Meng C. Lin

Hypoxia, overnight wear, and tear stagnation effects on the corneal epithelium: data and proposed model.

Purpose. To explore the possible mechanisms that may lead to overnight contact lens–associated corneal morbidity by examining data from several corneal epithelial permeability experiments obtained under different environmental and lens fitting paradigms. Methods. Epithelial permeability was assessed by using fluorometry to determine the fluorescein penetration rate from the tear film into the corneal stroma. Changes in this rate provide an index of the corneal epithelial status; increased permeability leads to decreased barrier function. Results. Hypoxia and tear stagnation during overnight lens wear play a significant role in altering the corneal epithelial barrier function (P<0.05). However, eliminating lens-induced hypoxia alone does not ameliorate changes in epithelial status (P<0.05). Conclusions. Based on data from these experiments, it is suggested that hypoxia and tear stagnation should be eliminated to minimize alteration of the corneal epithelium associated with overnight contact lens wear.

Diurnal Pattern of Tear Osmolarity and Its Relationship to Corneal Thickness and Deswelling

Purpose: To identify the diurnal variations of tear osmolarity (TO) and its relationship with central corneal thickness (CCT) and corneal deswelling over a 14-hour period. Methods: TO and CCT were measured using the TearLab Osmometer and Bioptigen spectral domain optical coherence tomography, respectively, on 38 healthy neophytes (mean age, 21.5 ± 2.2 years). TO and CCT were measured at bedtime (baseline), upon awakening, 20 minutes, 40 minutes, 1 hour, 2 hours, 4 hours, and 8 hours after awakening. Deswelling rate was estimated and expressed as percent recovery per hour (PRPH). Mixed-effect linear regression models describe the relationships among TO, CCT, and PRPH. Results: The tear film upon wakening (264 ± 14 mOsm/L) was hypoosmotic compared with baseline (297 ± 15 mOsm/L, P < 0.001). TO (in mOsm/L) at 20 minutes, 40 minutes, 1 hour, 2 hours, 4 hours, and 8 hours were 287 ± 10, 292 ± 16, 293 ± 12, 292 ± 10, 289 ± 10, and 286 ± 10, respectively. CCT (mean ± SD) at baseline was 552.2 ± 35.9 µm and increased to 572.0 ± 38.7 µm after sleep. CCT returned to baseline thickness 4 hours after awakening (P < 0.000) and remained stable throughout the day. A small but statistically significant association was found between higher TO and lower CCT (P < 0.0001) and between lower baseline TO and higher PRPH (faster deswelling; P < 0.0001). Conclusions: The diurnal pattern of TO has been established. The association of TO with corneal thickness and deswelling suggests that the tear film tonicity may be partly responsible for corneal hydration control; however, the effect may not be of clinical significance in a normal study cohort.

Contact Lenses Wettability In Vitro: Effect of Surface-Active Ingredients

Purpose. To investigate the release of surface-active agents (surfactants) from unworn soft contact lenses (SCLs) and their influence on the lens surface wettability in vitro. Methods. Surface tension (ST) of blister pack solutions was measured by pendant-drop technique. STs at the air-aqueous interface and contact angles (CAs) of four conventional and seven silicone hydrogel SCLs were evaluated in a dynamic-cycling regime using a modified captive-bubble tensiometer-goniometer. Measurements were performed immediately after removal from blister packs, and after soaking in a glass vial filled with a surfactant-free solution, which was replaced daily for 1 week. Lens surface wettability was expressed as adhesion energy according to Young equation. Results. STs of all blister pack solutions were lower than the reference ST of pure water (72.5 mN/m), indicating the presence of surfactants. When lenses were depleted of surfactants by soaking, the STs for all studied lenses and advancing CAs of selected lenses increased (p < 0.001). Receding CAs of all studied lenses were 12° ± 5° and were not affected by the presence of surfactants. For most of the conventional lenses, the surface wettability was largely dependent on surfactants, and reduced significantly after surfactant depletion. In contrast, most silicone hydrogel lenses exhibited stable and self-sustained surface wettability in vitro. Conclusions. The manufacturer-added surfactants affected wetting properties of all studied SCLs, although to different degrees.

