Backscattered light from the cornea before and after laser-assisted subepithelial keratectomy for myopia

Distribution of 50-layer corneal densitometry values and related factors

PURPOSE

To investigate the distribution of 50 layers of corneal densitometry and related factors.

METHODS

Clinical data, including age, sex, central corneal thickness, corneal keratometry, and diopters, were collected from 102 healthy participants (102 eyes) in this retrospective study. The cornea was divided into 50 layers, and densitometry of each layer at 19 points was measured by the Pentacam. The value versus the depth curve was plotted. Paired-sample t test and one-way analysis of variance were used to compare densitometry in different regions or depth. Statistical significance was defined as P < .05.

RESULTS

The densitometry values of the Bowman membrane (10-14% depth), anterior stroma (14-30% depth), epithelium (0-10% depth), and Descemet membrane (94-98% depth) decreased sequentially, and the densitometry values of the middle and posterior stroma (30-94% depth) and endothelium (98-100% depth) were the lowest. The higher the degree of astigmatism, the higher the second densitometry peak (R = 0.277, P < .001). The densitometry values of the vertex and superior parts of the cornea were higher than those in the periphery and inferior parts, respectively (all P < .001). In the Bowman membrane, the densitometry is lowest in the inferior nasal part, while in the Descemet membrane, it is lowest in the inferior temporal part.

CONCLUSION

Two densitometry peaks appeared near the Bowman membrane and Descemet membrane. For different depths, the distribution of densitometry within a layer is different. We provide a methodological reference and data basis for corneal research based on local changes in densitometry, and help understand the details of corneal structure from an optical perspective through detailed layering and zoning analysis of densitometry.

Comparison of Corneal Densitometry and Visual Quality after Small Incision Lenticule Extraction (SMILE) and Laser Epithelial Keratomileusis (LASEK): One-Year Comparative Study

Purpose

To investigate changes in corneal densitometry (CD) and visual quality following small incision lenticule extraction (SMILE) and laser epithelial keratomileusis (LASEK) in patients with mild-to-moderate myopia.

Methods

A retrospective analysis was performed on 24 and 25 patients (46 eyes each) who underwent SMILE and LASEK, respectively, for mild-to-moderate myopia. The visual quality and CD values were recorded. Using the Pentacam Scheimpflug system, CD values were collected in three concentric optical zones at the depths of the anterior, central, and posterior layers. Efficacy, safety, predictability, corneal wavefront aberrations, and QoV scores were measured to evaluate visual quality. A correlation analysis was performed between changes in CD and clinical characteristics.

Results

There were no statistical differences in efficacy and safety indices between the two groups. At 3 months postoperatively, a pronounced reduction in several zones was observed in the LASEK group (p < 0.05), whereas no obvious change was observed in the SMILE group. There were obvious changes in the CD values in several zones in the SMILE and LASEK groups (p < 0.05) after 1 year. The magnitude of the CD changes in the anterior and central corneal layers was smaller in the SMILE group than in the LASEK group (all p < 0.05). Lower HOAs, spherical aberration, and horizontal comas of the anterior and whole corneal surfaces were observed in the SMILE group. QoV scores were similar between the two groups.

Conclusion

CD decreased in the SMILE and LASEK groups after 1 year; there was a smaller reduction in SMILE than in LASEK. SMILE and LASEK did not differ significantly in terms of safety and effectiveness in correcting mild-to-moderate myopia.

Corneal Densitometry and In Vivo Confocal Microscopy in Patients with Monoclonal Gammopathy—Analysis of 130 Eyes of 65 Subjects

