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Dry eye disease is a common complaint, but diagnosing it accurately can be difficult for most practitioners. Owing to the many clinical manifestations, variable severity and factors which can contribute to its onset, it is not surprising that many practitioners find the condition challenging. This article will outline the present thinking on the subject, describe techniques that can be employed to identify the signs of dry eyes and offer some direction for its management in contact lens practice.

In 1995 the National Eye Institute/Industry Dry Eye Workshop proposed a definition: 'Dry eye is a disorder of the tear film due to tear deficiency or excessive evaporation, which causes damage to the interpalpebral ocular surface and is associated with symptoms of ocular discomfort.'1 For over a decade this definition has served to differentiate between the two major categories of dry eye - that created by a lack of aqueous production and that induced by excessive evaporation of the tears. However, this definition did not allow for the osmolarity changes and interference to visual function commonly found associated with the condition.

In 2007, following extensive research and consultation by a number of eminent authorities on the subject working together in the International Dry Eye Workshop, the DEWS Report was published which defined dry eye 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.' This clearly expands on the previous definition to encompass the effects on vision, osmolarity and induced inflammation commonly associated with the condition. The distinctions aqueous-deficient dry eye and 'evaporative dry eye' were removed from the definition, but are retained in the aetiopathogenic classification.2

Contact lens wear

Clearly the subject of dry eyes is complex and associated with some form of ocular disease. This is not to suggest that the wearing of lenses is a culprit in the formation of the disease. Rather that in the right environment, lens wear can create symptoms, even in a healthy wearer, similar to those of the true dry eye sufferer. Given that around 39 per cent of the asymptomatic non-contact lens wearing population have meibomian gland dysfunction, and another 20 per cent demonstrate incomplete blinking, it is quite plausible that at least some of these marginally dry eyed individuals might consider contact lenses at some time, so adding to the potential for contact lens complications.3 It follows that contact lens practitioners should at least have a basic understanding of the symptoms, signs and tests required to determine whether a potential new wearer is a suitable candidate.

Structure and function

The precise structure of the tear film, if there is one, requires further research. However, the easiest model to understand is that proposed by Wolff in the late 1940s of a three-layered structure, with the tear film consisting of a lipid layer, a central aqueous tear layer and a mucous layer (Figure 1).4

The external lipid layer is around 0.1µm thick and secreted by the meibomian glands found in the tarsal plates of both the upper and lower lids. Its function is to reduce aqueous tear evaporation. The main aqueous layer is about 7µm and contains glucose, electrolytes, antibacterial proteins, glycoproteins and antibodies. The aqueous is produced by the lacrimal glands found temporally in the bony roof of the orbit. The inner layer is made of mucin and is approximately 0.05µm thick. The mucin is secreted by the goblet cells of the conjunctiva and the non-goblet cells of the cornea. The superficial epithelial cells have microvilli and microplicae that are covered by the glycocalyx, a non-goblet cell mucin believed to anchor the aqueous to the anterior surface of the cornea and aid tear film spreading.5,6

The problem

Concern for the effects of dry eyes has been an issue within the contact lens fraternity for many years studies indicate that around 50 per cent of wearers suffer dryness related discomfort either during or at the end of the day. It may not be surprising to discover that with the effects of ageing on the mucous membranes of the conjunctiva, this figure increases to 68 per cent for presbyopic contact lens wearers.7 Commonly, wearers can describe an itchiness or scratchy sensation with mild redness of the conjunctiva when lenses are worn. When the eyes are examined with the slit-lamp biomicroscope the practitioner sees inferior punctate staining on the lower cornea, one of the most frequently seen observations in contact lens practice (Figure 2).8 In fact, these concerns are one of the main causes of contact lens discontinuation with around 35 per cent of wearers.9

Lens effects on tears

Contact lenses split the tear film in two, with the pre-lens tear film always thinner and more unstable than the pre-ocular tear film. The lipid layer is thinner or sometimes nonexistent which causes an increase in evaporation of the aqueous and a reduction in tear break-up times. It stands to reason that environmental factors can quite easily exacerbate this already compromised situation.

Environmental factors

Environments which commonly challenge contact lens wearers are air conditioned offices, extended use of computers, commercial aircraft cabins, windy weather and smoky atmospheres taking certain pharmaceutical medications and conditions such as chronic allergies can pre-dispose the wearer to complications. Age is also a factor.

Many modern office blocks have sealed, air-conditioned and poorly humidified environments where it is not possible to open windows. Add to this the use of computers and the reduced blink rate due to concentrated work and the majority of wearers will suffer some dryness symptoms.

