Dynamic imaging of accommodation by swept-source anterior segment optical coherence tomography

Abstract

Purpose: To study the accommodation process in normal eyes using a commercially available clinical system based on swept-source anterior segment optical coherence tomography (AS-OCT).

Setting: Ophthalmology Department, University of Parma, Italy.

Design: Evaluation of diagnostic technology.

Methods: Right eyes were analyzed using swept-source AS-OCT (Casia SS-1000). The optical vergence of the internal coaxial fixation target was adjusted during imaging to obtain monocular accommodation stimuli with different amplitudes (0, 3.0, 6.0, and 9.0 diopters [D]). Overlapping of real and conjugate OCT images enabled imaging of all the anterior segment optical surfaces in a single frame. Central corneal thickness (CCT), anterior chamber depth (ACD), and lens thickness were extracted from the OCT scans acquired at different static accommodation stimulus amplitudes. The crystalline lens was analyzed dynamically during accommodation and disaccommodation by acquiring sequential OCT images of the anterior segment at a rate of 8 frames per second. The lens thickness was extracted from the temporal sequence of OCT images and plotted as a function of time.

Results: The study analyzed 14 eyes of 14 subjects aged 18 to 46 years. During accommodation, the decrease in the ACD was statistically significant (P < .05), as were the increase in the lens thickness (P < .001) and the slight movement forward of the lens central point (P < .01). The CCT and anterior chamber width measurements did not change statistically significantly during accommodation. The lens thickness at 0 D was positively correlated with age (P < .01).

Conclusion: High-resolution real-time imaging and biometry of the accommodating anterior segment can be effectively performed using a commercially available swept-source AS-OCT clinical device.

Financial disclosure: No author has a financial or proprietary interest in any material or method mentioned.

Figure 1. SS-AS-OCT scans of an eye included into the study

1. Vertical B-scan showing the anterior part of the anterior segment: the posterior aspect of the crystalline lens is not included into the scan. 2. Vertical B-scan showing the posterior part of the anterior segment: the entire crystalline lens is shown while the anterior part of the cornea is not included into the scan. 3. Vertical B-scan showing the entire anterior segment of the eye by overlapping the real and the conjugate images: the posterior lens is shown vertically inverted and overlapped to the anterior lens. All the optical surfaces of the anterior segment are included into the scan and the following intraocular distances can be measured: central corneal thickness (CCT): distance from point A to point B; anterior chamber depth (ACD): distance from point B to point D; lens thickness (LT): distance from point D to point F plus distance from point C to point F.

Figure 2. Example of SS-AS-OCT dynamic analysis of the crystalline lens changes in shape during the accommodation process

A. Initial frame (vertical B-scan) of video 1 (accommodation) for an eye included into the study: the lens is shown at the relaxed state (accommodation stimulus amplitude or ASA=0D). B. Initial frame (vertical B-scan) of video 2 (relaxation from accommodation) for an eye included into the study: the eye is shown at the maximal ASA (ASA=9D).

Figure 3. Graph showing the intraocular distance changes during accommodation

The graph reports the mean axial distances of the corneal endothelium, lens anterior surface face, lens central point, and lens posterior surface from the corneal epithelium for different accommodation stimulus amplitudes (ASA): 0, 3, 6, and 9D.

Figure 4. Dynamic biometry of accommodation for four subjects at different age

Changes in the lens thickness during accommodation are plotted in function of time. The dynamic changes of the lens thickness are reported after the stimulus onset (t=0s). The graphs on the left-hand side show the accommodation process 0D to the maximal ASA (accommodation stimulus amplitude). The graphs on the right-hand side show the relaxation process from the maximal ASA to ASA=0D.