To misquote a famous English soccer coach, “Comfort may not be the most important feature of contact lens wear—but it’s in the top one.” Recent studies suggest that discomfort has been overtaken by poor vision as the most common reason for contact lens discontinuation.^1,2 However, this is likely a reflection of the growing number of lens wearers of presbyopic age; comfort remains the principal requirement for happy contact lens wear.

Approximately one-quarter of new contact lens wearers drop out within the first year, and about one-third of those cite discomfort as the primary reason for doing so.² From a practitioner’s perspective, the obvious thoughts are: What are the underlying reasons for the discomfort, and can these be alleviated? However, the mechanisms for discomfort are often unclear and even confusing. Patients are often uncertain about the location of their discomfort—is it the cornea, the conjunctiva, or the lids? Even if the location is pinpointed, there may be confusion about the type of irritation; various types of discomfort symptoms can be interpreted as dryness, and dryness itself can take several forms.

A few years ago, a large study attempted to understand the underlying causes of contact lens-associated dryness.³ More than 200 symptomatic contact lens wearers underwent a detailed examination involving a battery of dry eye tests. There was a wide diversity of clinical presentations, emphasizing the wide range of causative factors for contact lens-related dryness. Even those patients who had the same diagnosis exhibited a range of presentations. For example, some aqueous deficient patients showed a minimal tear meniscus but a relatively stable tear film, while others showed an apparently normal tear meniscus but a low Schirmer’s tear test score. This emphasizes the need for practitioners to use a wide range of assessment techniques to screen potential contact lens wearers. As expected, a range of tear film anomalies were diagnosed, including aqueous deficiency (30%), tear film instability (25%), and meibomian gland dysfunction (14%).

Interestingly, however, nearly one-quarter (23%) of these symptomatic contact lens wearers showed none of the signs normally associated with dry eye problems. Evidently, other problems were present but were interpreted by the patients as symptoms of dryness. This prompted speculation as to what factors other than dryness might be at play, and these were termed “the Four Fs”: fit, form, friction, and formulation.³

LENS FIT

Two aspects of lens fit, both of which involve the lens edge, that are important to comfortable lens wear can be easily overlooked. As the true corneal diameter is more than a millimeter larger compared to the visible iris diameter,⁴ it’s not always obvious when a lens is failing to provide full corneal coverage. Contact lens pressure is greatest at the edge and, therefore, can feel like a foreign body when placed on the cornea. Mass-produced soft lenses often come in a single diameter, whereas corneas vary widely within the population, and thus, it’s inevitable that some lens designs will be too small to provide full corneal coverage in every case. Keep in mind also that all lenses shrink when raised to eye temperature, and lenses of supposedly similar diameter can vary widely in size when settled on the eye.⁵ Any discomfort from the lens edge encroaching on the limbus will be exacerbated by a relatively loose fit. The problem may be intermittent and may help to explain some cases of end-of-day discomfort.

Another side effect of a relatively loose fit is edge stand-off, which can result in lid irritation (Figure 1). If the circumference of the lens edge is larger compared to the corresponding contour of the sclera, the lens will fail to align with the ocular surface. Stand-off is usually located at the inferior-nasal edge and can be intermittent, disappearing immediately after a blink. Edge stand-off can be subtle, and it’s worth taking the extra time to observe the edges under magnification, particularly when patients report some lens awareness.

LENS FORM

Lens “form” is a contrived way of saying that the front-surface design of a soft lens can be a source of mechanical irritation. This can arise from the optic zone junction, the peripheral bevel, engravings, or, in the case of toric lenses, the stabilization zones. The front-surface profile of toric lenses helps to explain the higher prevalence of symptoms⁶ and the higher proportion of comfort-related discontinuations¹ in toric lens wearers. The lid margin, which comes into contact with the lens surface during blinking, is highly sensitive; Collins et al found the upper lid margin to be only marginally less sensitive compared to the central cornea,⁷ whereas Lowther and Hill found that lower-lid-margin sensitivity sometimes exceeds that of the cornea.⁸

Clearly, the remedy is to try alternative designs, perhaps considering the configuration of a patient’s lids in relation to differing lens designs and how the two might interact. For example, a prism-ballasted design might not be the best option for a patient who has a high lower lid.

FRICTION

Recently, one of the most debated contact lens properties has been coefficient of friction. In any system of moving parts, poor lubrication leads to physical damage, and in the context of contact lenses, this manifests as lid wiper epitheliopathy. There is tantalizing evidence that variations in material coefficient of friction have a strong influence on contact lens comfort (Figure 2).⁹ This makes sense given the daily journey made by the lids (about the length of a football field).

