Visual disturbances during prolonged space missions.

PURPOSE OF REVIEW: During prolonged spaceflight, astronauts often experience ocular changes, due to constant head-ward fluid shifts in space as compared with Earth. This article reviews symptoms, likely causes, and potential solutions, such as lower body negative pressure, to counteract space-associated neuroocular syndrome (SANS). RECENT FINDINGS: Low gravity conditions and other aspects of spaceflight affect the eye detrimentally, causing SANS which is characterized by optic disc edema, choroidal thickening, cotton wool spots, and a hyperopic shift. SANS is probably caused by altered hemodynamic flows in the head and neck as well as mildly elevated intracranial and intraocular pressures. Carbon dioxide and other chemicals in space-craft may influence SANS as well. SANS may be counteracted by using lower body negative pressure, thigh cuffs, spacecraft engineering, and/or artificial gravity by a centrifuge. SUMMARY: Prolonged space missions are associated with optic disc edema, choroidal thickening, cotton wool spots, and a hyperopic shift. Possible causes and countermeasures are currently being researched to reduce the risk of SANS. Although many countermeasures to SANS are under investigation lower body negative pressure exhibits great promise in counteracting the headward fluid shifts in space. Understanding and prevention of SANS is critical to future space exploration, especially to long-duration missions to the moon and Mars.

Using the posterior to anterior corneal curvature radii ratio to minimize the risk of a postoperative hyperopic shift after Descemet membrane endothelial keratoplasty.

PURPOSE: To evaluate different corneal parameters in identifying patients at risk of a hyperopic shift after (DMEK). METHODS: This retrospective study included 92 eyes of patients with FECD after DMEK surgery. Pachymetry parameters, various tomographic parameters and densitometry values before and after DMEK were determined using a rotating Scheimpflug system (Pentacam HR, Oculus). For assessing the posterior to anterior corneal curvature relationship, we calculated the RPA (posterior to anterior corneal curvature radii ratio). RESULTS: The average keratometry reading of the posterior corneal surface (KmB) increased and the total corneal refractive power (TCRP) decreased significantly after surgery (P < .001). There was a significant difference between the preoperative and postoperative RPA (P < .001) and the posterior Q value (P < .001). The strongest correlation was found between the change in the KmB and the preoperative RPA (Spearman's correlation coefficient = 0.872, P < .001). In the receiver operating characteristic (ROC) analysis, the highest AUC values (for ∆KmB) among the different preoperative parameters tested were obtained for RPA and posterior Q value (Asph. QB) with AUROC (area under the ROC) values of 0.95 and 0.89, respectively. CONCLUSIONS: The Q value and the RPA showed the highest correlation with the change in corneal refractive power and the greatest AUC. These parameters could be used as surrogate markers to identify eyes that might be at risk of a greater postoperative hyperopic shift, which would allow more accurate setting of refractive goals.

T2 formula in a highly myopic population, comparison with other methods and description of an improved approach for estimating corneal height.

BACKGROUND: To determine the accuracy of the T2 formula as applied to highly myopic eyes, to compare the T2 formula to the SRK/T and Holladay 1 formulas, and to describe possible ways to improve the estimation of corneal height and prediction error in two settings, the Hadassah Hospital, Ophthalmology Department, Jerusalem, Israel and Clínica Barraquer, Bogotá, Colombia. METHODS: In this retrospective case series, optical biometer measurements were taken for 63 highly myopic patients (> 25 mm) undergoing uneventful crystalline lens phacoemulsification and insertion of an acrylic intraocular lens. Prediction errors were obtained, with estimations of ±0.50 D, ± 1.00 D, and greater than ±2.00 D. A method to improve the corneal height calculation is described. RESULTS: The SRK/T formula (mean absolute error [MAE] = 0.418; median absolute error [MedAE] = 0.352) was the most accurate, followed by the T2 (MAE = 0.435; MedAE = 0.381) and Holladay 1 (MAE = 0.455; MedAE = 0.389) formulas. Both the SRK/T and T2 formulas overestimated corneal height, but values were higher with the T2 formula. Corneal height was more precisely estimated using an alternative method that, when combined with axial length optimization, resulted in lower MAE (0.425) and MedAE (0.365) values than when applying the T2 formula alone. CONCLUSIONS: The T2 formula seems to be less accurate than the SRK/T formula in highly myopic eyes. An improved corneal height estimation method is described for the the T2 formula.

