Anterior Segment Study with the Pentacam Scheimpflug Camera in Refractive Surgery Candidates

Abstract

Purpose:

The purpose of this study was to evaluate the anterior segment measurements according to refractive status in a sample of the Iranian population.

Setting:

Rassoul Akram Hospital, Tehran University of Medical Sciences.

Material and Methods:

In this study, refractive surgery candidates were assigned according to the refractive error to one of three groups : emmetropia, myopia, and hyperopia. Myopic eyes were further divided to four subgroups : simple myopic group, simple myopic astigmatism group, high myopic group, and high myopia with astigmatism group. Anterior segment measurements with the Pentacam Scheimpflug system were performed in the right eye of all subjects.

Results:

The study sample was comprised of 283 subjects with a mean age of 29.1 ± 7.5 (standard deviation) years. Mean keratometry reading, Anterior chamber depth (ACD) and volume measurements were significantly higher in the myopic group and mean keratometry reading and anterior chamber angle measurements were significantly lower in the hyperopic group (P < 0.05, all comparisons). Maximum anterior elevation (AE_max) and maximum posterior elevation (PE_max), Q value, progression index, minimum corneal thickness, and corneal volume measurements were similar for all groups (P > 0.05, all comparisons). In the myopic subgroups, AE_max and PE_max and maximum keratometry (K_max) were significantly higher, and ACD was lower in the astigmatic groups (P < 0.05, all comparisons). The Q value was less negative in low myopia (P < 0.05).

Conclusions:

Myopic eyes had steeper corneas than hyperopic eyes and anterior chamber measurements were significantly higher in the myopic eyes. In myopic eyes, AE_max and PE_max and K_max measurements were higher, and ACD measurements were lower in the astigmatic groups.

INTRODUCTION

The Pentacam (Oculus Optikgerδte GmbH, Wetzlar, Germany) has become a popular device for evaluating the anterior segment of the eye since its launch in 2004. Previous studies have shown excellent reproducibility of this device for the automated measurements of the anterior segment structures.¹,²,³,⁴ The Pentacam system may be used for the diagnosis of keratoconus, monitoring post-surgical corneas, calculating the keratometric index, intraocular lens power, and assessment of intraocular lens implants.⁴,⁵,⁶,⁷,⁸,⁹,¹⁰ Despite several reports on corneal thickness (CT),⁵ anterior and posterior corneal curvature and elevation,¹,²,⁴ anterior chamber depth (ACD) and anterior chamber angle (ACA),³,¹¹,¹² and corneal volume (CV) measurements,⁵,¹² little is known about the distribution of these measurements in the normal population and its relation to the refractive status. We evaluated the distribution of the anterior segment measurements in a population of normal Iranian subjects with different types of refractive errors.

MATERIAL AND METHODS

From January 2008 to January 2010, all patients presented for corneal refractive surgery were evaluated. Exclusion criteria included a history of corneal injury, illness, previous ocular surgery, contact lens wear during the past 3 weeks, and pregnancy.

The study was performed in accordance with the Declaration of Helsinki guidelines for human research.

The right eye of each patient was selected for measurement with the Scheimpflug camera (Pentacam, software version 1.12) with the 50-image mode. The images were obtained using the automatic mode when correct alignment with the corneal apex and focus were achieved. To avoid miscalculations due to poor imaging quality, only measurements with “OK” quality statements were selected for the study. The CT, anterior elevation (AE), posterior elevation (PE), sagittal curvature, and keratoconus maps were evaluated. The maximum anterior elevation in 4 mm central circle (AE_max), maximum posterior elevation in 4 mm central circle (PE_max), minimum corneal thickness, 3, 5, and 7 mm corneal volumes (CV3, CV5, CV7), mean keratometry (Km), maximum keratometry (K_max) in 4 mm central circle, Q value (Q), average progression index, ACD, anterior chamber volume (ACV), and ACA measurements automatically provided by the software were recorded. For K_max, AE_max and PE_max, the cursor was moved through the screen to find the maximum number for the central 4 mm circle. From the AE and PE map options, automatic diameters were selected for the float best-fit spheres.

