Evaluation of corneal stromal changes in vivo after laser in situ keratomileusis with confocal microscopy

A new method for in vivo assessment of corneal transparency using spectral-domain OCT

Corneal transparency is essential to provide a clear view into and out of the eye, yet clinical means to assess such transparency are extremely limited and usually involve a subjective grading of visible opacities by means of slit-lamp biomicroscopy. Here, we describe an automated algorithm allowing extraction of quantitative corneal transparency parameters with standard clinical spectral-domain optical coherence tomography (SD-OCT). Our algorithm employs a novel pre-processing procedure to standardize SD-OCT image analysis and to numerically correct common instrumental artifacts before extracting mean intensity stromal-depth (z) profiles over a 6-mm-wide corneal area. The z-profiles are analyzed using our previously developed objective method that derives quantitative transparency parameters directly related to the physics of light propagation in tissues. Tissular heterogeneity is quantified by the Birge ratio Br and the photon mean-free path (ls) is determined for homogeneous tissues (i.e., Br~1). SD-OCT images of 83 normal corneas (ages 22-50 years) from a standard SD-OCT device (RTVue-XR Avanti, Optovue Inc.) were processed to establish a normative dataset of transparency values. After confirming stromal homogeneity (Br <10), we measured a median ls of 570 μm (interdecile range: 270-2400 μm). By also considering corneal thicknesses, this may be translated into a median fraction of transmitted (coherent) light Tcoh(stroma) of 51% (interdecile range: 22-83%). Excluding images with central saturation artifact raised our median Tcoh(stroma) to 73% (interdecile range: 34-84%). These transparency values are slightly lower than those previously reported, which we attribute to the detection configuration of SD-OCT with a relatively small and selective acceptance angle. No statistically significant correlation between transparency and age or thickness was found. In conclusion, our algorithm provides robust and quantitative measurements of corneal transparency from standard SD-OCT images with sufficient quality (such as 'Line' and 'CrossLine' B-scan modes without central saturation artifact) and addresses the demand for such an objective means in the clinical setting.

The intra- and inter-day repeatability of corneal densitometry measurements in subjects with keratoconus and in healthy controls

The healthy cornea is transparent, however, disease can affect its structure, rendering it more or less opaque. The ability to assess the clarity of the cornea objectively could thus be of considerable interest for keratoconus patients. It has previously been suggested that densitometry can be used to diagnose early keratoconus, and that the values of densitometry variables increase with increasing disease severity, indicating that densitometry could also be used to assess progressive keratoconus. Previous studies have only assessed the repeatability of corneal densitometry measurements on the same day, which does not reflect the clinical setting in which changes are evaluated over time. We have therefore evaluated the inter-day repeatability of densitometry measurements in both patients with keratoconus and healthy controls. Measurements in the middle layer of the 2-6 mm zone of the cornea showed the best repeatability. Although an objective measure of the corneal transparency could be interesting, the generally poor repeatability of densitometry measurements limits their use. The repeatability of corneal clarity measurements could be improved by using other approaches such as optical coherence tomography, but this remains to be investigated. Such improvements would allow the more widespread use of corneal densitometry in clinical practice.

Clinical Applications of In Vivo Confocal Microscopy in Keratorefractive Surgery

PURPOSE

To review the contribution of in vivo confocal microscopy (IVCM) to the understanding of corneal wound healing following refractive surgery, and its role in the diagnosis and management of complications arising from keratorefractive procedures.

METHODS

Review of the basic science and clinical literature relating to the study of keratorefractive surgical procedures using IVCM.

RESULTS

Extensive research using IVCM has generated a comprehensive understanding of tissue responses after corneal refractive surgery. Epithelial thickness and stromal keratocyte density can be quantified postoperatively and studied longitudinally. Corneal nerve loss and subsequent reinnervation has been characterized and differs significantly between laser refractive techniques. IVCM has also been used to study complications arising from postoperative inflammation (diffuse lamellar keratitis, central toxic keratopathy, ring keratitis, and ectasia), infection (microbial keratitis), and neuropathy (dry eye and neuralgia). This imaging technique can have a critical role in the diagnosis of these complications and subsequent monitoring of treatment response. Manual processing of IVCM images is time-consuming and there may be significant interobserver and intraobserver variability with poor repeatability. However, increasing automation and the use of artificial intelligence is improving the speed and accuracy of image analysis.