Can manipulation of orthokeratology lens parameters modify peripheral refraction?

PURPOSE: To investigate changes in peripheral refraction, corneal topography, and aberrations induced by changes in orthokeratology (OK) lens parameters in myopes. METHODS: Subjects were fitted with standard OK lenses that were worn overnight for 2 weeks. Peripheral refraction, corneal topography, and corneal surface aberrations were measured at baseline and after 14 nights of OK lens wear. Subsequent to a 2-week washout period, subjects were refitted with another set of lenses where one eye was randomly assigned to wear an OK lens with a smaller optic zone diameter (OZD) and the other eye with a steeper peripheral tangent. Measurements were taken again at a second baseline and after 14 days of overnight wear of the second OK lens set. RESULTS: Standard OK lenses with a 6-mm OZD and 1/4 peripheral tangent caused significant changes in both peripheral refraction and corneal topography. Significant hyperopic shift occurred in the central visual field (VF) while a myopic shift was found at 35 degrees in the nasal VF. OK induced significant reductions in corneal power at all positions along the horizontal corneal chord except at 2.4 mm nasal where there was no significant change and at 2.8 mm nasal where there was an increase in corneal refractive power. A positive shift in spherical aberration was induced for all investigated lens designs except for the 1/2 tangent design when calculated over a 4-mm pupil. Reducing OZD and steepening the peripheral tangent did not cause significant changes in peripheral refraction or corneal topography profiles across the horizontal meridian. CONCLUSIONS: OK lenses caused significant changes in peripheral refraction, corneal topography, and corneal surface aberrations. Modifying OZD and peripheral tangent made no significant difference to the peripheral refraction or corneal topography profile. Attempting to customize refraction and topography changes through manipulation of OK lens parameters appears to be a difficult task.

Predicting the child who will become myopic - can we prevent onset?

Myopia has become a global epidemic with significant public health impacts. Identifying the child at risk of developing myopia, i.e. the pre-myopic child and implementing strategies to prevent the onset of myopia, could significantly reduce the burden of myopia on an individual and society. This paper is a review of publications that have identified ocular characteristics of children at risk of future myopia development including a lower than age normal amount of hyperopia and accelerated axial length elongation. Risk factors associated with increased risk of myopia development such as education exposure and reduced outdoor time, and strategies that could be implemented to prevent myopia onset in children are also explored. The strong causal role of education and outdoor time on myopia development suggests that lifestyle modifications could be implemented as preventative measures to at-risk children and may significantly impact the myopia epidemic by preventing or delaying myopia onset and its associated ocular health consequences.

Protective effects of taurine against alloxan-induced diabetic cataracts and refraction changes in New Zealand White rabbits.

The present study examined the protective effects of taurine on alloxan-induced diabetic cataracts and lens damage in male New Zealand White rabbits. The animals were randomly divided into three treatment groups: (1) normal control (vehicle administration); (2) diabetes (100 mg/kg alloxan administration); and (3) diabetes + taurine (1% [w/v] taurine dissolved in drinking water and alloxan administration). The results showed that alloxan-induced diabetes caused significant (p < 0.05) hyperglycemia, hyperopic refraction shifts, cataract formation and lens damage compared with the normal control group. In contrast, the administration of taurine for 24 weeks significantly ameliorated the alloxan-induced elevated levels of blood glucose, level of hyperopic refraction error shifts in the eyes and progression of diabetic cataract formation in the lens in rabbits. Moreover, histopathology showed that the taurine supplement reduced the incidence of lens lesions induced by hyperglycemia. Overall, the studies demonstrate that taurine exhibits potent protective effects against alloxan-induced diabetic cataracts and refraction changes in rabbits.

Pachymetric ratio no-history method for intraocular lens power adjustment after excimer laser refractive surgery.

