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51 EW GLAUCOMA July 2018 by Maxine Lipner EyeWorld Senior Contributing Writer would cause the optic nerve head to tilt temporally," Dr. Demer said. Such temporal tilting is not present in children but is very common in adults, especially those with near- sightedness. "You could easily imag- ine that this is a repetitive strain injury because we make tens of thousands of these eye movements every day," he said. In the study, investigators looked at several groups of peo- ple including those with normal tension glaucoma who were losing their sight and age-matched controls without glaucoma. Patients under- went both MRI and OCT scanning. Their results were then compared to those of young, healthy people without glaucoma, as well as those with strabismus who have mis- aligned eyes with differing amounts of eye movements. Investigators found that for those with normal tension glauco- ma there was a tightening when they rotated the eye toward the nose beyond an angle of about 26 degrees, Dr. Demer noted. "When that occurs, the eyeball gets pulled back significantly into the socket in a way that we do not see in any oth- er group," he said. The only other group this was noticed in was those with high pressure glaucoma. In building the computer mod- el, investigators noted that the optic nerve and the sheath that surrounds it start out in youth thin and pli- able, but this changes with age. Dr. Demer pointed out that open angle glaucoma is much more prevalent in older people. "It may simply be a co- incidence, but it's certainly a strong coincidence that as people get older and the risk of chronic open angle glaucoma goes up, their optic nerves and sheaths get thicker and stiffer," he said. Dr. Demer theorized that with age the area becomes vulnerable to damage. "There's a vast amount of compelling data that the damage produced by the optic nerve occurs at the junction between the optic nerve and the eyeball in glaucoma, that the pattern of visual loss is consistent with that," Dr. Demer A closer look at the impact of eye movements F or patients with normal tension glaucoma, recent research suggests that the culprit may be common eye movements, according to Joseph Demer, MD, PhD, Arthur L. Rosenbaum Professor of Pediatric Ophthalmology, Stein Eye Institute, and professor of neurology, Univer- sity of California, Los Angeles. The research explored whether repetitive strain from eye movements could damage the optic nerve and lead to vision loss. This theory was an outgrowth of Dr. Demer's earlier work to better understand strabismus with the aid of MRI scanning. "We have been working on optimizing this tech- nique so that we can see the func- tion of the eye muscles by MRI," Dr. Demer said. "By making these MRI pictures in different gaze positions, it became obvious that the optic nerve was getting pulled tight in some positions." In some cases, the tugging of the optic nerve on the eye was sufficient to retract this back into the socket. It occurred to Dr. Demer that the part of the eye that was being pulled on the hardest was the temporal edge of the optic nerve as it exits the eye wall, which is an area where peripapillary atrophy develops. Studying eye movement Dr. Demer thought that the tension on the optic nerve during these eye movements may produce damage to the peripapillary region. With this in mind, investigators built a finite element analysis computer model out of thousands of tiny three-dimensional virtual pyramidal elements. Using this computer model, investigators then had the eyeball to rotate into the position where they found the optic nerve gets tethered and calculate the stress in each part. "It turns out that there's a lot of stress and strain in this area at the temporal edge of the optic nerve where the peripapillary atrophy happens, and the distribution of it Normal tension glaucoma: The result of repetitive strain? Optic nerve tethering in adduction in a case of advanced normal tension glaucoma with progressive visual field loss at low-normal IOP. Colorized, high resolution MRI shows that while amply long in abduction, the optic nerve is not long enough to allow adduction without damaging traction on the optic nerve head. Coronal MRI shows large areas of optic atrophy on the left. Source: Joseph Demer, MD continued on page 52 Research highlight

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