ANTIOXIDANTS IN OPHTHALMOLOGY

ANTIOXIDANTS IN OPHTHALMOLOGY
EVOLVING CONCEPTS

Prof. Dr M R Jain MS, FICS( USA), FAMS, FACLP ( LONDON)
MEDICAL DIRECTOR
M.R. J INSTITUTE & JAIN EYE HOSPITAL
JAIPUR ( INDIA)
E Mail: drmrjain55@gmail.com


ABSTRACT
As one ophthalmologist had mentioned way , a decade ago: ‘Advocating antioxidants is like shooting in the dark’. It is no more now. Today the pathophysiology of free radical mediated eye degenerative diseases like age-related macular degeneration and cataract are well established, and so is the definite role of antioxidants, particularly carotenoids. As far as eye is concerned, lutein and zeaxanthin have a vital role to play, and these two carotenoids are a must for the eye to be well protected form developing macular degeneration as well as cataract. In addition, lycopene, another carotenoids has a special place in eye defense since it is the best quencher of singlet oxygen - a reactive oxygen species which causes havoc particularly in the eye.


INTRODUCTION
Free radical chemistry began in 1900s when they were determined as cause for fat spoilage. Importance of free radicals in human diseases pathophysiology was first recognized in 1969 when McCord & Fridovich isolated the first antioxidant enzyme superoxide dismutase.

The controversy as regards the use of antioxidants, particularly carotenoids, in ophthalmic diseases seems to be resolving due to advances made in measuring their levels in foods and tissues. There is consistent experimental and epidemiological evidence to substantiate the role of particularly lutein and zeaxanthin in prevention and, to a certain extent, cure of early age-related macular degeneration (ARMD) and cataract formation. Also clinical observations depending upon the recommended dietary modification and therapeutic supplementation presently are encouraging.



FREE RADICALS
DEFINITION
A free radical is defined as any species capable of independent existence and contains one or more unpaired electrons.



HOW FREE RADICALS ARE FORMED?
The various tissues in the human body are formed by innumerable molecules. Each molecule consists of two or more atoms joined together by chemical bonds. An atom, the smallest particle of an element, consists of a core which contains positively charged protons (or positrons) as well as neutral neutrons. In the orbit of each atom (referred to as orbital) are present the electrons (or negatrons). Each orbital can accommodate a maximum of two electrons both of which spin in opposite directions.

Most molecules are non-radical since they contain a paired set of electrons. But oxygen is always electronegative. As a consequence, it pulls electrons away from other atoms (including oxygen itself) and renders these as free radicals.
Oxygen-derived free radicals have a lifespan of only a few microseconds. Their concentration at any single site is miniscule. However the danger lies in their ability to combine with another nonradical to render the latter as free radical. Normally, bonds don’t split in a way that leaves a molecule with an odd, unpaired electron. But when weak bonds split, free radicals are formed. Free radicals are very unstable and react quickly with other compounds, trying to capture the needed electron to gain stability.

Fig: Serial formation of free radicals.



Generally, free radicals attack the nearest stable molecule, thereby "stealing" its electron. When the "attacked" molecule loses its electron, it becomes a free radical itself. This leads to a process of chain reaction. Once such a process has started, it can cascade, finally resulting in the disruption of a living cell.

Radicals can react with other molecules in a number of ways. If two radicals meet, they can combine their unpaired electrons symbolized by.) and join to form a covalent bond (a shared pair of electrons). The hydrogen atom, with one unpaired electron, is a radical and two atoms of hydrogen easily combine to form the diatomic hydrogen molecule:
H. + H.

Radicals react with nonradicals in several ways. A radical may donate its unpaired electron to a non-radical (a reducing radical) or it might take an electron from another molecule in order to form a pair (an oxidizing radical). A radical may also join onto a nonradical. Whichever of these three types of reaction occurs, the nonradical species becomes a radical. A feature of the reactions of free radicals with nonradicals is that they tend to proceed as chain reactions, where one radical begets another.



SOURCES OF FREE RADICALS
Some free radicals arise normally during metabolism. Sometimes the body’s cells or its immune system purposefully create them to neutralize viruses and bacteria. However, environmental factors such as pollution, radiation, cigarette smoke and herbicides can also generate free radicals. Free radicals causing structural damage (to proteins) resulting in aging changes such as cataract and ARMD.

An adult utilizes 3.5 ml oxygen per kg body weight per minute. Assuming a body weight of 70 kgs, this works out to 352.8 liters per day. Even if 1% of oxygen is converted to free radicals, this amounts to 1.72 kg of free oxygen radicals per year!



EXAMPLES SOURCES OF FREE RADICALS
Some free radicals well studied free radicals are:
• SUPEROXIDE ANION (O2.)
• HYDROXYL RADICAL (OH.)
It is important to note that free radicals such as hydroxyl radical differ from hydroxyl ions in their content of electrons.



REACTIVE OXYGEN SPECIES
These are partially reduced oxygen species which do not contain any unpaired electron. Examples of reactive oxygen species are:

• HYDROGEN PEROXIDE (H2O2)
• HYDROPEROXY RADICAL (HOO-)
• HYPOCHLOROUS ACID RADICAL (HOCl)

Under certain conditions reactive oxygen species have potential to enter free radical reactions to form the more toxic free radicals. Another reactive oxygen species, which is not a free radical, is singlet oxygen (O). In this, a rearrangement of electrons has occurred which allows it to react faster with biological molecules - as compared to ‘normal’ oxygen.



ANTIOXIDANTS
DEFINITION
Antioxidants can be defined as substances whose presence in relatively low concentrations significantly inhibits the rate of oxidation of the targets.



HOW ANTIOXIDANTS WORK?
Antioxidants serve as natural protectors in the body, mopping up free radicals and reactive oxygen species, which are potentially damaging. Antioxidants protect the tissues in 4 ways:

• Physically separating the free radicals / reactive oxygen species from the susceptible molecules of the human body.
• Providing molecules which effectively compete for oxygen.
• Rapidly repair the damage caused by free radicals / reactive oxygen species.
• Lyse the free radicals / reactive oxygen species and rapidly remove these.