Background: Corneal imaging may support an early diagnosis of monoclonal gammopathy. The goal of our study was to analyze corneal stromal properties using Pentacam and in vivo confocal cornea microscopy (IVCM) in subjects with monoclonal gammopathy. Patients and methods: In our cross-sectional study, patients with monoclonal gammopathy (130 eyes of 65 patients (40.0% males; age 67.65 ± 9.74 years)) and randomly selected individuals of the same age group, without hematological disease (100 eyes of 50 control subjects (40.0% males; age 60.67 ± 15.06 years)) were included. Using Pentacam (Pentacam HR; Oculus GmbH, Wetzlar, Germany), corneal stromal light scattering values were obtained (1) centrally 0–2 mm zone; (2) 2–6 mm zone; (3) 6–10 mm zone; (4) 10–12 mm zone. Using IVCM with Heidelberg Retina Tomograph with Rostock Cornea Module (Heidelberg Engineering, Heidelberg, Germany), the density of hyperreflective keratocytes and the number of hyperreflective spikes per image were manually analyzed, in the stroma. Results: In the first, second and third annular zone, light scattering was significantly higher in subjects with monoclonal gammopathy, than in controls (p ≤ 0.04). The number of hyperreflective keratocytes and hyperreflective spikes per image was significantly higher in stroma of subjects with monoclonal gammopathy (p ≤ 0.012). Conclusions: Our study confirms that increased corneal light scattering in the central 10 mm annular zone and increased keratocyte hyperreflectivity may give rise to suspicion of monoclonal gammopathy. As corneal light scattering is not increased at the limbal 10–12 mm annular zone in monoclonal gammopathy subjects, our spatial analysis provides evidence against the limbal origin of corneal paraprotein deposition. Using IVCM, stromal hyperreflective spikes may represent specific signs of monoclonal gammopathy.

Age-Related Corneal Transparency Changes Evaluated With an Alternative Method to Corneal Densitometry

PURPOSE

To compare densitometry distribution analysis (DDA), a platform-independent method to assess corneal transparency, with traditional corneal densitometry.

METHODS

A total of 196 healthy participants aged 43.3 ± 18.0 years (range 18-79 years) were recruited for assessment. All participants were assessed using the corneal densitometry analysis add-on to the standard software of the Oculus Pentacam HR. In addition, the Scheimpflug image corresponding to the horizontal meridian of each participant was exported for further analysis. For each image, corneal pixel intensities were statistically modeled. The estimated output parameters, α and β, were compared with the corresponding densitometry values. The analysis was performed considering 3 concentric areas and 3 layers defined at fixed corneal depths. To demonstrate the platform independence of the DDA method, a randomly selected subset of 80 participants also had their eye measured with Oculus Corvis ST.

RESULTS

α and β were found to be well correlated with densitometry, especially α (overall cornea; r = 0.89, P < 0.001), independent of the corneal region investigated. Changes in α, β, and corneal densitometry were correlated with age.

CONCLUSIONS

In this work, we presented the relationship of DDA with age and traditional corneal densitometry. The α and β parameters, the output of DDA, are platform independent and can be used to investigate corneal clarity objectively.

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.

Scheimpflug Densitometry in Keratoconus: A New Method of Visualizing the Cone

PURPOSE

To report an observation made while performing Scheimpflug densitometry analysis on the corneal region affected in keratoconus (KC) that seems to delineate the base of the cone.

METHODS

Scheimpflug densitometries of 20 healthy subjects and 90 patients with KC were examined. Corneal densitometry was analyzed using both "1-layer" and "2-layer" approaches. The first considers the corneal transparency layer by layer at different depths, whereas the second averages densitometry between 2 corneal layers selected by the examiner. Fixed layers, 120 μm depth, and endothelium were selected. Repeated same-day scans and longitudinal series of scans were also evaluated to see whether the findings evolved over time.

RESULTS

Eighty-eight of 90 KC cases displayed a bright area on the densitometry map that corresponded to the cone location. The area's characteristics, such as its brightness, contrast, and the presence of a delimiting arc correlated with KC severity and was more noticeable in advanced cases. No similar marks were found in any of the normal subjects. The shape, location, and extent of the mark were consistent over consecutive measures taken on the same day. Changes over time were also seen in eyes with known clinical progression but was also seen in eyes considered clinically stable.