CONCLUSIONS

IVCM has historically been confined to a research setting because image capture and subsequent processing was extremely labor intensive. However, advances in both hardware and software capabilities promise to allow the use of IVCM in routine clinical practice. Real-time evaluation of the cornea at a cellular level will be particularly useful in patients with inflammatory, infectious, or neuropathic complications of keratorefractive surgery. [J Refract Surg. 2021;37(7):493-503.].

Confocal and Histological Features After Poly(Ethylene Glycol) Diacrylate Corneal Inlay Implantation

Purpose

To evaluate the in vivo biocompatibility of photopolymerized poly(ethylene glycol) diacrylate (PEGDA) intrastromal inlays in rabbit corneas.

Methods

Sixty-three eyes of 42 New Zealand rabbits were included. Manual intrastromal pockets were dissected in 42 eyes. PEGDA inlays were obtained using a specifically designed photomask and were inserted in the intrastromal pocket of 21 eyes (inlay group); the remaining 21 right eyes did not receive any implant (pocket-only group). Twenty-one eyes with no intervention were used as controls. In vivo confocal microscopy (IVCM) was performed at every visit. After 2 months, rabbits were sacrificed and corneas removed for histological analysis.

Results

Corneas remained clear in all but two animals, and five cases of corneal neovascularization were seen (P = 0.2). Inlays remained stable without evidence of lateral or anterior migration, and no other complications were observed. No changes in anterior and posterior keratocyte density (P = 0.3 and P = 0.1, respectively) or endothelial cell density (P = 0.23) were observed between groups during the study time by IVCM. On pathology samples, thinning of the epithelium over the inlay area and epithelial hyperplasia over the edges were observed. A polygonal empty space with no evidence of PEGDA hydrogel within the midstroma was seen in the inlay group. Keratocytes were normal in shape and number in the vicinity of the PEGDA implant area.

Conclusions

Photopolymerized PEGDA intrastromal inlays have shown relatively good safety and stability in rabbit corneas. Inlays were biostable in the corneal environment and remained transparent during follow up.

Translational Relevance

The investigated PEGDA is promising for the development of biocompatible intrastromal implants.

Quantitative measures of corneal transparency, derived from objective analysis of depth-resolved corneal images, demonstrated with full-field optical coherence tomographic microscopy

Loss of corneal transparency, as occurs with various pathologies, infections, immune reactions, trauma, aging, and surgery, is a major cause of visual handicap worldwide. However, current means to assess corneal transparency are extremely limited and clinical and eye-bank practice usually involve a subjective and qualitative observation of opacities, sometimes with comparison against an arbitrary grading scale, by means of slit-lamp biomicroscopy. Here, we describe a novel objective optical data analysis-based method that enables quantifiable and standardized characterization of corneal transparency from depth-resolved corneal images, addressing the demand for such a means in both the laboratory and clinical ophthalmology setting. Our approach is based on a mathematical analysis of the acquired optical data with respect to the light attenuation from scattering processes in the corneal stroma. Applicable to any depth-resolved corneal imaging modality, it has been validated by means of full-field optical coherence tomographic microscopy (FF-OCT or FF-OCM). Specifically, our results on ex-vivo corneal specimens illustrate that 1) in homogeneous tissues, characterized by an exponential light attenuation with stromal depth (z), the computation of the scattering mean-free path (ls) from the rate of exponential decay allows quantification of the degree of transparency; 2) in heterogeneous tissues, identified by significant deviations from the normal exponential z -profile, a measure of exponential-decay model inadequacy (e.g., by computation of the Birge ratio) allows the estimation of severity of stromal heterogeneity, and the associated depth-dependent variations around the average ls enables precise localization of the pathology.

Cornea donors who have had prior refractive surgery: data from the Eye Bank Association of America

PURPOSE OF REVIEW

Millions of Americans have undergone refractive surgeries, including radial keratotomy, photorefractive keratectomy, and laser-assisted in situ keratomileusis. Eye Bank Association of America medical standards do not permit corneas from patients who have undergone refractive procedures to be used in penetrating keratoplasty, anterior lamellar keratoplasty, or tectonic grafting procedures. Such corneas, can, however, be used for endothelial corneal transplantation. The objective of this article is to provide an update on current trends for the screening and usage of corneas that have undergone refractive surgery.

