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PubMed1995

Reproducibility of corneal astigmatism measurements with a hand held keratometer in preschool children.

AIMS: To evaluate the overall accuracy and reproducibility of the Alcon portable autokeratometer (PAK) measurements in infants and young children. METHODS: The accuracy of the Alcon PAK in measuring toric reference surfaces (1, 3, 5, and 7 D) under various suboptimal measurement conditions was assessed, and the reproducibility of PAK measurements of corneal astigmatism in newborn infants (n = 5), children (n = 19, age 3-5 years), and adults (n = 14) was evaluated. RESULTS: Measurements of toric reference surfaces indicated (a) no significant effect of distance (17-30 mm) on accuracy of measurements, (b) no systematic relation between amount of toricity and accuracy of measurements, (c) no systematic relation between angle of measurement and accuracy, (d) no difference in accuracy of measurements when the PAK is hand held in comparison with when it is mounted, (e) no difference in accuracy of measurements when axis of toricity is oriented obliquely than when it is oriented horizontally, with respect to the PAK, and (f) a small positive bias (+0.16 D) in measurement of spherical equivalent. The PAK did not prove useful for screening newborns. However, measurements were successfully obtained from 18/19 children and 14/14 adults. There was no significant difference in median measurement deviation (deviation of a subject's five measurements from his/her mean) between children (0.21 D) and adults (0.13 D). CONCLUSIONS: The PAK produces accurate measurements of surface curvature under a variety of suboptimal conditions. Variability of PAK measurements in preschool children is small enough to suggest that it would be useful for screening for corneal astigmatism in young children.

Astigmatism↗
PubMed1999

Topographic and keratometric astigmatism up to 1 year following small flap trabeculectomy (microtrabeculectomy).

AIM: To determine the induced corneal astigmatism by measuring the changes in manual keratometry and computerised corneal videokeratoscopy up to 1 year following small flap trabeculectomy (microtrabeculectomy). METHOD: A prospective study of a case series of small flap trabeculectomy procedures performed at the 90 degree meridian on 16 eyes of 16 patients, all followed to 1 year postoperatively. Changes in manual keratometry and computerised videokeratoscopy (Eyesys) readings were analysed by vector analysis and vector decomposition techniques. RESULTS: By vector analysis, the mean surgically induced refractive change (SIRC) cylinder power vectors induced at 1, 3, 6, and 12 months as measured by manual keratometry were 0.68, 0.38, 0.52, and 0.55 dioptres, and by keratography 0.75, 0.66, 0.59, and 0.64 dioptres. Vector decomposition on the induced vector cylinders on manual keratometry resulted in a "with the rule" mean vector of 0.52 and 0.22 dioptres at 1 and 3 months and an "against the rule" mean vector of 0.16 and 0.16 dioptres at the same time points (p=0.03 and 0.28 respectively). Vector decomposition at 6 and 12 months revealed no significant with the rule changes induced. Similar analysis on the videokeratoscopy results revealed significant induced with the rule astigmatism until 3 months, but not at 6 and 12 months postoperatively. CONCLUSION: Small flap trabeculectomy (microtrabeculectomy) produces smaller changes in corneal curvature that resolve sooner than previous reports of larger flap techniques.

Aged↗
PubMed2001

Corneal irregular astigmatism after laser in situ keratomileusis for myopia.

AIMS: To quantitatively evaluate the changes in corneal irregular astigmatism after laser in situ keratomileusis (LASIK) in relation to the amount of laser ablation. METHODS: In 189 eyes of 116 patients undergoing LASIK for myopia, corneal topography was obtained before and 1 month after surgery. Using Fourier harmonic analysis of the topography data, corneal irregular astigmatism (asymmetry and higher order irregularity) was calculated. RESULTS: By surgery, asymmetry component significantly increased (p < 0.0001, Wilcoxon signed rank test), while higher order irregularity did not (p = 0.767). The increases in the asymmetry component significantly correlated with ablation depth (Spearman rank correlation coefficient r(s) = 0.440, p < 0.0001). No significant correlation was found between changes in higher order irregularity and ablation depth (r(s) = 0.137, p = 0.074). CONCLUSION: LASIK significantly increases the asymmetry component of the cornea which is dependent on the amount of laser ablation.

Adult↗
PubMed1999

Significant immediate and long-term reduction of astigmatism after lateral rectus recession in divergent Duane's syndrome.

