12. Endothelial pump and barrier function.

The mammalian cornea maintains its thickness and transparency primarily by the activity of a fluid pump located in the endothelial cell layer. The accepted concept, the "pump-leak" theory, holds that the active transport of solute from the stroma to the aqueous humor leads to a steady state osmotic pressure gradient across the endothelium that balances the imbibition pressure created by the hydrophilic proteoglycans in the stromal ground substance. The details of this process are controversial and some of the classical in vitro studies aimed to explore the fluid pump using low temperature to challenge the regulatory behavior cannot be duplicated in vivo. The activity of sensory or sympathetic innervation may play a role in this low temperature tolerance. Asymmetry in endothelial cell volume regulation could be the basis for the fluid pump.

Fourier Analysis of Corneal Irregular Astigmatism Due to the Anterior Corneal Surface in Dry Eye.

OBJECTIVES: To evaluate corneal irregular astigmatism due to the anterior corneal surface using Fourier harmonic analysis with a Placido ring-based corneal topographer (Placido-based topographer) and three-dimensional anterior segment optical coherence tomography (OCT) in dry eyes. METHODS: Forty-four eyes of 44 subjects with dry eye and 20 eyes of 20 normal control subjects were enrolled. Corneal topographic data were obtained using a Placido-based topographer and OCT. Dioptric data from the central 3-mm zone of the anterior corneal surface were decomposed using Fourier harmonic analysis. Spherical, regular astigmatism, and irregular astigmatism (asymmetry and higher-order irregularity) refractive error components of the cornea from the two imaging modalities were compared. RESULTS: Both asymmetry and higher-order irregularity values were significantly greater in dry eyes than in control eyes for both the Placido-based topographer and OCT measurements (all P<0.05). In dry eyes, measured values of asymmetry and higher-order irregularities were significantly smaller when obtained with OCT than with the Placido-based topographer (both P<0.001). By contrast, these parameters were not significantly different between the two devices in control eyes. In dry eyes, severity of superficial punctate keratopathy in the central corneal region was correlated with irregular astigmatism. CONCLUSIONS: The amount of corneal irregular astigmatism, quantified using Fourier harmonic analysis, was significantly higher in dry eyes than in normal eyes. Measurements obtained with OCT and the Placido-based topographer differed in subjects with dry eyes. Therefore, caution should be practiced when trying to use these measurements interchangeably.

Prediction of the small aperture intraocular lens on visual acuity in patients with keratoconus.

PURPOSE: To investigate the impact of corneal higher-order aberrations (HOAs) on predicted corrected distance visual acuity (CDVA) in patients with keratoconus at varying simulated pupil apertures. SETTING: Ophthalmology clinics, Medical University of South Carolina, USA. DESIGN: Retrospective chart review study. METHODS: 56 eyes with keratoconus were examined using Scheimpflug tomography during routine examinations prior to medical intervention. The severity of keratoconus was graded using the Amsler-Krumeich classification. Zernike analysis was used to obtain corneal aberrations using simulated pupil diameters of 6-, 4-, and 2 mm. These data were extrapolated to obtain the total RMS HOAs for a 1.6 mm simulated pupil to evaluate the potential effect of a small aperture intraocular lens. Correlation analysis was used to study the impact and relative contributions of HOAs on CDVA. Convolution of HOAs from OPD-Scan III (NIDEK) provided a clinical method to predict CDVA with different simulated pupil sizes in corneas with irregular astigmatism. RESULTS: There were statistically significant positive correlations between photopic CDVA and the magnitude of total and individual (coma, spherical aberration and trefoil) HOAs in this cohort of keratoconus subjects. A keratoconus case with the small aperture IOL confirms the improvement in vision due to the pinhole effect. CONCLUSIONS: The small aperture IOL is expected to markedly reduce aberrations in keratoconus patients up to Amsler-Krumeich class 4 severity to levels consistent with the levels seen in healthy patients. Convolution of corneal HO aberrations with the ETDRS chart provides a useful simulation of the impact of pinhole optics in aberrated eyes.

