Resumen de Reference values, intertest correlations, and test-retest repeatability of selected tear film tests in healthy cats
Objective—To determine reference values, intertest correlations, and test-retest repeatability of Schirmer tear test 1 (STT-1), phenol red thread test (PRTT), tear film breakup time (TFBUT), tear osmolarity, and meibometry in healthy cats.
Design—Evaluation study.
Animals—135 healthy domestic cats aged 0.5 to 12.8 years.
Procedures—Each test was performed once in 120 cats and repeated in 40. Pearson correlation was used to assess correlation among tests. Intraclass correlation coefficients (ICCs) and 95% limits of agreement (LOA) were used to evaluate test-retest repeatability.
Results—Median (95% central range) values were 18 mm/min (9 to 34 mm/min) for STT-1, 29 mm/15 s (15 to 37 mm/15 s) for PRTT, 12.4 seconds (9.1 to 17.7 seconds) for TFBUT, 322 mOsm/L (297 to 364 mOsm/L) for osmolarity, and 32 meibometry units (MU; 11 to 114 MU) for peak meibometry value. The STT-1 and PRTT values were positively correlated. Age was weakly associated with TFBUT and osmolarity. Meibometry measurements were higher for strips that contacted the tear film (285 MU) than for those that touched the eyelid margin only (32 MU). All ICCs were < 0.75, and 95% LOA were wide.
Conclusions and Clinical Relevance—Tear deficiency should be suspected in cats with STT-1 < 9 mm/min, PRTT < 15 mm/15 s, or TFBUT < 9 to 10 seconds. Generally poor correlation among tests suggested that thorough tear film analysis requires performance of multiple tests in concert. Relatively poor test-retest repeatability should be considered when repeated tests are used to monitor tear film dysfunction and response to treatment.
The surface of the eye is coated by a thin film of tears critical for ocular comfort, optical clarity, and corneoconjunctival health. The tear film keeps the ocular surface moist, provides nutrients and protection against debris and pathogens, helps transport waste away from the ocular surface, and provides a smooth optical surface for refraction.1,2 The tear film has traditionally been described as a trilaminar structure, consisting of an inner mucinous layer secreted by conjunctival and corneal cells, a middle aqueous layer secreted by the lacrimal glands, and an outer lipid layer composed of meibum and secreted by the meibomian glands.1 However, the current concept is that the tear film is a bilayered structure, consisting of an inner aqueous-mucinous phase and an outer lipid phase.3–5 Normal tear film function demands that the volume and physiochemical qualities of all 3 tear film components (individually and relative to each other) remain within an appropriate physiologic range. Disruption of the aqueous component results in quantitative tear film deficiency manifested clinically as KCS (commonly described as dry eye syndrome). Disturbance of the lipid or mucus components of tears (jointly or individually) results in qualitative tear film deficiency, which causes instability and premature evaporation or breakup of the tear film with resultant keratoconjunctival injury.6 The diagnosis, treatment, and monitoring of quantitative and qualitative tear film deficiencies require a variety of tear film assays. Ideally, these techniques should be minimally invasive so that the test causes minimal discomfort to the animal and permits data collection from the ocular surface without inducing reflex tearing or otherwise altering the tear film. This is especially important if > 1 test is performed during the same examination. The aqueous component of the tears is commonly assessed on the basis of STTs (STT-1 and STT-2), PRTT, fluorophotometry, meniscometry, and assessment of tear osmolarity.7,8 Current techniques to characterize meibum include (in order of increasing invasiveness) evaporimetry, interferometry, meibometry, meibum expression with assessment of its physiochemical properties, and meibography.8,9 Mucins are typically assessed on the basis of TFBUT, biochemical analysis of tears, and cytologic or histologic evaluation of goblet cell density.8,10–12 Tear assays are constantly evolving, and a diversity of new techniques has recently been described in people, including wavefront aberrometry, corneal topography, advanced imaging (optical coherence tomography and confocal microscopy), and visual acuity testing.13 Use of these techniques has been increasingly described for people and, to a lesser extent, dogs. However, very few reports exist regarding tear evaluation in cats.
There is increasing evidence that quantitative and qualitative tear film deficiencies are an important cofactor or cause of some of the most common and frustrating ocular diseases of cats, such as KCS,6,14 chronic nonhealing corneal ulceration,15 corneal sequestration,15–17 conjunctivitis,18–20 and keratoconjunctivitis following feline herpesvirus-1 infection.19,21 Despite this, reference ranges for many of the tests commonly used to assess the human and canine tear film have not been established for healthy cats. As a result, it is likely that clinically important alterations in tear film health may be undiagnosed or underdiagnosed in cats. Therefore, the primary goal of the study reported here was to establish reference values for diagnostic tests commonly used to assess the tear film, including the STT-1, PRTT, TFBUT, tear osmometry, and meibometry, in healthy cats of various ages and various sex and neuter status combinations. Other aims included evaluation of correlations among these tests and, given that a highly repeatable measurement is essential for evaluating changes attributable to disease or treatment effects, assessment of test-retest repeatability of all 5 tests in a subset of these healthy cats.
