Astigmatism and vision:should all astigmatism always be corrected?

As technology and medical devices improve, there is much interest in when and how astigmatism should be corrected with refractive surgery. Astigmatism can be corrected by most forms of refractive surgery, such as using excimer lasers algorithms to ablate the cornea to compensate for the magnitude of refractive error in different meridians. Correction of astigmatism at the time of cataract surgery is well developed and can be achieved through incision placement, relaxing incisions and toric intraocular lens (IOL) implantation. This was less of an issue in the past when there was a lower expectation to be spectacle independent after cataract surgery, in which case the residual refractive error, including astigmatism, could be compensated for with spectacle lenses. The issue of whether presurgical astigmatism should be corrected can be considered separately depending on whether a patient has residual accommodation, and the type of refractive surgery under consideration. We have previously reported on the visual impact of full correction of astigmatism, rather than just correcting the mean spherical equivalent. Correction of astigmatism as low as 1.00 dioptres significantly improves objective and subjective measures of functional vision in prepresbyopes at distance and near.

Short-term changes in ocular biometry and refraction after discontinuation of long-term orthokeratology

OBJECTIVE: To assess refractive and biometric changes 1 week after discontinuation of lens wear in subjects who had been wearing orthokeratology (OK) contact lenses for 2 years. METHODS: Twenty-nine subjects aged 6 to 12 years and with myopia of -0.75 to -4.00 diopters (D) and astigmatism of ≤1.00 D participated in the study. Measurements of axial length and anterior chamber depth (Zeiss IOLMaster), corneal power and shape, and cycloplegic refraction were taken 1 week after discontinuation and compared with those at baseline and after 24 months of lens wear. RESULTS: A hyperopic shift was found at 24 months relative to baseline in spherical equivalent refractive error (+1.86±1.01 D), followed by a myopic shift at 1 week relative to 24 months (-1.93±0.92 D) (both P<0.001). Longer axial lengths were found at 24 months and 1 week in comparison to baseline (0.47±0.18 and 0.51±0.18 mm, respectively) (both P<0.001). The increase in axial length at 1 week relative to 24 months was statistically significant (0.04±0.06 mm; P=0.006). Anterior chamber depth did not change significantly over time (P=0.31). Significant differences were found between 24 months and 1 week relative to baseline and between 1-week and 24-month visits in mean corneal power (-1.68±0.80, -0.44±0.32, and 1.23±0.70 D, respectively) (all P≤0.001). Refractive change at 1 week in comparison to 24 months strongly correlated with changes in corneal power (r=-0.88; P<0.001) but not with axial length changes (r=-0.09; P=0.66). Corneal shape changed significantly between the baseline and 1-week visits (0.15±0.10 D; P<0.001). Corneal shape changed from a prolate to a more oblate corneal shape at the 24-month and 1-week visits in comparison to baseline (both P≤0.02) but did not change significantly between 24 months and 1 week (P=0.06). CONCLUSIONS: The effects of long-term OK on ocular biometry and refraction are still present after 1-week discontinuation of lens wear. Refractive change after discontinuation of long-term OK is primarily attributed to the recovery of corneal shape and not to an increase in the axial length.

Sympathetic innervation of ciliary muscle and oculomotor function in emmetropic and myopic young adults

Purpose: Evidence exists for an additional inhibitory accommodative control system mediated by the sympathetic branch of the autonomic nervous system (ANS). This work aims to show the relative prevalence of sympathetic inhibition in young emmetropic and myopic adults, and to evaluate the effect of sympathetic facility on accommodative and oculomotor function. Methods: Profiling of ciliary muscle innervation was carried out in 58 young adult subjects (30 emmetropes, 14 early onset myopes, 14 late onset myopes) by examining post-task open-loop accommodation responses, recorded continuously by a modified open-view infrared optometer. Measurements of amplitude of accommodation, tonic accommodation, accommodative lag at near, AC/A ratio, and heterophoria at distance and near were made to establish a profile of oculomotor function. Results: Evidence of sympathetic inhibitory facility in ciliary smooth muscle was observed in 27% of emmetropes, 21% of early-onset myopes and 29% of late-onset myopes. Twenty-six percent of all subjects demonstrated access to sympathetic facility. Closed-loop oculomotor function did not differ significantly between subjects with sympathetic facility, and those with sympathetic deficit. Conclusions: Emmetropic and myopic groups cannot be distinguished in terms of the relative proportions having access to sympathetic inhibition. Presence of sympathetic innervation does not have a significant effect on accommodative function under closed-loop viewing conditions. © 2005 Elsevier Ltd. All rights reserved.

