Magnification-corrected indirect biomicroscopy of the optic nerve head

BACKGROUND

Proper use of the new Super VitreoFundus and SuperPupil XL non-contact slit-lamp lenses for clinical disc biometry requires knowledge of their comparative magnification.

METHODS

The optical performance of each fundus lens is described in terms of axial magnification, lens position, and the correction factor p (in degrees per millimeter) using a calibrated Gullstrand-type model eye adjusted for axial ametropia between -12.5 D and +12.6 D.

RESULTS

The total change in axial magnification from myopia to hyperopia was -13.3% to +15.6% (Super VitreoFundus lens), and -13.9% to +14.1% (SuperPupil XL lens). When the fundus lens position was altered with respect to the model eye by +/-2 mm under myopic conditions the change in axial magnification was -5.1% to +7.7% (Super VitreoFundus lens), and -6.5% to +9.7% (SuperPupil XL lens). In the hyperopic condition the change was -3.8% to +5.8%, and -4.9% to +7.3%. Both fundus lenses exhibited a linear relationship between p and degree of ametropia of the model eye, while only the Super VitreoFundus lens displayed a constant relationship between p and ametropia of -5 D to +5 D.

CONCLUSIONS

Using the fundus lens correction factor (p), the clinician may be able to estimate the dimensions of optic nerve head features with sufficient accuracy to allow clinical decisions to be made in the evaluation of patients with diagnosed or suspected glaucoma.

Scanning laser polarimetry in myopic and hyperopic subjects

PURPOSE

To investigate the effect of refraction error and axial length on retinal nerve fiber layer (RNFL) measurements as obtained by scanning laser polarimetry (SLP).

METHODS

Besides ophthalmological standard examination (refractive error, keratometry, visual acuity, slit-lamp examination, applanation tonometry, funduscopy), perimetry, axial length measurement by means of ultrasound, and SLP were performed. Seventy-five myopic eyes (between -0.75 D and -8.5 D), 24 hyperopic eyes (0.75 D-6.5 D) and 40 emmetropic eyes were investigated. SLP parameters were compared in the different groups.

RESULTS

The statistical analysis of the absolute thickness values of SLP revealed highly significant ( P< 0.01) reductions in average thickness, ellipse average, superior average, inferior average, and superior integral in both myopic and hyperopic eyes in comparison with the emmetropic control eyes. The amount of reduction was between 12.9% (inferior average; myopia) and 30.2% (superior integral; hyperopia). There were no significant differences between myopes and hyperopes. A significant linear correlation for many of the SLP parameters with the refractive error (spherical equivalent) but not with axial length was found in both the hyperopic and the myopic group.

CONCLUSIONS

Despite a wide interindividual range, SLP measurement values decrease with increasing myopia and hyperopia. In clinical practice, such reduced RNFL thickness values should be viewed with the necessary caution and additional polarimetric signs for glaucomatous damage should be taken into consideration.

The relationship between intraocular pressure and refractive error adjusting for age and central corneal thickness

PURPOSE

To investigate the relationship between intraocular pressure (IOP) and refractive errors after adjusting for age, central corneal thickness (CCT), and other related factors.

METHODS

IOP, CCT and refractive errors were measured in the right eyes of 1855 subjects, aged 40-82 years, in a cross-sectional study design. Subjects were divided into groups by refractive status: hyperopia, emmetropia, mild myopia, moderate myopia, or high myopia. With adjustments for age, CCT, blood pressure, obesity, education, hypertension, diabetes, and smoking status, IOP was estimated for each refractive status using a general linear model.

RESULTS

IOP increased with advancing degrees of myopia, even after adjustment for age, CCT, and other related factors (p = 0.011). Estimated IOP of moderate myopia was significantly higher than that of emmetropia (p = 0.022).

CONCLUSIONS

Our results confirm the positive association between IOP and increasing degrees of myopia. This finding would support the hypothesis that the relationship between glaucoma and myopia might be pressure mediated.

Ocular shape and myopia

INTRODUCTION

To learn if eye shape might be a useful parameter in refractive research.

MATERIALS AND METHODS

Laboratory research on eye growth mechanisms is summarised. The available clinical literature relating refraction to eye shape and peripheral refraction is critically assessed in the context of the laboratory research on refractive development.

