Biomedical subjects

L G Carney

Publications and source records attributed to L G Carney.

At least 37 records · Page 2Linked to original sources

PubMed1991

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.

Aged↗

PubMed1991

Management of keratoconus: comparative visual assessments.

We compared the visual performance of keratoconus patients whose vision had been corrected with one of the following: RGP lenses alone, penetrating keratoplasty, or epikeratoplasty. Despite similar Snellen visual acuities with optimal correction, significant differences in visual performance were found in the three groups. Visual performance was assessed by measuring contrast thresholds for electronically-generated sinusoidal gratings and by measuring contrast thresholds in the presence of a glare source. These measurements were carried out on six bilateral keratoconus patients such that three eyes were corrected with RGP lenses alone, five eyes were corrected by penetrating keratoplasty, and four eyes by epikeratoplasty. There were statistically significant differences in the visual responses for the three correction types (ANOVA, F2,45 = 5.755, P = 0.0059), with the epikeratoplasty procedure providing the poorest results. Visual performance was further affected in the presence of a glare source. Statistically significant group differences were again found (ANOVA, F2,45 = 7.593, P = 0.0014), with the penetrating keratoplasty eyes being the most affected. These differences were spatial frequency dependent (p = 0.0293), the effect being most marked at medium spatial frequencies. These findings provide a ranking of the quality of vision provided by each management strategy.

Adult↗

PubMed1990

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.

Astigmatism↗

PubMed1990

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.

Adult↗

PubMed1990

Influence of rigid contact lens overall and optic zone diameters on tear pump efficiency.

To determine the effects of contact lens overall diameter on tear pump efficiency, oxygen uptake rates were measured for the open-eye condition and following steady-state periods of static (without blinking) and dynamic (with blinking once every 5 s) contact lens wear of oxygen-impermeable (polymethylmethacrylate) lenses of 5 overall diameters (8.2, 8.5, 8.8, 9.1, and 9.4 mm). The optic zone diameters were 1.4 mm smaller than the overall diameters in each case, while all other parameters remained constant. Differences in corneal oxygen demand following the static and dynamic conditions are a quantitative index of the tear pump efficiency for each lens size on the same cornea, and the differences were found to increase with reduction in lens overall diameter. This indicated that a more efficient tear pump was associated with small diameter designs. In addition, subjects with large palpebral aperture size, relative to lens overall diameter, were found to demonstrate better tear exchange.

Adult↗

PubMed1990

Tear buffering in contact lens wearers.

The buffering capacity of tears collected from contact lens wearers was compared to that from non-contact lens wearers. Three groups of subjects were investigated: 6 subjects not wearing contact lenses, 3 subjects wearing rigid contact lenses, and 3 subjects wearing hydrogel contact lenses. Each of the 12 subjects provided, on 6 separate occasions, 100 microliters of tears. The pH of the tears was measured with a closed chamber, temperature stabilized micro-electrode system firstly on collection and then after titration with acid and base. In all, 28 titration steps across the acid-base spectrum were completed for each subject's tears on each occasion. These tear pH responses to acid-base challenge were compared to that of unbuffered distilled water for the same titration procedure. There was no statistically significant effect of contact lens wear on tear buffering capacity, but considerable inter-subject variability was noted.

Adult↗

PubMed1990

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.

Adult↗

PubMed1990

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.

Adult↗

PubMed1989

Visual performance with aspheric rigid contact lenses.

Single vision aspheric contact lens designs may be able to enhance visual performance by controlling spherical aberration. In this study, we have compared the visual performance provided by two front surface aspheric rigid contact lenses with that of a spherical rigid contact lens and spectacle correction. These 4 corrections were worn by ten subjects while visual acuity was determined as a function of aperture size. The possibility that front surface aspheric lens designs may enhance visual performance was confirmed, although individual subjects varied significantly in this visual response. Contributions to the visual performance, other than contact lens surface shape, may include lens position and stability.

Adult↗

PubMed1989

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.

Age Factors↗

PubMed1989

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.

Adult↗

PubMed1988

Corneal oxygen availability during contact lens wear: a comparison of methodologies.

Three distinct methodologies have been used to assess corneal oxygen supply during contact lens wear--oxygen transmissibility (Dk/L), equivalent oxygen percentage (EOP), and corneal swelling. To examine the interelation among these methodologies, we measured the EOP and the amount of corneal swelling in response to a set of contact lenses of a range of Dk/L values on 10 subjects. We also measured the corneal swelling response after exposure to gas mixtures of 0.00, 1.01, 2.65, 5.13, and 10.3% oxygen concentration (balance nitrogen). Analysis of covariance showed that the relation between corneal swelling and EOP during lens wear was significantly different from the relation between corneal swelling and oxygen concentration after exposure to the gas mixtures, leading to different estimates of the minimum oxygen tension required to avoid corneal swelling (18.0% for the EOP technique vs. 10.9% for the gas mixtures). This discrepancy may be explained in terms of the underlying assumptions of the various techniques and by consideration of the mechanisms of corneal swelling during contact lens wear. The clinical implication of this result is that an EOP of 18% should be used as the basis for the development of a daily wear contact lens that provides an "edemafree" response.

Adult↗

PubMed1988

Effect of hydrogel lens wear on contrast sensitivity.

