Moving the retina: choroidal modulation of refractive state

The chick eye is able to change its refractive state by as much as 7 D by pushing the retina forward or pulling it back; this is effected by changes in the thickness of the choroid, the vascular tissue behind the retina and pigment epithelium. Chick eyes first made myopic by wearing diffusers and then permitted unrestricted vision developed choroids several times thicker than normal within days, thereby speeding recovery from deprivation myopia. Choroidal expansion does not occur when visual cues are reduced by dim illumination during the period of unrestricted vision. Furthermore, in chick eyes presented with myopic or hyperopic defocus by means of spectacle lenses, the choroid expands or thins, respectively, in compensation for the specific defocus imposed. Consequently, when the lenses are removed, the eye finds its refractive error suddenly of opposite sign, and the choroidal thickness again compensates by changing in the opposite direction. If a local region of the eye is made myopic by a partial diffuser and then given unrestricted vision, the choroid expands only in the myopic region. Although the mechanism of choroidal expansion is unknown, it might involve either a increased routing of aqueous humor into the uveoscleral outflow or osmotically generated water movement into the choroid. The latter is compatible with the increased choroidal proteoglycan synthesis either when eyes wear positive lenses or after diffuser removal.

Refractive error, axial length, and relative peripheral refractive error before and after the onset of myopia

PURPOSE

To evaluate refractive error, axial length, and relative peripheral refractive error before, during the year of, and after the onset of myopia in children who became myopic compared with emmetropes.

METHODS

Subjects were 605 children 6 to 14 years of age who became myopic (at least -0.75 D in each meridian) and 374 emmetropic (between -0.25 D and +1.00 D in each meridian at all visits) children participating between 1995 and 2003 in the Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error (CLEERE) Study. Axial length was measured annually by A-scan ultrasonography. Relative peripheral refractive error (the difference between the spherical equivalent cycloplegic autorefraction 30 degrees in the nasal visual field and in primary gaze) was measured using either of two autorefractors (R-1; Canon, Lake Success, NY [no longer manufactured] or WR 5100-K; Grand Seiko, Hiroshima, Japan). Refractive error was measured with the same autorefractor with the subjects under cycloplegia. Each variable in children who became myopic was compared to age-, gender-, and ethnicity-matched model estimates of emmetrope values for each annual visit from 5 years before through 5 years after the onset of myopia.

RESULTS

In the sample as a whole, children who became myopic had less hyperopia and longer axial lengths than did emmetropes before and after the onset of myopia (4 years before through 5 years after for refractive error and 3 years before through 5 years after for axial length; P < 0.0001 for each year). Children who became myopic had more hyperopic relative peripheral refractive errors than did emmetropes from 2 years before onset through 5 years after onset of myopia (P < 0.002 for each year). The fastest rate of change in refractive error, axial length, and relative peripheral refractive error occurred during the year before onset rather than in any year after onset. Relative peripheral refractive error remained at a consistent level of hyperopia each year after onset, whereas axial length and myopic refractive error continued to elongate and to progress, respectively, although at slower rates compared with the rate at onset.

CONCLUSIONS

A more negative refractive error, longer axial length, and more hyperopic relative peripheral refractive error in addition to faster rates of change in these variables may be useful for predicting the onset of myopia, but only within a span of 2 to 4 years before onset. Becoming myopic does not appear to be characterized by a consistent rate of increase in refractive error and expansion of the globe. Acceleration in myopia progression, axial elongation, and peripheral hyperopia in the year prior to onset followed by relatively slower, more stable rates of change after onset suggests that more than one factor may influence ocular expansion during myopia onset and progression.

Relative peripheral hyperopic defocus alters central refractive development in infant monkeys

Understanding the role of peripheral defocus on central refractive development is critical because refractive errors can vary significantly with eccentricity and peripheral refractions have been implicated in the genesis of central refractive errors in humans. Two rearing strategies were used to determine whether peripheral hyperopia alters central refractive development in rhesus monkeys. In intact eyes, lens-induced relative peripheral hyperopia produced central axial myopia. Moreover, eliminating the fovea by laser photoablation did not prevent compensating myopic changes in response to optically imposed hyperopia. These results show that peripheral refractive errors can have a substantial impact on central refractive development in primates.

Prevalence and risk factors for common vision problems in children: data from the ALSPAC study

OBJECTIVE

To estimate the distribution and predictors of some common visual problems (strabismus, amblyopia, hypermetropia) within a population-based cohort of children at the age of 7 years.

METHODS

Children participating in a birth cohort study were examined by orthoptists who carried out cover/uncover, alternate cover, visual acuity and non-cycloplegic refraction tests. Prospectively collected data on potential risk factors were available from the study.