RESULTS

Almost all refraction research assesses optical and length parameters exclusively along the visual axis. Contemporary laboratory research demonstrates a remarkable phylogenic conservation of the neural mechanisms regulating refractive development. On-axis image quality regulates central refractive development in animals and probably, to some extent, in humans. Off-axis image quality at the retina depends on anterior segment geometry and optics, and on the 3-dimensional conformation of the retina. In chicks, eye shape is a predictable parameter linked to the underlying neural mechanisms modulating eye development. Based on the sparse clinical literature in human adults and children, the eye shapes induced in chicks are also seen in human subjects in patterns suggesting that eye shape may be a useful parameter in clinical studies.

CONCLUSION

The diverse findings suggest that incorporating the 3-dimensional conformation of the eye into future clinical studies may help resolve many of the ambiguities in contemporary refractive research.

The effect of central corneal thickness on estimates of the anterior chamber depth

BACKGROUND

The slitlamp can be used to estimate the anterior chamber depth (ACD). The length of a slit object is increased until the corneal and iris/lens images appear to just touch. Multiplying the just-touching-slit-length (JTSL) by a conversion factor gives an estimate of the ACD as measured by ultrasonography. The purpose of this study was to determine if central corneal thickness (CCT) affects the accuracy of this technique.

METHODS

The ACD of 50 subjects was measured by A-scan ultrasonography and estimated by the slitlamp technique. CCT was measured by ultrasonic pachometry. The refractive error was determined subjectively.

RESULTS

The average ultrasonographic ACD for all subjects was 3.32 +/- 0.65 mm. The average JTSL was 2.46 +/- 0.38 mm. The conversion ratio between the ultrasonographic ACD and the average JTSL was 1.35. The predicted ACD using the regression equation of JTSL on the ultrasound anterior chamber depth (USACD) was 3.32 +/- 0.54 mm. The corresponding value using the regression equation of JTSL and CCT on USACD was exactly the same, that is, 3.32 +/- 0.54 mm.

CONCLUSION

Incorporation of CCT into a regression equation does not improve the accuracy of the Smith technique.

Anterior chamber depth in relation to refractive status measured with the Orbscan II Topography System

PURPOSE

To evaluate the anterior chamber depth (ACD) according to refractive status, assess the reliability of repeated ACD measurements using the Orbscan II Topography System (Bausch and Lomb), compare Orbscan II and IOLMaster (Carl Zeiss Meditec AG) ACD measurements, and investigate the correlation between refraction, axial length (AL), and ACD.

SETTING

Department of Ophthalmology, Ruprecht-Karls-University of Heidelberg, Heidelberg, Germany.

METHODS

In this clinical study, 60 patients with a mean age of 43.8 years +/- 18.74 (SD) were assigned to 1 of 3 groups of 20 patients each according to refraction: emmetropia group; hyperopia group (mean +4.84 +/- 1.60 diopters [D]); myopia group (mean -9.64 +/- 3.79 D). Using the Orbscan II system, 3 consecutive ACD measurements (apex and 3.0 mm zone) were performed. The IOLMaster was used to measure ACD and AL.

RESULTS

The mean ACD (from epithelium) with the Orbscan II and IOLMaster, respectively, was 3.61 +/- 0.24 mm and 3.61 +/- 0.24 mm in the emmetropia group, 3.03 +/- 0.21 mm and 3.06 +/- 0.24 mm in the hyperopia group, and 3.72 +/- 0.26 mm and 3.73 +/- 0.23 mm in the myopia group. The standard deviation of the repeated Orbscan II measurements increased from 13 to 15 microm from the apex to the 3.0 mm zone. The difference between the apex and 3.0 mm zone of the cornea in all groups ranged from 0.1 to 0.12 mm. The mean AL was 23.52 +/- 0.82 mm in the emmetropia group, 22.14 +/- 0.64 mm in the hyperopia group, and 27.44 +/- 1.67 mm in the myopia group. There was a significant correlation between the spherical equivalent and AL (r = 0.94).

CONCLUSIONS

Significantly lower ACD values were found in the hyperopia group than in the other 2 groups. There was no difference in ACD between the emmetropia and myopia groups even though the AL in the myopia group was 4.0 mm longer. No statistical difference in ACD measurements was found between the Orbscan II and IOLMaster.

The relationship between the Stiles-Crawford effect of the first kind (SCE-I) and myopia

The Stiles-Crawford effect of the first kind (SCE-I) was measured on both emmetropic and myopic subjects at six different retinal locations. The results revealed a number of significant discrepancies in receptor alignment between the groups of different refractive errors. In myopic subjects, the receptors in the nasal retina (i.e. between the fovea and the optic nerve head) were found to be aligned nasally towards the optic nerve head, whereas the receptors in the temporal retina were aligned towards the centre of the exit pupil. In emmetropic subjects, the receptors across the retina were finely tuned towards the centre of the exit pupil. The magnitude of the receptor displacement in myopic subjects was found to be directly associated with the length of the eyeball.

