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At least 415 records · Page 23Linked to original sources
PubMed1999

A study of aniseikonia and Knapp's law using a projection space eikonometer.

PURPOSE: Knapp's Law, which states that anisometropias due to varying abnormal axial lengths between eyes would not result in inequality in relative retinal image size, provided the correcting spectacle lens was placed at the far point of the eye, has been shown to fall short in clinical practice in several studies using "direct comparison eikonometry". To test these findings using space eikonometry and to further elucidate this Law's clinical applicability, the following study was conducted. METHODS: Thirteen patients with suspected axial anisometropia of at least 4 Diopters were identified, selected and examined. Cycloplegic refraction, A-scan ultrasonic ocular biometry and Essilor Projection Space Eikonometry were performed. RESULTS: Ten of the thirteen patients had their anisometropia due primarily to ocular axial length differences. Of these ten, seven (70%) had detectable levels of aniseikonia and 3 (30%) demonstrated no aniseikonia. The other three patients whose aniso-metropia was due to combined axial and refractive components, all had aniseikonia. CONCLUSION: As a geometric optics theory, Knapp's Law stands on its own merits. However, in clinical practice, reduction in retinal element density in high myopia limits its applicability. Such patients often do have significant aniseikonia which can produce ocular referable complaints and/or interfere with binocular vision.

Adolescent↗
PubMed1993

[Crystal morphological and habit investigations of crystalline drugs. II. Amobarbital, barbital and ephedrine hydrochloride].

Crystal morphology, habit and refractive indices necessary for the analyses of amobarbital, barbital, phase III and ephedrine hydrochloride were studied using a polarizing microscope and partly using a reflexing goniometer. These substances, market products of Japanese Pharmacopoeia XII, were measured for optical properties including key refractive indices to confirm their modifications, and their possible facial angles along the zone of which axis is parallel to the elongated direction was also measured using a goniometer. Stereographic projections were drawn and the crystal habits of market products were shown by orthographic projections. It was also found that one of the key refractive indices of each modification was specially usable for the analyses of crystal habits including their crushed powders.

Amobarbital↗
PubMedDate not supplied

Applications of confocal microscopy in refractive surgery.

PURPOSE: To evaluate applications of the Confoscan 2.0, a computerized confocal optical microscopy system by Nidek, in refractive surgery. METHODS: We used the Confoscan 2.0 to evaluate the tissue reparative response after PRK, LASEK, and LASIK in 60 eyes of 40 patients. Confoscan 2.0 provided highly detailed images of the corneal layers, up to the cellular level. Furthermore, it measured corneal thickness as well as precise positioning of each single recorded corneal layer. The Confoscan was also used to analyse the LASIK flap. RESULTS: Findings in the transition zone between epithelium and anterior stroma in PRK and LASEK eyes were divided into two groups: Group 1, with marked hyper-reflectivity, and Group 2, with less hyper-reflectivity, similar to normal eyes. Group 2 eyes belonged to patients that underwent smoothing with a masking solution after ablation. Group 1 eyes received no smoothing. In LASIK eyes, we evaluated the quality of the flap interface. Hyper-reflecting specks could be observed in the interface, showing a tendency to decrease over time. Depth of the LASIK flap was measured, and showed variable thickness with all microkeratomes tested. CONCLUSION: We hypothesize that a more regular corneal surface, obtained with smoothing in PRK and LASEK, allows a better reparative response after refractive surgery. In LASIK eyes, the Confoscan provided precise measurement of the residual stromal bed, information particularly valuable for the prevention of ectasia in retreatments.

Adult↗
PubMed1987

[Macrodisks with physiologic macrocups (pseudo-glaucoma disks). Papillometric characteristics in 17 eyes].

Characteristics of 17 optic disks larger than 4.4 mm2 and thus satisfying the papillometric criteria of macrodisks (larger than normal optic disks plus two-fold standard deviation) were as follows: (1) Form: similar to normal optic nerve heads but even more circular. The vertical diameter was about 6% larger than the horizontal. (2) Optic cup: abnormally large, therefore "pseudoglaucomatous." (3) Neuroretinal rim: normal-sized when correlated to the total optic disk area, larger in the temporal lower than in the temporal upper optic disk region, narrowest on the temporal optic disk side, larger nasally at the top, and widest at the upper optic disk pole. (4) Cup/disk ratio: abnormally high (0.76 +/- 0.06 horizontally, 0.69 +/- 0.05 vertically), larger horizontally than vertically. (5) Perimetry: the only abnormality was an increased blind spot corresponding to the enlarged optic disk area. (6) Refraction. -0.25 +/- 1.90 diopters (high myopics less than -8.00 diopters) had been excluded). (7) Central retinal artery and vein penetrating the superficial lamina cribrosa almost at the center of the cup, bayonetlike vessel bending at the border of the optic cup. (8) Increased number of cilioretinal arteries, corresponding to their correlation with the optic disk area. These macrodisks with physiologic macrocups can be differentiated by their morphology or by optic disk planimetry in absolute values from: normal-sized optic nerve heads with glaucomatous cupping, abnormally large optic nerve heads in highly myopic eyes, and abnormally large optic nerve heads with pitting of the optic disk.

