Scheimpflug anterior segment photography assessment of wound healing after myopic excimer laser photorefractive keratectomy

New algorithm for corneal densitometry assessment based on anterior segment optical coherence tomography

PURPOSE

To describe a new algorithm to measure corneal densitometry based on images obtained by swept source anterior segment ocular coherence tomography (SS-AS-OCT) and establish standard densitometry values in a group of normal eyes.

METHODS

A total of 111 healthy participants (195 eyes) were enrolled in this study. Using a MATLAB designed algorithm, the cornea was segmented into three layers: anterior, posterior and mid-stroma, and it was divided into two concentric areas, 0-2 and 2-4 mm, resulting in nine areas for the analysis. The mean corneal densitometry values were calculated and expressed as grayscale units (GSU).

RESULTS

The mean age was 57 years (range 22-87), with 100 (51.3%) right eyes and 95 (48.7%) left eyes. The total corneal densitometry was 86.9 ± 12.1 GSU. The mid-stroma layer had the highest densitometry values, 87.4 ± 12.1 GSU, and the anterior layer had the lowest values, 81.9 ± 14.2 GSU. Densitometry differences between the anterior layer and the mid-stroma layer (P < 0.001), as well as the anterior layer and the posterior layer (P < 0.05) were statistically significant. The 0-2 mm concentric area had higher mean densitometry values, 97.8 ± 12.7 GSU, and the differences were significant compared to the 2-4 mm concentric area (P < 0.001). No correlation was found between the corneal densitometry values and gender or age.

CONCLUSIONS

The new MATLAB segmentation algorithm for the analysis of corneal SS-AS-OCT images is capable to objectively assess corneal densitometry. We provide standard and normal data for better clinical and research approach.

Evaluation of corneal optical properties in subjects wearing hydrogel etafilcon A contact lenses and the effect of administering mannitol-enriched sodium hyaluronate ophthalmic solution

BACKGROUND

The purpose of this study was to evaluate the effect of daily administration of mannitol-enriched sodium hyaluronate ophthalmic solution on the corneal optical properties of subjects wearing low Dk hydrogel (etafilcon A) contact lenses (CLs).

METHODS

Forty-five subjects wearing etafilcon A CLs daily for more than 6 months were recruited into this pilot study. Fifteen of the subjects administered a 10% mannitol-enriched 0.05% sodium hyaluronate solution (study group) once daily and 30 subjects did not administer any ophthalmic solution (control group). The subjects were examined at baseline and one month after recruitment. Changes in central corneal thickness (CCT) and corneal light backscatter were evaluated by Scheimpflug imaging (Pentacam HR). Changes in corneal total high-order aberration, corneal spherical aberration, coma, and trefoil were evaluated using the OPD scan II.

RESULTS

At one month, corneal light backscatter decreased significantly in the study group (≤18.30 arbitrary units; P<0.05) and this was highly correlated with a decrease in CCT (R=0.81; P=0.04). The decrease in corneal total high-order aberration, spherical aberration, and coma was significantly higher in the study group than in the control group (P<0.05). No changes in corneal light backscatter or CCT were found in the control group during follow-up.

CONCLUSION

Once-daily administration of a mannitol-enriched lubricant ophthalmic solution was effective for improving the corneal optical quality and reducing corneal swelling in subjects wearing low Dk hydrogel (etafilcon A) CLs during one month follow-up.

Optical coherence tomography of the anterior segment

Perhaps no diagnostic technology has emerged as rapidly in ophthalmology as optical coherence tomography (OCT). A single clinical device for this noninvasive imaging technique was first released in 1996, and now at least ten clinical devices are available. Although the first clinical anterior segment OCT was marketed only 2 years ago, a substantial amount of work has been done using modified retinal imagers or prototype laboratory-based imagers. In this review, we discuss OCT imaging primarily of the cornea. We also highlight previous and current publications on nonclinical and clinical uses of the device to illustrate how anterior segment OCT can be used to understand corneal structure and function in health and disease.

Analysis of ectasia after laser in situ keratomileusis: Risk factors

PURPOSE

To examine a database of laser in situ keratomileusis (LASIK) procedures for preoperative and operative factors assumed to increase the risk for developing post-LASIK ectasia.