CLASSIFICATION OF ANTIOXIDANTS
 ANTIOXIDANT ENZYMES
• Superoxide dismutase
• Catalase
• Glutathione peroxidase
 PREVENTIVE ANTIOXIDANTS
• Ceruloplasmin
• Transferrin
• Albumin
 CHAIN-BREAKING ANTIOXIDANTS
 Water-soluble*
• Uric acid (200-400 mmol/L)
• Ascorbate (25-100 mmol/L)
• Thiols (400-500 mmol/L)
• Bilirubin (10-20 mmol/L)
• Flavanoids
 Fat-soluble*
• Tocopherols (20-30 mmol/L)
• Ubiquinol-10 (<2 mmol/L)
• Beta-carotene (1-2 mmol/L)
• Estrogens
* optimal blood level given in brackets


The most important antioxidants are three vitamins and three minerals.

 ANTIOXIDANT VITAMINS
• CAROTENOIDS
• VITAMIN E
• VITAMIN C
 ANTIOXIDANT MINERALS
• SELENIUM
• ZINC
• MANGANESE
• COPPER




CAROTENOIDS
Carotenoids circulate in lipoproteins; 53% of beta carotene occurs in low density lipoproteins. Besides the well known beta carotene, the other carotenoids of human importance are:

• Lutein
• Zeaxanthin
• Lycopene
• Alpha carotene
• Beta cryptoxanthin

As far as the eye is concerned, Lutein and zeaxanthin are exclusively concentrated in the macula, lens and iris. The retina and choroids additionally contain lycopene, alpha- and beta carotene. In the ciliary body, all the carotenoids taken in foodstuff or as dietary supplement get accumulated.



VITAMIN E
Being a fat soluble vitamin, alpha tocopherol is abundant in all cell membranes as well as in lipoproteins. In the eye, vitamin E is present in retina and choroids, and balance in iris and ciliary body. It is important in protection of rods and cones in retina, and also for preventing free radical damage to lens. Vitamin E acts synergistically with vitamin C, beta carotene and selenium for better functioning of glutathione.



VITAMIN C
Vitamin C is protective for the cytoplasm & is also most important for plasma defense. It also occurs in certain cells like muscle, adrenals and eye. Vitamin C has the capacity to regenerate vitamin E. It is more significant in combating free radicals formed due to pollution and cigarette smoke. Vitamin C especially concentrates in ocular tissues and is the first antioxidant to tackle free radicals.



ZINC, MANGANESE & COPPER
Zinc (Zn), manganese (Mn) and copper (Cu) are constituents of superoxide dismutase (SOD) antioxidant enzyme. SOD is widely distributed in tissues as well as fluid compartments. CuZnSOD is present in cytoplasm and nucleus, MnSOD in operates mitochondria whilst CuSOD is most distributed in plasma. SOD attacks free radicals like hydroxyl radical to convert these into hydrogen peroxide.

Besides, Zn serves an important structural role, whilst Cu is necessary for functioning of another antioxidant enzyme called as catalases. Hydrogen peroxide is converted by catalases into harmless water and molecular oxygen.

In the retina, SOD plays an important role by scavenging free radicals to prevent the oxidative damage which plays a role in the development of drusen, an early sign of ARMD. Catalases, on the other hand, are vital for lens protection.


SELENIUM
Selenium (Se) is the most important dictator of glutathione peroxidase activity. Glutathione peroxidase is concentrated in various tissues, besides blood and synovial fluid. In tisues, it operates in the cytoplasm and mitochondria principally. Like catalases, glutathione peroxidase breaks down hydrogen peroxide, besides reducing lipid peroxidation like vitamin E and beta carotene.

Glutathione peroxidase and related enzymes in the retina, plus the precursor amino acids (N-acetylcysteine, L-glycine, and glutamine and selenium) are protective against damage to human retinal pigment epithelium cells. Glutathione peroxidase prevents free radical-induced apoptosis (cell suicide) and helps prevent or treat ARMD.



CAROTENOIDS
DEFINITION
More than 500 distinct compounds are today identified as naturally occurring carotenoids. They include cyclic hydrocarbon-carotenoids (carotenes), acyclic hydrocarbon carotenoids (lycopene), and oxygenated hydrocarbon carotenoids (xanthophylls like lutein and zeaxanthin).

Handelman and associates noted carotenoids concentration in the macula to be 5-fold higher compared to peripheral retina and 500 times more than the concentration in other tissues. Lutein is the major carotenoid in the peripheral retina, whereas zeaxanthin becomes more and more dominant as the foveal centre is approached. The proportion of lutein to zeaxanthin in macula is 1:2 and the proportion is reversed in the peripheral retina. The distribution of xanthophyll carotenoids suggests a possible role of lutein in protecting the rods and for zeaxanthin in protecting the cones that are concentrated in the central retina. The human lens carotenoids content is 10-20 ng/gm of wet tissue, and the ratio is 1.6:2.2 for Lutein and zeaxanthin.

Another most important dietary antioxidant of ocular significance is lycopene, which is however, conspicuous by its absence in macula. Due to its presence in high concentration in circulating blood in the eye, lycopene plays a prominent role in prevention of macular degeneration mainly by its very potent singlet oxygen quenching capacity.





FOCUS ON CAROTENOIDS IN ARMD
ARMD - INTRODUCTION
In developed countries, ARMD is the leading cause of blindness amongst the elderly (more than 60 years) with a prevalence ranging between 2 to 7% for severe (wet) form and a range of 12 to 30% for the dry form. The disease has caused irreversible visual impairment in an estimated 1.7 million Americans over the age of 65 years. The number of cases of ARMD has been predicted to increase from 2.7 million in 1970 to 7.5 million by the year 2030.

In India, the incidence of ARMD affects approximately 4-5 per cent of the population over the age of 50 years and may be affecting 19-20 per cent of people above 70 years of age. Early disease is characterized by yellowish-colored subretinal drusen. Late disease, which may be ‘dry’ or ‘wet’, may lead to significant loss of central vision. Wet form occurs only in 10 percent of population.



ARMD - PATHOPHYSIOLOGY
The light must pass the macular pigment, which contains abundance of zeaxanthin and lutein before striking the photoreceptors. If any damage to the rods and cones is to be prevented the short wave length of light rays (<500 nm range) must be filtered. This is accomplished as follows:

• 5-286 nm wavelength (ultraviolet C rays): filtered by the earth’s ozone layer.
• 286-320 nm wavelength (ultraviolet B rays): filtered by cornea.
• 320-400 nm wavelength (ultraviolet C rays): filtered by lens.
• 400-500 nm wavelength (visible blue light): filtered by lutein / zeaxanthin in macula.

