*Department of Ophthalmology and Visual Sciences, University of Illinois-Chicago, Chicago, IL
†Cornea Research Foundation of America
‡Price Vision Group, Indianapolis, IN
§Wilmer Eye Institute, Johns Hopkins University Baltimore, MD
∥King Khaled Eye Specialist Hospital, Riyadh, Kingdom of Saudi Arabia
Disclosure: Carson Gabriel Funds and Komarek-McQueen-Hyde Glaucoma Research Funds created in honor of Dr Mark Lunde. K12HD055892 (T.S.V.), supported by the National Institute of Child Health and Human Development and the Office of Research on Women's Health, Bethesda, Maryland. The authors declare no conflict of interest.
Reprints: Thasarat S. Vajaranant, MD, Department of Ophthalmology and Visual Sciences, University of Illinois-Chicago, Chicago, IL 60612 (e-mail: firstname.lastname@example.org).
Received February 14, 2012
Accepted July 3, 2012
Purpose: To study the relationship of ocular pulse amplitude (OPA) and intraocular pressure (IOP) in eyes that have undergone successful Descemet stripping endothelial keratoplasty (DSEK).
Patients and Methods: Fifty eyes of 38 patients with successful DSEK at a single tertiary referral center were followed up for at least 3 months. At the time of the study all patients were carefully examined to rule out any clinically detectable corneal edema. IOP was measured by Goldmann applanation tonometry (GAT), pneumotonometry (PT), and dynamic contour tonometry (DCT). IOP, OPA, and quality measurements were recorded. Central corneal thickness (CCT) was measured by ultrasonic pachymetry.
Results: Mean IOP was 15.9±4.9 mm Hg by GAT, 20.3±4.6 mm Hg by PT, and 19.8±4.4 mm Hg by DCT. Mean OPA was 2.53±1.24 mm Hg. OPA was correlated with GAT (r=0.357, P=0.011) and PT (r=0.316, P=0.026). The correlation of OPA and DCT approached significance (r=0.270, P=0.058). Mean CCT was 701±65 µm (range, 529 to 928 µm). OPA was not associated with CCT (r=0.238, P=0.096).
Conclusions: In eyes with DSEK, our results showed that OPA was similar to that reported in normal eyes. Comparable with results in normal eyes, OPA was not associated with CCT but was associated with increasing IOP in DSEK eyes.
Many patients with corneal endothelial dysfunction share a diagnosis of glaucoma, and intraocular pressure (IOP) surveillance is important for disease management.
In a recent study, we found that increased central corneal thickness (CCT), as found after Descemet stripping endothelial keratoplasty (DSEK), in which donor stromal tissue is added to the posterior surface of the host cornea, did not cause falsely elevated measurements by GAT as might be expected.1 Furthermore, IOP measurements by pneumotonometry (PT) and dynamic contour tonometry (DCT) were significantly higher than by Goldmann applanation tonometry (GAT) in the DSEK eyes. In fact, both GAT and DCT are affected similarly by the increase in corneal thickness after DSEK and are both potential options as IOP indicators in post-DSEK eyes to identify pathologic IOP.2
DCT, a recently introduced technique for measuring IOP, is designed to directly measure IOP across the cornea. It features a tip that is contour matched to the cornea and thus reduces the influence of corneal properties such as thickness, contour, and hydration during IOP measurement.3 In addition to measuring IOP, DCT measures ocular pulse amplitude (OPA), which is the difference between IOP as measured in systole and diastole. On the basis of a twin study conducted in the United Kingdom, both corneal hysteresis (measure of viscoelasticity of the cornea) and OPA variability demonstrated a strong genetic heritability and may play a role in characterizing glaucoma.4 Corneal hysteresis, however, resulted in a positive correlation with CCT, whereas in that same study OPA had no correlation with CCT. OPA may be an indirect indicator of choroidal perfusion and a potentially important independent parameter in the clinical management of glaucoma.5
In this study, we examined the relationship of OPA to IOP and anterior segment characteristics in eyes that had undergone successful DSEK. To our knowledge, OPA has not been studied in DSEK eyes.
MATERIALS AND METHODS
In this prospective, cross-sectional study, patients who had undergone successful DSEK (performed by F.W.P. at Price Vision Group, Indianapolis, IN) were followed up for at least 3 months. The DSEK surgical technique for all 50 eyes was previously described in detail by Price and Price.6 In all but 1 eye, a microkeratome was used in donor graft preparation; manual dissection was used in the remaining eye. In order to maintain a watertight wound and avoid induced astigmatism, a scleral tunnel incision was used in most cases, except in combined DSEK and cataract extractions, where a clear corneal incision was used.
All patients underwent an anterior segment examination by an experienced corneal surgeon to evaluate for corneal edema on the day of testing. Exclusion criteria included patients with postoperative corneal edema, other acute corneal pathology, or poor fixation. Patients with IOPs higher than normal were included in the study as glaucoma was not considered to be exclusion criterion.
