Hyperopic Error Research Articles

Phacoemulsification in eyes with extreme axial myopia

To determine the outcomes of phacoemulsification of age-related cataract in eyes with extreme axial myopia. Moorfields Eye Hospital NHS Foundation Trust, London, United Kingdom. This consecutive case series of eyes with an axial length (AL) greater than 30.0 mm was identified retrospectively. Surgical procedure details, complications, and refractive outcome were recorded. The incidence of postoperative retinal detachment (RD) was assessed in a subgroup of eyes without previous RD or vitrectomy. One hundred seventy-seven eyes (124 patients) were identified from a dataset of 6882 cataract procedures; 21 eyes had previous RD or vitrectomy. Intraoperative complications were posterior capsule tear (5 eyes, 2.8%), zonular dehiscence (3 eyes, 1.7%), and anterior capsule tear (2 eyes, 1.1%). At a median follow-up of 25.4 months, 2 (1.3%) of 156 eyes without previous RD or vitrectomy developed RD. The median biometry prediction error in the 156 eyes (88.1%) with an intraocular lens (IOL) was 0.88 diopter (D) (range 0.00 to 5.75 D); 83 eyes (53.2%) were within +/-1.00 D. The median biometry prediction error was greater in the 61 eyes that received a negative-power IOL (1.38 D) than in the 95 eyes that received a plano or positive-power IOL (0.68 D) (P<.001). The incidence of complications after phacoemulsification in eyes with extreme axial myopia was similar to that in previously reported unselected case series. The refractive outcome showed a hyperopic error that increased with AL, an effect that was primarily the result of the eyes that received a negative-power IOL.

Stimulus Requirements for the Decoding of Myopic and Hyperopic Defocus under Single and Competing Defocus Conditions in the Chicken

The bidirectional nature of emmetropization, as observed in young chicks, implies that eyes are able to distinguish between myopic and hyperopic focusing errors. In the current study the spatial frequency and contrast dependence of this process were investigated in an experimental paradigm that allowed strict control over both parameters of the retinal image. Also investigated was the influence of accommodation. Defocusing stimuli were presented through lens-cone devices with attached targets. These devices were monocularly applied to 5-day-old chickens for 4 days. Defocus conditions included: (1) 7 D of myopic defocus, (2) 7 D of hyperopic defocus, and (3) a combination of the two. Two high contrast target designs, a spatially rich, striped Maltese cross (target 1) and a standard Maltese cross (target 2) were used, except in some experiments where target contrast or spatial frequency content was further manipulated. To test the role of accommodation, the treated eye of some chicks underwent ciliary nerve section before attachment of the device. Refractive error (RE) was measured by retinoscopy and axial ocular dimensions measured by A-scan ultrasonography, both in chicks under anesthesia. With imposed myopic defocus and high contrast, target 1 elicited significantly better compensation than did target 2. With imposed hyperopic defocus, both targets elicited near normal compensatory responses. Reducing image contrast to 32% for target 2 and to 16% for target 1 precluded compensation for myopic defocus, inducing myopia instead. The low-pass-filtered target also induced myopia, irrespective of the sign of imposed defocus. With competing defocus and intact accommodation, target 1 induced a transient hyperopic growth response, whereas myopia was consistently observed with target 2. When accommodation was rendered inactive, both targets induced myopia under these competitive conditions. Compensation to myopic defocus is critically dependent on the inclusion of middle to high spatial frequencies in the stimulus and has a spatial frequency-dependent threshold contrast requirement. With competing myopic and hyperopic defocus, the former transiently dominates the latter as a determinant of ocular growth, provided that the stimulus conditions include sufficient middle to high spatial frequency information and that accommodation cues are available.

