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Changes in the anterior chamber during accommodation assessed with a Scheimpflug system

Domínguez-Vicent, Alberto MSc*; Monsálvez-Romín, Daniel OD; Del Águila-Carrasco, Antonio J. MSc; Ferrer-Blasco, Teresa PhD; Montés-Micó, Robert PhD

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Journal of Cataract & Refractive Surgery: November 2014 - Volume 40 - Issue 11 - p 1790-1797
doi: 10.1016/j.jcrs.2014.02.043
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Abstract

Changes in the anterior chamber with accommodation have been studied in different populations using a wide range of measuring techniques. Table 1 shows the studies that measured the changes in the anterior chamber depth (ACD),1–28 anterior chamber angle,16 and pupil diameter3,7,26,27,29 with accommodation. These studies found that all 3 parameters decreased as a result of the eye’s natural accommodation process. All focused on the central ACD; that is, the ACD value measured at the pupil center. Figure 1 shows a side-by-side comparison of 2 Scheimpflug images; the white arrows indicate the central ACD for far vision and the ACD after −4.00 diopters (D) of accommodation.

Figure 1
Figure 1:
Changes in ACD for far vision and after −4.00 D of accommodation in the same subject (ACDac = central anterior chamber depth during accommodation. ACDfar = central anterior chamber depth for far vision).
Table 1
Table 1:
Results in peer-reviewed studies that assessed the ACD, anterior chamber angle, and pupil diameter with accommodation. The negative signs represent a decrease.
Table 1
Table 1:
Continued.

However, despite the wealth of data available in the literature, to our knowledge no published study to date has focused on the quantitative changes in the peripheral ACD with accommodation. This information would be useful to know if the anterior chamber of the eye is symmetric and to develop new intraocular lenses (IOLs) that preserve the anatomic structures. Only 1 study16 has evaluated the changes in the anterior chamber angle at the horizontal meridian and vertical meridian in pseudophakic eyes after cataract surgery. There are no data in the literature regarding changes with accommodation in young people. Determining whether the anterior chamber angle decreases significantly with accommodation would help in the design of more anatomically correct anterior chamber phakic IOLs. Finally, although several studies evaluated the change in pupil diameter with accommodation,3,7,26,27,29 none used a Scheimpflug imaging system, which allows assessment of the changes in pupil diameter with accommodation.

The aim of the present study was to evaluate changes in the ACD, anterior chamber angle, and pupil diameter with accommodation. Moreover, the ACD was measured 2.0 mm from the corneal center in the nasal, superior, temporal, and inferior directions, areas that have not been previously studied. Measurements were taken in young people, who have higher accommodation capability than an older population. Because of this, changes in the anterior chamber with accommodation should be greater in a young population.

Subjects and methods

The study included the right eyes of subjects aged 22 to 30 years. To avoid possible bias related to relaxation of accommodation, all subjects were trained and able to accommodate to a red target during testing. Written informed consent was obtained from all subjects in accordance with the Declaration of Helsinki.

Inclusion and Exclusion Criteria

The inclusion criterion was a corrected distance visual acuity (CDVA) of 20/25 or better. Subjects with a CDVA worse than 20/30, ocular or systemic disease, a history of ocular surgery, intraocular pressure above 21 mm Hg, or retinal or optic nerve pathology were excluded from the study.

