Tear film Artificial tears

Regardless of the cause, the mainstay of treatment for dry eye is artificial tears. Artificial tears augment the tear film topically and provide relief. If a patient uses artificial tears more than four times daily, recommend a preservative-free formulation. Preservative-free formulations are also appropriate if the patient develops an allergy to ophthalmic preservatives. Artificial tears are available in gel, ointment, and emulsion forms that provide a longer duration of relief and may allow for less frequent instillation. Ointment use is appropriate at bedtime.30... 

PVA can lower the surface tension of water, reduce interfacial tension at an oil/water interface and enhance tear film stability. These together with ease of sterilization, compatibility with a range of ophthalmic dmgs and an apparent lack of epithelial toxicity have led to the widespread use of PVA as a drag delivery vehicle and a component of artificial tear preparations. 

Mucoadhesive polymers increase the contact time of a formulation with the tear film and simulate the continuous delivery of tears due to a high water-restraining capacity. As such, they allow a decrease in the instillation frequency compared to common eye drops and are therefore useful as artificial tear products [1,197,198],... 

A single drop of 0.01% BAG can break the superficial lipid layer of the tear film into numerous oil droplets because it can interface with the lipid monolayer of the tear surface and disrupt it by detergent action. BAG reduces the breakup time of the tear film by one-half. Repeated blinking does not restore the lipid layer for some time.The inclusion of BAG in artificial tear formulations is questionable. It neither protects the corneal epithelium nor promotes a stable oily tear surface. 

Advances in chemical synthesis and in an understanding of the tear film of the eye have resulted in the development of compounds with two or more regions that vary in both their lipophilic nature and binding. The first of these to be tested in the eye was poloxamer 407, a block polymer vehicle with a hydrophobic nucleus of poly-oxypropylene,and hydrophilic end groups of polyoxyethylene. One advantage of poloxamers is their ability to produce an artificial microenvironment in the tear film, which can greatly enhance the bioavailability of lipophilic drugs such as steroids. 

The ideal artificial tear would reproduce the metabolic, optical, and physical characteristics of natmal tears. Additionally, it would have a long ocular residence time and would contain therapeutic additives to treat primary and secondary damage to the eye. Supplementation of natural tears with a substance that prolongs residence time generally improves tear film breakup time (TBUT) and is superior to tear replacement fluids of low retention time. 

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... 

Substituted Cellulose Ethers. Since their introduction for ophthalmic use, MC and other substituted cellulose ethers such as hydroxyethylcellulose, hydroxypropylcel-lulose, hydroxypropyl methylcellulose (HPMC), and carboxymethylcellulose (CMC) have been used in artificial tear formulations.These colloids dissolve in water to produce colorless solutions of varying viscosity. They have the proper optical clarity, a refractive index similar to the cornea, and are nearly inert chemically. Their relative lack of toxicity, their viscous properties, and their beneficial effects on tear film stability have made cellulose ethers useful components of artificial tear preparations. Historically, the most frequently used representative of this group was MC. 

The device is generally comfortable and well accepted by many patients, but its use does have certain disadvantages. Some patients have problems with discomfort (foreign body sensation) or expulsion of the Lacrisert.The insert can be wetted with saline before insertion to improve comfort, but this can make even more difficult the insert s placement into the lower cul-de-sac, which requires a moderate amount of dexterity. Supplementation with artificial tears after insertion may improve comfort. The most common patient complaint is blurred vision associated with the intense release of polymer during the first 4 to 6 hours after instillation, from a thickened tear film.Adding such fluid as drops of NaCl 0.9% or artificial tear solution can reduce the tear film viscosity and minimize the visual complaints. As the insert dissolves it releases debris that can blur vision and cause irritation. Most patients with mild signs and symptoms of dry eye do not experience improvement with use of the insert, as compared with the use of conventional tear solutions. Because some tear secretion is necessary to dissolve the Lacrisert, KCS patients with low basal tear secretion may not benefit from or tolerate its use. 

Murube J, Murube A, Zhuo C. Classification of artificial tears. II. Additive and commercial formulas. In Sullivan DA, Dartt DA, Meneray MA, eds. Lacrimal gland, tear film, and dry eye syndromes 2 basic science and clinical relevance. New York Plenum Press, 1998 705-715. 

Topical antibiotic use in MOD is controversial. Some authors recommend against topical applications to avoid further disruption of the tear film and also because efficacy is questionable.Topical steroids are unlikely to have any benefit. During the course of therapy, attention should be given to the KCS that occurs in nearly every case. Artificial tears or lubricating ointments are indicated to ensure improvement in symptoms. More recent treatment options include topical cyclosporine A 0.05% as well as soft steroids such as loteprednol or rimexolone. 

