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PubMed2006

Visual acuity in simple myopic astigmatism: influence of cylinder axis.

PURPOSE: The relationship between astigmatic refractive errors and their associated visual acuity has been studied in recent years in the context of refractive power space. The influence of the axis of astigmatism remains a matter of controversy. Our goal in this study is to provide additional experimental evidence to clarify this subject. The influence of the simulated axis orientation was compared with other factors that affect visual acuity such as the particular design of the test and the differences between eyes. METHODS: Simple myopic astigmatism from 0 to -3.00 D, in steps of -0.25 D, and with five different axes between 0 degrees and 90 degrees , were simulated on four healthy eyes of young observers. In each case, visual acuity was recorded for three different tests. Refractions were expressed in the form of vectors and visual acuity was represented as a function of strength. RESULTS: No significant differences in visual acuity were found for astigmatism of the same power but different axes. In fact, our results show these differences are even less important than those recorded for the same astigmatism induced in different eyes. The highest discrepancies in visual acuity were found when different charts were used to test the same astigmatic error. CONCLUSIONS: The strength of the vector representing the astigmatic refractive state describes very accurately the performance of visual acuity across simple myopic astigmatic errors. In these cases, visual acuity can be associated with a single refractive parameter. This fact could be useful, especially in statistical studies involving visual performance.

Adult↗
PubMed2005

Impact of decentration of astigmatic intra-ocular lenses on the residual refraction after cataract surgery.

PURPOSE: The purpose of this study is to assess the impact of decentration of astigmatic intra-ocular lenses on the residual refraction after cataract surgery, using a computing scheme with 5 x 5 system matrices. METHODS: Based on the definition of an optical system in the paraxial Gaussian space containing astigmatic surfaces without restrictions to coaxiality, we derived a method (using 5 x 5 refraction and translation matrices) for calculating the residual refraction and the compensating prism in the spectacle plane after decentred implantation of thin and thick astigmatic intra-ocular lenses. The 'optical system eye' may contain astigmatic refractive surfaces with their axes at random. RESULTS: The capabilities of this computing scheme are demonstrated with two examples. In example 1 we calculate the residual refraction of a decentred 'thin astigmatic lens' for compensation of corneal astigmatism to achieve a spherical target refraction. In example 2 we compute the residual refraction after implantation of a 'thick astigmatic lens', where the spherical and cylindrical power as well as the implantation axis of the lens do not fully match the pre-operative recommendations and the lens is decentred relative to the optical axis. For both examples, we derive the residual prismatic effect in the spectacle plane and the lateral displacement of a ray exiting the spectacle correction when starting coaxially at the retina. CONCLUSIONS: We have presented an en bloc matrix-based strategy for the calculation of the residual spherocylindrical refraction at the spectacle plane after implantation of a decentred thin or thick astigmatic intra-ocular lens without restrictions to coaxiality. The resulting system matrix is written as a product of 5 x 5 refraction and translation matrices.

Astigmatism↗
PubMed1997

Induced astigmatism after 4 and 6 mm scleral tunnel incision. A randomized study.

PURPOSE: To study the surgically induced astigmatism after phacoemulsification through either a 4 or a 6 mm scleral tunnel incision by using multiple analyses of astigmatism. METHODS: 197 eyes from 186 patients scheduled for phacoemulsification between October 1992 and March 1994 were randomly assigned two different-sized incisions with follow-ups at 1 day, 1 week, 2 weeks, 1 month and 4 months after surgery. The surgically induced astigmatism was evaluated using at each follow-up: 1) The subtraction method, 2) vector analysis, 3) vector decomposition, 4) Cravy's vertical vector, 5) Naeser's polar values, and 6) the algebraic method. RESULTS: By subtraction, without regard to axis, the induced astigmatism 4 months after surgery was +0.04 D and +0.18 D in the 4 mm and the 6 mm incision group, respectively. By vector analysis, the numerical value of the induced cylinder was stable one month after surgery at 0.61 D and 0.77 D in the 4 mm and in the 6 mm group, respectively. However, cylinder orientation was not found stable until 4 months after surgery, where 94% and 96% of the surgically induced astigmatism (vector decomposition) was against-the-wound in the two groups, respectively. By Cravy's method, the mean induced astigmatism changed from -0.08 D to -0.32 D and from -0.42 D to -0.60 D between 1 and 4 months in the 4 mm and the 6 mm group, respectively. Similar values were found with Naeser's method and with the algebraic method. CONCLUSION: We conclude the mean cylinder of the surgically induced astigmatism (vector analysis) to be stable 1 month after phacoemulsification with both the 4 mm and 6 mm scleral tunnel incision. However, the direction of the induced axis (vector decomposition) was still drifting between 1 and 4 months in both groups. These astigmatic changes were adequately described using vector analysis and vector decomposition.