SETTING

Private clinical practice.

METHODS

A computer database was queried for eyes that had LASIK for myopic refractive errors with the following characteristics: preoperative corneal thickness 500 microm or less, mean keratometry greater than 47.0 diopters (D), patient age 25 years or younger, attempted correction greater than -8.0 D, refractive astigmatism not with-the-rule and greater than 2.0 D, and residual stromal bed thickness (RST) 250 microm or less. Flap thickness and RST were measured using ultrasound pachymetry. All recorded information was exported to MS Excel and analyzed for eyes that had ectasia.

RESULTS

Of the 9700 eyes in the database, none with the above characteristics developed ectasia over mean follow-up periods exceeding 2 years. Seven eyes had multiple risk factors without ectasia. Three eyes with abnormal preoperative topography developed ectasia.

CONCLUSIONS

Individual preoperative and operative factors did not in and of themselves increase the risk for ectasia. Unmeasured and unknown factors that affect the individual cornea's biomechanical stability, in combination with some suspected risk factors as well as the current inability to identify corneas at risk for developing ectatic disorders, probably account for most eyes that develop ectasia today.

Corneal Light Backscatter Measured by Optical Coherence Tomography After LASIK

PURPOSE

To objectively quantify corneal light backscatter after LASIK using optical coherence tomography (OCT).

METHODS

Twenty-eight eyes of 14 patients (mean age: 39.9 +/- 8.6 years) underwent LASIK surgery. Corneal images were taken with a custom built anterior segment OCT at 1310 nm before and 1 day, 1 week, and 1 month after surgery. Backscattered light from the epithelium and 10 equally divided corneal stromal layers of the central cornea were analyzed using custom software. Light scattering of the interface area (defined as seven image pixels [46.2 microm] in depth centered by the peak corresponding to the interface between the corneal flap and bed) was also calculated and compared to the light backscatter at an equivalent depth of the respective preoperative cornea.

RESULTS

There were significant differences of light backscatter in different layers (analysis of variance [ANOVA]: F(10, 130) = 44.89, P = .0001), but no significant differences between right and left eyes preoperatively (ANOVA: F(10, 130) = 1.16, P = .32). After surgery, there were significant differences in light backscatter profiles of the central cornea (repeated measures ANOVA: F(30, 810) = 7.70, P = .0001) with significant increases at approximately 140 to 190 microm in depth from the corneal front surface at 1 day (post hoc test: P = .004) and 1 week (post hoc test: P = .001) postoperatively, compared to the baseline. One month after surgery, light backscatter increased significantly in the epithelium (post hoc test: P = .0001) and decreased significantly (post hoc test: P = .0001) at approximately 100 to 140 microm in depth. Light backscatter results of these interface areas (repeated measures ANOVA: F(3, 81) = 21.29, P = .0001) showed significant increases at 1 day and 1 week postoperatively (post hoc tests: P = .0001) compared to baseline results and 1-month postoperative results.

CONCLUSIONS

Objective and quantitative analysis of corneal light backscatter from OCT demonstrated increasing comeal light scattering at the interface and subsequent recovery.

Agreement and Repeatability of Central Thickness Measurement in Normal Corneas Using Ultrasound Pachymetry and the OCULUS Pentacam

PURPOSE

To compare the accuracy and repeatability of the OCULUS Pentacam (a new Scheimpflug-based imaging system) with ultrasound pachymetry in the measurement of central corneal thickness (CCT).

METHODS

CCT was measured in 21 subjects (21 normal corneas) on 2 separate occasions by the same examiner, using an Allergan-Humphrey 850 ultrasonic pachymeter and an OCULUS Pentacam instrument.

RESULTS

Mean values of CCT for both visits for each instrument were 534 +/- 47 microm and 528 +/- 45 microm using the ultrasonic pachymeter and the Pentacam, respectively. Plots of differences against means displayed relatively good agreement (limits of agreement were -13.0 to +26.6 microm). The repeatability (limits of agreement) of the ultrasound pachymeter was -18.3 to +17.7 microm, while for the OCULUS Pentacam it was -24.1 to +21.1 microm.

