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PubMed2003

Corneal astigmatism in applanation tonometry.

PURPOSE: This short study aims at quantitatively correlating scale readings of the Goldmann tonometer to corneal curvatures in one and the same eye. METHODS: A total of 30 eyes of 21 adult patients who presented themselves in clinical practice for routine examination were arbitrarily selected. Corneal curvatures were measured in the horizontal and vertical meridians and then applanation tension measured in the same order. The data were submitted to statistical analysis by computer using Microsoft Excel and SAS System programs. Results All cases exhibited a strong positive correlation of tonometric readings to corneal curvature, that is, the flatter the curvature, the lower the reading on the scale of the tonomer, and vice versa. CONCLUSION: The numbers on the scale of the Goldmann tonometer may not represent true intraocular pressure in mmHg because their values depend on a number of variables of the globe, including corneal astigmatism. Attention to these influences may help interpret the tonometric record in difficult cases.

Adult↗

PubMed2003

The streak retinoscopy pupil reflex in the presence of astigmatism.

Computer simulation is being increasingly used as a teaching tool. Having developed a computer-generated virtual focimeter, we are now in the process of developing a computer-generated virtual streak retinoscope to teach the principles of retinoscopy and the effect of residual refractive error and mirror movement on the pupil reflex. One of the important requirements was to provide as accurate a simulation as possible for the completely general case of an astigmatic patient and a streak in any orientation being moved also in any orientation. This required a thorough understanding of the optical theory of the retinoscope and equations that describe the behaviour of the pupil reflex. We have taken this opportunity to review the optics of the streak retinoscope and derive equations for the behaviour of the pupil streak reflex.

Astigmatism↗

PubMed1994

[Comparison of astigmatism due to ambulatory cataract surgery with 3 different techniques].

We show that there was little difference in the average astigmatism induced in three surgical groups (Extracapsular (EC), Phacoemulsification with and without scleral tunnelization) at the last control. An examination of a second group of patients shows a significant reduction in the time necessary for recovering at least 80% of the final visual function and a significant shortening of the time elapsed before the prescription of the glasses in the group of patients operated on using phaco with tunnellization. Just as there is a significant relationship between the post-operative cyclinder and postoperative phase 1, there is also a reduction in, and early and lasting stabilisation of, the cyclinder induced by surgery using small incisions. We show that the technique of vectorial calculation brings into play discontinuous trigonometric functions that require great prudence when interpreting statistics of cylinders induced by surgery.

Ambulatory Surgical Procedures↗

PubMed1992

[Intraoperatively-induced astigmatism after "small incision cataract extraction". An analysis of 4.0 mm, 5.0 mm and 6.5 mm incisions].

Surgically induced astigmatism in three series of cataract surgical procedures were compared. The three series comprised 120 eyes having phacoemulsification and implantation of: a) a foldable silicon lens through a 4.0 mm incision (32 eyes), b) a 5.0 mm PMMA lens through a 5.0 mm incision (48 eyes) and c) a 6.5 mm diameter PMMA lens through a 6.5 mm incision (40 eyes). There were no significant differences in surgically induced cylinder between the three groups after one month or 6 months postoperatively. No differences between these three groups with respect to visual rehabilitation were seen. We concluded that small diameter PMMA implants inserted through a 5.0 mm or 6.5 mm incision minimized surgically induced cylinder in a way comparable to the use of foldable silicone implants inserted through a 4.0 mm incision.

Adult↗

PubMed1990

[Astigmatism following cataract surgery: comparison following wound closure with nylon suture and Mersilene].

Mersilene (polyester fiber) is stronger and less elastic than nylon suture material. In contrast to nylon, Mersilene does not appear to be susceptible to biodegradation in the long term. After a follow-up period of 6 months, the regression of surgically induced astigmatism following extracapsular cataract extraction and lens implantation with a conventional 150 degree corneoscleral incision was in the same range with nylon monofilament 10-0 and Mersilene 10-0. With nylon single sutures (14 patients) there was a reduction from 3.5 and with Mersilene (12 patients) from 3.9 to 0.9 diopters.

Astigmatism↗

PubMed1977

Early treatment of anisometropic astigmatism and strabismus.

A 4-year-old boy with uncorrected anisometropic indirect astigmatism showed amblyopia and strabismus. Over a period of approximately 3 yr, neutralization of the refractive error and treatment of the amblyopia and strabismus resulted in single binocular vision with good stereopsis. The results suggest that at least up to age 7 the visual system is sufficiently plastic to be modified by treatment.

Astigmatism↗

PubMed1981

An algorithm for computing astigmatic refractive errors from ocular parameters.

Calculation of total refractive astigmatism by analyzing refractive parameters of the individual components can be very tedious, especially when the principal meridians among all the components are not orthogonal. A straightforward programming algorithm for accomplishing these calculations is presented. The algorithm is applicable to any programmable device which performs trigonometric functions.

