Tear replacements are designed to mimic the normal tear film and in doing so help prevent corneal damage and improve symptoms in diseases associated with low tear production. They can be classified as demulcents (a soothing material to protect and lubricate mucus membranes) or emollients (oils and fats to decrease evaporation).
Demulcents are available as solutions and gels, and emollients as ointments. Demulcent solutions and gels are formulated from various combinations of water soluble celluloses such as carboxymethylcellulose and hydroxypropylmethylcellulose; polyols such as polyvinyl alcohol, polyvinylpyrrolidone (povidone), glycerin, propylene glycol, and polyethylene glycol; glycerin; and dextran 70. The combinations are intended to thicken, stabilize, and strengthen the tear film; increase viscosity; and form a protective layer. Other constituents are often added, such as sorbitol (to lower viscosity), hyaluronic acid (acts as a humectant), sodium hyaluronate (protectant, lubricant, aids in wound healing), L-carnitine and erythritol (as osmoprotectants), polyacrylic acid (to increase retention time or tears), electrolytes to maintain osmolarity and help with epithelial metabolism, and buffering agents to maintain an appropriate pH.
Emollients are formulated from lanolin or other oily components such as mineral oil, paraffin, petrolatum, and white or yellow wax. They are usually used in combination to help seal in moisture, lubricate, and stabilize the tear film.
To control contamination, various preservatives are used. They are detergents (benzalkonium chloride, chlorobutanol, polyhexamethylene biguanide, polyquaternium-1) or oxidative (sodium perborate, stabilized oxychloro complex, sodium chlorite) or ionic buffering systems. Detergent preservatives are more toxic to the corneal epithelium, which can cause problems because products are applied so frequently. To minimize this toxicity, some products are available in preservative-free single-use containers or in multi-use bottles with specially designed openings.
Hypertonic (5% sodium chloride) drops and ointments are used to help decrease and manage corneal edema in animals affected with endothelial degeneration, bullous keratopathy, and changes secondary to corneal ulcers. The drops can sting on initial application.
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