Dry eye disease is observed often in our practice as primary eye care providers. In general, dry eye is characterized as a disease that occurs due to increased tear evaporation or decreased tear secretion that resulted in symptoms of ocular irritation.1
The recent International Dry Eye WorkShop (DEWS) defined dry eye disease as follows: Dry eye is a multifactorial disease of the tears and ocular surface that results in symptoms of discomfort, visual disturbance, and tear film instability with potential damage to the ocular surface. It is accompanied by increased osmolarity of the tear film and inflammation of the ocular surface.2 As outlined in the DEWS definition, dry eye disease not only causes ocular irritation, but also affects the quality of the patients visual acuity and can affect the integrity of the ocular surface.
Estimates of the prevalence of dry eye vary considerably between different populations of the world. In the
Dry eye disease does cause significant economic burdens on the patient through the costs for obtaining treatment, follow-up visits, additional testing and loss of working hours.4 Furthermore, it has been shown that dry eye disease affects the quality of life of a patient.5
A wide variety of therapeutic options are available for dry eye disease. Various treatment strategies, such as artificial tears or rewetting agents (henceforth called artificial tears), punctal plugs, anti-inflammatory agents (topical steroids or oral fatty acids), moisture goggles, secretogogues, autologous serum, prescription immunomodulatory drugs (topical and oral) as well as surgical options, are available for the treatment of dry eye disease.
Identification of the underlying cause of dry eye aids in successful treatment of this disease. However, its multifactorial nature makes it complicated to identify a single cause in a particular patient.
The diagnostic challenge is further increased for dry eye disease because no single test is available to correctly diagnose its various forms. Current research suggests that inflammation at the ocular surface plays an important role in the pathogenesis of dry eye.6
Among all therapeutic options for dry eye disease, artificial tears are still the mainstay in the initial management of a dry eye patient.7 Ophthalmic physicians usually begin a newly-diagnosed dry eye patient on a regimen of topical artificial tears and then add to or modify the course of treatment from there.
Currently, there are various artificial tears available over-the-counter; one brand (FreshKote, Focus Laboratories) is available by prescription only. Artificial tears are generally designed to lubricate the ocular surface and replace tear volume.
Due to the complex nature of the tear film, it is difficult to design an artificial tear solution that is identical to human tears. However, many artificial tear brands try to improve their quality by altering the composition, viscosity and/or osmolarity of the solution.
This article details some of the commonly available compositions of artificial tears and discusses their potential use in specific causes of dry eye disease. There is no single brand of artificial tears that works well for every form of dry eye. Rather, each option (organized here by formulation composition) has benefits for certain clinical situations.
CMC-Based Artificial Tears
Carboxy methylcellulose (CMC, discussed below) and hydroxypropyl methylcellulose (discussed later) are polysaccharides known as mucilages.8 The methyl and hydroxpropyl cellulose derivatives are widely used in artificial tear formulations. They increase the residence time of tears as well as increase the viscosity of tears.8 Interestingly, the refractive index of 1% methylcellulose is 1.336, which closely matches that of human tears.8
Commonly available CMC artificial tears include the Refresh brand of tears (Allergan) as well as TheraTears (Advanced Vision Research). CMC-based artificial tears may protect the integrity of the ocular surface.9
Refresh is available in both preserved and non-preserved as well as in liquigel formulations. It has been shown that the mid-viscosity (1.0% CMC) Refresh Liquigel promotes significant reduction in the signs and symptoms of dry eye compared to lower viscosity agents.9 So, CMC-based artificial tears may be indicated in patients who demonstrate ocular surface staining with vital dyes. TheraTears has the added benefit of being a hypotonic solution. Hyperosmolarity is a common feature of most forms of dry eye disease.1 Therefore, the hypotonic TheraTears solution may provide comfort in dry eye patients.
HMC-Based Artificial Tears
Several brands of artificial tears, such as Tears Naturale and Bion Tears (Alcon), GenTeal (Novartis) and Visine Tears (Pfizer) are hydroxypropyl methylcellulose (HMC) based. Tears Naturale is available both with and without preservatives. Bion Tears is preservative free and has been suggested for use in patients with severe dry eye.10 GenTeal is available in three different formulations, one each for mild, moderate or severe dry eye.11 Visine Tears is available with or without preservatives. HMC-based artificial tears work on the simple principle of lubricating the ocular surface in order to promote the integrity of the surface.10
HP Guar-Based Artificial Tears
Systane (Alcon) is formulated with hydroxypropyl guar (HP guar). The HP guar in this artificial tear brand is gel-forming and offers a unique mechanism to approach the problem. These properties of HP guar improve recovery of the ocular surface due to possible increased retention time of the artificial tear drop.12
Also, the increased retention time may be responsible for the increase in tear film break-up times observed with HP guar-based artificial tears compared to CMC-based tears.13 Tear evaporation may be reduced by the use of HP guar containing artificial tears.14 So, HP guar-based artificial tears may be helpful in patients with conditions causing evaporative dry eye (e.g., meibomian gland disease) as well as patients demonstrating ocular surface staining.