Astigmatism↗

PubMed1981

A system matrix for astigmatic optical systems: I. Introduction and dioptric power relations.

A single 4 X 4 system matrix is used to represent the para-axial properties of optical systems consisting of separated obliquely crossed spherocylindrical lenses. The 4 X 4 system matrix is a generalization and combination of the 2 X 2 Gaussian system matrix for spherical optical systems, and the 2 X 2 dioptric power matrix for a single spherocylindrical lens or for obliquely crossed spherocylindrical lenses in contact with each other. The 4 X 4 system matrix approach simplifies both the conceptual and numerical analysis of complicated astigmatic systems.

Astigmatism↗

PubMed1982

Unwanted astigmatism in lenses with a concentric variation in sagittal power.

A concentric variation in sagittal power, Fs, with zonal radius, y, has been used in some designs of varifocal and aphakic spectacle lenses. It is shown theoretically, using a paraxial approximation, that a gradient dFs/dy in sagittal power leads to the introduction of an amount y.dFs/dy of unwanted astigmatism. The validity of this result is illustrated experimentally.

Aphakia, Postcataract↗

PubMed1991

Inducing myopia, hyperopia, and astigmatism in chicks.

Myopia and hyperopia have been produced in chicks by applying specially designed convex and concave soft contact lenses to the eyes of newly hatched birds. After 2 weeks of wear, the eyes develop refractive states equivalent in sign and amount (+8 and -10 D) to the lens used. However, the lenses produce an artificial hyperopic shift during the first week of wear due to corneal flattening. We have developed a new approach involving the use of goggles with hard convex and concave contact lens inserts placed between the frontal and lateral visual fields. Myopia and hyperopia (+10 and -10 D) can be produced within days (4 days for hyperopia and 7 days for myopia) if the defocus is applied from the day of hatching. We can also produce significant amounts of astigmatism (1 to 5 D) axis at 90 degrees and 180 degrees by using cylindrical contact lens inserts. Although these last results are preliminary, they suggest that accommodation is not likely involved at this stage of refractive development because we do not believe that the accommodative mechanism can cope with cylindrical defocus. All spherical refractive errors produced using the goggle system appear to result from alterations in vitreous chamber depth.

Animals↗

PubMed1996

Ray vector fields, prismatic effect, and thick astigmatic optical systems.

The application of the concept of ray vector fields to optical systems is reexamined. Paraxial or linear optics defines a four-dimensional ray vector field for any optical system: the vector field maps the incident ray vector into the emergent ray vector. In the case of thin systems, including thin astigmatic lenses, one can define a vector field of reduced dimensionality: the vector field is two-dimensional and maps the ray's incident position into the change in reduced direction. When the index of refraction is the same before and after a thin system, the change in reduced direction is the reduced deflection through the system or the reduced prismatic effect. Contrary to what has recently been claimed, this type of two-dimensional vector field does not apply in general to thick systems. However, a number of different types of two-dimensional vector fields can be defined for various particular classes of optical systems. Thick systems differ qualitatively from thin systems. They do not have equivalent thin lenses and cannot generally be replaced by thin lenses. Equations are derived for the change in reduced direction and deflection for a ray through optical systems in general and through separated two- and three-lens systems in particular.

Astigmatism↗

PubMed1997

Equivalent dioptric power asymmetry relations for thick astigmatic systems.

For optical systems consisting of separated obliquely crossed toric interfaces, the equivalent dioptric power has principal meridians that are not necessarily orthogonal to each other. In this case it takes four parameters to specify the equivalent power. A set of parameters convenient for ophthalmic optics consists of three traditional spherocylindrical parameters SC x theta together with a dioptric asymmetry parameter g. The parameter g has been described as "so far" being entirely mathematical in nature. The purpose of this paper is to develop further optical knowledge about the equivalent power asymmetry g. The method was a theoretical and numerical study involving optics and the dioptric power matrix theory. Among the results of this study are a number of new equations involving g that clarify the relationships between the nonorthogonal principal meridians and the power and axis meridians of SC x theta, as well as explicitly illustrating the parameters that can increase or decrease g. It is also pointed out that the asymmetry g is formally identical to the circular astigmatism that has previously been presented in discussions of ray vector deflection fields (and is used in the Humphrey Lens Analyzer measurements). In conclusions, the theoretical relations presented here provide optical insight into the equivalent dioptric power asymmetry and the parameter g. The relations and insight can assist further developments.

Astigmatism↗

PubMed1993

The construction of sutureless cataract incision and the management of corneal astigmatism.