OBJECTIVE: To evaluate a new pachymetric method not requiring pre-refractive surgical data for adjusting the intraocular lens (IOL) power in eyes undergoing cataract surgery after excimer laser refractive surgery and comparing final refractive results with previously published formulas or methods. DESIGN: Retrospective noncomparative case series. PARTICIPANTS: Thirty-six eyes from 23 patients who had uneventful phacoemulsification cataract surgery after previous myopic (35) or mixed astigmatism (1) excimer laser photoablation. METHODS: A new corneal ratio (Geggel ratio) method was developed to estimate the diopters (D) of previous excimer treatment or change in spherical equivalent (SE) at the corneal plane. A regression formula, 0.40 (|DeltaSE|-1), predicted the correction factor to be added to the SRK/T (Sanders, Retzlaff, Kraff) formula. The IOL results from the Geggel and Geggel-real (modified for mild myopia) method were compared with the Masket, Koch double K table, Ladas, Walter, modified Maloney, clinical history, Feiz standardized and nomogram, Latkany average and flat, Ferrara, Rosa, Savini, Jin, Shammas no-history and regression formula, Seitz, and Awwad methods. The SRK/T, Hoffer, and Holladay formulas were tested in appropriate formulas. All IOL powers were converted to refractive results using IOL(exact) equations. MAIN OUTCOME MEASURES: Mean +/- standard deviation (SD), range, absolute mean +/- SD, and percent within +/-0.5 D, +/-1.0 D, and -1.0/+0.5 D. RESULTS: The pachymetric technique minimizes hyperopic surprises with 92% of eyes within -1.0/+0.5 D and no overcorrections >0.5 D with the Geggel-real modification. Final refractive results with the Geggel, Geggel-real, Masket, Koch double K tables, Latkany average and flat, Savini, Shammas no-history, Seitz Holladay, Seitz Hoffer, and Awwad Hoffer all had >55% SE +/-0.5 D and >85% SE +/-1.0 D of the surgical goal. The best results with fewer hyperopic overcorrections were found in 5 methods that comprise a new consensus group: Geggel-real, Shammas no-history, Savini, Latkany flat, and Seitz Hoffer. The consensus group had 96% of eyes within -1.0/+0.5 D of the surgical goal. CONCLUSIONS: The Geggel-real method is a new approach requiring no historical data to determine IOL power in this ever-enlarging and challenging group of former refractive surgery patients undergoing routine cataract surgery. FINANCIAL DISCLOSURE(S): The author(s) have no proprietary or commercial interest in any materials discussed in this article.

Corrected and uncorrected near and distance vision with ReSTOR compared to monofocal intraocular lens implantation after cataract surgery: a pooled analysis.

BACKGROUND/AIM: To evaluate distance and near vision-related benefit after implantation of ReSTOR multifocal intraocular lenses (IOLs) following cataract surgery. METHODS: Two prospective open-labeled nonrandomized studies were pooled. Patients' perception of benefit was assessed with the TyPE, administered at baseline and after each eye surgery. RESULTS: A total of 499 patients received ReSTOR IOLs, and 173 received monofocal IOLs. The distance vision of monofocal and ReSTOR patients improved equally with and without glasses. A greater improvement in near vision without glasses was reported by ReSTOR-implanted patients as early as after the 1st eye surgery (p < 0.0001). More ReSTOR patients than monofocal patients reported independence from glasses after the 1st eye surgery (64 vs. 52%; p = 0.0002). This difference had increased after the 2nd eye surgery (85 vs. 51%; p < 0.0001). CONCLUSIONS: The improvement in near vision without glasses was significantly more evident in ReSTOR patients, allowing the majority of them to be free of glasses.

Avoiding Hyperopic Surprises After Descemet Membrane Endothelial Keratoplasty in Fuchs Dystrophy Eyes by Assessing Corneal Shape.

PURPOSE: It is unclear which patients unexpectedly have a hyperopic refractive outcome after combined Descemet membrane endothelial keratoplasty and cataract surgery (triple DMEK). We assessed how corneal shape predicts hyperopia after triple DMEK. DESIGN: Retrospective cohort study. METHODS: Patients with Fuchs endothelial corneal dystrophy (FECD) with Scheimpflug examinations before uncomplicated triple DMEK at a tertiary referral center were included. The arithmetic error was calculated (stable postoperative refraction minus predicted refraction). Using multinomial logistic regression, risk ratios of > +0.5 diopter (D) hyperopic and > 0.5 D myopic arithmetic errors were calculated. RESULTS: In 112 eyes, the median predicted refraction was -0.43 D (interquartile range [IQR], -0.47 to -0.17) with an achieved refraction of -0.63 to 0.56 (IQR). The arithmetic error was 0.34 D (IQR, -0.22 to 0.81). A hyperopic arithmetic error was present in 46% of eyes. FECD eyes with an oblate posterior cornea (Q value >0) had a 3.0 times higher risk of hyperopic shift after triple DMEK (95% confidence interval [CI], 1.3-7.0; P = .011), compared to spherical or prolate corneas (Q value ≤ 0). In eyes with posterior Q > 0, the mean prediction error was +0.50 D higher than in eyes with negative Q values (95% CI, 0.19-0.82; P = .002), independent of corneal thickness. CONCLUSIONS: Hyperopic surprises after triple DMEK particularly occur in corneas that are flatter centrally than the periphery because of edematous changes (oblate posterior profile). Eyes with a positive Q value on Scheimpflug imaging should be considered for additional power at the intraocular lens level.