Commercial aircraft cabins are a particular problem especially for passengers on long-haul flights where the relative humidity can fall to an abnormally low 6 per cent. This low figure is predominantly due to air in the cabin being recycled and not humidified. The major source of moisture is derived from the breathing of passengers with a small additional amount coming from clothing and baggage carried on board. In contrast, the normal relative humidity of most homes in Europe is around 60 to 70 per cent.10

Climatic conditions vary considerably around the world and the driving force for evaporation from a hydrogel lens is dependent on the relative humidity and air temperature these factors directly influence the wearability of a lens. The differing effect of these global variations can be illustrated by comparing climatic conditions for cities near the equator, such as Bahrain, Mumbai and Calcutta where favourable wearing conditions exist all year round. In other cities conditions are unfavourable during almost the entire year, as in Las Vegas, Albuquerque and Winnipeg. For many cities, however, some months are more favourable than others in London the months between May and October are favourable.10

Dry eye evaluation

A review of the literature would suggest many tests are available to the practitioner to determine a dry eye. However, tests such as lactoferrin analysis, osmolarity measurements, impression cytology, tear ferning analysis and fluorophotometry, which may be of value in the laboratory, are not practical for general practice and are mainly used in the identification of a true pathological dry eye.

For identification of a potential marginal dry eye in the contact lens population, a more practical and generally accepted group of tests to consider might be those shown in Table 1.

Schirmer test

The Schirmer test measures tear secretion. A 35mm strip of filter paper with 5mm of the end folded over is placed under the inferior lid slightly on the temporal side. The patient is asked to look up and blink normally for five minutes. Dry eyed patients show averages of around 8mm of wetting over the five minutes. However, owing to the irritation created to the conjunctiva, the results commonly show much greater wetting of the strip and are more likely an indication of reflex tearing than normal tear secretion.

Phenol red thread test

In an effort to reduce reflex tearing, a fine cotton thread was introduced by Kurihashi et al in 1975 and refined by Hamano who impregnated it with phenol red. In a similar way to the Schirmer test this 70mm impregnated thread is inserted into the lower lid for 15 seconds while the eye is closed. Values of less than 9mm of wetting are considered to be indicative of a dry eye.11 However, there is some debate as to what exactly this test measures and whether it is actually secretion rate.

Fluorescein break-up time

A commonly used method to determining the quality of the tears is break-up time (TBUT) while viewing with a slit lamp. To enhance the image many practitioners view through a yellow Wratten filter after instillation of sodium fluorescein and observe the time it takes before dark spots appear, indicating that the tear film is breaking up (Figure 3). A normal TBUT by this method is considered to be greater than 10 seconds. This is of course an invasive technique and, it could be argued, destabilises the normal state of the tears. But this test has been shown to be reasonably reproducible. To improve accuracy, a timer of some kind should be used not just simply counting the time it takes for the break-up to occur.

Non-invasive break-up time

To maintain a more natural environment while measuring TBUT, many non-invasive techniques have been developed. A simple method is to use the keratometer as a readily available tool in the consulting room. More sophisticated instruments are available such as the Keeler Tearscope and a number of other devices containing grids. As these instruments do not interfere with natural tear production the measurements obtained are a truer reflection of the normal state and generally the TBUT is longer than with invasive methods, anything over 20 seconds being acceptable.

The Keeler Tearscope is an unusual instrument in that it strictly measures the thickness of the lipid layer, although it can also be used to view the tear meniscus or prism along the lid margin. It can either be used as a hand-held device or attached to a slit lamp. Irrespective of the instrument used, the process of measuring tear stability is the same, viewing and determining the time it takes for the continuous tear film to break up.

Tear meniscus height

Tear prism height measurement, as mentioned above, is an assessment of the quantity of tears produced and not quality. This simple test is achieved by viewing with a slit lamp the prism of tears formed between the lower lid margin and the eye. Two aspects can be observed, viewing the integrity of the prism along the lid margin to see if there are any breaks or dips in continuity and measuring the height of the prism using a graticule or the slit lamp beam set to a 1mm spot and estimating the prism height. A normal measurement would be greater than 0.2mm centrally with no breaks in the prism between the palpebral canthi.

Rose bengal or lissamine green

Rose bengal is a dye that has been used traditionally to stain the conjunctiva in the diagnosis of dry eye. It stains dead and damaged corneal and conjunctival cells where the mucin layer has been disrupted by surface disease. However, it stings on insertion, especially in the dry eyed patient, and takes many hours to dissipate, staining the eye and surrounding lid margins red.

A better alternative that is gaining acceptance is lissamine green which acts in a similar manner to rose bengal without the unpleasant side effects. This blue green dye is less intense and washes away quickly so needs to be observed within a few minutes of instillation for accurate results. It is also available as impregnated strips which makes it easy to apply (Figure 4).

