Soft contact lens extended wear affects corneal epithelial permeability: hypoxic or mechanical etiology?

A Review of Contact Lens-Induced Limbal Stem Cell Deficiency

Limbal stem cell deficiency (LSCD) is a pathologic condition caused by the dysfunction and destruction of stem cells, stem cell precursors and limbal cell niche in the corneal epithelium, leading to severe conjunctivalization of the cornea. Etiologies for LSCD span from congenital (aniridia), traumatic (chemical or thermal injuries), autoimmune (Stevens-Johnson syndrome) and iatrogenic disease to contact lens (CL) wear. Of these, CL wear is the least understood and is often a subclinical cause of LSCD. Even with recent advances in LSCD research, limitations persist in establishing the pathogenesis and treatment guidelines for CL-induced LSCD. A literature search was conducted to include original articles containing patients with CL-induced LSCD. This review will critically discuss the complex pathophysiology behind CL-induced LSCD, the underlying risk factors and epidemiology of the disease as well as methods to obtain a diagnosis. Various treatment options will be reviewed based on proposed treatment strategies.

Protection against corneal hyperosmolarity with soft-contact-lens wear

Hyperosmotic tear stimulates human corneal nerve endings, activates ocular immune response, and elicits dry-eye symptoms. A soft contact lens (SCL) covers the cornea preventing it from experiencing direct tear evaporation and the resulting blink-periodic salinity increases. For the cornea to experience hyperosmolarity due to tear evaporation, salt must transport across the SCL to the post-lens tear film (PoLTF) bathing the cornea. Consequently, limited salt transport across a SCL potentially protects the ocular surface from hyperosmotic tear. In addition, despite lens-wear discomfort sharing common sensations to dry eye, no correlation is available between measured tear hyperosmolarity and SCL-wear discomfort. Lack of documentation is likely because clinical measurements of tear osmolarity during lens wear do not interrogate the tear osmolarity of the PoLTF that actually overlays the cornea. Rather, tear osmolarity is clinically measured in the tear meniscus. For the first time, we mathematically quantify tear osmolarity in the PoLTF and show that it differs significantly from the clinically measured tear-meniscus osmolarity. We show further that aqueous-deficient dry eye and evaporative dry eye both exacerbate the hyperosmolarity of the PoLTF. Nevertheless, depending on lens salt-transport properties (i.e., diffusivity, partition coefficient, and thickness), a SCL can indeed protect against corneal hyperosmolarity by reducing PoLTF salinity to below that of the ocular surface during no-lens wear. Importantly, PoLTF osmolarity for dry-eye patients can be reduced to that of normal eyes with no-lens wear provided that the lens exhibits a low lens-salt diffusivity. Infrequent blinking increases PoLTF osmolarity consistent with lens-wear discomfort. Judicious design of SCL material salt-transport properties can ameliorate corneal hyperosmolarity. Our results confirm the importance of PoLTF osmolarity during SCL wear and indicate a possible relation between PoLTF osmolarity and contact-lens discomfort.

Corneal Health during Three Months of Scleral Lens Wear

SIGNIFICANCE

This study evaluated the effects scleral lens wear has on corneal health using fluorometry and in vivo confocal microscopy. No subclinical changes on healthy corneas of young subjects were observed during 3 months of scleral lens wear.

PURPOSE

This study aimed to evaluate the effects 3 months of scleral lens wear has on the corneal epithelial barrier function, dendritic cell density, and nerve fiber morphology.

