The relationship between corneal oxygen tension and hypoxic corneal edema.

PURPOSE: We sought to develop a direct, noninvasive measurement of tear oxygen tension behind soft contact lenses, and to use the tear oxygen tension data to determine if subjects with greater comeal oxygen consumption experience more comeal edema under hypoxic conditions. The relationship between tear oxygen tension and comeal thickness was also analyzed to determine if thicker comeas have higher metabolic rates. METHODS: Tear oxygen tension measurement was based on the quenching of a phosphorescent dye by oxygen. By coating a soft contact lens with the dye and placing it in the eye, the rate of decay of phosphorescence could be measured and the tear oxygen tension calculated. Corneal oxygen consumption could then be calculated from the tear oxygen tension. Central corneal swelling was measured after 3 hours of hypoxia induced by contact lens wear under closed-eye conditions. RESULTS: Seven right eyes of seven subjects were tested. Baseline central corneal thickness ranged from 517 to 616 microns. After 3 hours of hypoxia, central corneal swelling ranged from 24.5 to 74.3 microns (3.9% to 13.4%). Steady-state open-eye tear PO2 behind the lens varied between 42.7 and 67.1 torr Pearson correlation suggested possible inverse relationship between open-eye tear oxygen tension and central comeal swelling during hypoxia (r = -0.711, p = 0.0734). CONCLUSIONS: Phosphorescent-based measurements provided a measure of tear oxygen tension in human beings. Low tear film oxygen tensions (high corneal metabolic demand) appeared to predict greater amounts of swelling in subjects' corneas. No relation was found between corneal thickness and tear oxygen tension, suggesting that epithelial and endothelial metabolic rates are more important than stromal thickness in determination of comeal oxygen demand.     

Scuba decompression illness (DCI) and hydrophilic contact lens wear: a case report

A 36-year-old male occupational abalone diver wearing hydrophilic contact lenses presented for after-care. He had been diving earlier the same day and had exceeded accepted diving standards for depth, time of dive and ascent times. He exacerbated these problems by flying in an unpressurised aircraft afterwards. At the time of his optomet-ric examination, he was in the early stages of decompression illness (DCI). Both corneas exhibited gas bubble formation in the epithelial layers and these bubbles were also present within the tear film beneath the hydrophilic lenses. Areas involved gave punctate staining of the corneal epithelium. He refused referral for specialised hyperbaric treatment. Review two months later showed no residual signs of any corneal cellular irregularities. (Clin Exp Optom 1995; 78: 1: 14–17)     

Soft and hard contact lenses worn in combination

Each of 5 subjects with normal corneas was fitted with a Bausch & Lomb F3-series Soflens contact lens and with an experimental F3-series ultrathin Soflens. A best-fit PMMA lens was worn in combination with each of the soft lenses. In addition, a best-fit CAB lens and a tight PMMA lens were separately worn in combination with the ultrathin soft lens. Each of 3 subjects with keratoconus was fitted with an ultrathin soft lens combined with a PMMA lens. For the normal corneas, the combination that produced the least corneal edema after 5 hr was an ultrathin soft lens with either a PPM or CAB lens of best fit. Two of the 3 keratoconic subjects were able to wear their contact lens combination for the 5-hr test period; corneal swelling was 1.7% and 5.3% For all subjects, acuity with a combination co ntact lens system was better than with a soft or hard lens alone.     

Rigid gas permeable contact lenses in hyperbaric environments

We studied the wearing of hard gas permeable contact lenses during decompression in a hyperbaric environment. Twenty-four exposures of three different lens materials were carried out in a hyperbaric chamber. The dry air dive profiles were: 45.72 m (150 ft) for 30 min and 15 min; 22.86 m (75 ft) for 15 min; and 11.43 m (37.5 ft) for 15 min. Bubbles occurred under all lens types; they were seen at simulated depths as shallow as 1.98 m (6.5 ft) after the least stressful exposure. Due to the tissue offgassing half-time of the eye, it is unlikely that the bubbles are a result of nitrogen from the eye. They probably arise from the tear layer.     