CONCLUSIONS

Our data showed that the Pentacam instrument provided measurements that were slightly but systematically lower than the measurements provided by ultrasonic pachymetry, which is currently the clinical gold standard method. The results, coupled with a unique ability to image and analyze the anterior chamber in vivo, make the OCULUS Pentacam a promising new instrument for anterior eye evaluation.

Grading of corneal transparency

AIM

To examine the academic literature on the grading of corneal transparency and to assess the potential use of objective image analysis.

METHOD

Reference databases of academic literature were searched and relevant manuscripts reviewed. Annunziato, Efron (Millennium Edition) and Vistakon-Synoptik corneal oedema grading scale images were analysed objectively for relative intensity, edges detected, variation in intensity and maximum intensity. In addition, corneal oedema was induced in one subject using a low oxygen transmissibility (Dk/t) hydrogel contact lens worn for 3h under a light eye patch. Recovery from oedema was monitored over time using ultrasound pachymetry, high and low contrast visual acuity measures, bulbar hyperaemia grading and transparency image analysis of the test and control eyes.

RESULTS

Several methods for assessing corneal transparency are described in the academic literature, but none have gained widespread use in clinical practice. The change in objective image analysis with printed scale grade was best described by quadratic parametric or sigmoid 3-parameter functions. 'Pupil image scales' (Annunziato and Vistakon-Synoptik) were best correlated to average intensity; however, the corneal section scale (Efron) was strongly correlated to variations in intensity. As expected, patching an eye wearing a low Dk/t hydrogel contact lens caused a significant (F = 119.2, p < 0.001) 14.3% increase in corneal thickness, which gradually recovered under open eye conditions. Corneal section image analysis was the most affected parameter and intensity variation across the slit width, in isolation, was the strongest correlate, accounting for 85.8% of the variance with time following patching, and 88.7% of the variance with corneal thickness.

CONCLUSION

Corneal oedema is best determined objectively by the intensity variation across the width of a corneal section. This can be easily measured using a slit-lamp camera connected to a computer. Oedema due to soft contact lens wear is not easily determined over the pupil area by sclerotic scatter illumination techniques.

Ultrasound biomicroscopy and Scheimpflug photography of angle-supported phakic intraocular lens for high myopia

PURPOSE

To evaluate the equivalence of Scheimpflug photography (SP) and ultrasound biomicroscopy (UBM) in determining corneal epithelium-intraocular lens (IOL) and border IOL-iris distances.

SETTING

Universität Erlangen-Nuremberg, Erlangen, Germany.

METHODS

In 26 eyes of 17 patients who had a NuVita MA20 angle-supported anterior chamber intraocular lens (Chiron-Domilens), SP and UBM were used to evaluate the distance between the endothelium and the anterior IOL face in central and peripheral regions (12 o'clock and 6 o'clock positions) and between the border of the anterior IOL face and the iris. The Wilcoxon test was used for statistical analysis.

RESULTS

The mean central endothelium-anterior IOL face distance was 2.01 mm and 2.00 mm by SP and UBM, respectively. The mean peripheral endothelium-anterior IOL border distance was 1.28 mm and 1.58 mm, respectively, and the mean peripheral anterior IOL face-iris distance, 0.89 mm and 0.75 mm, respectively.

CONCLUSIONS

The difference between the 2 methods in the central endothelium-anterior IOL face distance was not significant (methods were equivalent), but the difference in the peripheral endothelium-anterior IOL face distance was. This may be the result of difficulty in obtaining the exact transition point between the IOL and the haptics by SP examination. The difference between the 2 methods in the IOL border-iris distance was also significant because of the irregularity of the iris surface; therefore, measurements were performed at different sites along this structure. The significant differences in the peripheral endothelium-IOL and IOL border-iris distances indicate that although both methods are useful, they are not equivalent.

Corneal optical coherence tomography before and after phototherapeutic keratectomy for recurrent epithelial erosions(2)

PURPOSE

To study the representation of corneal structures with optical coherence tomography (OCT) before and after excimer laser phototherapeutic keratectomy (PTK) for recurrent epithelial erosions.

SETTING

Departments of Ophthalmology, Vivantes Klinikum Neukölln, Berlin, and Medizinische Universität, Lübeck, Germany.

