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Elias Pavlatos, Xueliang Pan, Keyton Clayson, Richard T Hart, Paul Weber, Jun Liu; ONH Deformation in Human Eyes Using Ultrasound Speckle Tracking. Invest. Ophthalmol. Vis. Sci. 2017;58(8):3154.
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© ARVO (1962-2015); The Authors (2016-present)
To map and quantify the deformation of the human ONH and peripapillary (pp) sclera in response to intraocular pressure (IOP) elevation.
Eight human globes were tested within 36 hours postmortem. The optic nerve was trimmed to the outer surface of the pp sclera. The globes were held in place using a custom-built holder and immersed in 0.9% saline. Two 20G needles were inserted into the anterior chamber, one connected to a programmable syringe pump (PHD Ultra, Harvard Apparatus) to control IOP and the other to a pressure sensor (P75, Harvard Apparatus) to monitor IOP. The globes were preconditioned with 20 pressure cycles from 5 to 30 mmHg then equilibrated at 5 mmHg for 30 minutes. Inflation tests were performed from 5 to 30 mmHg with 0.5 mmHg steps every 15 seconds. Ultrasound images 8 mm in width were obtained along the nasal-temporal meridian of the ONH at each step (Vevo 660, VisualSonics). An ultrasound speckle tracking algorithm calculated vertical displacements and compressive strains (Tang & Liu, J Biomech Eng 2012, 134(9)).
The ONH showed nonlinear posterior (vertical) displacements during inflation with average magnitudes of 71.83 ± 37.48 µm at 15 mmHg and 120.23 ± 64.76 µm at 30 mmHg for all eyes. The ONH had larger posterior displacements compared to the pp sclera (Fig 1c). A consistent gradient of posterior displacements resulted in a nearly uniform layer of compressive strain (Fig 1d). The compressive strains at 15 and 30 mmHg were -0.0075 ± 0.0067 and -0.0218 ± 0.0146 respectively. At 15 mmHg, the anterior half of the ONH experienced larger displacements (64.29 µm vs 62.43 µm, p = 0.018) and strains (-0.0138 vs -0.0045, p = 0.001) compared to the posterior half (Fig 2). The same trend was observed at 30 mmHg.
Ultrasound speckle tracking revealed posterior displacement and through-thickness compression of the ONH and pp sclera during inflation. The largest deformations occurred in the anterior ONH, likely corresponding to the prelaminar neural tissue. The pp sclera had much smaller posterior displacements, but similar compressive strains (i.e. displacement gradients). These new findings may offer insight into the biomechanical mechanisms of glaucoma pathophysiology.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
Fig 1. Ultrasound image of ONH (a), anterior/posterior layers (b), displacement (c) and strain (d) color maps.
Fig 2. Average vertical displacement (a) and compressive strain (b) during inflation (n=8).
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