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At least 757 records · Page 42Linked to original sources

PubMed2007

Determinants of normal retinal nerve fiber layer thickness measured by Stratus OCT.

PURPOSE: To determine the effects of age, optic disc area, ethnicity, eye, gender, and axial length on the retinal nerve fiber layer (RNFL) in the normal human eye as measured by Stratus OCT (optical coherence tomography). DESIGN: Cross-sectional observational study. PARTICIPANTS: Three hundred twenty-eight normal subjects 18 to 85 years old. METHODS: Peripapillary Fast RNFL scans performed by Stratus OCT with a nominal diameter of 3.46 mm centered on the optic disc were performed on one randomly selected eye of each subject. MAIN OUTCOME MEASURES: Linear regression analysis of the effects of age, ethnicity, gender, eye, axial length, and optic disc area on peripapillary RNFL thickness. RESULTS: The mean RNFL thickness for the entire population was 100.1 microm (standard deviation, 11.6). Thinner RNFL measurements were associated with older age (P<0.001); being Caucasian, versus being either Hispanic or Asian (P = 0.006); greater axial length (P<0.001); or smaller optic disc area (P = 0.010). For every decade of increased age, mean RNFL thickness measured thinner by approximately 2.0 microm (95% confidence interval [CI], 1.2-2.8). For every 1-mm-greater axial length, mean RNFL thickness measured thinner by approximately 2.2 microm (95% CI, 1.1-3.4). For every increase in square millimeter of optic disc area, mean RNFL thickness increased by approximately 3.3 microm (95% CI, 0.6-5.6). Comparisons between ethnic groups revealed that Caucasians had mean RNFL values (98.1+/-10.9 microm) slightly thinner than those of Hispanics (103.7+/-11.6 microm; P = 0.022) or Asians (105.8+/-9.2 microm; P = 0.043). There was no relationship between RNFL thickness and eye or gender. CONCLUSIONS: Retinal nerve fiber layer thickness, as measured by Stratus OCT, varies significantly with age, ethnicity, axial length, and optic disc area. These variables may need to be taken into account when evaluating patients for diagnosis and follow-up of glaucoma, particularly at the lower boundary of the normal range. Due to the relatively small numbers of subjects of Asian and African descent in the normative database, conclusions regarding the effect of ethnicity should be interpreted with caution.

Adolescent↗

PubMed1998

The SRKII formula in the calculation of intraocular lens power.

This study compares the predictive accuracy of the SRKII formula for groups with average axial length and long eyes. 344 eyes which underwent cataract extraction with implantation of a posterior-chamber intraocular lens were included in the survey; 246 eyes had an average axial length (22.0-24.49 mm), and 98 eyes had an axial length of more than 24.50 mm. Mean absolute errors were 0.26+/-0.68 dpt in the first group and 1.16+/-1.78 dpt in the second group. 94.3% of the average eyes were within 1.0 dpt of the expected refraction, 98.8% within 2.0 dpt, whereas in the long-eye group 58.2% were within 1.0 dpt and 84.7% within 2.0 dpt. The results show that the SRKII formula performed credibly in average-length eyes; in the long eyes the ratio of large refractive deviations was greater than that of the other group.

Biometry↗

PubMed2006

Analysis of intraocular lens power calculation for eyes with previous myopic LASIK.