Variability of subjective classifications of corneal topography maps from LASIK candidates.

PURPOSE: To evaluate the variability of subjective corneal topography map classification between different experienced examiners and the impact of changing from an absolute to a normative scale on the classifications. METHODS: Preoperative axial curvature maps using Scheimpflug imaging obtained with the Pentacam HR (Oculus Optikgeräte, Wetzlar, Germany) and clinical parameters were sent to 11 corneal topography specialists for subjective classification according to the Ectasia Risk Scoring System. The study population included two groups: 11 eyes that developed ectasia after LASIK and 14 eyes that had successful and stable LASIK outcomes. Each case was first reviewed using the absolute scale masked to the patient group. After 3 months, the same cases were represented using a normative scale and reviewed again by the same examiners for new classifications masked to the patient group. RESULTS: Using the absolute scale, 17 of 25 (68%) cases had variations on the classifications from 0 to 4 for the same eye across examiners, and the overall agreement with the mode was 60%. Using the normative scale, the classifications from 11 of 25 (44%) cases varied from 0 to 4 for the same eye across examiners, and the overall agreement with the mode was 61%. Eight examiners (73%) reported statistically higher scores (P < .05) when using the normative scale. Considering all 550 topographic analyses (25 cases, 11 examiners, and two scales), the same classification from the two scales was reported for 121 case pairs (44%). CONCLUSION: There was significant inter-observer variability in the subjective classifications using the same scale, and significant intra-observer variability between scales. Changing from an absolute to a normative scale increased the scores on the classifications by the same examiner, but significant inter-observer variability in the subjective interpretation of the maps still persisted.

Advances in Corneal Surgical and Pharmacological Approaches to the Treatment of Presbyopia.

The purpose of this article is to review the current status of presbyopia amelioration with surgical and pharmacologic procedures that partially compensate for loss of accommodation in advance of cataract surgery and lens replacement. Over the last few years, several corneal surgical and topical pharmacological approaches for the treatment of presbyopia have been introduced to the marketplace or are in the developmental pipeline. The approaches vary in invasiveness, duration of effect, reversibility, risk/benefit ratio, and clinical results. The advantages and disadvantages for each are discussed. Corneal surgical interventions aim to provide improved near and intermediate vision in patients with presbyopia through refractive means that extend ocular depth of focus through shape modification. The use of miotic drops or corneal lamellar implants extend depth of focus with the "pinhole" aperture size reduction effect. Unlike in adults younger than 40 years, the refractive status of the patient with presbyopia is not stable. Hence, procedures that provide a permanent refractive change may not provide long-term full correction; eye drops or other treatments that are self-reversing in time or are easily reversible may be used as needed. On the horizon, procedures are being explored that may add years of functional lens accommodation by preserving the deformable gel properties of the lens. [J Refract Surg. 2021;37(6 Suppl):S20-S27.].

Long-term Chronological Changes in Very Asymmetric Keratoconus.

PURPOSE: To investigate chronological changes in patients with very asymmetric keratoconus (KC) by Scheimpflug-based corneal tomographic evaluation. METHODS: We retrospectively reviewed the clinical records of 382 patients with KC attending a university hospital. Patients who were diagnosed with KC in one eye and forme fruste keratoconus in the other eye at the last visit and followed up with Scheimpflug-based tomographic corneal imaging were included. The anterior radius of curvature (ARC) and posterior radius of curvature (PRC) obtained at a 3.0-mm optical zone and the thinnest pachymetry reading of corneal thickness (Tmin), were used to investigate the chronological changes in tomographic parameters. RESULTS: Twenty-six patients aged 16 to 66 years met the inclusion criteria. The mean observation period was 6 years and 4 months. There was a significant increase in annual change in PRC with a longer follow-up period after initial examination (P = 0.011). Moreover, a greater increase in annual change in ARC was observed in patients in whom the initial examination was performed at a younger age than in those in whom it was performed at an older age (P < 0.001). CONCLUSIONS: On the basis of tomographic evaluation, even if the measured values are almost stable and within normal limits, a long-term clinical follow-up is warranted in forme fruste keratoconus eyes. Younger patients with very asymmetric keratoconus should be monitored carefully.