Scheimpflug Imaging Device

The Pentacam HR (Oculus Optikgeräte GmbH) was used to study the changes in the anterior segment at different accommodation vergences. This noninvasive optical diagnostic device combines a slit illumination system that has a wavelength of 475 nm and a Scheimpflug camera that rotates around the eyeA (Figure 2). A thin layer in the eye is illuminated through the slit, creating a sectional image that is then photographed. The camera is oriented according to the Scheimpflug principle, producing an image of the illuminated plane that is in sharp contrast to the anterior surface of the cornea up to the posterior surface of the crystalline lens. Rotating around the eye, the slit camera generates a series of radially oriented images of the anterior chamber. The sectional images are saved, corrected in relation to a common reference point, and combined to create a 3-dimensional (3-D) model of the entire anterior chamber. This generates reproducible tomographic images of the anterior chamber in any desired plane. Eye movements during image acquisition are captured by a second camera (pupil camera) and also taken into account in the mathematic evaluation. This produces a set of 3-D measurement data that give a geometric description of the anterior segment. The Scheimpflug system has a red light–emitting diode that serves as a fixation target and can be set to induce an accommodation state ranging from +2.00 to −5.00 D in steps of 0.50 D.

Figure 2
Figure 2:
The Scheimpflug system used in this study.

The Scheimpflug system was used to automatically measure the central and peripheral ACD, anterior chamber angle, and pupil diameter for far, intermediate, and near vision. The maximum image resolution obtained in the minimum duration time was used to take each measurement. The central ACD was computed as the distance between the corneal endothelium and the anterior surface of the crystalline lens. The peripheral ACD was measured 2.0 mm from the center of the cornea in the nasal, temporal, superior, and inferior directions and was defined as the distance between the corneal endothelium and the iris. In all cases, the ACD measurements had a resolution of 0.01 mm. The anterior chamber angle was computed from the anterior segment reconstruction image as the angle of intersection between the posterior corneal surface and the iris surface with a measure resolution of 0.10 degree. Finally, the pupil’s mean diameter during the examination (resolution 0.01 mm) was considered the pupil diameter.

Measurements

The same technician, who was experienced in operating the Scheimpflug device, performed all measurements. The technician was not aware of the study’s goal.

The ACD, anterior chamber angle, and pupil diameter values were measured at a viewing distance ranging from +1.00 to −4.00 D in steps of 1.00 D. Far vision was measured from +1.00 to 0.00 D; intermediate vision from −1.00 to −2.00 D; and near vision, from −3.00 to −4.00 D. This allowed assessment of the changes in the anterior segment during the most common accommodation efforts. The +1.00 D step was used to ensure all patients did not accommodate during far vision measurements. For each scenario (subject, corneal location, and viewing distance), the measurement was repeated 3 times and the mean value was recorded.

Before the measurements were taken, the subject fixated on the target for 2 seconds to obtain an appropriate accommodation response, as reported by López-Gil et al.30 Moreover, during the centration procedure, the subject was asked to maintain focus on the fixation target and the technician waited 2 seconds before starting the measurement.

During the measurement, the subject was asked not to blink to avoid distortion of the measurement. All measurements of each subject were completed in a single session.

Statistical Analysis

Sigmaplot for Windows software (version 11, Systat Software, Inc.) was used for statistical analysis of the data. The study had a factorial design because the main goal was to compare the values obtained under different conditions, such as accommodative stimulus and anatomic location. Repeated-measures analysis of variance (ANOVA) used after the Shapiro-Wilk test confirmed the normality assumption. In cases in which the ANOVA showed statistically significant differences, post hoc multiple comparisons were performed to find the pair comparison with significant differences using the Holm-Sidak method. A P value of 0.05 was considered statistically significant.

Two-way ANOVA was used to assess the ACD; the 2 factors in the ANOVA were accommodative stimulus and location. For pupil diameter and the mean anterior chamber angle, 1-way ANOVA was performed with accommodative stimulus as the main factor.

Results

The study included 80 right eyes of 80 subjects. The mean age of the 39 men and 41 women was 25.30 years ± 2.98 (SD). The mean spherical equivalent (SE) was 0.04 ± 1.56 D.

Anterior Chamber Depth

Table 2 shows the mean ACD values for each accommodative stimulus and position. The relative change between far vision and near vision was −3.67%, −3.66%, −3.67%, −5.66%, and −6.22% for the central, nasal, temporal, superior, and inferior ACD, respectively; negative values represent a decrease in ACD. However, there were no statistically significant differences in ACD measured at far vision and at any accommodative stimulus.