Comparison of IOL power calculations by the IOLMaster vs theoretical calculations

This study evaluates the agreement between the calculations performed by IOLMaster and as performed according to the published methods, for three formulas, Holladay I, Hoffer Q, and SRK-T. Hypothetical and identical data were used as input variables for calculations by IOLMaster and by a computer program written in visual basic to perform calculations with the three formulas. Results were compared and statistical significance carried out. Significant disagreement was found using Hoffer-Q formula between the results shown by the study program and IOLMaster, while identical results were obtained using Holladay I and SRK-T formulas. For an average eye (AL 23.45 mm and K 45D, ACD constant 6.15, vertex 12 mm), the mean difference in the predicted postoperative (postop) SE between the study-set and IOLMaster dataset was 0.13 D (+/-0.03 D), which is a measure for the systematic error (relative bias). In such eyes, the mean predicted postop ACD in IOLMaster dataset was 0.11 mm (+/-0.04 mm) less than in the study-set, and the data spread of the predicted postop ACD was much higher (+/-0.042 mm) indicating more discrepancy in predicting ACD with IOLMaster. When using Hoffer-Q formula, the mean predicted postop SE with IOLMaster was 0.13 D less than the computation in this study, that is, IOLMaster was showing a consistently myopic overestimation, or, a mean undercorrection of necessary intraocular lens power by 0.13 D, or, a residual hyperopic error of 0.13 D. This translates to a consistently more anterior estimation (0.11 mm mean) of predicted postop ACD by IOLMaster, as compared with theoretical analysis.

Intraocular lens power calculation formulas in Chinese eyes with high axial myopia

Purpose To compare the accuracy of intraocular lens (IOL) power calculation formulas in Chinese eyes with high axial myopia. Setting Department of Ophthalmology, Tuen Mun Hospital, Hong Kong, China. Methods This retrospective study reviewed 125 Chinese patients with high myopia and axial lengths (ALs) longer than 25.0 mm who had cataract surgery during the year 2000. Eyes with pathology or operative complications affecting the refractive status and those with missing data were excluded. In each case, the power of the implanted IOL was used to calculate the predicted postoperative refractive error by 4 IOL power calculation formulas: SRK II, SRK/T, Holladay 1, and Hoffer Q. The predictive accuracy of the formulas was analyzed by comparing the difference between the “actual” and “predicted” postoperative refractive errors. The Student t test was used for statistical analysis. The performance of the formulas in subcategories of long AL was also tested. Results One hundred twenty-five eyes (110 patients) were studied. Thirty-seven eyes (29.6%) were excluded. The Hoffer Q, Holladay 1, and SRK/T formulas showed a slight tendency toward resultant hyperopia, with a mean of +0.36 diopters (D), +0.53 D, and +0.74 D, respectively. The SRK II caused the largest hyperopic error, with a mean of +1.47 D. All 4 formulas caused a refractive error shift toward myopia in the subcategories of AL >28.0 mm, minus-power IOL, and extracapsular cataract extraction (ECCE). Conclusions In Chinese eyes with high axial myopia with an AL longer than 25.0 mm, the 4 formulas caused a slight postoperative hyperopic refractive error that was less in eyes with a minus-power IOL or an AL longer than 28.0 mm and in those that had ECCE. The Hoffer Q formula provided the best predictive result, and Holladay 1 and SRK/T were comparable in IOL power calculation. The SRK II was the least accurate in all subgroups.

Continuous ambient lighting and eye growth in primates.

