Soft lens extended wear affects epithelial barrier function

Contact lens-related corneal infection: Intrinsic resistance and its compromise

Contact lenses represent a widely utilized form of vision correction with more than 140 million wearers worldwide. Although generally well-tolerated, contact lenses can cause corneal infection (microbial keratitis), with an approximate annualized incidence ranging from ~2 to ~20 cases per 10,000 wearers, and sometimes resulting in permanent vision loss. Research suggests that the pathogenesis of contact lens-associated microbial keratitis is complex and multifactorial, likely requiring multiple conspiring factors that compromise the intrinsic resistance of a healthy cornea to infection. Here, we outline our perspective of the mechanisms by which contact lens wear sometimes renders the cornea susceptible to infection, focusing primarily on our own research efforts during the past three decades. This has included studies of host factors underlying the constitutive barrier function of the healthy cornea, its response to bacterial challenge when intrinsic resistance is not compromised, pathogen virulence mechanisms, and the effects of contact lens wear that alter the outcome of host-microbe interactions. For almost all of this work, we have utilized the bacterium Pseudomonas aeruginosa because it is the leading cause of lens-related microbial keratitis. While not yet common among corneal isolates, clinical isolates of P. aeruginosa have emerged that are resistant to virtually all currently available antibiotics, leading the United States CDC (Centers for Disease Control) to add P. aeruginosa to its list of most serious threats. Compounding this concern, the development of advanced contact lenses for biosensing and augmented reality, together with the escalating incidence of myopia, could portent an epidemic of vision-threatening corneal infections in the future. Thankfully, technological advances in genomics, proteomics, metabolomics and imaging combined with emerging models of contact lens-associated P. aeruginosa infection hold promise for solving the problem - and possibly life-threatening infections impacting other tissues.

The importance of the Pseudomonas aeruginosa type III secretion system in epithelium traversal depends upon conditions of host susceptibility

Pseudomonas aeruginosa is invasive or cytotoxic to host cells, depending on the type III secretion system (T3SS) effectors encoded. While the T3SS is known to be involved in disease in vivo, how it participates remains to be clarified. Here, mouse models of superficial epithelial injury (tissue paper blotting with EGTA treatment) and immunocompromise (MyD88 deficiency) were used to study the contribution of the T3SS transcriptional activator ExsA to epithelial traversal. Corneas of excised eyeballs were inoculated with green fluorescent protein (GFP)-expressing PAO1 or isogenic exsA mutants for 6 h ex vivo before bacterial traversal and epithelial thickness were quantified by using imaging. In the blotting-EGTA model, exsA mutants were defective in capacity for traversal. Accordingly, an ∼16-fold variability in exsA expression among PAO1 isolates from three sources correlated with epithelial loss. In contrast, MyD88-/- epithelia remained susceptible to P. aeruginosa traversal despite exsA mutation. Epithelial lysates from MyD88-/- mice had reduced antimicrobial activity compared to those from wild-type mice with and without prior antigen challenge, particularly 30- to 100-kDa fractions, for which mass spectrometry revealed multiple differences, including (i) lower baseline levels of histones, tubulin, and lumican and (ii) reduced glutathione S-transferase, annexin, and dermatopontin, after antigen challenge. Thus, the importance of ExsA in epithelial traversal by invasive P. aeruginosa depends on the compromise enabling susceptibility, suggesting that strategies for preventing infection will need to extend beyond targeting the T3SS. The data also highlight the importance of mimicking conditions allowing susceptibility in animal models and the need to monitor variability among bacterial isolates from different sources, even for the same strain.

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.

Confocal microscopic observations of stromal keratocytes in soft and rigid contact lens wearers

PURPOSE

To determine the density of corneal stromal cells in wearers of soft contact lenses (SCLs) and rigid gas-permeable (RGP) contact lenses (CLs).