Dry eye symptoms and significant ocular surface disease frequently occur in patients taking isotretinoin.The associated symptoms may be accompanied by blepharocon-jimctivitis. The presence of isotretinoin in tear fluid decreases stability (and tear break-up time) of the lipid layer of the tear film but may also cause a decrease in aqueous production, leading to ocular surface dryness. These effects could be responsible for the dry eye symptoms, contact lens intolerance, superficial punctate keratitis, and conjunctival irritation accompanying isotretinoin therapy. Use of artificial tear preparations may help to alleviate the associated discomfort. 

Figure 26.31 depicts the same measuring procedure applied to a contact lens material sheet (A) untreated, (B) (CH4 + air) plasma treated, and (C) (CH4 + air) plasma treated and then O2 plasma treated. Characterizing aqueous film stability on untreated and plasma-modified contact lens materials using artificial tear fluid by... 

Methylcellulose is a vehicle used in eye-drops and contact lens solutions (SEDA-1, 369). It is non-irritant and has a good refractive index. A 1% solution is well retained in the conjunctival sac. Corneal cultures have been stimulated by methylcellulose, resulting in increased growth of cells (2) tear-film break-up time was increased four-fold with 2% methylcellulose. Hydroxypropyl methylcellulose is also used as artificial tears for people with dry eyes (3). 

An erodible insert developed as a potential ocular drug delivery system is marketed as a prescription drug for the lubricant properties of the polymer base. Lacrisert is a sterile ophthalmic insert that is used in the treatment of moderate-to-severe dry eye syndrome and is usually recommended for patients unable to obtain symptomatic relief with artificial tear solutions. The insert is composed of 5 mg of hydroxypropylceUulose in a rod-shaped form, about 1.27 mm in diameter and about 3.5 mm long. No preservative is used, since it is essentially anhydrous. The quite rigid cellulose rod is placed in the lower conjunctival sac and first imbibes water from the tears, and after several hours, forms a gel-like mass, which gradually erodes as the polymer dissolves. This action thickens the tear film and provides increased lubrication, which can provide symptomatic relief for dry eye states. It is usually used once or twice daily. 

Poly (2-hydroxypropyl. Me) ether Hydro-xypropyl methyl cellulose. HPMC. E463. Hypromellose, USAN. Hydroxypropyl-methylcellulose, JAN. Cellulose hydro-xypropyl methyl ether, BAN, INN [9(X)4-65-3] Prepared by etherification of alkaline cellulose with methyl chloride and propylene oxide. Commercial samples have methyl DS values of 1.1-2.0 and hydroxypropyl MS values of 0.1 - 1.0. Thickening and binding agent with widespread industrial uses in adhesives, building products, films, protective creams and colloids. Approved for food use in US and EU. Used in deep fried batters. Shows better water solubility and electrolyte tolerance than methylcellulose. Component of artificial tears. 

Dry eye affects about 4.9 million people aged fifty and older in the United States. It may seem like a minor eye condition, but dry eye results in discomfort, changes in vision, tear film instability, and serious damage to the surface of the eye. Treatments for dry eye used to be limited to continued use of artificial tear products or surgical blockage of a tear duct in the eye, which decreased the amount of tears that could drain from the eye. In 2002, the FDA approved cyclosporine ophthalmic solution (Restasis),... 

The surface chemistry of the eye is probably better understood than that of the ear. The cornea is covered with a thin, fluid film the so-called tear film, which is believed to consist of an aqueous phase, approximately 10" cm thick, with an adsorbed lipid and mucin layer at the air-water interface and an adsorbed mucin layer on the corneal side. The latter renders the cornea hydrophilic and enables the tear film to spread. In dry eye syndrome local areas of dewetting occur due to increased contact angle. Dry eye is sometimes precipitated by drug therapy, and there is a search for adequate artificial tear fluids. The surface chemistry of tear film components has been discussed by Holly [331]. Adsorption of cationic surfactants present in eye drops as preservatives can lead to the production of a hydrophobic surface due to electrostatic adsorption of the cations with the hydrocarbon chains oriented towards the tear film. Such a process can itself result in dewetting and thus cationics should be excluded from artificial tear fluids. 

PU is a strong, hard-wearing, tear-resistant, flexible, oil-resistant, and blood-compatible polymer. The functional properties of natural macromolecules can be merged with those of synthetic polymers having controllable structures and properties for the production of polymer/protein hybrids. In tissue engineering, silk fibroin/PU blend film can be used as scaffold material for artificial blood vessels [466] (Figure 2.62). Bacterial synthesized cellulose, which was designed... 

---