The light entering the retina is between the wavelengths of 400 to 700 nm. The eye would be in perfect focus for daylight only at 560 nm, and even at night 500 nm wavelength of light is optimal for functioning of rods. Hence, filtering out 400-500 nm wavelength of light prevents damage to macula without affecting vision.
Thus macular pigments represent a significant filtering element and hence protect against the light–initiated cumulative oxidative damage. The macular pigment also removes much of the blurry, short wave blue and blue-green light that results from the eye’s chromatic aberration. Apart from this the earth’s atmosphere through which we view objects almost always contain small-suspended particles, which scatters short wave length light more than other wavelengths and results in a bluish veiling luminance.
The eye and skin are the only structures which have dual exposure to oxygen and light. In presence of blue light (400-500 nm wavelength) the oxygen will be split into singlet oxygen which is one of the most deadly reactive oxygen species as far as the eye is concerned. The blue light has potential to split molecular oxygen due to the high energy contained in it.

The singlet oxygen and other free radicals formed inside the eye initiate lipid peroxidation of photoreceptors. The polyunsaturated fatty acids in the outer membrane of rods and cones are attacked by free radicals and singlet oxygen species to result in damage of these photoreceptors. As a consequence, there is accumulation of lipofuscin by retinal pigment epithelium which then contributes in druse formation.



ARMD - MEDICAL MANAGEMENT
The damage to macula and formation of druse can be prevented by filtering out the damaging blue light of the visible spectrum. This is possible by the macula, if its content of lutein and zeaxanthin are adequate. The additional available of lycopene in adequate amounts is of paramount importance in tackling the singlet oxygen single this carotenoids is the best antioxidant known for quenching this reactive oxygen species. In addition, glutathione peroxidase and SOD too have been shown to have preventive benefit in ARMD.



FOCUS OF CAROTENOIDS IN CATARACT
CATARACT - INTRODUCTION
Cataract is a multifactorial disease. Oxidative stress together with weakened antioxidant defense mechanism is attributed to the changes observed in human diabetic cataract. Oxidative damage to the lens has been recognized as a primary event in the pathogenesis of many forms of cataract. Consistent with this view, epidemiological reports have identified factors related to oxidative process that both increase (eg smoking and light exposure) and decrease (eg antioxidant intake) cataract risk.

Epidemiological studies provide evidence that nutritional antioxidants slow down the progression of cataract.



CATARACT - PATHOPHYSIOLOGY
Oxidative stress is high in the eye due to ultraviolet rays which promote liberation of free radicals and singlet oxygen. The epidemiological evidence to support the possibility that lutein and zeaxanthin have an important role in reducing the risk of cataract is somewhat consistent, and justifies the belief in free radical & reactive oxygen species mediated damage to the lens.

Few of the recent studies have stressed the significance of vitamin C, E and selenium in the etiology of cataract. Role of vitamin E has been more specifically stressed by several workers. Low blood levels of vitamin E are associated with approximately twice the risk of both cortical and nuclear cataracts, compared to median or high levels. Smokers are 2.6 times likely to develop posterior subcapsular cataracts more than nonsmokers. Patients with senile cataracts were found to have significantly lower blood and intraocular levels of the mineral selenium than control.



CATARACT - MEDICAL MANAGEMENT
Lower prevalence of nuclear cataract in women or men was associated with intake of lutein and zeaxanthin in high doses. Furthermore, in prospective cohort studies it was noted that people who consumed diet rich in lutein and zeaxanthin, had 20-25 percent lower risk of cataract extraction and 70 percent lower risk of cataract extraction under the age of 65 years.

Experimental study in human lens epithelial cells (HLEC) in culture was evaluated and it was concluded that addition of lycopene had a protective effect to prevent vacuolization of epithelial cells. It was observed that there was as positive effect of retardation of lens opacities due to lutein and zeaxanthin in the aging lenses.

In an 8 year prospective cohort study, Hankinson et al reported that an elevated intake of spinach, which is high in lutein and zeaxanthin (but low in beta carotene content) was most consistently associated with a lower risk of cataract extraction, whereas high beta carotene and vitamin E intakes alone had no beneficial effects against cataract prevention.

This study corroborated data from Jaques et al 1988 who demonstrated that persons with slightly elevated levels of plasma total carotenoids had a 25% lower risk for any type of cataract.



ANTIOXIDANTS IN RETINITS PIGMENTOSA
There is possibility that lutein may slow degeneration of vision in retinitis pigmentosa, a heterogeneous group of slow retinal degenerations. However, only preliminary data in a very small number of patients has been published in which lutein slowed vision loss associated with retinitis pigmentosa in one.






ANTIOXIDANTS IN DIABETIC RETINOPATHY
Several studies are in progress as regards role of antioxidants in diabetic retinopathy and glaucoma but as yet none is conclusive.



CONCLUSION
The overwhelming body of evidence points to significant beneficial effects of nutritional supplementation for most degenerative eye conditions. Important to remember is that most of the above studies used blood levels and food intakes associated with a normal diet. Taking supplements, specifically containing zeaxanthin, lutein and lycopene in adequate doses, which are theorized to provide protection to macula and lens with adequate doses, may have a much more protective effect than dietary levels alone. With so little risk, and the other potential health benefits from taking nutritional supplements, it would certainly seem prudent to try them, especially for macular degeneration where there are no real options.

Once the damage is done it cannot be reversed (except to a small degree), so prevention and early intervention is essential, especially if we have a family history of the disease. Of course, it's important to slow further progression at any stage of development. Prevention of lens and macula from the ultraviolet rays and hazard of smoking, however, needs to be over stressed.

DRY EYE SYNDROME: EMERGING CONCEPTS

DRY EYE SYNDROME: EMERGING CHALLENGE IN OPHTHALMOLOGY

Prof.DR. M. R. JAIN M.S, F I C S( USA), FACLP ( LONDON), FAMS

MEDICAL DIRECTOR
M. R. J INSTITUTE AND JAIN EYE HOSPITAL, JAIPUR ( India)
Email: drmrjain55@gmail.com

Dry Eye Syndrome, which has been recently termed as Dry Eye Disease (DED) (Beherens et al 2006; Lemp 2008), is the most frequent disorder in Ophthalmology. Fortunately, only infrequently it becomes most severe. Although the condition was recognized as a clinical disorder in the year 1920 and described clinically in the early 1930’s, the greatest amount of information both from an epidemiological and pathogenetic perspective has accrued during the last ten years.

What is Dry Eye Syndrome?

Dry Eye Syndrome is a disorder of the preocular tear film that results in damage to the ocular surface and is associated with symptoms of ocular discomfort. Dry eye is characterized by instability of the tear film that can be due to insufficient amount of tear production or due to poor quality of tear film, which results in increased evaporation of the tears.
Dry eye therefore can mainly be divided in two groups, namely
Aqueous production deficient
Evaporative

Prevalence of dry eye.