METHODS

This prospective study comprised 15 eyes of 14 patients with recurrent epithelial erosions. The central corneal and epithelial thickness as well as the wound-healing response in the anterior corneal stroma were assessed with slitlamp-adapted OCT before and after PTK.

RESULTS

After PTK, the symptoms improved in all patients without loss of best corrected, glare, or low-contrast visual acuity. The mean central corneal OCT thickness was 540 microm +/-28 (SD) preoperatively, 492 +/- 36 microm immediately after epithelial debridement and PTK, and 519 +/- 25 microm after 7 weeks (P <.01). The mean central epithelial OCT thickness changed from 70 +/- 13 microm preoperatively to 60 +/- 7 microm after 7 weeks (P >.01). Changes in the light-scattering properties in the anterior subepithelial stroma revealed a hyperreflective area with a mean thickness of 46 +/- 13 microm after 7 weeks.

CONCLUSIONS

Using noncontact corneal OCT, corneal and epithelial thickness changes and the wound-healing response in the anterior corneal stroma could be evaluated after PTK in patients with recurrent epithelial erosions.

Central corneal thickness, radius of the corneal curvature and intraocular pressure in normal subjects using non-contact techniques: Reykjavik Eye Study

PURPOSE

To establish a population profile of central corneal thickness (CCT), radius of the corneal curvature (CC) and intraocular pressure (IOP) and the relationships between them using non-contact techniques.

METHODS

We used a population-based random sample of 415 male and 510 female Caucasians aged 50 years and older. CCT and the radius of CC were measured with Scheimpflug anterior segment photography. IOP was measured with air-puff tonometry.

RESULTS

The mean IOP of right eyes was 15.1 mmHg (SD 3.3) among men and 15.8 mmHg among women (SD 3.1), which is a statistically significant difference. The mean radius of CC for male right eyes was 7.78 (SD 0.60) and for females 7.62 (SD 0.58) which is also statistically significant. Mean CCT for male right eyes was 0.528 mm (SD 0.041) and for females 0.526 mm (SD 0.037), which is not a significant difference. Linear regression analysis shows no relationship between the radius of CC and IOP or between age and radius of CC. Linear regression analysis of the relationship between CCT and IOP suggests higher IOP measurements with thicker corneas. There was no significant correlation between age and CCT.

CONCLUSION

IOP was found to be independent of age and significantly higher in females than in males. Radius of CC was found to be age-independent and significantly steeper in females than in males. CCT appears to be independent of age and gender. Greater CCT is associated with higher mean IOP.

Objective assessment of transient corneal haze and its relation to visual performance after photorefractive keratectomy

PURPOSE

Photorefractive keratectomy has the potential to cause transient corneal haze. The purpose of this study was to evaluate the relationship between transient corneal haze as measured by an objective means and high and low contrast visual performance.

METHODS

In a prospective study, 44 eyes of 28 patients were examined preoperatively and at 1, 3, 6, and 12 months after photorefractive keratectomy. Five laser in situ keratomileusis and two intrastromal corneal ring segments (Intacs [KeraVision, Fremont, CA]) were included for comparison, because these procedures are not expected to cause haze. Haze was measured using a prototype objective hazemeter, TSPC-3, a modification of the Nidek EAS-1000. Visual performance was measured using high-contrast visual acuity and the Rabin Small Letter Contrast Test.

RESULTS

Corneal haze was greatest at the 1-month examination and was consistent with a decrease in visual performance on both tests. Corneal haze resolved in 82% of eyes by 10 +/- 4 months after photorefractive keratectomy. However, visual performance had not returned to preoperative levels in 65% and 81% of these eyes on the high-contrast visual acuity test and the Small Letter Contrast Test, respectively. Eyes that underwent laser in situ keratomileusis and Intacs did not develop corneal haze; however, visual decrements were measured.

CONCLUSIONS

As a clinical tool, the TSPC-3 hazemeter objectively measures very subtle changes in haze levels. Corneal haze appears to account for only approximately 50% of visual performance changes in the early healing period after photorefractive keratectomy. Other factors, namely topographic abnormalities, are more likely to be an important cause of persistent visual disturbances.

