EyeWorld Asia-Pacific March 2011 Issue

continued on page 26 25 EW CATARACT/IOL March 2011 Ciliary body and zonule anatomy described by Michelle Dalton EyeWorld Contributing Editor Heralded as a potential “landmark” study, an in-depth description of the posterior zonule may help in future accommodating IOL designs T he more the ophthalmic industry learns about how accommodation is affected by aging and what the precise mechanism of action is, the better the future generations of IOLs can be. A recent paper described the anatomy of the ciliary body/zonule, finding the “stiffening of the vitreous zonular system may contribute to age- related loss of accommodation and offer a therapeutic target for presbyopia”. 1 Another group is using high resolution ultrasound biomicroscopy (UBM) to film normal human lens accommodation in an effort to better define how accommodation is accomplished. Science has previously shown ciliary muscle mobility in monkeys does not decrease with age, unlike what occurs in humans. It has been postulated that “posterior restriction of muscle movement in the aging eyes” plays a role in the pathophysiology of presbyopia, said Mary Ann Croft, corresponding author of the article. The findings from their study “further support age-related posterior restriction of ciliary muscle and zonular movement as both a pathophysiologic factor and a potential therapeutic target in presbyopia”, the authors concluded. Bringing this to the attention of readers/participants of EyeSpaceMD, Daniel B. Goldberg, MD, clinical associate professor of ophthalmology, Drexel University College of Medicine, Philadelphia, Penn., USA, noted: “The complex anatomy of the zonule has never been demonstrated before. How many of us have ever heard of the ‘vitreous zonule’? The Croft study using improved scanning electron microscope and UBM techniques has identified three components of the vitreous zonule—anterior, intermediate, and posterior vitreous zonule. The varying zonular origins and insertions create a complex set of interconnections with resultant force vectors during accommodation—and disaccommodation, a new term incorporating the concept of active forces during the process of ciliary body relaxation—visualize an interconnected system of tethers that can stretch due to the action of the ciliary body, and then when the ciliary body relaxes, the tethering forces of stretched vitreous zonules create disaccommodation. Since we never understood the anatomy, we never developed the right theory of accommodation.” “There is not enough emphasis or attention on this research right now,” said Paul “Butch” Harton, MD, Rome, Ga., USA. “Before we can create a consistently reliable and truly accommodating IOL, we need to better understand the mechanisms and physiology of human accommodation.” To that end, Dr. Harton described research currently underway by the Accommodation and Ocular Imaging Council (www.theaoic.com), of which he is a member, using a VuMax II high resolution ultrasound (Sonomed, Lake Success, NY, USA) unit. The group is “presently finishing a multicenter study where we have produced what we believe are the first high resolution video clips of normal human accommodation.” The group has captured images of 32 eyes thus far from participants ranging in age from 20 to 25 years old, he said. The preliminary results were presented earlier this year at the ASCRS meeting in Boston and won Best Paper of Session. The group has also seen the posterior zonule in some of their images. “There is so much we don’t understand right now, and the Croft study may help us understand why some of these movements occur,” Dr. Harton said. Dr. Goldberg agreed, saying the other part of the Croft study “correlates the electron microscopy findings with improved UBM techniques that can now visualize the ciliary body/zonules/ lens accommodating in vivo. Clearly, accommodation is a dramatic intraocular event and the UBM videos need to be seen to be appreciated,” he said. During accommodation, the ciliary muscle centration “results in anterior movement of the lens relative to the scleral spur, and anteriorly there is centripetal contraction, which results in reduced tension on the anterior zonule”, Dr. Goldberg said. The Croft study “describes the complexity of a vitreous zonular system that helps to smoothly translate to the lens the driving forces for accommodation and disaccommodation generated by the ciliary muscle contraction and relaxation. Our study also suggests how that structure allows and assists the very fine and rapid refocusing and maintenance of focus at multiple distances required by primate visual needs.” Ophthalmology as a science may now be able to address whether Schachar’s theory of accommodation/presbyopia is correct and whether scleral incisions can be beneficial in reversing presbyopia, said Clive Novis, MD, Benoni, South Africa. Schachar’s theory suggests that ciliary muscles contract during accommodation, thereby relaxing the anterior zonule. Schachar’s theory differs from the Helmholtz theory of accommodation in that the latter suggests all the zonules relax. In Schachar’s theory, presbyopia is caused by the aging lens’ increasing equatorial diameter and the inability of the ciliary muscle to therefore generate sufficient tension on those zonules to effect a change in central lens power, according to several published articles and book chapters. There is a decrease in the circumlenticular space due to equatorial growth of the lens with age. This potentially “reduces the length–tension relationship of the ciliary muscle/zonular complex so the forces on the lens are reduced with age, thereby reducing accommodation”, Dr. Harton said. “However, this assumption is controversial and remains to be validated.” In the AOIC’s series of participants, zonular tension was not measured, but the group did consistently observe similar movements during accommodation in virtually all eyes in the study. One of the most dramatic observations is a steepening of the central anterior lens radius of curvature during accommodation. “It would appear the complex zonular forces cause the anterior Electron microscopy of lens zonules and ciliary body Source: missionforvisionusa.org