RECENT FINDINGS

Several case reports have highlighted the difficulty in using postrefractive surgery corneas in penetrating keratoplasty. However, tissue with anterior stromal flaws, including a history of refractive surgery, has been used in endothelial keratoplasty with equivalent outcomes in topography, endothelial cell count, and visual acuity. Many modalities for proper identification of postmortem donor corneas that have undergone refractive surgery have been studied.

SUMMARY

Corneas with a history of refractive surgery have found use in endothelial keratoplasty. Multiple objective methods of tissue identification have been investigated to avoid the use of these corneas in penetrating or anterior keratoplasty surgeries.

Changes in Keratocyte Density and Visual Function Five Years After Laser In Situ Keratomileusis: Femtosecond Laser Versus Mechanical Microkeratome

PURPOSE

To determine the effects of keratocyte loss on optical properties and vision after laser in situ keratomileusis (LASIK) with the flap created with a femtosecond laser or a mechanical microkeratome.

DESIGN

Randomized clinical paired-eye study.

METHODS

Both eyes of 21 patients received LASIK for myopia or myopic astigmatism. One eye of each patient was randomized by ocular dominance to flap creation with a femtosecond laser and the other eye to flap creation with a mechanical microkeratome. Before LASIK and at 1, 3, and 6 months and 1, 3, and 5 years after LASIK, keratocyte density was measured using confocal microscopy, and high-contrast visual acuity and anterior corneal wavefront aberrations were measured by standard methods. At each visit, all variables were compared between methods of creating the flap and to the same variable before treatment using paired tests with Bonferroni correction for multiple comparisons.

RESULTS

Keratocyte density in the flap decreased by 20% during the first year after LASIK and remained low through 5 years (P < .001). High-order wavefront aberrations increased and uncorrected visual acuity improved immediately after surgery, but these variables did not change further to 5 years. There were no differences in any variables between treatments.

CONCLUSIONS

A sustained reduction in keratocyte density does not affect vision or optical properties of the cornea through 5 years after LASIK. The method of creating a LASIK flap does not influence the changes in keratocyte density in the flap.

Repeated monitoring of corneal nerves by confocal microscopy as an index of peripheral neuropathy in type-1 diabetic rodents and the effects of topical insulin

We developed a reliable imaging and quantitative analysis method for in vivo corneal confocal microscopy (CCM) in rodents and used it to determine whether models of type 1 diabetes replicate the depletion of corneal nerves reported in diabetic patients. Quantification was reproducible between observers and stable across repeated time points in two rat strains. Longitudinal studies were performed in normal and streptozotocin (STZ)-diabetic rats, with innervation of plantar paw skin quantified using standard histological methods after 40 weeks of diabetes. Diabetic rats showed an initial increase, then a gradual reduction in occupancy of nerves in the sub-basal plexus so that values were significantly lower at week 40 (68 ± 6%) than age-matched controls (80 ± 2%). No significant loss of stromal or intra-epidermal nerves was detected. In a separate study, insulin was applied daily to the eye of control and STZ-diabetic mice and this treatment prevented depletion of nerves of the sub-basal plexus. Longitudinal studies are viable in rodents using CCM and depletion of distal corneal nerves precedes detectable loss of epidermal nerves in the foot, suggesting that diabetic neuropathy is not length dependent. Loss of insulin-derived neurotrophic support may contribute to the pathogenesis of corneal nerve depletion in type 1 diabetes.

Confocal microscopy evaluation of the corneal response following AcuFocus KAMRA inlay implantation

PURPOSE

To describe the corneal appearance on confocal microscopy after AcuFocus KAMRA Inlay (AcuFocus, Inc., Irvine, CA) implantation and evaluate the visual acuity compared to the confocal microscopy data.

METHODS

Twelve eyes of 12 patients implanted with one of three models of the AcuFocus KAMRA Inlay (ACI 7000, 7000T, and 7000PDT) were prospectively evaluated by confocal microscopy 6 months after implantation. Additionally, 4 eyes of 4 patients explanted during the follow-up period were evaluated.

RESULTS

Among the eyes implanted, mean epithelial thickness was 54.6 ± 22 μm. The subbasal nerve plexus was detected in 10 patients. The corneal nerves per unit area were 2.73 ± 2.1 sprouts/mm(2). The branch pattern was found in 8 patients. The mean keratocyte density value was 540 ± 210 cells/mm(2). A low grade of keratocyte activation was found in all patients. Among the eyes explanted, the mean wound healing opacity was 1,092.75 ± 1,877.35 μm/pixel.