Adult↗
PubMed1994

Experimental myopia in chickens induced by corneal astigmatism.

Astigmatism has been suggested to be an important factor in the production and/or progression of myopia. Chickens have been used as a myopic animal model for several years. In the present investigation our aim was to evaluate the importance of surgically induced corneal astigmatism by peripheral corneal incision with compression suture in one eye, the right, of 3-day-old chicks. Vertical incision (2 mm) with compression suture (nylon 9-0, one stitch) induces against-the-rule astigmatism, horizontal incision with compression induces with-the-rule astigmatism. Four groups were studied 1) with one vertical cut, 2) one horizontal cut, 3) two vertical cuts, 4) two horizontal cuts. The eyes were measured by caliper after 8 weeks. Eye enlargements were induced in all groups, however, less by one cut (astigmatism around 5 diopters) than by two cuts (astigmatism 10 diopters, or more). In the latter groups the pattern of equatorial eye enlargement depended on the axis of the induced astigmatism, the greater elongation being associated with the strongest refracting corneal meridian. All considered, the results suggest that significant corneal astigmatism could be another factor in the production of myopia, possibly acting by way of optical degradation of retinal image quality.

Animals↗
PubMed1997

Laser intrastromal keratomileusis for high myopia and myopic astigmatism.

BACKGROUND: Laser intrastromal keratomileusis (LASIK) is an evolving technique which enables high degrees of myopia (> 8.0 dioptres) and myopic astigmatism to be corrected. This paper describes initial experience with this procedure. It also details the methodology, the results, the problems encountered, and discusses retreatment procedures. METHODS: 51 eyes (48 primary cases and three retreatments) underwent LASIK for simple myopia or compound myopic astigmatism. After the keratotomy was fashioned with a Chiron corneal shaper, the ablation was performed with either a Summit or Meditec excimer laser. The actual preoperative astigmatism ranged from -0.5 D to -6.0 D (in the astigmatic myopic LASIK (AML) series), while the range of preoperative myopia in the combined myopic LASIK (ML) and AML series was -8.0 D to -37.0 D. Of the ML cases, group 1 (-8.0 to 15.0 D (dioptres)), group 2 (> -15.0 to -20.0 D), and group 3 (> -20.0 D) had mean preoperative myopia values (spherical equivalent) of -11.26 D, -16.84 D and -27.78 D. The same groupings (1, 2, and 3) for the AML cases had respective values of -9.702, -17.4, and -23.08. In the AML series the mean preoperative astigmatism was -2.109 D. Follow up ranged from 8 to 27 months (mean 15.8 months). Six of the cases required retreatment. RESULTS: There was a reduction in best corrected visual acuity (BCVA) (of 1 Snellen line) in seven of the primary cases (14.5%) (three in the ML group and four in the AML group), and in one of the retreatment cases. The BCVA improved in 28 cases (58%) in the primary treatment group. The mean correction attempted (spherical equivalent) for the ML groups 1, 2, and 3 was 10.51 D, -14.5 D, and -27.78 D, versus a mean correction achieved of -9.445 D, -15.625 D, and -21.571 D. Similarly, for the AML groups, attempted correction values were -9.702 D, -17.4 D, and -23.08 D, while the values achieved were -6.95 D, -51.425 D, and -15.708 D. Regression was minimal and stabilisation of the refractive result was achieved in all groups, except group 3 of the ML series, by the 3 month examination period. The mean postoperative astigmatism in the AML series was -0.531 D. Vector analysis of the AML series showed that the mean surgically induced astigmatism was +0.93 D. The most common complication encountered was undercorrection, which occurred in 35 cases-23 cases in the ML group and 12 cases in the AML series. Twenty eight per cent of the ML cases, and 25% of the AML cases were within plus or minus 1.5 D of the attempted refraction. CONCLUSION: For the correction of high myopia and myopic astigmatism, LASIK results in less postoperative pain and relatively little subepithelial haze compared with high myopic photorefractive keratectomy. Furthermore, a stable refraction and reasonably predictable outcome occurs much earlier. High myopia up to -37.0 D can be corrected, albeit with some limitations at the extremes of myopia-in terms of the amount of myopia correctable; this represents a limitation of the technique. Retreatment is a technically straightforward and effective way to treat undercorrection. Undercorrection, the main complication seen in our series, should become less common when the ablation algorithms are further refined.

