CONTACT LENS DRY EYE
An In-Depth Look at Lens-Related Dry Eye
Managing patients' dry eye symptoms can prevent them from becoming contact lens dropouts.
By Ashley Wallace-Tucker, OD, FAAO
|Dr. Wallace-Tucker is a graduate of the University of Houston College of Optometry, where she is currently a visiting associate professor. Dr. Wallace-Tucker completed a residency in Cornea and Contact Lenses, and her research interests include contact lenses and anterior segment disease.|
Approximately 50 percent of all contact lens wearers experience contactlens-related dry eye symptoms. That number equates to about 17 million to 18 million contact lens wearers in North America (Nichols et al, 2002). In addition, contact lens wearers are five times more likely than are spectacle wearers to report symptoms of dry eye (Nichols et al, 2005). Because discomfort—usually synonymous with “dryness”— is the primary reason for contact lens discontinuation (Pritchard et al 1999; Richdale et al, 2007), it is imperative that practitioners understand the etiology and management of contact lens-related dry eye (CLRDE).
Although the exact pathogenesis of CLRDE is unknown, most agree that one of the major factors for successful contact lens wear is a stable tear film. The tear film lubricates and hydrates the contact lens; therefore, it is essential to contact lens comfort. Because CLRDE is complex and likely multifactorial, there are many potential mechanisms. Contact lenses alter the natural structure of the precorneal tear film by dividing it into the prelens and post-lens tear layers. The pre-lens layer likely consists of the superficial lipid and aqueous layers whereas the post-lens layer likely consists of aqueous and mucin.
In a study of 360 hydrogel and GP contact lens wearers, Nichols et al (2006) found that CLRDE may be explained by increased pre-lens tear film thinning times and increased tear film osmolality. An increase in pre-lens tear film thinning time can be attributed to evaporation of the tear film itself secondary to an altered lipid layer and lens dewetting secondary to hydrophobic regions on the lens surface. Furthermore, tear film osmolality, which we will discuss more in depth later, may be a key factor in diagnosing CLRDE. Nichols et al (2006), along with many others, found that the tear film osmolality was significantly higher in contact lens patients who have dry eye when compared to contact lens patients without dry eye.
The exact mechanism of increased tear film osmolality is unknown, but it has been suggested that it could be due to hyperosmolality of the contact lens or to increased tear film evaporation (Nichols, 2006). In addition, Tyagi et al (2012) found that both soft and GP contact lens wear adversely affect the tear film quality as measured by high speed videokeratoscopy. There is limited data on the frequency of dry eye symptoms and GP lens wear, so it is important to note that they found no statistically significant difference in the tear film surface quality between GP and soft lens materials.
Contact Lens Materials
It is commonly believed that patients who wear low-water-content contact lenses have fewer dry eye symptoms because low-water-content lenses lose less of their water than do high-water-content contact lenses (Efron et al, 1987). Although patients who wear low-water-content contact lenses typically have fewer symptoms of dry eye, there have been varying results on the correlation between lens dehydration and dry eye symptomology (Fonn et al, 1999; Pritchard et al, 1995).
There are potential hypotheses for the relationship between high-water-content contact lenses and dry eye. First, high-water-content lenses demonstrate significantly higher lipid and protein deposition when compared to low-water-content lenses (Jones et al, 1997). Contact lens deposition ultimately can lead to altered lens wettability and tear film instability, contributing to contact lens discomfort. For instance, polar lipids from the tear film may be more attracted to the high-water contact lens, resulting in an alteration in the pre-lens tear film lipid layer and contact lens surface. This would ultimately cause increased tear film evaporation and/or lens dewetting (Nichols et al, 2006).
In addition to water content, ionicity and oxygen transmission (Dk/t) are additional factors that may be associated with lens dehydration and subsequent contact lens comfort. Ramamoorthy et al (2010) found that high-water-content and ionic hydrogel lens materials demonstrated greater lens dehydration compared to lower-water and non-ionic materials. Thus, they concluded that not only water content, but also ionicity is positively related to lens dehydration. Furthermore, ionicity is a material property that affects lens biocompatibility. It has been hypothesized that ionic materials attract hydrophilic proteins such as lysozyme, which promotes wettability and, therefore, tear stability (Guillon et al, 1997). In contrast, non-ionic materials attract hydrophobic proteins such as lactoferrin and albumin (Guillon et al, 1997). Secondly, as a hydrogel lens dehydrates, there is a significant reduction in oxygen transmissibility. There is a relationship between oxygen transmissibility and water content, meaning that water loss in a low-water-content contact lens is less significant in terms of oxygen transmissibility when compared to high-water-content lenses because high-water-content lenses lose more water (Efron et al, 1999).
