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Biomedical subjects

N Efron

Publications and source records attributed to N Efron.

At least 73 records · Page 4Linked to original sources
PubMed1988

Visual decrement with deposit accumulation of HEMA contact lenses.

The vision decrement associated with deposit accumulation on hydroxyethyl methacrylate (HEMA) contact lenses was assessed in 51 patients presenting consecutively to a large clinic. Both high contrast visual acuity (HCA) and low contrast visual acuity (LCA) (logMAR) decreased with increased deposition (Rudko classification). Apart from being consistently lower than HCA, LCA offered no additional useful information. Both HCA and LCA worsened with lens age, whilst lens deposition increased with lens age (p less than 0.001). No associations between any of the above measures and patient symptoms were revealed. In general, unacceptable vision decrement and deposit formation occurred after 12 months or 4,000 h of daily lens wear. Clinicians can use the data presented in this paper to reconcile vision loss with deposit formation in patients wearing HEMA lenses.

Contact Lenses↗
PubMed1988

Temperature of the hyperemic bulbar conjunctiva.

The relationship between bulbar conjunctival hyperemia and temperature was investigated. Conjunctival redness was induced in eighteen volunteers by instilling hypertonic saline into the conjunctival sac. The degree of redness was estimated using a subjective grading scale. The subsequent changes in temperature of the nasal bulbar conjunctiva were monitored using an infra-red bolometer. Conjunctival hyperemia was significantly correlated with conjunctival temperature; the maximum response of a 3-grade change in redness was accompanied by an increase of 0.5 degrees C in temperature. These findings confirm the classic association between inflammation, rubor and calor.

Adult↗
PubMed1988

Dehydration of hydrogen contact lenses in vitro and in vivo.

An attempt was made to derive a model for predicting the extent of hydrogel lens dehydration in vivo (delta Wv) by using a contact lens refractometer to (1) measure the water content (W) of eight lenses ranging from 36.5 to 85.8% water before and after 90 min lens wear by six subjects; and (2) measure the rate of dehydration of the same lenses in vitro (delta Wt), in a controlled, randomized, double-masked experiment. A significant correlation was demonstrated between W and the rate of change of delta Wt (the dehydration rate, D), whereby lenses of higher water content dehydrate at a slower rate. No correlation was found between W and delta Wv or between D and delta Wv, thus precluding the development of a predictive model. Possible reasons for this failure, and directions for further research, are discussed.

Adult↗
PubMed1988

Dehydration of hydrogel lenses: environmental influences during normal wear.

To examine whether certain aspects of the environment can have a significant effect upon the steady-state water content of hydrogel contact lenses during normal wear, nine volunteers monitored ambient temperature, ambient humidity, and the water content of their contact lenses over a 5-day period of daily lens wear. Three subjects wore Hydron zero-6 lenses (38% nominal water content), three wore Hydrocurve II lenses (55%), and three wore Permalens contact lenses (71%). The results indicate that a knowledge of ambient temperature and humidity is of little value in predicting the water content of hydrogel lenses under normal wearing conditions, at least with respect to the range of environmental conditions encountered in the course of this experiment.

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

Strategies for minimizing the ocular effects of extended contact lens wear--a statistical analysis.

In a study reported recently, the effects of long-term extended wear of soft contact lenses on the human cornea were found to include epithelial thinning, a reduction in epithelial oxygen uptake, induction of epithelial microcysts, stromal thinning, and an increase in endothelial polymegathism. A multiple regression analysis was performed on data from this study to identify lens or patient characteristics associated with these effects. Although generalization to the population of conclusions based on associations identified using a small sample should be treated with caution, results from this analysis suggest that lens-induced effects on the cornea may be minimized by fitting lenses that are thinner and more mobile, and by encouraging more frequent lens removal and replacement. Patients with thinner corneas and with high endothelial cell density and low polymegathism before commencing lens wear tended to show fewer effects from extended lens wear.

Cell Count↗
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↗
PubMed1987

Corneal thickness changes following sleep and overnight contact lens wear in the primate (Macaca fascicularis).