CONCLUSIONS

The densitometry mark seems to correspond with the zone most affected by KC and could be a supplementary tool for documenting KC stage, alongside conventional parameters. Further studies are required to ascertain whether it could prove useful in KC detection, to determine progression, and to relate it to corneal biomechanical behavior.

Incidence and quantification of corneal haze by Pentacam Scheimpflug densitometry following photorefractive keratectomy for myopia in virgin and post corneal transplant eyes with dark irides

PURPOSE:

To assess the incidence and quantification of corneal haze after myopic photorefractive keratectomy in virgin and post corneal transplant eyes with dark irides.

METHODS:

In this retrospective study at the tertiary eye hospital, the corneal haze was evaluated by slit-lamp and quantified by Pentacam Scheimpflug densitometry after myopic photorefractive keratectomy with mitomycin C in virgin eyes (group I) and post corneal transplant eyes (group II) with dark irides over 12 months.

RESULTS:

Group I compromised 126eyes from 77patients (35males and 42females) aged 31.31 years (24.11–36.18 years), and group II compromised 44eyes from 33patients (18males and 15females) aged of 32.25 years (26.15–36.66 years). The incidence of corneal haze was 9.5% (95%CI: 5.0%–16%) and 6.8% (95%CI: 1.4–18.7%) in group I and II respectively (P = 0.587). The corneal densitometry was 22.69 ± 8.28GSU preoperatively and 17.98 ± 3.13GSU at 12 months postoperatively (P = 0.010) in group I. The corneal densitometry was 21.86 ± 6.22GSU preoperatively and 21.23 ± 4.29GSU at 12 months postoperatively (P = 0.815) in group II. High corneal maximal densitometry was associated with the thin central corneal thickness (P = 0.027), the presence of haze (P = 0.028), post-keratoplasty (P = 0.004), steep keratometry (P = 0.035).

CONCLUSION:

The incidence of corneal haze was comparable in virgin and post corneal transplant eyes. The Pentacam Scheimflug densitometry can be helpful in the diagnosis of corneal haze; however, the cutoff values need to be studied further in larger studies.

Methods for assessing forward and backward light scatter in patients with cataract

PURPOSE

To compare objective methods for assessing backward and forward light scatter and psychophysical tests in patients with cataracts.

SETTING

Moorfields Eye Hospital NHS Foundation Trust, London, United Kingdom.

DESIGN

Prospective case series.

METHODS

This study included patients scheduled for cataract surgery. Lens opacities were grouped into predominantly nuclear sclerotic, cortical, posterior subcapsular, and mixed cataracts. Backward light scatter was assessed using a rotating Scheimpflug imaging technique (Pentacam HR), forward light scatter using a straylight meter (C-Quant), and straylight using the double-pass method (Optical Quality Analysis System, point-spread function [PSF] meter). The results were correlated with visual acuity under photopic conditions as well as photopic and mesopic contrast sensitivity.

RESULTS

The study comprised 56 eyes of 56 patients. The mean age of the 23 men and 33 women was 71 years (range 48 to 84 years). Two patients were excluded. Of the remaining, 15 patients had predominantly nuclear sclerotic cataracts, 13 had cortical cataracts, 11 had posterior subcapsular cataracts, and 15 had mixed cataracts. Correlations between devices were low. The highest correlation was between PSF meter measurements and Scheimpflug measurements (r = 0.32). The best correlation between corrected distance visual acuity was with the PSF meter (r = 0.45).

CONCLUSIONS

Forward and backward light-scatter measurements cannot be used interchangeably. Scatter as an aspect of quality of vision was independent of acuity. Measuring forward light scatter with the straylight meter can be a useful additional tool in preoperative decision-making.

Straylight As The Result Of Refractive Correction

Purpose

To investigate the effect of refractive correction on straylight.

Patients and methods

Straylight values were measured with the C-Quant (Oculus Optikgeräte, GmbH, Wetzlar, Germany) in 1) near-emmetropic eyes (n=30) with various negative powered refractive lenses and in 2) myopic eyes (n=30) corrected with prescribed eyeglasses and contact lenses. The straylight measurements in each group were compared in the different conditions.

