The tear film is the primary interface between the ocular surface (corneal and conjunctival epithelium) and the outside environment. It plays a critical role in optical form and function. Quantitative and qualitative maintenance of an adequate tear film rests on a dynamic system consisting of tear production, tear drainage through the naso-Lacrimal duct, fluid absorption or exchange through the conjunctival and corneal epithelium and environmental interface evaporation.1
An adequate volume of tears is a prerequisite for a healthy ocular surface. A reduction in the volume of tears results in a greater chance of developing signs and symptoms of ocular dryness. Fluorophotometry is a technique which can be utilized to determine tear volume, tear flow and tear turnover. It uses the emitted photons that result from excitation of fluorophore (fluorescein) by electromagnetic radiation.2 Fluorescein is a hydrophilic and non toxic molecule that has the property of emitting light. The excitation peak for fluorescein (DSF) molecules is about 490 nm (blue part of the spectrum) and at this wavelength the maximal absorption of light energy by fluorescein takes place. Fluorescein molecules stimulated by this wavelength will be excited to a higher energy level and will emit light of a longer wavelength about 530nm (green portion of the spectrum).3
Utilizing a fluorophotometer (Fluorotron Master; Ocumetrics, Mountain View, California) a background fluorescence measurement is taken prior the instillation of 0.5 µl of 0.5 % sodium fluorescein onto the ocular surface. Eight measurements are subsequently taken for each eye to determine tear fluorescence. This data is used to calculate tear volume, tear flow, and tear turnover.4,5
Tear turnover is the percentage change in fluorescein concentration in a given amount of time; there are no units of measurements for these calculations. Tear flow is the amount of tear fluid flowing past the cornea in a given amount of time expressed as microliters per minute (µl/min).6 Tear volume refers to the initial tear volume determined by regressing the concentration to time zero, thereby obtaining the decay constant (k) and the concentration at instillation.7 The volume of tears in the eye reflects the secretion and drainage rates of the Lacrimal system.
An estimation of all these parameters provides useful insight into the underlying pathophysiology of dry eye syndrome.
- Pearce E, Keenan B, McRory C. An Improved Fluorophotometric method for tear Turnover Assessment. Optom Vis Sci 2001;78:30-36
- Fahim M, haji S, Koonapareddy G\C, Fan V, Asbell PA. Fluorophotometry as a diagnostic tool for the evaluation of dry eye disease. BMC Ophthalmology.2006;6:20
- McCulley JP, Shine WE, Aronowicz J, Oral D, Vargas J. Presumed hyposecretory/hyperevaporative KCS: tear characteristics. Trans Am Ophthalmol Soc 2003;101:141-152.
- McCulley JP, Uchiyama E, Aronowicz JD, Butovich IA. Impact of evaporation on aqueous tear loss. Trans Am Ophthalmol Soc 2006;104:121-128.
- Van Best JA, Benitez JM, Coulangeon L. Measurement of basal tear turnover using a standardized protocol European Concerted Action on Ocular Fluorometry. Graefe’s Arch Clin Exp Ophthal1995;233:1-7.
- Mishima S, Gasset A, Klyce S, Baum J. Determination of tear volume and tear flow. Invest Ophthalmol1966;(5)3:264-276.
- Eduardo Uchiyama, MD, Mario Di Pascuale, MD, Igor Butovich, MD, James McCulley, MD; "Impact on Ocular Surface Evaporation of an Artificial Tear Solution Containing Hydroxypropyl (HP)Guar," Eye & Contact Lens, November 2008, Vol. 34, No. 6, pgs. 331-334.
- Wojtowicz, J., McCulley, JP; “Assessment and Impact on the Time of Day on Aqueous Tear Evaporation in Normal Subjects,” Eye and Contact Lens, May 2009, Vol. 35, No. 3, pgs. 117-119.