Laser in situ Keratomileusis for Hyperopia and Hyperopic and Mixed Astigmatism With LADARVision Using 7 to 10-mm Ablation Diameters

PURPOSE

To evaluate the results of laser in situ keratomileusis (LASIK) performed to correct hyperopia, and hyperopic and mixed astigmatism using wider ablation diameters (optical zone diameter and overall ablation diameter) than those commonly used with the same and other lasers.

METHODS

After flap creation using an Alcon SKBM microkeratome set for a 10-mm flap diameter, 53 eyes (33 patients) with a mean spheroequivalent attempted correction of +2.34 +/- 2.09 D underwent LASIK (Alcon LADARVision 4000) using a 7-mm optical zone diameter and a 3-mm transition zone for an overall 10-mm total ablation diameter. The nasal hinge was prevented from undesired ablation by the use of proprietary hinge protector software. Eyes were followed for 6 months after surgery.

RESULTS

Six months after surgery, mean spheical equivalent refractive error was -0.22 +/- 0.41 D. There were 79.2% of eyes within +/- 0.50 D, and 98.1% within +/- 1.00 D of intended correction. Uncorrected visual acuity of 20/20 or better was achieved by 28 eyes (53%) and 20/40 or better by 50 eyes (94.3%). No meaningful visual complaints during nighttime hours, such as haloes or glare, were subjectively reported by patients.

CONCLUSION

The use of larger ablation diameters in LASIK for hyperopia, and hyperopic and mixed astigmatism produced accurate results, early refractive stability, and good visual performance.

Two-year follow-up of noncontact holmium laser thermokeratoplasty for the correction of low hyperopia

PURPOSE

To summarize the 2-year results of laser thermal keratoplasty with a holmium:yttrium-aluminum-garnet (Ho:YAG) laser and the Sunrise Corneal Shaping System and assess the procedure's safety, efficacy and predictability in correcting hyperopia in a phase III clinical intervention case series.

METHODS

The Ho:YAG laser was used to correct low hyperopia (manifest spherical equivalent of +0.75 to +2.50 dioptres [D], with manifest cylinder of 1.00 D or less) in 38 eyes of 28 patients 40 years of age or older. Laser pulses were delivered to the cornea in 2 radially placed, concentric, 8-spot rings 6.0 and 7.0 mm in diameter. The last follow-up was at 2 years.

RESULTS

The preoperative uncorrected visual acuity (UCVA) at distance was less than 20/40 in 82% of the eyes and at near was less than 20/32 in 42%. At 2 years the distance UCVA was 20/40 or better in 100% of the eyes and 20/20 or better in 84%, and the near UCVA was 20/32 or better in 97% of the eyes and 20/20 or better in 8%. The difference between the preoperative and postoperative UCVA was statistically significant (p < 0.01). The correction at 2 years was within 0.50 D of that intended in 92% of the eyes and within 1.00 D in 100%. The reduction in mean spherical equivalent at 2 years was statistically significant (p < 0.0001). No loss of 2 lines or more in best-corrected visual acuity occurred after the 1st month of follow-up.

INTERPRETATION

The Ho:YAG Sunrise Corneal Shaping System is safe and effective for the treatment of low hyperopia and astigmatism. It provides a predictable refractive outcome at 2 years of follow-up. However, our results show a tendency towards regression by 2 years.

Variability of retinal steepness at the posterior pole in children 7-15 years of age

PURPOSE

Considerable evidence suggests that both axial and peripheral refraction play important roles in eye growth control. The large variability in peripheral refraction seen in adults and children indicates that the peripheral retina is exposed to a wide range of refractive errors. The current lack of appropriate measurement techniques has hampered the determination of whether variability in peripheral refraction between individuals can be correlated with variability in retinal steepness. An Optical Low Coherence Reflectometer (OLCR) was developed to determine retinal steepness.

METHODS

Retinal steepness was assessed in right eyes of 63 children 7-15 years of age by measuring eye length (EL) and spherical equivalent refraction (SER) axially and at 15 degrees temporally, nasally, inferiorly and superiorly with OLCR and Binocular Auto-Refractometry, respectively, during cycloplegia. At each peripheral location, relative peripheral EL and SER (i.e., the difference between peripheral and axial readings) were compared between myopic, emmetropic and hyperopic eyes, and the correlation between relative peripheral EL and SER was analyzed.