Duane's syndrome is associated with anisometropia and amblyopia. We encountered 1 patient with right divergent Duane's syndrome (type 2 according to Huber's classification) with high astigmatism of the right eye and a head turn. In order to improve the head turn and thereby avoid eccentric gaze through the glasses, the right lateral rectus muscle was recessed by 7.75 mm. Postoperatively, the amount of astigmatism was reduced by 1.5 dpt with no change of the axis. This change of refraction remained stable over a follow-up time of 18 months.

Astigmatism↗
PubMed1994

Vector analysis of corneal astigmatism after scleral buckling surgery.

We conducted a short-term prospective study of 125 eyes with retinal detachment to examine changes of corneal astigmatism about 2 weeks after scleral buckling surgery, using a vector method on 2-fold-angle rectangular coordinates. Segmental buckles of one to less than two quadrants produced significantly greater changes in astigmatism (1.65 +/- 0.97 dptr) than those of less than one quadrant (0.88 +/- 0.75 dptr) and those spanning two quadrants or more (1.09 +/- 0.38 dptr) (p = 0.0005, Kruskal-wallis test; n = 73). The amplitude of differential vectors after explant buckling surgery (1.33 +/- 0.89 dptr, n = 24) was significantly greater than after implant buckling surgery (0.65 +/- 0.45 dptr, n = 15) (p = 0.009, Mann-Whitney U-test) with buckles of one quadrant or less. Differential vectors tended to direct toward the buckles. There was no obvious directional tendency in the case of encircling procedures (n = 52).

Adolescent↗
PubMed2004

Compensation of corneal horizontal/vertical astigmatism, lateral coma, and spherical aberration by internal optics of the eye.

Both the anterior surface of the cornea and the internal optics (the posterior cornea, crystalline lens) contribute to the aberration of a wavefront passing through the eye. Artal, Guirao, Berrio, and Williams (2001) reported that the wavefront aberrations produced by the internal optics offset, or compensate for, the aberrations produced by the cornea to reduce ocular wavefront aberrations. We have investigated the wavefront aberrations of the cornea, internal optics, and complete eye on both the population and individual level to determine which aberrations are compensated and probable paths leading to that compensation. The corneal and ocular aberrations of 30 young subjects at relaxed accommodation were measured with the Topcon Wavefront Analyzer, which simultaneously measures refraction, corneal topography (videokeratoscope), and wavefront aberrations (Hartmann-Shack sensor). We found strong evidence for compensation of horizontal/vertical (H/V) astigmatism (Zernike term Z5) lateral coma (Z8) and spherical aberration (Z12). H/V astigmatism compensation is scaled for each individual, suggesting that it is actively determined by a fine-tuning process. Spherical aberration shows no individual compensation, suggesting that is a passive result of genetically determined physiology. Lateral coma shows individually scaled compensation, some of which may be attributable to eccentricity of the fovea.

Accommodation, Ocular↗
PubMed2006

Astigmatism and its components in 6-year-old children.

PURPOSE: The purpose of the present study was to report the prevalence of refractive (RA), corneal (CA), and internal astigmatism (IA) in a population of 6-year-old children; examine their variation with gender, ethnicity, and refraction; and examine the effects of gender, ethnicity, and spherical equivalent refraction on the relationship between CA and RA in this population. METHODS: The Sydney Myopia Study is a population-based survey of refraction and eye health in 6-year-old children. A random cluster design was used to recruit children from schools across Sydney, Australia, during 2003 to 2004. Data collection used a detailed questionnaire and comprehensive eye examination. Keratometric and cycloplegic autorefraction data from right eyes were analyzed. RESULTS: Of 2238 eligible children, 1765 (78.9%; 50.7% boys) had parental consent to participate. Overall prevalence of RA (> or =1.0 diopter [D]) was 4.8% (95% confidence interval [CI] 3.8%-6.1%), CA (> or =1.0 D) 27.7% (CI 23.8%-32.3%), and IA (> or =1.0 D) 21.1% (CI 19.0%-23.5%). The RA axis was fairly evenly distributed, with predominance of oblique axis (39.1%; CI 35.9%-42.6%). CA axis was mainly with the rule (75.1%; CI 72.6%-77.8%), while IA axis was mainly against the rule (76.7%; CI 74.2%-79.3%). After adjustment for multiple variables, girls had significant, marginally greater mean CA and IA than boys. East Asian and South Asian children had significantly greater prevalence and mean RA and CA than European Caucasian children. There were no significant ethnic differences of mean IA. Compared to reference (spherical equivalent [SEq] 1.01-1.50 D), mean RA and CA increased significantly with more hyperopic and more myopic refractions. Mean IA was significantly greater only for hyperopic refractions (SEq > 2.00 D). CONCLUSIONS: The prevalence of astigmatism found in this population of 6-year-old children was relatively low, and showed significant variation with ethnicity. The data suggest that emmetropization for RA occurs by a compensatory process between CA and IA.

