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Biomedical subjects

N Efron

Publications and source records attributed to N Efron.

At least 19 recordsLinked to original sources
PubMed1992

Conjunctival flora in extended wear of rigid gas permeable contact lenses.

A longitudinal study was performed to examine the effect of rigid gas permeable (RGP) contact lenses (Boston Equalens II and Quantum II) on the conjunctival flora of 45 young healthy subjects. Microbial flora were determined before delivery of lenses. Subjects wore lenses on an extended wear basis, removing them every 7 days for cleaning and disinfection. Cultures were repeated after 2 months of lens wear and the microbial flora were found to be significantly altered compared to the prelens wear results (0.05 greater than p greater than 0.02). Changes to conjunctival flora included an increase in the number of eyes from which potentially pathogenic microorganisms were isolated, an increase in the number of eyes that were culture-negative, and a decrease in the number of eyes harboring only normal conjunctival flora. The increase in potentially pathogenic flora was not specific for Gram-negative bacteria, which are most often associated with infectious keratitis during contact lens wear.

Adolescent↗
PubMed1992

Microbial flora in eyes of current and former contact lens wearers.

Microbial flora from the right eye conjunctival sac of 84 consecutively presenting contact lens patients were compared with cultures from both surfaces of their lens after aseptic removal and with the flora of their storage cases. Similar results were obtained from contact lens and conjunctival cultures of each individual; however, there was no correlation between storage case isolates and lens or conjunctival flora, suggesting that in uncomplicated lens wear, the eye is highly efficient in eradicating microorganisms introduced via handling. Conjunctival flora during daily contact lens wear was similar to the conjunctival flora of a matched control group of non-lens wearers. However, bacteria that are considered to be part of the normal ocular flora were isolated significantly more often from former contact lens wearers. The data also indicated that the use of nonperoxide chemical lens disinfection was associated with a higher proportion of positive cultures for pathogenic microorganisms than the use of other forms of disinfection, for both current and former contact lens wearers. The isolation of potential pathogens was particularly common among elderly subjects using thick contact lenses for extended wear. These changes to conjunctival flora may contribute to the increased risk of ocular infection associated with contact lens wear.

Adult↗
PubMed1992

Extended contact lens wear enhances Pseudomonas aeruginosa adherence to human corneal epithelium.

Extended wear of soft contact lenses is associated with an increased risk of Pseudomonas aeruginosa infection of the cornea. To assess the role of bacterial adherence in the pathogenesis of these infections, superficial corneal epithelial cells and leukocytes from ten patients who use extended-wear soft lenses and ten control eyes were compared for their propensity to attach P. aeruginosa in vitro. Cells were washed from the cornea by saline irrigation, incubated with a 10-ml solution containing 10(7) colony-forming units/ml of bacteria at 35 degrees C for 30 min, collected on a filter, and prepared using a modified acridine orange staining method. Fluorescence microscopy showed bacterial adherence to corneal epithelial cells, leukocytes, and ocular mucus. The mean number of bacteria adhering to epithelial cells was 2.6 for control eyes and 6.6 for the lens-wearing eyes (P = 0.002). The percentage of epithelial cells attaching greater than or equal to four bacteria was higher for lens-wearing eyes than control eyes (57.4% versus 26.0%, P = 0.0005). There was no significant difference between contact lens-wearing eyes and control eyes in the number of leukocytes collected or in the number of bacteria attached to these cells. These results show that P. aeruginosa adherence to epithelial cells is enhanced in those who use extended-wear soft contact lenses, and this may contribute to the increased incidence of P. aeruginosa keratitis for this population.

Acridine Orange↗
PubMed1991

Clinical application of the oxygen transmissibility of powered contact lenses.

Stipulation of the oxygen transmissibility (Dk/L) of a contact lens based upon oxygen permeability (Dk) and nominal lens thickness (L) can be misleading since center thickness, average thickness, and hence Dk/L of a contact lens vary with lens power. While earlier confusion regarding determination of Dk has now been resolved, this data has yet to be fully incorporated into a clinically applicable model. We developed a graphical solution describing variations in Dk/L with power for a representative sample of currently available hydrogel and rigid contact lenses. The Dk/L is found to be optimal for lenses of low minus power (approximately -2.50 D). The model presented here will assist practitioners in assessing the potential oxygen performance of different lens designs with respect to a given power. We advocate that lens manufacturers print the average Dk/L on lens packaging, according to the model described in this paper, along with the parameters that are usually stated.