Decompression-induced ocular tear film bubbles reflect the process of denitrogenation

PURPOSE: To confirm that the tear film bubbles observed after decompression from hyperbaric exposure are due to denitrogenation and to assess the time course of denitrogenation based on the number of ocular tear film bubbles. METHODS: The study comprised two parts. In the first, subjects (n = 8) were compressed to a pressure of 2.0 ATA (atmospheres absolute; depth of 10 meters of sea water [msw]) for 60 minutes in a hyperbaric chamber on two separate occasions. On one occasion they breathed air, whereas on the second occasion they inspired pure oxygen. Before and within 30 minutes after each dive, the subjects' tear film was examined with a slit lamp microscope and the average number of bubbles recorded. Ultrasound reflectivity of the lens-vitreous humor compartments was also examined. In the second part of the study, subjects (n = 8) participated in two simulated dives in the hyperbaric chamber: 4.0 ATA (30 msw) for 15 minutes and 2.5 ATA (15 msw) for 180 minutes. The former was a no-stop decompression dive, whereas the latter required a 43-minute decompression stop at 3 msw. Ocular tear film examinations were conducted before the dive, as well as 30 minutes and 1 day, 2 days, and 3 days after the dives. RESULTS: The number of tear film bubbles increased significantly (P < 0.05) after the air dives to 2.0 ATA for 60 minutes, whereas there was no significant postdecompression increase in tear film when oxygen was inspired by the subjects during the dive. Posterior lens-vitreous humor reflectivity increased significantly after decompression from 2 ATA, when air was the breathing mixture, whereas no change in reflectivity was observed when oxygen was inspired during the dive. In the second part of the study, there was a significant elevation in tear film bubbles for 2 days after the two dives (P < 0.05). There was no significant difference in the number of ocular tear film bubbles between the two dives. CONCLUSIONS: After a hyperbaric air exposure, tear film bubbles reflect the process of denitrogenation, which may persist for up to 2 days after the decompression.     

Effect of hydrogel lens wear on tear film stability

Noninvasive break-up time (NIBUT) of the tears was measured in a controlled, randomized, double-masked study to assess: (1) the stability of the prelens tear film during wear of new high and low water content lenses and (2) the stability of the precorneal tear film following lens removal after 1 h of wear. The prelens tear film NIBUT of 6 subjects was found to be relatively constant over a 1-h wearing period, averaging 6.1 +/- 1.1 s (mean +/- SEM). These values were significantly (Scheffe's S test, p less than 0.05) lower than those recorded for the precorneal tear film before lens insertion (33.5 +/- 10.6; mean +/- SEM), although 85% of prelens tear film NIBUT's were greater than the 3-s average interblink period reported previously for soft lens wearers. After lens removal, precorneal tear film NIBUT was reduced significantly compared to prewear levels (Scheffe's S test, p less than 0.05) for up to 15 min. Application of the monomolecular growth model to the NIBUT recovery data revealed a half-time for recovery of 6.0 min, with recovery 95% complete 25.8 min after lens removal. Lens type was not a significant factor in tear film stability, either during wear or after lens removal. The basis for reduced precorneal tear film NIBUT after lens removal is unknown; however, a disruption of the mucin layer coating the corneal epithelium is the most likely mechanism. Indeed, the technique of measuring precorneal tear film NIBUT after lens removal may be a useful determinant of the extent to which contact lens wear disrupts the precorneal mucin layer, providing an indication of the susceptibility of the cornea to a variety of complications.     

Influence of palpebral aperture height on tear pump efficiency

Oxygen uptake rates were measured on 6 human corneas fitted with a standardized rigid contact lens in order to determine the relation between vertical palpebral aperture height and exchange efficiency of tears in such systems. The contribution provided by tear exchange to the oxygen available to the cornea was isolated through the use of lenses of negligible transmissivity (PMMA). Measurements of corneal oxygen uptake were made on the normal open eye, after 5 min of static (with no blinking) open eye contact lens wear, and after 5 min of dynamic (with blinking every 5 s) contact lens wear. The difference between the oxygen demand associated with static conditions (SC's) and that associated with dynamic conditions (DC's) is an index of the tear pump efficiency for each subject. A statistically significant correlation (p less than 0.0001) was found between the vertical palpebral aperture height and the tear bulk-flow of oxygen to the cornea, confirming that small palpebral apertures can restrict lens motion.     

