Using SD-OCT, the authors evaluated the changes in optic nerve head morphology during IOP increase obtained with an ophthalmodynameter applied against the sclera in a large cohort of Chinese subjects with open-angle glaucoma, angle-closure glaucoma, or without glaucoma. In particular, two anatomical parameters - lamina cribosa depth and minimum rim width (MRW) - were evaluated, and the regional and global relationships with the visual field defects were calculated. They found a significant relationship between structure change in response to IOP increase and function only in open-angle glaucoma subjects: a larger MRW reduction and larger anterior displacement of the lamina cribosa were associated with worse visual field defects.
It would be interesting to evaluate in longitudinal studies the biomechanical response of the optic nerve head in progressing glaucomaI think that the present study adds some valuable evidence to the growing literature about the possible role of the biomechanical response of the lamina cribosa and optic nerve head to IOP variations in the pathophysiology of open-angle glaucoma. Clearly, many studies report a different response of the lamina cribosa and optic nerve morphology during IOP increase in subjects with glaucoma compared to healthy. It should be mentioned, however, that the studies performed to date show conflicting results about the direction and magnitude of the lamina cribosa displacement. Some studies have rather reported a posterior movement of the lamina cribosa during IOP increase in subjects with glaucoma. Also, Quigley et al. have reported a smaller lamina cribosa displacement in subjects with severe glaucoma compared to subjects with early glaucoma. Some differences in the imaging device used, in the population studied, type and stage of glaucoma, and also in the reference plane used to calculate the lamina cribosa displacement could explain the discrepancies. Particularly, it should be mentioned that the reference structure or plane used in those studies are also likely impacted by IOP variations, and thus we do not have any invariable anatomical landmark.
As mentioned by the authors, further studies could evaluate the diameter of the Bruch membrane or scleral canal opening during IOP increase, and correlate the opening diameter to the lamina cribosa displacement. It could be hypothesized that a large increase in opening diameter during IOP increase leads to anterior displacement, and vice-versa. Also, it could be interesting to evaluate in longitudinal studies the biomechanical response of the optic nerve head in subjects with progressing glaucoma or subjects with OHT that have developed glaucoma, compared to non-progressing glaucoma or OHT.