Astigmatism analysis by the Alpins method.

PURPOSE: To determine the effectiveness of correcting astigmatism by laser refractive surgery by a vectorial astigmatism outcome analysis that uses 3 fundamental vectors: target induced astigmatism vector (TIA), surgically induced astigmatism vector, and difference vector, as described by the Alpins method. METHODS: A data set of 100 eyes that had laser in situ keratomileusis to correct myopia and astigmatism (minimum preoperative refractive astigmatism 0.75 diopter) was analyzed. The data included preoperative and 3 month postoperative values for manifest refraction and standard keratometry. Using the ASSORT or VectrAK analysis program, individual and aggregate data analyses were performed using simple, polar, and vector analysis of astigmatism and an analysis of spherical change. Statistical analysis of the results was used for means and confidence limits, as well as to examine the differences between corneal and refractive astigmatism outcomes. RESULTS: At an individual patient level, the angle of error was found to be significant, suggesting variable factors at work, such as healing or alignment. A systematic error of undercorrection of astigmatism is prevalent in the treatment of these 100 patients by a factor of between 15% and 30%, depending on whether refractive or corneal values are examined. Spherical correction showed systematic undercorrection of 11%, and parallel indices demonstrated it to be more effective than the astigmatic correction. CONCLUSION: This method of astigmatism analysis enables the examination of results of astigmatism treatment measured by both refractive and corneal measurements using vector analysis. By examining individual vector relationships to the TIA (ie, the correction index, index of success, and flattening index), a comprehensive astigmatism analysis is completed. Each index provides information necessary for understanding any astigmatic change. Astigmatic outcome parameters are more favorable when measured by subjective refractive than objective corneal methods.

Management of irregular astigmatism.

Using a liberal definition of corneal irregularity, modern videokeratoscopy may define approximately 40% of normal corneas with a toric refractive error as possessing primary irregular astigmatism. The causes of secondary forms of irregular astigmatism include corneal surgery, trauma, dystrophies, and infections. Internal refractive surface and media irregularity or noncorneal astigmatism (ocular residual astigmatism) contribute to irregular astigmatism of the entire refractive path of which crystaline lenticular astigmatism is usually the principal contributing component. Treatment options have increased in recent years, particularly, though not exclusively, through the advent of tailored corneal excimer laser ablations. However, discussion continues concerning the systematic approach necessary to enable treatment to achieve an optimal optical surface for the eye. Discussion also continues as to what constitutes the optimal corneal shape. Some refractive procedures may increase higher order aberrations in the attempt to neutralize refractive astigmatism. The way to further refinement of the commonly performed refractive techniques will ultimately lie in the integrated inclusion of a trio of technologies: topographic analysis of the corneal surface, wavefront analysis of ocular refractive aberrations, and vector planning to enable the appropriate balance in emphasis between these two diagnostic modalities. For the uncommon, irregularly roughened corneas, the ablatable polymer techniques show some promise.

Refractive versus corneal changes after photorefracive keratectomy for astigmatism.

PURPOSE: To define measures of assessing success and subsequent ways to improve excimer laser treatment of astigmatism. METHODS: We studied 97 eyes of 79 patients, followed for 12 months, that underwent photorefractive keratectomy (PRK) for myopia and astigmatism with a VISX 20/20 excimer laser. Preoperative spherical equivalent refraction at the corneal plane was between -1.00 and -15.00 D. Mean preoperative refractive astigmatism at the spectacle plan was -2.17 +/- 1.05 D (range, -1.25 to -6.00 D), which is -1.81 +/- 0.86 D (range -1.04 to -4.97 D) when calculated at the corneal plane. All patients were examined before and after surgery; examination included refraction, keratometry, and topography measurement. RESULTS: The success in treatment of astigmatism appeared measurably less than the treatment of sphere when analogous indices were used for assessment. Success in astigmatism surgery improved, as measured by all parameters, after an additional 20% was applied to astigmatism treatment magnitude indicated by the VISX computer algorithm. The sequential modes of treatment undercorrected astigmatism magnitude to a greater extent than elliptical, but equivalent success rates were present in view of the greater astigmatic changes attempted using the sequential mode. The elliptical mode tended to produce a greater undercorrection of associated sphere (p = 0.313). Results measured by refraction showed a larger change than those measured by topography and keratometry. CONCLUSION: During PRK with the VISX 20/20 laser, adjustment for undercorrection of astigmatism treatment achieves a fuller correction of astigmatism. When measuring astigmatic changes, results are different when comparing refractive astigmatism changes with corneal astigmatism changes measured by keratometry and topography.