Extrapolating information from equations that govern fluid flow, a theoretical formula is developed for a sutureless cataract incision. This theoretical formula defines the resistance of aqueous outflow as a function of three variables: length of cataract incision, the length of the scleral tunnel, the tortuosity of the outflow channel, and one constant friction factor. The nonlinear relationship of corneal incisions to length, depth, and distance from the visual axis is also examined with respect to their effect on central corneal curvature and control of astigmatism. Finite element analysis of differential equations is discussed as the most plausible technique for predicting these incisional effects.

Astigmatism↗

PubMed2003

Treating astigmatism at the time of cataract surgery.

An increasingly important goal of modern cataract and implant surgery is to obtain the most desirable refractive outcome for our patients, and in so doing, decrease their dependence upon spectacle correction. Refined spherical outcomes are now achieved by directing close attention to biometry technique, through the use of third generation intraocular lens (IOL) power formulae, and refinements in technology such as partial coherence interferometry. Similarly, improvements in astigmatic outcomes are now possible by focusing upon and obtaining more accurate preoperative cylinder measurements, careful surgical planning particularly in regard to incision(s) design, use of toric IOLs, and employing advanced postoperative enhancement techniques. Because of these refinements in technique and technology, the field of cataract and implant surgery has emerged as a true and recognized form of refractive surgery.

Astigmatism↗

PubMed2005

Infectious keratitis after astigmatic keratotomy in penetrating keratoplasty: review of three cases.

PURPOSE: To present 3 cases in 2 patients of infectious keratitis of the astigmatic keratotomy (AK) site in patients who had previous penetrating keratoplasty. METHODS: History, clinical examination, chart review, cultures, and laboratory results were obtained from 2 patients with infectious keratitis within the donor stroma of their graft after an AK was performed. RESULTS: One patient had an early infiltrate of the AK site that was culture negative. This cleared with fortified antibiotics, and the patient had a final visual acuity of 20/30-2. The second patient developed 2 late infections, the first with coagulase negative Staphylococcus and the second infection with Pseudomonas aeruginosa. Both of these infections cleared with fortified antibiotics, and the final visual acuity was 20/25. Neither patient developed a rejection episode from the infections, and both grafts survived. CONCLUSIONS: We report a series of 3 cases of infectious keratitis in 2 patients after AKs in corneal grafts. This, to the best of our knowledge, has not been previously reported. Patients who suffer infectious keratitis after AK in corneal grafts may have good visual outcomes if managed with appropriate antibiotic regimens and closely followed.

Astigmatism↗

PubMed1988

Post-cataract astigmatism: its control and correction.

The factors responsible for postoperative astigmatism--wound compression and wound gape--are discussed and illustrated for a number of suture incision combinations. Variability is pronounced. Mid-limbal incisions sutured with 8-0 virgin silk had a range of 8D at four days, 8.5D at four weeks and 9D at three months. Nylon 9-0 double continuous, tied normally, had 8D at four days, 7D at four weeks and 7.5D at three months. The same suture tied tightly gave a range of 5D at four days, 7.5D at four weeks and 6D at three months. Methods of control and correction are discussed and illustrated with case histories.

Absorption↗

PubMed1980

Astigmatism and surface phenomena in pterygium.

Examination of 39 eyes with pterygium and 17 contralateral eyes revealed increased astigmatism with the rule (greater than 0.5 D in 46%, greater than or equal to 4 D in 13%) without associated impairment of vision. Intensified vital staining was seen to be provoked over the pterygium by tetrazolium and rose bengal, whereas this was not the case with fluorescein or alcian blue. There were no signs of desiccation (Break up time normal). Stocker's line in front of the pterygium was observed in 46%. In 13% this was continuous with Hudson Stahli's line. A quantitative cytologic study disclosed keratinisation in 5%. Increased desquamation was seen in 10%. The central thickness of the cornea was the same as in normal eyes.

Adult↗

PubMed1990

A longitudinal study of a population based sample of astigmatic children. II. The changeability of anisometropia.

The variability of anisometropi in a sample of 310 children with astigmatism at the age of 1 year was longitudinally studied during a 3-year period between 1 and 4 years of age. The prevalence of anisometropia of 1 D or more at each year level was rather stable. When individual cases were examined we found that between the first and the last test session 19 of the 33 children with anisometropia at the first test session had become non-anisometropic and were substituted with 14 new cases which were non-anisometropic at the age of 1 year. In general, less than half of the cases, at all levels of anisometropia, remained anisometropic throughout the whole test period. We also found that children with anisometropia persisting through the whole test period were at considerable risk, about one out of four, of developing amblyopia. There was no simple relationship, however, between anisometropia at a certain age level between 1 and 4 years and amblyopia and/or strabismus. Non-persisting anisometropia in an emmetropizing eye is in most cases a benign sign and not connected with an increased risk for developing amblyopia.

Aging↗