Comparison of dynamic contour tonometry and Goldmann applanation tonometry following penetrating keratoplasty

Transcription

1 Comparison of dynamic contour tonometry and Goldmann applanation tonometry following penetrating keratoplasty Artemios Kandarakis,* MD, FACS; Vasileios Soumplis,* MD; Christos Pitsas,* MD; Stylianos Kandarakis, { MD; Jiannis Halikias, { MSc, PhD; Dimitrios Karagiannis,* MD ABSTRACT N RÉSUMÉ Objective: To evaluate dynamic contour tonometry (DCT) versus Goldmann applanation tonometry (GAT) intraocular pressure (IOP) measurements in eyes that underwent penetrating keratoplasty (PKP). Design: Prospective, cross-sectional, observational study. Participants: Thirty-one eyes of 28 patients were examined after PKP. Methods: All eyes had undergone PKP with interrupted sutures. The postoperative period was more than 1 year for 25 eyes and less than 1 year for 6. Sutures were removed based on corneal topography and refraction. IOP was measured by both DCT and GAT methods and was correlated to the number of remaining sutures. Results: IOP readings were successfully obtained in 25/31 (80.6%) with DTC and in 21/31 (67.7%) with GAT (p ). In eyes with fewer than 4 remaining sutures, both methods were successful. In eyes with more than 4 sutures, the success rates of DCT and GAT were 66.7% and 44.4%, respectively (p ). In PKPs with a postoperative period of more than 1 year, the success rates of DCT and GAT were 96% and 84%, respectively (p ). In 20 eyes, both methods measured the IOP. The mean IOP obtained by DCT (16.6 [SD 2.8] mm Hg) was higher than the mean IOP obtained from GAT (15.1 [SD 3.6] mm Hg). The IOPs from the 2 instruments correlated significantly (p, 0.05) and the mean difference was 1.5 mm Hg. Conclusions: The success rate in measuring IOP with DCT and GAT did not show any statistically significant difference. Both methods were less effective measuring the IOP after recent PKPs and regrafts. However, DCT seemed to be superior to GAT in corneas with more than 4 remaining sutures and in PKPs performed more than 1 year earlier. The absolute values of IOP were higher with DCT than with GAT. Objet : Évaluation des mesures de la tonométrie dynamique par contour (TDC) versus la tonométrie par aplanation de Goldman (TAG) à la suite de la kératoplastie pénétrante (KPP). Nature : Étude d observation transversale et prospective. Participants : Trente-et-un yeux de 28 patients qui ont examinés après une KPP. Méthodes : Les yeux ont tous subi une KPP avec sutures à points séparés. La période postopératoire a duré plus d un an pour 25 yeux et moins d un an pour 6. Les points ont étéretirés selon la topographie de la cornée et la réfraction. La pression intraoculaire (PIO) a été mesurée selon la TDC et la TAG et corrélée au nombre de sutures restantes. Résultats : La lecture de la PIO a été réussie dans 25 cas sur 31 (80,6 %) avec la TDC et dans 21 cas sur 31 (67,7 %) avec la TAG (p 5 0,25). Dans les yeux qui avait moins de 4 sutures restantes, les deux méthodes ont réussi. Dans les yeux qui avaient plus de 4 sutures restantes, les taux de réussiteaveclatdcetlatagontétéde 66,7 % et de 44.4 % respectivement (p 5 0,18). Chez les KPP avec une période postopératoire de plus d un an, les taux de réussite avec la TDC et la TAG furent de 96 % et 84 % respectivement (p 5 0,16). Dans 20 yeux, les deux méthodes ont servi à mesurer la PIO. La moyenne de PIO obtenue par la TDC (16,6 [ÉT 2,8]mmHg)était supérieure à celle obtenue par la TAG (15,1 [ÉT 3,6]mmHg). La corrélation des PIO obtenues des 2 instruments était significative (p, 0,05) et l écart moyenétait de 1,5mm Hg. Conclusions : Les taux de réussite de la mesure de la PIO avec la TDC et la TAG ne montrèrent pas d écart statistiquement significatif. Les deux méthodes furent moins efficaces pour mesurer la PIO après des KPP récentes et des regreffes. Toutefois, la TDC semblait supérieure à la TAG pour les cornées qui avaient plus de quatre sutures restantes et pour les KPP effectuées plus d une année plus tôt. Les valeurs absolues de la PIO étaient supérieures avec la TDC qu avec la TAG. One of the most frustrating complications of penetrating keratoplasty (PKP) is raised intraocular pressure (IOP) and glaucoma, which may result in loss of vision due to irreversible optic nerve damage and graft failure. Therefore, accurate measurement and monitoring of IOP play significant roles in patient management following corneal transplant. The assessment of IOP in patients with corneal abnormalities is difficult. The measurement of IOP can be challenging, especially in patients who have undergone PKP. From *the A Eye Clinic, Ophthalmiatrion Athinon, Athens Eye Hospital, Athens, Greece; { the Ophthalmology Department, Mount Sinai School of Medicine, New York, N.Y.; and { the Department of Marketing and Communication, Athens University of Economics, Athens, Greece Originally received July 20, Final revision Feb. 12, 2010 Accepted Feb. 25, 2010 Published online Sep. 13, 2010 Correspondence to Artemios Kandarakis, MD, FACS, Ophthalmiatrion Athinon, Athens Eye Hospital, A Eye Clinic, Sina 2, Athens, Attiki, , Greece; artemiskandarakis@yahoo.gr This article has been peer-reviewed. Cet article a été évalué par les pairs. Can J Ophthalmol 2010;45: doi: /i CAN J OPHTHALMOL VOL. 45, NO. 5,

