in the ONH. Top panel: baseline condition; Bottom panel: 5 uL of choroidal swelling.