Adolescent↗
PubMed1996

Selective broad-band spatial frequency loss in contrast sensitivity functions. Comparison with a model based on optical transfer functions.

PURPOSE: Contrast sensitivity functions (CSFs) were measured under various optical conditions in healthy observers together with CSFs from selected patients. Threshold increases across the spatial frequency range were compared with predictions of a theoretical optical model based on modulation transfer functions. METHODS: Contrast thresholds for various spatial frequencies were determined with a computer-automated method of ascending limits in a control group and a group of patients with various visual pathway diseases ranging from retinal disorders, such as diabetic retinopathy, to neural disorders, such as multiple sclerosis. For normal control subjects, simulated contrast sensitivity losses also were effected by manipulating pupil diameter and dioptric blur. Modulation transfer functions of the eye's optics in polychromatic light were calculated. The wave aberration function included standard spherical aberration, coma, and small amounts of irregular aberrations. RESULTS: Experimentally, slight dioptric blur (e.g., 0.4 to 0.75 D) introduced increased CSF thresholds within either a narrow or broad bandwidth. For the latter, decreased CSF sensitivity occurred across a spatial frequency range as broad as 1 log unit, from low spatial frequencies (2 cyc/deg), and for pupil sizes equal to or larger than 3 mm. Predictions based on an optical model are qualitatively and quantitatively in agreement with these findings. Contrast sensitivity losses of the patients were neither specific nor selective to the pathologic condition at hand. Furthermore, various CSF losses optically induced in the control subjects were indistinguishable from nonoptically induced pathologic CSF profiles. CONCLUSIONS: Selective broad-band contrast sensitivity loss may be optically induced by slight refractive error. As a result, selective contrast sensitivity loss at lower and intermediate spatial frequencies concurrent in patients with various pathologic, neuro-ophthalmologic conditions cannot be a priori attributed to neural factors without carefully controlled and well-defined optical variables.

Adult↗
PubMed1998

Photorefractive keratectomy for pediatric myopic anisometropia.

PURPOSE: To evaluate the visual results of excimer laser photorefractive keratectomy (PRK) to treat pediatric patients with amblyopic myopic anisometropia in whom conventional amblyopia treatments have failed. SETTING: Instituto Oftalmológico de Alicante, University of Alicante, Spain. METHODS: Six patients aged 5 to 7 years with amblyopic myopic anisometropia were treated by PRK in the more myopic eye using a VISX Twenty-Twenty excimer laser. Minimum preoperative refraction in the amblyopic eyes ranged from -4.00 to -13.00 diopters and best corrected visual acuity (BCVA), from 20/40 to 20/400. All cases completed a minimum of 2 years follow-up of antiamblyopic treatment. RESULTS: After PRK, preoperative BCVA was maintained without optical correction and complementary refraction resulted in increased BCVA in every case. One patient with high myopia developed severe haze. There were no other significant complications. CONCLUSION: Our results indicate that PRK may be an alternative for the correction of pediatric myopic anisometropia and an important aid in treating amblyopia. When other therapies have failed, using PRK in children of the ages in this study must be carefully evaluated; however, our results suggest a beneficial effect.

Amblyopia↗
PubMed2001

Refractive surgery for unilateral high myopia in children.