Contact Lenses, Hydrophilic↗
PubMed1991

Characteristics of the pre-lens tear film during hydrogel contact lens wear.

The integrity of the pre-lens tear film (PLTF) is an important determinant of the clinical performance of contact lenses. The PLTF of eight hydrogel lenses, ranging from 36.5% to 85% water content, was assessed on six asymptomatic lens wearers in a randomized, double-masked experiment. Using a specular reflection technique, the thicknesses of the lipid and aqueous phases were found to be greater on lenses of higher water content. Water content was measured before and after 90 min wear using a hand-held refractometer. Non-invasive tear break-up time was also found to be greater for higher water content lenses. No association was found between lens dehydration and PLTF characteristics. The differences in PLTF noted in this study may help to explain the differences noted between some lenses in their rates of surface deposition.

Adult↗
PubMed1991

The effect of soft lens lubricants on symptoms and lens dehydration.

In order to establish the efficacy of soft lens lubricants and their mode of action, we conducted a controlled, double-masked, randomized study. Symptoms of dryness as well as lens water content were assessed in 30 symptomatic hydroxyethylmethacrylate (HEMA) contact lens wearers who each applied either saline or one of two soft lens lubricants every 2 hours over a 6-hour wearing period. Short- and long-term symptomatic relief from ocular dryness was provided by both lens lubricants (ANOVA, P less than .05). However, neither lubricant was found to be significantly superior to saline. Contact lens dehydration was not significantly reduced by the instillation of any solution. A psychological rather than a physical (lens hydration) or physiological basis to the symptomatic relief provided by soft lens lubricants cannot be discounted.

Adolescent↗
PubMed1990

In vivo dehydration of disposable (Acuvue) contact lenses.

The in vivo dehydration of the Acuvue disposable contact lens was measured in six subjects using a contact lens refractometer. On average, the lenses dehydrated 6.2 +/- 1.9% after 20 min and 10.2 +/- 3.9% after 6 h of wear. The magnitude of dehydration is greater than has been reported for other hydrogel lens types of similar water content. This information may assist practitioners in the interpretation of clinical signs pertaining to lens behavior and associated patient symptoms.

Adult↗
PubMed1990

Soft lens lubricants and prelens tear film stability.

Soft contact lens wearers complaining of "dry eye" are frequently prescribed rewetting drops, although the mode of action and efficacy of this treatment have not been established. Enhancement of prelens tear film (PLTF) stability could provide a basis for symptomatic relief with solution instillation. Thirty symptomatic hydroxyethyl methacrylate (HEMA) lens wearers participated in 4 separate trials of no solution (baseline), saline, and 2 soft lens lubricants. Solutions were applied for a 6-h adaptation period, and PLTF noninvasive break-up time (NIBUT) was monitored for 1 h after a final instillation. PLTF NIBUT was significantly elevated 1 min after instillation of all solutions (p less than 0.001, logrank test) but this effect persisted for less than 5 min. The transient enhancement of PLTF stability arising from the instillation of these rewetting solutions is unlikely to provide a basis for prolonged symptomatic relief.

Adolescent↗
PubMed1990

The Dk project: an interlaboratory comparison of Dk/L measurements.

The oxygen transmissibilities (Dk/L) of a set of 48 contact lenses made from 8 different materials were measured by 4 laboratories. The L/Dk measurements from each laboratory were compared and correlated. Samples which were not masked with a fixed front surface aperture during measurement were corrected for edge effects. This paper shows that provided L/Dk is calculated for each lens using the same technique and Dk is derived using a graphical method of calculation, similar results can be obtained by all laboratories. However, the agreement was less good for materials of Dk greater than 70 x 10(-11) (cm2/s) (ml O2/ml x mm Hg).

Contact Lenses↗
PubMed1990

Corneal hypoxia and hypercapnia during contact lens wear.