Comparison of intraocular lens power calculation formulas in Chinese eyes with axial myopia.

PURPOSE: To assess the accuracy of intraocular lens (IOL) power calculation formulas in Chinese eyes with axial lengths (ALs) longer than 26.0 mm. SETTING: Department of Cataract Surgery, Shanxi Eye Hospital, China. DESIGN: Prospective case series. METHODS: This study evaluated (1) two new formulas (Barrett Universal II and Hill-RBF 2.0), (2) three vergence formulas (Haigis, Holladay 1, and SRK/T), and (3) the original and modified Wang-Koch AL adjustment formulas with Holladay 1 and SRK/T. The User Group for Laser Interference Biometry lens constants were used for IOL power calculation. The refractive prediction error was calculated by subtracting the predicted refraction from the actual refraction postoperatively. The mean numerical error (MNE), percentage of eyes with hyperopic outcomes, and mean absolute error (MAE) were determined. RESULTS: The study comprised 136 eyes. The Barrett and Hill-RBF formulas had MNEs close to zero (-0.09 D to 0.03 D), the Haigis, Holladay 1, and SRK/T produced hyperopic MNEs (0.25 to 0.70 D), and the original and modified Wang-Koch AL adjustment formulas induced myopic MNEs (-0.48 to -0.22 D). The original Wang-Koch formulas produced significantly lower percentages of eyes with hyperopic outcomes (15% to 18%) than all other formulas (28% to 91%). There were no significant differences in MAEs between the Barrett, Hill-RBF, Haigis, and original and modified Wang-Koch adjustment with the Holladay 1 (0.32 to 0.41 D). CONCLUSION: The performances of the Barrett and Hill-RBF were comparable in long eyes. The incidence of hyperopic outcome with the Wang-Koch AL adjustment formula was significantly lower than other formulas.

[Impact of continuous graduated slight myopathic defocusing on postnatal refractogenesis].

Based on today's theories of implication of visual feedback in the regulation of eye growth in postnatal refractogenesis, the authors propose new optical procedures for prevention of myopathy and its progression: continuous graduated slight myopathic defocusing of an image, by using specially chosen plus-lens spectacles. Spectacles for both eyes to be persistently worn were chosen for this defocusing in a risk group of children who had pseudomyopathy, slight or absent hyperopia reserve, and near-sighted parents. In a group of children with mild myopathy, the authors applied their developed and patented method of alternating monolateral defocusing, i.e., alternating distance anisocorrection by means of two pairs of persistently worn spectacles, which were changed every day. The methods were shown to have an inhibiting action on the rise of the anteroposterior axis, a refraction shift towards myopathy; the regular changes in anterior chamber depth and lenticular thickness, which lowered dynamic ocular refraction, were also revealed.

[Evaluation of the efficacy of long term topical steroid treatment to prevent regression after hyperopic LASIK]. / Valoración de la eficacia de tratamiento corticoideo tópico prolongado para prevenir regresión en LASIK hipermetrópico.

PURPOSE: To evaluate the efficacy of long-term versus short-term steroid treatment in preventing regression after hyperopic LASIK. MATERIAL AND METHODS: This is a prospective single masked study. Hyperopic patients who were candidates for LASIK surgery were randomized to be treated with one week (topical dexametasone) in control group or one month (topical dexametasone the first week and topical fluormetalone the next three weeks) in study group. The three month postoperative manifest refraction was compared between the two groups. RESULTS: 105 eyes were included in each group. The mean preoperative spherical equivalent was 3.17 D SD 2.82 D (standard deviation) and 3.39 D SD 2.65 D in the study and control group, respectively (p=0.6). The final manifest refraction in the three-month postoperative visit was 0.62 D SD 0.68 D and 0.6 D SD 0.3 D in the study and control group respectively (p=0.6). CONCLUSIONS: Long-term topical steroid therapy does not appear to improve the refractive result in hyperopic LASIK.