Functional optical zone of the cornea.

PURPOSE: When keratorefractive surgery is used to treat a central corneal diameter smaller than the resting pupil, visual symptoms of polyopia, ghosting, blur, haloes, and glare can be experienced. Progress has been made to enlarge the area of surgical treatment to extend beyond the photopic pupil; however, geometric limitations can pose restrictions to extend the treatment beyond the mesopic pupil diameter and can lead to impediments in night vision. The size of the treated area that has achieved good optical performance has been defined as the functional optical zone (FOZ). In this study the authors developed three objective methods to measure the FOZ. METHODS: Corneal topography examination results from 1 eye of 34 unoperated normal eyes and 32 myopic eyes corrected by laser in situ keratomileusis (LASIK) were evaluated in three ways. First, a uniform axial power method (FOZ(A)) assessed the area of the postoperative cornea that was within a +/-0.5-D window centered on the mathematical mode. Second, FOZ was determined based on the corneal wavefront true RMS error as a function of the simulated pupil size (FOZ(R)). Third, FOZ was determined from the radial MTF, established at the retinal plane as a function of pupil size (FOZ(M)). RESULTS: Means for each of the FOZ methods (FOZ(A), FOZ(R), and FOZ(M)) were 7.6, 9.1, and 7.7 mm, respectively, for normal eyes. For LASIK-corrected eyes, these means were 6.0, 6.9, and 6.0 mm. Overall, an average decrease of 1.8 mm in the functional optical zone was found after the LASIK procedure. Correlations between the FOZ methods after LASIK showed acceptable and statistically significant values (R = 0.71, 0.70, and 0.61; P < 0.01). CONCLUSIONS: These methods will be useful to more fully characterize corneal treatment profiles after keratorefractive surgery. Because of its ease of implementation, direct spatial correspondence to corneal topography, and good correlation to the other more computationally intensive methods, the semiempiric uniform axial power method (FOZ(A)) appears to be most practical in use. The ability to measure the size of the FOZ should permit further evolution of keratorefractive surgical lasers and their algorithms to reduce the night vision impediments that can arise from functional optical zones that do not encompass the entire mesopic pupil.

Absolute color scale for improved diagnostics with wavefront error mapping.

PURPOSE: Wavefront data are expressed in micrometers and referenced to the pupil plane, but current methods to map wavefront error lack standardization. Many use normalized or floating scales that may confuse the user by generating ambiguous, noisy, or varying information. An absolute scale that combines consistent clinical information with statistical relevance is needed for wavefront error mapping. The color contours should correspond better to current corneal topography standards to improve clinical interpretation. DESIGN: Retrospective analysis of wavefront error data. PARTICIPANTS: Historic ophthalmic medical records. METHODS: Topographic modeling system topographical examinations of 120 corneas across 12 categories were used. Corneal wavefront error data in micrometers from each topography map were extracted at 8 Zernike polynomial orders and for 3 pupil diameters expressed in millimeters (3, 5, and 7 mm). Both total aberrations (orders 2 through 8) and higher-order aberrations (orders 3 through 8) were expressed in the form of frequency histograms to determine the working range of the scale across all categories. The standard deviation of the mean error of normal corneas determined the map contour resolution. Map colors were based on corneal topography color standards and on the ability to distinguish adjacent color contours through contrast. MAIN OUTCOME MEASURES: Higher-order and total wavefront error contour maps for different corneal conditions. RESULTS: An absolute color scale was produced that encompassed a range of +/-6.5 microm and a contour interval of 0.5 microm. All aberrations in the categorical database were plotted with no loss of clinical information necessary for classification. In the few instances where mapped information was beyond the range of the scale, the type and severity of aberration remained legible. CONCLUSIONS: When wavefront data are expressed in micrometers, this absolute scale facilitates the determination of the severity of aberrations present compared with a floating scale, particularly for distinguishing normal from abnormal levels of wavefront error. The new color palette makes it easier to identify disorders. The corneal mapping method can be extended to mapping whole eye wavefront errors. When refraction data are expressed in diopters, the previously published corneal topography scale is suggested.