General fluorescein staining

The routine use of fluorescein can sometimes identify areas of compromised tissue where, for example, inferior punctuate staining is seen with incomplete blinking or lid parallel conjunctival folds stand out more clearly. A commonly encountered indicator of thinning tear layer is the inferior arcuate stain. This is sometimes described as a 'smiley' stain seen in soft lens wear, which is induced or exacerbated by poor environmental conditions such as air conditioning and extended use of computers. Inverting the upper lids and viewing the marginal conjunctiva can sometimes show a thin strip of staining which has been described in the literature as an indication of 'lid wiper epitheliopathy'. This is believed to be a sign of increased friction to the lid margin due to reduced lubrication as the lid wipes across the cornea or a lens. This can often be seen when symptoms of dry eye are experienced in the absence of other clinical findings.12

Lid parallel conjunctival folds

A relatively newly considered phenomenon, lid parallel conjunctival folds are seen as small indiscrete horizontal folds in the bulbar conjunctiva within close proximity to the margin of both the upper and lower lids, but are generally easier to view along the inferior temporal margin. There is believed to be a direct correlation between the height and number of folds and the severity of dryness. Researchers at the University of Saarland Hospital, Homburg, Germany proposed a grading scheme whereby any folds observed are compared against the height of the tear meniscus. A single fold smaller than the tear meniscus would be a mild dry eye, with multiple folds to the same height as the tear meniscus being a moderate dry eye and so on. In contact lens practice seeing folds should alert the practitioner to the possible consequences of fitting lenses.

Lenses and materials

Little can be done to alter directly the environments in which contact lenses are worn, but careful consideration of the type of lens, material and care products can dramatically influence the comfort and wearability experienced. All lenses interfere with the stability of the tear film causing an increased rate of evaporation the materials from which lenses are made dehydrate at different rates on insertion into the eyes. This impacts the length of time a lens can be worn comfortably and, of course, affects oxygen permeability.

Efron and Morgan measured the variation among 17 different lens types to determine the amount of dehydration that occurred after four hours of wear. Their results indicated that ionic materials were more likely to be associated with on-eye dehydration irrespective of water content.13 This suggests that some materials resist the effects of a dry environment better than others. For example, studies have shown that hioxifilcon A and omafilcon A lens materials can reduce the symptoms of dryness when prescribed to patients who present with contact lens related dryness. In one study comparing these two materials, nearly two-thirds of the subjects who reported frequent or severe symptoms of contact lens-related dryness or discomfort had an improvement of around 50 per cent to their symptoms after use of both the study lenses.14 Reduction of symptoms and increased wearing time can make the lens wearing experience more acceptable, reducing the likelihood of the wearer discontinuing lens wear.

Omafilcon A is the material used in all the Proclear range of lenses and was the first material to be approved by the FDA in the US as being suitable for use in dry eyes. A specific label claim allowed by the FDA is: 'The lenses may provide comfort for contact lens wearers who experience mild discomfort or symptoms related to dryness during lens wear associated with evaporative tear deficiency or from aqueous tear deficiency (non-Sjögren's only).'

The Proclear family now covers the full range of products from daily and monthly disposables to conventional forms in both toric and multifocal designs. The XR extension to the range covers sphere powers to ±20.00 with add powers to +4.00 and cylinder powers to -5.75, all in the unique material omafilcon A.

Silicone hydrogel lenses with their generally lower water content are known to be beneficial in the management of marginally dry eyed patients. However, keep in mind the material affinity for attracting lipids can exacerbate the problems of dryness discomfort in lens wear. This is not helped by the higher modulus first generation lenses.

Care products and storage solutions

Care products can have a significant effect on the comfort of the contact lens wearer who may describe dry eye symptoms which are in fact caused by a solution incompatibility. Unfortunately, it is not possible to pre-determine a wearer's sensitivity to any particular brand of solution. The majority of contact lens cleaning and storage products used today are of the multipurpose type usually containing either polyhexamethylene biguanide (PHMB) or polidronium chloride (polyquad) as preservatives. It is usually these preservatives that are implicated in the discomfort and staining associated with a solution sensitivity response. However, these products contain many other chemicals, disinfection, tonicity and chelating agents, buffers and lubricants all of which are capable of producing an adverse response to the wearer if not correctly balanced for the individual. Simply changing from one brand to another without having some knowledge of the actual ingredients and concentrations may not be sufficient to prevent further problems.

An alternative to the above is to use one of the peroxide-based systems which are often described as preservative free. These contain stabilised, buffered 3 per cent hydrogen peroxide solution as a disinfectant, but must be neutralised before the lens can be re-inserted. Neutralisation into oxygen and water is generally achieved by using either a catalase enzyme or a catalytic disc coated in platinum. By this means it is possible to disinfect lenses without introducing chemical into the wearer's eyes. The platinum coated discs are extremely efficient at neutralising the peroxide when new, but with time irritation can be created by progressive reduction in the performance of the disc to neutralise the peroxide as quickly.