Influence of rigid contact lens base curve radius on tear pump efficiency.

The effects of rigid contact lens base curve radius changes on tear pump efficiency were determined by measuring oxygen uptake on six with-the-rule corneas (with toricities between 0.37 and 1.00 D) under three conditions: (1) normal open eye, (2) after 5 min of static (without blinking) wear of polymethyl methacrylate (non-gas permeable) contact lenses, and (3) after 5 min of dynamic (with blinking once every 5 s) wear of the same lenses. The difference between the oxygen uptake rates measured under static and dynamic conditions provides an index of tear pump efficiency. Each subject wore a series of five lenses: fitted on K and 0.05 or 0.10 mm steeper and flatter than the flattest corneal meridian. The overall diameter of all lenses was 8.8 mm, the optic zone was 7.4 mm, the axial edge lift was 0.09 mm, and all other parameters were constant. The contact lens fitted on K was associated with the greatest central corneal oxygen debt under static conditions, whereas the same lens also provided for the best tear exchange with the blink. In addition, better tear exchange was found to be associated with larger palpebral aperture sizes.     

Ocular surface changes with applanation contact lens and coupling fluid use after argon laser photocoagulation in noninsulin-dependent diabetes mellitus

PURPOSE: To describe the effect of coupling solutions used during laser photocoagulation on the ocular surface of patients with noninsulin-dependent diabetes mellitus (NIDDM). DESIGN: A prospective case-controlled study. METHODS: Ninety-two eyes of 46 NIDDM patients with clinically significant macular edema, poor metabolic control of diabetes, and peripheral neuropathy and 100 eyes of 50 normal control subjects were studied. The patients' eyes were assigned to argon green focal/grid laser photocoagulation using an applanation contact lens and one of the coupling fluids; 2% methocel, Thilo-Tears Gel, 1.4% sodium hyaluronate, or 0.9% simple saline. The control subjects received time-matched three-mirror contact lens fundus examinations. All subjects underwent corneal sensitivity measurements, Schirmer test, tear film breakup time, and corneal fluorescein staining before as well as 3 and 8 days after the laser procedures and contact lens examinations. Patients with corneal problems persisting after 8 days were followed longer. RESULTS: Diabetic eyes assigned to 2% methocel and 1.4% sodium hyaluronate had significantly lower mean corneal sensitivities and break-up time values as well as significantly higher mean fluorescein staining scores at all examination points after laser photocoagulation. All diabetic eyes with aqueous deficiency assigned to 2% methocel and 1.4% sodium hyaluronate developed delayed corneal epithelial healing. CONCLUSION: The use of viscous coupling solutions during applanation contact lens-aided laser procedures may be detrimental for the corneal epithelium in poorly controlled NIDDM patients with peripheral neuropathy and coexisting aqueous deficiency.     

Precorneal and pre- and postlens tear film thickness measured indirectly with optical coherence tomography

PURPOSE: To demonstrate the feasibility of indirectly measuring the precorneal tear film thickness and pre- and postlens tear film (PLTF) thickness using optical coherence tomography (OCT). METHODS: Central corneal thickness (C(1)) which includes the tear film (T) of both eyes of 40 non-contact lens wearers was measured using OCT after calibration. The mean age of the 40 subjects was 31.2 +/- 9.3 years with a mean horizontal K-reading of 7.87 mm. Rigid contact lenses with base curves 0.3- to 0.5-mm steeper than the flattest K of the eye were fitted to measure real corneal thickness (C(2)), independently of the postlens tear film. T was calculated by T = C(1) - C(2). To measure pre- and postlens tear film thickness, Focus Night & Day and Acuvue lenses (Vistakon, Johnson & Johnson Vision Care, Jacksonville, FL) were fitted on both eyes. Central soft lens thickness (L(1)), which includes the prelens tear film (P), was measured by OCT in situ and in saline in a wet cell (L(2)). P was calculated by P = L(1) - L(2). Thickness of the central cornea plus the postlens tear film (C(3)) was measured during lens wearing. Postlens tear film (PLTF) was calculated by PLTF = C(3) - C(2). RESULTS. The mean +/- SD precorneal tear film thickness was 3.3 +/- 1.5 microm (range, 0-6.9) before lens insertion and 4.7 +/- 2.3 microm (range, 0.7-11.0) after lens fitting, which was significantly thicker (paired t-test: P < 0.01). The prelens tear film thickness was 3.9 +/- 2.6 and 3.6 +/- 2.1 microm (mean +/- SD; paired t-test: P = 0.52) and the postlens tear film thickness was 4.5 +/- 2.3 and 4.7 +/- 3.1 microm (paired t-test: P = 0.08) on and under Focus Night & Day and Acuvue lenses, respectively. Post hoc tests showed that precorneal (baseline) and prelens tear films were equivalent, and each was different (thinner; Tukey honestly significant difference P < 0.05) from the postlens tear film. CONCLUSIONS: OCT can noninvasively measure the thickness of the precorneal and prelens tear film as well as the postlens tear film. The thickness of the normal precorneal tear film is approximately 3 micro m and becomes thicker after lens fitting. The postlens tear film is thicker than the precorneal and prelens tear films with soft contact lenses. The thickness of both pre- and postlens tear films appears to be independent of the investigated lens types.     