Myopic foveoschisis:a clinical review

To review the literature on epidemiology, clinical features, diagnostic imaging, natural history, management, therapeutic approaches, and prognosis of myopic foveoschisis. A systematic Pubmed search was conducted using search terms: myopia, myopic, staphyloma, foveoschisis, and myopic foveoschisis. The evidence base for each section was organised and reviewed. Where possible an authors' interpretation or conclusion is provided for each section. The term myopic foveoschisis was first coined in 1999. It is associated with posterior staphyloma in high myopia, and is often asymptomatic initially but progresses slowly, leading to loss of central vision from foveal detachment or macular hole formation. Optical coherence tomography is used to diagnose the splitting of the neural retina into a thicker inner layer and a thinner outer layer, but compound variants of the splits have been identified. Vitrectomy with an internal limiting membrane peel and gas tamponade is the preferred approach for eyes with vision decline. There has been a surge of new information on myopic foveoschisis. Advances in optical coherence tomography will continually improve our understanding of the pathogenesis of retinal splitting, and the mechanisms that lead to macular damage and visual loss. Currently, there is a good level of consensus that surgical intervention should be considered when there is progressive visual decline from myopic foveoschisis.

Reply: 'Myopic foveoschisis: An ectatic retinopathy, not a schisis'

Full text: We thank Tsilimbaris et al1 for their comments on the appropriateness of the term ‘myopic foveoschisis’ to describe the condition that is characterized by the separation of neural retina layers associated with high myopia and posterior staphyloma. They have proposed the term ‘myopic ectatic retinopathy’ as a more literal and functionally more accurate descriptor of the condition to avoid the use of the word ‘schisis’, which may be misleading because it is also used to describe other conditions where there is separation of neural retina layers without the presence of staphyloma.2 Using the word ‘ectatic’ for this condition would imply that we are fairly certain about the pathogenesis and mechanistic factors that underlie its development and progression. However, this is not the case, unfortunately, as our review of the literature has shown. There are several theories ranging from vitreous traction to sclerosing changes of retinal vessels to progression of staphylomas as possible etiological factors. Therefore, it is likely to be multifactorial in nature—hence the success reported with different procedures that address either the vitreous traction factor using vitrectomy, peel plus tamponade or the scleral ectasia factor using posterior buckling techniques. In the absence of a good understanding of underlying pathogenesis, it is probably best to use purely descriptive names rather than mechanistic terms. The use of descriptive terms, even though similar, do not necessarily cause confusion as long as they are widely accepted as differentiating terminology, for example, postoperative pseudophakic cystoid macular edema (Irvine–Gass syndrome) vs cystoid macular edema associated with posterior uveitis in a phakic patient. The introduction of too many mechanistic or pathogenetic terms in the absence of clear understating of etiology can in fact cause more confusion, for example, serous chorioretinopathy vs central serous retinopathy vs serous choroidopathy. The confinement to broad descriptive terms can enhance communication and reduce confusion without committing to any presumption about etiology until it is better understood. This approach is probably best illustrated by the recent advances in the understanding of mactel21, a condition initially described and classified, using descriptive nomenclature, by Don Gass as bilateral, idiopathic acquired juxtafoveolar telangiectasis (Group2A) and as distinctly different from unilateral, congenital parafoveolar telangiectasis (Group 1A; Gass,3 pp 504–506 vs 127–128). Finally, it is worthy to note that for myopic foveoschisis associated with a staphyloma that is associated with outer layer macular detachment, Don Gass also descriptively included the additional observation (before the advent of OCT) that the retinal profile was concave rather than convex in shape, thereby differentiating it from rhegmatogenous detachments with recruitment of subretinal fluid that is associated with posteriorly located breaks and macular holes in myopic eyes. References 1.Tsilimbaris MK, Vavvas DG, Bechrakis NE. Myopic foveoschisis: an ectatic retinopathy, not aschisis. Eye 2016; 30: 328–329. 2.Powner MB, Gillies MC, Tretiach M, Scott A, Guymer RH, Hageman GS et al. Perifoveal müller cell depletion in a case of macular telangiectasia type 2. Ophthalmology 2010; 117(12): 2407–2416. 3.Gass DM. Stereoscopic Atlas of Macular Diseases: Diagnosis and Treatment, 4th edn. Mosby-Yearbook: St. Louis, 1997.

