Oxygen levels beneath the closed eyelid.

The level of oxygen at the anterior corneal surface beneath the closed eyelid is shown to be equivalent to an atmosphere of 7.7% oxygen. This finding is in good agreement with assumptions which have been based on the oxygen level at the palpebral conjunctiva. However, in some instances a significant amount of oxygen is derived not only from the palpebral conjunctiva but also from the atmosphere as a result of an imperfect palpebral aperture seal.

Vision through an abnormal cornea: a pilot study of the relationship between visual loss from corneal distortion, corneal edema, keratoconus, and some allied corneal pathology.

Visual function was assessed by classic acuity measures as well as contrast thresholds over a wide spatial frequency range for subjects with experimentally induced corneal distortion and induced corneal edema. These results were compared with similar results for patients with uniocular keratoconus, bilateral keratoconus, and allied corneal pathology. Distortion and edema were found to produce characteristically different types of contrast attenuation at threshold (amplitude) and have quite different suprathreshold abnormalities (phase) for objects within the resolution limit and therefore could form the basis of a useful functional classification of the visual degradation from corneal pathology.

Human tear responses to alkali.

With the use of a closed-chamber microelectrode system, the buffering capacities of over 100 tear samples were measured in response to alkali challenge of several strengths. Although distinctive variations were found among the seven subjects studied, a substantially greater buffer capacity was consistently noted in all their tear responses to pH conditions of near 10 and above. These results are consistent with the early observations of Friedenwald, who demonstrated the buffering capacity of corneal tissue when challenged in this higher pH range, and may here be due to the onset of protein denaturation in the tears.

Buffering in human tears: pH responses to acid and base challenge.

The buffering capacity of tears collected from six young, healthy subjects was assessed using a microtitration technique. Each subject provided, on six separate occasions, about 100 microliter of tears, collected in small amounts and with minimal mechanical stimulation over several hours. The pH of the total stirred pool of tears from each subject was determined at the outset. This pool of tears was then divided into two equal volume aliquots, the pH of each being determined following each titration step of one of them with acid, and of the other with base. In all, 28 titration steps across the acid-base spectrum were completed for each patient pool collected. A total of 1044 tear pH measurements were made, all being done in a closed, temperature stabilized (36 degrees C) microelectrode chamber having an accuracy of within 0.04 pH units. For a comparative reference, an identical titration procedure was used on degassed, demineralized distilled water (348 pH determinations). Buffering capacity was found to show considerable intersubject variations, but in all cases the effect was more pronounced and more uniform following acid titration. Local zones of enhanced buffering across the pH spectrum could be identified, presumably reflecting the multiple buffering components (bicarbonate, protein and others) present in tear fluid.

Corneal topography and myopia. A cross-sectional study.

PURPOSE: Central corneal curvature is known to vary with refractive error, but the relation between corneal topography and ametropia is less clear. The current study was conducted to determine whether a relation exists between corneal asphericity and myopia. Associations between corneal asphericity and each of the components of refraction also were examined. METHODS: Corneal asphericity and apical radius of curvature were determined for 113 eyes (spherical equivalent refractive error +0.25 diopter [D] to -9.88 D) by fitting a conicoid equation to videokeratoscopic data. Computerized videokeratoscopic images were recorded using a Topographic Modeling System. Keratometry also was performed on each eye. Anterior chamber depth, lens thickness, vitreous chamber depth, and axial length were measured with a hand-held biometric ruler. RESULTS: A low but statistically significant positive correlation was found between corneal asphericity (Q) and spherical equivalent refractive error (r = 0.275, P < 0.01). Significant relations also were observed between Q and vitreous chamber depth (r = 0.17, P < 0.1) and between Q and axial length (r = 0.24, P < 0.05). The association between Q and corneal radius of curvature was found not to be significant. Eyes with higher levels of myopia had steeper central corneal curvatures, deeper anterior and vitreous chambers, and greater axial lengths. CONCLUSIONS: A tendency for the cornea to flatten less rapidly in the periphery with increasing myopia was shown. Decreased peripheral corneal flattening also was observed in association with increasing vitreous chamber depth and increasing axial length. These findings have implications for refractive surgery outcomes, schematic eye modeling, contact lens design, and ocular aberration analysis.

Human tear buffering capacity.

With the use of a closed chamber microelectrode system, we measured the relative buffering capacities of 490 human tear samples from young healthy adults. The buffering capacities of the 457 waking-hour samples did show small but regular oscillations that were similar to those previously reported for blood and tear pH, but only rarely did the buffering capacity of one sample approach double that of another. The buffering capacities of 33 tear samples, associated with periods of prolonged eye closure (sleep), were not significantly different (P less than 0.5) from those of the open eye.

Visual losses after myopic epikeratoplasty.

