Purpose: To develop a measurement protocol using the Zeiss Visante™ AS-OCT to asses changes in the shape and size of the ciliary muscle with accommodation and to determine the test-retest repeatability of these measurements.
Methods: Twenty-five adults (17 females) ages 22-27 years (mean ± SD = 23.8 ± 1.1 years) were recruited. The ciliary muscle was imaged at two separate visits two weeks apart with the Zeiss Visante Anterior Segment OCT. While subjects viewed targets at two different stimulus levels (1 D and 4 D), the ciliary muscle was imaged six times and accommodation was monitored with the PowerRefractor. Ciliary muscle thickness (CMT) measurements were obtained at four muscle locations; 1 mm (CMT1), 2 mm (CMT2), and 3mm (CMT3) posterior to the scleral spur as well as (CMTMAX) at the point of maximal muscle thickness. The action of the ciliary muscle during accommodation was described by our data in two ways. The „raw change‟ was calculated as the change in ciliary muscle thickness calculated based on the accommodative target (the change in accommodation from distance to the 4D target). The „standardized change‟ was calculated as the ratio as (change in CMT/accommodative response) represented the amount of thickening or thinning that occurred in the ciliary muscle per diopter of accommodation. Finally, the repeatability measurements between visit 1 and visit 2 were analyzed using a Bland-Altman analysis.
Results: The mean accommodative response measured with the PowerRefractor at visit 1 was 3.97±0.87D with a range of -1.87 to -4.95D and -3.92±0.82 with a range of -2.79 to -5.39D at visit 2. The change in ciliary muscle thickness per diopter of accommodation was calculated based on both the accommodative stimulus and the measured accommodative response (PowerRefractor) at the four thickness locations. Ciliary muscle thinning from 1D to 4D of accommodation was statistically significant at CMT3. Thickening of the ciliary muscle with accommodation was observed at both CMT1 and CMTMAX while no statistical thickening or thinning was present at CMT2. Bland-Altman repeatability analysis between the two visits was non-significant for every muscle location. The 2D stimulus was not considered in these calculations because the majority of subjects failed to produce any accommodative effort to the 2D target.
Conclusion: The combination of the Visante Anterior Segment OCT and the PowerRefractor is a feasible tool for measuring the change in ciliary muscle thickness with accommodation. There was a wide range of accommodative responses during the time of image capture in this study. This indicates is it essential to know the accommodative response at the exact time of image capture for the most accurate calculations of change in ciliary muscle thickness with accommodation.