Screening patients with the corneal navigator.

PURPOSE: To present a corneal topography screening device for the detection of corneal ectasias and various refractive procedures based on corneal topography patterns. METHODS: A database of corneal topography patterns were analyzed and used to "train" a neural network on nine different corneal topography patterns using nineteen corneal topography indices of corneal shape and power. RESULTS: Sample normal and corneal topographies were recognized correctly. CONCLUSIONS: The use of the corneal navigator to screen various corneal topographies aids clinical diagnosis.

Goodness-of-prediction of Zernike polynomial fitting to corneal surfaces.

PURPOSE: To determine the goodness-of-prediction of the fitting routine by measuring the difference between topographic corneal surfaces and their Zernike reconstructions as a function of polynomial order and optical zone size for various corneal conditions. SETTING: Corneal research laboratory in a university eye center. METHODS: Corneal topography maps (N = 253) were obtained from the Louisiana State University Eye Center. A variety of corneal conditions were used: normals; astigmatism; laser in situ keratomileusis, photorefractive keratoplasty (PRK), and radial keratotomy (RK) postoperative cases (myopic spherical corrections); keratoconus suspect; mild, moderate, and severe keratoconus; pellucid marginal degeneration; contact lens-induced corneal warpage; and penetrating keratoplasty. The root-mean-square (RMS) error of the goodness-of-prediction of the Zernike representation of corneal surface elevation was extracted for 4, 6, and 10 mm optical zones, whereas Zernike radial orders were varied from 3 to 14 in 1-order steps. The mean +/- SEM of the RMS error was plotted as a function of Zernike order and compared with criteria for normal surface fits. RESULTS: Fitting accuracy improved as more Zernike terms were included, but some conditions showed significant errors (when compared with normal surfaces), even with many added terms. For a 6 mm optical zone, the normal cornea group had the lowest RMS error and did not require terms above the 4th order to achieve <0.25 microm RMS error. Astigmatism met the 0.25 microm threshold at the 5th order, whereas keratoconus suspect required 7 orders. Laser in situ keratomileusis and PRK met the 0.25 microm threshold at the 8th order, whereas RK required 10 orders. Contact lens-induced corneal warpage and mild keratoconus needed 12 orders to meet the 0.25 microm threshold, whereas pellucid marginal degeneration, moderate and severe keratoconus, and keratoplasty categories were not well fitted even at 14 orders. CONCLUSIONS: A 4th-order Zernike polynomial appeared reliable for modeling the normal cornea, but using a 4th-order fitting routine with an abnormal corneal surface caused a loss of fine-detail shape information. As more Zernike terms were added, the accuracy of the fit improved, and the result approached the minimum error found with normal corneas. Unless sufficient higher-order Zernike terms are included when analyzing irregular surfaces, some diagnostic applications of Zernike coefficients may not be rigorous. This conclusion also suggests that wavefront shape analysis is similarly dependent on the number of orders used. Current surgical corrections based on normal-eye wavefronts may fail to capture all visually relevant aberrations in abnormal eyes, such as those having laser retreatments or experiencing corneal warpage from contact lens wear. A clinical goodness-of-fit or goodness-of-prediction index would indicate whether the number of terms in use has fully accounted for all of the visually significant aberrations present in the eye.

Screening Patients With Cataract for Premium IOL Candidacy Using Microperimetry.

