Dry eye conditions are among the most common disorders treated by ophthalmologists. Key mechanisms involved in dry eye syndrome include tear hyperosmolarity and tear film instability. The major causes of tear hyperosmolarity are reduced aqueous tear (AT) flow due to lacrimal gland dysfunction, increased evaporation from the tear film, or a combination of the two. 1, 2 The environmental conditions of low humidity3 and high air flow also contribute to the hyperosmolarity of tears.
Another clinically relevant mechanism is meibomian gland dysfunction that leads to a presumed unstable tear film lipid layer (TFLL). During blinking, meibum produced by the meibomian glands is spread onto the ocular surface to form the TFLL. Functions attributed to pre-ocular TFLL include the tear film that spreads over the aqueous surface, retardation of evaporation, and provision of a smooth optical surface for the cornea. 4,5
Evaporation from the ocular surface is increased when a disturbance of the lipid layer is present.6-8 However, mechanisms involved in lipid layer dysfunction leading to increased evaporation are not completely evident and are likely complex. The evaporation of AT has been studied as a major factor in tear film dynamics and human AT evaporation rates have been reported in studies. The contribution of evaporation to AT volume loss ranges from 20 to 60 percent and is dependent on environmental conditions such as relative humidity (RH).9
For evaporation rate measurements, our lab utilizes an Evaporometer (Oxdata, Portland, Oregon) which incorporates a pump to direct ambient air through a drying tube into a form-fitting goggle, containing a humidity/temperature sensor.10 Dry air is pumped into the firm-fitting eye goggle to reduce the RH to 15%, at which time the pump is turned off. The RH within the goggle is then allowed to rise. The increase in humidity due to evaporation from skin or the evaporation of tears is subsequently measured over time. The process is carried out first with the eyelids closed and then with them open; the difference represented the ocular surface AT evaporation rate.11
Using the original formula published by Rolando and Refojo 12, we calculate the evaporative rates under two different ranges of increasing RH, 25% to 35% and 35% to 45%. The area of the interpalpebral ocular surface is used to calculate AT evaporation per unit area; the image of the area is captured with the use of a digital camera, and the area is calculated directly with use of computer software (Adobe Photoshop, version 184.108.40.2061; Adobe Systems, San Jose, California)11, expressed as μL/cm2/min.
Current results confirm that environmental conditions influence the AT evaporation rate.3 A reduction in RH is correlated with an increase in evaporation rate.13 Clinical trails are continuing and we anticipate further conclusions on tear film and its effects on evaporation rate.
- Li DQ, Chen Z, Song XJ. Stimulation of matrix metalloproteinases by hyperosmolarity via a JNK pathway in human corneal epithelial cells. Invest Ophthalmol Vis Sci. 2004;45:4302-4311.
- Luo L, li DQ, Corrales RM, Pfugfelder SC. Hyperosmolar saline is a proinflammatory stress on the mouse ocular surface. Eye Contact Lens 2005;31:186-193.
- Uchiyama E, Aronowicz JD, Butovich IA, McCulley JP. Increased evaporative rates in laboratory testing conditions simulating airplane cabin relative humidity: an important factor for dry eye syndrome. Eye Contact Lens. 2007 ;33(4):174-6.
- Mishima S, Maurice DM. The oily layer of the tear film and evaporation from the corneal surface. Exp Eye Res. 1961;1:39-45
- Bron AJ. Tiffany JM. The contribution of meibomian disease to dry eye. Ocul Surf. 2004;2(2):149-165.
- Iwata S, Lemp M, Holly FJ, Dohlman CH. Evaporation rate of water from the precorneal tear film and cornea in the rabbit. Invest Ophthalmol 1969;8:613-619
- Matsumoto Y, Dogru M, Goto E, Endo K, Tsubota K. Increased tear evaporation in a patient with Ectrodactyly-Ectodermal Dysplasia-Clefting Syndrome. Jpn J Opthalmol 2004;48:372-375.
- Goto E, Endo K, Suzuki A, Fujikura Y, Matsumoto Y, Tsubota K. Tear Evaporation dynamics in normal subjects and subjects with obstructive meibomian gland dysfunction. Invest ophthalmol Vis Sci2003;44:533-539
- McCulley JP, Uchiyama E, Aronowicz JD, Butovich IA. Impact of evaporation on aqueous tear loss. Trans Am Ophthalmol Soc 2006;104:121-128.
- Mathers WD, Binarao G, Petroll M. Ocular water evaporation and the dry eye. A new measuring device. Cornea 1993;12:335-340
- McCulley JP, Shine WE, Aronowicz J, Oral D, Vargas J. Presumed hyposecretory/hyperevaporative KCS: tear characteristics. Trans Am Ophthalmol Soc 2003;101:141-152.
- Rolando M, Refojo MF. Tear evaporometer for measuring water evaporation rate from tear film under controlled conditions in humans. Exp Eye Res 1983;36:25-33.
- McCulley JP, Aronowicz JD, Uchiyama E, Shine WE, Butovich IA. Correlations in a change in aqueous tear evaporation with a change in relative humidity and the impact. Am J Ophthalmol 2006;141:758-760.
- 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.