2 Therefore, many tonometers have been used, such as the Goldmann applanation tonometer (GAT), 1 the Mackay- Marg tonometer, 2 the pneumatic applanation tonometer, 3 and the Tonopen tonometer. 4 GAT is still considered the gold standard for measuring the IOP in the normal as well as glaucoma populations. In patients with corneal edema 5 or astigmatism greater than 3 4 diopters (D), GAT may cause an IOP measurement error of approximately 1 mm Hg per 4 D. 6,7 This potential error is avoided by using the average of 2 pressure readings, one taken with mires vertical and the other with mires horizontal. 6 However, any condition that alters the shape or thickness of the cornea can make the GAT readings uncertain or unobtainable. Dynamic contour tonometry (DCT) is a nonapplanation contact tonometer designed to be largely independent of corneal properties. The pressure-sensing device has been embedded within a contact tonometer that closely matches the corneal contour, thus minimizing the amount of corneal deformation. DCT has proven to be a reliable method for assessing IOP 8,9 independent of central corneal thickness and may be more accurate in measuring IOP in eyes with keratoconus. 11 The purpose of this study was to estimate the influence of PKP in obtaining IOP measurements with DCT and GAT and to compare the corresponding readings. METHODS In this prospective, cross-sectional, observational, singlecentre study, 31 eyes of 28 patients were examined following PKP. All PKPs were performed by a single surgeon (Artemios Kandarakis) using 16 interrupted sutures, and all patients attended our hospital cornea clinic. Data collected included patient age, sex, number of remaining sutures, refraction, corneal topography, and IOP measurement with both instruments during all phases of the postoperative period. The Ethical Board of our hospital approved the study and after detailed explanation, informed consent was obtained from each patient prior to examination. The age range was years, with a mean age of 53.1 years (14 males, 14 females). Twenty-five patients were operated in one eye and 3 were operated in both eyes. Three eyes were regrafts. During the postoperative period, all sutures were removed based on corneal topography and refraction. The postoperative period for 25 eyes was more than 1 year and for 6 eyes was less than 1 year. A full eye examination was performed on a regular postoperative basis, including slit-lamp biomicroscopy and identification of remaining sutures. Corneal topography and refraction were performed and sutures were removed when necessary. In addition, the IOP was measured by both the DCT and GAT methods. DCT was performed using a technically identical prototype of the model launched in November 2003 (Pascal dynamic contour tonometer; Swiss Microtechnology AG, Port, Switzerland; slit-lamp mounted, self-calibrating, 1 g appositional force, 100 Hz sampling rate, 7 mm tip diameter, 1.2 mm pressure sensor diameter). For each subject, a new disposable sensor tip cover was applied. DCT was chosen to assess IOP first, because the DCT tonometer has been described as closely matching the corneal contour, thus minimizing the amount of corneal deformation. 17 Three consecutive DCT measurements were obtained and averaged at 5-minute intervals. The quality of each reading was shown on the digital display of the DCT tonometer and was classified from Q1 (optimum) to Q5 (unobtainable). Quality readings of Q1 to Q3 were considered successful. The examiner performing the GAT measurement was different from the one who performed the DCT measurement and therefore was not biased by knowing the DCT readings. The GAT measurements were taken at least 30 minutes after the DCT ones and were performed on a slit-lamp (Haag-Streit, Koniz, Switzerland), with a tonometer calibrated according to the manufacturer s guidelines. Before each reading, the measuring drum was reset to