Fluorophotometric Assessment of Tear Volume and Turnover Rate in Healthy Dogs and Cats

Abstract

Purpose: The study establishes normative data of tear volume (TV) and tear turnover rate (TTR) in healthy dogs and cats, 2 species commonly used for translational research in ophthalmology. Methods: Thirty-six dogs and 24 cats were enrolled, encompassing a variety of breeds with diverse skull conformations (brachycephalic, mesocephalic, and dolichocephalic). Two microliters of 10% fluorescein were instilled onto the upper bulbar conjunctiva of both eyes, followed by tear collection with 2-μL capillary tubes at 0, 2, 4, 6, 10, 15, and 20 min. Fluorescein concentrations were measured with a computerized scanning ocular fluorophotometer. The TV and TTR were estimated based upon nonlinear mixed-effects analysis of fluorescein decay curves. Results: In dogs, median (interquartile range) TV, basal TTR (bTTR), and reflex TTR (rTTR) were 65.3 μL (42.3-87.9), 12.2%/min (3.7-22.1), and 50.0%/min (25.9-172.3), respectively. In cats, median (interquartile range) TV, bTTR, and rTTR were 32.1 μL (29.5-39.9), 10.9%/min (3.0-23.7), and 50.0%/min (28.4-89.4), respectively. Body weight (r = 0.44) and age (r = 0.30) were positively correlated (P ≤ 0.019) with TV in dogs. Age was negatively correlated (P ≤ 0.018) with TTR in dogs (r = -0.33) and cats (r = -0.24). However, TV and TTR were not associated with skull conformation in either species. Conclusions: Dogs have greater TV than cats but similar basal and rTTR. Tear parameters were impacted by body weight and age, but not by skull conformation. In both clinical and research settings, successive lacrimal tests should be spaced by ≥10 min to provide sufficient time for the tear film to replenish, as bTTR is ∼11%/min-12%/min in both species.

Keywords: canine; drainage; feline; fluorophotometry; tear film; tear flow.

FIG. 1.

Representative fluorescein decay curve in tear fluid of a dog or a cat, allowing for calculation of tear volume and TTR (reflex and basal) using parameters calculated with nonlinear mixed-effects model. C₀ represents the fluorescein concentration in tear fluid at t = 0 min, extrapolated from the fluorescein decay curve. NaFl, sodium fluorescein; TTR, tear turnover rate. Color images are available online.

FIG. 2.

Comparison of predicted tear fluorescence over time (purple curve) with observed data (blue points) for a random sample of dogs (A) and cats (B). Censored data are shown as vertical red bars. Color images are available online.

FIG. 3.

A positive association was found between canine body weight and tear volume (Pearson's correlation test). Estimated tear volumes are described in the table for body weights ranging from 1 to 65 kg.