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Objective assessment of nuclear cataract: Comparison of double-pass and Scheimpflug systems

Lim, Sung A. MD; Hwang, Jehyung MD; Hwang, Kyu-Yeon MD; Chung, So-Hyang MD, PhD*

Author Information
Journal of Cataract & Refractive Surgery: May 2014 - Volume 40 - Issue 5 - p 716-721
doi: 10.1016/j.jcrs.2013.10.032
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Abstract

To obtain the maximum optical quality in cataract surgery, the preoperative assessment should be objective and reproducible. Objective measurement of nuclear cataracts is especially important to predict phacoemulsification energy.

At present, the Lens Opacities Classification System III1 (LOCS III) is the standard grading system under a slitlamp despite concerns about observer bias and reproducibility.2 As an objective measurement, Scheimpflug-measured lens nuclear density has been correlated with the LOCS III system, visual function, and phacoemulsification energy.3–5

Because ocular scatter increases under cataract conditions, the measurement of scatter is considered a good tool to evaluate cataracts.6 In a study of 180 eyes, ocular scatter and LOCS III grades were strongly correlated.5 A new approach, double-pass technology, has also been used to measure optical aberrations and ocular scatter. We thought it possible that the ocular scatter analysis of the double-pass retinal image might provide objective information on cataracts.

The Optical Quality Analysis System (Visiometrics) is the only currently available clinical instrument that quantifies the combined effect of optical aberrations and light scatter resulting from the loss of ocular transparency in the human eye based on double-pass technology. This double-pass system provides several parameters, including the objective scatter index (OSI), modulation transfer function, and maximum visual acuity.7 The OSI shows the degree of objective scattering caused by loss of ocular transparency.

In this study, we used the Optical Quality Analysis System II and Pentacam Scheimpflug imaging (Oculus Optikgeräte GmbH) in eyes with age-related nuclear cataract to assess the correlation between the OSI and Scheimpflug-measured lens nuclear density, the relationship between the OSI and LOCS III nuclear grading, visual function measured by logMAR corrected distance visual acuity (CDVA), and phacoemulsification energy.

Patients and methods

This prospective observer-masked study evaluated patients who had age-related nuclear cataracts diagnosed between March 2012 and November 2012 at Seoul St. Mary’s Hospital, Seoul, Korea. The study protocol followed the guidelines of the Declaration of Helsinki and was approved by the hospital’s institutional review board. All patients provided written informed consent after they received a detailed explanation of the study and potential complications. No patient had a history of ocular disease, ocular surgery, or general disorders affecting vision (eg, diabetic retinopathy, uveitis).

Each patient had a comprehensive preoperative examination that included logMAR uncorrected distance visual acuity, logMAR CDVA, slitlamp evaluation, Goldmann applanation tonometry, fundus evaluation, optical quality analysis using the double-pass system, and Scheimpflug analysis.

Nuclear Grading

The nuclear sclerotic grade was classified by LOCS III.1 The same ophthalmologist examined all patients using a slitlamp microscope and graded each eye preoperatively. Eyes were maximally dilated with a combination of topical tropicamide 1.0% and phenylephrine 2.5%. The LOCS III nuclear opacity was graded on a scale of 0.1 to 6.9 by comparing a digital photograph of each lens with standard color photographic transparencies of nuclear opalescence (NO) and nuclear color (NC).

Double-Pass Measurements

With the double-pass system, a point of light (780 nm infrared laser diode) is projected and focused on the patient’s retina. An infrared video camera records the reflected light of the retina through the ocular media.7 In our study, the entrance pupil diameter was set at 2.0 mm and the exit pupil diameter at 4.0 mm. The OSI is defined as the ratio between the integrated light in the periphery and in the area surrounding the central peak of the double-pass image. This index enables calculation of the degree of scattering caused by loss of ocular transparency. The OSI scale ranges from 0 (no scatter) to 25 (maximum scatter). The same examiner (S.A.L.) took all eye measurements and included corrected sphere and cylinder to avoid artifacts such as defocus and astigmatism.

Scheimpflug Measurements

Preoperative lens density was measured with the Scheimpflug system. The system uses a blue light–emitting diode to detect anterior segment image, capturing 25 single slit images and collecting 25, 000 true elevation data points. The 3-dimensional representation of the anterior segment provides a lens image and density at a chosen point (range 0 to 100). The same examiner (S.A.L.) took 2 measurements of each eye after the pupil was dilated. The point of maximum density along the nucleus was chosen and calculated automatically.4 The mean value of the maximum density was used for analysis. Reproducibility of the lens density measured in 50 eyes using the Scheimpflug system was previously evaluated and showed a high correlation between 2 successive measurements in each eye (r = 0.942).

