Continuing Education
Preventing Contact Lens Dropouts
Addressing issues of discomfort, inconvenience and dryness may keep more patients in contact lens wear.
DESMOND FONN, DIP. OPTOM, M. OPTOM, FAAO

Discontinuation of contact lens wear still plagues the contact lens industry. The current estimates of drop-outs in the United States (US) range from 10 to 16 million people. The accuracy of this estimate is questionable because it is based on sampling, and some patients who abandon contact lenses return to wear after years of abstinence. Consumer advertising and, to some extent, proactive eye care practitioners can be credited with renewing the interest for patients who were dropouts. The picture is similar, and as dismal, in the United Kingdom (UK) according to a Vision Trak report in 2001. There were approximately 2.1 million drop-outs, which represents about 60 per cent of the lens wearing population. Morgan has suggested that this is the reason for the decline in the numbers of contact lens wearers in the UK.

The annualized permanent discontinuation rate is also questionable, but 10 percent in the United States is realistic. This does not include the temporary discontinuations which range from 30 to 50 percent, and at least half of these people may drop out for 2 years or longer. Attempts have been made by the industry and researchers to address the issues of discontinuation, but it is clear that more fundamental research is needed to understand the etiology. The payoff would be astronomical if the correct remedies were discovered.

The causes of contact lens discontinuation have been described previously, and the primary reasons have been consistent in all the studies. Those reasons are discomfort, handling/inconvenience, dryness and redness. There are other reasons, but they are considered less significant, e.g. decreased vision, expense and, more recently, the attraction of permanent refractive correction through refractive surgery. The studies by Weed et al. and Pritchard et al. compared the reasons for rigid and soft lens discontinuation and their results were remarkably similar. Table 1 shows the Weed study results from the survey that was conducted at the University of Waterloo Centre for Contact Lens Research (CCLR). The differences between rigid and soft lenses are not unexpected.

RIGID LENSES

It is well known that patients take much longer to adapt to rigid than soft contact lenses; and even when they have fully adapted, the majority of patients still report greater comfort with soft lenses compared to rigid. Although rigid lens wearers are able to achieve full day wear when they have passed through the adaptation phase, at least 30 percent will still ultimately drop out of lens wear because of discomfort, as compared to 25 percent for soft lenses. This higher number of drop-outs has been reported for as long as rigid gas permeable (RGP) lenses have been in existence. While the number of new rigid lens wearers is decreasing, they still represent about 15 percent of the United States contact lens wearers. Those patients that can wear them successfully would unlikely switch to soft lenses or refractive surgery. As discomfort is the chief cause of abandonment with rigid lenses, here are some strategies that maximize comfort and, in so doing, minimize the other common complications that are responsible for or result from discomfort.

Rigid lens discomfort can be acute or chronic. The acute cause is more obvious and the etiology is usually mechanical, such as lens defects, foreign body entrapment, denatured deposits on the posterior lens surface or very poor design and fitting characteristics. Provided a definitive diagnosis is possible through careful examination of the lens both on and off the eye, the management of these is usually uncomplicated and would unlikely result in discontinuation.

Chronic discomfort is much more difficult to alleviate because the symptoms are often less specific. Discomfort is invariably accompanied by dryness, and we at the CCLR have found a strong correlation between these two symptoms when they are graded independently by subjects using visual analog scales. These two symptoms and their relationship will be discussed in the wear of soft lenses.

Chronic discomfort of rigid lenses can be viewed as patients' failure to adapt to rigid lens sensation, but we do not know why some patients are able to adapt and others are not. It is probably linked to the neurophysiology of the cornea and, to a lesser extent, the interaction of the eyelids. While the mechanisms remain a mystery, here are some thoughts on how to maximize rigid lens comfort:

Oxygen permeability should not be critical even though patients have different oxygen demands. The vast majority of patients will be using rigid lenses on a daily wear (DW) basis, and exceeding 50 Dk units should be sufficient. As far as comfort is concerned, ensuring that the surfaces are wettable is probably more important than the permeability of the material.

Figure 1. An optimal fluorescein pattern.