Table 2
Table 2:
Mean anterior chamber depth, anterior chamber angle, and pupil diameter.

Although the deepest anterior chamber measurement was obtained at the central position, the temporal anterior chamber and inferior anterior chamber were significantly deeper than the nasal anterior chamber and superior anterior chamber; the superior-nasal ACD and temporal-inferior ACD were comparable as well. This was true for far vision and near vision.

Anterior Chamber Angle

Table 2 shows the mean anterior chamber angle values for each accommodative stimulus and position. The relative change in the anterior chamber angle around 360 degrees was 0.25%, 0.75%, 0.08%, and 1.58% with −1.00 D, −2.00 D, −3.00 D, and −4.00 D of accommodation, respectively. However, the anterior chamber angles were comparable between all vergences (P > .05).

Pupil Diameter

Table 2 shows the mean pupil diameter with accommodation. The pupil diameter changed by 2.19%, −7.29%, −5.84%, and −4.37% with −1.00 D, −2.00 D, −3.00 D, and −4.00 D of accommodation, respectively. The decrease in pupil diameter was statistically significant with −4.00 D and −3.00 D of accommodation. Statistically significant differences were also obtained between −1.00 D and −4.00 D.

Discussion

The aim of our study was to assess the changes in ACD, anterior chamber angle, and pupil diameter as a function of accommodation. We sought to determine whether accommodation modifies these ocular parameters. We also assessed the corneal location–related differences in the anterior chamber angle with different accommodative stimuli.

Neither the central ACD nor the peripheral ACD varied significantly with accommodation (P > .05). The relative change in the central ACD with −4.00 D of accommodation was −3.67%, meaning there was a decrease in the ACD. The mean decrease was −0.11 ± 0.17 mm. Of the many studies that measured central ACD changes with accommodation (Table 1),1–29 only 8 4,15,17,18,22,23,25,26 had a population similar to ours. Thus, our central ACD values were consistent with those in the 8 studies. Yan et al.26 obtained larger changes than the other 7 studies, reporting a mean decrease in the central ACD of 0.25 ± 0.09 mm. This varies by approximately 0.12 mm from our mean ACD change. Discrepancies between our results and Yan et al.’s could be due to the stimulus vergence range analyzed. Our highest vergence was −4.00 D; Yan et al.26 did not specify the vergence because they used the subject’s maximum amplitude of accommodation. A recent study by Nemeth et al.31 evaluated the ACD using 2 optical devices; that is, the Pentacam HR and the Visante anterior segment optical coherence tomographer (Carl Zeiss Meditec). The 95% limits of agreement between the 2 devices was approximately 0.2 mm, which is not clinically relevant. Thus, it is likely that the main reason for the discrepancy between Yan et al.’s26 results and ours were the differences in the maximum vergence, not the differences in the principle the device uses for measurements.

In our study, the ACD varied in an asymmetric manner across the eye; the superior–nasal quadrant was significantly shallower than the inferior–temporal quadrant. These results agree with those of Koç et al.,32 who studied the anterior chamber width with a Scheimpflug camera. They found that the superior–nasal quadrant was significantly narrower than the inferior–temporal quadrant. These results have important implications for the vault design of anterior chamber IOLs. In view of the above results, the IOL thickness should be asymmetric to avoid possible contact between the IOL and the corneal endothelium. Specifically, for far vision (0.00 D), in comparison to the central cornea, the IOL should be 0.26 mm closer to corneal endothelium at the temporal side and 0.76 mm closer to corneal endothelium at the nasal side.