METHODS

Twenty-seven neophytes (mean [standard deviation] age, 21.4 [3.9] years) wore scleral lenses of a fluorosilicone acrylate material bilaterally (97 Dk, 15.6 to 16.0-mm diameter) for 3 months without overnight wear. Subjects were randomized to use either Addipak (n = 12) or PuriLens Plus (n = 15) during lens insertion. Measurements of corneal epithelial permeability to fluorescein were performed with automated scanning fluorophotometer (Fluorotron Master; Ocumetrics, Mountain View, CA) on the central cornea of the right eye and the temporal corneal periphery of the left eye. Images of the distributions of corneal nerve fibers and dendritic cells and nerve fibers were captured in vivo with a confocal laser scanning microscope (Heidelberg Retina Tomograph, Rostock Cornea Module; Heidelberg Engineering, Heidelberg, Germany) on the central and inferior peripheral cornea of the left eye. Corneal measurements and imaging were performed at baseline and after 1 and 3 months of lens wear.

RESULTS

The corneal permeability values in natural log, dendritic cell densities, and nerve fiber morphology did not significantly change from baseline to 1 and 3 months of lens wear, for both central and peripheral corneal regions (P > .05). Dendritic cell density at the inferior cornea was higher than the central cornea throughout the study (P < .001). No relationships were observed between each outcome measurements and the saline solution groups (P > .05).

CONCLUSIONS

Scleral lens wear for 3 months on healthy cornea of young subjects did not affect corneal epithelial barrier function, nerve fiber, and dendritic cell densities. Buffered and nonbuffered saline solutions impacted the corneal health in similar ways.

Tear dynamics under scleral lenses

PURPOSE

To evaluate post-lens tear dynamics at two different time points during scleral lens wear in two cohorts with 10 neophytes each.

METHODS

All subjects wore bilaterally scleral lenses for 5 h on 3 separate visits. Post-lens tear exchange was measured using Out-in method, which required 5 μL of 2% FITC-Dextran instilled on the bulbar conjunctiva during lens wear. Time taken to observe the first sign of fluorescence in post-lens tear reservoir was recorded with a stopwatch. Out-in measurements were collected at 5-hour  post-lens insertion in Group 1 and compared with those obtained at 20 min of lens wear in Group 2. Tear dynamics under the lens was observed in Group 2 with fluorogram using a modified slit-lamp technique (Tan et al., 2018) to monitor post-lens fluorescence intensity and with high-resolution spectral domain optical coherence tomography (ENVISU 2300; Bioptigen Inc.) to measure post-lens tear thickness (PoLTT) over 5 h of lens wear.

RESULTS

60% of subjects in Group 1 achieved Out-in times less than 5 min at 5-hour post-lens insertion, compared with 67% of subjects at 20-min lens wear (Tan et al., 2018). Using qualitative analysis on 60 series of data in Group 2 to compare the changes in fluorescence intensity and PoLTT with respect to lens-wearing time, 27% was due to lens settling, 13% was due to tear exchange and mixing while 60% indicated tear dynamics under scleral lenses was due to a combination of tear exchange, mixing, and lens settling.

CONCLUSION

Tear flow into tear reservoir under a scleral lens on subjects with healthy cornea occurred at 20 min and 5 h after lens insertion. After 5 h of lens wear, roughly one third of the subjects had no tear flow into post-lens reservoir, as the observed decline in post-lens tear fluorescence was predominately due to lens settling.

Corneal epithelial permeability to fluorescein in humans by a multi-drop method

Purpose

The permeability of the corneal epithelium to fluorescein P_dc is an indicator of the health of the ocular surface. It can be measured in a clinical setting by determining the accumulation of fluorescein in the stroma following administration of the dye on the ocular surface. Here we demonstrate a new multi-drop method for the measurement of P_dc by a spot fluorometer.

Methods

Twenty-nine healthy participants were recruited for this study. First, a probe-drop of fluorescein (0.35%, 2 μL) was instilled on the conjunctiva. The clearance of the dye from the tears was immediately measured using the fluorometer. Following this, two loading drops (2%; 6 μL each) were administered 10 min apart. Fifteen minutes later, the ocular surface was washed and fluorescence from the stroma F_s was measured. Permeability was calculated using P_dc = (Q x F_s)/ (2 x AUC), where Q is the stromal thickness and AUC is the area under the fluorescence vs. time curve for the loading drops.