RESULTS

Data were available for 7825 seven-year-old children. 2.3% (95% CI 2.0% to 2.7%) had manifest strabismus, 3.6% (95% CI 3.3% to 4.1%) had past/present amblyopia, and 4.8% (95% CI 4.4% to 5.3%) were hypermetropic. Children from the lowest occupational social class background were 1.82 (95% CI 1.03% to 3.23%) times more likely to be hypermetropic than children from the highest social class. Amblyopia (p = 0.089) and convergent strabismus (p = 0.066) also tended to increase as social class decreased.

CONCLUSIONS

Although strabismus has decreased in the UK, it and amblyopia remain common problems. Children from less advantaged backgrounds were more at risk of hypermetropia and to a lesser extent of amblyopia and convergent strabismus. Children's eye-care services may need to take account of this socio-economic gradient in prevalence to avoid inequity in access to care.

Peripheral refractive errors in myopic, emmetropic, and hyperopic young subjects

To gain more insight into the relationship between foveal and peripheral refractive errors in humans, spheres, cylinders, and their axes were binocularly measured across the visual field in myopic, emmetropic, and hyperopic groups of young subjects. Both automated infrared photorefraction (the "PowerRefractor"; www. plusoptix.de) and a double-pass technique were used because the PowerRefractor provided extensive data from the central 44 deg of the visual field in a very convenient and fast way. Two-dimensional maps for the average cross cylinders and spherical equivalents, as well as for the axes of the power meridians of the cylinders, were created. A small amount of lower-field myopia was detected with a significant vertical gradient in spherical equivalents. In the central visual field there was little difference among the three refractive groups. The established double-pass technique provided complementary data also from the far periphery. At 45 deg eccentricity the double-pass technique revealed relatively more hyperopic spherical equivalents in myopic subjects than in emmetropic subjects [+/-2.73 +/- 2.85 D relative to the fovea, p < 0.01 (+/- standard deviation)] and more myopic spherical equivalents in hyperopic subjects (-3.84 +/- 2.86 D relative to the fovea, p < 0.01). Owing to the pronounced peripheral astigmatism, spherical equivalents (refractions with respect to the plane of the circle of least confusion) became myopic relative to the fovea in all three groups. The finding of general peripheral myopia was unexpected. Its possible roles in foveal refractive development are discussed.

Effects of optical defocus on refractive development in monkeys: evidence for local, regionally selective mechanisms

PURPOSE

To characterize the influence of optical defocus on ocular shape and the pattern of peripheral refraction in infant rhesus monkeys.

METHODS

Starting at 3 weeks of age, eight infant monkeys were reared wearing -3 diopter (D) spectacle lenses over one eye that produced relative hyperopic defocus in the nasal field (NF) but allowed unrestricted vision in the temporal field (NF group). Six infants were reared with monocular -3 D lenses that produced relative hyperopic defocus across the entire field of view. Control data were obtained from 11 normal monkeys. Refractive development was assessed by streak retinoscopy performed along the pupillary axis and at eccentricities of 15 degrees, 30 degrees, and 45 degrees along the vertical and horizontal meridians. Central axial dimensions and eye shape were assessed with magnetic resonance imaging.

RESULTS

In response to full-field hyperopic defocus, the eye developed relative central axial myopia, became less oblate, and exhibited relative peripheral hyperopia in both the nasal and the temporal hemifields. Conversely, nasal-field hyperopic defocus produced relative myopia that was largely restricted to the nasal hemifield; these alterations in the patterns of peripheral refraction in the NF monkeys were associated with local, region-specific alterations in vitreous chamber depth in the treated hemiretina.

CONCLUSIONS

Optically imposed defocus can alter the shape and pattern of peripheral refraction in infant primates. Like those of form deprivation, the effects of optical defocus in primates are dominated by mechanisms that integrate visual signals in a spatially restricted manner and exert their influence in a regionally selective manner.

Comparison of Ocular Component Growth Curves among Refractive Error Groups in Children

PURPOSE

To compare ocular component growth curves among four refractive error groups in children. methods Cycloplegic refractive error was categorized into four groups: persistent emmetropia between -0.25 and +1.00 D (exclusive) in both the vertical and horizontal meridians on all study visits (n = 194); myopia of at least -0.75 D in both meridians on at least one visit (n = 247); persistent hyperopia of at least +1.00 D in both meridians on all visits (n = 43); and emmetropizing hyperopia of at least +1.00 D in both meridians on at least the first but not at all visits (n = 253). Subjects were seen for three visits or more between the ages of 6 and 14 years. Growth curves were modeled for the persistent emmetropes to describe the relation between age and the ocular components and were applied to the other three refractive error groups to determine significant differences. results At baseline, eyes of myopes and persistent emmetropes differed in vitreous chamber depth, anterior chamber depth, axial length, and corneal power and produced growth curves that showed differences in the same ocular components. Persistent hyperopes were significantly different from persistent emmetropes in most components at baseline, whereas growth curve shapes were not significantly different, with the exception of anterior chamber depth (slower growth in persistent hyperopes compared with emmetropes) and axial length (lesser annual growth per year in persistent hyperopes compared with emmetropes). The growth curve shape for corneal power was different between the emmetropizing hyperopes and persistent emmetropes (increasing corneal power compared with decreasing power in emmetropes). conclusions Comparisons of growth curves between persistent emmetropes and three other refractive error groups showed that there are many similarities in the growth patterns for both the emmetropizing and persistent hyperopes, whereas the differences in growth lie mainly between the emmetropes and myopes.