METHODS

The keratocyte density (KD) was measured at different depths of the stroma by confocal microscopy. In study 1, 32 wearers of rigid gas-permeable (RGP) lenses and 30 wearers of SCLs were studied. Forty volunteers with no history of CL wear were studied as controls. In study 2, 16 volunteers with no history of CL wear were divided into 2 groups; 7 subjects wore RGP lenses (oxygen transmissibility, Dk/L, 35) and 9 subjects wore SCLs (Dk/L, 34). All subjects were asked to wear the CLs daily for 6 months.

RESULTS

In study 1, the KDs in the anterior stroma (AST) and the posterior stroma (PST) of the cornea were significantly lower in the RGP lens group than in the control group. The KD in the SCL group was significantly lower at all depths of the cornea than that of the control group. In study 2, the KD in the AST of the RGP lens group was significantly lower after 1 month of CL wear. The KD in the AST and PST of the SCL group was decreased significantly at 1 month, and all layers were decreased by 10% to 20% 6 months after wearing CLs. At 5 weeks after discontinuation of SCL wear, the KD in all layers was not significantly different from that at the baseline.

CONCLUSIONS

The change in the KD was greater in CL wearers than in volunteers with no history of CL wear and also greater in SCL wearers than in RGP lens wearers. Analysis of the KD by confocal microscopy may be a useful method for evaluating the effect of CL wear.

Fluorescein punctate staining traced to superficial corneal epithelial cells by impression cytology and confocal microscopy

PURPOSE

The basis of fluorescein-associated superficial punctate staining in dry eyes is controversial. Prior explanations include fluorescein pooling in surface erosive defects, intercellular trapping of fluorescein, and intracellular staining in dead cells. In this study, the hypothesis that punctate erosions are individual cells with enhanced fluorescence was tested.

METHODS

Ten impression cytology membrane materials were compared, to optimize cellular yield in buccal mucosa and cornea. Clinicocytologic correlation of punctate fluorescent spots was performed in four dry eye patients. Individual punctate spots were localized by fiducial marks in photographs, before and after removal with impression membranes, and were traced in fluorescence microscopy and cytologic staining. Two-way contingency table analysis was used to determine the correlation of punctate spots with cells removed by the membrane. Clinicopathologic correlation of punctate spots was performed in 10 corneas removed in dry eye patients by transplantation for concurrent diseases. Punctate fluorescence was tracked in specimens by fiducial marks and epifluorescence. The distribution of fluorescent spots in specific cell layers of the cornea was determined by confocal microscopy.

RESULTS

Cellular yield was greatest with impressions from polytetrafluoroethylene (PTFE [Teflon]; BioPore; Millipore, Billerica, MA) membrane compared with its closest rival (P = 0.019). Punctate fluorescent spots, most of which disappeared after impression cytology (71%), correlated with cells on the membranes (P = 0.009). The punctate spots were more frequent in the superficial cell layers of the cornea (80%) compared with the deepest two layers (0%) (P < 0.00049).

CONCLUSIONS

Punctate epithelial erosions correspond to enhanced fluorescence in epithelial cells predominantly in superficial layers of the cornea and would be more aptly named fluorescent epithelial cells (FLECs).

Microbial keratitis in kingdom of bahrain: clinical and microbiology study

Background:

Microbial keratitis is a potentially vision threatening condition worldwide. Knowing the predisposing factors and etiologic microorganism can help control and prevent this problem. This is the first study of its kind in Kingdom of Bahrain.

Objective:

To study the profile of microbial keratitis in Bahrain with special focus on risk factors, clinical outcome and microbilogical results.

Methods:

A retrospective analysis of all patients admitted in Salmaniya Medical Complex over a period of three years from January 2005 to January 2007 was performed. A total of 285 patients with keratitis were analysed. Non infectious corneal ulceration were excluded. Data collected from medical records were demographic features, predisposing factors, history of corneal trauma, associated ocular conditions, visual acuity at the time of presentation and the clinical course. Predisposing risk factors measured were contact lens use, presence of blepharitis, diabetes, lid abnormalities, dry eyes, keratoplasty and refractive surgery. For contact lens wearers any contact lens related risk factors that can lead to keratitis were measured. Pearson's chi-square test was used to carry out statistical analysis wherever required.