Lens design and fit are the most critical elements for optimizing comfort. Figure 1 is a fluorescein pattern that I consider as optimal. Note that the lens is fairly large and positions centrally, and it has some coverage by the superior lid. Williams ­ Lyn et al. found that larger RGP lenses (9.50 to 10.0mm diameter) are generally more comfortable than smaller lenses. Achieving complete alignment across the majority of the posterior lens surface with spherical base curves is not always possible because of the inevitable usually with the rule corneal astigmatism. Fitting the lens slightly steeper than the flattest corneal meridian should create minimal central clearance, mid-peripheral bearing and sufficient edge clearance with a gradual transition between each as depicted in Figure 1. This example also has significant inferior edge clearance, but any more than this can lead to excessive debris collection in the deep/wide peripheral zone. The debris will traverse between the back of the contact lens and the front of the cornea as a result of tear exchange that is activated by the vertical movement of the lens, which causes foreign body sensation.

The lens design should include an aspheric or a well blended multicurve posterior surface, an optic zone diameter about 8.5mm and a total diameter of 9.7 to 10.0 mm. Although center thickness is less critical for comfort, my recommendation would be to minimize it within a range of 0.10 to 0.15mm for minus lenses. Edge thickness and shape are very important, and the front surface should be continuous with the edge, the edge should be thin and rounded, and the apex centered between the front and the back surfaces.

	Figure 2. Three and nine o'clock staining (Grade 2 to 3) slightly lower than the mid-line.

Three and nine o'clock corneal staining (Figure 2) affects an estimated 40 to 90 percent of RGP contact lens wearers. The peripheral desiccation almost certainly results from blink inhibition, which is most likely due to discomfort. RGP lens wear causes accelerated tear film evaporation and decreased Tear Break Up Time (TBUT), which leads to peripheral corneal desiccation and damage and staining of the epithelial cells. This would naturally result in limbal and bulbar conjunctival hyperemia, as shown in Figure 3. The symptoms of these patients are discomfort and dryness and redness towards the end of the wearing period. Unfortunately optimizing the design and fit does not always alleviate the discomfort for RGP patients. Switching to soft lenses almost certainly improves comfort and definitely eliminates 3 and 9 o'clock staining.

SOFT LENSES
Inconvenience of lens wear

The majority of patients no longer have cause to complain about inconvenience. The concept of disposability is well entrenched in developed countries where it is the rule rather than the exception. Two week replacement frequency seems to be most popular in North America, but the ultimate "one use" lens surprisingly has not caught on as yet, in the UK daily disposables are much more popular, and they currently represent about 30 percent of all new fits. In Norway it is about 40 percent. Daily disposables eliminate the need for solutions and solution sensitivity, almost eliminate lens deposits and produce greater comfort for many patients.

Another method to reduce the inconvenience of lens wear is to prescribe silicone hydrogels for up to 30 days and nights of continuous wear. These lenses were approved by the Food and Drug Administration (FDA) in the United States within the last year, but they have been available internationally for over three years. It is estimated that there are about 400,000 silicone hydrogel wearers worldwide and it has been suggested that at least 60 percent wear the lenses continuously between 6 and 30 days and nights.

Inconvenience not only refers to cleaning and disinfection of lenses but patients are sometimes frustrated by lenses that are difficult to insert and remove, especially novices. Either higher modulus lenses, such as silicone hydrogels, or slightly thicker lens profiles in the higher water content hydrogel lens materials would overcome handling difficulty.

Discomfort and Dryness

Many patients state that when a soft lens is inserted, their eyes sometimes feel better than when no lenses are on the eyes. We correctly assume that this is because of the excess fluid that enters the eye with the lens. This improved comfort is more likely if the solution does not contain preservatives. Unfortunately ,the initial enhanced comfort does not continue, and most patients complain about discomfort and/or dryness towards the end of their wear. Many patients use rewetting drops to improve comfort as a remedy for the dryness or discomfort. If the beneficial effect of a tear supplement is temporary, patients usually remove their lenses before the desired wearing time and are unhappy with the limited wear.