Restriction of both oxygen influx to the cornea and carbon dioxide efflux from the cornea by contact lenses results in adverse tissue changes. We measured the extent of hypoxia and hypercapnia at the corneal surface of 10 human volunteers during static, dynamic (blinking), and closed-eye wear of hydrogel and nonhydrogel contact lenses of different gas transmissibilities. During open-eye wear, hypoxia and hypercapnia are lower beneath lenses of higher oxygen and carbon dioxide transmissibilities, respectively (p less than 0.001). Blinking plays a significant role in alleviating corneal hypoxia (p less than 0.05), but not hypercapnia, during lens wear. In the absence of lenses, the gaseous tensions at the anterior corneal surface during eye closure were: pO2 = 37.4 +/- 20.9 mm Hg and pCO2 = 39.3 +/- 3.1 mm Hg. Closed-eye lens wear resulted in greater levels of hypoxia and hypercapnia that were directly correlated with lens transmissibility (p less than 0.001). These data form the basis of models for predicting adverse tissue changes during contact lens wear and suggest prophylactic and therapeutic clinical strategies for alleviating lens-induced hypoxia and hypercapnia.

Adult↗
PubMed1989

Symptomatology of HEMA contact lens wear.

Identification of the determinants of patient symptoms is a requisite for successful patient management in contact lens practice. Symptoms reported by 104 patients wearing hydroxyethyl methacrylate (HEMA) contact lenses, who presented consecutively to a large contact lens clinic, were analyzed. Dryness was reported more frequently than the symptoms scratchy and watery (p less than 0.001). Females using oral contraceptives were more likely to experience the symptoms of scratchiness (p less than 0.01) and dryness (p less than 0.05) than females who were not using oral contraceptives. The symptoms dryness was reported more frequently by patients whose lenses were older than 6 months (p less than 0.05), and those wearing toric lenses (p less than 0.01). These findings will assist practitioners to anticipate those patients who are likely to develop contact lens-related symptoms and lay the foundations for a model of symptomatology during contact lens wear.

Acrylates↗
PubMed1989

Ocular surface temperature.

A wide-field color-coded infra-red imaging device was applied to the measurement of i) the temperature profile across the ocular surface and ii) the temporal stability of central corneal temperature, on 21 subjects. The thermographs showed a pattern of ellipsoidal isotherms (major axis horizontal) approximately concentric about a temperature apex (coldest point) which was slightly inferior to the geometric center of the cornea (GCC). The GCC had a mean temperature (+/- SD) of 34.3 +/- 0.7 degrees C (range 32.8 to 35.4 degrees C). Temperature increased towards the periphery of the cornea with the limbus being 0.45 degrees C warmer than the GCC (p less than 0.0001). Following a blink, the GCC cooled at a mean (+/- SD) rate of 0.033 +/- 0.024 degrees C/s (p less than 0.0001) over the first 15s. Subjects whose corneas cooled more slowly following a blink demonstrated a greater capacity to avoid blinking for a prolonged period (p less than 0.05). This improved method of measuring ocular surface temperature has important applications in modeling corneal physiology and pathology.

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

Ocular response to hydrogen peroxide.

A controlled, randomized, double-masked study was conducted on eight human subjects to determine the threshold level of hydrogen peroxide, which is toxic when introduced into the eye via a high water content (75%; Durasoft 4) hydrogel contact lens. Subjective comfort, conjunctival hyperemia, corneal and conjunctival epithelial staining, and corneal oxygen uptake were assessed in response to 5-min wear of lenses that were presoaked in isotonic saline solutions of physiologic pH containing 0, 25, 50, 100, 200, 400, and 800 parts per million (ppm) hydrogen peroxide. Higher levels of hydrogen peroxide were associated with greater discomfort (p less than 0.05) and increased conjunctival hyperemia (p less than 0.001). The highest level of hydrogen peroxide tested (800 ppm) did not induce significant corneal or conjunctival epithelial staining or alter the corneal aerobic response. We conclude that residual concentrations of hydrogen peroxide in contact lens care systems should not exceed 100 ppm. Practitioners can use these data to estimate the level of residual hydrogen peroxide to which a patient may have been exposed upon lens application after neutralization.

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↗
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↗