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At least 883 records · Page 49Linked to original sources
PubMed1997

[The optical power of the corneal epithelium. In vivo evaluation].

PURPOSE: The refracting power of the corneal epithelium is relatively unknown. We measured the keratometric refracting power of the cornea on 15 eyes of patients undergoing photorefractive keratectomy with and without epithelium. MATERIALS: The readings were performed with a videokeratoscope TMS-1. Three readings were averaged to obtain the mean and standard deviations. RESULTS: By removing the epithelium, there was an increase in corneal refracting power. The mean change was +1.19 D (standard deviation +/- 0.36 D) at 1 mm of the center of the cornea. On all patients the optical power of the cornea increased after epithelium removal. CONCLUSION: The curvature of the Bowman membrane is steeper than the cornea with epithelium. A change in the thickness of the epithelium is related to a modification of the optical power of the cornea. The optical role of the epithelium is to compensate surface irregularities of the Bowman membrane and could explained in part the hyperopia in the first months after excimer laser.

Cornea↗
PubMed2005

Accuracy of intraocular lens calculations in infants and children undergoing cataract surgery.

PURPOSE: We sought to evaluate the accuracy of intraocular lens calculations in children undergoing cataract extraction with intraocular lens (IOL) implantation. METHODS: This was a retrospective review of 101 consecutive cases of pediatric cataract extraction with posterior chamber IOL. Median age at the time of surgery was 4.8 years, and 23 of the children were younger than 2 years of age. Initial IOL calculations were made using either the SRK II or SRK T formulas. Preoperative predictive data were compared with the refractive result obtained at the 2-month postoperative visit. Prediction error was calculated for each patient, and lens calculation errors were back-calculated for each of 4 commonly used IOL calculation formulas, including the SRK II, SRK T, Holladay I, and Hoffer Q. RESULTS: The mean prediction error for all patients was 0.30 D. Despite this level of overall accuracy, significant variability did occur with outcomes ranging from -4.06 D to + 3.86 D of the desired spherical equivalent. Mean absolute prediction error was 1.16 D with no significant difference between the SRK II and SRK T formulas. Lens calculation errors predicted by each of the 4 formulas studied demonstrated a high degree of variability with the SRK II being the least variable and the Hoffer Q being the most variable, particularly among the youngest group of children with the axial lengths less than 19 mm. CONCLUSIONS: The accuracy of commonly used IOL calculation formulas is generally reasonable but highly variable within the pediatric population, Newer theoretic IOL calculation formulas did not outperform older regression models.

Adolescent↗
PubMed2006

Retinal function in infants with optic nerve hypoplasia: electroretinograms to large patterns and photopic flash.

AIMS: Optic nerve hypoplasia (ONH), which is defined as a congenital deficiency of retinal ganglion cells, may also involve more distal layers of the retina. We investigated electrophysiological function of the retina in ONH using electroretinograms (ERGs). METHODS: ERGs were recorded from 48 subjects (3.5-35 months) with unilateral or bilateral ONH. Pattern reversal (4 degrees checks) was presented under chloral hydrate sedation, using an optical system to correct a cycloplegic refraction. A photopic flash stimulus was also used. Fundus photographs were used to measure the disk diameter/disk macula ratio (DD/DM), and to document other clinical signs. Eyes were classified as moderate (0.15-0.3) or severe (<0.15) ONH, and those with DD/DM greater than 0.3 were used as reference eyes. RESULTS: Pattern ERG recording was completed in 89 eyes and was detectable in 80% of eyes with ONH (61/76 tested) and in all 13 reference eyes. Photopic flash ERGs were of good quality in all eyes. The severity of ONH correlates with the amplitude of the photopic flash b-waves and with the amplitude of the N95 component of the pattern ERG (P<0.01). However, the ERGs to large patterns were well preserved (>3.5 microV) in 10 of 35 eyes with severe ONH. Tortuous retinal vessels in eyes with either moderate or severe ONH were associated with smaller amplitude photopic b-waves and markedly diminished or undetectable pattern ERGs. CONCLUSIONS: This study supports the hypothesis that retinal dysfunction distal to the ganglion cells is common in ONH, but is not predictable on the basis of ONH severity alone. Additionally, tortuous retinal vessels in ONH may be a sign associated with retinal dysfunction.

