Cataract is a major public health issue, since it is the most prevalent condition afflicting patients who attend an optometric practice; according to the World Health Organization, it is one of the principal causes of blindness worldwide.1 Accordingly, reliable assessment of cataract is indispensable to anyone engaged in an epidemiological study or a clinical trial.2 3 Studies of cataract classification and quantitative measurement are vital for investigating the possible risk factors of cataract formation and evaluating the performance of anti-cataract drugs. Clinically, the methods used for cataract assessment may be divided into two types, subjective and objective. The former includes the Lens Opacities Classification System (LOCS),4 the Wisconsin system,5 the Wilmer system6 and the Oxford system.7 The latter, based on Scheimpflug photography or slit-lamp imagery, includes the Oxford Scheimpflug,8 the Topcon SL-45,9 the Zeiss Scheimpflug video camera10 and the Nidek EAS-100011 which had been used in clinical practice.

LOCS III is well recognised as an age-related cataract grading scheme and is widely used for clinical and research study.12^–15 Some studies have shown a good reproducibility with this method for cataract grading.4 12 16 However, it suffers, as do all other subjective assessments, from limitations rooted in observer bias and doubts concerning interobserver and intraobserver reliability.

Pentacam, a recently developed camera based on the Scheimpflug principle, captures images of the anterior eye segment in order to create a precise, three-dimensional view, and uses the digitally acquired data to evaluate the parameters of lens, cornea and anterior chamber. The purpose of this study was to investigate the relationship between lens density measured with Pentacam and LOCS III grading score as well as that between lens density and visual acuity in age-related nuclear cataract patients, with a view to evaluating the relative merits of the different approaches for the assessment of age-related nuclear cataract.

PATIENTS AND METHODS

A total of 138 patients (180 eyes) with age-related nuclear cataract from the People’s Eye Center, People’s Hospital of Peking University, aged 51 to 86 years (70.3 (SD 8.5) years) were recruited in this study; another 19 people (30 eyes) aged 50 and 85 years (68.0 (9.1) years) with clear lens and normal vision were enrolled as the control group. This study was approved by the Human Research Ethics Committee of the People’s Hospital of Peking University and adhered to the guidelines of the Declaration of Helsinki for doing research on human subjects. Written informed consent was obtained from all participants.

Inclusion criteria were as follows: subjects of the control group were emmetropia, with uncorrected visual acuity of 6/6 or better, and the lenses were clear; age-related nuclear cataract patients without other ocular abnormalities were evaluated by measuring the corneal power and axial length, which were in the normal range of corneal power (42–44 dioptres) and axial length (22–24 mm). The distribution of age and gender is shown in table 1. The differences among LOCS III groups were not statistically significant (F = 1.870, p = 0.087; χ² = 3.578, p = 0.965).

A comprehensive eye examination was performed on each participant, including uncorrected visual acuity (logMAR Early Treatment Diabetic Retinopathy Study (ETDRS) acuity chart, Precision Vision, Aurora, CO), refraction (Shin-Nippon auto refractometer SR-998, Shin-Nippon, Kinzoku Kagaku, Japan), slit-lamp examination (Topcon SL-1E, Tokyo, Japan), fundus examination (90Dioptre, Volk Opticals, Mentor, OH), axial length measurement (Ultrasound Ocuscan, Alcon, Fort Worth, TX) and the Pentacam Scheimpflug system (Oculus, Wetzlar, Germany).

LENS GRADING

The nucleus opacity of each eye was assessed according to LOCS III standards under pupil dilation with tropicamide 1%. Two ophthalmologists (YB and XP) graded every eye into nuclear opacity (NO) and nuclear colour (NC) and decided in which interval the lens feature fell under slit-lamp examination. The assigned score must be between two successive standards. Each interval between adjacent reference standards is divided into 10 equal parts. The scale ranges from 0.1 to 6.9.

Before the start of this study, the degree of agreement of scores determined by the two graders was checked, and their scores showed a high correlation (R = 0.971). The mean difference in LOCS III scores was 0.04, the standard deviation of difference was 0.24, and the 95% CI was −0.4 to +0.44 for 30 eyes.