No authentic prevalence survey has been conducted in India but it is noted that out of the patients above the age of 30 years attending the outdoor, one out of every four has a complaint pertaining to dry eye. A recent survey conducted in year 2007 (Lemp et al 2007), based upon a well – characterized population of adult men and women in the USA, identified a prevalence of 5 to 30 percent at various age groups. These rates extrapolate to potentially 9.1 million dry eye patients in USA alone. About 5 million Americans above 50 years of age have mild. to moderate dry eye disease.
In women at the age of 45 to 52 when menopause usually sets in, an imbalance occurs between the estrogen and androgen hormone due to decrease of androgens after the menopause. Decrease in androgen levels, excites inflammation in lachrymal gland and ocular surface, disrupting the normal homeostatic maintenance of the lacrimal gland and ocular surface.
The factors which has increased the incidence of dry eye can be narrated as under
a. increasing longitivity of the population
b.increased consumption of medication, both systemically and topically which have adverse effect on the production of high quality of tears
c. increased computer use ( Computer Vision Syndrome)
d. increased contact lens use and cosmetic surgery of LASIK/ LASEK
e better understanding and diagnosis of dry eye.
f possibly, adulteration in the food/ and pollution.
g. Increased use of systemic and topical drugs.

TEAR FLUID COMPOSITION

The tear is found to be composed of three fractions: albumin, globulin and lysozyme. The immunoglobulins found in normal tear fluid are lgA, lgG and IgE. lgA predominates in the secretory form. IgE levels increase in patients with allergic conjunctivitis and lgM is found in tears of patients with acute infections. Lysozyme may act synergistically with lgA in causing lysis of bacteria. Tears also contain lactoferrin, which has some antibacterial effect.

TEARS: VITAL STATISTICS:

Average glucose concentration of the tears is 2.5 mg/dl.
Average tear urea level is 0.04 mg/dl.
Electrolytes such as K, Na and Cl occur in higher concentration in the tears than in the blood.
Average pH of the tears is 7.25.
Osmolality is 309 mosm/ liter (hypertonic in patients with the dry eye syndrome).
Surface tension of the tear film is 40-42 mN/m.
Refractive index of the tear film is 1.336.

Under normal conditions, the tear fluid forms a thin layer over the cornea and conjunctiva, this is known as the pre ocular tear film. The pre ocular tear film measures 8 um thick and covers the corneal and conjunctival epithelial surface.

The pre-ocular tear film acts as an important component of the ocular defense mechanism.
It makes the cornea a smooth optical surface.
It helps to wet the cornea and the conjunctiva and prevents them from drying.
It flushes out the debris and organisms from the corneal surface.
It has bactericidal properties due to the presence of lysozyme, lactoferrin and betalysin.


Immunoglobulins (lgA) and specific antibodies in the tears defend the eye against external infections.
Frictional trauma between the tarsal and the bulbar conjunctiva and cornea is minimized by the lubricating action of the tear film.
It enables the anti-inflammatory cells to reach the injured areas of the cornea and the conjunctiva.
It provides the epithelial cells with glucose, oxygen and growth factors.


Distribution System:

The distribution system for the tear film consists of the eyelids and the tear meniscus along the lid in the open eye. Each blink compresses the superficial lipid layer. The mucous layer acts as a scavenger to pick up any lipid containing debris and carry it to the fornices. As the eyelid reopens, a new tear- film layer is spread across the ocular surface. Inadequacy of any layer of the tear film increases its instability and may accelerate tear breakup time (BUT).

The distribution system of the lids also acts as a pumping mechanism to draw tears into the excretory system.

EXCRETORY SYSTEM:

Blinking is an important factor in tear distribution and also plays a pivotal role in tear drainage. Crucial to proper lacrimal excretory function is the punctum, the entry point for lacrimal drainage. Proper tear elimination requires that the punctum be apposed to the globe.

Spontaneous blinking replenishes the fluid film by pushing a thin layer of fluid ahead of the lid margins as they come together. The excess fluid is directed into the lacrimal lake- a small triangular area lying in the angle bound by the innermost canalculi via the nasolacrimal duct, and then drained into the nasopharynx and oropharynx to be swallowed.

The drainage pathway may account for up to 90% of the fate of tears. The remainder evaporates. Thus, the act of blinking exerts a suction-free force action in removing tears to the lacrimal lake and emptying them into the nasal cavity.

Functions of Pre-ocular Tear Film

Traditionally a tear film comprises of three layers
Outer Lipid layer
It is formed by the oily secretion of Meibomian glands. It acts as a lubricant and prevents evaporation of tears.
Middle Aqueous Layer
It is the main tear fluid liberated from lacrimal gland and accessory glands. It contains proteins, immunoglobulins, lysozyme, lactoferrin and betalysin. It provides moisture to the eye, nutrition to the cornea and antibacterial activity. It provides the epithelial cells with glucose, oxygen and growth factors. It flushes out the debris and organisms from the corneal surface and drains into nasolacrimal canal.
Inner Mucous Layer
The innermost mucous layer of the tear film forms a highly hydrophilic wetting surface over the hydrophobic epithelial surface of the cornea and conjunctiva. The mucous also reduces the surface tension between the lipid layer of the tear film and the water layer, thus contributing to the stability of the tear film.

Recent Concept of Tear Film

The contemporary concept of the tear ocular-surface structure is that of a metastable tear film consisting of an aqueous gel with a gradient of mucin content decreasing from the ocular surface to the undersurface of the outermost lipid layer. The latter structure interacts with the underlying aqueous and mucin components, retarding evaporative loss of aqueous tears and contributing to the stability of the tear film between the blinks (Lemp, 1995).



Mucin Layer

At least three distinct types of mucin have been identified: transmembrane mucins produced by the corneal conjunctival cells, gel forming from the conjunctival goblet cells, and soluble mucins primarily from the lacrimal glands.(Gipson et al, 2004).The transmembrane mucins contribute to the surface structure of the epithelial cells, interact with the gel-forming and soluble mucins of the tear film to stabilize the film, and provide a cleansing pathway for the ocular surface; lipid-mucin interactions support relatively stable tear film between the blinks.
Tear film provides not only lubrication and nutrition to ocular surface but stable vision (Lemp, 2008). All tissues of the ocular surface, secretary glands, eye lids and outflow channels of the nasolacrimal pathway are linked via a neural network (the lacrimal functional unit). Sensory receptors monitor condition of tears and cells, sending afferent signals to the central nervous system. That in turn, sends efferent impulses to the secretary glands and cells, effecting changes in the composition and volume to maintain homeostasis and to respond to stress and injury. Additional factors supporting the tear film-ocular surface complex include bioavailability of hormones, primarily androgens, and an intact immune system. This exquisitely balanced system represents a highly complex unit providing visual access to the external environment. (Lemp et al, 2007). Derangement of anyone element leads to a breakdown in overall structure and function with significant clinical effects.



