There is a growing body of literature exploring changes in the ciliary muscle structure with accommodation. Recent advances in imaging technology have made it possible to more easily visualize in vivo
changes in the ciliary muscle with accommodation.11
The primary objective of this study was to determine if the Visante™
AS-OCT was capable of providing images with enough resolution to observe changes in ciliary muscle shape with accommodation. During the course of our efforts to develop a protocol using the Visante™
, we did determine that it is important to monitor accommodation during the imaging as it did differ slightly from measurements obtained prior to imaging on an autorefractor (data not shown).
In a published letter,18
we have attempted to encourage those who measure the ciliary muscle in accommodative research to discuss and consider how the ciliary muscle should be measured. Two previous studies have demonstrated that the Visante™
is capable of imaging the ciliary muscle in vivo
and reported changes in the ciliary muscle morphology that are similar to our results even though their data collection and image analysis techniques were different from the ones used in this report.18
While Sheppard and Davies (2010) did not use exactly the same locations for thickness measurements as we did, i.e., their thickness measurements were referenced to the overall length of the muscle rather than the scleral spur, their CM25 measurement is probably in the general area of our CMT1, and their CM75 is probably similar to our CMT3. Both studies included an evaluation of ciliary muscle thickness differences, comparing distance and 4.00-D accommodative stimuli. When the measurements from our study and Sheppard and Davies (2010) are compared, we found a 45.2 μm mean thickening at CMT1, and Sheppard and Davies (2010) found a mean increase in thickness from 550 μm to 571 μm (or 21 μm) at CM25. Similarly, we reported a mean thinning of −45.9 μm at CMT3 and Sheppard and Davies (2010) reported a mean thinning from 174 μm to 166 μm (or 8 μm) at CM75. Sheppard and Davies (2010) also reported a measurement at 2 mm posterior to the scleral spur (CM2), and they found a very small, but statistically significant thinning of 21 μm at this location where we observed no significant change (−7.3 μm, p = 0.5). It is possible that we might have observed a statistically significant change if we had included more than 25 subjects in our study.
Our results and previous studies confirm that it is possible to measure the action of the ciliary muscle during accommodation using images from the Visante™, and that there appears to be a thickening of the anterior portion and a thinning of the posterior portion of the ciliary muscle during accommodation. Still, there are improvements that could be made to the protocol for making these measurements. It is possible that the best protocol for obtaining these measurements in future studies would include having a measure of the accommodative response at the exact time of image capture. This suggestion is based on observations from a couple of subjects with a wide range of accommodative responses during Visante™ imaging. For example, one subject had similar mean ± SD PowerRefractor readings while viewing the distance target at the two study visits, but the range of responses was more variable than some other subjects [Visit 1: −0.92 ± 0.17 D (range = −0.28 to −1.44) and Visit 2: −0.98 ± 0.19 D (range = −0.33 to −1.62)]. This subject also had more variable ciliary muscle measurements than other subjects. For example, CMTMAX at visit 1 was 780 μm and was 700 μm at Visit 2. We noted at least two subjects with discrepancies as extreme as these. Of course when we are measuring human subjects and relying on their responses and attention in a study such as this, some variability is inevitable and likely unavoidable. Nonetheless, in future studies, we will use an imaging protocol that includes measurements of accommodative response at the exact time each image is obtained to try to reduce the variability of the measurements and improve estimates of how much change in ciliary muscle thickness is required per diopter of accommodative response.
While accommodative monitoring during image acquisition is certainly one major limitation of the estimates of the change in ciliary muscle thickness during accommodation for the present study, we should also acknowledge one additional limitation. In our original publication of the semi-automatic algorithm we use to analyze the dimensions of the ciliary muscle, we included only cycloplegic images. When we began analyzing images that had been obtained during accommodation, it was clear that the anterior portion of the ciliary muscle was, at times, much darker than the cycloplegic images. It would not be surprising that the spacing of fibers within the muscle might vary between the accommodative and cycloplegic states of the muscle, resulting in different levels of contrast in the two types of images. The process of adjusting the contrast for some of the 4.00-D images, as depicted in , did require some subjectivity on our part. For future studies, we are exploring how image capture and/or analysis might be adjusted so that all images could be analyzed with the same contrast settings in the algorithm.
While we are certain that refinements of the methods used to monitor in vivo
accommodation in humans is important, we would also like to remind the reader that we have already discussed the fact that our results were similar to those of Sheppard and Davies (2010), even though our measurement techniques were not identical.18
In addition, we have collected data similar to these for additional studies, i.e., a study of children20
and also in a study of pre-presbyopic and presbyopic adults (unpublished data), where we made protocol adjustments but still obtained remarkably similar results. Based on the fact that we have repeated these results in other samples, we are confident that the results presented in this study are a good estimate of the change in ciliary muscle thickness per diopter of accommodation, despite the limitations of the study that are discussed above.
In summary, the combination of the Visante™
AS-OCT and the PowerRefractor provide a feasible method for future studies to continue to explore the exact nature of ciliary muscle contraction in adult subjects. In future studies, we intend to further refine the estimates of how the shape of the ciliary muscle changes with accommodation, and refine our procedures for determining the subject’s accommodative response. The information about ciliary muscle contraction and also the evolution of our methodology and protocols provided here and in our companion paper in this issue of the journal describing these measurements in children,20
should allow for more definitive and insightful studies of the ciliary muscle during accommodation in the future.