PURPOSE: To evaluate the safety and efficacy of refractive surgery in children. METHODS: Pediatric patients with unilateral high myopia who were 9 years of age or older were offered refractive surgery to supplement optical correction. The patients and families were informed that the operation may not improve their best-corrected visual acuity. Photorefractive keratectomy (PRK) or laser in situ keratomileusis (LASIK) was performed on the more myopic eye with the use of topical anesthesia. Cycloplegic refraction, stereopsis, motility, and best-corrected visual acuity were measured before the procedure and at 2 months and 20 months after the procedure. All patients had completed amblyopia therapy before surgery. RESULTS: Fourteen eyes of 14 patients aged 9 to 14 years received refractive surgery. Average age at the time of surgery was 11.9 years (+/-1,6). Average corrected preoperative visual acuity was 20/147 (+/-0.065 in decimals). Average preoperative refraction was -7.96 D (+/-2,16) spherical equivalent. Twenty months after refractive surgery, the uncorrected visual acuity averaged 20/129 (+/-0.08 in decimals) and best-corrected vision averaged 20/121 (+/-0.08 in decimals). Average refraction was -0.46 D (+/-0,58) at 2 months and -0.67 D (+/-0,68) D at 20 months. An average myopic shift in refraction of -0.22 D was found in treated eyes during the 20 months of follow-up; this was not statistically significant (P =.69). Three patients had LASIK and 11 patients had PRK. LASIK patients averaged -0.875 D of myopic shift over 20 months of follow-up. Those with PRK averaged -0.025 D. This difference was not statistically significant (P =.10). The vision of 5 of 14 patients improved 1 or 2 lines after refractive surgery. Two patients who had 20/80 vision preoperatively improved to 20/60. No patients lost any lines of vision. Only 4 patients demonstrated stereopsis preoperatively, and all retained stereopsis postoperatively. No patient gained stereopsis. CONCLUSIONS: LASIK and PRK can be performed safely and effectively in children who are cooperative enough to undergo the procedures with topical anesthesia. Refractive surgery does not improve vision in densely amblyopic eyes but may give modest improvement in those that are mildly amblyopic. No significant complications were encountered aside from a myopic shift over time.

Adolescent↗
PubMed2004

Compensation for experimentally induced hyperopic anisometropia in adolescent monkeys.

PURPOSE: Early in life, the optical demand associated with the eye's effective refractive state regulates emmetropization in many species, including primates. However, the potential role of optical demand and/or defocus in the genesis of common refractive errors, like myopia, that normally develop much later in life is not known. The purpose of this study was to determine whether chronic optical defocus alters refractive development in monkeys at ages corresponding to when myopia typically develops in children. METHODS: A hyperopic anisometropia was produced in seven adolescent rhesus monkeys by photorefractive keratectomy (PRK) with an excimer laser. Standard treatment algorithms for correcting myopia in humans were used to selectively flatten the central cornea of one eye thereby producing relative hyperopic refractive errors in the treated eyes. The laser ablation zones were 5.0 mm in diameter and centered on the monkeys' pupils. The laser procedures were performed when the monkeys were 2 to 2.5 years old, which corresponded to onset ages between approximately 8 and 10 human years. The ocular effects of the induced anisometropia were assessed by corneal topography, retinoscopy, and A-scan ultrasonography. RESULTS: By approximately 30 days after PRK, the experimentally induced refractive errors had stabilized and the treated eyes were between +0.75 and +2.25 D more hyperopic than their fellow eyes. Subsequently, over the next 300 to 400 days, six of the seven monkeys showed systematic reductions in the degree of anisometropia. Although some regression in corneal power occurred, the compensating refractive changes were primarily due to relative interocular differences in vitreous chamber growth. CONCLUSIONS: Vision-dependent mechanisms that are sensitive to refractive error are still active in adolescent primates and probably play a role in maintaining stable refractive errors in the two eyes. Consequently, conditions that result in consistent hyperopic defocus could potentially contribute to the development of juvenile onset myopia in children.

Animals↗
PubMed1998

Optic disc size and retinal vessel characteristics in healthy children.

PURPOSE: To characterize the optic disc and retinal vessels in healthy children and adolescents. METHODS: Fundus photographs were evaluated by digital image analysis in one hundred healthy Swedish children and adolescents (3-19 years). RESULTS: Median optic disc area was 2.67 mm2 (range, 1.93 to 4.39), median cup area was 0.41 mm2 (range, 0 to 1.45) and median neuroretinal rim area was 2.24 mm2 (range, 1.60 to 3.05). The size of the cup and neuroretinal rim increased with increasing size of the optic disc. There were no correlations between the measured variables and age, sex or refraction. CONCLUSION: The measurements of optic disc size in healthy children, between 3 and 19 years of age, are in accordance with optic disc values found in most studies on adults. The resulting reference intervals may be helpful in clinical pediatric ophthalmology, to facilitate identification of children with abnormal optic disc and retinal vessel morphology.