Pathogenesis of Dry Eye

It is an established fact that any lacrimal gland damage would result in decreased tear flow. This leads to decreased washout of the tear surface debris and bacterias as well as increased presence of inflammatory cytokines and decreased growth factors to maintain ocular surface integrity.
Almost all tear flow is due to a reflex mechanism due to stimuli from cornea sending impulses to the brain and to the lacrimal gland. Any thing which disturbs corneal sensations like hormonal imbalance, contact lenses, LASIK surgery or any other trauma to the eye, may it be surgical or accidental.

Infection of the lacrimal gland may it be primary (dacryoadenitis) or immunological due to rheumatism of joints or prolonged conjunctivitis may result in decreased formation of aqueous. As a result of inflammation, activation of matrix metalloproteinase enzymes (MMP-9) was identified which has further potential to damage the ocular surface. It is now generally recognized that inflammation is an integral part of the pathogenesis of dry eye disease and a target for dry eye therapy.
The normal interaction of the tear film and ocular surface is conditioned by a background of androgenic hormonal support that prevents inflammation and an intact corneal sensation that stimulates secretion by the lacrimal gland to produce tears that nourish and protect the ocular surface. When there is perturbation of the normal homeostatic controls, dry eye occurs either as an aqueous tear deficiency or excess evaporative loss with subsequent damage to the ocular surface. This disease state creates a vicious spiral of increasing inflammation of the lacrimal gland and ocular surface that further suppresses normal corneal sensation and leads not only to suppression of tear secretion but to further damage to the ocular surface.
The aqueous deficient dry eye (keratoconjunctivitis sicca) is a disturbance of the neuro-humoral interaction of the ocular surface which interrupts secretomotor nerve impulses to the lacrimal gland that results in inflammatory suppression of aqueous secretion, a necessary component of the tear film, with subsequent damage to the ocular surface, producing symptoms of ocular irritation and discomfort. The evaporative dry eye is a disturbance of the stability of the tearfilm, which is usually due to abnormalities of Meibomian gland secretion or abnormal eyelid position and movement. Both types of dry eye results in damage to the ocular surface and symptoms of ocular discomfort and impaired visual function.


Classification Based On Etiology

Murube (1996) has subdivided dry eye in following 10 families. These are:

Age Related. Lacrimal secretion begins to decrease after the age of 30 years. At the age of 6o, we reach the borderline between the production and need. At the age of 90, almost all persons have dry eye.
Hormonal. At the age of menopause almost every women develops dry eye either mild or moderate. Recent research has shown that it is due to lowering of androgen levels produced by the ovaries. Men develop dry eye related to hormones with less frequency and intensity than women.
Pharmacological. There is adverse effect on production of tears due to preservatives in tear drops used for long period. Glaucoma patients are more prone to this problem due to prolonged therapy.
Systemic drugs like antidepressants, antihypertensive, antihistaminic, anticholinergics, antipsychotics, angiolytics, antiparkinsonians, diuretics and hormones too can cause dry eye.

4 Immunological: I This is related to autoimmune reaction in exocrine glands affecting outside body secretion like secretion of tears, saliva, sweat and vaginal secretions. The Sjogren’s syndromes are those in which patient’s immunological system attacks its own exocrine glands. Rheumatism, cicatricial pemphigoid and erythema multiform can lead to Sjogren’s syndrome.
5 Infection. Chronic infection of conjunctiva can affect mucous secretion leading to mucin deficiency and infection of lacrimal glands can affect aqueous secretion. Inflammation of lids may affect oily secretion. Any of the component if affected, tearfilm is disturbed.
6 Hypo nutrition. Avitaminosis A, and alcoholism that leads to poor intestinal absorption may give rise to dry eye.
7 Traumatic: Any trauma to the eye may it be accidental or surgical, can precipitate dry eye. Major surgeries like removal of tumour etc has more chances to cause dry eye. Even a cataract Phaco or glaucoma surgery can be responsible to give dry eye symptoms, especially in older persons.


8 Neurological.
a. Post LASIK. Lasik leads to the development of temporary dry eye in about 4 percent of patients. Wilson (2001) observed rose- Bengal staining and punctate erosions without pre-existing dry eye and labeled it as neurotrophic epitheliopathy. He believes that this change in epithelium is attributed to transection of a significant number of the afferent sensory nerves in the cornea during formation of the flap and, therefore, interruption of the cornea-trigeminal nerve-brainstem-facial nerve-lacrimal gland reflex arc that influence both basal and stimulated tear production. The Lasik induced dry eye tends to resolve approximately within 6 months. Laser in situ keratomileusis cause dry eye symptoms in 50 percent of eyes.

Contact lens wear. Contact lenses when worn for prolonged period affect corneal sensations and hence decrease tear secretion.
Hard and semi soft lenses cause marked corneal anesthesia. Moreover, soft lenses absorb tears and cause hypertonic tears, which further affects, the corneal epithelium. Semi soft lenses also affects lipid layer of the tear film.
Defective glands. Responsible for aqueous, mucin and lipid secretions.
Inability to utilize tears. There is normal production of tears but cornea is unable to use them due to:
Epitheliopathy or corneal dystrophy, which decreases corneal, wet ability.
Due to lipid defect the lids are unable to circulate the tears over the entire ocular surface (lid paralysis, extortion, lagophthalmos)

B. Classification Based on the Pathophysiology of Tear Film

Aqueous tear deficiency ( ATD)
Senile or idiopathic atrophy of lacrimal gland
Menopause
Hypo function of lacrimal gland associated with autoimmune diseases like Sjogren’s Syndrome
2 Lacrimal Surfactant (Mucin) deficiencies
a. Trauma to conjunctiva
b. Vitamin A deficiency
c. Conjunctival infections: trachoma, diphtheria
d. Pemphigoid, erythema, Stevens Johnson’s Syndrome
e. Chemical, thermal, radiation injury
f. Drug induced: sulpha, epinephrine
3. Lipid Layer Abnormality:
a. Chronic Blepharitis
b. Acne rosaecea
4 Impaired Lid Function or Blinking
Neuropralytic lesions of Trigeminal, Facial, Greater Superficial Petrosal Nerve etc.
5 Epitheliopathy
Disease of corneal epithelium
6. Other Causes
Drugs
VDTS : Visual Display Terminal Syndrome, Computer Vision Syndrome
Contact Lenses





Symptoms

Dry eye patient can present any one of them or multiple symptoms:
Itching, burning, irritation, pain, discomfort foreign body sensation. There may be pain and photophobia and blurred vision that improve with blinking. There is usually stringy ropy mucous discharge, which can increase in the afternoon. The discomfort in the eye usually increases while reading, watching T.V, air-conditioning system (lower levels of humidity) or working on the computer. At times there may be excess of watering, especially during breeze. The main cause of discomfort in the eyes is elevated electrolyte concentration in tears leading to hyperosmolarity and subsequent damage to the ocular surface.