Adolescent↗
PubMed1999

Screening for refractive errors in children: accuracy of the hand held refractor Retinomax to screen for astigmatism.

AIMS: To assess the reliability of the hand held automated refractor Retinomax in measuring astigmatism in non-cycloplegic conditions. To assess the accuracy of Retinomax in diagnosing abnormal astigmatism in non-cycloplegic refractive screening of children between 9 and 36 months. METHODS: Among 1205 children undergoing a non-cycloplegic refractive screening with Retinomax, 299 (25%) had repeated non-cycloplegic measurements, 302 (25%) were refracted under cycloplegia using the same refractor, and 88 (7%) using retinoscopy or an automated on table refractor. The reproducibility of non-cycloplegic cylinder measurement was assessed by comparing the cylindrical power and axis values in the 299 repeated measurements without cycloplegia. The influence of the quick mode on cylinder measurement was analysed by comparing the cylinder and axis value in 93 repeated measurements without cycloplegia where normal mode was used in one measurement and quick mode in the other. Predictive values of the refractive screening were calculated for three different thresholds of manifest astigmatism (> or = 1.5, > or = 1.75, and > or = 2 D) considering as a true positive case an astigmatism > or = 2 D under cycloplegic condition (measured by retinoscopy, on table, or hand held refractor). RESULTS: The 95% limits of agreement between two repeated manifest cylinder measurements with Retinomax attained levels slightly less than plus or minus 1 D. The 95% limits of agreement for the axis were plus or minus 46 degrees. The comparison of non-cycloplegic measurements in the quick and normal mode showed no significant difference and 95% limits of agreement plus or minus 0.75 D. The mean difference between non-cycloplegic and cycloplegic cylinder values measured by Retinomax reached 0.17 D and was statistically significant. Manifest thresholds of > or = 1.5 D, > or = 1.75 D, > or = 2 D cylinder value diagnosed 2 D of astigmatism under cyclplegia respectively with 71-84%, 59-80%, 51-54% of sensitivity (right eye-left eye) and 90-92%, 95%, 98% of specificity. CONCLUSION: Without cycloplegia, Retinomax is able to measure cylinder power with the same reproducibility as cycloplegic retinoscopy. No significant difference was found in the cylinder values obtained with the quick and the normal modes. Therefore, the quick mode of measurement is recommended as it is more feasible in children. No difference, which is significant from a screening point of view, exists between the non-cycloplegic and the cycloplegic cylinder value (< 0.25 D). Retinomax diagnoses abnormal astigmatism (> or = 2 D) in a non-cycloplegic refractive screening at preschool ages with 51-84% sensitivity rates and 98-90% specificity rates, depending on the chosen threshold of manifest astigmatism. If 2 D of manifest astigmatism is chosen as a positive test, the positive predictive value of the screening reaches 81-84% and the negative predictive value 91-90% (right eye-left eye).

Age Distribution↗
PubMed1992

Sudden against-the-rule astigmatism 6 months after intraocular lens implantation with the Kelman phacoemulsification procedure: 4 of 809 cases.

Of 809 eyes that underwent the Kelman phacoemulsification and aspiration (KPE) procedure, we report on 4 patients (0.49%) who developed sudden against-the-rule (AR) astigmatism after intraocular lens implantation. These patients had had with-the-rule astigmatism preoperatively to 3 months postoperatively and suddenly developed AR astigmatism 6 months after surgery. To study the development and progression of AR astigmatism in these patients (group A), we compared their 6-month time course with that of 257 patients (group B) with postoperative AR astigmatism. In group A, the degree of astigmatism was not significantly different from group B astigmatism preoperatively (1.03 +/- 0.16 vs. 1.04 +/- 0.04 dptr) and 6 months postoperatively (1.32 +/- 0.13 vs. 1.26 +/- 0.05 dptr). However, 1 week postoperatively, the astigmatism of the 4 cases was much greater (5.72 +/- 0.14 dptr) than that of the others (2.32 +/- 0.07 dptr). These rare 4 cases, which had no other unusual characteristics, are indicative of the importance of a careful follow-up even if the post-KPE course seems uneventful.