Age Distribution↗
PubMedDate not supplied

The early results of excimer laser photorefractive keratectomy for compound myopic astigmatism.

UNLABELLED: An excimer laser (VISX Twenty/Twenty Excimer Refractive System) was used to treat 51 eyes for myopia and astigmatism. Uncorrected pretreatment visual acuity was between 6/18 and 6/60 (log unit +0.45 to +1.0) in 59% and worse than 6/60 in 29%. The mean pretreatment spherical refractive error was -4.05 dioptre (range 1.25 to 13.25), and the mean pretreatment cylindrical error was -0.97 dioptre (range 0.25 to 4.00). RESULTS: Uncorrected visual acuity measured 6/6 or better (log unit 0.0 or less) in 80% at three months, and averaged 6/6 for all eyes at six months post-treatment, with 75% eyes obtaining 6/6 or better. The mean post-treatment spherical error decayed according to pre-treatment values, with a mean sphere of -0.20 dioptre for eyes initially less than -2.00 dioptre, -0.40 dioptre (for those between -2.25 and -3.00), -0.71 dioptre (for those between -4.25 and -5.00), and -1.15 dioptre for eyes initially above -6.25 dioptre. Vectored cylindrical correction exhibited response proportional to initial refraction, with a mean post-treatment cylinder of -1.83 dioptre for eyes formerly averaging -3.08 dioptre, -0.55 dioptre (eyes initially averaging -1.63 dioptre), and -0.51 dioptre (eyes initially averaging -0.67 dioptre). Vector analysis of post-treatment astigmatism showed 58% eyes exhibiting 51 or more degrees of axis shift, although 34% eyes remained within 20 degrees of their pretreatment axis. CONCLUSIONS: An effective reduction in spherocylindrical error was achieved with all eyes, although axis misalignment was a common event.

Adult↗
PubMedDate not supplied

Surgically induced astigmatism after manual extracapsular cataract extraction or after phacoemulsification procedure.

Astigmatic changes in three series of cataract surgical procedures were compared. The first two series comprised eyes having a manual extracapsular cataract extraction (ECCE) through a 12.0 mm or 8.0 mm incision and implant of a PMMA posterior chamber lens. The third series comprised eyes having phacoemulsification through a scleral pocket and implant of a PMMA posterior chamber lens. Preoperative keratometry measurements and corneal topography maps were compared with those obtained two days, one week, one, three and six months postoperatively. The keratometry measurements, obtained with computerized videokeratography, revealed: on day 2, 4.89 D (1st series), 3.95 D (2nd series), 2.66 D (3rd series); one week, respectively 4.46 D, 3.51 D and 2.14 D; one month, 0.65 D, 0.53 D and 0.05; three months, 1.44 D, 0.35 D and 0.36 D; six months, 1.36 D, 0.42 D and 0.48 D. The surgically-induced cylinder at three months was four times greater in the manual ECCE 12.0 mm incision series than in the phacoemulsification series. However, by six months all differences were markedly reduced. The results add to the growing evidence that the phacoemulsification procedure produces less astigmatism and more rapid visual rehabilitation than the manual ECCE procedure. Topographic analysis showed much less corneal steepening after phacoemulsification than after manual ECCE. However, all three surgical procedures offered satisfactory clinical results.

Aged↗
PubMed2002

Comparison of exact pupil astigmatism conditions with Seidel approximations.

The aberrations of axisymmetric imaging systems can be calculated to third order by use of the Seidel formula. The Coddington equations give aberrations that have quadratic dependence on the pupil, for all field points. The pupil astigmatism conditions were recently developed to predict and control aberrations that have quadratic field dependence and arbitrary pupil dependence. We investigate the relationship between the exact pupil astigmatism conditions and the classical Seidel treatment of pupil aberration.