All these symptoms are exaggerated during dry and windy conditions. Patient has frequent desire to remove mucous discharge from the eyes. Some of the patients give a typical history of desire to frequently sprinkle water into the eyes. Visual acuity can be significantly affected especially when corneal staining occurs. In early stages, there can be slight blurring of vision which requires frequent blinking, resulting in ocular fatigue.

Signs

Tear Lake. Normally at the lower lid margin there is there is concave tear meniscus of 0.3 to 0.5 mm, which is called Tear Lake. In dry eye, it is usually less than 0.1mm.
Debris. There is increased debris in the decreased tear lake. Mucous threads (strings of mucoid discharge) may be seen.
Other Signs. Redundant conjunctiva, injection of the conjunctival vessels, and sometimes mild chemosis may be present. In the fornix of the conjunctiva, the threads form owing to a slow tear flow and partly because of the increased number of the desquamated epithelial cells. In advanced cases, the conjunctival and corneal dryness may be very evident and may be associated with chronic blepharitis and blepharospasm.

Staining.

Fluorescein stain. Fluorescein may stain any denuded area of corneal epithelium. Staining is graded as 0,1,2and 3. 0= no corneal stain, 1=1/3 of corneal epithelium stained, 2= ½ of corneal area and 3=severe staining of ½ of corneal epithelium. The reduced tear lake could easily be appreciated with Fluorescein.
Rose Bengal Stain. Rose Bengal (solution 1 % or strip) stains the damaged devitalized epithelial cells of the conjunctiva and cornea. It can detect even mild cases of Keratoconjunctivis Sicca (KCS) by staining the palpabral conjunctiva in the form of two triangles with their base towards limbus. Rose Bengal gives stinging sensations but anesthetic drug should not be used as it may give false results. Alcian Blue has similar properties as Rose Bengal but is not usually available.
3 Film Break Up Time. (TBUT)
It is a quantitative measurement of tear film stability. A mucous deficiency results in beading of the aqueous tear around the small amount of available mucous on the epithelial surface and reduction of TBUT. The test is performed by asking the patient not to blink for 10 seconds after instillation of Fluorescein. Appearance of a dark spot (dry area) before 10 seconds is abnormal. Mild to moderate dry eye patients shall usually have TBUT of 2-3 seconds.

Diagnosis.

Diagnosis is most often based on the complaint of the patient without any evident cause in the eye. Quite often, persistent fishing for ropy mucous discharge is very classical and so is the importance of the complaint of increased discomfort in dry and windy environment.
Diagnostic tests mostly employed are as under
Schirmer Test. The test is used to quantitatively measure the tear secretions by the lacrimal gland, and should be done before any other examination as the manipulation of the eyelid and eye can alter the results of the test.
Shirmer I Test. Is used to measure tear secretion rate without anesthesia.
Shirmer II Test is done similar to Shirmer one but after instillation of anesthetic drops.
After instillation of anesthetic drops, the amount of tear secretion is closure to the basal secretion rate as there should be no stimulus from the filter paper strip placed in the inferior conjunctival sac. A value of less than 5.0 mm is considered abnormal. The test is quite often not conclusive.
b Tear Function Index (TFI) test. It is a more specific and sensitive test to quantitatively measure the tears. It takes into account the influence of tear drainage in the measurement of tears with Shirmer Test. Its numerical value is obtained by dividing the Shirmer II test value in millimeters by tear clearance rate. The higher the numerical value of TFI, the better the ocular surface. Values below 96 suggest dry eyes.
c Fluophotometery. It is another way to measure tear secretions. It uses decay of sodium fluorescein to measure the tear flow and the tear volume. This test is costly and not very informative.
d Tear Osmolarity. It provides qualitative assessment of tear formation. The reference value is 312 mosm/L. This value increases with the severity of the dry eye.
e Impression cytology, conjunctival and lateral salivary gland biopsy may be used to diagnose the etiology of the disease process. In dry eye states there is marked decrease in goblet cell count.

Classification of Dry Eye Syndrome:

Mild Dry Eye Syndrome: can be defined in patients who have a Shirmer Test of less than 10 mm in 5 minutes and less than one quadrant of staining of cornea
Moderate Dry eye Syndrome:
Shirmer Test results of 5-10 mm in 5 minutes with or without punctate staining of more than one quadrant of the corneal epithelium.
Severe Dry Eye Syndrome: Can be defined as diffuse punctate or confluent staining of the corneal epithelium, often its filaments. Schirmer Test mostly less than 5 mm in 5 minutes. Sjogren syndrome is classically associated with severe dry eye symptoms.





Treatment

Conservative
1. Patient Information. Patient must be educated and fully informed about the disease as well as he must be explained the limitations of medical management. This maintains the patient’s confidence in your line of treatment.
2. Controlling the surroundings. Special stress must be put to control the surroundings to minimize the severity of the condition.
a. Still Air. Patient must avoid sitting facing direct flow of air from air conditioners, ventilators, windows or fans. It is better that patient avoid sitting in front of door in a room. While driving car, the car window must be closed and the patient should use glasses. Car A.C. wind should not blow directly on the face.
b. Humid Air. Even if there is no refractive error, patient must wear glasses. Just by wearing spectacles, the humidity between the eyes and the spectacles rises by 2 %. Spectacles with side panels and moist chamber may be reserved for more severe cases. Humidifiers must be used in the rooms. There are air-conditioners available with attached humidifiers.
Special glasses with moist inserts ameliorate severe dry eye symptoms. The moist inserts on the side panels increase the ambient humidity, resulting in a decrease in the tear evaporation from the ocular surface. Another type of moist chamber is obtained more easily and less expensively by using swimming goggles. The most favorable range of relative humidity for minimizing tear evaporation is reported to be 40% to 50 %. Wet gauze mask is an alternative treatment modality.
c. Pure Air. Polluted air is very harmful for dry eye patients. Palpabral aperture must remain open as little as possible. Closed window in the car, helmet with a shield while driving scooter and covering your eyes with goggles while driving bicycle gives some relief. While reading books, the book should be kept as close to chest as possible so as to have minimum palpabral aperture. While looking down, ocular surface exposed to the air is just 1 square centimeter, whereas while looking straight, 2.0 sq. cm. and while looking up, 3,0 sq. cm.
Computer Vision Syndrome. While looking at the monitor, the eyes have the tendency to stare at the screen thereby reducing the blink to about 6-7 blinks a minute. If the computer is at a higher level than the eye, there is further increased evaporation of tears. To avoid computer vision syndrome, one must keep the computer at the lower level than the eyes and a habit must be formed to blink about 10-12 times per minute. When working for long period, one must close the eyes for some time or use some artificial teardrops.