The effects of constant and diurnal illumination of the pineal gland and the eyes on ocular growth in chicks.

PURPOSE: To investigate the effects of constant or 12-hour cyclic illumination of the pineal gland and the eyes on the growth of the chick eye. METHODS: Chicks (Gallus gallus, Cornell K Strain) were raised either under a 12-hour light-dark cycle of normal light or under constant light, with or without opaque removable hoods that covered the top of the head for 12 hours each day. A second group of chicks was raised under constant light with opaque eye covers that were worn on either both eyes or only the right eye for 12 hours each day. Chicks were placed in the experimental conditions on the third day after hatching and raised for 3 weeks. RESULTS: Pineal gland hoods and eye covers worn 12 hours a day significantly (P < 0.0001) protected the chicks from hyperopia under constant-light conditions. They also reduced the flattening of the cornea caused by constant light. Most striking was the protection afforded the uncovered eye from constant light's effects by the periodic covering of the opposite eye. CONCLUSIONS: A diurnal light-dark rhythm presented to one of three photosensitive organs (the pineal gland and both eyes) can protect the eyes from the effects of constant light. This is most probably due to the maintenance of a melatonin rhythm in the organ receiving the diurnal light rhythm.

Screening of myopic photorefractive keratectomy in eye bank eyes by computerized videokeratography.

BACKGROUND: In contrast to incisional keratotomy, corneas that have undergone photorefractive keratectomy may be difficult to detect by inspection with slitlamp biomicroscopy alone. Eye bank corneas that have undergone high myopic refractive surgical correction could potentially result in substantial postoperative hyperopic correction if used as donor tissue for corneal transplantation. Surface irregularities or displacement of the treated optical zone within the graft in relation to the entrance pupil of the recipient could result in significant induced astigmatism and distortion. This study examines computerized videokeratographic screening of eye bank globes as a strategy for detecting myopic photorefractive keratectomy. METHODS: Preoperative and postoperative corneal topographic maps of freshly enucleated human and rabbit eyes that have undergone myopic photorefractive keratectomy with an excimer laser were placed in a globe-fixating device and analyzed using a vertically oriented videokeratoscope. The same system was applied in an actual eye bank setting, and potentially transplantable globes from donors without a history of corneal surgery were analyzed. RESULTS: Computerized videokeratography using a vertically mounted system reliably detected photorefractive keratectomy in 12 of 12 human eye bank corneas treated by excimer photorefractive keratectomy in a range between -1.5 to -6.0 diopters. This method also detected similar changes on lased rabbit corneas enucleated 6 weeks after excimer surgery. Data processed with the tangential mode yielded a "bull's-eye" topography pattern reflecting central corneal flattening that was more sensitive in detecting myopic corrections than the conventional axial formula-based color maps. False-positive results were not detected in 96 cadaver globes sequentially screened in the eye bank. CONCLUSIONS: Computerized videokeratography represents a feasible method to screen donor globes for myopic photorefractive keratectomy as shown by the in vitro and rabbit models. However, only whole globes and not corneoscleral sections are amenable to processing with this technique. Tangential maps provided greater sensitivity in detecting low myopic corrections than the axial formula-based color maps.

Correcting the corneal power measurements for intraocular lens power calculations after myopic laser in situ keratomileusis.

PURPOSE: To describe and evaluate a refraction-derived method and a clinically derived method to calculate the correct corneal power for intraocular lens (IOL) power calculations after laser in situ keratomileusis (LASIK) and to compare the results to the commonly used history-derived method. DESIGN: Interventional case series. METHODS: Retrospective analysis of consecutive cases from clinical practice. Two hundred randomly selected eyes from 200 patients were evaluated before and after LASIK surgery. For each patient, we established the pre-LASIK and post-LASIK spectacle refraction, the pre-LASIK (Kpre) and post-LASIK K readings (Kpost). We then calculated for each case the pre- and post-LASIK refraction at the corneal plane and the amount of correction obtained by the refractive surgery (CRc). The cases were divided into two groups. Group I was used to derive the two formulas. The K values were calculated using the history-derived method (Kc.hd) in which Kc.hd = Kpre - CRc. Kc.hd was compared with Kpost. The average difference was 0.23 diopters for every diopter of myopia corrected. This value was used to calculate the corneal power using the refraction-derived method (Kc.rd) where Kc.rd = Kpost -0.23CRc. A regression equation was used to develop a clinically derived method (Kc.cd) where Kc.cd = 1.14Kpost -6.8. The values obtained with the two methods were compared with the Kc.hd values in group II to validate the results. RESULTS: Both Kc.rd and Kc.cd values correlated highly with Kc.hd when plotted on a scattergram (P <.001), and there was no statistically significant difference between the mean keratometric values (P >.5). CONCLUSIONS: The corneal power measurements for intraocular lens power calculations after LASIK need to be corrected to avoid hypermetropia after cataract surgery by either the history-derived method, the refraction-derived method, or the clinically derived method.