Results:

Contact lens wear, as a risk factor for microbial keratitis, formed 40% of the total study population. Other risk factors identified were dry eyes 24 cases (8%), 10 blepharitis (3%), 22 trauma (8%), abnormal lid position 14 cases (5%). 6 patients keratitis in a graft (2%), 3 had refractive surgery (1%). The most common causative organism isolated was pseudomonas aeroginosa (54%) followed by streptococcus 12%, staph 10%, other organisms 6%. 95% of contact lens wearers had pseudomonas Aeroginosa. This was statistically significant (p< 0.0001). The vast majority, 92% healed with scarring. 1% needed therapeutic keratoplasty and 7% lost to follow up. Risk factors in contact lens wearers were; 41 patients (36%) slept with the contact lenses. 12 (8%) had contact lens related trauma and 8 (7%) had poor hygiene. Sleeping with the contact lenses was statistically significant (p< 0.0001).

Conclusion & Recommendation:

Contact lens wear is the major risk factor for microbial keratitis in Bahrain. Pseudomonas aeroginosa was the commonest bacteria isolated. Sleeping with the contact lenses is the major risk factor among contact lens wearers. Majority of keratitis patients resulted in permanent scarring on the cornea. Educating the public, especially on contact lens care and precaution, can help reduce this visual morbidity.

The effect of long-term wear of soft lenses of low and high oxygen transmissibility on the corneal epithelium

PURPOSE

To measure the effects of long-term wear of soft lenses of low and high oxygen transmissibility (Dk) on basal epithelial appearance and epithelial thickness.

METHODS

Sixty-three subjects were enrolled in a cross-sectional study. Seventeen high Dk lens wearers and 24 low Dk lens wearers who had worn lenses on an extended wear basis for more than 3 years (range: 3-22) were compared to a group of 22 age-matched subjects who had never worn contact lenses. Cell regularity and the intensity of light backscattered by the basal epithelium were assessed using confocal microscopy. Epithelial thickness was measured at the centre and at four peripheral locations using modified optical pachometry.

RESULTS

Epithelial basal cells appeared less regular in low Dk lens wearers than high Dk wearers or controls (Mann-Whitney U-test, P=0.001). The intensity of backscattered light did not differ across groups (Kruskal-Wallis test, P=0.37). Low Dk wearers had the thinnest epithelium (46 (10) microm), followed by high Dk wearers (54 (14) microm) and controls (58 (9) microm; ANOVA, P<or=0.006). Topographical position did not affect epithelial thickness (ANOVA, P=0.10).

CONCLUSIONS

Visible alterations to the basal epithelial cells can only be detected in long-term extended wearers of low Dk soft lenses. Extended wear of high Dk soft lenses results in topographically uniform epithelial thinning that is significantly less than the thinning seen with low Dk lenses. Confirmation of these findings using groups with evenly matched duration of lens wear is required.

Effects of a 0.9% sodium chloride ophthalmic solution on the ocular surface of symptomatic contact lens wearers

BACKGROUND

Ocular surface-related discomfort is the main reason for stopping contact lens wear. We carried out a study to evaluate the efficacy of preservative-free artificial tears containing 0.9% sodium chloride on ocular surface signs and symptoms in contact lens wearers experiencing discomfort and its possible influence on the duration of contact lens wear.

METHODS

We studied 49 contact lens wearers experiencing discomfort who had normal results of slit-lamp biomicroscopy, a fluorescein tear film break-up time (BUT) of 10 seconds or more, and wetting greater than 5 mm in 5 minutes on the Schirmer 1 test with and without anesthesia. Twenty-nine subjects (16 men and 13 women with a mean age of 32.5 years [standard deviation (SD) 8.7 years]) received one instillation of the 0.9% sodium chloride solution four times daily in the lower conjunctival fornix for 21 days. Twenty subjects (12 men and 8 women with a mean age of 35.1 [SD 6.2] years) received no drops and served as a control group. The overall comfort and duration of contact lens wear, results of tear film analysis and adverse events were recorded on days 7 and 21. Patients rated their symptoms (while not receiving any medications or hydrating solutions) on a 100-mm visual analogue scale with "Excellent (lenses not felt)" at the left and "Very uncomfortable (lenses cause irritation or discomfort)" at the right. Measurement of corrected visual acuity, slit-lamp examination, determination of the tear film BUT, the Schirmer 1 test with and without anesthesia, and assessment of the colour and surface of the lens were performed at baseline and at day 21. We analysed the data for the more uncomfortable eye or, if the eyes were equally uncomfortable, the right eye.