The effect of the corneal epithelium on the biosynthesis of keratin sulphate in the stroma with regard to curvature variations important for wearing contact lenses (author's transl)

Intact or denuded bovine corneas were incubated in the presence of 14lC GlcN and parts of the proteoglycan-glycoprotein complexes of the stroma were extracted by 0.15M NaCl and successively by 1.0M CaCl2. The specific activities of total GAG and KS were determined. In the presence of the epithelial layer the biosynthesis of total GAG and KS was found to be significant higher compared to denuded corneas when light influence was excluded during the incubation. The GAG and especially KS play a role in the regulation of the dehydratation of the corneal stroma. In the young myope dehydratation and flattening of corneas after contact lens wear is reported by several authors. It may be suggested that suppression of the activating effect of the epithelium on the KS biosynthesis in the central part of the cornea is involved in the changes of corneal curvature after prolonged wearing of hard contact lenses. Since visible light decreases the activating effect of the epithelium in our experiments it may be suggested that the wearing of dark coloured glasses my be contraindicated in the young myope who wears hard contact lenses.     

Corneal hydration control in contact lens wearers with diabetes mellitus.

PURPOSE: The purpose of this study was to assess the acute swelling and recovery response to contact lens-induced corneal hypoxia in diabetic patients who wear contact lenses. METHODS: A thick, low water content, soft contact lens was fitted under a light patch to one eye of 23 diabetic patients and one eye of 23 nondiabetic control subjects in a single-masked, controlled clinical study. After 2.5 hours an ultrasonic pachymeter was used to measure the induced corneal edema and the rate at which the cornea recovered to baseline thickness. RESULTS: The induced corneal edema was significantly less for the diabetic patients compared with the nondiabetic control subjects (p = 0.004). The corneas of the diabetic patients were found to recover from the induced edema at a similar rate to the corneas of the nondiabetic control subjects (p = 0.28). CONCLUSIONS: Diabetes mellitus alters aspects of corneal hydration control.     

Potential acuity meter for predicting postoperative visual acuity in penetrating keratoplasty. A new method using a hard contact lens

An irregular corneal surface degrades the optical transmission and limits the effectiveness of the potential acuity meter (PAM) in predicting post-keratoplasty visual acuity. The authors investigated the efficacy of using a temporarily placed hard contact lens to neutralize surface irregularities in conjunction with the PAM in predicting post-keratoplasty visual acuity. A prospective evaluation was performed on 40 keratoplasty patients with a mean follow-up of 10.4 months. Of 27 patients with corneas clear enough to permit them to read letters on the PAM chart, 17 (63%) had very accurately predicted visual acuities by this new method, compared to only eight patients (30%) with the PAM alone. Three patients had predicted visions better than final vision, but this was due to a concurrent cataract in two cases and chronic cystoid macular edema (CME) in the third case. This study indicates that temporarily placing a hard contact lens for use with the PAM can increase its accuracy in predicting final visual acuity after penetrating keratoplasty.     

The effects of six months of contact lens wear on the tear film, ocular surfaces, and symptoms of presbyopes.