Excimer laser correction of astigmatism with multipass/multizone treatment. The Melbourne Excimer Laser Group.

PURPOSE: To evaluate the accuracy of excimer laser correction of myopic astigmatism by multipass/multizone photoastigmatic refractive keratectomy (PARK). SETTING: Tertiary referral ophthalmic hospital with an associated private laser facility. METHODS: This study comprised a consecutive series of 332 eyes of 289 patients who were followed for 6 months. All patients were 18 years or older, had stable myopic astigmatism (up to a -19.0 diopters [D] spherical equivalent [SE] at the spectacle plane), and had a best corrected visual acuity of at least 20/60 in both eyes. All eyes were treated with a VISX Twenty-Twenty excimer laser. The correction was divided between ablation zones using a multipass/multizone treatment paradigm based on the amount of myopia and astigmatism. Patients were examined 1 week, and 1, 3, and 6 months after surgery. RESULTS: Analysis of the mean percentage of spherical correction across the range of myopic preoperative SEs treated demonstrated 90% correction for most amounts of myopic astigmatism. Eyes with low myopia (mean preoperative SE < or = -5.0 D) treated with < or = -1.0 diopter cylinder (DC) of astigmatism achieved a mean percentage of spherical correction of 91% versus 93% in eyes with high myopia (> -5.0 D mean preoperative SE). Eyes with low myopia treated with > -1.0 DC of astigmatism achieved a mean percentage spherical correction of 90% versus 89% in eyes with high myopia. The differences between the two groups were not statistically significant. Patients with high relative cylinder (> 80% of total sphere treated) achieved comparable results. Analysis of the astigmatic component of the treatment, independent of the spherical result, showed a trend toward overcorrection in the high myopia group with less than -1.0 DC and a mean astigmatic correction of 89 and 98%, respectively, in the low and high myopic astigmatism groups. The mean angle of error was +2.0 degrees. CONCLUSION: Multipass/multizone PARK for myopic astigmatism demonstrated a high degree of predictability and stability with desirable results for low and high levels of astigmatism across the range of myopic astigmatism treated by surface ablation.

Treatment of irregular astigmatism.

PURPOSE: To treat irregular astigmatism by applying separate appropriate treatments in each of the two distinct hemidivisions of the cornea. SETTING: Cheltenham Eye Centre, Melbourne, Australia. METHODS: Two general surgical strategies are presented. The first applies the principles of optimization separately to each corneal hemidivision to achieve the maximum reduction in astigmatism when measured topographically and refractively. The second is for targeting symmetrical orthogonal topographic goals for each semimeridian to create the regular state in differing ways. These are performed in one of the following ways: without changing refractive astigmatism; by reducing the associated ocular residual astigmatism; by shifting the less favorably placed topography semimeridian to the other more favorably located one; by shifting both topographic semimeridians to more favorably located sites. This is an alternative when a potential improvement in the best corrected visual acuity is sought and the maximum reduction of astigmatism is not the priority. RESULTS: The calculated treatments necessary to achieve various improved astigmatic states, together with each of their respective separate refractive astigmatism targets, are presented. A single refractive astigmatism value for the entire cornea is also calculated by vector summation. CONCLUSION: Consideration of each of the two distinct hemidivisions of the eye enables improved treatment of irregular astigmatism, potentially resulting in improved visual outcomes.

Three multizone photorefractive keratectomy algorithms for myopia. The Melbourne Excimer Laser Group.