RESULTS

Significant lessening of ocular discomfort was observed in the treatment group during the study: the mean rating on the visual analogue scale at baseline was 60.2 mm (SD 12.7 mm), compared with 35.8 mm (SD 18.0 mm) at day 21 (p < 0.001, Student's t test). The duration of contact lens wear was significantly longer at day 21 than at baseline (7.0 [SD 2.6] hours vs. 6.4 [SD 2.6] hours, p < 0.05, Student's t test), and the proportion of subjects with conjunctival hyperemia was significantly lower at day 21 (48.3% vs. 82.8%, p < 0.05, chi2 test). No statistically significant changes were observed in tear film BUT, results of the Schirmer 1 test, corneal punctate staining by fluorescein or results of tear film analysis. The treatment was well tolerated by all patients. No significant differences in any of the variables studied were observed in the control group.

INTERPRETATION

Treatment with a preservative-free 0.9% sodium chloride ophthalmic solution reduced ocular surface discomfort and extended the duration of contact lens wear without interfering with the tear film or contact lens materials. Long-term studies are needed to confirm the role of this solution in reducing discomfort experienced by contact lens wearers.

What Does Overnight Lens Wear Do to the Corneal Epithelium?

The corneal epithelium plays a central role in preventing infectious keratitis. It is a layer that functions both as a physical barrier and as an active component of the ocular surface defense system. Contact lens wear is capable of altering the structure and physiology of the corneal epithelium, potentially increasing the risk of infectious keratitis. This brief review summarizes the effects of continuous contact lens wear and corneal refractive therapy on the physiology and structure of the corneal epithelium.

Contact lens-associated corneal infiltrates

PURPOSE This review article examines recent studies pertaining to contact lens-associated corneal infiltrates (CLACI) that occur in the absence of culture-proven microbial infection. METHODS The literature was reviewed in regard to the clinical appearance, incidence and risk, etiology, pathophysiology, differential diagnosis, and management of CLACI. Recent insights are presented in the context of future directions for prevention of CLACI. RESULTS Contact lens-associated corneal infiltrates may manifest in various forms that require careful observational skills to ensure proper diagnosis. Although the reported incidence of CLACI varies widely, even a low percentage of contact lens wearers would constitute a substantial number of affected individuals. Any one or a combination of multiple mechanical, hypoxic, or toxic stimuli associated with contact lens use can induce proinflammatory responses that lead to infiltration of inflammatory cells into the cornea. A number of candidate cytokines, chemokines, adhesion molecules, and so forth have been identified. In addition to differentiation from microbial keratitis, CLACI also should be differentiated from ocular disorders not associated with contact lenses but involving corneal infiltrates and from contact lens-associated disorders that may resemble infiltrates. Management of CLACI can range from simple monitoring of the patient to the use of pharmacologic intervention. CONCLUSIONS The small percentage of affected lens wearers translates into a notable number of individuals who, although not experiencing a vision-threatening event, are inconvenienced by the development of infiltrates. Design of preventive measures for CLACI should focus on the elimination of various mechanical, hypoxic, and toxic stimuli that can induce infiltrates and on the approaches for molecular intervention of the inflammatory cascade initiated by the stimuli.

Corneal epithelial thinning profile induced by long-term wear of hydrogel lenses

PURPOSE

To characterize the epithelial thickness profile and study the effects of long-term wear of hydrogel lenses on this profile.