PURPOSE: To assess the tear film, ocular surfaces, and symptoms of ocular discomfort in a presbyopic population before and after contact lens wear. METHODS: A total of 150 presbyopes (49% were previous soft contact lens wearers) participated in a clinical trial in which they wore either monovision (single vision Acuvue lenses) or Acuvue Bifocal contact lenses. Clinical measurements of tear film, biomicroscopy, and corneal sensitivity as well as subjective ratings using the Dry Eye Questionnaire were collected at the initial visit and repeated after 6 months. Comparisons were made between age groups (40 to 51 years and 52 to 71 years) and genders before and after contact lens wear. Associations between objective and subjective tests were sought. RESULTS: After 6 months of contact lens wear, clinical signs had worsened by less than one-half of a grade, and tear break up time (TBUT) worsened by 3 s. Only TBUT was lower for the older age group. Females had less bulbar hyperemia, more sensitive eyes, more lissamine green staining, and lower TBUT and phenol red thread measurements (all p < 0.04). Twenty-eight percent experienced dryness before contact lens wear, but this figure increased to 68% when wearing contact lenses. There were no age differences, but almost twice as many females as males reported dryness. Reporting symptoms of dryness was associated with gender, corneal sensitivity, and type of corneal staining. CONCLUSIONS: These results provide a representation of the ocular surface condition and symptoms of ocular discomfort in the middle-aged population and seem similar to reports of younger populations. Wearing contact lenses seems to influence dry eye symptoms more than age or gender. Therefore, presbyopes should not be excluded from consideration for contact lens fitting.     

Effect of softperm lens wear on corneal thickness and topography: a comparison between keratoconic and normal corneae.

PURPOSE: Complications related to contact lens-induced anoxia in the keratoconic eye are well documented, but the underlying mechanisms are poorly understood. This study compared topographical changes in corneal thickness and anterior corneal radius following SoftPerm (Ciba Vision, Atlanta, GA) (rigid-and-soft combination) lens wear in a group of keratoconic and normal individuals. METHODS: Video keratoscopic (VKS) (EyeSys, Houston, TX) and ultrasonographic pachometry measurements were taken at nine ocular locations, at central, mid-peripheral, and peripheral corneal areas of keratoconic and normal subjects. Subjects were subsequently fitted with SoftPerm lenses and further measurements of corneal topography and thickness were noted after 1 day, 2 weeks, and 1 month of daily wear. RESULTS: Corneal thickness increased significantly in normal and keratoconic eyes following SoftPerm lens wear. Radius values obtained from the VKS measurements indicated that a significant and progressive degree of corneal flattening occurred for the keratoconic subjects at the 1-day, 2-week, and 1-month intervals, in contrast to the normal group, whose corneal radii remained unchanged. CONCLUSIONS: Corneal swelling occurs in both normal and keratoconic corneas following SoftPerm lens wear. Differences in the apparent central edema response within the keratoconic group may relate to the ease with which these corneas may be molded by a rigid contact lens.     

Clinical experience with the therapeutic hydrophilic contact lens.

Hydrophilic contact lenses were prescribed for 62 patients with a variety of corneal diseases. The lens was must successful for relief of pain in bullous keratropathy and in the healing of recurrent erosions. Its value in improving vision in bullous keratopathy depends on the stage of corneal edema and is not impressive in long-standing edema with scarring. The lens may be of value in patients with sterile indolent ulcers and in alkali-burned corneas with large epithelial defects after conventional therapy has failed. Caution is advised in fitting patients with the dry eye syndromes. Patients with minimal corneal scars due to keratoconus may benefit from a soft lens. Complications related to the use of hydrophilic lenses in this group of 62 patients are also discussed.     