PURPOSE: To evaluate the effectiveness of a manual keratometry (K) adjusted value for intraocular lens (IOL) power calculation in patients who underwent cataract surgery following previous myopic LASIK. METHODS: Sixteen eyes of 14 consecutive patients who underwent cataract surgery after previous LASIK were evaluated retrospectively. All IOL powers were calculated using an adjusted K value (K minus 1.0 diopter [D]) with the Binkhorst II formula aiming for -0.75 to -1.00 D final refraction. Additionally, the IOL power for each eye was retrospectively calculated using K, refractive-derived K, and adjusted K with the Binkhorst II, Holladay I, and SRK/T formulas. The final refraction was used as a criterion of accuracy of each approach. RESULTS: Uncorrected visual acuity > or = 20/40 was achieved in 14 (87.5%) of 16 eyes. The mean postoperative spherical equivalent refraction was -0.41 +/- 0.57 D (range: +0.50 to -2.00 D). Twelve (75%) of 16 eyes were within +/- 0.50 D of emmetropia and 15 (94%) of 16 eyes were within +/- 1.00 D. No eye was > +1.00 D. CONCLUSIONS: Using an adjusted K with the Binkhorst II formula, aiming for -0.75 to -1.00 D, and with the Holladay I formula, aiming for -0.50 to -1.00 D, measuring K with a regular manual keratometer permits determination of an IOL power after myopic LASIK without the need of preoperative LASIK refractive data.

Adult↗

PubMed1992

Ocular manifestations of Noonan syndrome.

Noonan syndrome is a genetic condition inherited in an autosomally dominant manner, characterised by congenital heart disease, short stature, abnormal facies and the somatic features of Turner's syndrome, but a normal Karyotype. The ophthalmological and orthoptic findings on 58 patients with Noonan syndrome are reported. External features were hypertelorism (74%), downward sloping palpebral apertures (38%), epicanthic folds (39%) and ptosis (48%). The orthoptic examination revealed strabismus in 48%, refractive errors in 61%, amblyopia in 33%, and nystagmus in 9% of cases. Sixty-three per cent of cases had anterior segment changes consisting of: Prominent corneal nerves (46%), anterior stromal dystrophy (4%), cataracts (8%) and panuveitis (2%). Fundal changes occurred in 20% of the study group, including optic nerve head drusen, optic disc hypoplasia, colobomas and myelinated nerves. Forty-seven per cent required non surgical treatment and a further 16% had undergone surgery for strabismus or ptosis. Only three patients had no visual defects. With such a high incidence of ophthalmic abnormalities it is clearly important that children with Noonan syndrome are screened by an ophthalmologist at an early age.

Adolescent↗

PubMed2000

Comparison of immersion ultrasound biometry and partial coherence interferometry for intraocular lens calculation according to Haigis.

BACKGROUND: The precision of intraocular lens (IOL) calculation is essentially determined by the accuracy of the measurement of axial length. In addition to classical ultrasound biometry, partial coherence interferometry serves as a new optical method for axial length determination. A functional prototype from Carl Zeiss Jena implementing this principle was compared with immersion ultrasound biometry in our laboratory. PATIENTS AND METHODS: In 108 patients attending the biometry laboratory for planning of cataract surgery, axial lengths were additionally measured optically. Whereas surgical decisions were based on ultrasound data, we used postoperative refraction measurements to calculate retrospectively what results would have been obtained if optical axial length data had been used for IOL calculation. For the translation of optical to geometrical lengths, five different conversion formulas were used, among them the relation which is built into the Zeiss IOL-Master. IOL calculation was carried out according to Haigis with and without optimization of constants. RESULTS: On the basis of ultrasound immersion data from our Grieshaber Biometric System (GBS), postoperative refraction after implantation of a Rayner IOL type 755 U was predicted correctly within +/- 1 D in 85.7% and within +/- 2 D in 99% of all cases. An analogous result was achieved with optical axial length data after suitable transformation of optical path lengths into geometrical distances. CONCLUSIONS: Partial coherence interferometry is a noncontact, user- and patient-friendly method for axial length determination and IOL planning with an accuracy comparable to that of high-precision immersion ultrasound.

Adult↗

PubMed1976

Blurred imagery and the cylinder sine-squared law.

The sine-squared law is shown to describe changes in spherical power which either minimize the blur or maximize the visibility of gratings oriented at various angles to the principal meridians of an astigmatic eye. These results help justify the use of the sine-squared law in applications such as meridional refraction. This derivation and its interpretation is quite different from the usual approach which emphasizes the meridional changes in curvature of a toric surface.

Astigmatism↗

PubMedDate not supplied

Contemporary management of aniseikonia.