OBJECTIVE: To compare the efficacy and complications of three different excimer laser algorithms for multizone photorefractive and photoastigmatic keratectomy. METHODS: Three different software algorithms were applied to treat myopia and myopic astigmatism with the VISX 20/20 excimer laser. Each algorithm had a maximum ablation zone of 6 mm but differed in the number of zones employed, the proportion of the total treatment allocated to each ablation zone, and the treatment of astigmatism. The Melbourne multizone technique equally divided myopia correction into a maximum of three ablation zones. The Pop multizone technique biased myopia treatment into the smaller diameter zones to a maximum of six ablation zones, with one central island pretreatment. The Alpins multizone technique equally divided myopia treatment through all zones up to a maximum of six, with one central island pretreatment. RESULTS: A total of 585 patients (780 eyes) were treated and 625 eyes (80%) were followed for more than 6 months. The mean baseline spherical equivalent refractive error was -5.63 D (-1.00 to -19.50 D). Between 71 and 79% of eyes were treated for astigmatism. There was no statistically significant differences in baseline refractive error or other characteristics among the three groups. At 6 months, the Alpins multizone algorithm had more eyes with a refractive error within +/- 1.00 D of emmetropia (p = 0.01) and more within +/- 2.00 D of emmetropia (p < 0.01). This new algorithm produced more eyes with an uncorrected visual acuity of 20/20 or better at 6 months (p < 0.01). When multiple logistic regression was used to correct for any differences in baseline myopia among the three groups, this algorithm also had a higher odds ratio for achieving 20/20 or better uncorrected visual acuity (OR = 1.58). CONCLUSION: At 6 months, all three algorithms were effective in the reduction of myopia. Significantly better visual acuity and refractive results were achieved with the Alpins multizone algorithm that spread the total treatment over a larger number of ablation zones, with an equal number of diopters of treatment in each zone.

New method of targeting vectors to treat astigmatism.

PURPOSE: To describe a method for optimizing the treatment of astigmatism using vector analysis of both refractive and corneal topographic values. SETTING: Cheltenham Eye Centre, Melbourne, Australia. METHODS: This study evaluated a method of vector analysis for planning surgery that uses both preoperative topographic and refractive values and determined how to select the relative treatment emphasis to be given to each. In addition, the significance of the phenomenon of ocular residual astigmatism (ORA) was explored. Its presence provides an inherent limitation on eliminating astigmatism from the eye's optical system. RESULTS: Various comparisons of preoperative and ORA values are plotted in a series of 100 excimer laser photoastigmatic refractive keratectomy patients. These ORA values are equivalent to the expected corneal astigmatism resulting from surgery where treatment is performed by refractive astigmatism values alone. A theoretical example is given in which the corneal astigmatism remaining from surgery is reduced by giving less emphasis to completely eliminating refractive astigmatism and consequently greater emphasis to completely eliminating topographic astigmatism. CONCLUSION: Using vectors in astigmatism surgery enables the incorporation of topography and refractive values into the surgical plan. This would achieve a greater reduction in corneal astigmatism and potentially a better visual outcome than using refractive astigmatism values alone.

Vector analysis of astigmatism changes by flattening, steepening, and torque.

PURPOSE: To understand the effect of astigmatism surgery by analyzing astigmatic changes according to their component parts. SETTING: Cheltenham Eye Centre, Melbourne, Australia. METHODS: The component parts of the astigmatic changes considered were flattening, steepening, clockwise torque, and counterclockwise torque. Calculations to determine the astigmatic change were performed by vector analysis using rectangular coordinates after doubling the astigmatism and surgical vector axes. A reference axis was used for the resolution of astigmatic change to ascertain its effect along a selected meridian. RESULTS: When correcting astigmatism, the orientations of incisional (tissue addition) or nonincisional (tissue ablation) techniques in any cornea are at right angles to each other. Since differences exist in the measured astigmatism depending on whether it is measured topographically or by manifest refraction, an on-axis correction in one will occur with an off-axis effect in the other. The net result is a reduced flattening effect and a proportionately increased torque effect for the off-axis component. CONCLUSION: When treatment is applied off one of the four primary axes, the treating vector can be resolved into its component parts of flattening, steepening, and torque. Analyzing changes in this way provides a uniform means of assessing astigmatic changes for all types of cataract and refractive surgery and quantifies the flattening effect when treatment is applied off the intended meridian.

Astigmatic change 1 year after excimer laser treatment of myopia and myopic astigmatism. Melbourne Excimer Laser Group.