Limited lateral spread of stromal edema in the human cornea fitted with a ('donut') contact lens with a large central aperture

Topographical corneal thickness changes were monitored in 10 subjects who each wore a hydrogel contact lens with a large central aperture ("donut" lens) for 6 hours. Analysis of local corneal thickness changes indicates that no corneal swelling occurred in the central exposed area of the cornea, but significant swelling occurred in the area of the cornea covered by the lens. The lateral cut-off point of corneal swelling was well-defined, indicating that the contact lens-induced corneal edema did not spread laterally to the exposed area of the cornea over the six-hour wearing period. Swelling of the peripheral cornea covered by the lens was found to be significantly greater with a tightly-fitting, immobile donut lens than with a loosely-fitting lens, suggesting that tear mixing may explain in part the apparent averaging of edema during open-eye wear of hydrogel lenses of varying thickness profile. The possibility that lateral spread of lactate within the stroma may contribute to this apparent averaging of edema was not confirmed in this study. We suggest that rapid metabolism or elimination of lactate in the exposed region of the cornea, or evaporation through the central lens aperture, may have contributed to the maintenance of normal central corneal thickness during open-eye wear of the donut lens.     

Effect of contact-lens-induced hypoxia on lactate dehydrogenase activity and isozyme in rabbit cornea.

Lactate dehydrogenase (LDH) levels and subunit isozyme patterns in cornea were monitored in 36 albino rabbits wearing thick, rigid, gas-permeable contact lenses for periods of 24 h, 2 and 7 days, and 1 and 3 months. The oxygen transmissibility of the contact lens was 15.3 x 10(-9) (cm/s) (ml O2/ml mm Hg). The activity of LDH in corneal tissue decreased according to the duration of lens wear. The LDH isozyme patterns shifted with lens wear from LDH1,2,3 (heart type, aerobic) to LDH4,5 (skeletal muscle type, anaerobic). The cornea swelled 8.8% with overnight contact lens wear, with increased swelling (11-12%) after further continuous lens wear. After contact lens removal, however, the LDH activity and the isozyme pattern returned to normal, and the corneal thickness quickly returned to normal. Based on these observations, it is suggested that LDH in rabbit corneas was physiologically affected by lens-induced hypoxia, but these changes were reversible. These results might further suggest that tear LDH levels in the human contact lens wearer could provide an ongoing assessment of the tolerance of the lens by the ocular surface.     

Effects of diabetes on anterior ocular structure and function

Diabetes is a systemic disease that affects multiple organs including the entire eye. Although diabetic retinopathy is the main cause for vision loss in patients with diabetes, many anterior segment abnormalities have been described, including structural changes in the tear film, the conjunctiva, and the corneal epithelial and endothelial cells. Functional alterations in corneal wound healing, corneal sensitivity, and recovery from contact lens-induced edema also accompany diabetes, and this raises the issue of whether contact lens wear may be contraindicated in these patients. To address this issue, eye care providers must be able to recognize the effects of diabetes on the anterior ocular surface, understand the biochemical mechanism(s) believed to be responsible for these alterations, and determine the relevance of observed changes with respect to contact lens wear.     

Pilot study on the influence of corneal biomechanical properties over the short term in response to corneal refractive therapy for myopia

PURPOSE: To study the short-term corneal response to corneal refractive therapy for myopia and correlate it with corneal biomechanical properties as measured with the ocular response analyzer. METHODS: Eight eyes from 8 young subjects were fitted with a reverse geometry contact lens, attempting a myopic correction of -4.00 D. Corneal resistance factor and corneal hysteresis (CH) were measured before contact lens fitting with the ocular response analyzer. These parameters were correlated with the degree of change in apical curvature, simulated keratometry, and central corneal thickness after 3 hours of contact lens wear (effect) and 3 hours after lens removal (recovery). RESULTS: There was a trend toward a faster effect and faster recovery of the orthokeratologic effect for corneas with less resistance in terms of biomechanical properties. Corneal resistance factor did not correlate significantly, however, with any of the topographic and pachymetric parameters. Conversely, CH was significantly correlated with changes in steep keratometry (0.758; P = 0.029) and central corneal thickness (0.755; P = 0.030) during lens wear and with changes in steep keratometry (-0.835; P = 0.010) during recovery. Overall, higher values of CH meant slower effect and recovery of the orthokeratologic effect. CONCLUSIONS: Short-term response of human cornea to corneal refractive therapy is correlated with the biomechanical properties of the cornea. Of the different theories supporting such involvement of corneal response to reverse geometry contact lenses, the most likely one seems to be the one assuming a faster response and faster recovery for corneas with lower resistance. Larger sample studies would be needed to clarify the involvement of corneal biomechanical properties on corneal response to orthokeratology.