All IOP measurements were performed by one of the authors (T.S.V.) using GAT, PT, and DCT. The sequence of these measurements was randomized, and each measurement was taken approximately 1 to 2 minutes apart. For each technique 2 measurements were obtained and averaged. DCT was performed using the Pascal (SMT Swiss Microtechnology, Switzerland). A DCT/Pascal tonometer was attached to a Haag-Streit slit lamp and a disposable sensor cap was fitted and attached. After corneal anesthesia, the measuring tip was aligned in order to center the pressure sensor over the contact zone for 3 to 5 seconds (approximately 5 oscillating sounds representing ocular pulses). The IOP, OPA, and quality measurement were recorded. Quality measurements are based on 5-point scale with Q1 being optimum, Q2 and Q3 acceptable, Q4 questionable, and Q5 of low quality.7 Quality levels Q1 to Q3 were accepted.
Total CCT was measured by ultrasonic pachymetry (Pachmate; DGH 55, Exton, Pennsylvania). Three readings were produced by each measurement, and an average of 6 to 9 readings were used for analysis. Anterior segment optical coherence tomography (OCT) was also performed to produce high-resolution corneal scans of each quadrant (Visante; OCT 1000, Carl Zeiss, Meditec Inc, Dublin, California). The donor/recipient interface was manually identified, and the donor and recipient thickness was calculated by the OCT software. The average corneal thickness of all 4 quadrants was used.
Pearson correlation coefficients were computed using SPSS software (Chicago, IL). P-values <0.05 were considered significant.
Fifty eyes of 38 patients who underwent successful DSEK at least 3 months before the study were included. Mean age of the patients was 69±10 years. Two thirds (25/38) were female (66%) and nearly 90% were white (34/38). Twelve patients (31.6%) had bilateral DSEK. Mean follow-up was 12.6±8.0 months, ranging from 3 to 37 months. The indications for DSEK included Fuchs endothelial dystrophy (41/50; 82%) and bullous keratopathy (9/50; 18%). Two participants had prior glaucoma procedures, whereas 8/50 (16%) were using glaucoma medications and 47/50 (94%) were using topical steroids.
The mean IOP was 15.9±4.9 mm Hg by GAT, 20.3±4.6 mm Hg by PT, and 19.8±4.4 mm Hg by DCT. Mean OPA was 2.53±1.24 mm Hg. OPA was weakly correlated with GAT (r=0.357, P=0.011) and PT (r=0.316, P=0.026). The correlation of OPA and DCT approached significance (r=0.270, P=0.058) (Figs. 1A–C).
Mean CCT was 701±65 µm (range, 529 to 928 µm). Mean thickness of the recipient was 502±46 µm, whereas mean thickness of the graft was 175±42 µm by OCT. OPA was not associated with CCT (r=0.238, P=0.096) or donor (r=0.111, P=0.443) or recipient (r=0.130, P=0.125) thicknesses.
Statistical analysis was repeated using only 1 eye from each patient (the right eye for those patients with bilateral DSEK) to show similar associations as reported above in all 50 post-DSEK eyes.
The use of OPA as measured by DCT has the potential to become a useful parameter in the clinical management of glaucoma; however, to our knowledge the accuracy of OPA in post-DSEK eyes has not yet been evaluated. Factors such as ocular blood flow, choroidal rigidity, and scleral rigidity are reflected in OPA and might reveal a relationship between blood flow to the optic nerve head and glaucoma severity.8 Our study shows OPA in eyes that have undergone DSEK correlates weakly with GAT and PT and that OPA measurement is not associated with CCT or the thickness of the graft or recipient.
Although there is no universal agreement on whether OPA is associated with CCT, our study supports the conclusion that OPA and CCT are not significantly correlated post-DSEK. Furthermore, the observation that OPA measurements in post-DSEK eyes, which have increased CCT due to the addition of donor tissue, may behave like OPA measurements in normal eyes supports the conclusion that OPA is unrelated to corneal thickness irrespective of corneal endothelial pathology. Prior studies have evaluated the relationship between OPA and anterior segment parameters, including CCT, with a range of conclusions describing the relationship as either uncorrelated or positively associated among both normal and glaucomatous eyes.8–10
The positive trend of increased OPA with elevated IOP in post-DSEK eyes established by this study supports other studies comparing OPA values to IOP. Erickson et al11 demonstrated that OPA measurements (range, 0.90 to 5.33 mm Hg) in healthy eyes was positively correlated with IOP—suggesting that post-DSEK eyes may behave similar to healthy eyes when comparing OPA to other ocular parameters. In an evaluation of eyes with various types of glaucoma, including open angle glaucoma, normal tension glaucoma, exfoliation glaucoma, and ocular hypertension, Punjabi et al12 found that OPA was highest in eyes with ocular hypertension and OPA increased with IOP, after controlling for the use of glaucoma medications. This data in conjunction with the results from our study suggest that post-DSEK eyes behave similar to both healthy and glaucomatous eyes when measuring OPA and IOP.
The weak positive correlation between OPA and GAT and PT, and lack of a statistically significant correlation between OPA and CCT in this sample population suggest that thicker corneas may not discredit OPA measurements on post-DSEK eyes. It could be that the additional thickness, which is not anchored to the globe structure but just to the cornea, does not alter corneal movement in response to the vascular pulse. Prior publications have also shown that CCT demonstrates poor correlation with DCT measurements, especially in thicker corneas and that CCT is one of several structural factors including axial length and corneal curvature that affects the accuracy of applanation.13
This study shows that comparable with findings in normal eyes, OPA was not associated with CCT but was associated with increasing IOP in DSEK eyes. These findings suggest that DSEK does not significantly alter OPA readings and an elevation in OPA in post-DSEK eyes should raise a true concern for glaucomatous advancement.
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