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.

Impact of acute conjunctivitis on ocular surface homeostasis in dogs

“…Tear volume (TV) and tear turnover rate (TTR) were estimated based upon nonlinear mixed-effect analysis of fluorescein decay curves in tear samples (Monolix ® version 2018R2; Lixoft). 21,27,28 The biphasic decay of fluorescein allowed for calculation of reflex (first phase) and basal (second phase) values for both tear parameters. mild conjunctivitis vs severe conjunctivitis eyes were assessed with the Kruskal-Wallis test and post hoc Dunn's.…”

“…• Fluorophotometry: Tear film fluorophotometry was performed in each eye as previously described. 21 Briefly, 2 µL of 10% fluorescein (Akorn Inc) was instilled onto the dorsal bulbar conjunctiva using a pipette, followed by 3 conjunctival fornix of each eye, and tear quantity was recorded at 1 minute (mm/min) using a stopwatch. Strips were left until the ≥20-mm mark was reached, followed by centrifugation for 2 minutes at 3884 g in a punctured 0.2-mL tube (secured to 2-mL tube) to extract tear fluid for the ferning test.…”

Albumin Levels in Tear Film Modulate the Bioavailability of Medically-Relevant Topical Drugs

“…Two canine albumin ophthalmic solutions (0.4% and 1.5%) were formulated by mixing canine albumin lyophilized powder (Animal Blood Resource International, Stockbridge, MI) with lubricating eye drops (Artificial tears solution, Rugby, Rockville Center, NY, USA) in a sterile manner under a laminar flow hood. Of note, albumin concentrations selected herein (0.4% and 1.5%) aimed to achieve tear film albumin levels of~1 to 5 mg/ml (i.e., after dilution of the instilled drop with the canine tear film, accounting for~3-fold dilution; Sebbag et al, 2019b), thus representing a spectrum of albumin levels in tears of dogs with spontaneous or experimentally-induced conjunctivitis (Sebbag et al, 2019a). Artificial tears solution without albumin (vehicle only) was used as control for the experiment.…”

“…Such discrepancy is likely explained by two important differences in study designs. In the present experiment, the concentrations of albumin (0.4% and 1.5%) were specifically chosen based on clinically-relevant albumin levels detected in canine patients with diverse ocular diseases (Sebbag et al, 2019a), taking into account the threefold dilution effect from resident tears (Sebbag et al, 2019b) to better account for the true binding constant seen in dialysis experiments (Chrai and Robinson, 1976). In contrast, the concentrations of albumin used in rabbits were higher (1% and 3%) and may not reflect the range of biological concentrations of albumin at the ocular surface.…”

“…Drug binding to proteins in tear fluid could partly explain the aforementioned variability in pupil size, a phenomenon our group investigated in the present study to help guide future diagnostic and therapeutic applications in ophthalmology. The present work was conducted in dogs, a species that represents an excellent large animal model for translational research in humans given similarities in ocular anatomy (Vézina, 2013) and physiologic parameters pertinent to topical route of drug administration (Sebbag et al, 2019b, Sebbag et al, 2019c, as well as spontaneous disease development such as glaucoma (Grozdanic et al, 2010), dry eye (Kaswan et al, 1989) and conjunctivitis (Sebbag et al, 2019a).…”