Axial eye growth and refractive error development can be modified by exposing the peripheral retina to relative myopic or hyperopic defocus

PURPOSE

Bifocal contact lenses were used to impose hyperopic and myopic defocus on the peripheral retina of marmosets. Eye growth and refractive state were compared with untreated animals and those treated with single-vision or multizone contact lenses from earlier studies.

METHODS

Thirty juvenile marmosets wore one of three experimental annular bifocal contact lens designs on their right eyes and a plano contact lens on the left eye as a control for 10 weeks from 70 days of age (10 marmosets/group). The experimental designs had plano center zones (1.5 or 3 mm) and +5 diopters [D] or -5 D in the periphery (referred to as +5 D/1.5 mm, +5 D/3 mm and -5 D/3 mm). We measured the central and peripheral mean spherical refractive error (MSE), vitreous chamber depth (VC), pupil diameter (PD), calculated eye growth, and myopia progression rates prior to and during treatment. The results were compared with age-matched untreated (N=25), single-vision positive (N=19), negative (N=16), and +5/-5 D multizone lens-reared marmosets (N=10).

RESULTS

At the end of treatment, animals in the -5 D/3 mm group had larger (P<0.01) and more myopic eyes (P<0.05) than animals in the +5 D/1.5 mm group. There was a dose-dependent relationship between the peripheral treatment zone area and the treatment-induced changes in eye growth and refractive state. Pretreatment ocular growth rates and baseline peripheral refraction accounted for 40% of the induced refraction and axial growth rate changes.

CONCLUSIONS

Eye growth and refractive state can be manipulated by altering peripheral retinal defocus. Imposing peripheral hyperopic defocus produces axial myopia, whereas peripheral myopic defocus produces axial hyperopia. The effects are smaller than using single-vision contact lenses that impose full-field defocus, but support the use of bifocal or multifocal contact lenses as an effective treatment for myopia control.

Developing eyes that lack accommodation grow to compensate for imposed defocus

The eyes of growing chicks adjust to correct for myopia (eye relatively long for the focal length of its optics) or hyperopia (eye relatively short for the focal length of its optics). Eyes made functionally hyperopic with negative spectacle lenses become myopic and long, whereas eyes made functionally myopic with positive spectacle lenses become hyperopic and short. We report here that these compensatory growth adjustments occur not only in normal eyes but also in eyes unable to accommodate (focus) because of lesions to the Edinger-Westphal nuclei. Thus, at least in chicks, accommodation is not necessary for growth that reduces refractive errors during development, and may not be necessary for the normal control of eye growth.

Effects on the compensatory responses to positive and negative lenses of intermittent lens wear and ciliary nerve section in chicks

This study examined the ocular compensation to lens-induced defocus in chick and the effect of interrupting lens wear on a daily basis. Eyes fitted with +10 D lenses at hatching compensated rapidly, with almost complete compensation after 4 days of lens wear; they had decreased vitreous chamber depth compared to normal eyes and were thus hyperopic when the lenses were removed. In contrast, adaptation to the -10 D lenses was much slower, was still incomplete after 9 days of lens wear, and in this case, eyes had increased vitreous chamber depth and were myopic without the lenses. Adaptation improved when lens wear was delayed until 7 days after hatching. The effect of interrupting lens wear by periods of normal vision varied with the sign of the lenses worn. Hyperopia was always seen in response to +10 D lenses, although the magnitude of the response decreased as the duration of lens wear was decreased. In contrast, even brief periods of normal vision, i.e., 3 hr, prevented the development of myopia in response to the -10 D lenses; this apparent sensitivity to normal vision is similar to that reported for form-deprivation myopia. Ciliary nerve section used here to eliminate accommodation did not alter these response patterns.

Wavefront-optimized ablation profiles

PURPOSE

To describe a method for calculating wavefront-optimized ablation profiles to precompensate for the spherical aberration and higher-order astigmatism induced by myopic, hyperopic, and astigmatic corneal laser corrections.

SETTING

IROC-Institut für Refraktive und Ophthalmo-Chirurgie, and Institute for Biomedical Engineering, Swiss Federal Institute of Technology, Zürich, Switzerland.

METHODS

The basic ablation profile for myopic, hyperopic, and astigmatic correction is derived from the 2nd-order Zernike representation of wavefront aberrations. Including 4th-order spherical aberration and higher-order astigmatism in the theoretical calculation of the ablation profile allows precompensation for the expected amount of higher-order aberrations (HOAs). The shapes of wavefront-optimized ablation profiles are compared with the shapes of "classic" ablation profiles for myopic and astigmatic corrections.