Malyugin et al.17 studied the accommodative changes in the ACD in emmetropic subjects and myopic subjects; the mean SE was −0.39 ± 0.33 D and −12.52 ± 4.07 D, respectively. They found that the maximum decrease in the ACD during accommodation in myopic eyes was on average 1.5 times lower than that obtained in emmetropic eyes. Yan et al.26 evaluated the changes in anterior segment parameters with accommodation in emmetropic, myopic, and hyperopic subjects. In that study, the mean ACD was −0.25 ± 0.09 mm, −0.15 ± 0.07 mm, and −0.06 ± 0.04 mm, respectively. Thus, the lowest decrease in ACD with accommodation occurs in hyperopic eyes while the greatest changes occur in emmetropic eyes.

The anterior chamber angle varied 1.58% from far vision (0.00 D) to near vision (−4.00 D); however, the change was not significant (P > .05). Only 1 study in the literature16 evaluated the changes in the anterior chamber angle with accommodation. In the study, the authors assessed elderly patients who had IOL implantation. With 3.00 D of accommodation, the mean decrease in the anterior chamber angle was 1.22 degrees at the horizontal meridian and 0.75 degrees at the vertical meridian. These values were higher than in our study; however, the main difference between our results and those of Marchini et al. was the anterior chamber angle measurement. Although Marchini et al.16 report changes in the anterior chamber angle at the horizontal meridian and vertical meridian, in our study the mean change in the anterior chamber angle was around the entire eye. Thus, it is possible that differences in the anterior chamber angle were significant. Other possible factors for discrepancies between the studies include the measurement method (ultrasound biomicroscopy versus Scheimpflug imaging) and the differences in the 2 cohorts (pseudophakic subjects versus phakic subjects with different age ranges).

In our study, the pupil diameter decreased significantly between far vision (0.00 D) and near vision (−4.00 D) (P<.05); the relative change was −4.38%. Yan et al.26 measured the pupil diameter up to the maximum amplitude of accommodation in emmetropic subjects using the slitlamp optical coherence tomography (OCT) system; the relative pupil diameter decrease was 20.83%. Yuan et al.27 measured changes in the pupil diameter with accommodation in myopic subjects using a spectral-domain OCT system; the relative decrease was 15.80%. The discrepancy between the results in these studies and ours was probably due to the measuring procedure. The other 2 studies used the full accommodation interval without specifying the vergence value; the highest accommodative stimulus used in our study was −4.00 D. Another possible reason is the difference in luminance conditions with each device.

Baikoff et al.3 used anterior chamber OCT to measure how pupil diameter varies after accommodating −10.00 D in dim light. They report a mean change in pupil diameter of 1.5 mm, which is 8 times higher than the result we obtained. This discrepancy was probably because of differences in the maximum vergence.

In conclusion, although the ACD and mean anterior chamber angle did not vary significantly with accommodation, the pupil diameter did. Moreover, in light of the location-dependent ACD distribution, we conclude that the eye’s anterior chamber is asymmetric, with the nasal–superior areas being significantly shallower than the temporal–inferior areas. Limitations of our study are that only emmetropic subjects were included, which prevents application of our findings to myopic cases and hyperopic cases, and the use of a single measuring device. Further studies should evaluate the changes in the anterior segment with accommodation in myopic eyes and hyperopic eyes, use alternative measuring devices, and differentiate between male subjects and female subjects.

What Was Known

  • Changes in the anterior chamber with accommodation have been thoroughly studied; however, changes in the anterior chamber angle have not been studied.
  • Changes in the peripheral ACD at different meridians with accommodation in young subjects have not been studied.

What This Paper Adds

  • Anterior chamber depth and anterior chamber angle did not vary significantly with accommodation, while pupil diameter did.
  • The anterior chamber of the eye is asymmetric, with the nasal–superior area being significantly shallower than the temporal–inferior area.

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Other Cited Material

A. Oculus Optikgeräte GmbH. The Pentacam®. The gold standard in anterior segment tomography. Available at: http://www.pentacam.com/sites/messprinzip.php. Accessed June 12, 2014
© 2014 by Lippincott Williams & Wilkins, Inc.