The patients were divided into three groups according to the manifest refractive error. Emmetropic eyes consisted of patients within ±0.5 diopters (D) of plano and uncorrected visual acuity of 20/20. The myopic group included eyes with a spherical equivalent refraction of –0.75 D or more and hyperopic group included eyes with a spherical equivalent refraction of +0.75 or more. The myopic group was further divided into 4 subgroups; the simple myopic subgroup consisted of patients with a spherical refraction of –6.0 D or less and a cylindrical refraction of 1.00 D or less; the simple myopic astigmatism subgroup consisted of patients with a spherical refraction of –6.0 D or less and a cylindrical refraction greater than 1.0 D; the high myopic subgroup consisted of patients with a spherical refraction greater than –6.0 D and a cylindrical refraction of 1.0 D or less and; the high myopia with astigmatism subgroup consisted of patients with a spherical refraction greater than –6.0 D and a cylindrical refraction greater than 1.0 D.

Data were entered using the SPSS software (version 15, SPSS Inc.). Analyses were performed with the Chi-square test, ANOVA test, t-test, and correlation test. A P value less than 0.05 was considered statistically significant.

RESULTS

A total of 283 subjects comprised the study cohort. The mean age of the participants was 29.1 ± 7.5 years (range 18-56 years). One hundred seven participants (37.5%) were men. Patient demographics are presented in Table 1. There were 21 emmetropic eyes, 20 hyperopic eyes, and 242 myopic eyes. From the myopes, there were156 eyes in the simple myopic group, 47 eyes in the simple myopic astigmatism group, 17 eyes in the highly myopic group, and 22 eyes in the highly myopic with astigmatism group.

Table 1.

Patient demographics

Table 2 present the different measurements between the emmetropic, myopic, and hyperopic groups. Km reading, ACD, ACV, and ACA measurements were significantly different between the three groups. Post-hoc analysis revealed higher values in the myopic group for ACD, ACV (P < 0.001 and P < 0.001, respectively) and lower values in the hyperopic group for Km and ACA (P = 0.01 and P < 0.001, respectively).

Table 2.

Measurement of structures in the anterior segment with the Pentacam Scheimpflug camera in emmetropic, myopic, and hyperopic eyes

Subgroup analysis revealed a significant difference in AE_max, PE_max, K_max, Q, and ACD between the four myopic groups [P < 0.05 all comparisons; Table 3]. AE_max, PE_max, and K_max were significantly higher, and ACD was significantly lower in the astigmatic subgroups (P < 0.05, all comparisons). The Q value was less negative in low myopia (P > 0.05)

Table 3.

Measurement of structures in the anterior segment with the Pentacam Scheimpflug camera (mean±SD) in four myopic subgroups

In the entire study population, statistically significant correlations were found for AE_max and corneal astigmatic measurements [r = 0.67, P < 0.001; Figure 1], PE_max and corneal astigmatic measurements (r = 0.57, P < 0.001) and K_max and corneal astigmatic measurements (r = 0.27, P = 0.01).

Figure 1.

A significant correlation was found between the maximum anterior elevation and corneal astigmatic measurements

DISCUSSION

Adequate information on normal anterior segment measurements is essential for the diagnosis and treatment of abnormal ocular conditions. Obtaining and appropriately analyzing data on anterior segment structures have become more critical as various refractive surgical procedures continue to evolve and gain popularity. Different instruments are available to measure the optical and anatomical properties of the anterior segment of the eye. Among these, the Pentacam is unique in its ability to evaluate the entire anterior eye segment. The Pentacam provides clinically relevant and reproducible information from the anterior corneal surface to the posterior lens surface in a single scan without contact with the cornea.¹,²,³,⁴ Our study evaluated the distribution of the parameters available from the Pentacam system in eyes with various refractive disorders in a sample of the Iranian population, in an attempt to better discriminate normal from abnormal values. The data from our study will aid in screening candidates for refractive surgery.