PURPOSE: To explore the use of microperimetry in the evaluation of macular dysfunction in patients with cataract, to aid in proper intraocular lens selection (multifocal vs monofocal), and to set expectations for postoperative visual function. METHODS: One randomly chosen eye of 10 patients diagnosed as having mild (1+) to moderate (3+) cataract was evaluated before and 1 week after cataract surgery with the MAIA microperimeter (Centervue S.p.A, Padova, Italy). Corrected distance visual acuity, dilated and undilated biomicroscopy and indirect fundus examinations, intraocular pressure measurement, and a MAIA microperimeter test to measure macular sensitivity and fixation pattern were also performed. RESULTS: Two patients showed macular abnormalities before and after surgery that were noted on MAIA testing: one patient showed reduced average threshold sensitivity and abnormal percent reduced threshold, whereas the other patient showed fixation drift demonstrating eccentric fixation. Both eyes had reduced postoperative visual performance after uneventful cataract surgeries. CONCLUSIONS: Because not every macular lesion noted on fundus examination or optical coherence tomography may be clinically significant, microperimetry can be useful to detect clinically significant retinal dysfunction prior to cataract surgery. The outcome may aid in the selection of intraocular lens type and may yield a better documented surgical prognosis. Further studies are necessary to confirm these findings.

Comparison of objective and subjective refractive surgery screening parameters between regular and high-resolution Scheimpflug imaging devices.

PURPOSE: To compare objective and subjective metrics from regular and high-resolution Scheimpflug devices (Pentacam) to determine their equivalence and interchangeability for refractive surgery screening. SETTING: Emory Vision at Emory University, Atlanta, Georgia, USA. DESIGN: Retrospective comparative case series. METHODS: Eyes of consecutive screened refractive surgery patients were evaluated with high-resolution and regular Scheimpflug devices. Objective parameters evaluated included keratometry (K) values, central corneal thickness (CCT), and device-generated keratoconus screening indices. Masked expert reviewers subjectively graded images as normal, suspicious, or abnormal. RESULTS: One hundred eyes of 50 patients were evaluated. The mean K values were not significantly different (anterior K: high resolution 1.21 diopters [D] ± 1.13 (SD) versus regular 1.15 ± 1.16 D, P = 0.73; posterior K: 0.34 ± 0.23 D versus regular 0.35 ± 0.23 D, P = .67). The mean CCT was significantly thinner in the high-resolution group (514.7 ± 26.6 µm versus 527.6 ± 27.6 µm (P < .0001) with limits of agreement of -12.9 to +17.5 µm. Most keratoconus screening indices were more suspicious with the high-resolution device than with the regular device except the indices of height asymmetry and height deviation. Subjectively, 60% of cases received the same score, high resolution was more suspicious in 28% of cases, and regular was more suspicious in 12% of cases; there was only slight subjective agreement between technologies (κ = 0.26 to 0.31). CONCLUSIONS: Regular and high-resolution Scheimpflug imaging devices generated different objective values and significantly different subjective interpretations with poor inter-reviewer agreement. The high-resolution device provided a more conservative overall output. For refractive surgical screening, the 2 devices are not interchangeable. FINANCIAL DISCLOSURE: Proprietary or commercial disclosures are listed after the references.

Effect of inhibition of inflammatory mediators on trauma-induced stromal edema.