We are being myopic about myopia control

Editorial

Prevalence of refractive errors in students in Northeastern Brazil

To determine the prevalence of refractive errors in the public and private school system in the city of Natal, Northeastern Brazil. Methods: Refractometry was performed on both eyes of 1,024 randomly selected students, enrolled in the 2001 school year and the data were evaluated by the SPSS Data Editor 10.0. Ametropia was divided into: 1- from 0.1 to 0.99 diopter (D); 2- 1.0 to 2.99D; 3- 3.00 to 5.99D and 4- 6D or greater. Astigmatism was regrouped in: I- with-the-rule (axis from 0 to 30 and 150 to 180 degrees), II- against-the-rule (axis between 60 and 120 degrees) and III- oblique (axis between > 30 and < 60 and >120 and <150 degrees). The age groups were categorized as follows, in: 1- 5 to 10 years, 2- 11 to 15 years, 3- 16 to 20 years, 4- over 21 years. Results: Among refractive errors, hyperopia was the most common with 71%, followed by astigmatism (34%) and myopia (13.3%). Of the students with myopia and hyperopia, 48.5% and 34.1% had astigmatism, respectively. With respect to diopters, 58.1% of myopic students were in group 1, and 39% distributed between groups 2 and 3. Hyperopia were mostly found in group 1 (61.7%) as well as astigmatism (70.6%). The association of the astigmatism axes of both eyes showed 92.5% with axis with-the-rule in both eyes, while the percentage for those with axis againstthe- rule was 82.1% and even lower for the oblique axis (50%). Conclusion: The results found differed from those of most international studies, mainly from the Orient, which pointed to myopia as the most common refractive error, and corroborates the national ones, with the majority being hyperopia

Prevalence of refractive errors in students in Northeastern Brazil

To determine the prevalence of refractive errors in the public and private school system in the city of Natal, Northeastern Brazil. Methods: Refractometry was performed on both eyes of 1,024 randomly selected students, enrolled in the 2001 school year and the data were evaluated by the SPSS Data Editor 10.0. Ametropia was divided into: 1- from 0.1 to 0.99 diopter (D); 2- 1.0 to 2.99D; 3- 3.00 to 5.99D and 4- 6D or greater. Astigmatism was regrouped in: I- with-the-rule (axis from 0 to 30 and 150 to 180 degrees), II- against-the-rule (axis between 60 and 120 degrees) and III- oblique (axis between > 30 and < 60 and >120 and <150 degrees). The age groups were categorized as follows, in: 1- 5 to 10 years, 2- 11 to 15 years, 3- 16 to 20 years, 4- over 21 years. Results: Among refractive errors, hyperopia was the most common with 71%, followed by astigmatism (34%) and myopia (13.3%). Of the students with myopia and hyperopia, 48.5% and 34.1% had astigmatism, respectively. With respect to diopters, 58.1% of myopic students were in group 1, and 39% distributed between groups 2 and 3. Hyperopia were mostly found in group 1 (61.7%) as well as astigmatism (70.6%). The association of the astigmatism axes of both eyes showed 92.5% with axis with-the-rule in both eyes, while the percentage for those with axis againstthe- rule was 82.1% and even lower for the oblique axis (50%). Conclusion: The results found differed from those of most international studies, mainly from the Orient, which pointed to myopia as the most common refractive error, and corroborates the national ones, with the majority being hyperopia

Online Supplement to ‘Myopic Allocation Policy with Asymptotically Optimal Sampling Rate’

This document is the Online Supplement to ‘Myopic Allocation Policy with Asymptotically Optimal Sampling Rate,’ to be published in the IEEE Transactions of Automatic Control in 2017.