RESULTS

Although the standard deviations were large, significant differences in relative peripheral EL and SER between refractive groups as well as a significant correlation between relative peripheral EL and SER were observed at several of the assessed locations. CONCLUSIONS. The results strongly suggest that peripheral refraction is correlated with retinal steepness and that previously observed variability in peripheral refraction chiefly reflects variability in retinal steepness. If peripheral refraction represents a determining parameter in the control of eye growth, the precise measurement of retinal steepness could be used not only to improve estimates of myopic progression, but also to identify children who are at high risk of developing myopia. It may lead to specialized clinical/optical treatments, e.g. the correction of not only axial but also peripheral refractive errors, which are more effective than current treatments in individuals who are at risk of myopia development or progression.

[Correlation between refraction and ocular biometry]

PURPOSE

To study the correlation between subjective refraction and biometry obtained by Orbscan and echography in normal eyes. To compare biometric parameters with the subjective spherical equivalent.

MATERIAL AND METHODS

Subjective refraction, biometric parameters using Orbscan, and echography were recorded in 190 normal eyes (including eyes with ametropia) of 95 patients. Biometric parameters (i.e., corneal diameter, anterior chamber depth, central pachymetry, iridocorneal angle, corneal and lens radii of curvature, and axial length of the eye) were compared in different refractive groups and were correlated with the subjective refraction.

RESULTS

Corneal biometric parameters did not correlate with the subjective spherical equivalent and showed no differences between the refractive groups except for the central pachymetry. In the high myopic group (<-6D), the central cornea was significantly thinner (531 micro m versus 549 micro m, p=0.016). The correlation between corneal radius and axial length was strong in emmetropic eyes (r(s)=0.63, p<0.001) and poor but significant in ametropic eyes (r(s)=0.28, p=0.002). Axial length, anterior chamber depth, and iridocorneal angle showed significant differences between the refractive groups (p<0.001) and correlated with the subjective spherical equivalent (r(s[[/INF=0.44, p<0.001). Subjective spherical equivalent showed the strongest correlation with the axial length (rs)=0.82, p<0.001). Subjective spherical equivalent, central corneal thickness, axial length, anterior chamber depth, and anterior corneal radius showed a strong correlation between both eyes (rs[[/INF=0.94, p<0.001).

CONCLUSION

Biometric characteristics of the eye (excluding cornea characteristics) vary with subjective spherical equivalent. Axial length presents the strongest correlation with the subjective spherical equivalent and correlates with the other ocular biometric parameters. Axial length plays a major role in the ocular biometry and refraction.

Quantification of chick lens alphaA- and delta-crystallins in experimentally induced ametropia

PURPOSE

The role of the lens in experimentally induced ametropia is not known. A recent study of the chick lens demonstrated optical quality deterioration with the induction of refractive errors, without alteration in lens morphology, size or shape. A change in lens gradient of refractive index (which is dependent on alpha-, beta-, and delta-crystallin concentration and arrangement), could underlie this observation. The purpose of this work was to quantify the concentrations of alphaA- and delta-crystallin in lenses from chick eyes with induced high myopia or hyperopia.

METHODS

White Leghorn chicks were unilaterally fitted on the day of hatching either with translucent plastic goggles to induce form-deprivation myopia (n=21) or with +15 D defocus goggles to induce hyperopia (n=14). The ungoggled contralateral eyes were used as controls. The chicks were refracted twice, once on the day of hatching and again seven days later, using streak retinoscopy. On day 7 chicks were sacrificed, lenses decapsulated, and soluble proteins were isolated. Western blot assays were optimized and used to assess crystallin concentration.

RESULTS

Analysis revealed no significant difference in alphaA- or delta-crystallin concentration in lenses from eyes induced with form-deprivation myopia and hyperopia as compared to their respective control eyes. Analysis of the difference in medians of delta-crystallin between the control and treated groups of the myopia and hyperopia experiments revealed significance (p=0.030).

CONCLUSIONS

This study suggests that with the induction of ametropia, the increased lens spherical aberration previously noted is not due to a change in the absolute concentration of lens alphaA- or delta-crystallin. However, results suggest that the myopic and hyperopic treatments had different effects on lens delta-crystallin concentration. Further investigation is necessary to expand the current knowledge of the role played by the lens in experimental ametropia.