Results

In the near-emmetropic group, a significant effect (p<0.001) of each added negative diopter was found to increase straylight values with 0.006 log-units. In the second group, no significant correlation with type of correcting lens was found on straylight values.

Conclusion

Refractive correction with high minus power (contact) lenses result in subtle increase of straylight values. These changes are relatively small and do not lead to visual disability in a clinical setting.

Quantitative evaluation of corneal epithelial edema after cataract surgery using corneal densitometry: a prospective study

PURPOSE

The optical density of the cornea can be evaluated quantitatively by "densitometry" using a rotating Scheimpflug camera. Densitometry allows evaluation of corneal opacity in the anterior segment of the eye by quantitative measurement of scattering light. In the present investigation, we evaluate quantitatively minimal subclinical corneal edema after cataract surgery using densitometry.

METHODS

Fifty four eyes of 34 patients who underwent cataract surgery were enrolled. Measurement of corneal density was performed using Pentacam® before and on days 1, 3 and 7 after surgery.

RESULTS

Densitometry scores increased from 18.12 ± 1.76 before cataract surgery to 21.03 ± 3.84 on day 1 (P < 0.001) and 19.90 ± 2.46 on day 3 (P = 0.018), but recovered to 19.44 ± 1.58 on day 7 (P = 0.131). Total corneal thickness was 549.1 ± 32.7 μm before surgery and increased to 582.7 ± 46.3 μm on day 1 (P = 0.001), but recovered to 566.4 ± 29.7 μm on day 3 (P = 0.097). Densitometry reading correlated positively with corneal thickness (correlation coefficient = 0.13, P = 0.003).

CONCLUSIONS

Densitometry is useful to detect corneal edema that is not detectable by slit-lamp examination.

Comparison of densitometric profile after deep lamellar keratoplasty with two different techniques for treatment of keratoconus

PURPOSE

To compare densitometric outcome of two techniques of deep lamellar keratoplasty using Pentacam HR corneal densitometry software.

METHODS

Postoperative outcomes of 31 patients with big bubble (BB) and 28 patients with Melles (M) surgery for the treatment of keratoconus were evaluated. Data were gathered at least 3 months after the removal of all sutures including demographic data, uncorrected distance visual acuity (UCDVA), best spectacle-corrected visual acuity (BSCVA), refraction and endothelial cell count. Moreover, the Scheimpflug images were taken by Pentacam^® HR to assess keratometric profile, densitometric outcomes in different depths and zones of the cornea and central corneal thickness. Anterior segment OCT was performed in ten cases of M group.

RESULTS

Thirty-one eyes with BB and 27 eyes M surgery were enrolled. Comparison of densitometric profile between groups revealed no statistical significance in zone 0-2 mm, zone 2-6 mm and zone 10-12 mm in all depths (all P values >  0.05). Zone 6-10 mm of M group had lower densitometric readings (more transparent) in anterior, central and posterior layers of the cornea in comparison with BB group (all P <  0.05). Also no statistical difference was found in UCDVA, BSCVA, spherical or cylindrical refractive error, and spherical equivalent, keratometric readings, endothelial cell count and central corneal thickness in two groups (all P values >  0.05).

CONCLUSION

Densitometric profiles were identical in both methods in visually important zones so as visual and refractive outcomes.

A Three-Year Observation of Corneal Backscatter After Small Incision Lenticule Extraction (SMILE)

PURPOSE

To investigate changes in corneal backscatter after small incision lenticule extraction (SMILE).

METHODS

One eye was randomly selected from 34 patients who underwent SMILE procedures. Corneal backscatter was analyzed using Scheimpflug technology before surgery and at postoperative 1 day, 1 week, 1 month, 3, 6, and 12 months, and 3 years. Three different corneal depths (the anterior 120 µm, the center, and the posterior 60 µm) and three concentric radial zones (0 to 2, 2 to 6, and 6 to 10 mm) were analyzed. Another 40 eyes of 40 patients who underwent femtosecond laser-assisted LASIK (FS-LASIK) were examined before surgery and at postoperative 1 day and 3 years as the control group.