CONCLUSIONS

The corneal tolerance to the KAMRA Inlay appeared to be good. The inlay modified the normal structure of the corneal layer, but it was not associated with severe complications of the eye. Keratocyte activation was the finding most associated with a negative visual outcome. Confocal microscopy can be useful to evaluate the long-term evolution of the corneal layer changes following KAMRA Inlay implantation.

Cellular effects after laser in situ keratomileusis flap formation with femtosecond lasers: a review

PURPOSE

To provide an overview of the cellular effects of femtosecond laser in laser in situ keratomileusis flap formation.

METHODS

Literature review.

RESULTS

The IntraLase is the only femtosecond laser with sufficient histopathological and confocal studies to allow review of the cellular effects of laser application. Histopathological analyses have demonstrated that the energy per pulse and total energy delivered play important roles in the inflammatory reaction to the surgery. The IntraLase laser triggers cellular necrosis (death accompanied by the release of lysosomal enzymes and other components from membrane-bound intracellular compartments) in the corneal stroma surrounding the lamellar cut rather than apoptosis (gentler form of cell death in which most intracellular components remain confined to membrane-bound apoptotic bodies) that is predominant with the microkeratome. Necrosis is a more inflammatory form of cell death that attracts more inflammatory cells. This is likely why earlier femtosecond lasers, such as the 15-kHz IntraLase laser, which requires higher total energy delivery to cut a flap, are associated with more corneal inflammation and diffuse lamellar keratitis. The design of the 60-kHz IntraLase model allows for much lower energy delivery to cut the flap and, therefore, a substantial reduction in keratocyte necrosis to the point that the overall inflammatory response is not significantly different from the microkeratome. Histopathological analysis performed with the Femtec femtosecond laser noted little change in the corneal stromal structure. Confocal microcopy studies performed with the IntraLase laser showed keratocyte "activation" in the stroma and greater fibrotic scarring at the interface than that induced by a mechanical microkeratome.

CONCLUSIONS

The morphologic alterations in the corneal stroma produced by currently available models of the IntraLase laser are comparable to those produced by mechanical microkeratomes. Advances that have resulted in a reduction in the total amount of energy delivered by the laser when it cuts the flap have resulted in a decrease in the inflammatory response associated with femtosecond flap formation to the point that it is indistinguishable from the microkeratome at the cellular level. Further study of each of the femtosecond laser models, including the 150-kHz IntraLase laser, is needed to fully characterize the corneal response to these lasers.

Keratocyte density 3 months, 15 months, and 3 years after corneal surface ablation with mitomycin C

PURPOSE

To study the effects of surface ablation with mitomycin C (MMC) on keratocyte population.

DESIGN

Prospective, nonrandomized, interventional, comparative case series.

METHODS

Thirty two eyes treated with surface ablation with 0.02% MMC were compared with nontreated eyes at Vissum Santa Hortensia, Madrid, Spain. Keratocyte density was measured with the Heidelberg Retina Tomograph II (Rostock Cornea Module) 3, 15, and 36 to 42 months after the surgery in the anterior, mid, and posterior stroma, and compared with control eyes.

RESULTS

Three months postoperatively, we found a lower stromal bed density compared to controls (16 993 ± 8001 vs 29 660 ± 5904 cells/mm(3), P = .0001), while there was a significantly higher cell density in the mid (30 783 ± 9300 vs 18 505 ± 1996 cells/mm(3), P = .0001) and deep stroma (30 268 ± 8321 vs 18 438 ± 2139 cells/mm(3), P = .0001). Three years after the surgery, the cellularity in the stromal bed had not significantly changed from the 3-month follow-up, but the density in the mid (18 889 ± 3474 cells/mm(3)) and posterior stroma (18 992 ± 3402 cells/mm(3)) had decreased to show no difference from controls. The mean cell density between the anterior, mid, and posterior stroma was not significantly different from controls 15 months and 3 years after the surgery.

CONCLUSION

Our study suggests that there is a reorganization of the stromal cell population soon after surface ablation with MMC, with a decrease in the stromal bed compensated initially with an increase in the mid and posterior stroma. Corneal cellularity tends to normalize over time, and 3 years postoperatively the mean cell density throughout the cornea seems to maintain normal values.