Astigmatism↗
PubMed2002

Application of the pupil astigmatism criteria in optical design.

We developed the pupil astigmatism criteria for correcting the quadratic field-dependent aberrations. These criteria provide an elegant way to determine and correct aberrations that have quadratic field dependence and arbitrary pupil dependence in the same way that the Abbe sine condition is used for aberrations with linear field dependence. Like the sine condition, the pupil astigmatism criteria involve only the properties of the rays originating from the on-axis object point, so it is convenient to implement them in optical design. We introduce an algorithm to apply the criteria in designing new well-corrected optical systems. Some example designs are presented.

Algorithms↗
PubMed1991

Theory of eccentric photorefraction (photoretinoscopy): astigmatic eyes.

An optical analysis of eccentric photorefraction (photoretinoscopy) of astigmatic eyes is presented. The size and the angular tilt of the dark crescent appearing in the subject's pupil are derived as a function of five variables: the ametropia of the eye (Dsph, Dcyl, axis), the eccentricity of the flash, e, and the distance of the camera from the subject's eye, dc. A simplified solution and a solution of the inverse problem, which enable one to calculate the degree of ametropia from the size and the tilt of the crescent, are also presented. If the crescent is smaller than the pupil, both the size and the tilt of the dark crescent are independent of the pupil size. The angular tilt of the crescent is also independent of the eccentricity. Characteristic changes of the crescent as a function of the cylinder axis are illustrated for compound and mixed astigmatisms. The validity of the theoretical predictions was experimentally verified on a model eye.

Astigmatism↗
PubMed2005

[Abnormal head position caused by incorrect prescription for astigmatism: case report].

Abnormal head position is a compensatory condition which improves patients' vision. It can be caused by ophthalmological problems such as oculomotor imbalances (strabismus, nystagmus) and high astigmatism. However, it results in esthetic impairment, orthopedic trouble and facial asymmetries. We describe a case of a girl, JL, 8 years, with abnormal head position tilted to the left since the last glasses were prescribed. The correction used by the patient was: right eye = +2.00 sph diopter -5.5 cyl 180 degrees and left eye = +2.25 sph diopter -5.75 cyl 180 degrees. In tilted position, the correct visual acuity was: right eye 6/12 and left eye 6/9. No deviations were noted by the cover test and the remaining ophthalmological examination was completely normal. Retinoscopy under cycloplegia and subjective test showed right eye = +3.50 sph diopter -6.00 cyl 10 degrees; and left eye = +3.50 sph diopter -6.00 cyl 170 degrees, with visual acuity 6/6 in both eyes. With adequate prescription, the head position was normalized. Wrong cylindrical positions for correction of high astigmatism may cause abnormal head position. Retinoscopy under cycloplegia and subjective test are essential for precise diagnoses and prescriptions.

Astigmatism↗
PubMedDate not supplied

Changes in astigmatism after surgery for congenital ptosis.

To determine the effect of ptosis surgery on the astigmatic refractive error in pediatric patients, we assessed 88 eyes postoperatively at 3, 6, and 12 months. Thirty-six eyes of unilateral cases were used as controls. There was an overall increase in average astigmatic refractive error of the study group by 0.30 diopters, while the control group decreased 0.15 D. Thirty-six percent of study eyes changed by more than 0.75 D. Results were similar for both fascia lata slings and levator resections. Those patients older than 4 years showed an increase in average cylinder (0.50 D), while those younger than 4 years decreased their average cylinder (0.20 D). Careful monitoring with postoperative refractions is required in pediatric ptosis patients.

Astigmatism↗
PubMedDate not supplied

Anterior polar congenital cataract and corneal astigmatism.

Seven eyes of six patients with anterior polar congenital cataract were examined. All patients presented with corneal astigmatism greater than 1.50 diopters. The combination of these two anomalies has often been responsible for amblyopia and occasionally for squint. The association of corneal astigmatism with this type of congenital cataract is possibly due to a delayed separation of the lens from the surface ectoderm during fetal development and thus may be considered a new syndrome.

Adolescent↗
PubMedDate not supplied

Photoastigmatic refractive keratectomy for compound myopic astigmatism with a Nidek laser.

