Study cohort

Eighty right eyes from eighty subjects (39 men and 41 women), whose ages ranged from 20 to 40 years (mean ± standard deviation = 30.36 ± 7.32 years), with a mean spherical equivalent of 0.80 ± 0.73 diopters (D) were included in the study. All patients were informed of the details of this study and a written informed consent was obtained in accordance with the Helsinki Declaration. An Institutional Review Board approval was not required for this study. Subjects with best-corrected visual acuity below than 20/40, ocular or systemic disease, history of ocular surgery, intraocular pressure above 21 mmHg, or retinal or optic disk pathology were excluded from the study.

Measuring device and study Metrics

Measuring device

The rotating Dual Scheimpflug and a Placido disc system (Galilei G4, Ziemer Ophthalmic Systems AG, Switzerland) (Figure 2) is one of the latest versions of the Galilei topographer. This device integrated both the Placido and rotated Dual Scheimpflug images to generate a three-dimensional model of the anterior eye segment. This device also recorded images of the cornea, iris, pupil, limbus, anterior chamber, and the lens. Additionally, the Galilei G4 has a red-LED near/far adjustable fixation target, which can be moved in 0.25-D steps from -20 D to +20 D.

Figure 2 Photograph of the measuring device used in this study. 

Experimental procedure

Before each measurement, the central Placido rings were focused and the instrument was aligned. Following this, a volunteer was requested to blink and look at the fixation target for 2 seconds to get an appropriate accommodation response⁽³⁰⁾. Then, to avoid distortion, the subject was requested to not to blink during the measurement. To ensure that all measurements were made under the same lighting conditions, the experiments were performed in dark.

The ACD, ACA, and PD were measured for a viewing distance that ranged from +1 D to -4 D, in 1-D steps. Far, intermediate, and near vision ranged from +1 D to 0 D, -1 D to -2 D, and -3 D to -4 D, respectively. These parameters were used so as to study the changes in the anterior eye segment during the most common accommodative effort. Three measurements were made for each accommodation stimuli and the average value was retained. A single specialist, who had prior experience operating this device and who also was not aware of the study's goal, carried out all experiments. For each subject, all measurements were made within a single session.

Statistical analysis

SigmaPlot for Windows version 11 (Systat Software, Inc.) was used for the statistical analysis of the data. Since the main goal was to compare the values obtained under different conditions such as accommodative stimulus and anatomical location, the study had a factorial design. Therefore, once the normality assumption was confirmed using the Shapiro-Wilk test, the repeated-Measures ANOVA was chosen as the most suitable test. The Post-Hoc multiple-comparison test by means of the Holm Sidak method was then performed in those cases where the ANOVA revealed differences. The threshold of statistical significance was set at P=0.05.

For ACD, a Two-Way Analysis of Variance was performed, whose two factors were "accommodative stimulus" and "location". For the analysis of the pupil size and mean ACA, the One-Way Analysis of Variance was chosen, with "accommodative stimulus" as its main factor. Finally, for location-dependent ACA, the Two-Way Analysis of Variance was used, with "location" and "accommodative stimulus" as the two factors.

Anterior chamber depth

Table 2 summarizes the ACD values for each accommodation stimuli and position. The percentage of relative change between far and near vision was -4.11%, 7.57%, -2.11%, 7.56%, and -0.51% for central, superior, inferior, nasal, and temporal ACD, respectively, where the negative values indicate a decrease in the ACD and the positive values suggest an increase in ACD. No statistical significant differences were observed in this eye metric among far vision and each accommodation stimuli.

The highest ACD was obtained at the central position. The quadrant nasal-superior was significantly shallower than the temporal-inferior quadrant. Further, comparable values were obtained between each pair (nasal-superior, and temporal-inferior). This behaviour was also observed at far and near vision.