RESULTS

The introduction of precompensating spherical aberration and higher-order astigmatism leads to a more aspheric ablation profile with a significant increase in ablation depth (up to 35%) in the midperiphery of the optical zone. The central ablation depth remains unchanged in the myopic correction but increases by 3% in cylinder correction.

CONCLUSIONS

Wavefront-optimized ablation profiles provide a simple method to precompensate for the expected 4th-order spherical aberration and higher-order astigmatism in the average eye. Further clinical studies must be performed to prove the theoretical results; demonstrate the reduction in HOAs; and predict safety, predictability, and stability of wavefront-optimized ablation profiles.

Intraocular Lens Power Calculation in Eyes After Corneal Refractive Surgery

PURPOSE

The purpose of this review article is to discuss the major reasons for postoperative hyperopia after cataract surgery following radial keratotomy (RK) and photorefractive keratectomy (PRK) and to illustrate potential methods for improvement of intraocular lens (IOL) power prediction after keratorefractive surgery based on exemplary model calculations.

METHODS

We previously performed model calculations in eyes after PRK for myopia (-1.50 to -8.00 D, mean -5.40 +/- 1.90 D) using keratometry readings as measured by the Zeiss keratometer and the TMS-1 topography unit and as calculated using the "clinical history method" (spherical equivalent refraction change) and change in anterior surface keratometry readings.

RESULTS

We found that after PRK, mean measured keratometry readings were significantly greater than respective calculated values considering the preoperative to postoperative change of anterior corneal surface (P < .001), which itself was significantly greater than calculated keratometry readings considering the preoperative to postoperative change of spherical equivalent refraction (P < .001). IOL power underestimation correlated significantly with the difference between preoperative and postoperative spherical equivalent refraction (P = .001).

CONCLUSIONS

For correct assessment of keratometric readings to be entered into more than one modern third-generation IOL power calculation formula (but not a regression formula), the clinical history method should be applied whenever refraction and keratometric diopters before the keratorefractive procedure are available to the cataract surgeon. If preoperative keratometric diopters and refraction are not known, average central power on the postoperative videokeratograph may be used after RK, but refined calculation of keratometric diopters from radius of anterior and posterior corneal surface should be used after PRK and/or LASIK.

Myopic versus hyperopic eyes: axial length, corneal shape and optical aberrations

This study investigated differences in geometrical properties and optical aberrations between a group of hyperopes and myopes (age-matched 30.3+/-5.2 and 30.5+/-3.8 years old, respectively, and with similar absolute refractive error 3.0+/-2.0 and -3.3+/-2.0, respectively). Axial length (AL) was measured by means of optical biometry, and corneal apical radius of curvature (CR) and asphericity (Q) were measured by fitting corneal topography data to biconic surfaces. Corneal aberrations were estimated from corneal topography by means of virtual ray tracing, and total aberrations were measured using a laser ray tracing technique. Internal aberrations were estimated by subtracting corneal from total aberrations. AL was significantly higher in myopes than in hyperopes and AL/CR was highly correlated with spherical equivalent. Hyperopic eyes tended to have higher (less negative) Q and higher total and corneal spherical aberration than myopic eyes. RMS for third-order aberrations was also significantly higher for the hyperopic eyes. Internal aberrations were not significantly different between the myopic and hyperopic groups, although internal spherical aberration showed a significant age-related shift toward less negative values in the hyperopic group. For these age and refraction ranges, our cross-sectional results do not support evidence of relationships between emmetropization and ocular aberrations. Our results may be indicative of presbyopic changes occurring earlier in hyperopes than in myopes.

Femtosecond laser-assisted keyhole endokeratophakia: correction of hyperopia by implantation of an allogeneic lenticule obtained by SMILE from a myopic donor

PURPOSE

To describe endokeratophakia in which a small incision lenticule extraction (SMILE) lenticule from a myopic patient is implanted into a recipient eye through a small incision to correct hyperopia.

METHODS

A 23-year-old aphakic woman presented following cataract surgery to remove a childhood congenital cataract with hyperopia of +12.00 -1.50 × 155, corrected distance visual acuity of counting fingers, and exotropia. A SMILE procedure using the VisuMax femto-second laser (Carl Zeiss Meditec, Jena, Germany) was performed on a donor patient with high myopia and the extracted lenticule was stored (power -10.50 diopter sphere, optical zone 5.75 mm, central lenticule thickness 127 μm). In the recipient eye, a pocket lamellar incision was created using the VisuMax SMILE software. The upper interface was separated and the donor lenticule was inserted through the small incision.

RESULTS

One year postoperatively, retinoscopy refraction was +7.50 -3.00 × 150, a spherical equivalent refraction reduction of 5.25 diopters. Mean keratometric power increased by 2.91 diopters. The posterior surface elevation changed significantly with a central bulge into the anterior chamber. Central corneal thickness by Pentacam (Oculus Optikgeräte, Wetzlar, Germany) increased by 121 μm. Central lenticule thickness was 130 μm and central epithelial thickness was 43 μm measured by RTVue OCT (Optovue Inc., Fremont, CA). The cornea remained clear over the 1-year postoperative period.