Medical Management

Tear Substitutes.

Tear substitutes are the mainstay in the medical management of dry eye. Variety of tear substitutes is available. Hypotonic non-viscous solutions counteract the hyper tonicity in dry eye syndrome and can last up to two hours. Viscous solution contains cellulose as their base and thus last longer. Preservatives are added to increase the shelf life and the stability of the solution. The commonly used preservatives include benzalkonium chloride, thimerosal, and chlorhexidine. In spite of their low concentration, they can produce toxic effect on the cornea and conjunctiva and adversely affect the dry eye condition.

Preservative Free Drops

The use of unpreserved collyria, and more recently preservatives that are transient or which rapidly oxidize to non-toxic compounds upon exposure to air and the ocular surface, has become routine for those patients requiring more than three or four lubricant drops per day. The tear supplements have focused on maintaining a hypotonic collyrium with normalization of electrolyte concentration to combat the damaging effects of hyper tonicity.

In India, such non- reactive tear substitutes are marketed as:

Refresh Tear Drops (Allergan) it contains carboxymethyl cellulose sodium 5 mg with stabilized Oxychloro Complex 0.05mg. (Purite)
Gen Teal drops and Gel (Novartis) it contains hydroxypropylmethyl Cellulose 0.3 % with stabilized H2O2.
Eyemist Drops (Avesta) it contains hydroxypropylmethyl
Cellulose 0.3 % with stabilized Oxychloro Complex 0.005 %.
Tear Drops (Milmet) Contains sodium
Carboxymethyl Cellulose 5.0 mg with stabilized Oxychloro
Complex 0.005 %)
Celluvisc 1 % (Allergan) it contains carboxymethyl cellulose
1 percent.
Refresh Liquigel (Allergan) it contains Carboxymethyl
Cellulose Sodium 1 %.

Hyvisc 0.1 and 0.18 percent Sodium Hyaluronate is considered more soothing to the conjunctival epithelium. It has Ph of 7.3. Increases TBUT and aids healing of superficial keratitis.
Ocumoist, Ecotear, Lubrex , Aquaray, Vel Drops, CMC, Add Tears, Tear Drops, Flogel, Moisol-Z are few of the other preservative free drops.
Systane ( Alcon ) contains Polyethylene glycol and propylene glycol.

Imported Tear Substitutes

Refresh PM (Allergan)
Gel Viscous Tear (Ciba)
Tears Naturale Free (Alcon)
Bion Tears (Alcon)
Lagricel Ofteno (Sophia Laboratories) it contains Sodium
Hyaluronate.
Hyalein Mini 0.1 % and Hyalein Mini 0.3 % (Santen’s
Japan) contain Sodium Hyaluronate.
Refresh Endura Drops (Allergan). It is lipid emulsion, which reduce tear evaporation and stabilize the tearfilm, thereby reducing frequency of tear instillation.

Tear substitutes are instilled in the eyes 3- 6 times a day,
depending on the severity of the condition. If necessary, Refresh
Liquigel or Celluvisc is instilled at bedtime.
Androgens

Role of androgen as a therapy is yet not well established though it is known that in females, lack of Androgens play important role in its etiology.
Topically, androgenic supplementation of artificial tears appears to lower the Osmolarity of patient’s tears either by retarding evaporation or possibly stimulating tear secretion. This gives an indication that adding androgenic hormones to artificial tears might benefit dry eye patients.

Tear Stimulants
The use of oral or sublingual pilocarpine (Salagen, MGI Pharma) has proven useful in some patients but has been associated with systemic side effects of sweating and gastrointestinal upset. Cevimeline (Evosac, Daiichi Pharmaceuticals, Inc) also stimulate tear and salivary secretion and may be better tolerated than pilocarpine.
Systemic use of congeners of bromhexine have been tried in Europe with unsatisfactory results.
Recent trials with purinergic P2Y2 agonist has reached phase three trials in USA. The medication designated diquafosol tetrasodium (Inspire Pharmaceuticals, USA) has been extremely well tolerated and increases tear film volume and mucin content. The pharmacological action is to increase fluid transport across the conjunctiva and stimulate mucin release from goblet cells.

Cyclosporin A

Looking to the immunological aspect of the disease, cyclosporin A in the form of topical drops (0.05 % & 0.1 %) is being used in moderate to severe form of DES to treat inflammation of the ocular surface and lacrimal gland. The drops are instilled twice a day and the beneficial results are observed within four to six months. The drug may have to be used for whole life. Cyclomune is an immunomodulator. It selectively suppresses lymphocytic functions involved in a disease without actually suppressing the entire immune system. It inhibits T helper cells that are known to cause inflammation of the ocular surface and lacrimal glands in patients with dry eye. The main indication for the use of Cyclomune is surface staining of the cornea. Instillation of drops is associated with stinging sensations, which gradually decrease.
Cyclosporine drops are marketed by Allergan as ResStasis in USA and by Avesta in India as Cyclomune( 0.05 & 0.1% drops )
Omega 3 Fatty Acids (Omecard); or Cap. C S N given orally said to decrease the dependence on Tear substitutes. Fish eaters are said to be relatively resistant to dry eye. Cod Liver Oil may be useful .

Meibomitis.
A recent study in USA has shown that about 38 % patients with dry eye have concurrent Meibomian gland involvement. (Mathers M. D. 2000). Hot wet compresses, betadain scrub, eyelid massage and oral tetracycline or doxycycline, may treat Meibomian inflammation. Tetracycline is effective as an antibacterial and it makes the oily secretion more liquid and hence it flows out freely from Meibomian glands. Tetracycline is given 2 hours before meals in divided doses. It is given as capsule 500 mg B.D.

