Feature Of The Week 05/28/2017: In Vivo 3-Dimensional Strain Mapping Confirms Large Optic Nerve Head Deformations Following Horizontal Eye Movements
Glaucoma is characterized by an irreversible damage of retinal ganglion cells within the optic nerve head (ONH) at the back of the eye. Currently we know that elevated intraocular pressure (IOP) is associated with increased prevalence of glaucoma but not all glaucoma patients have an elevated IOP. The biomechanical theory of glaucoma hypothesizes that elevated (or fluctuating) IOP deforms the ONH tissues, including the lamina cribrosa (LC), and that these deformations drive retinal ganglion cell injury and death. However, IOP is not the only load that can deform the ONH. Eye movements have recently been hypothesized to be able to deformed the optic nerve head but no studies have demonstrated this hypothesize in vivo.
In this study, we employed in vivo OCT imaging to measure the LC strains (deformations) following abduction and adduction in healthy human subjects and to compare them with those resulting from a relatively high acute intraocular pressure (IOP) elevation. Specifically, 16 eyes from 8 healthy subjects were included. For each subject, ONHs were imaged using spectral-domain OCT at baseline (twice), in different gaze positions (adduction and abduction of 20°) and following an acute IOP elevation of approximately 20 mmHg from baseline (via ophthalmodynamometry). Eye rotation was achieved by rotating the subject’s head while keeping the eye aligned with the fixed OCT objective. The subject’s head was fixed on a custom-built 3D-printed chin rest, which is rotatable and translatable. This chin rest allowed precise control of the head rotation magnitude to make sure that the eye rotation angles were accurately achieved. Each set of images (raster scan) comprised 97 serial horizontal B-scans (each composed of 384 A-scans) covering a rectangular region of 15°×10° centered on the ONH. LC strains for all loading scenarios were mapped using a 3D tracking algorithm.
In all 16 eyes, LC strains induced by adduction and abduction were 5.83%±3.78% and 3.93%±2.57%, respectively, and both significantly higher than the control strains measured from the repeated baseline acquisitions (p < 0.01). LC strains in adduction were on average higher than those in abduction but the difference was not statistically significant (p = 0.07). LC strains induced by IOP elevations (on average 21.13±7.61 mmHg) were 6.41%±3.21% and significantly higher than the control strains (p < 0.0005). Gaze-induced LC strains in the PPA group were on average larger than those in the non-PPA group, however, the relationship was not statistically significant.
Our results confirm that horizontal eye movements generate significant ONH strains. Further studies are needed to explore a possible link between ONH strains induced by eye movements and axonal loss in optic neuropathies.
For more information see recent Article. Courtesy Xiaofei Wang and Michaël J. A. Girard from National University of Singapore.