Results

After the probe drop, the tear fluorescence followed an exponential decay (elimination rate constant; k_d = 0.41 ± 0.28 per min; 49 eyes of 29 subjects), but the increase in F_s was negligible. However, after the loading drops, the measured F_s was ~ 20-fold higher than the autofluorescence and could be recorded at a high signal to noise ratio (SNR > 40). The intra-subject variability of k_d was insignificant. Since fluorescein undergoes concentration quenching at > 0.5%, the value of AUC for the loading drops was estimated by scaling the AUC of the probe drop. The calculated P_dc was 0.54 ± 0.54 nm/sec (n = 49). A Monte Carlo simulation of the model for the multi-drop protocol confirmed the robustness of the estimated P_dc.

Conclusions

The new multi-drop method can be used in place of the single-drop approach. It can overcome a lack of sensitivity in fluorometers of high axial resolution. The P_dc estimated by the multi-drop method is ~ 11-fold higher than previously reported but closer to the value reported for other drugs with equivalent octanol/water partition coefficient.

Accounting for Ethnicity-Related Differences in Ocular Surface Integrity as a Step Toward Understanding Contact Lens Discomfort

Contact lens discomfort is a common problem that can lead to unsuccessful or limited contact lens wear. Although many factors may contribute to contact lens discomfort, limited research has explored the influence of ethnicity-related differences in the anatomy and physiology of the ocular surface. Therefore, we performed a search of the literature in PubMed using key words related to "ocular surface" paired with the terms "race" and "ethnicity." The goal of this review was to determine potential areas of research regarding ethnicity differences, particularly between Asian and non-Asian eyes, in ocular surface integrity to advance our understanding of contact lens discomfort.

Objective Assessment of Ocular Surface Response to Contact Lens Wear in Presbyopic Contact Lens Wearers of Asian Descent

Introduction:

Contact lens wearers of Asian descent may be predisposed to experience microtrauma of the ocular surface as a result a thinner post-lens tear film and higher eyelid tension, and these effects would be anticipated to be most marked in an older population. The objective of this study was to quantify the mechanical effects of the study contact lenses on the ocular surface in a population of presbyopic contact lens wearers of Asian descent.

Methods:

Twenty established presbyopic contact lens wearers (hydrogel n=5, none habitual wearers of etafilcon A lenses; silicone hydrogel n=15) of Asian descent were refitted with etafilcon A multifocal daily disposable contact lenses (1-DAY ACUVUE MOIST MULTIFOCAL) for a period of 1 month of daily lens wear. The habitual modalities of wear were 45% daily disposable and 55% planned replacement. Digital photographs of the upper lid margins, nasal and temporal conjunctiva, and superior cornea were taken after 6 hr of wear of the participants' habitual contact lenses, after 1 day without contact lens wear, and after 6 hr of wear of the study contact lenses at the end of the 1-month period. The photographs were masked according to study visit and the staining extent measured using proprietary software.

Results:

Lid margin staining was significantly lower with the study contact lenses (2.0±1.0 mm²) than with the participants' own contact lenses (3.2±3.0 mm²) after 6 hr of wear, representing a mean staining decrease of 38% (P=0.010). Lid margin staining after 6 hr of wear of the study contact lenses was not different from that measured after 1 day without contact lenses (P=0.507). Limbal staining was also significantly less with the study contact lenses than with the participants' own contact lenses after 6 hr of wear (P=0.009). There was minimal upper corneal staining, and the degree was similar with the study and habitual lenses.

Conclusions:

Etafilcon A material, worn under a daily disposable modality, was shown to reduce upper lid margin and limbal staining in presbyopic contact lens wearers of Asian descent compared with the wearers' own contact lenses. Because of the high preponderance of dry eye amongst presbyopes, material selection is of importance and consideration should be given to the lens–ocular surface interaction.