approximately 2 mm Hg and the mean of 4 pressure readings, 2 taken with mires vertical and the other 2 with mires horizontal, was recorded. The GAT reading was regarded as unacceptable (not successful) only when the fluoresceinstained semicircles were impossible to identify on the tonometer head. SPSS statistical software, v.10.0 for Windows (SPSS Inc, Chicago, Ill.) was used for statistical analysis. To test the differences between the success percentages of IOP measurements, we employed the z test statistic. A p value of, 0.05 was considered statistically significant. A Bland Altman plot 18 of the difference between the DCT and GAT readings against the average of the 2 was drawn to assess the agreement between the 2 methods. RESULTS IOP readings were successfully obtained in 25/31 (80.6%) eyes with DTC and in 21/31 (67.7%) eyes with GAT (z , p ). In eyes with fewer than 4 sutures, both methods were successful. In eyes with more than 4 sutures, the success rates of DCT and GAT measurements were recorded as 66.7% (12/18), and 44.4% (8/18), respectively (difference statistically nonsignificant; z , p ). Regarding the PKPs performed during the last year (6 eyes), the success rate of DCT was 16.7 % (1/6) and of GAT, 0% (0/6), which was not statistically significant (z , p ). From a different perspective, in PKPs with a postoperative period of more than 1 year it was easier to measure the IOP, with the success rates being 96% (24/25) for DCT and 84% (21/25) for GAT (difference statistically nonsignificant; z , p ). In 20 eyes, both the DCT and the GAT method measured the IOP (Fig. 1). Thirteen of these had fewer than 490 CAN J OPHTHALMOL VOL. 45, NO. 5, 2010

3 4 remaining sutures and the rest (7 eyes) were PKPs performed in a time period of more than 1 year. Mean IOP measurements by DCT (16.6 [SD 2.8] mm Hg) were significantly higher than IOP readings obtained from GAT (15.1 [SD 3.6] mm Hg). IOP measurements by the 2 instruments correlated significantly with each other (r , p, 0.05). The mean difference between DCT and GAT was 1.5 (+5.9 to 26.0) mm Hg with a 95% CI of 0.3 mm Hg to 2.8 mm Hg. The Bland Altman plot of the difference between the 2 methods against their mean sets the upper limit of agreement at 6.9 mm Hg and the lower limit of agreement at 23.8 mm Hg. Regarding the intratest variability of each method, we found that the coefficient of repeatability was 4.1 mm Hg for DCT and 4.2 mm Hg for GAT. Moreover, we computed Cronbach s coefficient a, which was 0.92 for both DCT and GAT. DISCUSSION The prevalence of high IOP after PKP varies from 9% to 31% in the early postoperative period and from 18% to 35% in the late postoperative period. 19,21 24 The etiology of the raised IOP is related to many factors, such as preexisting glaucoma, 25 distortion of the angle and collapse of the trabecular meshwork, 26 suturing technique, postoperative inflammation, formation of peripheral anterior synechiae, 27 residual viscoelastic in the anterior chamber, and use of steroids following the operation. 19 Raised IOP may turn out to be visually threatening because it can cause graft failure as well as irreversible damage to the optic nerve. Therefore, the application of an accurate, userfriendly, and reliable method in measuring the IOP following PKP is considered mandatory, not only in the diagnosis of glaucoma, but also in the follow-up of the IOP fluctuations in the early and late postoperative periods. Previous studies have assessed other tonometers in the measurement of IOP following keratoplasty 1,28 30 but none has shown an advantage over GAT. In a cross-sectional study, we examined 31 eyes that had undergone PKP, to detect if a significant difference between the measurements of IOP by GAT and DCT existed. In our study, the success rate in measuring IOP with DCT and GAT after PKP did not show any statistically significant difference. We further analyzed the results by categorizing the eyes with reference to the number of remaining sutures (, 4 and. 4) and postoperative period (, 1 year and. 1 year). The DCT method seemed to be superior in all subgroups, although the difference was not statistically significant. Better results were more pronounced in corneas with more than 4 remaining sutures and in PKPs performed more than 1 year earlier. In addition, both methods were shown to be less effective in measuring the IOP in recent PKPs (, 1 year) and in regrafts. This finding could be explained by the increased irregularity of the cornea surface in the early postoperative period. After PKP, the alteration in corneal geometry influences the capability of GAT to