Evidence for a potential role of glucagon during eye growth regulation in chicks

Eye growth and refraction are regulated by visual processing in the retina. Until now, the messengers released by the retina to induce these changes are largely unknown. Previously, it was found that glucagon amacrine cells respond to defocus in the retinal image and even to its sign. The expression of the immediate-early gene product ZENK increased in this cell population in eyes wearing plus lenses and decreased in minus lens-treated chicks. Moreover, it was shown that the amount of retinal glucagon mRNA increased during treatment with positive lenses. Therefore, it seems likely that these cells contribute to the visual regulation of ocular growth and that glucagon may act as a stop signal for eye growth. The purpose of the present study was to accumulate further evidence for a role of glucagon in the visual control of eye growth. Chicks were treated with plus and minus lenses after injection of different amounts of the glucagon antagonist des-His1-Glu1-glucagon-amide or the agonist Lys17,18,Glu21-glucagon, respectively. Refractive development and eye growth were recorded by automated infrared photorefraction and A-scan ultrasound, respectively. The glucagon antagonist inhibited hyperopia development, albeit only in a narrow concentration range, and at most by 50%, but not myopia development. In contrast, the agonist inhibited myopia development in a dose-dependent fashion. At high concentrations, it also prevented hyperopia development. The amount of glucagon peptide in the retinae and choroids of lens-treated chicks and its diurnal variation was measured by using a radio-immunoassay. Retinal glucagon content decreased after minus lens treatment and choroidal glucagon content increased after plus lens treatment. No diurnal variation in the retinal amount of glucagon was detected. In addition, using an optokinetic nystagmus paradigm, the effect of glucagon and the antagonist des-His1-Glu9-glucagon-amide on suprathreshold contrast sensitivity was studied. Glucagon reduced contrast sensitivity (which might be linked to a signal for growth inhibition) whereas the antagonist des-His1-Glu9-glucagon-amide increased contrast sensitivity. The results of the study are in line with the hypothesis that glucagon plays a role in the visual control of eye growth in the chick.

Corneal asphericity after hyperopic laser in situ keratomileusis

PURPOSE

To analyze corneal asphericity after hyperopic laser in situ keratomileusis (LASIK) and its relationship to the clinical outcomes.

SETTING

Corneal and Refractive Surgery Service, Massachusetts Eye and Ear Infirmary, Boston, Massachusetts, USA.

METHODS

In a retrospective case series, 23 patients (33 eyes) with hyperopia or hyperopic astigmatism who had LASIK were evaluated. A computer program (Holladay Diagnostic Summary, EyeSys Laboratories) was used to analyze corneal asphericity (Q) before and after LASIK. Corneal asphericity was evaluated to determine the association with the postoperative refractive error, best spectacle-corrected visual acuity (BSCVA), uncorrected visual acuity (UCVA), achieved refractive correction, mean corneal power (K), refractive yield (achieved/attempted correction), and keratometric yield (change in keratometry/attempted correction).

RESULTS

After hyperopic LASIK, all corneas exhibited increased negative central Q. The postoperative corneal radius of curvature, BSCVA, and refractive and keratometric yields were not significantly correlated with the preoperative Q values. The asphericity change, Delta Q, was highly correlated with the achieved correction (r = 0.747, P <.0001). The postoperative Q value correlated well with the preoperative value (r = 0.534, P <.05) and the achieved correction (r = 0.601, P <.05) but not with the Delta Q. Neither the postoperative Q nor the Delta Q was correlated with the spherical equivalent, K, BSCVA, or UCVA.

CONCLUSIONS

Asphericity may be a useful quantitative descriptor of the corneal optical contour after hyperopic LASIK. Negative central Q increased after hyperopic LASIK, especially when greater degrees of refractive correction were attempted.

Factors that influence outcomes of hyperopic laser in situ keratomileusis

PURPOSE

To analyze the influence of preoperative corneal curvature, postoperative keratometric power, and the amount of correction on the outcomes of hyperopic laser in situ keratomileusis (LASIK).

SETTING

Clínica Baviera, Instituto Oftalmológico Europeo, Madrid, Spain.

METHODS

In this retrospective study, the records of 376 eyes that had LASIK for hyperopia using the Moria LSK-One microkeratome and the Technolas-Keracor 217C excimer laser were reviewed. The results were analyzed by preoperative hyperopia (5 subgroups) and by preoperative (more than and less than 43.0 diopters [D]) and postoperative (more than and less than 48.0 D) mean keratometry.