Surface ablation techniques

The ongoing quest for a safe, simple, effective, minimally invasive, and stable refractive surgical procedure to correct refractive errors has stimulated the development of surface ablation techniques and laser in situ keratomileusis. In this review, we describe the history, patient assessment, techniques, outcomes, and complications of surface ablation (photorefractive keratectomy, laser-assisted sub-epithelial keratectomy, epithelial laser-assisted in situ keratomileusis) and compare the results of various surface techniques. Surface ablation procedures will continue to evolve, with potential improvements in outcomes accompanying future sophisticated ablation profiles and laser technology.

Keratocyte density after laser-assisted subepithelial keratectomy with mitomycin C

PURPOSE

To study the effects of laser-assisted subepithelial keratectomy (LASEK) with mitomycin C (MMC) on the keratocyte population.

DESIGN

Prospective, nonrandomized, interventional, comparative case series.

METHODS

Fifty-six eyes treated at Vissum Santa Hortensia, Madrid, Spain, were included in the study. We compared 28 eyes treated with LASEK with intraoperative 0.02% MMC versus 28 non-treated eyes. Keratocyte density was measured 3 months after the surgery in the anterior, mid, and posterior stroma and was compared with the corresponding layers in the control eyes. The anterior layer in the LASEK group was compared with 2 layers in the control group: the most anterior stromal layer and the 80 μm-deep layer, because that was the mean ablation depth performed in eyes that underwent LASEK.

RESULTS

We found a statistically significantly lower keratocyte population in the most anterior stromal layer after LASEK with MMC compared with both the most anterior stromal layer and the 80 μm-deep layer in controls. On the contrary, the treated group showed a significantly higher keratocyte density in both the mid stroma and the deep stroma. The comparison between the average densities through the entire cornea showed a significantly higher keratocyte population in the LASEK with MMC group.

CONCLUSIONS

LASEK with MMC seems to cause a decrease in the anterior stromal cells 3 months after the surgery compared with nonoperated corneas. There seems to be a compensating proliferation of keratocytes in the deeper corneal layers, suggesting that the ability of keratocytes to repopulate the cornea is maintained after the surgical procedure.

Persistent subepithelial haze in thin-flap LASIK

PURPOSE

To report persistent subepithelial haze in two patients following femtosecond LASIK associated with creation of a thin flap.

METHODS

Subepithelial haze was assessed by slit-lamp photography, high-resolution Scheimpflug imaging, and corneal confocal microscopy.

RESULTS

Two patients showed distinct subepithelial haze and reduced corrected distance visual acuity at 3 months after LASIK with a think-flap generated by a femtosecond laser. The extent of haze was documented, and the haze was treated topically with steroids up to 12 weeks. The haze was localized approximately 20 to 40 microm below Bowman's layer and dissolved slowly during the 2 months of treatment. At 6 months after surgery, uncorrected visual acuity was 20/20.

CONCLUSIONS

Subepithelial haze formation represents a new potential complication in the thin-flap LASIK.

Automated assessment of keratocyte density in stromal images from the ConfoScan 4 confocal microscope

Purpose. To develop a program to determine cell densities in images from the ConfoScan 4 (Nidek, Inc., Freemont, CA) confocal microscope and compare the densities with those determined in images obtained by the Tandem Scanning confocal microscope (Tandem Scanning Corp., Reston, VA). Methods. A program was developed that used image-processing routines to identify stromal cell nuclei in images from the ConfoScan 4 confocal microscope. Cell selection parameters were set to match cell densities from the program with those determined manually in 15 normal corneas of 15 volunteers. The program was tested on scans from 16 other normal volunteers and 17 volunteers 3 years after LASIK. Cell densities were compared to densities determined by manual assessment and to those in scans by the Tandem Scanning confocal microscope in the same corneas. Results. The difference in cell density between the automatic and manual assessment was -539 +/- 3005 cells/mm(3) (mean +/- SD, P = 0.11) in the 16 test corneas. Densities estimated from the ConfoScan 4 agreed with those from the Tandem Scanning confocal microscope in all regions of the stroma except in the anterior 10%, where the ConfoScan 4 indicated a 30% lower density. Conclusions. Differences in anterior stromal cell density between the ConfoScan 4 and the Tandem Scanning confocal microscope can be explained by the different optical designs. The lower spatial resolution of the ConfoScan 4 limits its ability to resolve thin layers. The adaptation of our earlier cell-counting program to the ConfoScan 4 provides a timesaving, objective, and reproducible means of determining stromal cell densities in images from the ConfoScan 4.