METHODS

A cross-sectional study was designed. Epithelial thickness was evaluated in 15 subjects who had worn one of two types of low oxygen transmissibility (Dk) hydrogel contact lenses for an average of 10 years (range, 7-16) and compared with a group of 18 control subjects who had never worn contact lenses. Epithelial thickness was measured at the center and at four mid-peripheral and four peripheral locations in the vertical and horizontal meridians of the cornea using a modified optical pachymeter.

RESULTS

Lens wearers had significantly thinner epithelium than controls [analysis of variance (ANOVA), p < 0.001] in the central (41 +/- 7 microm versus 48 +/- 5 microm), mid-peripheral (41 +/- 7 microm versus 48 +/- 7 microm), and peripheral (42 +/- 9 microm versus 48 +/- 6 microm) cornea. The extent of this difference ranged from 8.7% to 18.4% of the total epithelial thickness, was not associated with the duration of wear (ANOVA, p = 0.87, power = 0.05), and was significantly greater for the lower Dk contact lens type (ANOVA, p < 0.001). Topographical position did not have a significant effect on epithelial thickness (ANOVA, p > 0.13, power > 0.22).

CONCLUSION

This study establishes that the epithelial thinning associated with hydrogel lens wear is topographically uniform. It also confirms that this effect is inversely related to lens oxygen transmissibility but does not appear to increase with longer duration of wear.

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.

The pathogenesis of bacterial keratitis: studies with Pseudomonas aeruginosa

Bacterial keratitis is a sight-threatening corneal disease that is most commonly associated with the extended wear of soft contact lenses. Over the past decade, we have investigated the pathogenesis of infectious keratitis involving the opportunistic pathogen Pseudomonas aeruginosa. Our research has focused on understanding the respective roles of bacteria and host in the establishment of this infection. Here, we provide a current perspective on P. aeruginosa keratitis, reviewing some of the research developments that have helped shape our views on the mechanisms by which pathogen and host response cause corneal disease. P. aeruginosa may provide a model for the pathogenesis of bacterial keratitis and help further elucidate the complex array of host factors that normally protect the cornea from infectious agents.

The Berkeley Contact Lens Extended Wear Study. Part II : Clinical results

OBJECTIVE

To describe the principal clinical outcomes associated with 12 months use of rigid gas-permeable (RGP) extended wear contact lenses and address two primary study questions: (1) does extended wear (EW) of high oxygen transmissibility (Dk/t) RGP lenses reduce the incidence of ocular complications, and (2) does the wearing of high-Dk/t lenses reduce the rate of failure to maintain 6-night RGPEW over 12 months?

DESIGN

A randomized, concurrently controlled clinical trial.

INTERVENTION

Subjects who adapted to EW with high Dk (oxygen permeability) RGP lenses were randomized to either high Dk or medium-Dk RGP lenses for 12 months of 6-night EW.

MAIN OUTCOME MEASURES

Contact lens-associated keratopathies (CLAK), changes in refractive error and corneal curvature, and survival in EW.

RESULTS

Two hundred one subjects were randomized to medium or high-Dk lenses for 12 months of EW. Sixty-two percent of the subjects in each group completed 12 months of EW; however, the probability of failure was significantly greater for the medium-Dk group. Although the risk of complications was similar for the two groups, the number of CLAK events that led to termination were 16 versus 5 for the medium-Dk and high-Dk groups, respectively. This suggests that the type of adverse response or the inability to reverse an adverse event was different for the group being exposed to the lower oxygen dose.

CONCLUSIONS

The level of oxygen available to the cornea has a significant impact on maintaining successful RGP extended contact lens wear, but not on the initial onset of CLAK. The number of clinical events leading to termination was substantially higher for the medium Dk group, which suggests that corneal hypoxia is an important factor in the development of CLAK. Although overnight contact lens wear should be recommended with caution and carefully monitored for early detection of ocular complications, it appears that high-Dk RGP lenses can be a safe and effective treatment for correction of refractive error for most individuals who can adapt to EW.

Short term wear of high Dk soft contact lenses does not alter corneal epithelial cell size or viability

BACKGROUND/AIMS

Current contact lenses (CLs) when worn on an extended wear basis cause corneal epithelial alterations. The aim of this study was to evaluate changes in corneal epithelial cell morphology and physiology following short term (3 months) wear of highly oxygen permeable CLs and to compare this with disposable CLs.