Contact Lens-induced Limbal Stem Cell Deficiency

Limbal stem cell deficiency (LSCD) is a pathologic condition caused by the dysfunction and/or destruction of stem cell precursors of the corneal epithelium, typified clinically by corneal conjunctivalization. The purpose of this review is to critically discuss a less well-known cause of limbal stem cell disease: contact lens (CL) wear. A literature search was conducted to include original articles containing patients with CL-induced LSCD. This review describes epidemiology, diagnostic strategies, pathogenesis, differential diagnosis, and treatment modalities for this condition.

Tear exchange and contact lenses: a review

Tear exchange beneath a contact lens facilitates ongoing fluid replenishment between the ocular surface and the lens. This exchange is considerably lower during the wear of soft lenses compared with rigid lenses. As a result, the accumulation of tear film debris and metabolic by-products between the cornea and a soft contact lens increases, potentially leading to complications. Lens design innovations have been proposed, but no substantial improvement in soft lens tear exchange has been reported. Researchers have determined post-lens tear exchange using several methods, notably fluorophotometry. However, due to technological limitations, little remains known about tear hydrodynamics around the lens and, to-date, true tear exchange with contact lenses has not been shown. Further knowledge regarding tear exchange could be vital in aiding better contact lens design, with the prospect of alleviating certain adverse ocular responses. This article reviews the literature to-date on the significance, implications and measurement of tear exchange with contact lenses.

Corneal cell adhesion to contact lens hydrogel materials enhanced via tear film protein deposition

Tear film protein deposition on contact lens hydrogels has been well characterized from the perspective of bacterial adhesion and viability. However, the effect of protein deposition on lens interactions with the corneal epithelium remains largely unexplored. The current study employs a live cell rheometer to quantify human corneal epithelial cell adhesion to soft contact lenses fouled with the tear film protein lysozyme. PureVision balafilcon A and AirOptix lotrafilcon B lenses were soaked for five days in either phosphate buffered saline (PBS), borate buffered saline (BBS), or Sensitive Eyes Plus Saline Solution (Sensitive Eyes), either pure or in the presence of lysozyme. Treated contact lenses were then contacted to a live monolayer of corneal epithelial cells for two hours, after which the contact lens was sheared laterally. The apparent cell monolayer relaxation modulus was then used to quantify the extent of cell adhesion to the contact lens surface. For both lens types, lysozyme increased corneal cell adhesion to the contact lens, with the apparent cell monolayer relaxation modulus increasing up to an order of magnitude in the presence of protein. The magnitude of this increase depended on the identity of the soaking solution: lenses soaked in borate-buffered solutions (BBS, Sensitive Eyes) exhibited a much greater increase in cell attachment upon protein addition than those soaked in PBS. Significantly, all measurements were conducted while subjecting the cells to moderate surface pressures and shear rates, similar to those experienced by corneal cells in vivo.

Contact lens care solutions: a pilot study of ethnic differences in clinical signs and symptoms

OBJECTIVES

To determine whether Asian and white subjects differ in clinical signs or subjective symptoms in response to the use of different biguanide-preserved contact lens care solutions.

METHODS

Forty-two subjects (15 Asian and 27 white) wearing lotrafilcon B silicone hydrogel contact lenses used a preservative-free lens care solution bilaterally for 2 weeks, then used two biguanide-preserved solutions (solution 1: ReNu MPS; solution 2: AQuify MPS) contralaterally in randomly assigned eyes for 4 weeks. Comprehensive ocular surface examinations were performed and symptomatology questionnaires were administered every 2 weeks. Investigators were masked as to solution assignment during examinations, whereas subjects were not to avoid potential difficulties in compliance with the protocol.

RESULTS

With solution 1, most Asian and white subjects had grade 2 or greater corneal staining after 2 weeks (67% and 59%, respectively) and 4 weeks (60% and 67%, respectively). With solution 2, grade 2 or greater corneal staining occurred in 40% of Asians after 2 weeks and in 13% after 4 weeks, but in only 4% of whites after 2 weeks and 0% after 4 weeks. Whites reported significantly better average comfort (P=0.046) and less dryness (P<0.001) than did Asians.