measure the IOP and introduces technical difficulties in obtaining the measurement itself. Over time, the graft surface becomes smoother, the epithelium is intact, and the mires become more regular, allowing GAT to increase its effectiveness. Previous studies 31,32 reported an underreading of IOP by GAT in grafted eyes, compared with DCT, with continuous sutures of 2.7 mm Hg and 2.5 mm Hg, respectively. Our data, in a larger number of patients with a longer follow-up period, showed a tendency to underestimate the IOP using GAT in comparison to DCT. In addition, because interrupted sutures were used, our data were compared during all phases of the postoperative period and results were correlated to the number of remaining sutures. IOP measurements using DCT were found to be significantly higher than those obtained by GAT. As stated above, DCT was performed first, followed by GAT. It is possible that the DCT could have caused Fig. 1 Scatter diagram of intraocular (IOP) measurements (mm Hg) obtained by dynamic contour tonometry (DCT) and Goldmann applanation tonometry (GAT) (r , p, 0.05). Solid line indicates linear function of the data. Dashed line indicates 100% correlation. Fig. 2 Bland-Altman plot of the agreement between dynamic contour tonometry (DCT) intraocular pressure (IOP) measurements and Goldmann applanation tonometry (GAT) IOP measurements (mm Hg). The difference between the measurements is plotted against the average of the measurements. The solid lines represent the average of the 2 methods and 95% limits of agreement ([SD 1.96] D). The dashed lines represent the upper and lower limits ([SD 3.0] mm Hg). CAN J OPHTHALMOL VOL. 45, NO. 5,

4 a decrease in the choroidal volume and a reduction in IOP, which may, in part, explain the lower IOP found with GAT. Repeated IOP measurements have been shown to introduce a reduction of IOP, possibly caused by neurologic, vascular mechanisms 33 or by displacement of aqueous humour. 34 To avoid such an effect, we implemented a 30-minute interval between the 2 tonometers. In our study, the mean difference in IOP measured by DCT against GAT was +1.5 mm Hg, with a 95% CI of 0.3 mm Hg to 2.8 mm Hg. Therefore, DCT tends to give a higher reading by between 0.3 and 2.8. In the screening of IOP measurements, a mean error of 3 mm Hg has been suggested as acceptable. 35,36 In this study, agreement between the 2 methods within the SD of 3 mm Hg of the GAT was observed in 70% of the readings (Fig. 2). By increasing the upper and lower limits of agreement up to mean (SD 1.96) to include 95% of the readings, the difference between the 2 methods increased to 5.3 mm Hg (Fig. 2). Although this may initially appear to be clinically unacceptable, in irregular corneas the error rate with any tonometer can be expected to be higher than normal. 37 Further analysis of our data regarding the intratest variability of each method revealed that the coefficient of repeatability was 4.1 mm Hg for DCT and 4.2 mm Hg for GAT. In a normal population, the reproducibility of GAT has been shown to vary from 2 to 4 mm Hg. 38 Our results are slightly higher, which can be explained by the unique properties of the post-pkp cornea, and are similar to the range of values found in the literature for post-pkp patients. 1 Moreover, we computed Cronbach s coefficient a, which was 0.92 for both DCT and GAT. A Cronbach s coefficient a value of more than 0.90 reflects a high consistency of IOP readings for both tonometers. These results suggest that DCT and GAT are equally reliable for IOP measurements following PKP. One of the limitations of our study was that corneal thickness measurements were not performed to assess if they were related to the tonometry readings or the difference between the 2 tonometers. However, several studies 29,31,32 showed that DCT, GAT, and DCT-GAT difference measurements were statistically independent of central corneal thickness, probably because corneal thickness after PKP may respond in a different way from that of a normal cornea. 29 In conclusion, DCT may have been more effective than GAT in measuring IOP in corneas that have undergone PKP, although the difference was not statistically significant. As expected, both methods were less effective in recent PKPs (, 1 year) and in regrafts. We suggest that DCT could challenge GAT as the new gold standard for measuring IOP in post-pkp eyes. Further studies should be undertaken comparing DCT with GAT in post-pkp eyes. The authors have no proprietary or commercial interest in any materials discussed in this article. 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