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

Contact lens extended wear increases the permeability of epithelium to sodium fluorescein (P(dc)). The exact mechanism is not known. However, changes in P(dc) likely result from either corneal hypoxia or mechanical trauma, or both. We explored the effects of one-night continuous wear with either high- or low-Dk/t soft lenses on P(dc). The results show that corneal epithelial barrier function decreases significantly with both lens groups. We also observed that Asian eyes had higher P(dc) after overnight wear compared to non-Asian and that for both Asian and non-Asian eyes, the elimination of corneal hypoxia did not prevent changes in epithelial permeability.

Tear lipids interfacial rheology: effect of lysozyme and lens care solutions.

Purpose. To evaluate the interfacial properties of ex vivo tear lipid multilayers with controlled and varying thickness. The influence of lysozyme and surfactant-containing multipurpose lens care solution (MPS) on interfacial rheology of lipids and mixed lipid-protein films were studied. Methods. Lipids were extracted from lotrafilcon A lenses worn continuously for 1 month. Interfacial properties of the lipids without and with lysozyme in the aqueous phase were examined using tensiometry and step-strain relaxation. Lipid-lysozyme multilayers were exposed to either diluted opti-free express (OFX) or opti-free replenish (OFR) for 30 min, and then MPS was displaced from the bulk phase. Surface tension and rheological parameters before and after MPS exposure were measured and compared. Results. Thick lipid films exerted high surface pressure at the air-aqueous interface, 50 ± 2 mN/m, with little inter- and intrasubject variability. The rheological storage modulus (E∞; 25.3 ± 2 mN/m) and relaxation time (&tgr;; 87 ± 25 s) were similar among subjects. Neither lysozyme nor MPS changed the surface tension of the lipid multilayers. Lysozyme adsorbed irreversibly onto multilayers without changing E∞, but increased &tgr; 2.5 times. Exposure of mixed multilayers to OFX reduced E∞ to less than a half of its original value (13 ± 4.5 mN/m; p < 0.001), whereas after OFR exposure, small but statistically significant changes in E∞ were found (21 ± 3.0 mN/m; p = 0.0044). MPS-treated mixed films relaxed much slower than an untreated one, &tgr; increased 4.5 times after OFX and 7 times after OFR exposure. Conclusions. Thick multilayers of the ex vivo tear lipids have exhibited surface tension 15 ± 1 mN/m lower than reported in the literature for meibomian lipids. The lipid-lysozyme interaction altered the interfacial rheology of the ex vivo lipids. OFX and OFR changed rheological properties of the mixed films to different extents.

The effects of ocular and lens parameters on the postlens tear thickness.

Purpose. To assess the effects of soft contact lens base curve radius (BCR), sex, ethnicity, central corneal curvatures, and vertical palpebral aperture size (PAS) on the postlens tear thickness (PLTT). Methods. The PLTT was measured using optical pachometry on 114 experienced lens wearers who were fitted with lathe-cut soft lenses (Alden 47, polymacon, 35.5% H2O, −2.00 diopter, and 14.0 mm). Each subject was randomly allocated to one lens group receiving a BCR of 7.9, 8.3, or 8.7 mm. Pachometry measurements were taken at 30 min after lens insertion. Vertical PAS and keratometry readings were measured for 94 of the 114 subjects. Results. The mean (95% confidence interval) PLTT was 15.7 &mgr;m (13.2–18.0 &mgr;m), 12.8 &mgr;m (10.9–14.7 &mgr;m), and 12.1 &mgr;m (10.2–14.0 &mgr;m) for the 7.9-mm, 8.3-mm, and 8.7-mm BCR groups, respectively. The differences in PLTT among the three BCR groups was significant (analysis of variance F-test;P =0.039). Post hoc testing using the Tukey honestly significant difference statistic showed that only the two extreme BCR groups (7.9 mm and 8.7 mm) were significantly different. Sex had no effect on the PLTT; however, the PLTT was significantly thinner for the Asian compared with non-Asian eyes (P =0.0001). The Asian PLTT did not vary with different soft lens BCRs. The non-Asian PLTT was thicker with lenses of the steep BCR compared with the flat BCR. Conclusion. These results show that the base curve radius of a soft contact lens and several ocular characteristics can affect the thickness of the postlens tear film.

Mechanical complications induced by silicone hydrogel contact lenses.