Aged↗
PubMed2004

Effects of optically imposed astigmatism on emmetropization in infant monkeys.

PURPOSE: Although astigmatism is prevalent during early infancy, the influence of astigmatism on early refractive development is unclear. This study was undertaken to determine the effects of astigmatism on emmetropization in monkeys. METHODS: Infant rhesus monkeys (n = 39) were exposed to optically simulated astigmatism in one or both eyes from approximately 1 to 4 months of age. With-the-rule, against-the-rule, and oblique astigmatisms were optically simulated by appropriately orienting the principal meridians of the spherocylindrical treatment lenses (+1.50 -3.00 D x 90 degrees, 180 degrees, 45 degrees, or 135 degrees; i.e., +1.50 and -1.50 D powers in the two principal meridians). Refractive development was assessed every 2 to 3 weeks by cycloplegic retinoscopy, keratometry and corneal videotopography, and A-scan ultrasonography. Data from 19 control monkeys, including 3 animals that were reared with binocular plano lenses, were used for comparison purposes. RESULTS: Most of the cylinder-lens-reared monkeys, regardless of the orientation of the imposed astigmatism, showed clear signs of either hyperopic or myopic growth compared with control monkeys. The distributions of refractive error and vitreous chamber depth both showed bimodal patterns that differed from normal by amounts equivalent to the optical powers of the principal meridians of the treatment lenses. More frequently, refractive development was biased toward the eye's least-hyperopic focal plane. The refractive changes were mainly axial. After lens removal, the lens-reared monkeys recovered and as a group exhibited refractive errors and axial dimensions similar to those in control monkeys. CONCLUSIONS: In the presence of significant amounts of astigmatism, emmetropization is directed toward one of the two focal planes associated with the astigmatic principal meridians and not the circle of least confusion. These results suggest that the mechanisms responsible for emmetropization are insensitive to stimulus orientation and the global form of the retinal image. It appears that emmetropization seeks out the image plane that contains the maximum effective contrast integrated across spatial frequency and stimulus orientation.

Animals↗
PubMed2004

Pseudophakic residual astigmatism.

We investigated pseudophakic residual astigmatism in order to minimize postoperative refractive astigmatism. We examined 110 eyes of 87 patients who had undergone phacoemulsification with small incision and posterior chamber intraocular lens (IOL) implantation. Corneal astigmatism was measured using an autokeratometer (RK-5, canon), refractive astigmatism by manifest refraction, and residual astigmatism by vector analysis. Mean pseudophakic residual astigmatism was +0.47 x 176 degrees, predominantly against-the-rule. Variations of pseudophakic residual astigmatism according to sex, age and IOL type were not statistically significant. When performing cataract surgery as refractive surgery, we may consider that pseudophakic residual astigmatism is approximately 0.50D against-the-rule.

Adult↗
PubMedDate not supplied

Astigmatism in infancy and childhood.

PURPOSE: To evaluate the prevalence of astigmatism in infancy and childhood in a Spanish population. METHODS: A total of 478 children with astigmatism from Valencia Province, Spain were examined. The patients, ranging in age from 2-12 years, were classified into groups according to age. Astigmatism error and axis orientation were determined using noncyclopegic retinoscopy over each eye. RESULTS: The overall prevalence of astigmatism (> or = 1.00 diopter of cylinder) decreased in relation to increasing age from 44.3% to 5.2%. The prevalence of against-the-rule astigmatism decreased from 61.5% (at 2 years) to 33.4% (at 12 years), with-the-rule astigmatism increased in prevalence from 23.1% (at 2 years) to 66.6% (at 12 years), and oblique astigmatism decreased with age from 15.4% (at 2 years) and 0% (at 12 years). CONCLUSION: These results do not vary from those observed in other populations. Factors such as race, nutrition, and environmental condition did not vary from the literature regarding the prevalence of astigmatism in infants and children.

Age Distribution↗
PubMedDate not supplied

Combined wedge resection and relaxing incisions for astigmatism after penetrating keratoplasty.