Corneal thickness was monitored on seven cynomolgus monkeys (Macaca fascicularis) over a 76 hour period. During this time, corneal thickness measurements were also made on six monkeys after overnight wear of a hydrogel contact lens in one eye. Mean corneal thickness was 417 +/- 12 microns. An overall diurnal variation of 16 +/- 5 microns (3.8 +/- 1.2%) was found. During the first half hour after waking, corneal thickness changes ranged from -9 microns to +14 microns. After overnight contact lens wear, the cornea had swelled an average of 42 +/- 24 microns (9.1 +/- 4.8%). Following lens removal, the cornea returned to normal thickness within approximately 90 minutes. These results are similar to those found in humans and indicate that with respect to contact lens induced corneal thickness changes, the cynomolgus monkey is a suitable model for the physiological response to contact lens wear.

Animals↗
PubMed1987

Vascular response of the cornea to contact lens wear.

The subject of contact lens-induced corneal vascularization has attracted considerable interest in recent times in view of the relatively high prevalence of this condition with the use of extended wear soft lenses. This paper reviews clinical features and underlying mechanisms of the vascular response to lens wear. The concept of a "normal" vascular response to contact lens wear is proposed; the limits of this response are defined and incorporated into a simple grading system. The clinical presentation of vessel growth in the cornea is described together with appropriate methods of slit lamp evaluation. Current theories of the pathology and etiology of lens-induced vascularization are reviewed, and a dual-etiology model of stromal vessel formation is described. It is concluded that the prognosis for halting the progression of vascularization is excellent; however, little information is available concerning the long-term regression of "ghost" vessels.

Contact Lenses, Extended-Wear↗
PubMed1987

A review of the theoretical concepts, measurement systems and application of contact lens oxygen permeability.

The interest in rigid gas-permeable contact lenses for extended wear has promoted a re-evaluation of the theoretical and practical aspects of the oxygen transport characteristics of contact lens materials. Oxygen permeability has been considered by some authors to be a parameter which is dependent upon the measurement conditions; however, this paper demonstrates that an intrinsic permeability coefficient may be derived for a given material when the basic principles of the transport mechanisms are considered. Two basic methodologies have been described for measuring oxygen permeability--dual-chamber and electrochemical methods, the latter being the more popular. Although consistency in measured oxygen transmissibility of a given lens may be achieved among laboratories, current analytical techniques do not allow the intrinsic permeability coefficient of materials to be determined. Future research should be directed towards developing methods to fully correct for surface and edge effects, and to achieve standardization among laboratories.

Contact Lenses↗
PubMed1986

Determinants of the initial comfort of hydrogel contact lenses.

A double-masked, randomized study was conducted to determine the effects of power and water content on the initial comfort of hydrogel contact lenses in 10 unadapted subjects. Three lens powers (-0.50, -5.00, and -10.00 D) were used in each of three water contents (38, 55, and 70%). A significant negative correlation (p less than 0.05) was found between lens comfort and lens water content; that is, lower water content lenses of lesser bulk were more comfortable than higher water content lenses. These data will allow practitioners to predict patient awareness to various lens types. When fitting hydrogel lenses to an apprehensive patient who has not worn contact lenses previously, it may be advisable to insert a thin, low water content lens initially, thereby maximizing lens comfort.

Adult↗
PubMed1986

Intersubject variability in corneal swelling response to anoxia.

A controlled, double-masked, randomized experiment was conducted on 9 human subjects, to determine whether there is an intersubject difference in the corneal swelling response to anoxia. A statistically significant intersubject variability was observed (P less than 0.001), with oedema ranging from a mean of 3.6% to 7.3% following 3 h exposure to humidified nitrogen gas (anoxia). This result supports the hypothesis of previous workers that the intersubject difference in the corneal oedema response to a thick HEMA "stress lens" is due to oxygen deprivation and not some other physiological variable resulting from lens wear.

Adult↗
PubMed1986

Definitions for hydration changes of hydrogel lenses.

Authors of studies on the hydration characteristics of hydrogel lenses have used a variety of definitions to describe the changes observed with exposure of the lenses to different environments. This has resulted in difficulties in the interpretation of published data. In particular, the single term "percentage dehydration" has been used to denote different mathematical expressions for alterations to the lens composition. A detailed theoretical analysis of hydrogel hydration levels is presented to examine definitions for lens hydration changes, compare results obtained under different definitions and propose appropriate usage for the alternative definitions according to the aspect of lens performance under consideration. The interrelationships between the commonly used definitions of dehydration are shown to be independent of initial lens mass but dependent on initial water content. Typically, higher water content lenses undergo considerably larger mass changes than lower water content lenses, an effect that may be masked if these changes are presented as changes of water content. There are a number of clinical consequences of lens dehydration, the importance of which will vary depending on the initial lens water content. It is therefore essential when comparing different water content lenses on the basis of the extent of dehydration to do so with respect to a specific clinical consequence rather than in general terms.