BACKGROUND: With advances in the delivery of excimer laser energy to the cornea, spherocylindrical ablations are now possible. The refractive and visual outcome of eyes undergoing photoastigmatic refractive keratectomy with a minimum of 12 months follow-up are presented. METHODS: A retrospective analysis of 160 consecutive eyes that underwent photoastigmatic refractive keratectomy using the Nidek EC5000 excimer laser was undertaken. One year follow-up data were available on 89 eyes. Vector analysis of the change in cylindrical error, by the Alpins method, was performed. Before surgery, the mean spherical equivalent refraction was -5.68 diopters (D) (SD 2.67 D) with a mean cylinder power of -1.40 D (SD 0.75). RESULTS: At 1 year after surgery, the mean spherical equivalent was -0.44 D (SD 0.87). Seventy-one eyes (79.8%) had a spherical equivalent within 1.00 D of the target refraction and 79 eyes (89%) achieved 6/12 or better, unaided. Four of 89 eyes (4.5%) lost more than two lines of spectacle-corrected visual acuity with 9 eyes (10%) gaining Snellen acuity, comparing preoperative spectacle-corrected acuity with postoperative uncorrected visual acuity. The mean coefficient of adjustment (targeted induced astigmatism vector magnitude divided by surgically induced astigmatism vector magnitude) was 1.11 (SD 1.33), indicating undercorrection of the cylinder. The mean angle of error was 0.73 degree (+/- 10.91). CONCLUSIONS: Refractive visual acuity outcome after photoastigmatic refractive keratectomy was good. Current algorithms undercorrect the cylinder power, but are adequately aligned. Algorithms for toric ablations in the Nidek EC5000 need to be improved.

Adult↗
PubMed1998

Long-term results of photorefractive keratectomy for hyperopia and hyperopic astigmatism.

PURPOSE: This study was conducted to determine the safety and efficacy of using the Nidek EC-5000 excimer laser for photorefractive keratectomy to correct hyperopia and hyperopic astigmatism. METHODS: We treated 67 eyes of 44 patients for hyperopia and hyperopic astigmatism with the Nidek EC-5000 excimer laser. The algorithm provided an ablation zone of 5.5 mm diameter with the addition of a tapered transition zone of 3.5 mm diameter, for a total ablation of 9 mm diameter. RESULTS: Uncorrected visual acuity (geometrical mean) changed from 0.16 to 0.37 at 12 months; corrected visual acuity (geometrical mean) changed from 0.8 to 0.89; mean sphere decreased by 2.08 D from 3.76 to 1.40 D (range, 1.70 to 1.68 D) and cylinder by 1.40 D from 2.20 to 1.00 D. Refractive results for < or = 3.00 D were reasonably accurate and stable, but for > +3.00 D, undercorrection and regression over l year were the rule. CONCLUSION: Hyperopic PRK proved to be a safe technique in regard to the risk of loss of visual acuity with no central corneal opacities and with a generally rapid recovery of baseline spectacle-corrected visual acuity, but the predictability of correction greater than +3.00 needs improvement.

Adult↗
PubMedDate not supplied

Photorefractive keratectomy to correct myopic or hyperopic astigmatism with a cross-cylinder ablation.

PURPOSE: To assess the efficacy and safety of a combined ablation of the steep and flat meridian to correct astigmatism with the excimer laser. METHODS: Twenty-two eyes with myopic, mixed, or hyperopic astigmatism (mean preoperative spherical equivalent refraction -4.30 +/- 4.70 D [range, -12.50 to +1.50 D] and mean preoperative cylinder magnitude -3.40 +/- 1.40 D [range, -1.50 to -6.00 D]) underwent PRK with the Nidek EC-5000 excimer laser. The surgical strategy involved ablating half the amount of the cylinder (in diopters) along steepest meridian, the other half in a subsequent step along the flattest meridian; thereafter, the spherical equivalent was corrected. RESULTS: Six months postoperatively, mean spherical equivalent refraction was -0.07 +/- 0.87 D and mean cylinder was -0.44 +/- 0.36 D. Mean spectacle-corrected visual acuity was 0.86 +/- 0.13 compared to 0.75 +/- 0.22 preoperatively. Mean corneal haze was 0.67 +/- 0.31. No patient lost 2 or more lines of spectacle-corrected visual acuity and there were no complaints about night halos or glare. CONCLUSIONS: Unlike other ablation strategies, the cross-cylinder method creates a smooth transition (low dioptric gradient) between the treated and untreated cornea. This is achieved by first treating the cylinder and making the corneal surface spherical and then ablating the spherical component of the refractive error.

Adult↗