PURPOSE: To determine the specific biochemical pathways involved in the initial-phase inflammatory response that causes stromal edema after epithelial debridement of the rabbit cornea. METHODS: Adult New Zealand White rabbit corneas were treated with 2 mM synthetic inhibitor of metalloproteinase (SIMP)-1, 1 mM DFU (a specific cyclooxygenase [COX]-2 inhibitor) in 50/50 dimethyl sulfoxide (DMSO)/Ringer's solution, 300 KIU aprotinin (a serine protease inhibitor), 0.05% or 0.10% IL-1 receptor type II solution, 1 mM gliotoxin (a Ras farnesyltransferase inhibitor), or vehicle alone (the control). These were applied topically in vivo in five doses over a 3-hour period except IL-1 receptor type II, which was applied in vitro. After rabbits were killed, the corneas were mounted in perfusion chambers with the endothelium bathed in a modified Ringer's solution and the epithelium bathed with silicone oil. Corneal thickness was measured with an automatic specular microscope. The corneal thickness typically stabilized 1 hour after mounting. After stabilization, the corneal epithelium was removed with a rotating bristle brush and stromal thickness monitored for 1 hour. Paired control corneas were treated similarly. RESULTS. Stromal swelling after epithelial debridement was significantly less in most treated corneas, compared with untreated controls: 18.4 +/- 5.3 microm vs. 28.6 +/- 7.7 microm (n = 6, P = 0.004); SIMP-1, 18.7 +/- 10.2 microm vs. 34.3 +/- 10.2 microm (n = 7, P = 0.02); DFU, 19.3 +/- 10.2 microm vs. 23.5 +/- 8.4 microm (n = 6, P = 0.01); and IL-1 receptor type II (0.05%), 26.2 +/- 5.6 microm vs. 30.4 +/- 5.6 microm (n = 5, P = 0.03) and (0.10%), 26.6 +/- 5.6 microm vs. 32.1 +/- 7.4 microm (n = 8, P = 0.03). Gliotoxin was not effective (21.5 +/- 8.0 microm vs. 21.9 +/- 6.2 microm; n = 5, P = 0.94). CONCLUSIONS: The reduction of stromal edema after topical administration of the inhibitors demonstrates the involvement of the COX-2 enzyme, the matrix metalloproteinase family, plasminogens, and the IL-1 system in the trauma-induced inflammatory response of the rabbit cornea. The inflammatory process in the cornea associated with trauma can proceed along multiple redundant parallel pathways.

Zernike polynomial fitting fails to represent all visually significant corneal aberrations.

PURPOSE: It is assumed that wavefront error data arising from aberrometry are adequately described by a Zernike polynomial function, although this assumption has not been extensively tested. Inaccuracies in wavefront error may compromise clinical testing and refractive correction procedures. The current retrospective study correlates visual acuity with corneal wavefront error and with the residual surface elevation error after fitting with the Zernike method. METHODS: Corneal topography maps were obtained from 32 keratoconus cases, 27 postoperative penetrating keratoplasty cases, and 29 postoperative conductive keratoplasty cases (88 total). The best spectacle-corrected visual acuity (BSCVA) for each case ranged from -0.2 to 1.3 logarithm of the minimum angle of resolution (logMAR) units (20/12.5-20/400). Topography was analyzed to determine wavefront error and the elevation fit error for a 4-mm optical zone. The 4th and 10th expansion series were analyzed with the 0th-order (piston) and 1st order (tip and tilt) removed. Linear regression analysis was performed. The difference in root mean square (RMS) error between the 4th- and 10th-order analyses was assessed for both wavefront and elevation fit error. RESULTS: The correlation of BSCVA to wavefront error for 4th-order terms was moderately strong and significant (R2=0.581; P<0.001). The 10th-order correlation for wavefront error had a similar result (R2=0.565; P<0.001), but the regression was not significantly different from the 4th-order result. The correlation of BSCVA to the elevation fit error was strong and significant for the 4th order (R2=0.658; P<0.001). The 10th-order data had a similar result (R2=0.509; P<0.001), and there was no significant difference between the two regressions. Only 72% of the cases showed a shift toward increased wavefront error with the 10th-order series, whereas 18% lost wavefront error. All cases showed a shift toward improved elevation fit with the 10th-order expansion. CONCLUSIONS: The wavefront error correlation to acuity was moderately strong, but the corneal elevation fit error also strongly correlated with visual acuity, indicating that Zernike polynomials do not fully characterize the surface shape features that influence vision and that exist in postsurgical or pathologic eyes. In addition, the change in wavefront error when using a larger expansion series was found to increase or diminish somewhat unpredictably. The authors conclude that Zernike polynomials fail to model all the information that influences visual acuity, which may confound clinical diagnosis and treatment.

Inattention to nonsuperimposable midline symmetry causes wavefront analysis error.

