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EW GLAUCOMA 72 November 2015 by Lauren Lipuma EyeWorld Staff Writer decreases after LVC. IOP correction formulae are available, but these no- mograms do not consider effects of corneal rigidity and therefore should be avoided at all costs, Dr. Kotecha said. On the other end of the spec- trum, corneal edema, keratoplasties, and keratoprostheses thicken the cornea and alter its structure, adding a host of other situations where GAT measurements may be unreliable. Physicians are now exploring alter- natives to GAT, looking for methods that are accurate, precise, and less affected by corneal physical and biomechanical properties. One available option is dynamic contour tonometry (DCT). Rather than flattening the cornea, as in applanation tonometry, the PASCAL dynamic contour tonometer (Ziemer, Port, Switzerland) uses a concave surface to contour-match the central 3 mm of the cornea. The DCT probe stays on the eye for 5–8 seconds, and is therefore able to detect changes in IOP caused by the cardiac cycle, so this method also measures the ocular pulse ampli- tude. Because it does not deform the cornea, DCT is less likely to be influ- enced by corneal biomechanics, and research has shown that DCT mea- surements come close to matching true IOP measurements determined by manometry. Some evidence suggests that DCT is superior to GAT in post-refractive surgery patients, but it does have difficulty measuring pathologic or transplanted corneas. The downside to DCT is that it requires skill and good patient preparation and could be time consuming in a busy practice, Dr. Kotecha said. Studies show that DCT measurements are more precise when trained technicians perform the measurements, so it may be best for physicians to dedicate trained staff to performing DCT in their practice. The Ocular Response Analyzer (ORA, Reichert Technologies, Depew, N.Y.), by contrast, is a non-contact tonometry method that measures the corneal deformation from a puff of air. This method uses the same principle as traditional non-contact tonometry, but the ORA is able to not introduced by an expert phys- icist or mathematician but by an ophthalmologist, who invented a tonometer and looked for a logical foundation to support the rational of the instrument and the method … never has even a single experi- ment (on any kind of a sphere) been reported which could support this law." In essence, Goldmann "in- voked" this law to describe how his tonometer worked, Dr. Kotecha said. "If we did that today, I'm not sure how well that would be received," she said. "But nonetheless, we use the Goldmann and we've been using it for the last 50 years, and we hav- en't caused any major patient harm by measuring IOP with this device." Alternative methods GAT works well enough for healthy corneas, but procedures that alter the cornea's biomechanical proper- ties—such as laser refractive surgery and corneal transplants—can affect GAT measurements. Ablation of corneal tissue during laser vision correction (LVC) decreases the central corneal thickness (CCT), and physicians are well aware that IOP measured by GAT significantly Reexamining a "gold standard" of measurement A patient is examined with a Goldmann applanation tonometer by Dr. Julia Song. Source: Julia Song, MD continued on page 74 that for a fluid-filled, encapsulated sphere, the force required to flatten an area of that sphere is proportion- al to the pressure inside the sphere. While this makes sense in theory, Hans Goldmann introduced the Imbert-Fick principle himself in 1957—the "law," as it is stated, is not described by any physics textbook and is found nowhere in medicine outside of ophthalmology. In addition, the principle assumes that the sphere's encapsu- lated surface is dry, perfectly elastic, flexible, and infinitely thin—proper- ties that do not apply to the cornea. Goldmann stated that he accounted for the cornea's properties when he designed his tonometer, but it has become clear that corneal physical and biomechanical properties—such as curvature, thickness, and rigidity —greatly influence GAT measure- ments. When the cornea is hard and stiff, GAT will overestimate the IOP, and when it is thin, soft, and flat, GAT will underestimate the IOP. Soon after Goldmann intro- duced his tonometry method, some physicians began to question the physics behind it. In a letter to the editor of Archives of Ophthalmology in July 1960, Harri H. Markiewitz, MD, stated, "The so-called law was Is Goldmann tonometry the best way to measure IOP? L owering IOP is the mainstay of glaucoma treatment, and Goldmann applanation tonometry (GAT) is the gold standard for measuring IOP. Since GAT was introduced in 1957, its use in clinical practice has be- come nearly universal—but just how reliable of a method is it? During the 2015 European Society of Cataract & Refractive Surgeons (ESCRS) meeting, Aachal Kotecha, PhD, senior research associ- ate, University College London Institute of Ophthalmology, London, and Thasarat Vajaranant, MD, director, glaucoma service, Department of Ophthalmology and Visual Sciences, University of Illinois at Chicago, described situations where GAT measurements may be unreliable and presented several alternative methods that could pro- vide more accurate measurements of IOP. The Imbert-Fick 'law' and the cornea GAT measurements are based on the Imbert-Fick "law," which states

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