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Optical consequences of asymmetries in normal corneas
Abstracts Eighty-nine patients, following baseline pachymetry measurements, were fit with Acuvue extended wear lenses. Cornea1 thickness measurements were made on the moming after the first night of overnight wear, and again 6 days later. Nine subjects were unable to continue lens wear for a full week due to cornea1 decompensation. Two subjects showed signs and symptoms of cornea1 decompensation (pain, redness, tearing, cornea1 haze, striae, and fluorescein staining) after the first night, and the remaining subjects presented with signs and symptoms during the ensuing week. Seven subjects discontinued lens wear due to inadequate vision. There were no failures (within a week of overnight wear) from the group of subjects who exhibited less than 5% swelling after one day of extended wear. All four patients with cornea1 swelling of more than 13% in one night were unable to continue lens wear for the week. In the 24 patients who exhibited between 6 and 13% corneal swelling, the failure rate for the week-long wear was about 50%. Thicker, hyperopic lenses were found to be associated with a higher failure rate than myopic lenses. From the results of the study, the authors recommend that patients exhibiting more than 5% cornea1 swelling with the lenses in place for one night, should be discouraged from extended wear. Those with less than 5% swelling were able to tolerate at least 7 nights of extended wear. This study has illustrated an objective, in-office provocative test for identifying patients unsuitable for extended contact lens wear.
Optical Consequences of Asymmetries in Normal Corneas. Hemenger RI’, Tomlinson A, Oliver KM: Ophthalmic Physiol Opt (1996);16:124-129. This paper discusses cornea1 aberrations as calculated from videokeratographic data taken from the EyeSys Cornea1 Analysis System, with particular attention to curvature asymmetries. Earlier studies have shown that the eye suffers
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/CLc, Vol. 23, July/August
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from substantial optical aberrations. These calculations indicate that the cornea may be the major source of ocular aberrations, contributing significantly to spherical aherration and the asymmetric aberration coma. Using topographic data from 21 subjects, optical properties were determined with a technique described previously by the same authors. This method assumes that the cornea is a single surface, and calculates the optical properties of the system in terms of the distribution of phase angles over the pupil. The phase angles are then used to calculate the optical properties of the system by comparing the relative phases, at the image point, of rays passing through different regions of the pupil. Different optical aberrations may then be determined from the amount that these phases deviate from the perfect situation (in which the optical path length for each ray would be equal for all locations). The use of Zernike functions as a means of describing the different aberration terms is discussed. Illustrations are given showing the wavefront errors for spherical aberration, astigmatism, and coma. These illustra, tions help to visualize the effect of the aberrations on vision. It is also demonstrated that removing the coma error markedly improves the calculated cornea1 modulation, transfer function, and therefore is likely to enhance con, trast sensitivity function. A coefficient for the gradient of change in dioptric power per mm over the cornea is used to illustrate the effect of coma on the optical performance of the eye. Of the 21 subjects, 28% had substantial cornea1 asymmetry with a gradient of more than 0.5 diopters per mm. The figure showing estimated modulation-transfer function with and without this degree of coma shows the improvement that might be expected if this optical aberration were to be corrected. The authors suggest that the aberrations induced by corneal asymmetry could be corrected, at least in theory, by the fitting of carefully designed contact lenses. This type of correction would have the greatest significance for large pupils for which coma causes the greatest detriment to vision.
Craig and Blades
Jennifer P. Craig, PhD, MCOptom, is currently a postdoctoral research fellow in the Department of Vision Sciences at Glasgow Caledonian University in Scotland. Her main research interest is the physiology of the tear film in normal and dry eyes. Jennifer obtained a first-class honors degree in optometry in 1991 from the same institution, then known as Glasgow Polytechnic. After a year’s experience within the Hospital Eye Service, she attained a professional membership in the British College of Optometrists in 1992. Returning to Glasgow Caledonian University the same year, Jennifer undertook a 3-year studentship, under the supervision of Professor Alan Tomlinson, and was awarded a PhD, for research into tear physiology, in 1995. She has won numerous prizes to date, including the Naylor Prize and several prizes from the Scottish Committee of Optometrists. Kenny Blades, BSc( Hans), is a postgraduate student in the Department of Vision Sciences at Glasgow Caledonian University in Scotland. Under the supervision of Sudi Patel, he is currently researching the potential role of antioxidant therapy in the management of marginal dry eye and is hoping to complete his PhD program, within the next year. Studying physiology at St. Andrew’s University in Scotland, Kenny was awarded an upper second-class honors degree in 1993. During the 4-year course, he also studied psychology, logic, and pholosophy of scienc as minor subjects and his senior honors project involved the investigation of a 5-HT (serotonin)-dependent sensory pathway in the skin of Xeno@s embryos.
