EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.
Issue link: https://digital.eyeworld.org/i/119916
32 EW CORNEA April 2013 Device focus Quantitative metrics for dry eye by Michelle Dalton EyeWorld Contributing Writer Osmolarity, inflammatory markers, lipid layers, and topography—each has its pros and cons in diagnosing dry eye C linical signs and subjective symptoms of dry eye often do not correlate, making it difficult at best for physicians to diagnose the disorder. What is now known is that "most dry eye is evaporative, or at least has a significant evaporative component, and somewhere around 86-87% of all dry eye patients have the evaporative form," said Marguerite McDonald, MD, in private practice, Ophthalmic Consultants of Long Island, Lynbrook, N.Y., and clinical professor of ophthalmology, Tulane University School of Medicine, New Orleans. (The other subtype is aqueous deficient dry eye.) It is also well established now that "ocular surface inflammation is a pathophysiologic variable in the definition of dry eye," said Christopher E. Starr, MD, associate professor of ophthalmology, director of the refractive surgery service, director of the cornea, cataract, and refractive surgery fellowship, and director of ophthalmic education, Weill Cornell Medical College, New York. In terms of actually diagnosing the disease, some tests (such as Schirmer's) are more than a 100 years old, said Michael A. Lemp, MD, clinical professor of ophthal- mology, Georgetown University and George Washington University, Washington, D.C., and chief medical officer, TearLab Corp, San Diego. "The most important determinant of dry eye remains the patient," said John D. Sheppard, MD, president, Virginia Eye Consultants, Norfolk, and clinical director, Thomas R. Lee Center for Ocular Pharmacology, Eastern Virginia Medical School, Norfolk. "If the patient has signs, symptoms, and visual disturbances, it doesn't matter what the quantities are. The most important piece of this picture is the informed, highly concerned eyecare specialist who synthesizes all the data and comes up with a plan for the patient that makes sense." Before the plethora of diagnostic tools became available, Dr. Starr said most corneal specialists thought diagnosing dry eye was relatively simple. "That's not the case. It's not easy to make an accurate diagnosis with just your clinical acumen in all cases. But having more objective point-of-care tests available that are accurate and easy to use has moved us rapidly forward," Dr. Starr said. Too many practitioners rely on the Ocular Surface Disease Index, but patients may be in the office on a "good" day, or after a return from a high humidity (or low humidity) environment, skewing the subjective opinions, Drs. Lemp and Sheppard both said. Corneal sensation is another modality that is more commonly employed in clinical research than in clinical practice, Dr. Sheppard The TearLab Osmolarity Test is used to measure the osmolarity of human tears. Source: TearLab said. Lactoferrin "can also act as a marker of either inflammation or tear solution," he said, but using lactoferrin as a test for dry eye is still in its infancy. "While the jury is not in on this one, we are all looking forward to gaining experience there," Dr. Sheppard said. "Ocular surface treatment and dry eye present an intellectual challenge to the clinician and a therapeutic challenge to the patient." Dr. Lemp added as many as 30% of people with clear, objective signs of dry eye are asymptomatic on subjective evaluations. Normal eyes don't fluctuate on any of these scales, he said, whereas people with dry eye will be "all over the charts." And it is precisely that variability that should alert clinicians to the presence of dry eye, he said. EyeWorld asked these specialists for their take on some of the newer diagnostic tests. Osmolarity Tear osmolarity is considered the best metric currently available, the experts said. Published literature confirms that assessment,1 and the American Academy of Ophthalmology now specifically recommends tear osmolarity testing for the diagnosis and management of dry eye syndrome.2 "Osmolarity is the most reliable metric we have," Dr. Starr said. "It doesn't distinguish between evaporative or aqueous deficient dry eye, however. Osmolarity as an objective measurement is the best single diag- nostic that clinicians currently have to diagnose dry eye." TearLab's device has "demonstrated that any variable results are biological in origin. In other words, it's not a crummy test, it's a crummy disease with moment-to-moment variability in tear osmolarity. Any inter-eye differences of 8 mOsmol/L or greater are a clear indication of dry eye," Dr. McDonald said. "I order a tear osmolarity test for anyone with a history of dry eye, anyone with complaints that sound like dry eye, anyone who is 40 or over, and anyone who is presenting preoperatively," she said. Dr. Sheppard said in addition to the TearLab device, LacriSciences (Washington, D.C.) is developing the LacriPen, a handheld tear osmometer. Inflammatory markers Rapid Pathogen Screening (RPS, Sarasota, Fla.) has developed the InflammaDry, "the first and only rapid, point-of-care test to detect MMP-9, an inflammatory marker that is consistently elevated in the tears of patients with dry eye disease," the company says on its website. "But it's a nonspecific marker for inflammation," Dr. McDonald said. So, for instance, a patient with a normal osmolarity test but a positive MMP-9 test has confirmed inflammation on the ocular surface—it's just not dry eye, she said. Inflammation may be caused by surgery or by some other ocular disease, she said. continued on page 34 The LipiFlow Thermal Pulsation System treats the primary cause of evaporative dry eye. Source: TearScience