The authors sought to determine the effect of continuous light exposure on ocular growth and emmetropization in infant monkeys. Nine infant rhesus monkeys were reared with the normal vivarium lights on continuously. The 24-hour light cycle was initiated between 1 and 4 weeks of age and maintained for 6 months. The ocular effects of continuous light were assessed by cycloplegic retinoscopy, keratometry, and A-scan ultrasonography. Longitudinal control data were obtained from 23 normal infants that were reared with an illumination cycle that included defined light and dark phases (either 12-hour light: 12-hour dark or 8.5-hour light: 15.5 hour dark). In contrast to previous studies involving light-reared chickens, no monkeys exhibited exaggerated ocular growth. There were no significant differences between treated and control monkeys in corneal radius, overall eye size, or the axial dimensions of individual ocular components. At the end of the treatment period, eight of the nine experimental monkeys also exhibited the moderate hyperopic errors (range, +1.5 to +3.4 D) that are typically found in normal animals. Aspects of emmetropization were, however, unusual for three monkeys. One monkey manifested a −0.50 D myopic error that was associated with an abnormally steep cornea but had normal axial lengths. Two additional monkeys developed persistent axial anisometropias. The authors conclude that in infant primates constant light exposure does not promote the constellation of ocular changes (in particular corneal flattening, a decrease in anterior chamber depth, and an increase in vitreous chamber depth) that has been observed in light-reared chickens. The slight variations from the expected developmental sequence observed in three infants may reflect individual differences. It is also possible, however, that aspects of the emmetropization process may not operate as effectively under constant light as they do under an ordinary light/dark cycle.—Thomas J. Liesegang

Accuracy and predictability of intraocular lens power calculation after laser in situ keratomileusis

Purpose To study the accuracy and predictability of intraocular lens (IOL) power calculation in eyes that had laser in situ keratomileusis (LASIK). Setting Gimbel Eye Centre, Calgary, Alberta, Canada. Methods Refractive outcomes in 6 cataract surgery and lensectomy eyes after previous LASIK were analyzed retrospectively. Target refractions based on measured and refraction-derived keratometric values were compared with postoperative achieved refractions. Differences between target refractions calculated using 5 IOL formulas and 2 A-constants and achieved refractions were also compared. Results The refractive error of IOL power calculation in postoperative LASIK eyes was significantly reduced when refraction-derived keratometric values were used for IOL power calculation. Persistent residual hyperopia still occurred in some cases; this was corrected by hyperopic LASIK. Refractive results appeared more accurate and predictable when the Holladay 2 or Binkhorst 2 formula was used for IOL power calculation. Conclusion Hyperopic error after cataract surgery in post-LASIK eyes was significantly reduced by using refraction-derived keratometric values for IOL power calculation. Persistent hyperopic error was corrected by hyperopic LASIK.

Refractive error in cataract surgery after previous refractive surgery

Bilateral cataract extraction with posterior chamber intraocular lens (IOL) implantation was performed in a patient after previous photorefractive keratectomy, radial keratotomy (RK) combined with astigmatic keratotomy, and retreatment of RK. Significant hyperopic error was observed after cataract surgery, and the IOLs were eventually exchanged in both eyes. A review of this case found that the refractive error was smaller when a refraction-derived keratometric value was selected for IOL power calculation. Nevertheless, hyperopic error still occurred.

PRK-induced anisometropia in the rabbit as a model of myopia.

Current animal models of myopia, such as the chick and the tree shrew, have characteristics that limit their applicability to human myopia and/or their use among researchers. The purpose of this study was to establish a rabbit model of myopia based on photorefractive keratectomy (PRK)-induced anisometropia. A group of five pigmented rabbits was treated with a monocular -5 D PRK at 5 weeks of age. At 10 weeks of age, two of the eyes were retreated with a second -5 D PRK procedure to compensate for partial regression of the refractive effect. A second group of six pigmented rabbits was treated with a monocular -6 D PRK at 10 weeks of age. Longitudinal measurements of corneal curvature, refraction, and axial length were performed until the rabbits were 13 and 21 weeks of age in groups 1 and 2, respectively. The rabbits in each group were from the same litter. Keratometry and retinoscopy measurements confirmed the refractive effect of the PRK procedures. At the final measurement point in group 1, the PRK-treated eyes were significantly longer than the untreated eyes (16.01 +/- 0.45 mm vs 15.45 +/- 0.56 mm). In group 2, the PRK-treated eyes were significantly longer by 0.19 mm and 0.20 mm at ages 19 and 21 weeks, respectively. PRK-induced anisometropia is an effective technique to induce hyperopic error compensation in the rabbit as a model of myopic development. The technique is effective if the PRK procedure is performed at either 5 or 10 weeks of age. However, after PRK at 5 weeks of age, partial retreatment may be necessary due to regression of the PRK effect.