BACKGROUND: Videokeratography may provide information for surgical correction of astigmatism after penetrating keratoplasty. We used a combination of wedge resection and relaxing incisions to treat high refractive astigmatism after penetrating keratoplasty. METHODS: Videokeratography using the normalized scale of the Topographic Modeling System was used as a guide in determining the location and the length of incisions and resections. Nine eyes were treated with both relaxing incisions and a wedge resection. All patients had more than 3.00 diopters (D) of refractive astigmatism. All patients were intolerant of spectacles or contact lenses. The depth of the corneal relaxing incisions was constant at 0.5 mm and the width of the corneal wedge resections was constant at 0.75 mm. RESULTS: The relaxing incisions produced flattening of the steeper meridian and the wedge resection produced steepening of the flatter meridian. The average preoperative keratometric astigmatism was 7.44 D (range, 3.50 to 11.00 D) and the average refractive astigmatism was 5.56 D (range, 4.00 to 8.00 D). The average preoperative spherical equivalent was 0.08 D (range, -7.00 to 4.25 D). Postoperatively, the average keratometric astigmatism was 2.97 D (range, 1.00 to 5.00 D) and the average refractive astigmatism was 2.58 D (range, 0.00 to 5.00 D). The average postoperative spherical equivalent refraction was -0.32 D. CONCLUSIONS: Combined corneal wedge resection and relaxing incisions appears to be effective in reducing high refractive astigmatism following corneal transplantation.

Astigmatism↗
PubMed1996

Early results of photorefractive keratectomy for myopic astigmatism.

BACKGROUND: The Technolas Keracor 116 excimer laser has been used to correct myopic astigmatism. The purpose of this study was to evaluate the early results in our patients. METHODS: The Technolas Keracor 116 excimer utilizes a scanning beam to correct myopic astigmatism. Following the correction of the astigmatic error, spherical myopic component is then treated. Twenty-eight eyes with myopic astigmatism were treated utilizing a 4 x 12 mm ablation zone for astigmatism and 4.5 mm to 6.6 mm transition zone for the spherical component at the same session. The mean preoperative spherical equivalent refraction was -4.87 +/- 3.48 diopters (D) (range: -1.00 to -14.25) and the mean preoperative astigmatic error was -2.53 +/- 1.49 D (range: -1.00 to -6.00 D) All eyes received dexamethasone eye drops for 3 months. RESULTS: At 6 months, the mean spherical equivalent refraction was +0.23 D (range +1.38 to -1.50 D), the mean postoperative refractive cylinder was -0.16 +/- 0.99 D (range: +1.50 to -2.00 D) and 78% of the eyes had a refraction within +/- 1 D. Fifty-five percent achieved 20/40 or better uncorrected visual acuity. In three cases there was one line loss and in two cases there was two line loss in best corrected spectacle visual acuity. CONCLUSIONS: Early results from this study suggest that excimer laser photorefractive keratectomy effectively reduces myopic astigmatism. The treatment of astigmatism offers a new challenge for excimer laser corneal surgery. Lasers utilize different methods to control the beam that reshapes the cornea.

Adult↗
PubMedDate not supplied

Relationships between preoperative astigmatism and corneal optical power, axial length, intraocular pressure, gender, and patient age.

BACKGROUND: Preoperative astigmatism is likely to influence the outcome of cataract surgery, but it is only rarely considered in studies of cataract surgery outcomes. METHODS: As part of a population-based study of 5878 consecutive cataract surgeries performed from 1986 through 1990, I studied the relationship between preoperative astigmatism and corneal shape, axial length, intraocular pressure, gender, and age. RESULTS: The men had more against-the-rule astigmatism than the women, and the preoperative astigmatism shifted for both genders toward against-the-rule with age. Long eyes tended to have more with-the-rule astigmatism. Also, astigmatism deviated more from normal in both long and short eyes, and the with-the-rule astigmatism decreased with increasing intraocular pressure. Finally, I found that a second-degree polynomial model better described the relation between the axial lengths and keratometric powers better than a linear one. CONCLUSIONS: Axial length is related to the direction and amount of astigmatism. Because in the present study a second-degree polynomial described the relationship between the axial lengths and keratometric powers better than a simple linear regression, a polynomial model also probably should be used for calculating the power of the intraocular lens to be inserted.