RESULTS

A statistically significant keratometry regression was found in the +3.00 to +3.90 D range (P <.01), a significant decrease in predictability in the +4.00 to +4.90 D range (P <.05), and a significant worsening in safety in the highest range (+6.00 to +7.90 D; P <.05). Comparative analysis of the > or = +4.00 D and <+4.00 D groups showed statistically significant differences in most measurement parameters. The preoperative keratometry did not influence postoperative results with the exception of poorer predictability in the group of preoperative flat corneas in which a high degree of hyperopia was corrected; ie, spherical equivalents within +/-0.50 D were found in 40.4% and 61.0% of cases with flat and steep corneas, respectively (P <.05). The efficacy and safety in eyes that achieved a postoperative keratometry >48.00 D did not differ significantly from the efficacy and safety in eyes that had a lower final keratometric power.

CONCLUSIONS

The factor that negatively influenced the outcome of hyperopic LASIK was the degree of hyperopia corrected. Preoperative keratometry did not significantly influence the postoperative results, and postoperative keratometry >48 D did not result in significant worsening of visual results when the attempted correction was less than +4.00 D.

Echobiometric study of ocular growth in patients with amblyopia

BACKGROUND

The natural evolution toward emmetropia is governed by genetic factors, but experiments with animals have demonstrated that a feedback mechanism that depends on visual function may regulate ocular growth. We attempted to verify this hypothesis in school-age patients with amblyopia by observing the ocular growth and the refractive state in the fixating and nonfixating eye in myopia and hypermetropia. This situation simulates the animal model.

METHODS

In 30 consecutive school-age patients with amblyopia, we prospectively evaluated axial length with echobiometry and the refractive state during cycloplegia induced by atropine.

RESULTS

We found a statistically significant increase in axial length in the fixating eye of patients with hypermetropia compared with the amblyopic eye (P = .0008). In patients with myopia, we found less of an increase in axial length in the fixating eye compared with the amblyopic eye (P = .0048).

CONCLUSION

Good vision seems to influence the evolution of ocular growth toward emmetropization.

Volume estimation of excimer laser tissue ablation for correction of spherical myopia and hyperopia

PURPOSE

To determine the theoretical volumes of ablation for the laser treatment of spherical refractive errors in myopia and hyperopia.

METHODS

The cornea was modeled as a spherical shell. The ablation profiles for myopia and hyperopia were based on an established paraxial formula. The theoretical volumes of the ablated corneal lenticules for the correction of myopia and hyperopia were calculated by two methods: (1) mathematical approximation based on a simplified geometric model and (2) finite integration. These results were then compared for optical zone diameters of 0.5 to 11.00 mm and for initial radii of curvature of 7.5, 7.8, and 8.1 mm.

RESULTS

Referring to a simplified geometrical model, the volume of ablated corneal tissue was estimated to be proportional to the magnitude of treatment (D) and to the fourth power of the treatment diameter (S(4)). For refractive correction of myopia and hyperopia, volume estimations using our formula, V congruent with D. (S/9)(4), were similar to those obtained by finite integration for optical zone diameters of 0.5 to 8.5 mm and for corneal radii of curvature within the clinical range (7.5, 7.8, and 8.1 mm).

CONCLUSIONS

The theoretical volume of corneal tissue ablated within the optical zone for spherical corrections can be accurately approximated by this simplified formula. This may be helpful in evaluating factors that contribute to corneal ectasia after LASIK for myopia and hyperopia. Treatment diameter (S) is the most important determinant of the volume of tissue ablation during excimer laser surgery.

Comparison of Placido-based, Rasterstereography, and Slit-scan Corneal Topography Systems

PURPOSE

Elevation-based topography systems have received growing recognition. We compared a Placido-based (EyeSys), a rasterstereography (PAR), and a slit-scan system (Orbscan) for human subjects.

METHODS

Measurements were obtained from 221 eyes of 119 human subjects. We made statistical comparisons central curvature, keratometric curvatures, and meridians between the three systems. We also compared the optical pachymetry of the slit-scan system with an ultrasonic device. We analyzed cases of subclinical keratoconus, central island, photorefractive keratectomy for myopia, and hyperopia correction with arcuate keratotomy.

RESULTS

The statistical comparison showed better correlation between the Placido-based and the rasterstereography systems in measurement of central curvature (R=0.95). The slit-scan system showed better correlation with the Placido system in the preoperative and postoperative radial keratotomy group (R=0.73) than in the group that included postoperative PRK eyes (R=0.69). Similar results were obtained for keratometric curvatures and meridians. The Orbscan pachymetry correlation with ultrasonic was R=0.69 and increased when the postoperative PRK eyes were excluded (R=0.95).