It is obvious that the volume of fluid in the eye decreases after lens insertion, but it is unclear if the decrease is as gradual through the day as are the sensation of discomfort and dryness. Although contact lens wear increases the evaporation rate of tears and produces a relatively dry eye, we cannot assume that this is the major or sole cause of the most common complication of contact lens wear. The reduction in tear volume is confounded by the associated increase in tear osmolarity, which may be the origin of the symptoms. It is as likely that factors in addition to the quantitative and qualitative changes of the tear film contribute to discomfort/dryness e.g. increased limbal hyperemia, elevated sensitivity of the bulbar and tarsal conjunctivas and the mechanical effect of increased friction of the front and back surfaces of the lens with the eyelids and cornea, respectively, and relatively low oxygen transmissibility insufficient to meet the specific corneal oxygen demands of individuals.

Contact Lens Wear and the Tear Film

Figure 3. Marked hyperemia due to 3 and 9 o' clock staining from rigid lens wear.

Cederstaff and Tomlinson and others have demonstrated that the evaporation of the tear film increases with contact lens wear. This probably accounts for the decreased TBUT on the lens surfaces of both rigid and soft lenses compared to the same measure on the corneal surfaces prior to lens insertion. It has also been demonstrated that TBUT on soft lens surfaces decreases significantly during the day for patients who are symptomatic of discomfort and dryness, as opposed to asymptomatic patients whose TBUT does not decrease. Corneal TBUT always decreases after lens removal compared to the measure before lens insertion, which suggests that the contact lens has caused tear film instability. One of the reasons that the tear film is unstable on the contact lens surface is the inability of the mucin layer to adhere to the lens surface, which cause the faster TBUT. Without the binding force of mucin and the faster dissipation of the aqueous, the lipid layer contaminates the aqueous and the lens surface. This yields a color-fringed image with a Tear Scope.

Gilbard found that tear film osmolarity increases with contact lens wear and this causes the symptoms of dryness. It also has been suggested that contact lens wear decreases corneal sensitivity, which down-regulates the lacrimal gland and results in decreased tear production. Most current, thin, spherical hydrogel lenses are very unlikely to affect corneal sensitivity worn on a DW basis because the lenses are sufficiently transmissible to oxygen.

Soft Lens Dehydration, Discomfort and Dryness

Hydrogel lenses dehydrate on the eye as a function of water content and ionicity. High water content lenses tend to dehydrate more than low water lenses and ionic lenses more than non-ionic lenses. Although dehydration is related to water content, dehydration can be controlled by altering the chemistry of the material without affecting the water content. Proclear (omafilcon A, CooperVision, formerly Biocompatibles) is an example where Phosphorylcholine (a water-retaining polymer) is incorporated in a 60 percent water content lens to minimize dehydration and studies at the CCLR and those of others show about a 2 percent decrease in water content after seven hours of wear with Proclear lenses. Acuvue (etafilcon A, Vistakon) is an ionic lens with a water content of 58 percent and we have found that these lenses dehydrate between 5 to 7 percent over a seven-hour wearing period. This is probably typical of other high water/ionic soft lens materials.

	Figure 4. Lens dehydration vs. visual analog dryness ratings for Acuvue and Proclear lenses worn by symptomatic and asymptomatic subjects for seven hours.

The most important question is whether dehydration impacts comfort, and the results of clinical studies are inconsistent. Our studies fail to establish a correlation between lens dehydration and dryness or comfort (Figure 4), whereas other studies establish a correlation. Proclear is FDA approved for patients who experience mild discomfort or symptoms of dryness during lens wear. Although many patients have had relief of these symptoms when switched to Proclear, others have not. The results are unpredictable, which is not that surprising based on our correlation data. We and others have found that most dehydration occurs within the first few hours of lens wear, but discomfort and dryness occur more than five or six hours after lens application. A useful strategy for patients who are symptomatic without a known underlying cause is to empirically dispense two different lens materials and have the patient report which eliminates or minimizes the symptoms.

Other factors can also affect and exacerbate dehydration, e.g. environmental conditions of temperature, humidity and wind, large palpebral apertures and blink rate. Consideration should be given to altering the patient's environment, e.g. by using a humidifier, punctal occlusion and routine lid hygiene. Although tear film function declines throughout life, Du Toit et al. have shown that wearing lenses has more of an effect on increasing symptoms than age does.

Prediction of Discomfort/Dryness

Between 20 and 50 percent of patients are symptomatic of dryness with contact lens wear So it would be advisable to treat every second or third neophyte lens wearer with the most appropriate lenses to avoid discomfort/dryness.