Digital colposcopy: ready for use? An overview of literature

The aims of this review were to summarise the various methods of digital colposcopy and to provide an overview of their efficacy. We conducted a literature search and focused on papers that described a technique for colposcopy, other than conventional colposcopy, and compared this with conventional colposcopy and/or histology and included digitalisation of the process. All papers have been classified in one of the following categories: digital imaging and telecolposcopy, spectroscopy, computerised colposcopy, optical coherence tomography and confocal microcolposcopy. Among the most promising developments is spectroscopy, allowing a more or less automated analysis and interpretation of the colposcopic image.

Thin-flap (sub-Bowman keratomileusis) versus thick-flap laser in situ keratomileusis for moderate to high myopia: case-control analysis

PURPOSE

To compare the refractive and visual outcomes of sub-Bowman keratomileusis (SBK) and thick-flap laser in situ keratomileusis (LASIK) for moderate to high myopia and evaluate the effect of corneal flap thickness on outcomes.

SETTING

Massachusetts Eye and Ear Infirmary, Boston, Massachusetts, USA.

METHODS

Two studies were performed. In the first study, the refractive and visual outcomes in 33 eyes that had SBK (flap thickness 82 to 120 microm) and 62 eyes that had thick-flap LASIK (flap thickness >or=160) were retrospectively analyzed. Inclusion criteria were spherical equivalent -4.0 to -10.0 diopters (D), astigmatism 3.0 D or less, and follow-up 3 months or more. In the second study, the influence of flap thickness was evaluated. A case-control matched study (21 pairs) that controlled for residual stromal bed (RSB) thickness was performed.

RESULTS

The mean flap thickness was 110.2 microm+/-9.2 (SD) in the SBK group and 179.2+/-19.5 microm in the thick-flap LASIK group. There were no significant differences in visual outcomes. In the second study with equivalent RSB thickness, case-control matched comparisons between SBK (mean 108.6+/-8.0 microm) and thick-flap LASIK (mean 165.7+/-12.6 microm) showed no differences in preoperative and postoperative refractive and visual outcomes. Comparison of the intended versus achieved correction showed no significant differences between the 2 groups.

CONCLUSION

Retrospective analyses showed that the safety, efficacy, and predictability of SBK were similar to those of conventional thick-flap LASIK in corneas with equivalent RSB thickness.

Effect of flap thickness on higher order wavefront aberrations induced by LASIK: a bilateral study

PURPOSE

To investigate the effect of flap thickness on wavefront aberrations induced by LASIK.

METHODS

LASIK was performed on 56 eyes of 28 patients with refraction errors that were well matched between the right and left eyes. For each patient, a planned 160-microm flap was created for one randomly selected eye using the Moria M2 microkeratome (130-microm head; thick flap group), and a 110-microm flap was created for the contralateral eye with the same microkeratome (90-microm head; thin flap group). Flap thickness was measured using ultrasonic pachymetry. Wavefront aberrations were measured in the anterior cornea using the Orbscan II (Bausch & Lomb) and in the whole eye using the Wavefront Supported Custom Ablation (WASCA) aberrometer (Carl Zeiss Meditec) preoperatively and at 1 month and 1 year postoperatively.

RESULTS

Mean flap thickness was 155 +/- 13 microm in the thick flap group and 112 +/- 11 microm in the thin flap group. Mean root-mean-square of higher order wavefront aberrations were changed for the different test times in the cornea (F = 29.9, P < .0001) and the whole eye (F = 48.0, P < .0001). There was no significant difference between the two flap groups for the cornea (F = 0.76) or for the whole eye (F = 0.07). Similar results were observed for higher order Zernike aberrations such as spherical aberration and comas.

CONCLUSIONS

The results suggest higher order wavefront aberrations induced by LASIK are independent of flap thickness. Complications in visual outcome for patients with different flap thickness should be attributed to factors other than aberrations.

Clinical applications of corneal confocal microscopy

Corneal confocal microscopy is a novel clinical technique for the study of corneal cellular structure. It provides images which are comparable to in-vitro histochemical techniques delineating corneal epithelium, Bowman's layer, stroma, Descemet's membrane and the corneal endothelium. Because, corneal confocal microscopy is a non invasive technique for in vivo imaging of the living cornea it has huge clinical potential to investigate numerous corneal diseases. Thus far it has been used in the detection and management of pathologic and infectious conditions, corneal dystrophies and ecstasies, monitoring contact lens induced corneal changes and for pre and post surgical evaluation (PRK, LASIK and LASEK, flap evaluations and Radial Keratotomy), and penetrating keratoplasty. Most recently it has been used as a surrogate for peripheral nerve damage in a variety of peripheral neuropathies and may have potential in acting as a surrogate marker for endothelial abnormalities.