Oxygen permeability (Dk) is another key factor in contact lens comfort. Lenses with a low Dk can cause up to a 50 percent decrease in corneal sensitivity (Millodot et al, 1980). Corneal hypoesthesia is related to damaged corneal nerve innervation, which ultimately can result in symptoms of dry eye because of the lack of stimulation to blink and tear (Bourcier et al, 2005). High-Dk soft contact lenses (i.e. silicone hydrogel lenses) and GP lenses have not been shown to cause corneal hypoesthesia (Liesegang, 2002).
Silicone hydrogel (SiHy) lenses represented about two-thirds of lens sales in 2011, and that percentage has remained fairly consistent over the last few years (Nichols, 2012). Although there is conflicting data, some studies have found that patients wearing SiHy contact lenses have fewer dry eye symptoms when compared to those wearing hydrogel lenses (Schafer et al, 2003; Sweeney et al, 2004; Fonn et al, 2000). There are several reasons for this finding, including increased oxygen supply, improved wettability, and reduced dehydration. Because SiHy lenses are low-water-content lenses, they exhibit less lens dehydration. Furthermore, oxygen transmissibility does not decrease with lens dehydration as it does with hydrogel lenses, it actually increases. This paradoxical finding is explained by the fact that the silicone component of SiHy lenses is the primary facilitator of oxygen transmissibility as opposed to water in hydrogel lenses (Morgan et al, 2003).
Several studies have confirmed that SiHy contact lenses have significantly less protein deposits, which may result in increased contact lens wettability (Senchyma et al, 2004; McKenney et al, 1998; Suwala et al, 2006). To further enhance wettability, some SiHy contact lens manufacturers have incorporated hydrophilic humectants such as polyvinyl alcohol (PVA) and polyvinyl pyrrolidone (PVP) into the contact lens materials. The high-molecular-weight molecules within these materials attract and maintain moisture, resulting in improved hydration throughout the patient's wear time (Fonn, 2007).
Because the symptoms of CLRDE are highly variable and patients are often unsure of how to express their concerns, a thorough case history is the most effective method of eliciting any symptomology. There are many dry eye questionnaires that have proven to be important diagnostic tools in both clinical practice and research, including but not limited to the Contact Lens Dry Eye Questionnaire (CLDEQ), the Dry Eye Questionnaire (DEQ), the Ocular Surface Disease Index (OSDI), and the McMonnies' questionnaire. Although McMonnies' questionnaire is arguably the most well-known and established dry eye questionnaire, Nichols et al (2002) found that the CLDEQ (sensitivity of 61 percent and specificity of 83 percent) was superior to McMonnies' questionnaire (sensitivity of 34 percent and specificity of 86 percent) in discerning CLRDE.
There are several clinical tests that can aid in the diagnosis of dry eye, but a standard protocol has yet to be established and the value of the data obtained from theses clinical tests is debatable. Many of the tests rely on subjective judgment, lack specificity, and are prone to reliability issues. Despite this, practitioners often use a combination of objective and subjective testing as baseline data and then subsequently to determine the efficacy of treatment. The most commonly utilized tests include tear breakup time (TBUT), vital dye staining of the cornea and/or conjunctiva, Schirmer testing, and meibomian gland assessment.
A recent study published by Sullivan et al (2012) determined that tear osmolarity was the least variable test among the following objective dry eye tests: TBUT, staining, meibomian gland grading, and Ocular Surface Disease Index. Tear film hyperosmolarity is often suggested to be the one underlying factor that is common to all forms of dry eye, resulting in corneal epithelial cell apoptosis and stimulation of inflammatory cells (Gilbard et al, 1994). Now that the TearLab Osmolarity System (TearLab Corp.) is commercially available, we can utilize tear osmolarity as a key dry eye diagnostic tool. The TearLab osmometer can measure osmolarity on a 50nL sample and record an average within run coefficient of variation of 1.47 percent, which corresponds to a precision of ±4.5 at 308.7mOsm/L (Sullivan et al, 2010).
Until fairly recently, there were no clinical tests to predict whether a patient would be a successful contact lens wearer. Pult et al (2008) found that lid wiper epitheliopathy (LWE) and lid parallel conjunctival folds (LIPCOF) were significantly more common in experienced, symptomatic, soft lens wearers than were corneal staining, conjunctival hyperemia, and pre-lens TBUT, indicating that LWE and LIPCOF are good predictors of CLRDE.