The suction bridge for radial keratotomy may avoid late hyperopic shift.

BACKGROUND: Radial keratotomy may induce late hyperopic shift. We present data on 140 consecutive eyes with a follow-up of up to 3 years that underwent radial keratotomy with the RK suction bridge. METHODS: We conducted a retrospective study of 140 consecutive eyes that had radial keratotomy between 1987 and 1994. Mean preoperative spherical equivalent was -5.21 D (range -2.00 to -9.75 D). All operations were performed by one surgeon (JHK) with the RK suction bridge. A suction ring maintaining physiological intraocular pressure immobilized the eye and left a peripheral rim of uncut cornea. The ring incorporated an eccentric bridge that guided the radial keratotomy knife. The knife setting was 90% of the central corneal thickness, measured by pachymetry. Spherical equivalent refraction and spectacle corrected visual acuity were measured at 1 week, 1, 3, 6 months, 1 year, and 3 years after radial keratotomy. RESULTS: The mean preoperative spherical equivalent refraction of -5.21 D dropped to -0.43 D at 1 week (n = 136), -0.71D at 1 month (n = 120), -0.85 D at 3 months (n = 95), -0.74 D at 6 months (n = 73), -0.77 D at 12 months (n = 79), and -0.85 D at 3 years (n = 67). Compared to 1 month spherical equivalent, at 3 years three eyes (4.4%) had moved > = or 1.00 D toward hyperopia. One eye (1.4%) shifted by 1.25 D. Paired t-tests of mean spherical equivalent refraction did not reveal significant shifts toward hyperopia. Mean preoperative spectacle-corrected visual acuity was slightly diminished at 1 week and was equal or better thereafter. CONCLUSIONS: Our 3-year data suggest that a late hyperopic shift following radial keratotomy may be prevented if an intact peripheral rim is maintained and cutting depth does not exceed 90% of the lowest corneal thickness.

Influence of ablation parameters on refractive changes after phototherapeutic keratectomy.

PURPOSE: The aim of the current study was to control the hypothetical effects of decreased laser energy delivered to the peripheral cornea during phototherapeutic keratectomy (PTK) and provide quantitative calculation of induced low and high order aberrations. METHODS: We employed a model eye to simulate the refractive effect of homogeneous laser corneal irradiation, as in PTK, for different laser fluences (range 125 to 225 mJ/cm2) and treatment depths up to 200 microm. RESULTS: The hyperopic shift induced by the relatively lower energy delivered at the peripheral ablation zone during PTK was proportional to the treatment depth and inversely proportional to the energy fluence. The hyperopic shift calculated using the above ablation parameters was lower compared to previously reported clinical results. Higher order wavefront aberration (total root mean square) changes were of minimal significance for treatment depths up to 200 microm. CONCLUSIONS: After PTK, a hyperopic shift cannot be attributed to the energy delivery method alone. Modification of laser energy delivery algorithms may only minimize PTK-induced hyperopia.

Diurnal illumination patterns affect the development of the chick eye.

Exposure to continuous illumination disrupts normal ocular development in young chicks, causing severe corneal flattening, shallow anterior chambers and progressive hyperopia ('constant light (CL) effects'). We have studied the minimum requirements of a diurnal light cycle to prevent CL effects. (1) Seven groups of 10 chicks were reared under a 0 (constant light, CL), or 1, 2, 3,4, 6, or 12/12 h (normal) light-dark cycles. It was found that CL effects were prevented if the dark period was 4 h or longer. Below 4 h, the effects were dose-dependent and inversely correlated with the amplitude of the Fourier component of illumination at 1 cycle per day (CPD). (2) Three groups of 20 chicks were exposed to 4 h of darkness distributed differently over 24 h to vary the amplitude of the Fourier component at 1 CPD. It was found that complete suppression of the CL effects required that the 4 h of darkness were given in one block and at the same time each day. Our results show that normal ocular development in the chick requires a minimum of 4 h darkness per day, provided at the same time of the day without interruption, and suggest that the light-dark cycle interacts with a linear or weakly nonlinear oscillating system.