Conflicting reports on the adequacy of the visual performance provided by hydrogel contact lenses continue to appear in the literature. We conducted a study to determine the effect on visual performance, as assessed by contrast sensitivity measurements, of (1) lens wear of optimally fitting 38% water content lenses and (2) altered lens parameters, including fitting characteristics, of 38% and 67% water content lenses. During normal wear of lenses, neither the lenses themselves nor any contact lens-induced corneal changes produced measurable visual losses. The visual performance was also independent of lens fit or lens thickness over the ranges used here. The higher water content lenses provided marginally poorer visual performance.

Contact Lenses, Hydrophilic↗

PubMed1987

Dehydration of hydrogel lenses during overnight wear.

We studied the extent of dehydration of hydrogel lenses during overnight wear. Seven subjects used a hand refractometer to measure the water content of five different lenses (Hydron zero-6 (nominal water content 38.6%), -0.50 D; Snoflex 50 (52.5%), -0.50 and + 15.00 D, and Hydron Z-67 (67.5%), -0.50 and + 15.00 D) before and after 7 h of both open- and closed-eye wear. No statistically significant difference was observed in dehydration between open and closed eye lens wear. Thick and thin lenses made of the same material were found to dehydrate to an equal extent. Contrary to expectations, the medium water content Snoflex 50 lenses displayed a greater absolute decrease in water content than the higher water content Hydron Z-67 lenses (p less than 0.01). Factors that may influence the extent of dehydration under open- and closed-eye wearing conditions, and the clinical implications of these results, are discussed.

Adult↗

PubMed1987

Critical oxygen requirements to avoid oedema of the central and peripheral cornea.

A randomized double-masked experiment was performed to compare the critical oxygen concentrations required to avoid corneal thickening at the central and peripheral cornea. Pachometry was performed on 10 subjects before and after 3 h of corneal exposure to the following gasmixtures: 0.00, 1.01, 2.65, 5.13 and 10.3% oxygen, balance nitrogen. Widely ranging critical values (from 4.4 to 11.6% for the central cornea and 1.7 to 15.8% for the peripheral cornea) could be obtained depending upon the analytical procedure used to treat the data; that is, the choice of reference baseline, the curve fitting procedure adopted and the method of comparing the reference baseline with the fitted curve. The estimates of the critical oxygen requirement of the central cornea are consistent with previous studies when compared using the same analytical procedure. Similar critical values were derived for the central and peripheral cornea for any given analytical procedure (e.g. 9.8% for the central cornea, and 12.3% for the peripheral cornea for zero mean change in corneal thickness). Our findings indicate that anatomical and physiological differences between the central and peripheral cornea do not have clinically significant effects on the minimum oxygen requirements at these respective sites. This result has implications with respect to lens design considerations for alleviating physiological stress of the cornea during contact lens wear.

Adult↗

PubMed1986

Patient compliance and its influence on contact lens wearing problems.

One hundred consecutively presenting patients, fifty from each of two contact lens clinics, were questioned about the procedures encountered in care and maintenance of their contact lenses and asked to demonstrate their use of those procedures. Their clinic records were then analyzed for the occurrence of signs and symptoms that were related potentially to noncompliance with instructions and procedures, and that could not be otherwise explained. Only 26% of patients were fully complaint. Noncompliance with instructions was related strongly to the occurrence of signs and symptoms indicative of potential wearing problems. Improvements in the level of patient compliance with instructions is likely to bring about increased patient success with contact lens wearing.

Contact Lenses↗

PubMed1986

Use of tear enzyme activities to assess the corneal response to contact lens wear.

A non-invasive biochemical method for assessing the effects of contact lens wear on the in vivo corneal epithelium is described. A fluorometric technique is used to measure the activities of lactate dehydrogenase (LDH) and malate dehydrogenase (MDH) in human tear fluid. In view of the corneal epithelial origin of these enzymes, changes in their activity can indicate the severity of environmental stresses on the corneal epithelium. Following short-term wear of contact lenses, LDH and MDH activities are altered so that the tear LDH/MDH ratio is elevated. The magnitude and time course of this elevation are influenced by contact lens type, fit and duration of wear. The technique can yield more specific measures of the corneal response to contact lens wear than previous techniques.

Circadian Rhythm↗

PubMed1985

Human tear enzyme changes as indicators of the corneal response to anterior hypoxia.

A non-invasive biochemical technique for quantifying the effects of anterior corneal hypoxia on the in vivo corneal epithelium of the human eye is described. Following short-term exposure of the cornea to low atmospheric oxygen pressures, lactate dehydrogenase (LDH) and malate dehydrogenase (MDH) activities in tears are altered so that the tear LDH/MDH ratio is elevated. The degree of elevation of the ratio and its timing are related to the severity of hypoxia. Possible explanations for the elevation of the tear LDH/MDH ratio include unbinding of intracellular LDH and increased cell membrane permeability. For severe hypoxia, de novo LDH synthesis may also contribute. These changes suggest that control mechanisms within the corneal epithelium may be responding to optimize the efficiency of anaerobic metabolism during conditions of environmental stress, and offer the prospect of a non-invasive technique for monitoring epithelial metabolic stress.

Circadian Rhythm↗