LENS DENSITY MEASUREMENT

The lens density of each participant was evaluated by one examiner using the Pentacam system. After pupil dilation (tropicamide 1% for half an hour), the participant sat in front of the camera and placed their chin on a chin rest. Next, the image of each eye was focused and centred manually. The Pentacam uses a blue light-emitting diode to image the anterior eye segment, capturing 25 single-slit images in 2 s while rotating around the eye from 0° to 180°. The instrument collects 25 000 true elevation data points (corresponding to 500 data points per slit image), which are processed to generate a three-dimensional representation of the anterior eye and provides an image of the whole lens along with an objective measurement of the lens density in the chosen point, reading from 0 to 100.

On the three-dimensional plot of anterior segment with each section running through the corneal vertex, the required lens density was taken as the peak value on an image at 120–300° for the right eye and at 240–260° for the left eye.

Two measurements were performed on each eye, and the mean value was recorded. The interval between the two measurements was several minutes, the time needed for the instrument to process the data, and each patient was asked to sit back and relax during the interval. The joystick of the camera was fully retracted and then realigned to ensure proper resetting of the instrument.

Reproducibility of the lens density evaluation in two scans for 30 eyes was checked and showed a high correlation between two successive scans (R = 0.986). The 95% limits of agreement, defined as the mean intraoperator difference (±1.96SD of differences), were −0.78 to +0.75.

VISUAL ACUITY TESTING

Visual acuity was measured on each eye of the participants by a single examiner with the ETDRS acuity chart and recorded as logMAR. The ETDRS chart illuminated for photopic (85 cd/m²) light level was displayed in a consistent indoor environment under diffuse artificial light. Visual acuity was measured in the right eye and then in the left eye, using the ETDRS chart which has 0.1 logMAR steps with five letters on each line. With this particular chart configuration, each letter was equivalent to 0.02 logMAR. Participants were examined with monocular uncorrected vision without any spectacle correction and responded to each optotype by indicating the direction of the letter E using their hands. Charts were viewed from a distance of 4 m unless the participant made any incorrect responses on the top line of a given chart, indicating an acuity worse than +1.00 logMAR. In this event, the participant was moved to 1 m, and the remainder of the testing process was performed at this distance. After all the optotypes on the chart had been attempted, the logMAR acuity score was recorded and used in statistical analysis.

STATISTICAL ANALYSIS

Statistical analysis was performed using SPSS 11.0 (SPSS, Chicago). A p value less than 0.05 was considered to be statistically significant. The normality of continuous variables distributions of all the data was checked using the Shapiro–Wilk test. After the test, the lens density was normally distributed in each group, and the mean values and standard deviation of lens density were calculated. The logMAR visual acuity and LOCS III score were not normally distributed, so the medians and interquartile range of logMAR visual acuity were calculated. The relationship between lens density and LOCS III score was analysed using Spearman correlation analysis for non-normally distributed variables. In addition, the partial correlation analysis was performed to evaluate the relationship between lens density, LOCS III score and logMAR visual acuity while controlling for age.

RESULTS

The mean axial length was 23.13 (0.79) mm (range 22.02 to 23.96 mm), and the mean central corneal power was 43.2 (0.71) dioptres (range 42.10 to 44.00 dioptres). The image of age-related nuclear cataract of one patient obtained using the Pentacam system is shown in fig 1.

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Figure 1 Pentacam Scheimpflug image of nuclear cataract with a lens density reading 35.6 and Lens Opacities Classification System III grading score of 6.2.

For nuclear opalescence of age-related nuclear cataract, 30 eyes were selected every 10 equal NO parts. Then, for NC grading, all the data were grouped again every 10 equal NC parts. Parameters including LOCS III score, lens density and logMAR visual acuity were obtained, and the results are displayed in table 2.

Figure 2 shows that there is a highly positive linear correlation between lens density and LOCS III nuclear opacity score; the Spearman correlation coefficient is 0.965 (p = 0.000). Figure 3 shows the correlation between lens density and LOCS III nuclear colour score, and the Spearman correlation coefficient is 0.842 (p = 0.000).

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Figure 2 Relationship between lens density value and Lens Opacities Classification System III nuclear opacity (NO) score (R = 0.965, p = 0.000).

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Figure 3 Relationship between lens density value and Lens Opacities Classification System III nuclear colour (NC) score (R = 0.842, p = 0.000).

The increasing lens density is associated with decreased logMAR visual acuity controlling for age (R = 0.867, p = 0.000). The correlation between NO score and logMAR visual acuity (R = 0.741, p = 0.000) and between NC score and logMAR visual acuity (R = 0.653, p = 0.000) controlling for age is shown in fig 4.