RESULTS

At postoperative day 1, a significant increase in corneal backscatter was detected in the SMILE group compared to preoperative values in the central 0 to 6 mm zone of the anterior layer and the 0 to 2 mm zone of the central layer (all P < .01). This increase was higher than the increase in the FS-LASIK group in the central 0 to 6 mm zone of the anterior and central layers (all P < .01). No significant differences in corneal backscatter were detected between preoperative values and the 1-week, and 1-, 3-, 6-, and 12-month follow-up visits in the SMILE group (all P > .05). Moreover, a significant decrease in corneal backscatter was detected in the three zones of all three layers in the two groups at 3 years postoperatively compared to preoperative values (all P < .001) and no significant difference was found between the two groups (all P < .05).

CONCLUSIONS

Corneal backscatter peaks and declines to the baseline in the early period after SMILE and corneal transparency achieves an improvement within 3 years postoperatively. [J Refract Surg. 2017;33(6):377-382.].

Long-Term Changes in Backscattered Light Measurements in Keratoconus Corneas Treated with Collagen Cross-Linking

PURPOSE

Our aim was to compare densitometry data between keratoconus and normal corneas and to assess the long-term changes in corneal backscattered light values after corneal collagen cross-linking (CXL) treatment.

MATERIAL AND METHODS

Retrospective analysis was performed on 26 eyes of normal patients (age: 32.71 ± 12.68 years) and 39 eyes of keratoconus patients (28.93 ± 7.59 years) using the Pentacam HR corneal densitometry module before CXL and during the postoperative period, which lasted a mean of 2.53 years (range: 1-4 years). Corneal backscattered light values in grey scale unit (GSU) were recorded for the anterior 120 µm, the center layer, and the posterior 60 µm of the cornea in four concentric, central rings. An additional calculation was performed with converting GSU values to GSU/cubic millimeters (mm³) with surface area calculations and using corneal thickness data.

RESULTS

Statistically significant differences were observed between normal and keratoconus group densitometry values (in GSU/mm³) in all annuli of the center layer; all, but anterior layer of the 10-12 annuli; all annuli of the total thickness and in all values of the total diameter. In all these cases, the normal cornea showed higher GSU/mm³ values compared to keratoconus corneas (p < 0.05). One month after the CXL treatment, the GSU/mm³ data increased significantly in the anterior, the center, and the posterior zone (p < 0.05), followed by its slow decrease. At the end of the follow-up period, the GSU/mm³ values were significantly higher in all three zones compared to those before the CXL treatment (p < 0.01).

CONCLUSION

Densitometry measurements can reflect the changes of optical quality of the cornea. These measurements may play a valuable role in assessing keratoconus and optical changes of the corneas after CXL treatment.

Effects of Atopic Syndrome on Keratoconus

PURPOSE

To evaluate the effects of atopic syndrome on manifestations of keratoconus.

METHODS

In this retrospective study, we reviewed patient files and data generated by Scheimpflug imaging of 670 eyes of 434 keratoconus patients. Patients were divided into a study group consisting of patients suffering from atopic syndrome (110 eyes of 75 patients), namely allergic asthma, atopic dermatitis, and/or allergic rhinitis, and a control group of patients without known atopic syndrome (560 eyes of 359 patients).

RESULTS

We found a significant difference with the mean age being 36.1 ± 11.7 for the control group, 32.8 ± 9.6 for the atopic group (P = 0.002) with 1 atopic trait, and 30.4 ± 7.5 for patients with 2 or more atopic traits (P = 0.002). No statistically significant differences were found in the mean corrected distance visual acuity, corneal pachymetry, minimum relative pachymetric progression (RPImin), mean refraction, keratoconus index, anterior chamber depth and volume, Kmax, and location of Kmax in relation to the corneal apex. However, we found a significantly higher corneal density for the anterior 120 μm of the cornea in the atopic group (control: 20.74 ± 4.68, atopic group: 21.92 ± 4.65 P = 0.016).