Adolescent↗
PubMed2001

Optical response to LASIK surgery for myopia from total and corneal aberration measurements.

PURPOSE: To evaluate the optical aberrations induced by LASIK refractive surgery for myopia on the anterior surface of the cornea and the entire optical system of the eye. METHODS: Total and corneal aberrations were measured in a group of 14 eyes (preoperative myopia ranging from -2.5 to -13 D) before and after LASIK surgery. Total aberrations were measured using a laser ray-tracing technique. Corneal aberrations were obtained from corneal elevation maps measured using a corneal system and custom software. Corneal and total wave aberrations were described as Zernike polynomial expansions. Root-mean-square (RMS) wavefront error was used as a global optical quality metric. RESULTS: Total and corneal aberrations (third-order and higher) showed a statistically significant increase after LASIK myopia surgery, by a factor of 1.92 (total) and 3.72 (corneal), on average. This increase was more pronounced in patients with the highest preoperative myopia. There is a good correlation (r = 0.97, P < 0.0001) between the aberrations induced in the entire optical system and those induced in the anterior corneal surface. However, the anterior corneal spherical aberration increased more than the total spherical aberration, suggesting also a change in the spherical aberration of the posterior corneal surface. Pupil centration and internal optical aberrations, which are not accounted for in corneal topography, play an important role in evaluating individual surgical outcomes. CONCLUSIONS: Because LASIK surgery induces changes in the anterior corneal surface, most changes in the total aberration pattern can be attributed to changes in the anterior corneal aberrations. However, because of individual interactions of the aberrations in the ocular components, a combination of corneal and total aberration measurements is critical to understanding individual outcomes, and by extension, to designing custom ablation algorithms. This comparison also reveals changes in the internal aberrations, consistent with the posterior corneal changes reported using scanning slit corneal topography.

Adult↗
PubMedDate not supplied

Differences between objective and subjective refractions after radial keratotomy.

BACKGROUND: In patients who are free of pathology, automatic refractions have shown close agreement with the subjective refractions. Clinical experience indicated that the normally strong relationship between objective and subjective refraction is significantly weakened as a result of radial keratotomy. METHODS: Seventy-two patients were refracted before and after surgery, objectively with a Humphrey Model #510 autorefractor and subjectively using a binocular refraction procedure without cycloplegia. All patients were free of ocular disease and had preoperative myopia ranging from -1.00 to -9.00 diopters as determined by the subjective spherical equivalent. RESULTS: The results indicated that the preoperative difference between the mean spherical automatic and subjective refractions was a clinically acceptable 0.25 diopter. However, postoperatively, there was a statistically significant difference of 1.25 D with the automatic refractor determining more myopic refractions. Subsequent analysis revealed that the age of the patient had a direct bearing on this finding with patients less than 40 years of age showing more minus in the automatic refraction than patients 40 years and older. CONCLUSIONS: The postoperative discrepancy between the two refractions may be explained by induced optical aberrations and may contribute towards the visual fluctuations experienced by radial keratotomy patients. It is postulated that the inconsistency in refractive determination is due to optical distortion since the age dependence of this effect may be related to the reduction of pupil size that occurs with aging. In the radial keratotomy patient, the practitioner is faced with a more complex and uncertain refraction that may vary according to refractive procedures used and other factors such as pupil size.

Accommodation, Ocular↗
PubMedDate not supplied

Optics of the corneal epithelium.

BACKGROUND: The refractive effect of the corneal epithelium in relation to the cornea as a whole is relatively unknown, yet it may affect the efficacy of keratorefractive surgical procedures, most notably photorefractive keratectomy and epikeratoplasty. This article investigates the refractive effect of the epithelium on the cornea. METHODS: We measured the corneal keratometry in 10 fresh human eye-bank eyes with and without epithelium. All readings were performed at 2.0- and 3.6-millimeter diameter zones with an automated keratometer. RESULTS: When the epithelium was removed, there was an increase in corneal refractive power in every case. The mean change was +1.03 D (range, 0.55 to 1.85 D) at the central 2.0-millimeter diameter zone and +0.85 D (range, 0.29 to 1.60 D) at the 3.6-millimeter diameter zone. We also found a change in both the power and axis of astigmatism in most cases. CONCLUSIONS: The corneal epithelium plays an active role in determining the final surface power of the cornea. It accounts for an average of 1.03 D of the power of the eye at the central 2-millimeter diameter zone. The difference in astigmatism suggests that the epithelium does not form a layer of uniform thickness over Bowman's layer. It appears that the epithelium attempts to minimize abrupt changes in surface contour. These findings suggest that in refractive procedures such as photorefractive keratectomy the refractive contribution of the epithelium must be taken into account to improve predictability.