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Figure 4 (A) Relationship between lens density value and logMAR visual acuity in nuclear cataract (R = 0.867, p = 0.000). (B) Relationship between Lens Opacities Classification System III (LOCS III) nuclear opacity (NO) score and logMAR visual acuity (R = 0.741, p = 0.000). (C) Relationship between LOCS III nuclear colour (NC) score and logMAR visual acuity (R = 0.653, p = 0.000).

The lens density value had a stronger statistical significance with the LOCS III NO score (R = 0.965, p = 0.000) than with the NC score (R = 0.842, p = 0.000). LogMAR visual acuity correlated more strongly with the lens density value (R = 0.867, p = 0.000) than with the NO or NC score.

DISCUSSION

It is important to quantify lens opacity objectively in studies of cataract formation and growth; the lack of objective measurements of lens opacity is a serious impediment to research activity in this area as well as clinical trials.8 17 18 19 In this study, we conducted quantitative measurements of lens opacity in nuclear cataract patients using the Pentacam Scheimpflug system, to evaluate the performance of the traditional, subjective assessment scheme, LOCS III criterion and visual acuity testing. In comparison with the LOCS III grading score (0.1 to 6.9 in steps of 0.1) and visual acuity measured by logMAR ETDRS visual acuity chart (−0.08 to 2.0), the lens density value was presented as a continuous percentage scale. These data showed a high reproducibility of lens density measured with Pentacam. There is a linear, increasing relationship between lens density and LOCS III NO score. The lens density value in the nuclear cataract had a significant correlation with visual impairment, and decreased visual acuity was associated with an increased lens density value. These results are in good agreement with the findings of Hall et al,16 who used a laser slit lamp and reported a high correlation between compensated pixel intensities analysed with software to assess lens opacity and LOCS III NO score.

Our study used the peak value of lens density, but other studies using different instruments have shown that both peak and mean values can be used with equal confidence for assessing lens opacity in cataract patients.9^–11 Foo and Maclean20 reported that the peak value and mean value of the lens density with Nidek EAS-1000 were able to present changes over a 6-month period in nuclear cataract but not in cortical or PSC cataract. Although the peak value of lens density was evaluated at one point of the entire lens nucleus, it may be considered as a representative index of the whole nucleus because, according to West and Taylor,21 NO is likely to be homogenous, and it is reasonable to suppose that visual impairment in nuclear cataract patients is most likely dependent on the opacity of the central nucleus rather than mean opacity. A further study of the correlation between mean value of lens density and lens opacity in different types of cataract is needed to grade cataract clinically and to assess visual impairment due to cataract.

There are three important issues in estimating the severity of cataract and the need for cataract surgery, including the level of lens opacity, visual acuity and patient concern.22 In age-related nuclear cataract, the mechanism of visual impairment is multifactorial, but light scattering due to random fluctuations of the refractive index of the lens material plays a major role. Light scattering can be produced in the lens by the formation of opalescence and condensation of the nucleus. Therefore, NO is dramatically responsible for visual impairment in emmetropic nuclear cataract patients who undergo cataract surgery. Our study showed that logMAR visual acuity correlated more strongly with the lens density value than with the LOCS III NO or NC score. In the literature, Datiles et al23 reported that lens density could present nuclear change earlier than LOCS II and visual acuity with 1-year follow-up by the NEI Scheimpflug system. According to the above results, an objective method would seem to be more valid and sensitive in estimating lens opacity and visual impairment.

Although many objective classification systems for nuclear opacities have been developed based on Scheimpflug photography or slit-lamp imagery, which was digitised and subjected to densitometric analysis along a slice through the nucleus, a subjective assessment scheme is still an economical and convenient method, and is widely used in estimating cataract growth and the need for cataract surgery. Our study concerning the reproducibility of LOCS III was successful, and the results agreed with those of other researchers.12 16 LOCS III grading score was well correlated with lens density and moderate associated with logMAR visual acuity. Meanwhile, the high correlation between LOCS III grading score and lens density presented in nuclear cataract assessment is similar to other studies reported by Rouhiainen et al24 and Duncan et al.25

In summary, there was a very high correlation between the peak value of lens density measured with the Pentacam Scheimpflug system and the grading score using the LOCS III system as well as the correlation between lens density and logMAR visual acuity in age-related nuclear cataract patients. It is also reassuring that the LOCS III criterion is a cheaper cataract-grading system and provides data that are in satisfactory concordance with the results obtained using far more expensive instrument. Therefore, LOCS III can be a reliable and economic method in estimating the severity of age-related nuclear cataract in societies with low medical resources.