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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.

    Keratoconjunctivitis sicca, or dry eye disease (DED), is a multifactorial disorder of the tears and the ocular surface characterised by symptoms of discomfort, tear film instability and sometimes inflammation.1 Among the most common conditions encountered by eye care professionals, estimates of DED prevalence vary from 7.4% to 33.7% worldwide.2–4 Even using a very restrictive definition, the disease is reported to affect almost 5 million Americans aged 50 years and older; tens of millions more experience episodes of dry eye, typically when exposed to adverse environmental factors such as low humidity.5

    Tear film

    Traditionally, the tear film has been described as having three distinct layers (Figure 1): an outermost lipid (oily) layer; an aqueous (watery) layer that makes up 90% of the tear film volume; and a mucin layer that coats the corneal surface.

    However, it is now recognised that, rather than having three distinct layers, the tear film has a surface lipid component and then phases of aqueous with differing concentrations of mucins suspended throughout (Figure 2). In addition, mucins in the tear film have a much more active role in maintaining tear-film stability than was once thought.6

    Dry eyes

    DED is prevalent in patients with autoimmune diseases, which affect around 8% of the population; 78% of patients with autoimmune diseases are women. DED also affects postmenopausal women and elderly people. The prevalence of DED is estimated to be anywhere from 7.4% to 33.7% depending which study is cited, how the disease is diagnosed and what population is surveyed. The Beaver Dam population-based study found the DED prevalence rate to be 14% in adults aged 48 to 91 years. The study also found that DED affects more women (16.7%) than men (11.4%). Reliable epidemiological studies, the large Women’s Health Study and the Physician’s Health Study, indicate that the prevalence of symptomatic dry eye in people over the age of 50 in the United States is about 7% in women and 4% in men. These numbers translate into approximately 3.2 million women and 1.05 million men with DED. Estimates of those affected by DED of any severity amount to approximately 20 million or more in the United States alone.

    International epidemiological studies report similar or higher rates around the world. The prevalence of DED is approximately 7.4% in Australia, with significantly increased prevalence in older patients and a significant decrease in tear production in women aged 50 to 59 years. In Indonesia, dry eye prevalence is around 27.5%, with increased prevalence associated with age, cigarette smoking and pterygium. In Taiwan, the prevalence of DED is 33.7% in a tested elderly population, with significantly more women than men reporting dry eye symptoms. The prevalence of dry eye is also estimated at 25% in Canada and 33% in Japan. However, the prevalence and incidence of DED might be under-reported, as patients might fail to recognise the symptoms of DED or to report the problem to a physician.

    Many contact-lens wearers experience dry eyes at some point, with many discontinuing or limiting lens wear.

    Common symptoms experienced by the patient vary but can include foreign body sensation, discomfort, dryness, stinging and blurred vision. The diverse array of dry eye symptoms and their effect on quality of life has prompted the use of numerous patient questionnaires, such as the ocular surface disease index (OSDI),7 the dry eye questionnaire8 and the IDEEL,9 to objectively quantify symptom improvement in response to treatment.

    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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