Child, Preschool↗
PubMed1991

Refraction, including prisms.

The literature in the past year on refraction is replete with several isolated but very important topics that have been of interest to strabismologists and refractionists for many decades. The refractive changes in scleral buckling procedures include an increase in axial length as well as an increase in myopia, as would be expected. Tinted lenses in dyslexia show little positive effect in the nonasthmatic patients in one study. The use of spectacles or bifocals as a way to control increase in myopia is refuted in another report. It has been shown that in accommodative esotropia not all patients will be able to escape the use of bifocals in the teenage years, even though surgery might be performed. The hope that disposable contact lenses would cut down on the instance of giant papillary conjunctivitis and keratitis has been given some credence, and the conventional theory that sclerosis alone is the cause of presbyopia is attacked. Also, gas permeable bifocal contact lenses are reviewed and the difficulties of correcting presbyopia by this method outlined. The practice of giving an aphakic less bifocal addition instead of a nonaphakic, based on the presumption of increased effective power, is challenged. In the review of prisms, the majority of articles concern prism adaption. The most significant report is that of the Prism Adaptation Study Research Group (Arch Ophthalmol 1990, 108:1248-1256), showing that acquired esotropia in particular has an increased incidence of stable and full corrections surgically in the prism adaptation group versus the control group.(ABSTRACT TRUNCATED AT 250 WORDS)

Adaptation, Ocular↗
PubMed1993

Intraoperative raster photogrammetry--the PAR Corneal Topography System.

The PAR Corneal Topography System (CTS) is a computer-driven corneal imaging system that uses close-range raster photogrammetry to measure and produce a topographic map of the corneal surface. The CTS determines distortion in a projected two-dimensional grid. Unlike Placido-disc-based videokeratoscopes, the PAR CTS produces a true topographic map (elevation map) and requires neither a smooth reflective surface nor precise spatial alignment for accurate imaging. Because the system uses two noncoaxial optical paths, it can be integrated into other optical devices. A modified CTS was integrated into an experimental erbium: YAG photoablative laser. The CTS successfully imaged corneas before, after, and during laser photoablation. Its ability to image nonreflective surfaces and to be integrated into other optical systems may make it suitable for intraoperative refractive monitoring.

Animals↗
PubMed2003

Bifocal optical coherenc refractometry of turbid media.

We propose and demonstrate a novel technique, which we term bifocal optical coherence refractometry, for the rapid determination of the refractive index of a turbid medium. The technique is based on the simultaneous creation of two closely spaced confocal gates in a sample. The optical path-length difference between the gates is measured by means of low-coherence interferometry and used to determine the refractive index. We present experimental results for the refractive indices of milk solutions and of human skin in vivo. As the axial scan rate determines the acquisition time, which is potentially of the order of tens of milliseconds, the technique has potential for in vivo refractive-index measurements of turbid biological media under dynamic conditions.

Animals↗
PubMed2003

A new fundus camera technique to help calculate eye-camera magnification: a rapid means to measure disc size.