The diagnosis and management of aniseikonia has become a lost art. Refractive surgery and intraocular lens implantation has created a new patient population of anisometropia. Measurement of aniseikonia through instrumentation such as the space eikonometer is not readily available. Therefore, estimating the degree of aniseikonia based on the dioptral differences between the two eyes may be the most practical approach. The optical principles of spectacle magnification can be translated by the prescribing eye doctor to reduce ocular image disparity. Contemporary lens designs, such as hi-index and aspherics, allow the practitioner to modify shape and power factors to achieve higher levels of aniseikonic correction in a cosmetically fashionable pair of spectacles.

Aniseikonia↗

PubMedDate not supplied

Corneal polarimetry after LASIK refractive surgery.

Imaging polarimetry provides spatially resolved information on the polarization properties of a system. In the case of the living human eye, polarization could be related to the corneal biomechanical properties, which vary from the normal state as a result of surgery or pathologies. We have used an aberro-polariscope, which we recently developed, to determine and to compare the spatially resolved maps of polarization parameters across the pupil between normal healthy and post-LASIK eyes. The depolarization distribution is not uniform across the pupil, with post-surgery eyes presenting larger levels of depolarization. While retardation increases along the radius in normal eyes, this pattern becomes irregular after LASIK refractive surgery. The maps of slow axis also differ in normal and post-surgery eyes, with a larger disorder in post-LASIK eyes. Since these changes in polarization indicate subtle structural modifications of the cornea, this approach can be useful in a clinical environment to follow the biomechanical and optical changes of the cornea after refractive surgery or for the early diagnosis of different corneal pathologies.

Adult↗

PubMed1995

[Visual acuity, refraction and color vision after implantation of foldable silicon lenses].

Implantation of intraocular lenses through a small opening is possible with phacoemulsification and foldable silicon lenses. There is little traumatic effect, which is positive for the healing process and the postoperative results of visual acuity and refraction. Because of alteration of the optic media, deteriorated sensation is possible. We investigated 28 patients with foldable silicon lenses after phacoemulsification. Pre- and postoperatively (1 day, 1 week, 3, 6, 12, 18 and 24 months) visual acuity and astigmatism were measured. As the testing of color vision is a sensitive method for the detection of disturbance in sensation because of altered optic media this was performed 24 months postoperatively using the pseudoisochromatic plates of Ishihara and Ichikawa and the color arrangement tests, Farnsworth Panel D-15 desaturated and the Farnsworth Munsell 100 hue test. The results of this study confirm quicker rehabilitation of visual function and a lower rate of operatively induced astigmatism. A slight blue color vision deficiency results as consequence to altered optic media, but there is no effect on daily life.

Aged↗

PubMed2003

IOLMaster biometry: refractive results of 100 consecutive cases.

AIMS: To study the refractive outcome of cataract surgery employing IOLMaster biometry data and to compare it with that of applanation ultrasonography in a prospective study of 100 eyes that underwent phacoemulsification with intraocular lens implantation. METHODS: The Holladay formula using IOLMaster data was employed for the prediction of implanted intraocular lenses (IOLs). One month after cataract surgery the refractive outcome was determined. Preoperative applanation ultrasonography data were used retrospectively to calculate the IOL prediction error. The two different biometry methods are compared. RESULTS: 100 patients, 75.42 (SD 7.58) years of age, underwent phacoemulsification with IOL implantation. The optical axial length obtained by the IOLMaster was significantly longer (p<0.001, Student's t test) than the axial length by applanation ultrasound, 23.36 (SD 0.85) mm v 22.89 (0.83) mm. The mean postoperative spherical equivalent was 0.00 (0.40) D and the mean prediction error -0.15 (0.38) D. The mean absolute prediction error was 0.29 (0.27) D. 96% of the eyes were within 1 D from the intended refraction and 93% achieved unaided visual acuity of 6/9 or better. The Holladay formula performed better than the SRK/T, SRK II, and Hoffer Q formulas. Applanation ultrasonography after optimisation of the surgeon factor yielded a greater absolute prediction error than the optimised IOLMaster biometry, 0.41 (0.38) D v 0.25 (0.27) D, with 93% of the eyes within 1 D from the predicted refraction. CONCLUSION: IOLMaster optical biometry improves the refractive results of selected cataract surgery patients and is more accurate than applanation ultrasound biometry.