Long-term efficacy of orthokeratology contact lens wear in controlling the progression of childhood myopia

Purpose: The primary outcome of this study is to compare the axial length growth of white European myopic children wearing orthokeratology contact lenses (OK) to a control group (CT) over a 7-year period. Methods: Subjects 6–12 years of age with myopia −0.75 to −4.00DS and astigmatism ≤1.00DC were prospectively allocated OK or distance single-vision spectacles (SV) correction. Measurements of axial length (Zeiss IOLMaster), corneal topography, and cycloplegic refraction were taken at 6-month intervals over a 2-year period. Subjects were invited to return to the clinic approximately 5 years later (i.e., 7 years after the beginning of the study) for assessment of their ocular refractive and biometric components. The CT consisted of 4 SV and 12 subjects who switched from SV to soft contact lens wear after the initial 2 years of SV lens wear. Changes in axial length relative to baseline over a 7-year period were compared between groups. Results: Fourteen and 16 subjects from the OK and CT groups, respectively, were examined 6.7 ± 0.5 years after the beginning of the study. Statistically significant changes in the axial length were found over time and between groups (both p <0.001), but not for the time*group interaction (p = 0.125). The change in the axial length for the OK group was 22% (p = 0.328), 42% (p = 0.007), 40% (p = 0.020), 41% (p = 0.013), and 33% (p = 0.062) lower than the CT group following 6, 12, 18, 24, and 84 months of lens wear, respectively. Conclusion: A trend toward a reduction in the rate of axial elongation of the order of 33% was found in the OK group in comparison to the CT group following 7 years of lens wear.

Neural networks and statistical analysis for classification of corneal videokeratography maps based on Zernike coefficients: a quantitative comparison

PURPOSE: The main goal of this study was to develop and compare two different techniques for classification of specific types of corneal shapes when Zernike coefficients are used as inputs. A feed-forward artificial Neural Network (NN) and discriminant analysis (DA) techniques were used. METHODS: The inputs both for the NN and DA were the first 15 standard Zernike coefficients for 80 previously classified corneal elevation data files from an Eyesys System 2000 Videokeratograph (VK), installed at the Departamento de Oftalmologia of the Escola Paulista de Medicina, São Paulo. The NN had 5 output neurons which were associated with 5 typical corneal shapes: keratoconus, with-the-rule astigmatism, against-the-rule astigmatism, "regular" or "normal" shape and post-PRK. RESULTS: The NN and DA responses were statistically analyzed in terms of precision ([true positive+true negative]/total number of cases). Mean overall results for all cases for the NN and DA techniques were, respectively, 94% and 84.8%. CONCLUSION: Although we used a relatively small database, results obtained in the present study indicate that Zernike polynomials as descriptors of corneal shape may be a reliable parameter as input data for diagnostic automation of VK maps, using either NN or DA.

Resultados preliminares de um sistema de topografia de grande-ângulo usando videoceratógrafo com discos de Plácido

OBJETIVO: Desenvolver a instrumentação e o "software" para topografia de córnea de grande-ângulo usando o tradicional disco de Plácido. O objetivo é permitir o mapeamento de uma região maior da córnea para topógrafos de córnea que usem a técnica de Plácido, fazendo-se uma adaptação simples na mira. MÉTODOS: Utilizando o tradicional disco de Plácido de um topógrafo de córnea tradicional, 9 LEDs (Light Emitting Diodes) foram adaptados no anteparo cônico para que o paciente voluntário pudesse fixar o olhar em diferentes direções. Para cada direção imagens de Plácido foram digitalizadas e processadas para formar, por meio de algoritmo envolvendo elementos sofisticados de computação gráfica, um mapa tridimensional completo da córnea toda. RESULTADOS: Resultados apresentados neste trabalho mostram que uma região de até 100% maior pode ser mapeada usando esta técnica, permitindo que o clínico mapeie até próximo ao limbo da córnea. São apresentados aqui os resultados para uma superfície esférica de calibração e também para uma córnea in vivo com alto grau de astigmatismo, mostrando a curvatura e elevação. CONCLUSÃO: Acredita-se que esta nova técnica pode propiciar a melhoria de alguns processos, como por exemplo: adaptação de lentes de contato, algoritmos para ablações costumizadas para hipermetropia, entre outros.