BACKGROUND: The nonsuperimposable mirror-image symmetry of the body (enantiomorphism) is reflected in the wavefront error maps of eyes. Averaging the wavefront errors of right and left eyes has the potential to adversely affect correlations made between wavefront error and visual acuity or other factors. Not only are the results of past studies using Zernike terms suspected of being invalid, there is concern about possible errors in the algorithms used to create customized corneal ablations. OBJECTIVE: To compare the results of analysis with and without correction for enantiomorphism. METHODS: Fourteen TMS-1 corneal topographic maps from 7 patients having with-the-rule astigmatism in both corneas were selected for Zernike decomposition to 45 terms. The maps were distributed among 3 groups: 7 right eye maps, 7 left eye maps, and 7 left eye maps in which the topography was transposed about the vertical axial to correct for enantiomorphism (left eye-corrected). The wavefront error difference between the right and left eyes was compared with the difference between the right eyes and the left eyes in which enantiomorphism was corrected (right eye vs left eye-corrected). The left eye wavefront error was then compared with the left eye wavefront error after correction (left eye vs left eye-corrected). RESULTS: Correcting for enantiomorphism produced a statisticially significant difference in the first 5 radial orders of Zernike terms (P=.02). Of the 45 Zernike terms analyzed, 7 terms were significantly different at the P<.05 level in the right eye vs left eye category, compared with 4 terms in the right eye vs left eye-corrected category. Eleven terms were significantly different at the P<.05 level in the left eye vs left eye-corrected category. CONCLUSIONS: Correcting for enantiomorphism makes the Zernike terms in right and left eyes appear more similar. Failure to correct for enantiomorphism causes certain terms to cancel each other when averaged across right and left eyes. Wavefront error studies that do not consider enantiomorphism, including those used to adjust laser surgical nomograms, will introduce significant errors to certain Zernike terms.

Evaluation of the tear film stability after laser in situ keratomileusis using the tear film stability analysis system.

PURPOSE: To evaluate the tear film stability of patients before and after laser in situ keratomileusis (LASIK) using the tear film stability analysis system (TSAS). DESIGN: Prospective observational case series. METHODS: New videokeratography software for a topographic modeling system (TMS-2N) was developed that can automatically capture consecutive corneal surface images every second for 10 seconds. Thirty-four subjects (64 eyes) who underwent myopia LASIK were enrolled in this study. All subjects were examined with the new system before LASIK and at 1 week, 1 month, 3 months, and 6 months after the surgery. Corneal topographs were analyzed for tear breakup time (TMS breakup time) and breakup area (TMS breakup area). Based on pre-LASIK TSAS analysis, subjects were separated into normal and abnormal TSAS value groups. The criteria for the normal group were either TMS breakup time more than 5 seconds or TMS breakup area less than 0.2. The percentage of the occurrence of superficial punctuate keratitis was compared between the two groups with regard to subject's dry eye signs and symptoms. RESULTS: Tear film stability decreased significantly during the early period after LASIK, as indexed by decreased TMS breakup time and increased TMS breakup area. Tear film instability resolved at 6 months after surgery. Before LASIK, 22 subjects (43 eyes) had normal TSAS evaluation and 12 subjects (21 eyes) were abnormal. After LASIK, among normal TSAS value eyes, 8 of 43 (18.6%) eyes developed superficial punctuate keratitis. In sharp contrast, 14 of 21 (66.7%) eyes in the abnormal group displayed superficial punctuate keratitis, correlating well with the patients' dry eye symptoms. The difference in the presence of superficial punctuate keratitis after LASIK between normal and abnormal TSAS value groups was statistically significant (P <.001). Subjects with abnormal TSAS evaluation also displayed resistance to dry eye treatment and had extended period of recovery. CONCLUSIONS: Tear film stability analysis can be a useful means for screening LASIK candidates before surgery and for prognosticating dry eye symptoms thereafter.

A new method for tear film stability analysis using videokeratography.

