Tear Volume-based Diagnostic Classification for Tear Dysfunction

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Tear production, distribution, and clearance are regulated by the Lacrimal Function Unit that consists of the main and accessory lacrimal glands, the meibomian glands, the conjunctiva, eyelids, and blink apparatus.1 Tear dysfunction develops when one or more of these components is no longer able to maintain a stable tear film.1 Tear dysfunction is one of the most common eye conditions with a reported prevalence of 6 to 43 million in the United States, depending on the diagnostic criteria.2–6 Tear dysfunction causes eye irritation and visual symptoms, including photophobia and blurred and fluctuating vision. Tear dysfunction can cause pathologic changes to the ocular surface epithelium with disruption of corneal epithelial barrier function and loss of mucus secreting conjunctival goblet cells. It may also decrease quality of life and in the most severe cases, it can cause functional and occupational disability. Indeed, the impact of dry eye on quality of life was rated to be equivalent to unstable angina using utility assessments.7
Tear dysfunction can result from a myriad of causes. Disease or dysfunction of the lacrimal glands results in aqueous tear deficiency (ATD), whereas aqueous tear production and tear volume are typically normal in meibomian gland disease (MGD) and conditions altering tear distribution, such as conjunctivochalasis (CCh). Consequently, there is no need to stimulate aqueous tear production or block tear drainage in patients with a normal or elevated tear volume. Consensus-based classification schemes for dry eye and tear dysfunction have been proposed. An international Delphi panel proposed a classification of tear dysfunction with 3 major subsets: without lid margin disease, with lid margin disease, and altered tear distribution.8 One problem with this classification scheme is that lid margin disease is prevalent in older individuals, is difficult to grade objectively, and is frequently not accompanied by eye irritation or ocular surface disease. The International Dry Eye Workshop (DEWS I) took a different approach by proposing 2 major categories of dry eye: aqueous deficiency and increased tear evaporation, the latter caused primarily by MGD.9 Although evidence of increased tear evaporation has been found in MGD,10–12 there are no commercially available instruments to measure tear evaporation rate and the evidence suggests that increased tear evaporation alone is not sufficient to decrease tear volume below normal levels in patients with MGD.13 A more practical approach is to stratify tear dysfunction by objective measurement of tear volume into aqueous deficient and aqueous sufficient groups. Evidence supports this approach to classification because studies have found distinct differences between aqueous deficient and sufficient groups in pattern and severity of ocular surface disease, conjunctival goblet cell density, response to desiccating environmental stress, corneal sensitivity, blink rate, and profile of inflammatory mediators in tears and conjunctiva.13–18 The advent of simple, rapid, noninvasive commercially available methods to measure tear volume facilitate the use of diagnostic and treatment guidelines based on tear volume. Rationale for this approach and guidelines for treating tear dysfunction associated with low or normal aqueous volume are presented herein.
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