OBJECTIVE: To find a simple means for calculating eye-camera magnification to permit estimation of true retinal object size from a retinal photograph. METHODS: The position of the focusing knob on 3 different retinal cameras (TRC-50F and TRC-50X; Topcon America Corp, Paramus, NJ; and the CR6-45NM Non-mydriatic Retinal Camera; Canon Inc, Tokyo, Japan) was measured during optic nerve photography and correlated with the refractive error, or spectacle refraction, of the subject (N = 11 for each camera). RESULTS: A strong correlation was found between focusing knob position and spectacle refraction for each of the 3 cameras tested (r = 0.96, r = 0.99, and r = 0.97, respectively). CONCLUSIONS: The focusing knob position reflects the spectacle refraction of the eye being photographed, and spectacle refraction is known to correlate well with eye-camera magnification. Therefore, focusing knob position can be used to help calculate eye-camera magnification and, hence, true retinal object size. CLINICAL RELEVANCE: The true size of the optic nerve head is important for the diagnosis of glaucoma from a retinal photograph. This technique is a simple means to calculate optic nerve head size, which may be especially useful in mass retinal photographic screening programs.

Anthropometry↗
PubMed1990

Is the quality of vision with contact lenses adequate? Not only adequate but often superior.

Two factors govern the quality of vision with contact lenses (CLs): the fit and movement of the lens and the adequacy of its optics. Studies have not demonstrated any inherent inadequacies in material or design parameters that might account for visual difficulties in lens wearers. To the contrary, thickness, water content, and fit can be varied within clinically acceptable ranges without compromising vision. A poorly fitted lens, however, can cause excessive corneal edema and consequent visual problems. Poor vision may also result from uncorrected refractive error, the development of an unstable optical surface, or the use of a lens material that is inherently inadequate for a given patient. Such causes of visual dysfunction can usually be avoided through better fitting, better lens care, more careful refraction, or better patient-lens matching. Specialized lens designs can sometimes be used to correct vision problems. Certain types of rigid front-surface aspheric lenses, for example, provide astigmats with even better visual correction than spherical rigid lenses or spectacles, but the improvement is small and highly patient dependent. Similarly, although soft toric lenses can provide good vision, it is often technically difficult to orient and stabilize the correcting cylinder.

Astigmatism↗
PubMed1986

Polysulfone corneal lenses.

Polysulfone is a thermoplastic compound first synthesized in 1965. A unique characteristic of polysulfone is its high refractive index (1.633), which allows very thin optical lenses to be manufactured. Over the last five years, D. Peter Choyce has surgically implanted over 40 polysulfone lenses into eyes of his patients. Analysis of his data indicates that polysulfone intraocular lenses are capable of correcting large refractive errors. Based on his work, a multicenter study was undertaken to evaluate the safety and efficacy of polysulfone as an intracorneal lens material in laboratory models. Four monkeys, eight baboons, and 24 cats were used as laboratory models; 5.0-mm to 6.0-mm diameter hyperopic (+28.5 diopter) and myopic (-17.0 and -25.5 diopter) lenses were surgically implanted within the corneal stroma in one eye of each of the laboratory models while a sham lamellar dissection was performed in the other eye. One hundred percent (4/4) of monkey eyes, 12.5% (1/8) of baboon eyes, and 70.0% (18/24) of cat eyes maintained clear media by ophthalmoscopic examination at follow-ups ranging from three to six months. Complications included both visually and nonvisually significant interface opacities, lens extrusion, anterior corneal necrosis, refractile particles, and epithelial thinning.

Animals↗
PubMed1998

Quantitative evaluation of the optic nerve head in early glaucoma.

AIMS: Progressive loss of neuroretinal rim tissue is known to occur early in glaucoma and measurement of the neuroretinal rim area is possible by magnification corrected analysis of optic disc photographs (planimetry). This study was performed to determine whether the facility to distinguish between glaucomatous and normal optic discs could be improved upon by: (a) taking into account the known relation between optic disc size and neuroretinal rim area, and (b) measuring rim area in a number of segments, in order to detect focal changes. METHODS: Planimetric examination of the optic disc photographs of 88 control subjects and 51 patients with early visual field defects was performed. In the control group, multiple linear regression analysis was performed between neuroretinal rim area and optic disc area, age, sex, eye side, refraction, and keratometry. This was repeated for the whole disc and for each of twelve 30 degree segments. Normal ranges were defined by the 98% prediction intervals of the regression analysis and the sensitivity and specificity for correct identification of optic discs in the two groups determined. RESULTS: Multiple linear regression demonstrated significant associations between the neuroretinal rim area and optic disc area and age in normal subjects. Sensitivity and specificity for glaucoma diagnosis, using the cut off derived from the 98% prediction intervals, was 37.7% and 98.9% respectively when total neuroretinal rim area alone was considered, and 88.7% and 94.3% respectively when the 30 degree segments were included. The most frequent pattern of neuroretinal rim loss was diffuse, followed by thinning in more than one sector and then by thinning in the inferotemporal sector alone. CONCLUSIONS: This method of optic disc analysis enables the examiner to identify glaucomatous optic discs at the stage of early perimetric loss with a high degree of precision. Optic disc photography is simple, and fundus cameras are widely available. This method for glaucoma case identification may therefore be suitable for the primary care setting as well as hospital practice.

