Corneal hysteresis but not corneal thickness correlates with optic nerve surface compliance in glaucoma patients.
OBJECTIVE
To investigate relationships between acute intraocular pressure (IOP)-induced optic nerve head surface deformation and corneal hysteresis and thickness in glaucomatous and nonglaucomatous human eyes.


METHODS
This was a prospective experimental study of 100 subjects (38 with glaucoma, 62 without glaucoma). Data collected included spherical equivalent, optic disc diameter, central corneal thickness (CCT), axial length, cylinder, Goldmann IOP, Pascal IOP, and ocular pulse amplitude and ocular response analyzer (ORA) measurements of corneal hysteresis (CH). Elevation of IOP was induced in the right eye of each subject with a modified LASIK suction ring to an average of 64 mm Hg for less than 30 seconds. Heidelberg Retina Tomography II (HRT) was used to map the optic nerve surface before and during IOP elevation. Mean cup depth was calculated using built-in HRT data analysis software. Change in optic disc depth during IOP elevation was calculated for all right eyes, and tests for correlation with the parameters listed were performed.


RESULTS
Both CH and CCT were lower in the glaucoma group (8.8 mm Hg and 532 microm) than in the control group (9.6 mm Hg, P = 0.012; 551 microm, P = 0.011, respectively). There were no statistically significant differences in spherical equivalent, cylinder, axial length, optic disc size, or ocular pulse amplitude between the glaucoma and the control groups. There was no difference between the amount of IOP elevation between the two groups (P = 0.41), and the average difference in mean cup depth between baseline (mean cup depth, 247 microm) and during IOP elevation was 33 microm (29.8 microm in glaucoma and 36.1 microm in control; P = 0.5). Multiple variable analysis, controlling for age and sex, showed that CH was correlated with mean cup depth increase (P = 0.032). This relationship persisted (P = 0.032) after controlling for glaucoma status in addition to age and sex. Other factors, including CCT (P = 0.3), axial length (P = 0.9), ocular pulse amplitude (P = 0.22), and spherical equivalent (P = 0.38), were not significant in this model.


CONCLUSIONS
In the glaucoma patients but not the control patients, CH but not CCT or other anterior segment parameters was associated with increased deformation of the optic nerve surface during transient elevations of IOP. (ClinicalTrials.gov number, NCT00328835.).
Wells
Anthony P
AP
Capital Eye Specialists, Wellington, New Zealand. twells@eyetext.net
Garway-Heath
David F
DF
Poostchi
Ali
A
Wong
Tracey
T
Chan
Kenneth C Y
KC
Sachdev
Nisha
N
D003187:Compliance; D003315:Cornea / Q000503:physiopathology*; D003941:Diagnostic Techniques, Ophthalmological; D005109:Extracellular Matrix / Q000502:physiology; D005260:Female; D005901:Glaucoma / Q000503:physiopathology*; D006801:Humans; D007429:Intraocular Pressure / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D009798:Ocular Hypertension / Q000503:physiopathology; D009898:Optic Disk / Q000503:physiopathology*; D009901:Optic Nerve Diseases / Q000503:physiopathology*; D011446:Prospective Studies; D013314:Stress, Mechanical; D014065:Tonometry, Ocular