Patients with obvious dry eye are easy to diagnose, but the clinical tests we use may not be sufficiently discriminate to tease out patients with borderline/marginal tear and ocular surface irregularities. Willcox and Glasson have shown that careful examination separates contact lens intolerant patients from tolerant. Intolerant patients have decreased Phenol Red Thread test, maximum forced blink interval, TBUT, tear meniscus height and tear flow rate. Patients who exhibit inferior corneal staining and partial blinking characteristics are at risk as well as those with poor lid hygiene and obvious meibomian gland disease.

Conjunctival Hyperemia and Discontinuation and Discomfort

Figure 5. An unacceptably high level of limbal and bulbar conjunctival hyperemia due to prolonged wear of a low Dk hydrogel lens.

Limbal hyperemia is characterized by filling and engorging of vessels and often accompanies DW and EW of soft lenses. According to Efron, conjunctival hyperemia represents the norm in hydrogel lens wearers. This was indirectly supported by Sweeney et al. who correctly identified 85 percent of soft lens wearers based on their levels of limbal hyperemia from a cohort of long term RGP, soft lens and spectacle wearers.

Figure 5 is an example of moderate/severe limbal hyperemia from soft contact lens EW. The photograph shows slightly more limbal than bulbar injection because a greater concentration of capillaries are at the limbus; however, they are linked. Sweeney's result is typified in the comparison between this figure and the example of conjunctival hyperemia (Figure 3) that is induced by rigid lenses. Patients are concerned about their eyes being "red," and our CCLR studies indicate that more hydrogel than rigid lens wearers discontinue lens wear because of this. Practitioners should be concerned about the chronic effects of limbal hyperemia as there is evidence that the consequence is corneal vascularization.

Soft lens-induced central corneal swelling and mechanical lens pressure have been labelled causes of limbal hyperemia. However, it has only recently been demonstrated that there is a relationship between limbal hyperemia and lens oxygen transmissibility. The advent of silicone hydrogel lenses has made this possible. Numerous experiments and clinical studies have shown decreased limbal hyperemia with silicone hydrogels compared to conventional hydrogels. Papas et al. were the first to document that silicone hydrogel lenses caused significantly less limbal injection than a control hydrogel lenses worn over a 16-hour period during the day. Du Toit et al's. study comparing overnight wear of these two lens material types showed that the limbal injection of eyes wearing the lower Dk soft lens took significantly longer to recover to baseline levels than eyes wearing the higher Dk silicone hydrogel. Based on his studies, Papas has suggested that the peripheral oxygen transmissibility of a soft lens should be about 125 x 10^-9 (cm x mlO₂)/(s x ml x mmHg) to avoid limbal hyperemia. This is considerably higher than 87 x 10^-9 and 24 x 10^-9 proposed by Holden and Mertz for EW and DW, respectively, to avoid lens-induced corneal swelling. Similar corneal vascularization differences have been observed between higher Dk silicone hydrogels and lower Dk soft lenses. Ghosting of vessels that have penetrated the cornea when previous lower Dk soft lens wearers have been refitted with silicone hydrogels has also been reported.

Although the relationship between discomfort/dryness and limbal hyperemia is merely an hypothesis, we cannot ignore the facts that they both independently represent a high incidence of the complications of lens wear. It is now possible to test this hypothesis because we have the technology to objectively measure conjunctival hyperemia and to scale discomfort/dryness. However, before that occurs, and even if we fail to establish a correlation, we now can and should prescribe silicone hydrogel lenses that will minimize limbal hyperemia, and thus corneal vascularization, particularly when thicker lens designs are required.

Corneal Staining from Lens Dehydration

	Figure 6. Punctate staining of the inferior corneal region associated with lens dehydration.

When soft contact lenses dehydrate during wear, fluid can be drawn from the posterior tear film due to osmotic force. This phenomenon has been described as "pervaporation." The effect on the cornea can be dramatic as a number of studies have shown, and the erosion-like staining pattern has been associated with high water, very thin lenses. This dramatic effect is not seen with current higher water content hydrogel lenses (55 to 60 percent) as the lenses are not extremely thin. However the superficial punctate inferior corneal staining pattern (smile stain) (Figure 6) is the most common type of corneal insult associated with soft lens wear. Although the staining with currently-worn hydrogels is not as severe, the etiology may well be the same as the erosion pattern.