CONCLUSIONS

Slit-projection topography and pachymetry seemed to be affected by hydration in the postoperative PRK group. The pachymetric map was useful in predicting the final outcome of refractive surgery and in the diagnosis of keratoconus.

Differences in time course and visual requirements of ocular responses to lenses and diffusers

PURPOSE

Myopia can be induced in chickens by having them wear either negative lenses (lens-compensation myopia [LCM]) or diffusers (form-deprivation myopia [FDM]), whereas positive lenses cause lens-compensation hyperopia (LCH). These three conditions were compared with respect to (i) their early time course and (ii) the effect of two manipulations of the lighting.

METHODS

Longitudinal changes in ocular dimensions and refractive error were measured in chicks maintained under three different conditions: (i) wearing either -15 D lenses or diffusers in a normal light/dark cycle; (ii) wearing either +15 D lenses, -15 D lenses, or diffusers with brief periods of stroboscopic lights at the beginning and end of the dark period; (iii) wearing either +6 D lenses, -6 D lenses, or diffusers with the nights interrupted by brief periods of white light. In addition, scleral and choroidal proteoglycan synthesis was measured in eyes that wore positive lenses, negative lenses, or diffusers for 3 hours followed by different periods of darkness.

RESULTS

(i) The time course of the changes in axial length over the first 72 hours was significantly faster in LCM than in FDM. Indeed, the diffusers did not begin to significantly affect the total length of the globe for 3 days, although the vitreous chamber had deepened after 9 hours, because the choroid thinned extremely rapidly (within 1 hour) with either diffusers or negative lenses. (ii) Scleral proteoglycan synthesis was higher in eyes with negative lenses than in those with diffusers at 11 hours, but the reverse was true at 27 hours. (iii) Brief periods of stroboscopic light attenuated FDM more than LCM. (iv) In contrast, interruption of the nights by brief periods of light attenuated LCM more than FDM. (v) Neither lighting manipulation affected LCH. (vi) Choroidal proteoglycan synthesis decreased similarly with 3 hours of wearing either diffusers or negative lenses.

CONCLUSIONS

Although both negative lenses and diffusers cause similar increases in the rate of ocular elongation, the responses differ in time course and in the effect of manipulations of the daily lighting. The responses to positive lenses differ from both of these.

Laser in situ keratomileusis flap margin: wound healing and complications imaged by in vivo confocal microscopy

PURPOSE

To examine the healing response of laser in situ keratomileusis flap margin in vivo.

METHODS

Forty-three eyes of 43 patients who had undergone myopic (n = 39) or hyperopic (n = 4) laser in situ keratomileusis were examined once after surgery. The flap margin was imaged by in vivo confocal microscopy at various depths, and the wound healing response, flap alignment, and complications were evaluated. Ten eyes were examined on day 3 postoperatively, 13 eyes at 1 to 2 weeks, 10 eyes at 1 to 2 months, five eyes at 3 months, and five eyes at 6 months or later.

RESULTS

At 3 days after laser in situ keratomileusis, the surface epithelium and basal epithelium appeared normal. Keratocyte activation was strongest at 1 to 2 weeks and 1 to 2 months, and an increased amount of haze was observed correspondingly. Intrastromal epithelial cells forming a plug could occasionally be perceived in the wound gape. Wound constriction was completed in most cases by 3 to 6 months or later. Good alignment was observed in 12 of 43 flaps (27.9%) and moderate and poor alignment in 17 of 43 flaps (39.5%) and 13 of 43 flaps (30.2%), respectively. Poor alignment was not associated with lamellar epithelial ingrowth. Epithelial ingrowth was associated with dense haze at the interface. Diffuse lamellar keratitis was imaged in two corneas after hyperopic laser in situ keratomileusis.

CONCLUSIONS

The laser in situ keratomileusis incision wound at the flap margin appears to heal after the sequence observed in incisional wounds in nonhuman primates. Complications, such as lamellar epithelial in growth and diffuse lamellar keratitis, were often observed, particularly after hyperopic laser in situ keratomileusis.

Anterior chamber depth measured by two methods in myopic and hyperopic phakic IOL implant

AIM

To evaluate the accuracy and reliability of the optical versus ultrasonic measurement of anterior chamber depth (ACD) in a group of patients affected by high myopia or hyperopia, before phakic IOL implant.