Corneal optical coherence tomography before and immediately after excimer laser photorefractive keratectomy

PURPOSE

To investigate the representation of the corneal structure with optical coherence tomography before and immediately after excimer laser photorefractive keratectomy.

METHODS

Twenty-four eyes of 24 patients with myopia and myopic astigmatism were prospectively studied. The corneal thickness and the corneal profile were assessed with slit-lamp-adapted optical coherence tomography preoperatively and immediately after excimer laser photorefractive keratectomy.

RESULTS

The attempted mean spherical equivalent of the refractive corrections was -6.7 +/- 3.6 (mean +/- SD) diopters with a mean calculated stromal ablation depth of 91 +/- 38 microm. The corneal optical coherence tomography was reproducible in all patients, demonstrating a mean decrease of central corneal thickness after epithelial debridement and excimer laser photorefractive keratectomy of 118 +/- 45 microm. The comparison of the calculated stromal ablation depth and the corneal thickness changes determined by corneal optical coherence tomography revealed a significant linear relationship with a correlation coefficient of 0.88 (P <.001). The flattening of the corneal curvature was confirmed in all patients with the optical coherence tomography system and correlated with the attempted refractive correction (r =.82, P <.001).

CONCLUSIONS

The slit-lamp-adapted optical coherence tomography system presented in this study allowed noncontact, cross-sectional, and high-resolution imaging of the corneal configuration. This initial clinical evaluation demonstrated that corneal optical coherence tomography could be a promising diagnostic modality to monitor corneal changes of thickness and curvature before and after excimer laser photorefractive keratectomy.

Surgical correction of moderate myopia: which method should you choose? I. Radial keratotomy will always have a place

This set of "Viewpoints" articles examines the relative merits of radial keratotomy (RK), photorefractive keratectomy (PRK), and laser assisted in-situ keratomileusis (LASIK). Drs. Rowsey and Morley review advances in RK techniques, long-term results, and complications, and explain why RK will remain a viable method for correction of moderate myopia, notably its minimal cost. Drs. Steinert and Bafna review both PRK and LASIK, discussing techniques and results and comparing their advantages and disadvantages with each other and with RK. Dr. Dutton, as "Viewpoints" section editor, summarizes clinical, technologic, and economic aspects of all three techniques, concluding that all will find a place among refractive surgeons for some time to come.

Quantification of stromal thinning, epithelial thickness, and corneal haze after photorefractive keratectomy using in vivo confocal microscopy

PURPOSE

The authors establish, for the first time, observer-independent quantification of stromal thinning, epithelial thickness, and corneal haze after excimer laser photorefractive keratectomy (PRK) using a unique, new form of in vivo confocal microscopy.

METHODS

Rapid, continuous z-scans of high-resolution confocal images, termed confocal microscopy through focusing (CMTF), were performed in the central corneal area of 17 patients before and 1 month after PRK for low- to moderate-grade myopia (-2.88-9.13 diopters [D]). Corneal, epithelial, and stromal thickness measurements and an objective haze estimate were obtained from each CMTF scan by digital image analysis.

RESULTS

Epithelial thickness averaged 51 +/- 4 microns before and 45 +/- 10 microns 1 month post-PRK (P < 0.005), whereas stromal thinning ranged from 20 to 154 microns, representing a direct estimate of the actual photoablation depth. Corneal thickness averaged 560 +/- 36 microns before PRK and 462 +/- 52 microns at 1 month. The change in corneal thickness correlated closely with the change in spherical equivalent refraction (r = 0.94, P < 0.0001); linear regression analysis revealed a value of 14.3 microns corneal thinning per diopter of correction. A significant correlation was found between the objective CMTF haze estimate and a clinical haze grading obtained by slit-lamp examination (r = 0.73, P < 0.001).

CONCLUSIONS

Confocal microscopy through focusing is a new, powerful in vivo tool that enables quantitative, unbiased evaluation of PRK procedures over time by providing epithelial and stromal thickness analysis, photoablation depth assessment, and unbiased haze measurement. The method is uniquely valuable in the pre- and postoperative assessment of PRK patients and for determining the optimal treatment strategy, especially in assessing refractive and visual outcomes in individual cases.