Biomedical subjects
Publications and source records attributed to L G Carney.
Tear protein deposition on contact lenses can lead to interruption or even cessation of lens wear. Yet, there are positive roles that tear proteins play in the maintenance of ocular integrity, with and without contact lenses present. One of these roles, that of tear buffering, is explored here.
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.
The effect of topical anaesthetics on human corneal integrity was assessed by measurement of the alterations they cause in corneal damage threshold. The anaesthetics investigated were 1.0% Amethocaine HCl, assessed immediately after instillation and after delays of 20 min and 60 min, and 0.5% Amethocaine HCl and 0.4% Benoxinate HCl, assessed 20 min after instillation. At 20 min after instillation of 1.0% Amethocaine HCl, the corneal damage threshold was significantly decreased. At other times for this anaesthetic, and for 0.5% Amethocaine HCl and 0.4% Benoxinate HCl, the corneal damage threshold was unaltered.
A fluorometric technique for measuring the activities of lactate dehydrogenase (LDH) and malate dehydrogenase (MDH) in human tear fluid is described. Measurement of separate LDH and MDH activities during the day resulted in fluctuating values with no definite pattern. However, a consistent diurnal pattern emerged when the tear LDH/MDH ratio was measured for individual tear sample. The tear LDH/MDH ratio was highest upon waking, declining soon after to a stable level for the remainder of the day. Evidence for a corneal epithelial origin of tear LDH and MDH is discussed, as well as the ability of the technique to detect differences in the metabolic status of the epithelium between the open-eye and closed-eye environments. Possible causes of the elevation of the tear LDH/MDH ratio following lid closure are unbinding of intracellular M-type LDH within epithelial cells and increased cell membrane permeability, both processes occurring in response to reduced cellular energy content under the more hypoxic conditions of lid closure. Although the nature of the present experiment did not allow any conclusive statement, de novo LDH synthesis may also be contributing to the increased tear LDH/MDH ratio. Deliberate corneal epithelial surface disruption was found to have no quantifiable effect on the tear LDH/MDH ratio.
The corneal shape of 196 eyes was determined by photokeratoscopy and subsequent fitting of a conicoid equation to four individual corneal meridians. This conicoid was found in almost all instances to be ellipsoidal, with the asphericity not varying between meridians. The corneal radius of curvature did vary as expected between meridians. The radii of curvature of these corneal meridians also varied with the age of the subjects, resulting in reduced corneal astigmatism, while the asphericity of each meridian did not vary substantially with age.
Recent clinical and experimental evidence suggests that a better physiological response to hydrogel lens wear can be obtained with fenestrations. To determine whether those observations could be attributed to increased corneal oxygenation, the equivalent oxygen percentage (EOP) was measured at the cornea of eight subjects wearing hydrogel contact lenses which had multiple fenestrations of 0.8 and 1.8 mm. It was possible to increase the mean EOP by 1.7 +/- 1.3% O2 (p less than 0.002) beneath a standard thickness lens using four 1.8-mm fenestrations. However, such lenses were uncomfortable to wear. Fenestrations that provided comfortable wear (0.8-mm diameter) did not significantly increase the EOP beneath standard thickness lenses (-0.1 +/- 1.0% O2, p greater than 0.1 for four fenestrations, and +0.2 +/- 1.2% O2, p greater than 0.1 for eight fenestrations) or ultrathin lenses (-0.1 +/- 1.3% O2 p greater than 0.1 for four fenestrations). It is concluded that fenestrations do not provide a clinically efficient means of increasing the oxygen tension beneath hydrogel contact lenses.
Two studies were performed to investigate dimensional stability of the cornea throughout the female menstrual cycle. In the first study, changes of corneal curvature, topography, and thickness were measured for six women for one complete menstrual cycle. Steepening of horizontal and vertical curvatures occurred at the beginning of the cycle, and flattening occurred after ovulation. Corneal thickening occurred on the second day of the cycle and around the time of ovulation, then thinning and another slight thickening on day 21. In the second study, more detailed information was sought on the timing of corneal thickness changes throughout any cycle. Corneal thickness was studied for two subjects throughout three consecutive cycles. Both subjects showed a slight decrease in thickness toward the end of the menses; thickening occurred at ovulation followed by thinning. Thickening also occurred 4 days after ovulation. Consideration of changes in urine levels of estrogen and pregnanediol suggests that rises in estrogen are accompanied by increases in corneal thickness.
The oxygen tension, which can also be stated as the equivalent oxygen percentage (EOP), was measured beneath a variety of hard and soft gas-permeable contact lenses following static and dynamic wearing conditions. The significant increases in EOP beneath rigid lenses upon blinking were to be expected in view of the tear pumping mechanisms known to exist with such lenses. However, blinking was found to have a minimal effect on the EOP beneath hydrogen lenses, confirming earlier predictions.
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Measurements of central and peripheral corneal thickness, corneal topography, and central corneal curvature were made for 21 subjects at hourly intervals for 12 hr of 1 day for each subject. Values for corneal radius and asphericity were obtained from keratometry and photokeratoscopy, and their stability throughout the day was evaluated. Corneal radius was observed to steepen during the day, whereas asphericity remained constant. Corneal thickness was measured by pachometry and was found to be at its thickest on first awakening. Corneal curvature and central corneal thickness were highly correlated; low correlation was found between corneal asphericity and central corneal thickness. Some subjects exhibited large fluctuations in one or all of the parameters considered.
Oxygen availability under hydrogel lenses was followed for 19 subjects over the first 6 months of wear. Each subject was assigned to a specific cleaning group, through which the method and intensity of cleaning was varied. The mean oxygen availability did not change significantly over the first six months of wear, regardless of the cleaning system used. However, the stability of the oxygen supply was dependent on the intensity of the cleaning system.
The blinking patterns of 20 normal subjects were examined under controlled experimental conditions. While the mean blink rate was 12.55 blinks/min, there were marked variations between individuals. As well, each subject displayed a characteristic pattern of blinking, mixing inter-blink periods of shorter and longer durations in a regular fashion.
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.
An equivalent oxygen technique was used to measure the level of oxygen beneath one daily wear lens (HEMA) and two extended wear lenses (Permalens and Gelflex 75) under the closed lid of the in vivo human eye. The cornea responded as if it was in an anoxic environment with the HEMA lens, whereas the extended wear lenses resulted in responses just beneath the level of the Mandell-Farrell limit. These findings are compared with model predictions, and their physiological consequences are discussed.
The measurement of anterior corneal surface topography before and during the occurrence of monocular diplopia can be used to predict the angular positions of any secondary images resulting from corneal distortion. The predictions correlate at a statistically significant level with direct measurements of the positions of the secondary images.
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.
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.
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.