EyeWorld India March 2016 Issue

Glaucoma treatment March 2016 35 EWAP SECONDARY FEATURE A patient is examined with a Goldmann applanation tonometer by Dr. Julia Song. Source: Julia Song, MD Reexamining a ‘gold standard’ of measurement by Lauren Lipuma EyeWorld Staff Writer 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 become 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 associate, 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 provide more accurate measurements of IOP. The Imbert-Fick ‘law’ and the cornea GAT measurements are based on the Imbert-Fick “law,” which states that for a fluid-filled, encapsulated sphere, the force required to flatten an area of that sphere is proportional to the pressure inside the sphere. While this makes sense in theory—Hans Goldmann introduced the Imbert- Fick principle 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 encapsulated surface is dry, perfectly elastic, flexible, and infinitely thin—properties 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 measurements. 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 introduced 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 not introduced by an expert physicist 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 experiment (on any kind of a sphere) been reported which could support this law.” In essence, Goldmann “invoked” 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 haven’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 properties—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 decreases after LVC. IOP correction formulae are available, but these nomograms do not consider effects of corneal rigidity and therefore should be avoided at all costs, Dr. Kotecha said. On the other end of the spectrum, 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 alternatives 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 amplitude. Because it does not deform the cornea, DCT is less likely to be influenced by corneal biomechanics, and research has shown that DCT measurements 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 continued on page 36