Tear substitutes

Because they are not produced constantly, as are natural tears, tear substitutes should have properties to enhance their retention time in the tear film. The addition of various types of polymers to artificial tears not only improves retention of added fluid but also increases corneal surface wettability, decreases blink friction, and minimizes surfece tension. Natural tears contain glycoproteins and other surfectant macromolecules to decrease surface tension. Although polymers may enhance tear film stability without enhancing viscosity, there appears to be no correlation between retention time and viscosity. Polysaccharides, including mucilages, dextrans, and... 

In addition to their use in tear substitutes, cellulose ethers are used to moisten contact lenses and, as... 

Use of preserved tear substitutes with contact lenses is a concern because the preservatives may bind to the lens polymer, prolonging ocular retention and exposure, which may result in toxic or hypersensitivity reactions. BAK is more readily absorbed than are thimerosal and chlorhexidine in most hydrogel lenses. 

Nutrients. In natural tears nutrients are necessary for corneal and conjunctival epithelial metabolism as well as for the synthesis of mucin and the glycocalyx. Water is the most important nutrient contained in tear substitutes, but other nutrients include dextrose, sodium lactate, sodium citrate, and vitamins A, B12, and C. 

Lacrimal occlusion can benefit patients who have symptoms of dryness or other ocular abnormalities that topical therapy alone does not resolve. The procedures are indicated in moderately severe to severe dry eye patients to prevent drainage and thereby conserve natural tears as well as instilled tear substitutes, reducing the frequency of application. Lacrimal occlusion also can improve contact lens tolerance in mild dry eye cases. Various studies have demonstrated that plugs may increase the aqueous tear component of the tear film, decrease corneal and conjunctival staining, and improve patient symptoms. Tear osmolarity decreases, probably due to increased tear volume. 

Lemp MA, Goldberg M, Roddy MR. Effect of tear substitutes on tear film breakup time. Invest Ophthalmol 1975 14 225-258. 

Women who are taking or considering hormone replacement therapy should be informed of the potential increased risk of dry eye syndrome with this therapy. Women on oral contraceptives who complain of dry eye or in whom ocular surface disease is noted should be advised of the potential causative aspects of their medication. In either case, therapies for dry eye are usually based on topical administration of tear substitutes, lid treatments, environmental modification, and immunomodulatory agents for inflammatory-related ocular surfece disease. Hormone-based eyedrops are being studied with respect to targeting postmenopausal women with dry eye. 

Treatment is usually with tear substitutes ( artificial tears ), containing compounds that enhance wetting, viscosity and stability of tears. These are hypromellose, polyvinyl alcohol (PVA), carbomer 940, and hydrophobic ocular lubricants containing liquid and soft paraffins, such as Simple Eye Ointment. All preparations are available as P medicines. 

The current management of dry eyes usually includes instilling artificial tears and ophthalmic lubricants, hi general, tear substitutes are hypotonic or isotonic solutions... 

Felt, O. Carrel, A. Baehni, P. Buri, P. Gurny, R. (2000) Chitosan as tear substitute a wetting agent endowed with antimicrobial efficacy. Journal of Ocular Pharmacology and Therapeutics. VoL 16, pp. 261-270. 

As with all cellulose ethers, methylcellulose increases the residence time of the preparation. In addition, methylcellulose possesses wound healing properties. Therefore the polymer is suitable as a tear substitute for dry eye especially for those with punctate lesions. A disadvantage are irritating insoluble cellulose particles present in methylcellulose. The amount of insoluble particles depends on the quality of the product... 

A. Sharma and E. J. Ruckenstein, The role of lipid abnormalities, aqueous and mucus deficiencies in the tear film breakup, and the implications for tear substitutes and contact lens tolerance. Colloid Interface Sci., Ill, 8 (1986). 

Geerling, G. Daniels, J. X Dart, J. K. Cree, I. A. Khaw, P. X. Toxicity of natural tear substitutes in a fiiUy defined culture model of human corneal epithehal cells. Invest. Ophthalmol. Vis. Sci. 2001, 42, 948-956. 

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