METHODS

18 consecutive patients (34 eyes) were enrolled in this study, and asked to undergo phakic IOL implant to correct high myopia or hyperopia. The mean age was 29.5 (SD 3.4) years, the male/female ratio was 10/8. 13 patients (24 eyes) were myopic (mean myopia -16.17 (4.39) D, range -9 to -24 D), whereas five patients (10 eyes) were hyperopic (mean hyperopia 7.4 (2.01) D, range 5.5-11 D). For each patient, ACD was evaluated comparing an optical system (Orbscan topograph system) with a standard A-scan ultrasound system. To evaluate the reliability of the two methods, the average of three optical and 10 ultrasonic consecutive measurements were considered. Statistical analysis was performed by means linear regression.

RESULTS

The average difference between optical and ultrasound values was 0.17 (0.1) mm (4.68% (2.52%)). There was a constant underestimation of the ACD with the optical system compared with the ultrasound measurement, more evident in the hyperopic eyes (5.20% (1. 95%)) than in myopic ones (4.46% (2.72%)). The reliability of measurements, as showed by the standard deviation values, was higher in the optical system (0.03 (0.02)) than in the ultrasound (0.1 (0. 05)), with a statistical difference (p<0.001). The linear regression analysis between optical and ultrasound measurements was 0.8992 (p<0. 0001).

CONCLUSIONS

The optical measurement of ACD is an accurate and reliable technique in high myopic and hyperopic eyes. It gives a small underestimation of the ACD values, but it could be preferable to the ultrasound technique, because it demonstrates more repeatability and has the advantages of a non-contact technique.

Vision-dependent changes in the choroidal thickness of macaque monkeys

PURPOSE

To determine whether changes in the eye's effective refractive state produce changes in the thickness of the choroid in infant monkeys.

METHODS

Normal developmental changes in choroidal thickness were studied in 10 normal rhesus monkeys. Hyperopia or myopia was induced by rearing 26 infant monkeys with either spectacle or diffuser lenses secured in front of one or both eyes. The treatment lenses were worn continuously beginning at approximately 3 weeks of age for an average of 120 days. Refractive status and ocular axial dimensions, including choroidal thickness, were measured by retinoscopy and high-frequency A-scan ultrasonography, respectively.

RESULTS

Three lines of evidence indicate that the normal increase in choroidal thickness that occurs during early maturation can be altered by the eye's refractive state. First, in monkeys experiencing form deprivation or those in the process of compensating for imposed optical errors, choroidal thickness and refractive error were significantly correlated with eyes developing myopia having thinner choroids than those developing hyperopia. Second, the choroids in eyes recovering from binocularly induced myopia increased in thickness at a faster rate than the choroids in recovering hyperopic eyes. Third, monkeys recovering from induced anisometropias showed interocular alterations in choroidal thickness that were always in the appropriate direction to compensate for the anisometropia. These changes in choroidal thickness, which were on the order of 50 microm, occurred quickly and preceded significant changes in overall eye size.

CONCLUSIONS

Changes in the eye's effective refractive state produce rapid compensating changes in choroidal thickness. Although these choroidal changes are small relative to the eye's refractive error, they may play an important role in the visual regulation of axial growth associated with emmetropization.

Choroidal thickness changes during altered eye growth and refractive state in a primate

PURPOSE

In the chick, compensation for experimentally induced defocus involves changes in the thickness of the choroid. The choroid thickens in response to imposed myopic defocus and thins in response to imposed hyperopic defocus. This study was undertaken to determine whether similar choroidal changes occur in the primate eye with induced refractive errors.

METHODS

Thirty-three common marmosets were used. Eyes in 26 monkeys served as untreated control eyes, and eyes in 7 received 3 weeks of monocular lid suture to induce changes in eye growth and refractive state. Refractive errors were measured using refractometry and retinoscopy, and axial ocular dimensions, including choroidal thickness, were measured using high-frequency A-scan ultrasonography. Eyes were measured before the lids were sutured and at frequent intervals after lid opening.

RESULTS

In the marmoset, choroidal thickness ranges from 88 to 150 microm and increases significantly during the first year of life. Monocular lid suture initially results in short, hyperopic eyes that then become elongated and myopic. In these animals the choroids of both the experimental and the fellow control eyes also increase in thickness with age but additionally show interocular differences that vary significantly with the relative changes in vitreous chamber depth and refraction. In eyes that are shorter and more hyperopic than control eyes the choroids are thicker, and in eyes that are longer and more myopic than control eyes the choroids are thinner.

CONCLUSIONS

In marmosets, the thickness of the choroid increases during postnatal eye growth. Superimposed on this developmental increase in choroidal thickness there are changes in thickness that are correlated with the induced changes in eye size. These changes are small (<50 microm) in comparison with those observed in the chick, contributing to less than a diopter change in refractive error.