The lid wiper is the portion of the marginal conjunctiva of the upper eyelid that reaches from the upper punctum to the outer canthus. The lid wiper is separated from the ocular surface by the tear film and thus is responsible for the even spread of tears during blinks. Because contact lenses alter the tear film, the epithelial cells of the lid wiper are subject to mechanical damage during each blink as the lid wiper comes into contact with the contact lens. LWE is seen clinically by everting the upper lid and staining the area with lissamine green or fluorescein (Korb et al, 2002; Figure 1).
Figure 1. Lid wiper epitheliopathy stained with lissamine green.
LIPCOF are subclinical folds of the inferior bulbar conjunctiva, oriented parallel to the lower lid margin. It is hypothesized that these folds result from friction during blinking (Pult et al, 2008). In a follow-up study in 2009, Pult et al discovered that the best combination of tests to predict success in new contact lens wearers is non-invasive breakup time (NIBUT) plus LIPCOF Sum and the ocular surface disease index (OSDI), coined the Contact-Lens-Predictive-Test. NIBUT is considered the time between a complete blink and the first observed break in the tear film. LIPCOF Sum is the summation of the grading score of the nasal and temporal LIPCOF. The OSDI is a 12-item questionnaire designed to both diagnose and grade the severity of dry eye disease (Schiffman et al, 2000). Because the Contact-Lens-Predictive-Test has shown impressive predictive potential, practitioners should consider it when managing new lens wearers.
To provide your patients with the best probability for lens success, all pre-existing anterior segment conditions such as blepharitis, meibomian gland dysfunction, and allergies should be diagnosed and properly treated. These conditions among others can directly cause dry eye or exacerbate the symptoms. Many patients will be able to undergo treatment while concurrently wearing contact lenses, but if the condition is severe, fully treat the ocular condition prior to initiating contact lens wear. Make every effort to ensure that a patient's ocular health can adequately support contact lens wear; patients who do not respond well to treatment should not be considered contact lens candidates.
Chalmers et al (2008) found that refitting daily hydrogel wearers into a SiHy contact lens reduced the number of symptomatic wearers by half. Therefore, practitioners should consider switching symptomatic hydrogel wearers to a SiHy lens. In addition, strongly discourage extended or overnight contact lens wear. Stagnant tears during overnight wear contribute to the release and retention of inflammatory products on the contact lens and ocular surface, resulting in discomfort and dry eye symptoms. Other contact lens options might be considered for soft lens wearers who have dry eye. Lastly, daily disposable lenses may be the best option for patients who have ocular allergies, those with high levels of lipid deposition, and for patients who have been unsuccessful with other contact lens wearing schedules.
Educate patients on the many external factors that could be impacting their dry eye symptoms including, but not limited to: smoking, relative humidity, and extended visual tasking. Exposure to cigarette smoke damages the lipid layer of the precorneal tear film, leading to tear film instability and ultimately to dry eye symptoms (Altinors et al, 2006). Environments with low relative humidity (less than 40 percent to 45 percent) such as offices, air conditioned cars, and airplanes negatively impact the tear film by promoting water evaporation (Wolkoff, 2008). Kojima et al (2011) found that lens wearers who spent extended hours working at a computer had a lower tear meniscus volume and significantly higher dry eye and visual symptoms when compared to those who did not wear contact lenses.
Despite our best efforts to prevent CLRDE, additional, more aggressive treatment is often inevitable. Because there seems to be an inflammatory component to dry eye, essential fatty acids (EFAs), including omega-3 and omega-6, are often utilized as adjunct therapy. EFAs are the precursor eicosanoids, which act as anti-inflammatory agents in the body (Rand et al, 2011). As of yet, there is no U.S. Food and Drug Administration-approved dosing for dietary supplementation of EFAs for the treatment of dry eye.
In addition, don't undervalue lens rewetting drops and lens solutions; both can effectively improve wettability and comfort. Lastly, medicinal supplementation such as topical anti-inflammatory agents and/or topical or oral antibiotics may be necessary to achieve complete relief from CLRDE symptoms.
Overnight orthokeratology is one approach used by some, particularly as it may eliminate end-of-day dryness associated with soft lens wear. Consider scleral lenses for patients who have severe dry eye syndrome or for those who have been unsuccessful in other lens types. Modern scleral contact lenses are manufactured in GP lens materials and are designed to have a fluid-filled chamber, providing the ideal environment for a compromised ocular surface.
Because dry eye complaints are often vague, be prudent in eliciting this information from your patients. Oftentimes, clinical testing can provide conflicting data; therefore, a thorough history and/or a dry eye questionnaire can provide valuable insight into your patient's dry eye status. Managing any anterior segment disease prior to contact lens fitting, daily disposable or frequent replacement SiHy lenses, and aggressively caring for lenses with modern care solutions will provide the best likelihood for success. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #200.