The vision of seven patients who had undergone epikeratoplasty for myopia was evaluated by measuring their contrast sensitivity functions without and with the presence of a glare source. Compared with findings for a normal control population, these epikeratoplasty patients were found to have statistically significant contrast sensitivity losses. To evaluate the clinical relevance of these losses, the results were also compared with those for two other patient groups with abnormal corneal optics, seven radial keratotomy patients and seven keratoconus patients wearing rigid contact lenses. Contrast thresholds for the epikeratoplasty and keratoconus patients were similar and usually statistically indistinguishable, and their losses were far more profound than those of the radial keratotomy patients. The addition of glare did not alter this pattern of losses. The results document the extent of vision losses in these patients and suggest that increased intraocular light scattering is not a major contributor to the observed contrast sensitivity decrements.

Mechanisms of visual loss in corneal edema.

Although visual disability from corneal edema is a well-recognized clinical finding, it is not easily confirmed by routine visual assessment. A more reliable assessment of visual changes resulting from corneal edema of varying origins is achieved by measurement of changes in the contrast sensitivity function. The measurements can be sensitized by the presence of a glare source to allow a more accurate determination of the nature of the visual loss. Even in the absence of a measurable loss of visual acuity, corneal edema leads to diminished visual function. The magnitude and type of this loss is dependent on the origin of the edema and the site of the corneal response.

Human tear pH. Diurnal variations.

Using a closed chamber microelectrode system, the tear pH levels of sixteen subjects were monitored throughout the waking extent of five days. In addition to the absolute mean pH differences found among subjects, diurnal patterns of pH change could be identified for the majority. The amplitudes and periods of these cycloid patterns, however, were distinctive to each individual. Also, tear pH levels following periods of prolonged eye closure were found to be notably more acid than those associated with the waking hours.

Corneal oxygenation: blink frequency as a variable in rigid contact lens wear.

Using a micropolarographic system, we measured the responses of six human corneas to nine oxygen exposure conditions: to air (continuous open-eye) with no contact lens in place, and to eight interblink intervals (1, 2, 3, 4, 5, 6, 7, and 300 s durations) with an oxygen impermeable lens in place. The corneal oxygen uptake rates immediately following each of those conditions were direct indices of tear bulk-flow exchange under a rigid contact lens as an oxygen route. Greatest efficiencies in reducing corneal oxygen demand were associated with the two highest blink frequencies examined (namely, for interblink intervals of 2 s or less). Even at those frequencies oxygen demands ranging from 4 to 6 times the open-eye, non-wearing, baseline rate for each eye typically occurred, clearly justifying the need for a supplementary oxygenation route, for example, directly through rigid contact lens materials having inherently high oxygen permeabilities.

Is the quality of vision with contact lenses adequate? Not only adequate but often superior.

Two factors govern the quality of vision with contact lenses (CLs): the fit and movement of the lens and the adequacy of its optics. Studies have not demonstrated any inherent inadequacies in material or design parameters that might account for visual difficulties in lens wearers. To the contrary, thickness, water content, and fit can be varied within clinically acceptable ranges without compromising vision. A poorly fitted lens, however, can cause excessive corneal edema and consequent visual problems. Poor vision may also result from uncorrected refractive error, the development of an unstable optical surface, or the use of a lens material that is inherently inadequate for a given patient. Such causes of visual dysfunction can usually be avoided through better fitting, better lens care, more careful refraction, or better patient-lens matching. Specialized lens designs can sometimes be used to correct vision problems. Certain types of rigid front-surface aspheric lenses, for example, provide astigmats with even better visual correction than spherical rigid lenses or spectacles, but the improvement is small and highly patient dependent. Similarly, although soft toric lenses can provide good vision, it is often technically difficult to orient and stabilize the correcting cylinder.

Extended contact lens wear in the developing cynomolgus monkey eye. Recovery from adverse responses.

Soft extended wear contact lenses were designed for and fitted to 12 eyes of 11 infant semiwild cynomolgus monkeys. The lenses were satisfactorily worn over observation periods of up to 465 days, the longest continuous period of lens wear being 69 days. However, some of the corneas were subject to a pattern of complications analogous to that in human contact lens wear. Nine eyes showed adverse responses, and of these, the ocular complications were severe enough in four cases to cause abandonment of lens wear. The most remarkable feature of the complications was the rapid recovery of severe reactions to near normality in less than 24 h, indicating an impressive ability of the cornea to tolerate and survive insult. Another feature was the recurrent nature of mild complications in certain monkeys despite apparent resolution from previous episodes. There appeared to be no correlation between duration of lens wear and appearance of the first or recurrent episodes of corneal infiltration and vascularization.

Visual performance in high myopia.

The visual performances of eight low myopic subjects (-2 D to -7 D) and eight high myopic subjects (greater than -7 D) with normal visual acuities were analyzed by contrast sensitivity testing with both spectacle and contact lens corrections to determine if any differences in function exist between those groups. Statistically significant (p = 0.0382) contrast sensitivity losses, particularly at the higher spatial frequencies, were found for the high myopic subjects when corrected with spectacles. However, with contact lens correction, no statistically significant overall difference between the two groups was evident (p = 0.5083). We believe that these results confirm for highly myopic subjects the lack of any consistently present loss of resolution. The differences with spectacle correction are attributable to the nature of the optical correction.