PURPOSE: To report a new tear film stability analysis system using videokeratography. DESIGN: Observational case series. METHODS: New videokeratography software for TMS-2N (topographic modeling system; TOMEY Corporation, Nagoya, Japan) was developed that can automatically capture consecutive corneal surface images every second for 10 seconds. Forty-eight adult volunteers (80 eyes) were recruited for this study, and all subjects were examined with the new system. Corneal topographs were analyzed for tear breakup time (TMS-BUT) and the ratio of breakup area to entire color-code area (TMS-BUA) was calculated. Routine methods for tear film breakup time evaluation using slit-lamp microscope and fluorescence staining (SLE-BUT) were performed for comparison purposes. Regressive correlations of TMS-BUT or TMS-BUA with SLE-BUT were analyzed. Based on SLE results, subjects were separated into two groups with normal and short BUT, respectively. TMS-BUT and TMS-BUA were compared with SLE-BUT data with regard to the sensitivity and specificity of evaluation of dry eye symptoms. RESULTS: Topographic modeling system-tear breakup time (TMS-BUT) had a positive correlation with SLE-BUT (R = 0.7219, P <.0001), whereas TMS-BUA showed a negative correlation (R = 0.6317, P <.0001). Among 34 eyes with normal SLE-BUT, 11 eyes (32.35%) displayed abnormal TMS-BUT, 9 (81.82%) of which were associated with dry eye symptoms. The sensitivities of TMS-BUT and TMS-BUA were 97.5% and 95%, respectively, significantly higher than that of SLE-BUT (75%), with P =.008 and 0.01, respectively. Topographic modeling system-BUT and TMS-BUA displayed a similar rate of specificity in comparison with SLE-BUT. CONCLUSIONS: This new videokeratography system is a noninvasive and objective method with increased sensitivity for tear film stability analysis.

Corneal topographic and pachymetric screening of keratorefractive patients.

PURPOSE: To review the incidence of corneal abnormalities detected in the preoperative examination, using videokeratography and pachymetry that excluded patients from laser in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK). METHODS: We conducted a chart review of consecutive patients who had a preoperative examination and were excluded from refractive surgery. RESULTS: Eighteen patients from 1,392 refractive candidates (1.3%) were designated as poor candidates for LASIK or PRK based on corneal topography and/or pachymetry. Thirteen patients (0.9%) were identified as having keratoconus suspect, keratoconus, or pellucid marginal degeneration. Five patients (0.4%) were excluded due to thin corneas (<490 microm) despite normal corneal topography. CONCLUSIONS: Corneal topography and pachymetry are indispensable tools in the preoperative screening of refractive surgery candidates.

Advantages and disadvantages of the Zernike expansion for representing wave aberration of the normal and aberrated eye.

PURPOSE: Zernike expansion has been selected for use in describing wavefront aberrations in the human eye. The advantages and limitations of this approach are assessed for eyes with varying degrees of aberration. METHODS: Corneal topography examinations were taken with the Nidek OPD-Scan topographer/aberrometer. These higher data density corneal topography examinations were converted to height data and subsequently to wavefront representations. System noise was evaluated with a 2D frequency analysis of 43-D test balls. Both Zernike polynomials and 2D Fourier transforms were used to evaluate fidelity in the presentation of the point spread function. A display format for potential clinical use was developed based upon Zernike decomposition. RESULTS: Systematic noise from the corneal topographer was found to be minimal and, when eliminated, produced small changes in the point spread function. Using Zernike decomposition up to the 30th order failed to preserve the higher frequency aberrations present in aberrated eyes. Use of a Zernike decomposition display with a fixed micron scale presented only clinically significant details of spherical aberration, coma, trefoil, irregular components above third order and total higher-order aberrations (above second order). CONCLUSIONS: Zernike polynomials excel in extracting the low-order optical characteristics of visual optics. Zernikes accurately represent both low- and high-order aberrations in normal eyes where high-order aberrations are clinically insignificant. For eyes after corneal surgery or eyes with corneal pathology such as keratoconus that have significant higher-order aberrations, the Zernike method fails to capture all clinically significant higher-order aberrations.