CONCLUSIONS

Endokeratophakia appears to be a viable procedure for correcting hyperopia on the cornea by implantation of an extracted myopic SMILE lenticule from a donor patient. However, posterior surface changes and epithelial remodeling resulted in only 50% of the intended correction. No adverse side effects were observed following implantation of donor tissue for 1 year.

The human eye is an example of robust optical design

In most eyes, in the fovea and at best focus, the resolution capabilities of the eye's optics and the retinal mosaic are remarkably well adapted. Although there is a large individual variability, the average magnitude of the high order aberrations is similar in groups of eyes with different refractive errors. This is surprising because these eyes are comparatively different in shape: Myopic eyes are longer whereas hyperopic eyes are shorter. In most young eyes, the amount of aberrations for the isolated cornea is larger than for the complete eye, indicating that the internal ocular optics (mainly the crystalline lens) play a significant role in compensating for the corneal aberrations, thereby producing an improved retinal image. In this paper, we show that this compensation is larger in the less optically centered eyes that mostly correspond to hyperopic eyes. This suggests a type of mechanism in the eye's design that is the most likely responsible for this compensation. Spherical aberration of the cornea is partially compensated by that of the lens in most eyes. Lateral coma is also compensated mainly in hyperopic eyes. We found that the distribution of aberrations between the cornea and lens appears to allow the optical properties of the eye to be relatively insensitive to variations arising from eye growth or exact centration and alignment of the eye's optics relative to the fovea. These results may suggest the presence of an auto-compensation mechanism that renders the eye's optics robust despite large variation in the ocular shape and geometry.

In a matter of minutes, the eye can know which way to grow

PURPOSE

The fitting of chick eyes with positive or negative lenses causes eye growth to decelerate or accelerate, respectively, thereby minimizing the imposed blur. This study was conducted to determine whether the eye can initially assess the correct direction of growth or whether it relies on trial and error, reversing its direction if the magnitude of blur increases. The rapid changes in choroidal thickness in response to brief periods of defocus were measured.

METHODS

After their eyes were measured by ultrasound biometry, chicks wore either a +10-D lens over one eye for 10 minutes while restrained in the center of a 60-cm drum (to ensure myopic blur), or a negative lens (-7 or -8.6 D) over one eye for 10 minutes or 1 hour in a normal cage environment. They were then kept in darkness until they were remeasured 2 hours, 1 day, or 2 days after the first measurement. Other chicks wore +10 or -8.6-D lenses briefly and were measured several times over the next 7 hours in darkness.

RESULTS

Wearing positive or negative lenses for only 10 minutes produced significantly different effects on choroidal thickness measured 2 hours later. Wearing positive lenses for 10 minutes caused an increase in choroidal thickness (in 28 of 32 eyes) and a concomitant decrease in vitreous chamber depth, relative to the amount of change in the untreated fellow eye over the same period. Wearing negative lenses for 1 hour caused significant changes in the opposite direction. Wearing lenses for 2 hours resulted in choroidal changes that persisted in darkness for up to 6 hours after positive lens wear, but returned to normal after negative lens wear. Finally, 1 hour of positive lens wear caused significant inhibition of ocular elongation over the next 2 days.

CONCLUSIONS

The eyes of chicks require only a brief period of lens wear to initiate compensation in the appropriate direction. Because the refractive status changes little during the period of lens wear, the authors conclude that eyes can rapidly determine the sign of the imposed blur without resorting to a trial-and-error method.

Hemiretinal form deprivation: evidence for local control of eye growth and refractive development in infant monkeys

PURPOSE

To determine whether refractive development in primates is mediated by local retinal mechanisms, the authors examined the effects of hemiretinal form deprivation on ocular growth and the pattern of peripheral refractions in rhesus monkeys.

METHODS

Beginning at approximately 3 weeks of age, nine infant monkeys were reared wearing monocular diffuser lenses that eliminated form vision in the nasal field (nasal field diffuser [NFD]). Control data were obtained from the nontreated fellow eyes, 24 normal monkeys, and 19 monkeys treated with full-field diffusers. Refractive development was assessed by retinoscopy performed along the pupillary axis and at eccentricities of 15 degrees, 30 degrees, and 45 degrees. Central axial dimensions and eye shape were assessed by A-scan ultrasonography and magnetic resonance imaging, respectively.

RESULTS

Hemiretinal form deprivation altered refractive development in a regionally selective manner, typically producing myopia in the treated hemifields. In particular, six of the NFD monkeys exhibited substantial amounts (-1.81 to -9.00 D) of relative myopia in the nasal field that were most obvious at the 15 degrees and 30 degrees nasal field eccentricities. The other three NFD monkeys exhibited small amounts of relative hyperopia in the treated field. The alterations in peripheral refraction were associated with local, region-specific alterations in vitreous chamber depth in the treated hemiretina.