Adult↗

PubMed1978

Optical characteristics of the eye of the spiny dogfish (Squalus acanthias).

On the basis of the failure of attempts to induce refractive accommodation changes electrically and with drugs, it appears that the eye of the spiny dogfish (Squalus acanthias) lacks the capacity to accommodate. Retinoscopic measurements indicate moderate hyperopia (+6D). However, measurements made with light restricted to the short end of the visible spectrum indicate only one or two dioptres of hyperopia and it is possible that the latter measurements are more coincident with natural conditions. In contrast to the somewhat flattened lens of other elasmobranchs the lens of the spiny dogfish is nearly spherical. The fact that its refractive index is high (1.66) suggests that the static optical system of this eye is very similar to that of the teleosts.

Accommodation, Ocular↗

PubMed2006

Laser trapping in anisotropic fluids and polarization-controlled particle dynamics.

Anisotropic fluids are widespread, ranging from liquid crystals used in displays to ordered states of a biological cell interior. Optical trapping is potentially a powerful technique in the fundamental studies and applications of anisotropic fluids. We demonstrate that laser beams in these fluids can generate anisotropic optical trapping forces, even for particles larger than the trapping beam wavelength. Immersed colloidal particles modify the fluid's ordered molecular structures and locally distort its optic axis. This distortion produces a refractive index "corona" around the particles that depends on their surface characteristics. The laser beam can trap such particles not only at their center but also at the high-index corona. Trapping forces in the beam's lateral plane mimic the corona and are polarization-controlled. This control allows the optical forces to be reversed and cause the particle to follow a prescribed trajectory. Anisotropic particle dynamics in the trap varies with laser power because of the anisotropy of both viscous drag and trapping forces. Using thermotropic liquid crystals and biological materials, we show that these phenomena are quite general for all anisotropic fluids and impinge broadly on their quantitative studies using laser tweezers. Potential applications include modeling thermodynamic systems with anisotropic polarization-controlled potential wells, producing optically tunable photonic crystals, and fabricating light-controlled nano- and micropumps.

Journal Article↗

PubMed2006

Accuracy of intraocular lens power prediction using the Hoffer Q, Holladay 1, Holladay 2, and SRK/T formulas.

PURPOSE: To compare the accuracy of intraocular lens (IOL) power calculations using 4 formulas: Hoffer Q, Holladay 1, Holladay 2, and SRK/T. SETTING: Tertiary care center. METHODS: This study was a retrospective comparative analysis. Immersion ultrasound biometry (axial length, anterior chamber depth, and lens thickness), manual keratometry, and postoperative manifest refraction were obtained in 643 eyes of consecutive patients who had routine uneventful cataract surgery with implantation of 1 of 2 IOLs using the same operative technique by the same surgeon. Biometric data were entered into each of the 4 IOL power calculation formulas, and the results were compared to the final manifest refraction. An optimized lens constant was used for each formula. Results were also stratified into groups of short, average, medium long, and very long axial length <22.0 mm, 22.0 to <24.5 mm, 24.5 to 26.0 mm, and >26.0 mm, respectively). RESULTS: No formula was more accurate than the others as measured by mean absolute error. The formulas were also equally accurate when eyes were stratified by axial length. CONCLUSION: The 4 IOL power formulas provided equivalent refractive results in the entire group of eyes and in the subsets of axial lengths tested.

Biometry↗

PubMed2006

Refractive index of carcinogen-induced rat mammary tumours.