CONCLUSIONS

Keratoconus patients suffering from atopic syndrome were significantly younger but showed no topographical changes except in corneal densitometry compared with keratoconus patients without an atopic disease. This suggests atopic syndrome is a factor, which can trigger earlier manifestation of keratoconus.

Changes in Corneal Densitometry in Patients with Fuchs Endothelial Dystrophy after Endothelial Keratoplasty

PURPOSE

The aim of the study was to quantify corneal densitometry in patients with Fuchs endothelial dystrophy (FED) after endothelial keratoplasty.

MATERIALS AND METHODS

We retrospectively reviewed the charts and anterior segment data of patients with FED before and after endothelial keratoplasty. Patients were examined using the Scheimpflug-based Oculus Pentacam corneal densitometry module. Densitometry parameters in different corneal layers and in different annuli were extracted and analyzed.

RESULTS

27 eyes of 27 patients after endothelial keratoplasty (11 DSAEK, 16 DMEK) were included. After endothelial keratoplasty the total corneal light backscatter at total corneal thickness in the central cornea (0-2 mm annulus) was significantly lower than before (DSAEK: p = 0.026, DMEK: p = 0.001). In the entire group the total corneal light backscatter at total corneal thickness and at total diameter before surgery correlated with the postoperative values (Pearson correlation = 0.49, p = 0.01). The strongest correlation was found in the central layer in the DMEK group (Pearson correlation = 0.79, p < 0.001).

CONCLUSIONS

Corneal densitometry is a useful, objective method for quantification of the outcome of posterior lamellar keratoplasty irrespective of visual acuity. There is a significant correlation between preoperative and postoperative corneal light backscatter values after endothelial keratoplasty, especially in the case of the DMEK procedure.

Changes in Corneal Transparency After Cross-linking for Progressive Keratoconus: Long-term Follow-up

PURPOSE

To determine long-term changes in corneal transparency after riboflavin-ultraviolet A-induced corneal collagen cross-linking (CXL).

METHODS

Charts and anterior segment data of patients after CXL for progressive keratoconus were retrospectively reviewed. Patients were examined using the Scheimpflug-based Pentacam corneal densitometry module (Oculus Optikgeräte, Wetzlar, Germany) before CXL and at five postoperative follow-up visits: 1 to 3, 3 to 6, 6 to 12, 12 to 24, and 24 to 36 months.

RESULTS

Forty-two eyes of 28 patients (mean age: 27.9 ± 8.6 years) were included. Total corneal light backscatter was higher 1 to 3 months after CXL than before CXL (P < .001). There were significant differences, especially in the anterior (P < .001) and central (P < .001) layer at total diameter and posterior layer (P = .014) and the three central annuli at total corneal thickness (0 to 2 mm: P < .001; 2 to 6 mm: P < .001; 6 to 10 mm: P = .002). Total corneal light backscatter at total corneal thickness and total diameter faded over time following CXL. The backscatter was significantly lower 24 to 36 months after CXL than before CXL (P < .001).

CONCLUSIONS

Corneal densitometry peaks in the first months after CXL and returns to preoperative values approximately 1 year after CXL. Two years after CXL, corneal densitometry reaches values obtained for healthy, untreated corneas, thus achieving an improvement in corneal clarity over untreated keratoconic corneas.

Effect of corneal light scatter on vision: a review of the literature

The cornea is the transparent connective tissue window at the front of the eye. The physiological role of the cornea is to conduct external light into the eye, focus it, together with the lens, onto the retina, and to provide rigidity to the entire eyeball. Therefore, good vision requires maintenance of the transparency and proper refractive shape of the cornea. The surface structures irregularities can be associated with wavefront aberrations and scattering errors. Light scattering in the human cornea causes a reduction of visual quality. In fact, the cornea must be transparent and maintain a smooth and stable curvature since it contributes to the major part of the focusing power of the eye. In most cases, a simple examination of visual acuity cannot demonstrate the reduction of visual quality secondary light scattering. In fact, clinical techniques for examining the human cornea in vivo have greatly expanded over the last few decades. The measurement of corneal back scattering qualifies the degree of corneal transparency. The measurement of corneal forward-scattering quantifies the amount of visual impairment that is produced by the alteration of transparency. The aim of this study was to review scattering in the human cornea and methods of measuring it.