Surgical Technique

All cataract surgery was performed using the same technique by the same surgeon (S.-H.C.). Surgery was performed through a self-sealing 2.2 mm temporal clear corneal incision under topical anesthesia. An Akahoshi phaco prechopper (Katena Products, Inc.) was used to divide the nucleus in half. Phacoemulsification was performed in the capsular bag using an Ozil phaco machine (Alcon Laboratories, Inc.); the torsional continuous mode with 100% amplitude was selected. The intraoperative cumulated dissipated energy (CDE) was obtained. The mean CDE indicates the mean percentage of power used during ultrasound (US). The CDE in torsional mode is as follows: torsional amplitude × torsional time × 0.4. The mean CDE was automatically calculated and displayed on the monitor of the phaco machine.

Statistical Analysis

All data are expressed as the mean ± standard deviation. Statistical analysis was performed using SPSS software (version 17.0, International Business Machines Corp.). The Pearson correlation analysis was used to compare the OSI with the Scheimpflug-measured lens density, CDE, and CDVA. Spearman correlation analysis was used for statistical comparisons of LOCS III scores. A P value less than 0.05 was considered statistically significant.

Results

This study enrolled 70 eyes of 47 patients (41 women, 29 men). Table 1 shows the patients’ preoperative characteristics. No intraoperative complications occurred.

Table 1
Table 1:
Patient characteristics.

The OSI had a high positive linear correlation with the LOCS III NO and NC scores (r = 0.772 and r = 0.752, respectively; both P<.01) The Scheimpflug-measured lens nuclear density was also positively correlated with the LOCS III NO and NC scores (r = 0.754 and r = 0.748, respectively; both P<.01). Two objective lens-density assessment systems (OSI and the Scheimpflug-measured lens nuclear density) were highly correlated with each other (r = 0.764, P<.01) (Figure 1).

Figure 1
Figure 1:
Relationship between the OSI measured with the double-pass system and Scheimpflug-measured lens nuclear density (OSI = objective scatter index).

The CDVA was correlated with the LOCS III NO and NC scores (r = 0.550 and r = 0.563, respectively; both P<.01) and with the Scheimpflug-measured lens nuclear density value (r = 0.583, P<.01) (Figure 2). However, the CDVA had a stronger relationship with the OSI value (r = 0.626, P<.01) than with the Scheimpflug-measured lens density value or with the LOCS III NO or NC scores (Figure 2).

Figure 2
Figure 2:
Relationships between (A) double pass–measured OSI and CDVA, (B) Scheimpflug-measured lens nuclear density and CDVA, (C) LOCS III NO score and CDVA, and (D) LOCS III NC score and CDVA (CDVA = corrected distance visual acuity; LOCS = Lens Opacities Classification System; NC = nuclear color; NO = nuclear opalescence; OSI = objective scatter index).

In terms of phacodynamics, the mean CDE was 8.92 ± 6.70 (range 0.53 to 24.80). Table 2 shows the relationships between the CDE and the following: LOCS III NO and NC scores, the OSI, and the Scheimpflug-measured lens nuclear density. The CDE had stronger relationships with the OSI and Scheimpflug-measured lens nuclear density (r = 0.768 and r = 0.753, respectively; both P<.01) than with LOCS III NO and NC scores (r = 0.719 and r = 0.712, respectively; both P<.01).

Table 2
Table 2:
Relationships between CDE and LOCS III NO and NC scores, the OSI, and Scheimpflug-measured lens nuclear density.

Discussion

In this study, we found that the OSI based on the Optical Quality Analysis System II double-pass system was correlated with objective lens nuclear density measured using the Pentacam Scheimpflug system in eyes with age-related nuclear cataract. The OSI was positively correlated with the NO and NC scores of the LOCS III subjective lens grading system, the CDVA, and the CDE. Because the ocular scatter analysis of double-pass retinal images reflects cataract density if other ocular pathology is excluded, this information is clinically useful for objective quantification of cataracts and may aid in predicting the outcomes of cataract surgery.