PURPOSE: To evaluate the surgically induced astigmatism (SIA) 1 year after excimer laser photorefractive astigmatic keratectomy (PARK) and photorefractive keratectomy (PRK). SETTING: Royal Victorian Ear and Eye Hospital, Melbourne, Australia. METHODS: This study comprised 333 PARK patients and 155 PRK patients treated with a VISX 20/20 excimer laser and followed prospectively for 12 months. Vector analysis of the change in astigmatism was used to calculate the SIA in the PRK group and the percentage of astigmatism corrected in the PARK group. RESULTS: Among patients with low cylinders astigmatic correction varied greatly, particularly in those treated for large amounts of myopia. The spherical PRK treatments yielded a mean induced postoperative astigmatism of 0.47 diopter. There was a linear relationship between this inadvertent SIA and increasing myopia. CONCLUSION: Excimer laser surgery for myopia creates a low degree of random, unpredictable SIA that may be the result of irregular epithelial thickening during postoperative healing. This creates a background noise of astigmatic change upon which the targeted astigmatic correction is superimposed.

Excimer laser correction of myopic astigmatism.

The excimer laser allows the controlled ablation of corneal tissue to correct refractive error. By using a combination of spherical and slit apertures, it is possible to correct both myopia and astigmatism. We report the results of 139 consecutive eyes that had photoastigmatic refractive keratectomy (PARK) for myopic astigmatism (myopia < or = -15.00 diopters [D] with astigmatism < or = -6.00 D) and compare these results with 107 consecutive and concurrent eyes that received photorefractive keratectomy (PRK) for myopia (< or = -15.00 D). The same excimer laser was used by 27 different surgeons. All patients were followed for at least three months. In the PARK group, 68% were within +/- 1.00 D at six months and 77% were within +/- 2.00 D. In the PARK group, these figures were 87% and 97%, respectively. Uncorrected visual acuity of 20/40 or better was achieved in 72% of PARK and 90% of PRK patients at six months. Minor adverse reactions occurred in 6% of PARK and 11% of PRK patients. No significant surgeon effect was seen. Photoastigmatic refractive keratectomy provides a realistic approach to the surgical correction of myopic astigmatism and is comparable to PRK in safety and efficacy.

Comparison of excimer laser treatment of astigmatism and myopia. The Excimer Laser and Research Group.

OBJECTIVE: To assess the safety and efficacy of excimer laser treatment of myopic astigmatism and to compare this with the excimer laser treatment of myopia. DESIGN: A prospective, open study of consecutive patients having excimer laser treatment of myopic astigmatism or myopia. SETTINGS: Patients were recruited from 18 private ophthalmic practices. PATIENTS: Fifty-four eyes received treatment for astigmatism and 66 eyes for myopia. One patient was lost to follow-up, and another underwent an ineffective ablation. INTERVENTIONS: A VISX Twenty/Twenty excimer laser was used to perform either photoastigmatic refractive keratectomy or photorefractive keratectomy. MAIN OUTCOME MEASURES: Refraction and visual acuity with and without correction were assessed preoperatively and postoperatively. RESULTS: At 6 months, 17 (85%) of the 20 patients receiving photoastigmatic refractive keratectomy were within 1 diopter of plano refraction, and 19 (95%) of 20 had uncorrected visual acuity of 6/12 (20/40) or better. For patients receiving photorefractive keratectomy, these figures were 28 (88%) of 32 patients and 28 (88%) of 32 patients, respectively. CONCLUSIONS: Excimer laser surgery offers an effective option in the treatment of myopic astigmatism.

A new method of analyzing vectors for changes in astigmatism.

This method of astigmatism analysis recognizes the need to define an astigmatism goal, thus allowing the surgeon to obtain precise, separate measures of the magnitude and the angle of surgical error. From this, the surgeon can evaluate what surgery may be required to achieve the initial preoperative goal. An index that measures surgical success is adjusted for the level of preoperative astigmatism. The resulting data allow statistical comparison of multiple surgeries and techniques. This method also assists in resolving the case when spectacle and corneal astigmatism do not coincide.

The supranuclear disturbances of gaze in kernicterus.

Five patients with kernicterus and ocular motility disturbances are reported. All of the patients showed some defect in vertical gaze but not always upgaze. The most severely affected patient exhibited slowness of horizontal saccades in addition to a total vertical gaze palsy. In general, the severity of the ocular motility disturbance paralleled the severity of hearing loss. It is suggested that damage to the periaqueductal area accounts for disturbances in vertical gaze and the infrequent horizontal saccadic disturbances results from interruption of the descending centrifugal fibers. It is emphasized that kernicterus must be included in the differential diagnosis of any supranuclear vertical gaze disturbance.