Anterior chamber angle

Table 2 summarizes the ACA values for each accommodation-stimuli and position. The percentage of relative change in ACA between far and near visions was -4.55%, -3.72%, -4.71%, -5.07% and -3.73% for nasal, temporal, superior, inferior, and mean ACA, respectively. Whereas the mean ACA at the nasal position and of the whole eye did not vary significantly with accommodation, significant variation was observed in other positions. A significant reduction in ACA at temporal position was obtained among -3.00 D and -1.00 D and 0 D; among -4.00 D and -1.00 D and -2.00 D at superior position, and among -4.00 D and 0.00 D, -1.00 D and -2.00 D at the inferior position.

The nasal-superior quadrant had significantly lower ACA than the temporal-inferior quadrant. Further, comparable ACA was obtained between each pair (nasal-superior and temporal-Inferior).

Pupil Diameter

Table 2 also highlights the absolute PD with accommodation. The PD was reduced -8.31% between far and near visions, and the difference became significant between -4.00 D and -3.00 D and between -1.00 D and -4.00 D.

Anterior chamber depth

The central ACD did not change significantly (P>0.05) with accommodation, the mean change between far and near vision being 0.13 ± 0.13 mm. A number of studies (Table 1) have assessed the changes in central ACD during accommodation. Our results are in agreement with those found previously in emmetropic volunteers^{(4,15,17,18,22,23,25,26)}. Bolz et al.⁽⁴⁾, who used partial coherence interferometry, found a relative ACD decrease of 4.92% after -4.00 D of vergence. Mallen et al.⁽¹⁵⁾ studied the changes in ACD during accommodation using the IOLMaster. The ACD obtained after -6 D accommodation was 5% shallower than that measured for far vision. Malyugin et al.⁽¹⁷⁾, Read et al.⁽²³⁾, and Tsorbatzoglou et al.⁽²⁵⁾ studied changes in ACD after -3.00 D of accommodation and found that the relative changes were 3.80%, 3.20% and 2.23%, respectively. Finally, Ni et al⁽¹⁸⁾ and Read et al⁽²²⁾ also studied the changes in ACD with accommodation in young eyes with the help of Pentacam HR. After -5 D accommodation, the mean ACD decreased approximately by 0.11 mm and 0.14 mm, respectively.

However, Yan et al.⁽²⁶⁾ found greater changes than that observed in previous studies. The central ACD decreased by 0.25 ± 0.09 mm. This magnitude of change is approximately 0.12 mm greater than that found in our study. The difference between our results and that of Yan et al.⁽²⁶⁾ could be due to the stimulus vergence range under analysis. Whereas the highest vergence in our study was -4 D, a vergence was not reported by Yan et al. since these authors used the subject's maximum amplitude of accommodation.

Our results also showed that the ACD is asymmetrical along the eye (Table 2), where the quadrant superior-nasal was significantly shallower than the quadrant inferior-temporal. These results are in agreement with those reported by Koç et al.⁽²⁷⁾, who studied the anterior chamber width with the help of a Scheimpflug camera. These authors found that the superior-nasal quadrant was significantly narrower than the inferior-temporal quadrant. However, our results contradict the results of Marigo et al.⁽³¹⁾, who used ultrasound biomicroscopy (UBM) for their studies. These authors found that the angle opening distance at 250 and 500 μm from the scleral spur was shallower at the horizontal direction than that in the vertical direction. These differences could be related to the different methodologies used in the studies. It can be concluded that ACD is asymmetrical along the anterior chamber eye. Further studies are needed to clarify these contradictions and to propose safer IOL designs, where thickness should be asymmetrical in order to avoid possible damage in the corneal endothelium.