Corneal structure and transparency

The corneal stroma plays several pivotal roles within the eye. Optically, it is the main refracting lens and thus has to combine almost perfect transmission of visible light with precise shape, in order to focus incoming light. Furthermore, mechanically it has to be extremely tough to protect the inner contents of the eye. These functions are governed by its structure at all hierarchical levels. The basic principles of corneal structure and transparency have been known for some time, but in recent years X-ray scattering and other methods have revealed that the details of this structure are far more complex than previously thought and that the intricacy of the arrangement of the collagenous lamellae provides the shape and the mechanical properties of the tissue. At the molecular level, modern technologies and theoretical modelling have started to explain exactly how the collagen fibrils are arranged within the stromal lamellae and how proteoglycans maintain this ultrastructure. In this review we describe the current state of knowledge about the three-dimensional stromal architecture at the microscopic level, and about the control mechanisms at the nanoscopic level that lead to optical transparency.

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.

Changes in Forward and Backward Light Scatter in Keratoconus Resulting From Corneal Cross-Linking

PURPOSE

To study the forward and backward light scatter in keratoconic corneas before and after cross-linking.

DESIGN

An institutional, prospective, cross-sectional study was conducted.

METHODS

This study includes 35 eyes of 25 patients with keratoconus scheduled for either standard corneal collagen cross-linking with riboflavin (CXL) or transepithelial corneal cross-linking (TE-CXL). A group of 26 healthy myopic eyes from 26 prerefractive patients was included as normal reference. Before and 6 months after cross-linking, forward light scatter was measured using the compensation comparison method, whereas backward light scatter was measured using Scheimpflug imaging.

RESULTS

In keratoconic eyes, backward light scatter originating from the corneal stroma was [mean (SD)] 27.2% (8.2%) higher than in the normal eyes (P < 0.001). In the anterior stroma, this increased backscatter was significantly correlated with the maximum corneal curvature Kmax as a measure of keratoconus severity (Pearson ρ = 0.582, P = 0.003). For forward light scatter, no significant difference was seen between the normal and keratoconus groups, nor was there any correlation with keratoconus severity. After CXL, the backscatter increased significantly by [mean (SD)] 33.0% (9.5%) in the entire corneal stroma (P = 0.001), whereas for TE-CXL, no significant increase was seen. Forward scatter increased significantly by [mean (SD)] 0.10 (0.10) log units (P = 0.009) and 0.09 (0.10) log units (P = 0.003) for CXL and TE-CXL, respectively, which is near the detection limit for an average patient.

CONCLUSIONS

Unlike TE-CXL, CXL increases the already-elevated stromal backscatter in keratoconus. Forward scatter increases equally for both techniques.

History of ocular straylight measurement: A review

The earliest studies on 'disability glare' date from the early 20(th) century. The condition was defined as the negative effect on visual function of a bright light located at some distance in the visual field. It was found that for larger angles (>1 degree) the functional effect corresponded precisely to the effect of a light with a luminosity equal to that of the light that is perceived spreading around such a bright source. This perceived spreading of light was called straylight and by international standard disability glare was defined as identical to straylight. The phenomenon was recognized in the ophthalmological community as an important aspect of the quality of vision and attempts were made to design instruments to measure it. This must not be confused with instruments that assess light spreading over small distances (<1 degree), as originating from (higher order) aberrations and defocus. In recent years a new instrument has gained acceptance (C-Quant) for objective and controllable assessment of straylight in the clinical setting. This overview provides a sketch of the historical development of straylight measurement, as well as the results of studies on the origins of straylight (or disability glare) in the normal eye, and on findings on cataract (surgery) and corneal conditions.