Mission for Vision- Complications of LASIK

The complications of LASIK with links to scientific articles for patients, ophthalmologists, medical students, optometrists and those contemplating the procedure.

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Saturday, December 31, 2005

Dry Eye Disease

Structure and Composition of the Tear Film.
The human tear film serves to protect and lubricate the cornea and conjunctiva. Although several models of the tear film have been proposed recent observations suggest that the mucins exist as a network distributed in the aqueous body of the tear film and covered by 2 layers of lipid. Nichols described a mucin layer that measures 2-7 microns above the corneal surface and intimately associated with corneal microvilli and presumably anchored to the glycocalyx of the conjunctiva. Three membrane associated mucins, MUC 1, 4, and 16, as well as two secretory mucins, MUC5AC and MUC 7, have been identified at the ocular surface. Laser confocal microscopy suggests that the human aqueous-mucin layer may be much thicker than first estimated, 41-46 microns (15, 16). In the current model of the tear film, the aqueous-mucin layer is covered by 2 thin layers of lipid. Polar lipids such as phospholipids and fatty acids lie adjacent to the aqueous-mucin layer and non-polar lipids such as cholesterol esters, and triglycerides are present at the tear-air interface. The thickness of the lipid layer, estimated by observation of interference patterns, measures between 0.06-0.18 microns in the open human eye. The aqueous-mucin layer of tears contains many proteins but there are 3 major protein components: lysozyme (24-47%), lactoferrin (23-29%) and tear lipocalin (15-33%) (25, 26). IgA becomes the predominant protein when the lids are closed for prolonged periods. Other components play a larger role in the exposed open eye. Any concept of tear film structure requires an understanding of the individual components and their specific interactions with other components. Tear lipocalin interacts with the lipid components of the tear film. The detailed mechanisms of this interaction are not known.
Functions of Tear Lipocalin.
Tear lipocalin (TL) , also called von Ebner’s gland protein, accounts for about 15-33% of the protein in tears. As the major lipid binding protein in tears, TL binds a broad array of lipids including fatty acids, cholesterol, phospholipids and glycolipids. The relative affinity of tear lipocalin to tear film lipids has been determined from displacement assays using fluorescent ligands. Tear lipocalin has the greatest affinity for relatively insoluble long-chain fatty acids and phospholipids that are abundant in meibomian glands. Tear lipocalin interacts with lipids to contribute to the surface pressure in the tear film, promote optical clarity, and may possibly transport lipids from the lacrimal gland to the tear film (32, 36). The lipid binding and transport functions of tear lipocalin are critical to understanding the physiology of the ocular surface.
There is strong evidence that tear lipocalin is a multi-functional protein. TL may have a role in the transport of retinol, Vitamin E, sapid molecules, and may inhibit cysteine proteinases. Holo-TL has anti-microbial activity; fatty acid endogenous ligands have antibacterial activity. Recent work shows that TL binds fungal and bacterial siderophores to deprive microbes of iron. TL is a Mg+2 dependent endonuclease, and may be important in destroying viral DNA before it transfects the corneal epithelium.
Evidence that Tear Lipocalin Is Important in Dry Eye Diseases (keratoconjunctivitis sicca).
Dry eye diseases are quite common and account for significant morbidity including irritation, pain, decreased vision, predisposition to corneal ulceration, infections and in severe cases, blindness. Approximately 7-10 million Americans currently use artificial tear preparations ) and the prevalence of the dry eye has been estimated to range from 8.4-35% in the elderly. Over $100 million are spent each year for various treatments of dry eye including the hundreds of available drops and viscous substances to cover the dry eye. Tear lipocalin is the principal lipid binding protein in tear and evidence is mounting that TL has a significant role in protection of the ocular surface from desiccation. Tear lipocalin scavenges lipid from the cornea to prevent dry spots from forming on the cornea.Lipids bound to tear lipocalin in aqueous provide a reservoir of lipid molecules in equilibrium with the surface and could reduce evaporation of water. The concentration of tear lipocalin as well as other protein components are decreased in dry eye disease. Furthermore, the concentration of tear lipocalin correlates with tear film stability in dry eye patients. The ocular surface is abnormal in dry eye disease. One of the objective criteria used widely for the diagnosis of dry eye is the presence of fluorescein staining of the cornea in areas where the epithelium has been disrupted (54). Punctate epithelial erosions occur in seborrheic blepharitis, a disease associated with aqueous tear deficiency. Epithelial erosions are quite common in forms of meibomian gland dysfunction. Tear lipocalin is reduced in both seborrheic blepharitis and meibomian gland dysfunction. Most treatments are not derived from fundamental knowledge of the composition, functions, mechanisms, and interrelationships of the tear components at the abnormal human ocular surface. This proposal will test the role of the protein component in ocular surface physiology with an emphasis on features that are potentially important in dry eye diseases.
TL interacts with lipids that are important to the function of the tear film.
Disruption of the tear film may result from thinning of the tear film or lipids contaminating the corneal surface. Normally, the membrane associated mucins form a glyocalyx that protects the apical epithelium and presumably prevents lipid binding to the surface of the cornea. However, the natural processes of apical epithelial shedding and ectoshedding of membrane associated mucins as well as the loss of corneal epithelium from minor trauma may leave a portion of the surface unprotected. Lipid contamination of the unprotected corneal surface lowers the surface tension and renders the cornea unwettable. This situation is possible whenever the tear film thins such as in dry eye diseases. The mucin covering the cornea may be compromised in dry eye disease as is suggested by the Rose Bengal staining of epithelial cells. The membrane-associated mucins and secreted mucins are altered by a host of dry eye diseases. Epithelial erosions are common in dry eye diseases. A mechanism to remove meibomian lipids that inadvertently come to rest on an area depleted of mucins, on the corneal epithelium, or on the de-epithelialized cornea is necessary to prevent drying of the ocular surface. Tear lipocalin acts to extricate these lipids from corneas with intact epithelium. This proposal presents a plan to investigate the interaction of TL with lipids on the abnormal corneal surface associated with dry eye disease. Tear lipocalin is promiscuous in binding a broad array of lipids and is ideally suited for scavenging the wide range of meibomian lipids that spill onto the abnormal corneal surface. In meibomian gland dysfunction there are decreased triglycerides, cholesterol, and monounsaturated fatty acids in tears. Recently tear lipocalin has been reported to be decreased in symptomatic meibomian gland dysfunction. Since tear lipocalin binds avidly to saturated lipids, it is plausible that meibomian gland disease would be manifest if the mechanism to remove and solubilize saturated lipids is compromised. The functions of tear lipocalin and its pH regulated mechanism of lipid binding is currently being investigated.
Tear Lipocalin is the major lipid carrier in tears and understanding its functions and interactions with other tear film components may provide insight into the prevention and possible treatment of dry eye diseases. Study of the structure of tear lipocalin has led to the development of hypotheses about molecular mechanisms and permits comparison to other lipocalins. Lipocalins have a wide range of physiologic roles including retinol transport, olfaction, pheromone transport, immune regulation, invertebrate coloration, cold acclimation of plants, anti-microbial activity via iron depletion, endonuclease activity, nitric oxide transport in insects, and nutrition. Because TL is a very promiscuous ligand binder and some of its putative functions overlap those of other lipocalins, it is an excellent paradigm for the study of structure-function relationships in the lipocalin family. Knowledge of the solution structure, interaction with other tear film components, and mechanisms will provide insight into the normal function of the tear film as well as the function of other lipocalins.
For more detailed scientific information click to link to the references.


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