Intraocular lens power prediction in patients who undergo cataract surgery following previous radial keratotomy.

Previous experience has shown that there is no technical difficulty in performing cataract surgery on patients who have previously undergone radial keratotomy. However, some researchers have reported inaccuracy in intraocular lens (IOL) power selection. To assess the visual and refractive outcomes of our patients and to compare different formulas and variables to improve accuracy in power determination. Ten eyes subjected to phacoemulsification with in-the-bag posterior chamber lens implantation 79 months (range, 36-118 months) after radial keratotomy were evaluated in this study. The IOL power was retrospectively calculated for each eye using the Binkhorst, SRK II, and Holladay formulas with the current keratometry reading, the refractive-derived keratometric value (K), the current refractive-derived K, and the adjusted K. The final refractive result was used as a criterion to judge the accuracy and predictability for each approach. Three eyes underwent an IOL exchange after initial surgery. Among the 7 eyes that did not undergo an IOL exchange, a hyperopic shift that regressed approximately 3 months after surgery occurred in the early postoperative period. At the final examination, 5 of the 7 eyes had a hyperopic error, with 2 eyes showing more than 1.00 diopter (D). Overall, in an average of 27 months (range, 9-80 months) of follow-up, an uncorrected visual acuity of 20/40 or better was obtained in 6 (60%) of the eyes. All 10 eyes had a 20/25 or better postoperative best-corrected visual acuity. The mean (+/-SD) spherical equivalent refraction was changed from -0.78 +/- 3.49 D preoperatively to 0.45 +/- 1.31 D postoperatively. We found that the Binkhorst and Holladay formulas are more accurate than the SRK II formula. With the use of an adjusted K (ie, the current average K minus 1.0 D) in combination with the Binkhorst and Holladay formulas, most of the eyes would achieve a refraction of -2.00 to +0.50 D. A corneal flattening effect caused by cataract surgery tends to occur in eyes that have undergone previous radial keratotomy. The use of an average between the Binkhorst and Holladay formulas, aiming for -0.75 D with an adjusted K, seems to be a more accurate and predictable method for IOL power calculation. This approach could reduce the chance of postoperative hyperopia.

Photographic screening for strabismus and high refractive errors of children aged 1-4 years.

182 children (age 1-4 years) were photographed by health sisters at the annual screening at the children's health care center as described by Kaakinen (1979). The photographs were examined by an ophthalmologist to diagnose squint and/or refractive error cases based on the typical changes in the corneal and fundus reflexes. The cases found in this way were controlled clinically. 7 pathological cases were detected: 2 esotropias, 1 microesotropia, 2 hyperopias of both eyes, 2 anisocorias. False positive were 6 cases, from which 4 were false positive microstrabismus suspects and 2 cases with asymmetrical fundus reflexes, which were normal. 14 cases failed (9 (4.9%) looked to the side, and 5 (2.7%) were technical failures). For the reliability of the method, a random sample of 20 children, who had normal corneal and fundus reflexes were examined clinically. No anisometropia of over 0.5 diopters appeared among these 20 control cases, and the highest hyperopic and astigmatic errors that appeared were +1.75 diopters. All the astigmatic cases were combined with a slight myopia of -0.5- -1.0 diopters in each case. No higher myopias occurred in this group, and all these 20 children had straight eyes with the cover test at near fixation.

Tonography in normal and malnourished children.

Tonographic studies were performed while 43 severely marasmic children and 33 normal controls ranging in age between 1 month and 1 year were under pentobarbital sodium sedation. The results showed no significant difference in the levels of Po and C values in the two groups. There was a statistically significant negative correlation between the levels of Po and weight in the normal children. These findings suggest similar dynamics in the eyes of the normal and marasmic children. The significance of these findings in relation to the smaller hyperopic error previously described in the marasmic children is discussed.