Results of a study using a non-contact applanation tonometer (Ocular Response Analyzer, Reichert) to evaluate keratoconic eyes during and after corneal collagen crosslinking demonstrate changes in tonometer measurements that are consistent with changes in corneal thickness, hydration, and regularity.
"Previous investigators using the [tonometer] have reported that the biomechanical properties of keratoconic eyes, measured by corneal hysteresis [CH] and corneal resistance factor [CRF], are significantly different than in normal eyes, and it has also been reported that the values for Peak 1 and Peak 2, which depend on the area of applanation and the uniformity of corneal deformation to the emitted air puff, are negatively correlated with keratoconus severity," Dr. Albé said. "We were interested in investigating changes in these [tonometer] parameters in keratoconic eyes undergoing collagen crosslinking."
Goldmann-correlated IOP (IOPg) and corneal-compensated IOP (IOPcc) were measured with the tonometer in addition to CH, CRF, and Peak 1 and Peak 2. All eyes also underwent Scheimpflug imaging (Pentacam, Oculus) to determine corneal pachymetry and topographic evaluation using a wavefront aberrometer (OPD Scan, Nidek). All of these assessments were performed prior to the crosslinking procedure, during the procedure after epithelial removal, and again after impregnation with riboflavin immediately before UVA irradiation, at the conclusion of the crosslinking procedure prior to bandage contact lens placement, after corneal re-epithelialization was achieved, and then at 1, 6, 12, and 24 months after crosslinking.
Mean CH and CRF values were not significantly different from baseline when measured after epithelial removal. Both values, however, were significantly increased when measured both after riboflavin application and after the irradiation. These changes were inversely correlated with corneal thickness, and they correlated positively with changes in apical K readings. Both Peak 1 and Peak 2 were decreased significantly compared with baseline at all measurement times during and immediately after the crosslinking procedure.
"Epithelial removal to allow penetration of riboflavin does not affect corneal biomechanical properties, although it increases corneal curvature, reduces corneal thickness, and decreases the light detected during applanation. This information could explain why there were no significant changes in CH or CRF after de-epithelialization but significant decreases in Peak 1 and Peak 2," Dr. Albé said.
"The application of riboflavin in dextran causes dehydration of the cornea, which also reduces the light detected during the applanation peaks," she continued. "However, the dehydration also affects corneal biomechanical properties and so could account for the significant increases in CH and CRF observed at this stage during the procedure."
Re-epithelialization occurred at about 4 days post-crosslinking, and the measurements taken at that follow-up visit showed that both CH and CRF were significantly increased from baseline. Peak 1 and Peak 2 remained significantly decreased. A significant positive correlation existed between the changes in CH and CRF and corneal thickness, which was significantly increased.
"Corneal thickness is increased at the time of re-epithelialization as a result of postoperative edema, and this edema might explain the changes in the [tonometer] values," Dr. Albé said.
Beginning at 6 months through 2 years, no significant differences were observed in CH, CRF, or corneal thickness compared with baseline. Peak 1 and Peak 2, however, were significantly increased, and those changes correlated positively with changes in apical keratometry.
"Examination of the signal curve in these crosslinked eyes shows [that] its shape is improved beginning at 6 months after the procedure," she said. "This suggests the cornea is stiffer and deforms less, but it is also consistent with the more regular shape that is consistent with the changes observed on topography and the significant decrease in mean apical corneal power."
Results from IOP measurement showed that both IOPg and IOPcc were increased significantly after the procedure through the 1-month visit but were no longer elevated compared with baseline at later visits.
"These increases in IOP measured with both of the [tonometer] parameters is a true change that can be explained by the fact that all patients are treated with a 20-day course of topical corticosteroid for 20 days after the crosslinking procedure," Dr. Albé said.