Adolescent↗
PubMedDate not supplied

Laser in situ keratomileusis for correction of high astigmatism after penetrating keratoplasty.

PURPOSE: To evaluate the safety and efficacy of laser in situ keratomileusis (LASIK) for correction of high astigmatism after penetrating keratoplasty, and to assess the refractive results and predictability of the procedure. METHODS: LASIK was performed on 19 patients (19 eyes) with high astigmatism after penetrating keratoplasty, using the Chiron Automated Corneal Shaper and the Chiron-Technolas Keracor 116 excimer laser. The amount of preoperative refractive astigmatism ranged from 6.50 to 14.50 D (mean, 9.21 +/- 1.95 D) and the spherical component of manifest refraction ranged from -7.00 to +1.25 D (mean, -2.14 +/- 2.11 D). All patients completed a minimum follow-up of 12 months. RESULTS: Refraction was stable after 3 months. At 1 year after LASIK, the amount of refractive astigmatism was reduced to a mean of 1.09 +/- 0.33 D (range, 0.50 to 1.75 D), with 57.9% of the eyes within +/- 1.00 D of refractive astigmatism. The mean percent reduction of astigmatism was 87.9 +/- 3.7%. The postoperative spherical component of manifest refraction ranged from -1.00 to +1.75 D with a mean of +0.43 +/- 0.82 D. Vector analysis showed that the mean amount of axis deviation was 1.1 +/- 1.3 degrees and the mean percent correction of preoperative astigmatism was 92.6 +/- 8.4%. There were no intraoperative complications. Spectacle-corrected visual acuity was not reduced in any eye, and improved by 2 or more lines in 42.1% of eyes after LASIK. CONCLUSION: LASIK with the Chiron-Technolas Keracor 116 excimer laser was effective for correction of both astigmatism and myopia after penetrating keratoplasty. The procedure proved to be safe and gave fairly predictable and stable refractive results.

Adult↗
PubMedDate not supplied

Laser in situ keratomileusis with the Nidek EC-5000 excimer laser for astigmatism greater than 4.00 D.

PURPOSE: To evaluate laser in situ keratomileusis (LASIK) performed with the Nidek EC-5000 excimer laser in 66 astigmatic eyes with cylinder from 4.00 to 8.00 D. METHODS: LASIK was performed with the Nidek EC-5000 excimer laser in 66 astigmatic eyes with cylinder from 4.00 to 8.00 D. Based on Chayet's nomogram, ablation zone diameters were 5.5 mm (steep meridian ablation) to 7.5 mm (flat meridian ablation). In eyes with simple myopic astigmatism, mixed astigmatism, and compound myopic astigmatism (low sphere), ablation was performed in both meridians. In eyes with simple hyperopic astigmatism, ablation was performed in the flat meridian alone. In eyes with compound hyperopic astigmatism, ablation zone diameter was 5.5 mm and the transition zone was 7.5 mm. RESULTS: Uncorrected visual acuity was 20/40 or better in 89% of eyes and was equal to preoperative best spectacle-corrected visual acuity in 62% of eyes. No eyes lost lines of best spectacle-corrected visual acuity. CONCLUSIONS: Using the Nidek EC-5000 excimer laser, we had satisfactory results in the treatment of astigmatic eyes with cylinder from 4.00 to 8.00 D, based on Chayet's nomogram.

Astigmatism↗
PubMed1998

Late post-operative astigmatism after extracapsular cataract extraction and intraocular lens implantation.

The purpose of the study is to establish the amount of surgically induced astigmatism after extracapsular cataract extraction (ECCE) and intraocular lens (IOL) implantation. Eighty three eyes of 79 patients who had ECCE + IOL implantation were examined in the period between the 6th and the 16th month after the operation. The pre- and post-operative auto keratometry readings (K1 and K2) were recorded for each patient. These readings were compared between themselves, as well as with the post-operative cylinder power estimated by an autorefractor (total astigmatism). The mean pre-operative corneal astigmatism (K1-K2) was 1.00 +/- 0.09 Dpt Cyl, and the mean post-operative -1.74 +/- 0.14 Dpt Cyl. The difference is statistically significant. The mean autorefractor cylinder power was 1.70 +/- 0.14 Dpt Cyl. It correlates strong with the post-operative corneal astigmatism. There is a moderate correlation between the degree of the post-operative corneal and total astigmatism. The surgically induced corneal astigmatism is the main component of the total post-operative astigmatism after ECCE + IOL implantation. The total astigmatism is mainly with the rule (vertical plus cylinder) and it does not seem to impair severely the post-operative visual acuity of the patients.

