Tear Dynamics in Healthy and Dry Eyes

Abstract

The compartmental periodic-steady tear-dynamics model accurately predicts tear behavior in normal and dry eyes. Inclusion of osmotic water flow is crucial to match measured tear osmolarity. Tear-dynamics predictions corroborate the use of TTR as a clinical discriminator between ADDE and EDE. The proposed model is readily extended to predict the dynamics of aqueous solutes such as drugs or fluorescent tags.

Human tear-production rate from closed-eye Schirmer-strip capillary dynamics

“…If we accept a reasonable tear-production rate of Q L = 1 L/min [6,7,9], the 0.025 dimensionless induction time corre-sponds to about 6 s in real time. Zero clinical time is difficult to assess precisely since strip insertion is not exactly reproducible.…”

“…For ease of analysis, the tear-prism cross section is approximated as an equilateral triangle with an apex height, h TL , located at the gray line (i.e., at the wetting transition [9]), about 0.7 mm above the cornea [9]. For a tear-prism margin of about 30 mm [9], approximately 2-3 L of tear are gathered into the prism after each blink [7][8][9]. Anterior-eye anatomy and physiology are outlined elsewhere [9].…”

“…where is the length of the lid margin [7,9] and R m is the transient arc-menisci radius in the tear prism defined in Fig. 4.…”

“…A simple, clinical test for measuring human tear production dates back to Schirmer in 1903 [1]. As now practiced in a Schirmer tear test (STT) [2], a 0.2-mm thick paper strip (Standard Schirmer Tear Test Strip, Whatman standard filter paper # 41, Alcon Laboratories, Fort Worth, TX), 5-mm wide and 35-mm long, is folded at a notch located 5 mm from the rounded end of the strip and gently inserted into the inferior fornix (Table 1 lists pertinent properties of a standard Schirmer strip [2][3][4][5][6][7][8][9][10][11][12][13][14][15]). After 5 min of eye closure, or until the patient's tear fills the entire strip, the Schirmer strip is removed, and the wetted length beyond the notch is recorded [2].…”

Flow Evaporimeter To Assess Evaporative Resistance of Human Tear-Film Lipid Layer

“…15 As tear aqueous evaporates, salinity increases. 16,17 At elevated evaporation rates, increased salinity stimulates afferent corneal epithelial nerves, thereby signaling irritating dry-eye symptoms. 15,18−20 The human tear film, deposited during each blink at approximately 5 μm thick, 21−23 consists mainly of dilute proteins, salts, and mucins.…”