Although the LOCS III is the most widely used classification system, there is ongoing concern about observer bias and reproducibility of results.1,2 Objective quantification of a cataract is vital to evaluate the possible risk factors for cataract formation and for predicting phacodynamics.8 There have been attempts to objectively classify lens density using Scheimpflug images. This system enables quantitative cataract grading from the anatomic lens density point but not from the functional retinal image point.4,5 In a previous study, we found that the objective method based on the use of Pentacam Scheimpflug images was more strongly correlated with CDE than with LOCS III NO or NC scores.4

Others report that ocular scatter increases with cataract density.9,10 In the double-pass system, light travels in 2 directions. It passes forward and then backward through the crystalline lens, providing general information about light scatter in the entire eye and about visual performance.11 Analysis of light distribution in the double-pass image can quantify intraocular scattering.12,13 Artal et al.14 found a significant correlation between the OSI and the LOCS III NO and NC scores. The type of cataract may affect the ocular scatter. Vilaseca et al.15 found that nuclear, cortical, and posterior subcapsular cataracts led to a decrease in vision and an increase in the OSI. However, in box plots to compare the OSI and CDVA, there was a significant dispersion in both parameters in the cortical group and posterior subcapsular cataract group because some patients with these cataracts still have a clear lens center and the double-pass system measured the optical quality using a pupil diameter of 4.0 mm. In our current study, the OSI based on the double-pass system was also significantly correlated with the LOCS III NO or NC scores and was well correlated with objective Scheimpflug-measured lens density. The correspondence between OSI values and lens density suggests the applicability of using these objective parameters to assess the severity of nuclear cataracts and to make decisions about surgery.

The double-pass technique is useful in examining forward-scattered light, which causes degradation of retinal images in eyes with cataract.16 There is a linear correlation between CDVA and ocular scatter, especially in eyes with nuclear cataract.17 In this study, we found positive correlations between the CDVA with the OSI, LOCS III NO and NC scores, and Scheimpflug-measured lens density (P<.01). The CDVA had a stronger positive correlation with the OSI (r = 0.626) than with the LOCS III NO score (r = 0.550), the LOCS III NC score (r = 0.563), and the Scheimpflug-measured lens density (r = 0.583). A recent study that used the double-pass system12 found that the correlation coefficient of the OSI with CDVA was 0.7 in cases of nuclear cataract. These results indicate that the OSI is more closely related to the patient’s subjective symptoms related to optical quality than the LOCS III classification or Scheimpflug-measured lens density.

The relationship between forward scatter and backward scatter was inconclusive.18,19 The OSI represents the forward-scattered light that reaches the retina and lens density using the Pentacam Scheimpflug system, and the LOCS III score using the slitlamp represents backward scatter. In in vitro studies,10,20 forward scatter was well correlated with backward scatter only when cataract areas were selected. In our study, the high correlation between the OSI and lens density may be explained by the fact that optically regular nuclear cataracts were included in the study.

In terms of phacodynamics, more phacoemulsification energy and time are needed for hard nucleus removal. In the torsional mode, the mean CDE indicates the mean percentage of power used during US and reflects the hardness of the nucleus. In our study, the CDE had stronger correlations with the OSI and Scheimpflug-measured lens density than with the LOCS III NO or NC scores.

In addition to ocular scatter, ocular aberration had an effect on visual performance. In nuclear cataract, higher-order aberrations such as spherical aberration are high and cause visual disturbance. Experimental double-pass images found that ocular aberration has some contribution to the OSI parameter related to intraocular scatter.21 Also Pérez et al.22 suggest that the combined presence of positive spherical aberration and scatter in old age could provide a mild protective compensatory effect. The OSI parameter is largely affected by refractive errors and is needed to correct defocus or astigmatism. Further studies are needed to evaluate the relationship between ocular scatter, aberrations, and visual performance.

We compared the OSI of nuclear cataracts with Scheimpflug-measured lens density primarily because of the difficulty in measuring the posterior cortex and posterior capsule with the Scheimpflug system, even with the pupil fully dilated. Similarly, with respect to phacodynamics, the CDE is related to the extent of nuclear cataracts. Thus, nuclear cataracts were selected to compare the CDE with the OSI, Scheimpflug-measured lens density, and LOCS III NO and NC scores.

In conclusion, the OSI based on the double-pass system was useful for the quantification of nuclear cataracts and for predicting phacoemulsification energy. It was correlated with the subjective LOCS III classification and with the objective Scheimpflug-measured lens density. In terms of visual performance, the OSI had a stronger correlation with CDVA than with the subjective LOCS III classification or objective Scheimpflug-measured lens density. Thus, the OSI based on the double-pass system may be a good tool for objective lens opacity grading, enabling monitoring of cataract progression and aiding in predicting phacodynamics in cataract surgery.

What Was Known

  • Objective quantification of a cataract is vital in evaluating the possible risk factors for cataract formation and predicting phacodynamics.

What This Paper Adds

  • Ocular scatter was well correlated with lens density in nuclear cataracts and can be a useful tool in predicting phacodynamics in cataract surgery.

References

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