CONCLUSIONS

Asians and whites differ in both ocular response to the use of contact lens care solutions and in reporting subjective symptoms. Racial and ethnic differences should be considered when evaluating and treating contact lens patients in a clinical setting.

Ethnic differences in dry eye symptoms: effects of corneal staining and length of contact lens wear

PURPOSE

To explore the relationships among length of contact lens (CL) wear, degree of corneal staining and severity of dryness symptoms, and to determine whether these relationships differ between Asians and non-Asians.

METHODS

Adapted soft CL wearers (n=395; 180 Asian, 215 non-Asian) were required to discontinue CL wear for at least 24h and report to the University of California, Berkeley Clinical Research Center (UCB-CRC). Fluorescein corneal staining was graded according to Brien Holden Vision Institute scales. Length of CL wear was reported by subjects and subjective dryness ratings were collected using the UCB-CRC Dry Eye Flow Chart (DEFC).

RESULTS

More Asian CL wearers exhibited corneal staining compared to non-Asians, and Asian CL wearers had a higher mean grade of corneal staining (p<0.001), as well as a higher mean DEFC classification (p<0.001). The difference between Asians and non-Asians in grades of corneal staining extent and depth were significant (p<0.001). Among non-Asian CL wearers, dryness symptoms decreased with more years of CL wear and increased in the presence of corneal staining, which was not the case for Asian CL wearers.

CONCLUSIONS

Asian soft CL wearers reported more severe dryness symptoms and demonstrated more severe corneal staining overall compared to non-Asians. Among non-Asians, dryness symptoms were less severe on average with increased years of CL wear and more severe in the presence of corneal staining. Dryness severity does not appear to be related to years of CL wear or corneal staining among Asians.

Corneal epithelial permeability: ethnic differences between Asians and non-Asians

PURPOSE

To ascertain whether a difference in the permeability of the corneal epithelium to fluorescein (Pdc) exists between Asians and non-Asians.

METHODS

From a multi-study database we extracted 632 records of baseline, open-eye Pdc measurements taken on both eyes of 176 subjects. Subjects were awake for a minimum of 4h before measurement, and were free of ocular disease and central corneal staining. Pdc was transformed by natural logarithm to better approximate normality for statistical tests.

RESULTS

The mean ln(Pdc) in the Asian group was significantly greater than in the non-Asian group [-2.34 ln(nm/s) vs. -2.58 ln(nm/s); p<0.001].

CONCLUSIONS

Compared with non-Asians, Asians exhibited a less negative ln(Pdc), which translates to a higher Pdc and a more permeable corneal epithelium. We speculate that this may be related to anatomic differences responsible for greater eyelid tension in Asians.

Scalloped Channels Enhance Tear Mixing Under Hydrogel Contact Lenses

PURPOSE

Tear exchange under a soft contact lens is directly related to the amount of lateral and transverse lens motion. Hydrodynamic modeling suggests that channels placed on the back surface of a soft lens will reduce fluid resistance and increase transverse lens movement. This study measured the effect of posterior lens surface scalloped channels on tear exchange.

METHODS

Tear exchange in the postlens tear film (PoLTF) was estimated using a fluorometer to measure the exponential depletion of high-MW fluorescein under the lens expressed as the time to deplete 95% of dye (T95). A total of 32 subjects wore two pairs of identical lenses except that the experimental lens had 12 scalloped channels placed radially in the midperiphery of the posterior lens surface, whereas lenses without channels served as controls.

RESULTS

The mean +/- standard error T95 values for the channel lenses was 28 +/- 2 minutes compared with 32 +/- 2 minutes for the control lenses (p = 0.107). There was a marginally significant difference in T95 between two lens groups in Asian eyes (p = 0.054).

CONCLUSION

Placing scallop-shaped channels on high-H2O content soft lenses improved the postlens tear pumping in Asian eyes.