Lubricants and re-wetting drops

There are so many comfort and lubricating solutions now available that it is difficult for the practitioner to decide which is best for the patient. It is beyond the scope of this article to mention them all so only a brief review of the more commonly available products will be made here. In reality, certain products work better for some individuals than others, so it is necessary to have a selection of solution with different compositions.

The first important decision to make when choosing which of these lubricants to use is whether it is more appropriate to consider an unpreserved or preserved solution and again there are many options under each heading. This is especially important for some atopic wearers. As previously mentioned, the preservatives used can induce symptoms and signs similar to those of a dry eye for individuals sensitive to the chemicals employed.

AQuify from CIBA Vision is an isotonic preservative-free lubricant. It contains sodium perborate to preserve the solution in the bottle. This decomposes to water and oxygen when placed into the eyes. The main active ingredient or lubricant within this product is sodium hyaluronate which is the substance responsible for retaining moisture in skin. Sodium hyaluronate is a non-Newtonian fluid which has a high viscosity when stationary but low viscosity when pressure is applied, such as during a blink, mimicking the behaviour of natural tears. Another unpreserved re-wetting drop that comes in a unit dose containing sodium hyaluronate and having a similar action to AQuify is Blink Contacts Eye Drops from AMO.

The sensation of dry eyes and lens discomfort are created when water is lost from the tears causing a change in osmolarity. The hyper-osmotic tears drag water from the contact lens structure. One product which purports to address these problems is TheraTears. This unit dose preservative-free product is hypotonic to re-establish a more normal tear balance. Additionally, the solution contains electrolytes to better match the natural tear layers. All lubricating drops require some kind of wetting agent and in the case of TheraTears it is sodium carboxymethylcellulose which reduces the shear forces between the lids and the lens or eye.

Conclusion

Once any potential contact lens wearers with more apparent indicators of a true dry eye have been rejected by the pre-screening ocular examination and any meibomian gland dysfunction or blepharitis treated, the practitioner can proceed with lens fitting. It is during subsequent aftercare appointments that vigilance is required to identify environmentally induced marginal dry eye symptoms and signs. As with any aspect of contact lens aftercare, careful questioning of the wearer can identify potential symptoms of a marginal dry eye problem. The many tests listed in the text can be employed to confirm any suspicions. With knowledge and ingenuity the practitioner should be able to determine which lens design, material and care system are most suitable to secure the best possible performance to minimise environmentally induced dry eyes ensuring the long term comfort, health, success and loyalty of their contact lens wearers. ●

References

1. Lemp MA. Report of the National Eye Institute/Industry Workshop on Clinical Trials in Dry Eye. CLAO J, 199521:221-32.
2. DEWS report. Definition and Classification of Dry Eye Disease. April 2007, Vol 5, No 2.
3. Fonn D, Pritchard N, Brazeau D. Discontinuation of contact lens wear. Invest Ophth Vis Sci, 1995 36: 312.
4. Wolff E. Anatomy of Eye and Orbit. IV Ed. New York: Ed. Blakiston Co 1954 207.
5. Albietz JM. Dry eye: an update on clinical diagnosis, management and promising new treatments. Clinical & Experimental Optometry 84.1 January-February 2001.
6. Efron N. Contact Lens Complications, 1999. Part III, 61.
7. Doughty MJ, Fonn D, Richter D, Simpson T, Caffery B, Gordon K. A patient questionnaire approach to estimating the prevalence of dry eye symptoms in patients presenting to optometric practice across Canada. Optom Vis Sci, 1997 74:624-631.
8. Hamano H et al. A study of the complications induced by conventional and disposable contact lenses. CLAO J, 1994 20: 103-108.
9. Hom MH, Martinson JR, Knapp LL. Prevalence of meibomian gland dysfunction. Optom Vis Sci, 1990 67:710.
10. Fatt I, Rocher P. Contact Lens Performance in Different Climates. Optometry Today, 1994 34: 26-31.
11. Hamano H, Hori M, Hamano T, Mitsunaga S,Maeshima J, Kojima S, Kawabe H. A new method for measuring tears. CLAO J, 1983 9: 281-9.
12. Korb DR, Herman JP, Greiner JV, et al. Lid wiper epitheliopathy and dry eye symptoms. Eye Contact Lens, 2005 Jan 31(1):2-8.
13. Efron N and Morgan PB, Hydrogel contact lens dehydration and oxygen transmissibility. CLAO J, 1999 25 (3): 148-151.
14. Riley C, Chalmers RL, Pence N. The Impact of lens Choice in the Relief of Contact Lens Related Symptoms and Ocular Surface Findings. Published on the internet by Indiana University School of Optometry. 2004 Nov 13.

● John Rogers is European clinical affairs manager for CooperVision

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