Abstract: With the introduction of silicone hydrogel (SiHy) lenses over a decade ago, clinicians have seen both improvements and challenges in contact lens (CL) wear. Regardless of lens design or material, the presence of a CL on the ocular surface induces mechanical complications. Although some of these complications have diminished in frequency and severity with newer generations of SiHy lenses, others persist at previously reported levels. The aim of this review is to provide up-to-date information on mucin balls, superior epithelial arcuate lesions, corneal erosions, CL-induced papillary conjunctivitis, conjunctival epithelial flaps, lid wiper epitheliopathy, and meibomian gland dropout. The conclusions in this review should provide a sound basis for identifying the future areas of research to help minimize mechanically driven adverse events during CL wear with SiHy lenses.

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.

Relationships among Tear Film Stability, Osmolarity, and Dryness Symptoms

Purpose To examine the relationships among tear osmolarity, tear film stability, and several measures of dry eye (DE) symptoms in a multivariable analysis. Methods A cross-sectional study was conducted with 137 subjects (68 non–contact lens [CL] wearers and 69 soft CL wearers) recruited from a university campus. Tear breakup time (TBUT) was measured noninvasively (NITBUT) and with fluorescein (FTBUT). Tear osmolarity was measured by an osmometer. Dry eye symptoms were assessed using the Dry Eye Flow Chart and several different questionnaires. Results Subjects ranged in age from 18 to 67 years, with a mean of 28 years. Subjects had a mean (SD) osmolarity of 293 (10) mOsm/L, NITBUT of 14.1 (10.9) seconds, and FTBUT of 14.8 (12.6) seconds. Shorter NITBUT and FTBUT were significantly associated with female sex (p = 0.001 and p = 0.027, respectively) and Asian ethnicity (p = 0.030 and p = 0.004, respectively). There were no clinically significant relationships between tear osmolarity and FTBUT, NITBUT, or DE symptoms. Higher Dry Eye Flow Chart score (i.e., worse symptoms) was associated with older age (p < 0.001), female sex (p = 0.014), CL wear (p < 0.001), shorter NITBUT (p < 0.001), and shorter FTBUT (p = 0.028). The sensitivities and specificities for using clinical measurements to diagnose moderate to severe DE were as follows: osmolarity, 0.67 and 0.46, respectively; NITBUT, 0.72 and 0.52, respectively; and FTBUT, 0.68 and 0.57, respectively. Conclusions In a population of asymptomatic, mild and moderate DE patients, increased tear osmolarity was not significantly associated with reported symptom severity and frequency. Tear osmolarity, NITBUT, and FTBUT exhibited similar sensitivities and specificities when used to diagnose moderate to severe DE.

Factors in Contact Lens Symptoms: Evidence from a Multistudy Database

Purpose This study aimed to examine the effects of demographic, lens performance, and ocular surface response measures on contact lens–related discomfort and dryness, using a large contact lens study database. Methods A total of 4164 records were extracted from a database of 220 subjects participating in 46 silicone hydrogel contact lens studies. Subjects discontinued lens wear for 24 hours and were then fit with either comfilcon A or enfilcon A lenses. The fit and performance of the lenses, along with subjective comfort and dryness, were assessed on insertion and after 3 and 6 hours of wear. After 6 hours of wear, ocular surface health was also assessed by fluorescein slitlamp examination. Results Decreased comfort at 3 hours after insertion was associated with excessive lens movement (p < 0.001), front surface deposits (p = 0.004), poor wettability (p = 0.014), and Asian ethnicity (p < 0.001). After 6 hours of wear, decreased comfort remained associated with greater lens movement (p = 0.032) and Asian ethnicity (p < 0.001), along with inferior corneal staining (p < 0.001). Dryness after 3 hours of wear was associated with greater lens movement (p < 0.001), Asian ethnicity (p < 0.001), increased deposits (p < 0.001), and poor wettability (p < 0.001). Dryness after 6 hours of wear remained associated with greater lens movement (p < 0.001) and Asian ethnicity (p < 0.001), along with inferior corneal staining (p < 0.001) and inferior lens decentration (p = 0.001). Conclusions Excessive lens movement, inferior lens decentration, poor surface wettability and deposits, inferior corneal staining, and Asian ethnicity are associated with discomfort and dryness. Clinicians should consider all these factors to achieve the most comfortable and successful contact lens fit.