An efficient heuristic for total flowtime minimisation in no-wait flowshops

In this paper, we address the problem of scheduling jobs in a no-wait flowshop with the objective of minimising the total completion time. This problem is well-known for being nondeterministic polynomial-time hard, and therefore, most contributions to the topic focus on developing algorithms able to obtain good approximate solutions for the problem in a short CPU time. More specifically, there are various constructive heuristics available for the problem [such as the ones by Rajendran and Chaudhuri (Nav Res Logist 37: 695-705, 1990); Bertolissi (J Mater Process Technol 107: 459-465, 2000), Aldowaisan and Allahverdi (Omega 32: 345-352, 2004) and the Chins heuristic by Fink and Voa (Eur J Operat Res 151: 400-414, 2003)], as well as a successful local search procedure (Pilot-1-Chins). We propose a new constructive heuristic based on an analogy with the two-machine problem in order to select the candidate to be appended in the partial schedule. The myopic behaviour of the heuristic is tempered by exploring the neighbourhood of the so-obtained partial schedules. The computational results indicate that the proposed heuristic outperforms existing ones in terms of quality of the solution obtained and equals the performance of the time-consuming Pilot-1-Chins.

Minimizing total tardiness in a stochastic single machine scheduling problem using approximate dynamic programming

This paper addresses the non-preemptive single machine scheduling problem to minimize total tardiness. We are interested in the online version of this problem, where orders arrive at the system at random times. Jobs have to be scheduled without knowledge of what jobs will come afterwards. The processing times and the due dates become known when the order is placed. The order release date occurs only at the beginning of periodic intervals. A customized approximate dynamic programming method is introduced for this problem. The authors also present numerical experiments that assess the reliability of the new approach and show that it performs better than a myopic policy.

Customized Topography-guided Photorefractive Keratectomy With the MEL-70 Platform and Mitomycin C to Correct Hyperopia After Radial Keratotomy

PURPOSE: To evaluate topography-guided photorefractive keratectomy (PRK) for correcting hyperopia and astigmatism after radial keratotomy (RK). METHODS., Prospective study of 12 consecutive patients (19 eyes) who were treated with topography-guided PRK with 0.02% mitomycin C using an Asclepion-Meditec MEL-70 excimer laser with a 9.5-mm ablation zone. All eyes were operated by the same surgeon and followed for 1 year. RESULTS: Thirteen eyes had complete epithelialization by day 7 and all eyes by day 10. At 1 year, uncorrected visual acuity was 20/25 or better in 42.1% of eyes and 20/40 or better in 68.4%. Preoperative mean spherical equivalent refraction was +3.80 +/- 2.47 diopters (D) and +0.24 +/- 2.36 D (P <.001) 1 year postoperative, with 47.4% of eyes being within +/- 1.00 D and 73.7% within +/- 2.00 D. Preoperative mean cylinder was -2.30 +/- 1.41 D and -0.62 +/- 0.73 D (P <.1001) 1 year postoperative. At 1 year, 68.4% of eyes gained at least 1 line of best-spectacle corrected visual acuity, 36.8% gained more than 1 line, and only 2 eyes lost 1 line (one due to corneal haze). Three eyes developed central haze. Mean regression from 6 to 12 months in these 3 eyes was +1.83 D and in the remaining 16 eyes was -0.50 D. CONCLUSIONS: Topography-guided PRK with mitomycin C was safe and reasonably effective for the treatment of hyperopia after RK [J Refract Surg. 2008;24:911-922.]