/CLc, Vol. 23, July/August
1996
157
Optical Consequences of Asymmetries in Normal Corneas. Hemenger RI’, Tomlinson A, Oliver KM: Ophthalmic Physiol Opt (1996);16:124-129. This paper discusses cornea1 aberrations as calculated from videokeratographic data taken from the EyeSys Cornea1 Analysis System, with particular attention to curvature asymmetries. Earlier studies have shown that the eye suffers
156
/CLc, Vol. 23, July/August
1996
from substantial optical aberrations. These calculations indicate that the cornea may be the major source of ocular aberrations, contributing significantly to spherical aherration and the asymmetric aberration coma. Using topographic data from 21 subjects, optical properties were determined with a technique described previously by the same authors. This method assumes that the cornea is a single surface, and calculates the optical properties of the system in terms of the distribution of phase angles over the pupil. The phase angles are then used to calculate the optical properties of the system by comparing the relative phases, at the image point, of rays passing through different regions of the pupil. Different optical aberrations may then be determined from the amount that these phases deviate from the perfect situation (in which the optical path length for each ray would be equal for all locations). The use of Zernike functions as a means of describing the different aberration terms is discussed. Illustrations are given showing the wavefront errors for spherical aberration, astigmatism, and coma. These illustra, tions help to visualize the effect of the aberrations on vision. It is also demonstrated that removing the coma error markedly improves the calculated cornea1 modulation, transfer function, and therefore is likely to enhance con, trast sensitivity function. A coefficient for the gradient of change in dioptric power per mm over the cornea is used to illustrate the effect of coma on the optical performance of the eye. Of the 21 subjects, 28% had substantial cornea1 asymmetry with a gradient of more than 0.5 diopters per mm. The figure showing estimated modulation-transfer function with and without this degree of coma shows the improvement that might be expected if this optical aberration were to be corrected. The authors suggest that the aberrations induced by corneal asymmetry could be corrected, at least in theory, by the fitting of carefully designed contact lenses. This type of correction would have the greatest significance for large pupils for which coma causes the greatest detriment to vision.
Craig and Blades
Jennifer P. Craig, PhD, MCOptom, is currently a postdoctoral research fellow in the Department of Vision Sciences at Glasgow Caledonian University in Scotland. Her main research interest is the physiology of the tear film in normal and dry eyes. Jennifer obtained a first-class honors degree in optometry in 1991 from the same institution, then known as Glasgow Polytechnic. After a year’s experience within the Hospital Eye Service, she attained a professional membership in the British College of Optometrists in 1992. Returning to Glasgow Caledonian University the same year, Jennifer undertook a 3-year studentship, under the supervision of Professor Alan Tomlinson, and was awarded a PhD, for research into tear physiology, in 1995. She has won numerous prizes to date, including the Naylor Prize and several prizes from the Scottish Committee of Optometrists. Kenny Blades, BSc( Hans), is a postgraduate student in the Department of Vision Sciences at Glasgow Caledonian University in Scotland. Under the supervision of Sudi Patel, he is currently researching the potential role of antioxidant therapy in the management of marginal dry eye and is hoping to complete his PhD program, within the next year. Studying physiology at St. Andrew’s University in Scotland, Kenny was awarded an upper second-class honors degree in 1993. During the 4-year course, he also studied psychology, logic, and pholosophy of scienc as minor subjects and his senior honors project involved the investigation of a 5-HT (serotonin)-dependent sensory pathway in the skin of Xeno@s embryos.
/CLc, Vol. 23, July/August
1996
157