New formula for calculating intraocular lens power after laser in situ keratomileusis.

PURPOSE: To assess the validity and accuracy of a proposed formula for keratometry (K) readings after laser in situ keratomileusis (LASIK). SETTING: The Eye Center and the Eye Foundation for Research, Riyadh, Saudi Arabia. METHOD: This studied comprised 34 eyes that had LASIK surgery. Refraction and an automated K-reading (auto-K) were performed preoperatively. Refraction, auto-K, and K-reading assessment by the clinical history method and the proposed formula were performed 4 to 12 weeks postoperatively. The proposed formula is K(postop) = K(preop) - [(N(c) - 1) x (R(a-postop) - R(a-preop))/(R(a-postop) x R(a-preop))], where K(postop) is the K-reading after LASIK, K(preop) is the K-reading before LASIK, N(c) is the index of refraction of the cornea (1.376), R(a-postop) is the radius of curvature of the anterior corneal surface after LASIK, and R(a-preop) is the radius of curvature of the anterior corneal surface before LASIK. RESULTS: Twenty patients (10 men, 10 women) were included in the study. The mean age of the patients was 30.58 years +/- 17.68 (SD) (range 18 to 44 years). Preoperatively, the mean spherical equivalent (SE) was -4.99 +/- 2.82 diopters (D) (range -1.12 to -15.00 D), the mean R(a) was 7.76 +/- 0.32 mm (range 7.33 to 8.50 mm), and the mean auto-K reading was 43.45 +/- 1.73 D (range 39.62 to 46.00 D). Postoperatively, the mean SE was +0.02 +/- 0.63 D (range -2.75 to +1.00 D), the mean R(a) was 8.63 +/- 0.53 mm (range 7.80 to 9.92 mm), and the mean K-reading assessed by auto-K, clinical history method, and the proposed formula was 39.17 +/- 2.35 D (range 34.00 to 43.25 D), 38.79 +/- 2.52 D (range 33.1 to 42.78 D), and 38.69 +/- 2.51 D (range 33.1 to 43.0 D), respectively. The results obtained by the proposed formula were similar to those obtained by the clinical history method (P =.098). Auto-K readings significantly overestimated the K-values (P<.0001) when compared to the proposed formula and clinical history method. CONCLUSION: The proposed formula was simple, objective, not dependent on refraction, and as accurate as the clinical history method in determining K-readings after LASIK.

Analysis of intraocular lens power calculation in post-radial keratotomy eyes.

PURPOSE: To determine whether refractive complications can be prevented by applying the currently most accurate method of intraocular lens (IOL) power calculation in the post-radial keratotomy (RK) patient. SETTING: Department of Ophthalmology, University of California, Davis, Sacramento, and American Eye Institute, Cedars Sinai Medical Center, Los Angeles, California, USA. METHODS: Twenty-four eyes having cataract surgery after RK were studied retrospectively for the final postcataract refraction and for the target refraction used in selecting the IOL. Nine of the eyes were further studied for the keratometry (K) values obtained with different methods and for the theoretical postoperative refraction with an IOL aiming for plano or -1.50 diopters (D) based on the known flatter calculated K, axial length, power of the implanted IOL, and refraction after cataract surgery. RESULTS: Implantation of an IOL aiming for plano in the 24 post-RK eyes would have resulted in a hyperopic refraction in 83.4% cases. The choice of an IOL targeted at myopia reduced the frequency of hyperopia to 42.0% (24 cases). Selection of an IOL calculated with a flatter calculated K and aiming for plano decreased the frequency of hyperopia to 44.4%; however, aiming for -1.50 D still resulted in hyperopia in 44.4% of eyes (9 cases). CONCLUSIONS: Unintentional hyperopia can be significantly decreased but not eliminated as a complication of post-RK cataract surgery. The accuracy of the IOL power determination can be improved if myopia is targeted as the postcataract surgery refractive error and the flatter calculated K is used in the IOL determination.