SH-Based Artificial Tears
Blink Tears and Blink Contacts (Abbott Medical Optics) and AQuify comfort drops (CIBA Vision) are brands that include sodium hyaluronate (SH) as an inactive ingredient. Blink Tears and AQuify comfort drops have been advertised for use in contact lens wearers. Blink Tears has a unique OcuPure preservative that dissipates upon exposure to light. Therefore, Blink Tears become preservative free upon instillation in the eye. AQuify comfort drops are blink-activated.15 Sodium hyaluronate in the AQuify comfort drops aids in a gradual release of water molecules, which increases the duration of wettability as well as relieves lens-related dryness.15
Recent studies have shown that SH eye drops are useful in improving subjective symptoms (as well as the ocular health) of dry eye patients, treating lipid tear-deficient patients and managing Sjgrens syndrome patients.16-20 Sodium hyaluronate also seems to have protective effects on the corneal epithelium.21
Based on the above evidence, it appears that SH-based artificial tears may be useful in a wide variety of patients with dry eye. Interestingly, Blink Tears may also provide relief to dry eye patients post-LASIK surgery or post-cataract surgery.22 So, this formulation appears to provide a wide range of beneficial effects for dry eye patients.
PVA-Based Artificial Tears
Polyvinyl alcohol (PVA) based artificial tears, such as Murine Tears (Murine Eye Care), Tears Again (Cynacon/Ocusoft) and HypoTears (Novartis), also work on the principle of lubricating the ocular surface. Tears Again is available in the eyedrop as well as the gel formulation.
HypoTears, as the name suggests, is a hypotonic solution. It is available with or without preservatives. So, HypoTears has the added advantage of addressing hyperosmolarity issues observed in dry eye patients (similar to TheraTears, which is CMC based).
One recent article concluded that Murine Tears provided the drop volume closest to the volume of the natural tear fluid and was also the least expensive per year of treatment compared to artificial tears from seven other manufacturers.23
Refresh Endura (Allergan) and Soothe XP Emollient eye drops (Bausch & Lomb) are the two brands that use oils in their composition. Refresh Endura is a preservative-free castor oil-based formulation. Castor oil aids in the reformation of the lipid layer of the tear film and prevents evaporation of the existing tear film.24,25
Castor oil-based eye drops also improve tear stability and aid in the treatment of meibomian gland disease.26 So, it is evident that the use of castor oil-based artificial tears is indicated in evaporative dry eyes. Unfortunately, the production of Refresh Endura was discontinued as of December 2008.
Soothe XP Emollient eye drops, on the other hand, are mineral oil-based. This brand of artificial tears has been shown to increase the thickness of the lipid layer of the tear film.27 So, Soothe XP eye drops can also be considered for use in evaporative dry eye patients who have a poor quality of the lipid layer.27
FreshKote eyedrops (Focus Laboratories) are the only brand of artificial tears that require a prescription. The main ingredients of FreshKote include PVA as well as polyvinyl pyrrollidone. FreshKote also includes the companys proprietary Amisol Clear which is intended to stabilize the lipid layer of the tear film and prevent evaporation of the tear fluid. Because the manufacturer of FreshKote describes Amisol Clear as a phospholipid, FreshKote functions somewhat akin to the oil-based tears. Additionally, these eyedrops are designed to achieve better wetting by aiding the combination of all three layers of the tear film. FreshKote has a high oncotic pressure, which helps in increasing ocular surface integrity.28 And, FreshKote can be used daily on an as needed frequency.28
No single artificial tear solution will work for all dry eye patients. Each patients symptoms, complaints and diagnosis will play a role in your selection of the best dry eyedrop for his or her needs.
The different components of artificial tears offer distinct advantages, and it is your responsibility to choose the best formulation for your patients dry eye therapy.
Dr. Narayanan is an Assistant Professor at the Pennsylvania College of Optometry at
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