Topical Steroids (Soft steroids)

Topical steroids are being tried in some of the resistant or advanced cases of dry eye or in patients who have severe itching. Loteprednol etabonate 0.2 % is a good choice for long-term use. It is soft steroid that is activated by enzymes as it passes through the cornea. It seems to have very little effect on IOP. It is marketed as Alrex (0.2 %) by Bausch & Lomb and as Lotepred Drops 0.5 percent by Sun Pharmaceutical in India.

Immunosuppressant Therapy

In advanced cases of DES, systemic cyclosporin A, prednisolone, methotrexate, infliximab may have to be given.

Lasik Induced Dry Eye

Clinically post Lasik patients may show punctate epithelial erosions and rose Bengal staining of the flap. (Neurotrophic epitheliopathy). All cases of Lasik has to be put on liberal use of preservative free tear substitute drops immediately after the surgery and continued for a period of 4- 6 months. It is noted that almost all cases recover within six months. Only few patients, who already had dry eye symptoms before surgery, may require punctual plugs.
Mucolytics.
Topical 5 percent Acetylcysteine drops are recommended for instillation four times a day. It is effective in eyes with excessive mucous.
Future Therapies.
Apart from tear substitutes, anti-inflammatory therapy, androgen hormone replacement, and tear stimulant diquafosol tetrasodium may form main therapeutic measures. Herbal supplements such as oil of primrose and flax seed oil are reported to be help in relieving symptoms of dry eye and Meibomitis. Essential fatty acids of omega –3 and specially omega-3 category as food supplements are showing some promising results.


Surgical Management

Canalicular Obstruction by Punctal Plugs
It is a simple procedure that decreases the tear drainage markedly and improves the qualitative and quantitative component of tears. A decrease in osmolarity of the tears is noted. Improvement can be seen by Shirmer and TBUT test.
Several methods of punctal occlusion have been described including dissolvable collagen stents, cyanoacrylate adhesive, removable silicon or Teflon plugs, or intracanalicular plugs. The most recently approved innovation is Smart Plug (Medennium Inc) that is a thermolabile polymer that when inserted into the canaliculus conforms to the diameter of the canaliculus to produce occlusion.
Canalicular block is obtained by inserting a silicon plug in the puncta. There are two types of plugs:
a. Punctal plug A. In this part of the plug remains visible over the puncta
b. Punctal plug resides completely within the canalicular canal. (Herrick plug)
Almost 75 percent of patients tolerate the plugs well. In some of the patients, we may have to remove the plugs. The insertable variety can be eliminated from the canaliculus by irrigating the canal with saline.

Canalicular Obstruction by cautry. Puncta can be temporarily blocked by thermal or diathermy cautry or by Argon Laser. An Argon Laser focused on the punctal surface causes overheating and destroys the punctum. (results not reliable)
Punctal Patch Technique This is most efficacious surgical technique for long lasting occlusion of the lacrimal drainage system. In this technique a raw area is created surrounding upper and lower puncta. A piece of bulbar conjunctiva is taken and transplanted to the punctal wound with its raw surface in contact with the lid and sutured to it with four 9. 0 stitches.

Summary

Dry Eye Disease appears to be on increase due to multiple factors. Inspite of great advance in understanding and diagnosing the disease, the disease remains a challenge to medical profession. Preservative free drops have significantly improved the quality of life of dry eye patients. Anti-inflammatory therapy, androgen hormones and tear stimulants, namely, diquafosol tetrasodium and probably some herbal drugs hold great hope for a DES patient. Cyclosporine has proved to be a boon to the management of moderate to severe dry eyes.




ILLUSTRATIONS





Fig. Showing all the components of Lacrimal Secretary System.

Fig Showing damaged tear film.


Dry Eye Showing Rose Bengal staining

Fig Tear Film showing three layers of tears.

Use Of Goggles in Dry Eye Condition

Further Reading

Foulks G.N. Der Eye Part I: Understanding the epidemiology and Pathogenesis. Highlights of Ophthalmology. Vol.31 (1) 2003, Pg 21-26
Boyd B F New Horizons in the relief and control of Dry Eye Vol 29 (5) 2001 Pg 55-65
Bairagi D Dry Eye Syndrome. Sight, Mediworld Publication 2004 Pg 6-10
Symposium on Changing Paradigms in the Diagnosis and Treatment of Dry Eye. World Eye View July 2004. Pg2-11.
Pflugfelder SC: Anti – inflammatory therapy of Dry Eye. The Ocular Surface 2003: 1: 31-36
Foulks GN: Dry Eye- Part II: Management and new treatment options. Highlights of Ophthalmology. Vo. 31 (2), 2003, pg.1-8
Murube J, Tsubota K: Dry Eye: What is new in understanding its nature and effective management? Highlights of Ophthalmology Bimonthly Journal Vol 24, No 5, 1996.
Wilson Se: Lasik induced neurotrophic epitheliopathy. Ophthalmology, June 2001.
Murube J Advances in Diagnosis and management of the dry eye. Highlights of Ophthalmology 1993: 21: 10: pg.81-88.Dilly P. N: Structure and function of Tear Film. Adv Exp. Med Biol 1994; 350:239-247.
Kanski J. J. Clinical Ophthalmology ed. 4 Butterworth, 1999.
Keshner. Ophthalmic Medication and Pharmacology, Slack. Inc. 1994
Zimmerman, Text Book of Ocular Pharmacology, Lippincott and William and Wilkins 1997
A Comprehensive Review of Dry Eye Syndrome: A monogram by FDEC Ltd
Ocular Surface Disease: Dry Eye. Chapter 2 & 3 by J. M. Castillo and M.
Rolando. Published by Novartis Ophthalmic 2004
Lemp MA. Report of the National Eye Institute/Industry Workshop on Clinical trials in dry eyes. CLAO J 1995; 2:221-232
Gipson IK, Hori I, Argueso P. Character of ocular surface mucins and their alterations in dry eye disease. Ocular Surf. 2004; 2:131-148
Lemp MA, Baudouin C, et al. The definition and classification of dry eye disease: report of the Definition and Classification Subcommittee of the International Dry Eye Workshop. (2007). Ocular Surf.2007;5:75-92
Beherens A, Doyle JJ, Stern L, et al. Dysfunctional tear syndrome: a Delphi approach to treatment recommendations. Cornea 2006;25:900-907