Astigmatism↗
PubMed2000

Astigmatism and Amblyopia among Native American Children (AANAC): design and methods.

The overall goal of the AANAC study is to improve detection of astigmatism and prevention of amblyopia in populations with a high prevalence of astigmatism. To meet this goal, the study will evaluate four methods of screening for astigmatism in preschool children and will assess both the short-term and long-term benefits of early correction of astigmatism in improving acuity and preventing amblyopia. This paper presents an overview of the design and methodology of the AANAC study. Subjects are members of the Tohono O'Odham Nation, a Native American tribe with a high prevalence of astigmatism. Preschool-age children who attend Head Start are screened with four tools: the Marco Nidek KM-500 autokeratometer, the MTI photoscreener, the Nikon Retinomax K-Plus autorefractor, and the Lea Symbols acuity chart. Sensitivity and specificity for detection of significant astigmatism, as measured by a technique that uses both cycloplegic retinoscopy and cycloplegic autorefraction, is determined for each of the four screening tools. Presence of amblyopia is evaluated by measurement of best-corrected recognition acuity and acuity for orthogonal gratings. Spectacles are provided to all 3-year-old children with > or =2.00 diopters (D) of astigmatism and all 4- and 5-year-old children with > or =1.50 D of astigmatism. Persistence of amblyopia after glasses wearing is evaluated by follow-up measurement of best-corrected recognition acuity and acuity for orthogonal gratings, conducted 2-5 months after glasses are prescribed. Long-term effectiveness of early screening and glasses prescription is evaluated through measurement of recognition acuity in two groups of first-grade children: one group who participated in the Head Start program before the intensive vision screening program was initiated, and a second group who participated in the study's Head Start vision screening program.

Amblyopia↗
PubMed2001

[Correlation between corneal topography and subjective refraction in idiopathic and surgery-induced astigmatism].

PURPOSE: To study the correlation between subjective refraction and corneal topography. To compare the topographic analysis of surgically induced astigmatism (cataract and penetrating keratoplasty) with that of idiopathic astigmatism. METHODS: Subjective astigmatism, subjective spherical equivalent, best spectacle-corrected visual acuity (LogMAR units), and videokeratoscopy using the EyeSys 2000((R)) device (axial, tangential, and refractive power) were recorded in 100 eyes with idiopathic astigmatism, 100 eyes after cataract surgery, and 100 eyes after penetrating keratoplasty. Topographies were classified according to pattern (Bogan classification) and asphericity (shape factor: prolate or oblate). RESULTS: The asphericity shape distribution was significantly different between the 3 groups (p<0.001). The shape of idiopathic astigmatism was almost always prolate (90%) whereas the oblate shape was more frequent in the penetrating keratoplasty group (75%). There was no significant difference in topographic pattern distribution between the 3 groups (p=0.11). The asymmetric bow tie pattern was the most common topographic pattern. Topography pattern classification was significantly correlated with the subjective astigmatic cylinder. (r(s)=0.60, p<0.001). Unlike the round and oval patterns, the bow tie pattern was associated with the high subjective cylinder. Correlation between the subjective cylinder, the refractive power cylinder, and the axial power cylinder was strong (r(s)=0.92 p<0.001), but it was weak for the tangential power cylinder (r(s)=0.72 p<0.001). The correlation between the subjective spherical equivalent and central cornea power was poor (r(s)<0.37, p<0.001). Subjective astigmatic cylinder showed the strongest correlation with best spectacle-corrected visual acuity (r(s)=0.70, p<0.001), whereas the predicted corneal acuity, corneal uniformity index, asphericity, and refractive power symmetry index were poorly correlated with it (r(s)<0.54, p<0.001). CONCLUSION: Despite the difference in the asphericity shape, the topographic pattern was similar in the 3 groups. The pattern type was significantly correlated with the subjective astigmatic cylinder. Topographic indices failed to predict visual acuity, while the subjective cylinder showed a strong correlation with visual acuity.

Astigmatism↗