CONCLUSIONS

The effects of form deprivation on refractive development and eye growth in primates are mediated by mechanisms, presumably retinal, that integrate visual signals in a spatially restricted manner and exert their influence locally.

Femtosecond lasers for LASIK flap creation: a report by the American Academy of Ophthalmology

OBJECTIVE

To review the published literature to assess the safety, efficacy, and predictability of femtosecond lasers for the creation of corneal flaps for LASIK; to assess the reported outcomes of LASIK when femtosecond lasers are used to create corneal flaps; and to compare the differences in outcomes between femtosecond lasers and mechanical microkeratomes.

METHODS

Literature searches of the PubMed and Cochrane Library databases were last conducted on October 12, 2011, without language or date limitations. The searches retrieved a total of 636 references. Of these, panel members selected 58 articles that they considered to be of high or medium clinical relevance, and the panel methodologist rated each article according to the strength of evidence. Four studies were rated as level I evidence, 14 studies were rated as level II evidence, and the remaining studies were rated as level III evidence.

RESULTS

The majority of published studies evaluated a single laser platform. Flap reproducibility varied by device and the generation of the device. Standard deviations in flap thicknesses ranged from 4 to 18.4 μm. Visual acuities and complications reported with LASIK flaps created using femtosecond lasers are within Food and Drug Administration safety and efficacy limits. Of all complications, diffuse lamellar keratitis is the most common after surgery but is generally mild and self-limited. Corneal sensation was reported to normalize by 1 year after surgery. Unique complications of femtosecond lasers included transient light-sensitivity syndrome, rainbow glare, opaque bubble layer, epithelial breakthrough of gas bubbles, and gas bubbles within the anterior chamber.

CONCLUSIONS

Available evidence (levels I and II) indicates that femtosecond lasers are efficacious devices for creating LASIK flaps, with accompanying good visual results. Overall, femtosecond lasers were found to be as good as or better than mechanical microkeratomes for creating LASIK flaps. There are unique complications that can occur with femtosecond lasers, and long-term follow-up is needed to evaluate the technology fully.

Measurement of Angle Kappa With Synoptophore and Orbscan II in a Normal Population

PURPOSE

To obtain normative values of angle kappa in a normal population by synoptophore and Orbscan II and to compare the reliability of these devices.

METHODS

Three hundred consecutive healthy individuals were enrolled in the study. A complete orthoptic and ophthalmologic examination was performed. Synoptophore and Orbscan II corneal topography were used to measure angle kappa. To evaluate the association of the angle kappa and refraction measures, individuals were further classified according to the degree of myopia and hyperopia. The spherical equivalent error measures were grouped into six categories: > or = -3.00 diopters (D); -2.75 to -1.50 D; -1.25 to -0.50 D; +0.50 to +1.25 D; +1.50 to +2.75 D; and > or = +3.00 D. Paired t test and Pearson's correlation test were used for statistical analysis.

RESULTS

The mean age of the individuals was 28.74 +/- 1.63 years (range: 20 to 40 years). The angle kappa values obtained by synoptophore and Orbscan II were normally distributed. In the myopic group, angle kappa values decreased significantly towards negative refractive errors. In contrast, a correlation existed between large positive angles and positive refractive errors in the hyperopic group. Angle kappa values obtained by Orbscan II were significantly higher in all groups when compared to synoptophore (P < .0001). A significant correlation was noted between synoptophore and Orbscan II measurements (r = 0.932, P < .0001).

CONCLUSIONS

A significant correlation exists between positive refractive errors and large positive angle kappa values. Refractive surgeons must take into account angle kappa, especially in hyperopic patients, to avoid complications related to decentration of the ablation zone.

Mathematical model of corneal surface smoothing after laser refractive surgery

PURPOSE

To construct a quantitative model of corneal surface smoothing after laser ablation for refractive correction.

DESIGN

Experimental study, interventional case series, and meta-analysis of literature.

METHODS

A theory of epithelial smoothing in response to corneal contour change is derived from differential equations that describe epithelial migration, growth, and loss. Computer simulations calculate the effects on postoperative epithelial thickness, topography, refraction, and spherical aberration. Model parameter is matched with laser in situ keratomileusis (LASIK) outcome in literature and in a retrospective study of primary spherical myopic (77 eyes) and hyperopic (19 eyes) corrections. Surgically induced refractive change was the main outcome measure.

RESULTS

Simulated epithelial remodeling after myopic ablation produces central epithelial thickening, reduction in achieved correction, and induction of oblate spherical aberration. Simulation of hyperopic ablation shows peripheral epithelial thickening, a larger reduction in correction, and induction of prolate spherical aberration. Simulation using a minus cylinder laser ablation pattern shows decreased astigmatism correction and increased hyperopic shift. In the LASIK series, linear regression of achieved correction vs ablation setting in hyperopic and minus cylinder corrections shows slopes of 0.97, 0.71, and 0.74, respectively. These clinical results match model predictions when the smoothing constant is set at 0.32, 0.63, and 0.55 mm, respectively.