Astigmatism↗
PubMed1987

Heterogeneous, ideal associations at sedimentation equilibrium.

A general approach to analyze associations of the type mA + nB = AmBn at sedimentation equilibrium under ideal conditions was derived. A simple transform to generate a function Q was defined in which the optical measurement (absorbance or refractive index increment) at a given radial distance is divided by the value that would result if no association occurred. Explicit calculations for the cases of m = 1, n = 1 and m = 2, n = 1 were used to simulate centrifuge experiments. Nonlinear least-squares regression of Q vs. the optical measurement showed that accurate estimation of parameters could be obtained in selected instances for both types of association only if the appropriate model was used. Additionally, the effect of random experimental and systematic error (e.g., the presence of contaminants) was evaluated.

Biopolymers↗
PubMed1992

Axial myopia in eyes with optic nerve hypoplasia.

Given that abnormal visual experience during post natal development interferes with emmetropization, we proposed that eyes with hypoplastic optic nerves were predisposed to the development of refractive errors. Six of 14 patients with unilateral optic nerve hypoplasia and 5 of 22 patients with bilateral involvement had at least 4 D of myopia. Nine of the 11 patients with asymmetric bilateral involvement had relative myopia in the eye with the more abnormal optic nerve; none of the patients with symmetric bilateral involvement had a significant interocular refractive difference. Analysis of axial length measurements obtained in 10 of the 11 patients with high myopia showed a significant increase in total axial length. The presumed normal eye of patients with unilateral involvement was significantly smaller than the mean value for age-adjusted normals. We suspect that visual input to the central nervous system is one of the feedback signals involved in the regulation of ocular growth.

Adolescent↗
PubMed2002

The effects of optical defocus on the legibility of the Tumbling-E and Landolt-C.

BACKGROUND: The Tumbling-E and Landolt-C are both used to test visual acuity in young, illiterate, or non-English-speaking patients. In this study, we sought to determine how optical blur affects visual acuity thresholds measured with both of these optotypes. METHODS: Ten subjects were tested with 4-position Tumbling-E and Landolt-Cs using the method of constant stimuli. The subjects were tested wearing their habitual corrections and with +1.50 D spherical and cylindrical lenses to simulate myopic refractive errors in the optical blur test conditions. RESULTS: Visual acuity thresholds for the Tumbling-E were slightly better for all test conditions, while the largest threshold difference between the test letters was found for the simulated against-the-rule astigmatism. For both test letters, vertical orientations were more difficult to resolve than horizontal. Also, the slopes of the psychometric functions were steeper for the Tumbling-E than for the Landolt-C. CONCLUSIONS: The Tumbling-E can be considered comparable with the Landolt-C except for the simulated against-the-rule-astigmatism test condition. Although the ranges are quite large, the average psychometric functions of the Landolt-Cs are flatter, which gives rise to the possibility that more confidence can be placed in the visual acuity thresholds obtained with the Tumbling-E.

Astigmatism↗
PubMed1990

Pupillary irradiance distributions for light backscattered in the human eye.

An investigation of linear knife-edge photoscreening devices for detecting focusing errors, refractive defects and other optical artifacts in the human eye is described. Pupillary irradiance distributions calculated for a model eye are shown to agree qualitatively with distributions recorded experimentally using video frame grabbing equipment. The form of the pupillary irradiance distribution from a cyclopleged human eye supports a diffuse-like characterization for the reflection/scattering processes at the retina. The extreme sensitivity of the instruments enables real-time detection of refractive effects from tear films on the cornea and real-time tracking of floaters.

Eye Diseases↗
PubMed2003

Method for measuring the refractive index and the thickness of transparent plates with a lateral-shear, wavelength-scanning interferometer.

A new method for measuring simultaneously the thickness and the refractive index of a transparent plate is proposed. The method is based on a simple, variable lateral-shear, wavelength-scanning interferometer. To achieve highly accurate measurements of both refractive index n and thickness d we use several means to determine these two quantities. We finely tune a distributed-feedback diode laser light source to introduce a phase shift into the detected signal, whereas we make the sample rotate to produce variable lateral shearing. Phase shifting permits precise determination of the optical thickness, nd, whereas refractive index n is obtained from the retrieved phase of the overall interference signal for all incidence angles.

Journal Article↗