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    Tear volume – a neglected issue?

    We report a new tool for measuring tear volume, which is essential for selecting the appropriate treatment for a patient with symptoms of dry eye.

     

    In recent years, advances in cataract and refractive surgery have dramatically improved patients’ postoperative quality of vision. The benefits of these advances may be lost, however, when the ocular surface deteriorates even slightly. During clear corneal cataract surgery, which now accounts for 73% of cataract surgery according to this year’s trends survey by the American Society of Cataract and Refractive Surgery, many of the corneal nerves are cut. With typical cataract surgery, the primary incision may be about 3.0 mm in width and the paracentesis about 1.0 mm in width, giving a total arc length of 4.0 mm of full-thickness corneal incisions. If we add to this the incisions for astigmatic correction, such as limbal relaxing incisions, or the three-incision technique of bimanual cataract surgery advocated by some surgeons, the cutting of corneal nerves is even more severe.10

    Often when patients present with cataracts, surgeons are so focused on planning for the surgical procedure that they overlook dry eye symptoms. It is important to preoperatively assess patients for concurrent problems, especially dry eye, and moderate to severe dry eye needs to be addressed prior to surgery. Even patients with no history of dry eye commonly experience dry eye after surgery: a recent study conducted by Cal Roberts and Eleanor Elie found that a clinically significant proportion of patients report experiencing at least some dry eye symptoms after cataract surgery.11

    Turning to refractive surgery, it is widely accepted that the act of cutting (with a microkeratome) or ablating (via a laser) the corneal nerves during refractive surgery leads to an iatrogenic dry eye syndrome in nearly all patients. It is now the standard of care to treat all patients for dry eye for at least a few months after corneal refractive surgery until the corneal nerves have had a chance to regenerate.10

    In both cataract and refractive surgery there is a growing necessity to measure tear volume and prescribe tear-enhancing products preoperatively and, postoperatively, it is also useful to investigate the patients’ tear volume.

    The successful development of pharmaceutical agents targeting DED requires a definitive demonstration that the drug can induce a significant improvement in signs and symptoms of the disease. To this end, reproducible and sensitive assessment of dry eye is central to the drug development process.12

    Measuring tear volume

    Tear quantity or volume is measured seldom in ophthalmic practice and almost never in general medical practice. Until now, doing so has been time consuming and somewhat bothersome for the patient. Several methods, including the Schirmer test (dated from 1905), the phenol red thread (PRT) and tear meniscus height, are available for checking tear volume. Tear clearance tests are also available but these have had low uptake rates by clinicians.

    Regarding the Schirmer test, much of the difficulty in defining wetting limits for diagnostic purposes can be summarised by a statement paraphrased from Cho:13 “Schirmer values are too variable, such that no definite limit for normal tear production can be determined.” Despite significant efforts, only a very small number of studies have found a wetting cut-off point that correlates with another sign or symptom of dry eye. Furthermore, the range of values is such that, regardless of cut-off point, false negative and/or positive identification of dry-eyed patients is common.

    Claes Feinbaum
    Claes Feinbaum is Professor Emeritus Optometry at the University of Rostock, Germany. He is an Optometric Consultant for Specspavers ...

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