Visual performance after aphakic epikeratoplasty.

While good visual acuity and refractive results after epikeratoplasty for aphakia have been reported, particularly for adults and older children, limited detailed information on the nature of the resulting vision is available. We have evaluated the visual performance of seven aphakic subjects corrected by epikeratoplasty by measuring contrast sensitivity, with and without the presence of glare. These results were compared with those for other aphakic corrections, namely spectacles (n = 5 eyes), contact lenses (n = 5 eyes), or intraocular lens implantation (n = 5 eyes). There were statistically significant differences among these four aphakic correction types (p = 0.0330), with a consistent trend for diminished visual performance after epikeratoplasty. Threshold elevations occurred in the presence of glare, but they were not statistically different between the groups (p = 0.1631). Based on these visual assessments, epikeratoplasty does result in statistically significant visual losses. Despite this, it may still offer an acceptable alternative to other managements of the aphakic patient when those others are contraindicated.

Hyperopia is predominantly axial in nature.

PURPOSE: Myopia has been found to be predominantly axial in nature, i.e. myopic eyes have longer than normal axial lengths, with corneal radius variations having only a small influence on the magnitude of the refractive error. In this study we assess whether a similar relationship exists for hyperopia. METHODS: Biometric data were collected on 57 subjects with either emmetropic or hyperopic refractive errors ranging in magnitude from -0.37 D to +17.25 D. Our main analysis concentrated on subjects with less than +10 D of hyperopia (group 1, n = 53), as subjects with +10 D of hyperopia or more (group 2, n = 4) exhibited marked differences in their biometric characteristics. RESULTS: Analysis of group 1 data revealed a significant relationship (r2 = 0.611, p = 0.0001) between the degree of hyperopia and the measured axial lengths. A weak but statistically significant relationship (r2 = 0.128, p = 0.009) was also found between mean corneal radius measures and mean spherical refractive errors, with the mean corneal radius flattening with increasing hyperopia. In group 2, three of the four subjects exhibited much steeper corneal characteristics than predicted from the group 1 data. CONCLUSIONS: Our results suggest that hyperopia, like myopia, is predominantly axial in nature, although the corneal radius also plays a role in determining refractive error magnitude. These results have implications for refractive surgery and visual performance in hyperopic eyes.

The effects of fenestrating soft contact lenses on corneal swelling: a re-examination.

Previous studies have suggested that central corneal oedema during hydrogel lens wear can be reduced by placing peripheral fenestrations in the lens. In this study, optical pachometry was used to monitor central and peripheral corneal swelling in response to a peripherally fenestrated hydrogel lens. The experiment was conducted in a double-masked, randomized manner on 10 unadapted subjects. Fenestrations did not alter central corneal oedema, but peripheral corneal oedema was reduced by 34% (p > 0.001). Fenestrations may therefore provide a useful strategy for minimizing lens-induced corneal oedema. Clinical trials with fenestrated lenses are now indicated.

Flexure of thin rigid contact lenses.

The flexure of spherical rigid contact lenses was measured on the eyes of 10 young subjects using a videokeratoscope. Five subjects had little or no with-the-rule astigmatism (<0.75 D) andfive had moderate levels of with-the-rule astigmatism (1.00-2.00 D). Two lens materials (polymethylmethacrylate [PMMA] and Boston XO) in three centre thicknesses (0.05, 0.10 and 0.15 mm) were used in the study. No significant difference in the amount of flexure was found between the two materials tested. The degree of regular astigmatism on the lens front surface was found to increase as the centre thickness of the contact lens decreased. For the astigmatic group, the lenses with centre thicknesses of 0.05 mm had levels of front surface astigmatism similar to those of the underlying cornea. On spherical corneas the level of regular astigmatism can exceed that of the cornea for thinner lenses. When sphero-cylinder variations are accounted for, residual higher order aberration (root mean square) levels were found to approach those of the cornea when the lens thickness was reduced to 0.05 mm.

[Environmental pH and corneal oxygen flux (author's transl)]. / pH ambiant et flux d'oxygène cornéen.

The instillation of ophthalmic solutions can cause substantial change to tear pH. Wilst alkaline changes of appropriate magnitude to the anterior corneal environment appear to have little effect on corneal oxygen uptake, acidic changes in the same range can cause a decrease in this measure of corneal integrity. The effect takes some time to develop, and is a function of both the pH and molarity of the bathing solution.

Contact lens correction of visual loss in keratoconus.

Visual acuity and contrast threshold functions of 5 bilateral keratoconic patients were measured following spectacle correction and rigid contact lens correction. Visual function was improved by contact lens correction, but some contrast at attenuation remained. This residual loss could not always be related to routine visual acuity findings.