CONCLUSIONS

Epithelial thickness modulations after ablation can be modeled mathematically to explain clinically observed regression and induction of aberration. The cornea appears to smooth over ablated features smaller than approximately 0.5 mm. The model provides an approach for designing ablation patterns that precompensate for the smoothing to improve final outcome.

5-year follow-up of LASIK for hyperopia

PURPOSE

To assess the long-term efficacy and stability of LASIK for hyperopia (+0.75 to +7.00 diopters [D]).

DESIGN

Retrospective follow-up study of a previous phase III multicenter clinical trial (unpublished data).

PARTICIPANTS

Patients who had been treated for hyperopia (33 individuals, 47 eyes) attended follow-up 5 years after surgery. The preoperative mean spherical equivalent at the spectacle plane was +3.58 D (range, +0.75 to 7.00 D), and the attempted mean spherical correction at the corneal plane was +3.18 D (range, +1.00 to +6.00 D).

INTERVENTION

Treatments were performed using a Moria LSK One microkeratome and a Summit Technology SVS Apex Plus excimer laser fitted with an Axicon.

MAIN OUTCOME MEASURES

Manifest refraction, uncorrected visual acuity, best spectacle-corrected visual acuity, corneal transparency, complications, and patient satisfaction were recorded.

RESULTS

At 5 years, for treatments between +1.00 to +3.00 D, 71.0% of eyes were within +/-1.00 D of the intended correction, and for treatments between +3.5 to +6.0 D, 37.5% of eyes were within +/-1.00 D of intended correction. From 12 to 54 months after surgery for all patients, there was a hyperopic shift of +0.53 D (range, -0.13 to +3.13 D), with 51.1% of eyes experiencing an increase of +0.50 D or more and 27.7% of eyes showing a hyperopic shift of more than +1.00 D. This hyperopic shift was +0.67 D (range, 0 to +1.125 D) for patients younger than 40 years of age and +0.44 D (range, -1.33 to +1.50 D) for patients between 43 and 55 years of age.

CONCLUSIONS

LASIK was moderately effective for the correction of low degrees of hyperopia. However, there was regression throughout the 5-year follow-up that was greater than would be expected as a result of aging. Long-term stability of hyperopic LASIK refractive corrections, therefore, is uncertain.

The prevalence of refractive errors among schoolchildren in Dezful, Iran

AIM

To determine the prevalence of refractive errors among schoolchildren in urban and rural areas of Dezful County, Iran.

METHODS

In a cross-sectional study, using random cluster sampling, 5721 Dezful schoolchildren were selected from 39 clusters. The participants in the study totalled 5544; 3673 elementary and middle school students and 1871 high school students. For the former group, cycloplegic refraction and for the latter, non-cycloplegic refraction was tested. In all participants, uncorrected visual acuity and best corrected visual acuity were determined, and those with a visual acuity of 20/40 or worse, underwent a complete ophthalmic examination to determine the cause of visual impairment. A spherical equivalent of -0.5 diopter (D) or worse was defined as myopia, +2.0 D or more was defined as hyperopia, and a cylinder refraction greater than 0.75 D was considered astigmatism.

RESULTS

The uncorrected visual acuity was 20/40 or worse in the better eye of 224 schoolchildren (3.8% of participants). This figure (percentage) was 14 (0.03%) based on their best corrected visual acuity and 96 (1.7%) with their presenting vision. According to results of cycloplegic refraction, 3.4% (95% confidence interval (CI), 2.5 to 4.4) of the primary and middle school students were myopic and 16.6% (95% CI, 13.6 to 19.7) were hyperopic. For high school students, these rates were 2.1% (95% CI, 0.7 to 3.5) and 33.0% (95% CI, 24.9 to 41.1), respectively, with non-cycloplegic refraction. In the multivariate logistic regression for primary and middle school students, myopia was correlated with age (p = 0.030), and hyperopia was correlated with age (p<0.001) and area of residence (p = 0.007). In high school students, hyperopia again showed a correlation with their area of residence (p = 0.029).

CONCLUSION

The present study reveals the considerable prevalence rates of refractive errors among schoolchildren in Dezful County and the high rate of an unmet need for their correction. Although myopia is not very prevalent, the high rate of hyperopia in the studied population emphasises its need for attention.

Neural pathways subserving negative lens-induced emmetropization in chicks--insights from selective lesions of the optic nerve and ciliary nerve

PURPOSE

Active emmetropization describes the process by which young eyes regulate their growth to eliminate refractive errors. The purpose of this study was to re-investigate the role of the brain in compensation to imposed hyperopic defocus (negative lenses), specifically, to assess whether a retina-brain link and/or an intact ciliary nerve are required for this emmetropizing response. Data from previous related studies are equivocal.