Contact Lenses, Hydrophilic↗
PubMed1985

Direct in vivo measurement of corneal epithelial metabolic activity using a polarographic oxygen sensor.

The component oxygen fluxes that constitute a single recording of corneal oxygen uptake when a polarographic oxygen sensor (POS) is placed in contact with the cornea were determined in the in vivo rabbit eye by removing the epithelium and injecting an air bubble into the anterior chamber. The relative contribution of epithelial oxygen consumption, stromal oxygen consumption and diffusion were approximately 55, 5 and 40%, respectively. It is demonstrated that differences in corneal oxygen uptake rates recorded with a POS primarily indicate differences in epithelial metabolic activity. Changes in epithelial metabolism can be determined by measuring both corneal oxygen uptake and corneal thickness. For example, if corneal thickness is constant, differences in epithelial oxygen flux (a) between two eyes of one person, or (b) before and after a procedure in the same eye, can be calculated by multiplying the difference in oxygen consumption measured with a POS by a factor of 1.8.

Animals↗
PubMed1985

Effects of long-term extended contact lens wear on the human cornea.

The effects of long-term extended wear of soft contact lenses on the human cornea were determined by examining 27 patients who had worn a high water content hydrogel contact lens in 1 eye only for an average of 62 +/- 29 months (mean +/- SD). The other eye, which was either emmetropic or amblyopic, acted as a control. The lens-wearing eye showed a 14.8% reduction in epithelial oxygen uptake (P less than 0.001), a 5.6% reduction in epithelial thickness (P less than 0.05), a 2.3% reduction in stromal thickness (P less than 0.05), the induction of epithelial microcysts, and a 22.0% increase in endothelial polymegathism (P less than 0.001). Endothelial cell density was unaffected by extended lens wear. No interocular differences in any of these physiological characteristics were found in a matched control group of anisometropic and amblyopic subjects who did not wear contact lenses. The patients ceased lens wear for up to one month and recovery of corneal function was monitored during this period. Epithelial oxygen uptake and thickness recovered within 33 days of lens removal. The number of microcysts increased over the first 7 days, but decreased thereafter; some microcysts were still present 33 days after lens removal. Recovery from stromal thinning had not occurred after 33 days following lens removal. There was a slight reduction in polymegathism in some patients, but overall this was not statistically significant. These findings establish (1) that the extended wear of hydrogel lenses induces significant changes in all layers of the cornea; (2) that lens wear suppresses aerobic epithelial metabolism, which may compromise the epithelial barrier to infection; and (3) that changes to the stroma and endothelium are long-lasting. Lens-induced effects on corneal physiology can be minimized by fitting lenses that have greater oxygen transmissibility (are thinner), are more mobile, more frequently removed, and more regularly replaced.

Adolescent↗
PubMed1985

Diurnal variation of corneal thickness in the cat.

Central corneal thickness of both eyes of seven cats was measured hourly for 72 hr using ultrasonic pachometry. The mean corneal thickness was 569 +/- 36 micron (mean +/- SD), and the diurnal variation was 49 +/- 14 micron (8.6% of corneal thickness). In a separate experiment, the corneal thickness of one eye of each of five cats was measured following 2 hr of natural sleep; 2 1/4 hr after eye opening, the corneas had thinned an average of 43 +/- 22 micron. The authors conclude that corneal swelling induced by eye closure during periods of sleep is the prime determinant of the diurnal variation in cat corneal thickness. In studies where corneal thickness is to be monitored over a period of time, it is possible to control against this large diurnal variation by ensuring that the cat is active for a period of two hours prior to pachometry measurements.

Animals↗
PubMed1984

Physiological response of the contralateral cornea to monocular hydrogel contact lens wear.

Previous authors have suggested that monocular contact lens wear can induce changes in the contralateral eye. However, most of these contralateral responses have yet to be substantiated or satisfactorily explained. This paper reports a randomized, controlled, single-masked experiment in which corneal thickness, corneal oxygen consumption, and the endothelial bleb response were monitored for 3 hr in both eyes of eight unadapted subjects who wore thick hydrogel lenses on one eye only. No significant contralateral effect was observed in any of the parameters examined. These findings reaffirm the validity of using the contralateral eye as a control in experiments with hydrogel contact lenses.

Adult↗