Coefficient of friction is a useful indicator of the quality of lubrication (or lubricity). Because this is the ratio of the frictional force to the normal force between two sliding surfaces, a lower coefficient indicates better lubrication. However, there is no recognized standard method of measurement for contact lenses.^10,11 In fact, the only International Organization for Standardization (ISO) standard for friction in medical devices is for the rubber ends of walking sticks. Various methodologies have been used for measuring contact lenses that involve different set-ups (lens mounts, counter surfaces, environment) and protocols (speed, number of motions, etc.).

Coles and Brennan¹² combined the data from a range of studies and noted a significant negative correlation between median comfort scores and coefficient of friction. Clearly, other factors also come into play, such as lens design, but the data showed a surprisingly high correlation. Not quite believing it, and being mindful of the importance of methodology, they replotted the data using coefficient of friction measurements from another source, but still noted similarly high correlations (Figure 2). Unfortunately, their results have not yet been replicated due to the difficulties in designing an appropriate clinical study and producing lenses with a range of friction properties.

This finding applied to normal subjects wearing contact lenses of varying surface properties; however, dryness-related discomfort can also be seen in terms of lubricity. In addition to providing less lubricating fluid, a thin, unstable tear film can produce dry patches, which again clearly impact lubricity.

FORMULATION—CARE SYSTEM

The Tear Film & Ocular Surface Society (TFOS) International Workshop on Contact Lens Discomfort (available online at https://iovs.arvojournals.org/issues.aspx?issueid=933599#issueid=933599 and at www.tearfilm.org ) noted that care system formulations can result in either reduced comfort or enhanced comfort, the former likely due to uptake and release of solution and the latter due to the adsorption of additives (e.g., poloxamers) designed to enhance comfort. It is not surprising, therefore, that some studies report variations in comfort for different lens/solution combinations.^13-16 This difference often manifests as end-of-day dryness. With one lens type, one study noted better comfort with daily disposable use compared with both hydrogen peroxide and multipurpose solution lens care systems.¹⁷

Many of these differences may relate to differences in physical, wetting, and cleaning properties; however, their interaction with the corneal surface is also a factor. Solution-induced corneal staining (SICS) is an effect of solution released when the lens is first placed on the eye, and it is, therefore, most noticeable during the first few hours of lens wear.¹⁸ It is often asymptomatic, producing only mild corneal staining and disappearing by mid-morning. However, in severe cases, the staining is widespread, and the corneal surface loses its smooth appearance (also termed “rough SICS”). Several studies have therefore noted an association between SICS and reduced comfort; a large study noted reduced mean comfort of nearly one grade on a 1 to 10 scale in subjects who had SICS.¹⁴ They also noted increased overall dryness and end-of-day dryness.

On the positive side, care product companies have invested much effort into developing products with better clinical performance. A recent large study evaluated three of the newer multipurpose solutions in comparison with a peroxide system and reported no differences in comfort levels or corneal staining.¹⁹

OBSERVATIONS

Contact lens-associated dry eye is multifactorial and, therefore, the clinical appearance is highly variable. Currently, there is no single sign or test to reliably predict dryness in contact lens wearers. The most useful assessment is observation of the pre-lens tear film, both in terms of its general appearance and its stability.²⁰ This is best undertaken before any other assessment, as touching the lids can stimulate meibomian fluid and alter the tear film. Diagnostic instruments for enhancing the appearance of the lipid layer are helpful, but much can be observed with careful slit lamp observation. The thicker the lipid layer, the brighter its appearance and the more stable the tear film is likely to be. Observing the pre-lens tear film over several blink cycles will show whether it maintains its structure between blinks. If the drying time is relatively short, this may manifest as a ground glass appearance (indicating uniform thinning [Figure 3]) or, alternatively, spot drying, but either is likely to be problematic and to cause symptoms.

The use of fluorescein is helpful, not only for revealing corneal staining, but it is beneficial in several ways for tear film assessment. If the fluorescein is slow to spread, this can indicate a thin tear film. If the fluorescein is difficult to visualize once it has spread through the tear film, again, this could indicate a thin tear film. Fluorescein also helps to assess blink efficiency. An incomplete blink often leaves a horizontal line in the tear film, and this frequently corresponds to a line of desiccation staining. Other patterns of staining also provide clues as to the cause of any symptoms.

However, as noted, not all comfort issues relate to dryness. Careful slit lamp observation of the lens fit might suggest subtle shortcomings, particularly relating to edge fit and corneal coverage. Observation of the interaction of the lids, the lens edge, and other surface features can be helpful. As always, we should be prepared to change lens design, wearing modality, or care system in search of the optimum lens.

Three out of four contact lens fittings are relatively straightforward, and a 75% success rate might satisfy many practitioners. However, the more diligent contact lens specialists will ensure that the trickier 25% of patients have the best likelihood of success. This means listening to patients, taking time to find the most suitable lens, and monitoring the subtleties of contact lens performance. Another colorful coach (baseball this time) once said, “You can observe a lot by just watching.” Certainly, this applies to contact lens practice. CLS

REFERENCES

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