METHODS

Unilateral lesion surgery involving either or both optic nerve section (ONS) and ciliary nerve section (CNS), was performed on 2-3 day old White-Leghorn chicks to interrupt communication between the eye (retina in the case of ONS) and brain. After a recovery period of 4 days, lesioned eyes were fitted with either -5 or -15 D lenses or diffusers (6-9 per group). An additional lesion group underwent unilateral CNS and was fitted with -5 D lenses bilaterally. Finally 3 groups that underwent the same unilateral optical treatments but no surgery were included as controls for analyzing lesion-induced changes. Complete sets of measurements, involving retinoscopy for refractive errors, and high frequency A-scan ultrasonography for axial ocular dimensions, were made at the beginning (baseline), and end of a 4 day treatment period. Additional ultrasonography data were collected after 1 and 2 days of treatment. Optical treatment effects were expressed as changes in interocular differences from baseline values.

RESULTS

All three lesions produced hyperopic shifts in refraction (evident in baseline values), although this effect was minimal for the ONS+CNS group. Choroidal thickening as well as increased anterior chamber depth and lens thinning were observed in all cases but vitreous chamber depth was reduced in only the ONS group. In response to the -5 D lens, the control (nonlesioned) group showed nearly complete compensation, while full compensation was not achieved to the -15 D lens over this short treatment period. The diffuser group showed the largest change, which was also in the direction of myopia. Both the ONS and CNS groups showed near normal compensation, as indexed by the changes in refractive errors relative to their respective baseline values. In contrast, the ONS+CNS lens groups overcompensated, by 130% and 54% for the -5 D and the -15 D lens groups respectively. Form deprivation responses were slightly exaggerated in both ONS and ONS+CNS groups, the latter group again showing the largest response. Enhanced vitreous chamber growth was evident under all conditions and correlated well with the refractive changes across the groups.

DISCUSSION

The data imply that an intact retina-brain link is not required for compensation to hyperopic defocus and thus emmetropization. However, the data also imply interactions between higher centers and the eye. The emmetropization set-point appears to be recalibrated after ONS surgery. The data also indicate a role of the ciliary nerve as an important conduit for signals that exercise a restraining influence on eye growth.

Changes in refraction between the ages of 1 and 3 1/2 years

A study has been made of the changes in refraction as a sample of 148 children grew between the ages of 1 and 3 1/2 years. There was no decrease in hypermetropia, but there was a significant decrease in the incidence of astigmatism. Study of the changes in the refraction in the horizontal and vertical meridia of individual eyes gave clear evidence of a trend towards emmetropia if the initial refraction in either meridian was myopic or less than +2.50 D. Above that level the refraction became more or less hypermetropic.

Comparison of Corneal Higher-Order Aberrations Induced by Myopic and Hyperopic LASIK

OBJECTIVE

To compare the change in anterior corneal higher-order (third- to fifth-order) aberrations (HOAs) induced by myopic and hyperopic LASIK.

DESIGN

Retrospective comparative case series.

PARTICIPANTS

One hundred eyes (50 myopes and 50 hyperopes) of 59 patients were included. The mean preoperative spherical equivalent (SE) was -4.22+/-1.78 diopters (D; range, -1.25 to -8.00 D) in the myopic group (group A) and +2.72+/-1.25 D (range, +0.25 to +5.00 D) in the hyperopic group (group B).

INTERVENTION

LASIK was performed using a conventional spherocylindrical laser algorithm (Planoscan V2.9992, Bausch & Lomb/Technolas, Munich, Germany). Optical zone diameter was 6.70+/-0.32 mm (range, 6-7 mm) in group A and 6.59+/-0.19 mm (range, 6.5 to 7 mm) in group B. Third to fifth corneal HOA were computed for a pupil diameter of 6 mm from corneal topographic examinations before and 1 month after surgery.

MAIN OUTCOME MEASURES

Change in corneal HOAs, derived from corneal topography.

RESULTS

Total HOA root mean square (RMS) changed in group A by 0.167+/-0.180 microm (factor 1.53) and in group B by 0.341+/-0.341 microm (factor 1.89). The mean induction of coma RMS was significantly different in both groups (myopes, 0.092+/-0.195 microm; hyperopes, 0.252+/-0.305 microm; P<0.05). For spherical aberration (Z 4,0), the myopic group showed a significant increase (0.130+/-0.120 microm; factor 1.6; P<0.001), whereas the hyperopic group showed a significant decrease (-0.317+/-0.158 microm; factor 0.76; P<0.001). Fifth-order aberrations showed an increase in both groups, which was higher in group B (0.069+/-0.120 microm; factor 2.46) than in group A (0.005+/-0.065 microm; factor 1.49).

CONCLUSIONS

Myopic and hyperopic LASIK had different patterns of HOA induction. Myopic LASIK induced positive spherical aberrations and positive secondary astigmatism, whereas hyperopic LASIK induced negative spherical aberrations and negative secondary astigmatism. Hyperopic LASIK induced more third- and fifth-order comalike aberrations than myopic LASIK.