Near-infrared optical techniques for clinical breast cancer screening in humans are rapidly advancing. Based on the computational inversion of the photon diffusion process through the breast, these techniques rely on optical tissue models for accurate image reconstruction. Recent interest has surfaced regarding the effect of refractive index variations on these reconstructions. Although many data exist regarding the scattering and absorption properties of normal and diseased tissue, no measurements of refractive index appear in the literature. In this paper, we present near-infrared refractive index data acquired from N-methyl-N-nitrosourea-induced rat mammary tumours, which are similar in pathology and disease progression to human ductal carcinoma. Eight animals, including one control, were employed in this study, yielding data from 32 tumours as well as adjacent adipose and connective tissues.

Animals↗

PubMed1990

A simple modified SRK formula for severely myopic eyes.

We describe our modification of the most accurate intraocular lens power calculation formula currently used, the SRK formula, to improve the accuracy with which it can be used to calculate the power of IOLs that are implanted in severely myopic eyes, especially in those with exceptional axial length. This modified formula, which we call the L-SRK, is I = A - 2.5 L - 0.9 K - 1.69R - 1.69 (where I = the actual implanted IOL power; A = the A-constant; L = the axial length; K = the average keratometer reading; and R = the predicted postoperative refraction). The results achieved using this modified formula demonstrate its superior accuracy in calculating lens powers for severely myopic eyes.

Humans↗

PubMed1997

Multimeridional refraction: dependence of the measurement accuracy on the number of meridians refracted.

A Monte Carlo simulation of multimeridional refraction measurements was used to investigate the dependence of the accuracy of the measurement on the number of meridians refracted, N, and on the standard deviation of a measurement in a single meridian, sigma. For the description of the measurement errors, the residual refraction values were used, i.e., the parameters of the refraction remaining after application of the measured correction. The distributions of the residual refraction values were found to be independent of the "true" refraction values; in addition, by means of a factor square root of N/sigma, reduced residual refraction values could be defined which also were independent of N and sigma. A vector space proposed by Lakshminarayanan and Varadharajan (based on Long's power matrix) was used to represent the joint distribution of the residual refraction values in three-dimensional space. It was found to be a three-variate Gaussian distribution with zero mean and diagonal covariance matrix. It could further be shown that the vector space proposed by Harris is identical to the one used, up to a linear transformation. Several criteria, based on the one- and three-dimensional distributions and corresponding to different levels of accuracy, are discussed resulting in a wide range of answers about the number of meridians to be refracted.

Analysis of Variance↗

PubMed1988

Effect of chromatic dispersion of a lens on visual acuity.

Materials with a high refractive index have a considerable level of chromatic dispersion which, around the periphery of corrective lenses, may affect the wearer's visual acuity. By measuring visual acuity through prisms of increasing chromatic power we have established the relation between chromatic power and visual acuity. The maximum chromatic dispersion of materials useable in ophthalmic optics can be deduced from these results.

Adult↗

PubMed1990

Monocular simultaneous refraction.

A system was developed to give an alternative to the standard Jackson Cross Cylinder test. This system eliminates the major difficulty of the Jackson Cross Cylinder test, which is alternative viewing of the images. The advantages of this system are: 1) ease of patient choice, 2) speed of testing, 3) patient preference, and 4) ease of understanding the test. The major disadvantage of this test system was that a few patients had difficulty viewing two images. Most difficulties could be easily resolved by a variation in the examination technique. (It helped if the patients were told they would now see double, or two rows of letters.) Occasionally, patients who had a field loss found this test difficult. The second problem was similar to one encountered with the Jackson Cross Cylinder test in that some patients would report one target clearer, regardless how one changed the lens. This was overcome by turning the testing lens system over so that the testing lens on the bottom was now on the top. Recently, we have relied only on the MSR technique as our subjective refraction, and the final prescription was based on it. These patients have reported no problems and have had good acuities. The MSR technique, when tested on a large number of patients, is demonstrated to be both efficient and reliable. The simultaneous refraction technique is not meant to be used to the exclusion of all other cylinder subjective tests, but, rather as an adjunct to our standard technique when conditions warrant its use.

Astigmatism↗