Anterior chamber angle

The ACA of the whole eye changed during accommodation. The relative decrease in ACA was 3.73%. However, the local ACA varied significantly with accommodation. The highest and lowest relative decreases in the ACA after -4 D of accommodation was 5.07% (at inferior position) and 3.72% (at temporal position), respectively. The superior-nasal ACA was significantly narrower than the temporal-inferior ACA. Therefore, it can be concluded that ACA varies in an asymmetrical manner along the anterior chamber eye. These results are in agreement with those of Koç et al.⁽²⁷⁾, who studied the ACA width at different quadrants. These authors found that the superior-nasal quadrant was significantly narrower than the temporal-inferior quadrant. Further, these results are in agreement with ACD ones because the nasal-superior ACD was significantly shallower than the temporal-inferior ACD. However, our results contradict those reported by Marigo et al.⁽³¹⁾, who found shallower angle opening distance at the horizontal direction than in the vertical direction. As discussed earlier, these discrepancies might be related to the different devices used in these studies. To propose a suitable method for placing an angular support IOL, future studies should clarify the ACA symmetry.

To the best of our knowledge, this is the first report that describes the changes in ACA during accommodation⁽¹⁶⁾ in subjects who underwent IOL implantation and whose mean age was 76 ± 6 years. After 3 D of accommodation, the ACA decreased by approximately 1.22 degrees at the horizontal meridian and by 0.75 degrees in the vertical direction. Considering the mean changes in ACA along the horizontal and vertical directions, the mean ACA were approximately 1.35 and 0.95 degrees, respectively. These values are higher than that reported by Marchini et al. These disagreements could be related to the differences in sample population. We included phakic eyes, whereas Marchini et al. used aphakic eyes with an implanted IOL. Additionally, the mean age of our sample population was 25.30 ± 2.98 years, whereas the mean age of subjects in the study reported by Marchini et al. was 76.60 ± 6.0 years.

Pupil diameter

The mean relative change in PD between far and near vision was 8.31%. Yan et al.⁽²⁶⁾ measured PD up to the maximum amplitude of accommodation in emmetropic subjects using an SL-OCT system and found that the relative decrease in PD was 20.83%. Yuan et al.⁽²⁸⁾, who measured the changes in PD during accommodation using a spectral-domain OCT system in myopic subjects, found a relative decrease of 15.80%. The differences between the results of earlier studies and that of the present study are likely due to the different methods of measurements adopted. The earlier studies used the full accommodation interval without specifying the maximum vergence value. Another possible reason for the discrepancies may be the differences in the luminance conditions used in each device.

Using ultralong scan depth optical coherence tomography, Du et al.⁽⁷⁾ measured dimensional changes in the anterior segment of human eyes during accommodation. The relative change in the pupil diameter with a maximum accommodation effort of -5 D was 21.74%. The differences between the PD values obtained in the present study and that of Du et al. likely occurred due to the different samples studied. Whereas Du et al. included myopic subjects, the subjects in our study were emmetropic.

Baikoff et al.⁽⁴⁾ used anterior-chamber OCT (AC-OCT) to measure variations in PD at -10 D of accommodation in dim light. These authors reported a mean change in PD of 1.5 mm. The differences between our results and those of Baikoff et al. could be related to the different experimental conditions used. Baikoff et al. used -10 D as the maximum accommodation, whereas the maximum accommodation stimulus in our study was -4 D. Chartman et al.⁽²⁹⁾ studied changes in pupil size during accommodation using a Hartman-Shack wavefront sensor. The mean rate of change in PD during accommodation was -0.21 ± 0.15 mm·D^-1. Our results are in agreement with those reported by Chartman et al., as the mean change in PD after -1 D accommodation in our study was 0.16 ± 0.31 mm.

One limitation of our study is that only emmetropic subjects were considered. A second limitation is that all measurements were performed with only one device. Future efforts should focus on studying the changes in the anterior segment during accommodation in myopic and hyperopic subjects using other devices.

In summary, the ACA and PD varied significantly with accommodation, whereas variations in ACD were not observed. The anterior chamber eye was found to be asymmetrical, with the nasal-superior area being significantly shallower than the temporal-inferior area.