Adult↗
PubMed1998

Spectacle lenses and emmetropization: the role of optical defocus in regulating ocular development.

The purpose of our investigation was to determine whether early ocular growth and refractive development was regulated by visual feedback in infant monkeys. Specifically, we examined the ability of infant monkeys to compensate for optically induced changes in the eye's refractive state and to recover from experimentally induced refractive errors. For moderate-powered anisometropic lenses, infants exhibited differential interocular axial growth rates that reduced the lens-induced refractive imbalance between the two eyes. Infants treated with equal-powered lenses over both eyes also showed compensating growth. For lens powers between approximately -3 and +6 D, the resulting refractive-error changes, which were primarily due to alterations in vitreous chamber growth rates, were well correlated with the effective refractive state produced by the treatment lenses. When the stimulus for altered eye growth was removed and the infants were provided unrestricted vision, monkey eyes consistently grew toward emmetropia. The remarkable degree of adaptability exhibited by the eyes of infant monkeys demonstrates that emmetropization in this higher primate is an active process that is regulated on a continuous basis by optical defocus. Consequently, early in life spectacle lenses by changing the eye's effective focus can predictably alter ocular growth and the refractive status of one or both eyes.

Animals↗
PubMed1984

Measurement of refractive index in an intact crystalline lens.

A knowledge of the refractive index distribution within the crystalline lens is necessary to define the optical properties of the eye. This report describes the first nondestructive method for measuring the refractive index of intact crystalline lenses based on the work of Chu (1977) [Electron Lett. 13, 736-738] and Barrell and Pask (1978) [Opt. Commun. 27, 230-234] for optical fibre preforms. The method is simpler and more accurate than previous methods and shows that the rat crystalline lens has a smooth and almost parabolic distribution of refractive index.

Animals↗
PubMed1980

Causes of loss of vision in congenital glaucoma.

Of 51 patients with congenital glaucoma, 13 had unilateral and 38 bilateral disease. Pressure was controlled at 8 to 24 mm Hg in 76 of the 89 affected eyes. Forty-one of these 76 eyes had visual acuity poorer than 6/15 (20/50); optic nerve damage as shown by visual field testing was the cause in 20, and medial opacities, including irregular corneal astigmatism, was the cause in 15. In only six eyes did the poor vision result from anisometropic amblyopia.

Adolescent↗
PubMed1981

Accuracy of ultrasound intraocular lens calculation.

A review of the literature was made to evaluate the prediction accuracy of intraocular lens calculations. A uniform standard formula (percentage +/- 1.0 diopter/percentage +/- 2.0 D/range of error) for reporting accuracy was used to evaluate the accuracy of reports in the literature, as well as the accuracy of an ultrasound unit (Kretz 7100 MA), the HOffer formula, use of a standard 3.5-mm anterior chamber depth, and addition of a retinal thickness factor to the measured ultrasound axial length. The appropriateness of technicians performing this procedure also was evaluated. Results of previous studies show that the use of another ultrasound unit (Sonometrics DBR) and Binkhorst's formula is the least clinically accurate method.

Eye↗