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Editors Selection IGR 11-2

Intraocular Pressure: Corneal hysteresis, CCT and ONH surface compliance

Comment by Claude Burgoyne on:

21753 Corneal hysteresis but not corneal thickness correlates with optic nerve surface compliance in glaucoma patients, Wells AP; Garway-Heath DF; Poostchi A et al., Investigative Ophthalmology and Visual Science, 2008; 49: 3262-3268

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In this cross-sectional study performed on the right eye of 100 patients (38 with glaucoma, 62 without glaucoma), Wells et al. (939), measured a series of ocular parameters including corneal hysteresis (CH) and corneal thickness (CT), then performed optic nerve head surface compliance testing by acquiring a single HRT mean image (mean of n = 3 images) at actual IOP, and then a second HRT mean image following acute (30-second) IOP elevation induced by a modified LASIK suction ring. IOP was measured at actual pressure, but only in about half of the normal and glaucomatous eyes at high IOP. For each eye, the mean cup depth was calculated within the low and high-IOP image using HRT software and change in optic disc depth during IOP elevation was calculated as a single value. Both CH (expressed in mmHg) and CCT were lower in the glaucoma group (8.8 mmHg and 532 microns) than in the controls (9.6 mmHg, P = 0.012; 551 microns, P = 0.011, respectively). In those in which it was measured, there was no difference between the amount of IOP elevation between the two groups (P = 0.41), and the average change in mean cup depth between following IOP elevation was 29.8 microns in glaucoma and 36.1 microns in the controls; P = 0.5). Among all patients considered together, CH was correlated with mean cup depth increase (P = 0.032). This relationship persisted among the glaucoma patients only (P = 0.032), but not among the control patients when considered individually. There are many levels on which these data should be interpreted cautiously. Firstly, corneal hysteresis is at present a manifestation of some aspect of the material properties of the cornea, but what those properties are remains unclear. Secondly, optic disc surface compliance is also, at present, a manifestation of some aspect of the compressibility of the prelaminar neural tissues combined with the material properties of the lamina and the peripapillary sclera. Which of those dominate in an individual eye also remains unclear. An alternative interpretation of these data might conclude that hysteresis was lower in the glaucoma eyes compared to the normals on the basis of the thickness differences rather than biomechanical alteration. The fact that there was no statistically significant difference in the magnitude of acute IOP-induced surface deformation between groups, suggests no measurable alteration of ONH biomechanics in the glaucoma eyes. The fact that the CH only weakly correlated with ONH surface deformation in the glaucoma eyes and not with the normals, may not hold up in larger populations in which the effects of age and severity of glaucoma damage are more rigorously assessed. The clinical estimation of optic nerve head biomechanical risk factors for glaucomatous damage at all levels of IOP is an important subject. The authors are to be commended for making a very serious attempt at studying the potential relationship between corneal and optic nerve head biomechanics in normal and glaucomatous human patients. The study is well presented and the existing literature is well discussed. The study's weaknesses are carefully noted and their implications for the robustness of these findings beyond these patients are acknowledged. The present study paves the way for next-generation testing of both the cornea and the ONH neural and connective tissues which may achieve more direct measures of corneal material properties and should utilize spectral domain OCT imaging of the anterior laminar and peripapillary scleral surface and scleral portion of the neural canal, at IOP elevations of longer duration and in the lower end of the physiologic range (where deformation per change in IOP will likely be greatest due to tissue non-linearity).

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