If the insult and staining are moderate to severe and accompanied by symptoms of discomfort and or dryness, the remediation process is to use lubricating drops frequently, especially towards the end of the day when the staining is usually at its worst. This treatment should be accompanied by changing the lenses to minimize the effect of dehydration, which might mean prescribing lenses that have minimal dehydration properties and or thickening the design, but not at the expense of insufficient oxygen transmission. This may be another example where silicone hydrogel lenses can be used effectively. They are low water, have a very high permeability and are stiffer. Tear exchange should be more efficient because silicone hydrogels tend to move slightly more than conventional hydrogel lenses.

Although tear exchange is desirable so that debris and other by-products of metabolism can be efficiently pumped from between the lens and the cornea, its effect on oxygenating the cornea is negligible. Many years ago Polse showed that tear exchange under a soft lens brings about 1 percent of fresh oxygen per blink compared to about 20 percent per blink with rigid lenses.

Inferior corneal staining complications are often unaccompanied by symptoms, which makes the condition difficult to manage. Patients are asymptomatic, especially with mild insult, because the lens acts as a bandage.

Conjunctival Insult and Soft Lenses

Figure 7. Conjunctival staining due to mechanical trauma from the edge of a soft lens.

Figure 7 is a dramatic example of conjunctival staining due to an imperfect edge. However, the symptoms of this patient were typically nebulous and not consistent with the sign. Conjunctival compression is more common and likely with soft lenses if the lens dehydrates sufficiently to cause a steepening effect. This can be compounded by a thick edge. Conjunctival compression does not necessarily damage the epithelial cells, but it can blanch the vessels and result in hyperemia. Correct by altering the design and fit of the lens by flattening the base curve.

Contact lens papillary conjunctivitis (CLPC) has become something of a rarity with soft hydrogel lenses because of frequent planned replacement, and daily disposables should further decrease the incidence. However, silicone hydrogels have a higher incidence than low Dk hydrogels in the continuous wear clinical trials that the CCLR and others have conducted. The reason is unclear, but we believe that it is a mechanical disorder because silicone hydrogel materials are stiffer, and perhaps the lens design may be more critical for some patients. Although it is not clinically apparent, the surfaces of silicone hydrogels are also different from hydrogels in that surface modifications have been made to overcome the inherent hydrophobic properties of silicone. This is another potential source of irritation of the conjunctiva.

CLPC has manifested in two distinct forms. It can be localized to a small area of the palpebral conjunctiva or generalized covering a much larger area of the conjunctival surface. We do not know whether the etiology of localized CLPC is different from generalized or whether localized eventually becomes generalized with continued wear and insult. The obvious remedy in these cases is to discontinue lens wear, and the localized condition usually recovers after two to four weeks of abstinence from lens wear. These patients should then be switched to daily disposable hydrogels.

Vision and Discontinuation of Contact Lens Wear

This is a rarity for average myopes, but the number is greater for complicated and higher prescriptions. Contact lenses have generally not been very successful for the correction of presbyopia. Our estimates indicate that out of 100 patients, 40 might choose monovision, 32 bifocal lenses and 28 spectacles The choice would be based primarily on visual results if all three options were tried with patients and if the motivation for contact lens wear were high enough. Useful tools to include in the battery of tests for measuring visual response are low contrast visual acuity and contrast sensitivity. The may assist in being able to predictably screen out patients who subjectively report poor vision but whose high contrast acuity results are acceptable. This is another example of a lack of correlation between subjective and objective results. Perseverance, optimizing the choice of lens design and understanding the time commitment by the patient is the suggested strategy for success.

Anecdotal reports have linked subjective vision degradation with tear film instability. Not only would the image blur or degrade as the tear film breaks prematurely or transparency decreases, but blink frequency increases. Most recently Thai et al found that contrast sensitivity decreases when the pre-lens lens tear film dries or breaks.

Conclusion

This article highlighted the primary causes of contact lens discontinuation and how to prevent or remediate these problems. Discomfort/dryness continues to remain an enigma and the Holy Grail of contact lenses must surely be a contact lens surface that can support a stable tear film as does the cornea and conjunctiva.

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