METHODS

Subjects were wearers of highly oxygen permeable CLs (n=11, wearing CLs on a 30 night schedule), disposable CL users (n=6, wearing CLs on a 6 night schedule), and non-CL wearers (n=20). Mean CL wear experience was 3 months. Epithelial cells were harvested using corneal cytology and were stained using acridine orange and ethidium bromide. Epithelial cell size and viability were determined.

RESULTS

The majority of epithelial cells recovered were non-viable (71%), and the mean longest cell diameter was 38 (SD 8) microm. Disposable CLs caused an increase in cell size (42 (7) microm) compared with both non-wear (39 (7) microm, p=0.01) and wear of highly oxygen permeable CLs (37 (10) microm, p=0.0049). There was no difference in cell viability between groups.

CONCLUSIONS

Extended wear of disposable CLs caused an 8% increase in cell diameter in harvested corneal epithelial cells following 3 months of CL wear. Cells harvested following 3 months' wear of highly oxygen permeable CLs were indistinguishable from those recovered from non-CL wearers.

Twenty-five years of contact lenses: the impact on the cornea and ophthalmic practice

PURPOSE

The history of contact lenses has occurred in the latter half of the 20th century. In particular, events in the 1970s through the 1980s related to the invention of soft, hydrogel contact lenses have revolutionized the contact lens industry and the eye care attached to it. This article recounts that history from the perspective of market forces, inventions, and discoveries about the physiologic functioning of the cornea.

METHODS

The relevant literature is critically reviewed.

RESULTS

Discoveries about the oxygen needs of the cornea and consumer pressure for clear, comfortable, around-the-clock vision have resulted in a history of rigid gas permeable and soft lenses that leads to today's contact lens picture. The short-term and long-term effects of chronic hypoxia and the levels of lens oxygen transmissibility necessary to avoid them have been well-described. The advent of the soft lens, followed by the "human experiment" with initial extended-wear modalities, led to the advent of the disposable soft contact lens.

CONCLUSIONS

In the past 25 years, the development and wide acceptance of soft contact lenses have revolutionized the management of refractive error and corneal diseases.

Effects of hypoxia on corneal epithelial permeability

PURPOSE

Epithelial permeability to fluorescein (Pdc) increases with closed-eye soft contact lens wear. Possible mechanisms responsible for this change include corneal hypoxia (and acidosis) during overnight wear and interference of the lens with normal tear exchange. In this study we used a local environmental chamber to examine the effects of hypoxia on Pdc.

METHODS

After baseline slit-lamp examination, autofluorescence, and central corneal thickness measurements, 35 subjects were fitted with a pair of airtight swimming goggles. One eye was exposed to a humidified gas mixture containing 95% nitrogen (N2) and 5% carbon dioxide (CO2), and the fellow (control) eye was exposed to air. The experimental eye and the eye measured first were assigned using a randomized block design. After the subjects wore the goggles for 1 hour, corneal thickness measurements were repeated and Pdc was assessed as previously described. A final slit-lamp examination was performed by a masked examiner to assess epithelial integrity.

RESULTS

There was no significant difference in the mean ln(Pdc) (95% confidence interval) for eyes exposed to N2/CO2 when compared with paired eyes exposed to air (-2.70 [-2.99, -2.41] vs -2.51 [-2.72, -2.30] ln[nm/sec], P = .272). Although there was also no substantial difference in the slit-lamp appearance of the experimental and control eyes, the mean (95% confidence interval) change in corneal thickness for the hypoxic eyes was 19.78 (15.72, 23.84) microm compared with 3.40 (-0.67, 7.47) microm (P < .0001) in the control eyes.

CONCLUSION

One hour of hypoxia caused a significant increase in corneal thickness with no apparent change in Pdc. This suggests that other factors, either alone or in combination with hypoxia, may be responsible for increased Pdc during closed-eye contact lens wear.