Biomedical subjects
Publications and source records attributed to N Efron.
A randomized, double-masked, placebo-controlled study was conducted to determine the effect of the prostaglandin-inhibitor naproxen on the bleb response of the corneal endothelium during hydrogel contact lens wear. Naproxen did not significantly alter the bleb response. This finding suggests that the endothelial bleb response is not prostaglandin-mediated.
Previous studies have demonstrated that the prostaglandin-inhibitor naproxen can reduce post-surgical corneal oedema, which is largely due to an inflammatory process. To determine whether there is an inflammatory component in the oedematous response of the cornea to contact lens wear, we conducted a randomized, double-masked, placebo-controlled study on the effect of naproxen on contact lens-induced corneal swelling. This drug did not have a significant effect, suggesting that prostaglandins are not involved in the hypoxic oedema response. The discrepancy between the effect of naproxen on lens- and surgically-induced corneal oedema can be attributed to the different aetiologies of these oedema responses.
Both corneas of 12 unilateral graft patients were subjected to 9% hypoxic oedema. Following removal of the hypoxic stimulus, the grafted corneas deswelled at a significantly slower rate (P less than 0.01) than the fellow unoperated corneas. We suggest that this can be attributed to a change in the barrier properties and/or pump efficiency of the endothelium. Monitoring corneal deswelling following hypoxic oedema could form the basis of a clinical test of corneal endothelial function.
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.
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.
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 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.
The effect of pupil size on the relation between Snellen visual acuity and corrected and uncorrected myopia was examined for 22 young subjects with degrees of myopia ranging from 0.75 D to 7.5 D. Effective pupil size was varied by inducing mydriasis and then placing artificial pupils of between 1.0 and 8.0 mm diameter before the eye. Both a constant chart luminance of 120 cd/m2 and a constant retinal illuminance of 2150 trolands were used. There was little difference in results for the two lighting conditions. For the corrected myopes considered as a group, maximum visual acuity occurred for 2--3 mm diameter pupils, but larger pupils reduced acuity only marginally. For the uncorrected myopes, variation in pupil size produced a large variation in visual acuity, and for refractive errors greater than about 1.5 D, the optimum pupil diameter was less than 1 mm. For uncorrected myopes of 3.0 D or less, visual acuity was nearly as good with a 1-mm pupil as for corrected myopes. The presented data are a useful guide to the clinician.
The presence of epicapsular stars (congenital epicapsular pigmentation) is a common occurrence, but it can become clinically significant when vision is impaired. A case history reporting such a condition is presented.
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.
In celebration of the centenary of August Müller's pioneering work on contact lenses, we present, for the first time, a full translation into English of Part III of his inaugural dissertation. Müller describes his attempts to correct his own -14.00 diopters of myopia with "cornea-lenses," which were essentially glass scleral lenses. His inability to tolerate lens wear for more than half an hour was probably due to a) limbal pressure as a result of the primary optic diameter being too small, and b) the use of water to occupy the space between the lens and cornea, which would have induced severe corneal edema. Many of the observations and recommendations embodied in Müller's thesis, such as the importance of a good fit, adequate tear flow and rounded edges, are still relevant to contact lens practice today.
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