Our results show that hyperopic eyes had lower AE_max and PE_max (although not significant), and keratometry readings. The mean PE_max in our study (15.4 ± 5.7 in myopic eyes and 15.4 ± 5.4 in hyperopic eyes) are different from those reported by de Sanctis et al.,¹³ (19.8 ± 6.37). We used automatic diameters for the float best fitted spheres as recommended by Belin (Belin M. Proposed Screening Parameters. Pentacam Interpretation Guideline. Oculus Optikgerδte GmbH). However, de Sanctis et al.,¹³ used the 9 mm measurements, which may explain the difference from the data in our study. The Km readings are automatically generated by the instrument, and our findings are similar to the previous reports.³,⁴,¹⁴ There have been conflicting reports regarding the relationship between refractive error and corneal curvature. Parssinen,¹⁵ reported no correlation between the degree of myopia and corneal curvature. Our results; however, concur with the majority of studies that show corneal power in myopia is on average about 1.0 D greater than in hyperopia.¹⁶,¹⁷,¹⁸ Interestingly, we found higher AE_max and PE_max and K_max readings in astigmatic eyes. When interpreting the elevation and keratometry maps in astigmatic eyes, one must consider that the high elevations and readings may represent the normal shape of the astigmatic band. To remove the effect of this astigmatic band on the elevation maps, Holladay recommends using the best-fitted toric ellipsoid option as a reference shape (Holladay J. Using the Holladay report on the Oculus Pentacam. In: The Pentacam: Precision, Confidence, Results, and Accurate “Ks!” Cataract and Refractive Surgery Today January 2007).

CT has been identified as a major determinant for measurement error of intraocular pressure and an independent risk factor for open-angle glaucoma.¹⁹ It is also important for the diagnosis of subclinical keratoconus in patients presenting for refractive surgery.²⁰ Values outside the accepted normal range may suggest that the patient is a poor candidate for refractive surgery or that further evaluation is warranted before undergoing a refractive procedure.²⁰,²¹ Studies that attempted to evaluate the effect of refractive errors, in particular degree of myopia on the central corneal thickness show conflicting results.¹⁴,²²,²³,²⁴ In contrast to Murata et al.,²² and similar to those reported by Uçakhan et al.,¹⁴ we found there was no significant difference between hyperopic and myopic CT measurements (P > 0.05). Although the thinnest CT measurements were lower in high myopic eyes, the difference was not statistically significant (P > 0.05).

Recently, measurement of CV has been proposed to be useful in detecting keratoconus and planning intracorneal ring implantation.⁵,¹² Ambrósio et al.,⁵ reported that the CV measurements in eyes with mild to moderate keratoconus were significantly lower than those in a group of normal eyes. Emre et al.,¹² suggested that evaluating the CV within the 7.0 mm optical zone is essential to reduce the risk for implant extrusion. We found no significant difference between the myopic and hyperopic CV measurements. Although not significant, the CV of high myopic eyes was less than other myopic eyes.

The aspheric nature of the normal cornea is well-established.²⁵,²⁶ A negative Q-value (prolate) is defined as progressive corneal flattening from center to periphery, whereas a positive Q-value (oblate) is defined as increasing corneal curvature moving from center to periphery.²⁷ Previous studies reported mean Q-values in normal corneas between –0.11 and –0.33, with a mean of –0.26.¹⁸,²⁵ Our results support the findings of Sheridan and Douthwaite²⁵ who found no significant difference in asphericity among emmetropes, myopes, and hyperopes.

The anterior chamber parameters are clinically useful for planning intraocular lens (IOL) power calculation for cataract surgery and phakic IOL placement in refractive surgery.²⁶,²⁷,²⁸

Our results on the association between deeper anterior chamber, higher chamber volume, wider chamber angles, and myopia concur with previous studies on other ethnic populations.²²,²⁹,³⁰ These data further explain why hyperopia is a predominant risk factor for a primary angle-closure glaucoma.³¹ Among the different subgroups of myopic eyes in the current study, higher ACD, and ACV measurements were found in low myopic eyes. These outcomes are similar to findings reported by Murata et al.,²² who reported a shallower anterior chamber in highly myopic eyes (>–9 D). However, Uçakhan et al.,¹⁴ reported significantly higher ACD and ACV measurements in the high myopic group. Different refractive definitions for the study eyes may be responsible for this discrepancy.

This study has some limitations, and the sample size is small especially in the emmetropic and hyperopic groups. This is comparable to some previous studies¹⁴,²² and actually represents the normal distribution of patients presenting for refractive surgery. The age range of patients presenting for refractive surgery is limited; some anterior segments parameters may be affected by age.

In conclusion, in this study, myopic eyes were found to have steeper corneas than hyperopic eyes and anterior chamber measurements (ACD, ACV, and ACA) were significantly higher in myopic eyes. In myopic eyes, maximum elevations and keratometry measurements were higher, and ACD measurements were lower in astigmatic groups. The information gained from this study may provide a general estimate of anterior segment parameters in the Iranian population as obtained from Pentacam.