Automated assessment of keratocyte density in clinical confocal microscopy of the corneal stroma

Cell density in the corneal stroma is typically determined by counting the number of bright objects, presumably keratocyte nuclei, in images from clinical confocal microscopy. We present a program that identifies bright objects and counts those that most likely represent cells. Selection variables were determined from 125 normal corneas with cell densities that had been assessed manually. The program was tested on 17 corneas of patients before and at several intervals to 5 years after laser in situ keratomileusis (LASIK) surgery. In these corneas, which showed a decrease in cell density after surgery, the program identified cells as well as human observers did.

Detection of an abnormally thick LASIK flap with anterior segment OCT imaging prior to planned LASIK retreatment surgery

PURPOSE

To report a case of an abnormally thick flap that was detected with the use of anterior segment optical coherence tomography (OCT) prior to planned LASIK retreatment surgery.

METHODS

A 43-year-old man presented 6 years after bilateral LASIK surgery with uncorrected visual acuity (UCVA) of 20/20 in the right eye and 20/100 in the left eye, and best spectacle-corrected visual acuity (BSCVA) of 20/20 in the right eye and 20/20 in the left eye (refraction -2.25 -1.25 x 131). Anterior segment OCT was performed before LASIK retreatment surgery in the left eye.

RESULTS

The flap was found to be 394 microm and the residual stromal bed was too thin (152 microm) to allow for safe LASIK retreatment surgery. After waiting 4 months to ensure refractive stability, photorefractive keratectomy was performed. Postoperative UCVA was 20/20 plano.

CONCLUSIONS

The use of anterior segment OCT in the preoperative examination for planned LASIK retreatment surgery provided more reliable data.

Microsporidium Stromal Keratitis: In Vivo Confocal Findings

PURPOSE

To relate the clinical signs, histopathologic features, and in vivo confocal biomicroscopy findings of a case of stromal microsporidial keratitis and to describe the use of in vivo confocal microscopy to monitor treatment effect.

METHODS

An immunocompetent male patient presented with unilateral indolent stromal keratitis. Stromal microsporidiosis was confirmed after corneal biopsy. He underwent examination that used in vivo confocal microscopy (Heidelberg Retina Tomograph II and Rostock Cornea Module) before and after treatment with topical fumagillin and oral albendazole. Clinicopathologic correlation of the confocal scan was performed.

RESULTS

Corneal biopsy showed extracellular microsporidium spores aligned along keratocytes and corneal lamellae. In vivo confocal scans showed similar morphology, with bright dots aligned along keratocytes. Treatment with antimicrobials and topical steroid gave resolution of active keratitis, correlating with disappearance of the bright spores on repeat in vivo confocal scanning.

CONCLUSIONS

The in vivo confocal microscopy appearance of microsporidial keratitis corresponds to the histologic features from biopsy material. Treatment response may be monitored by using this technique, although definitive diagnosis requires corneal biopsy.

In Vivo Corneal Confocal Microscopy in Marfan Syndrome

PURPOSE

To examine the cornea of patients with Marfan syndrome in comparison with a control group by using the in vivo confocal microscope.

METHODS

Twenty-four eyes of 12 patients with Marfan syndrome had their corneas examined using the in vivo confocal microscope Heidelberg Retina Tomograph (HRT) II/Rostock Cornea Module. The control group included 24 eyes of 12 subjects who had their corneas examined by the same in vivo confocal microscope.

RESULTS

Epithelium and neural plexus examination did not show any difference between the 2 groups. Examination of the stroma showed no significant differences concerning the morphology and density of keratocytes. The extracellular matrix of 16 of the 24 eyes of the Marfan group was clearly visible and showed thin highly reflective interconnected lines between keratocytes. In the healthy eye group, reflective lines were observed in only 5 of the 24 eyes. The endothelium of 14 corneas of the Marfan group showed brightly reflective particles. In no cornea of the control group were such particles observed.

CONCLUSIONS

Highly reflective extracellular matrix of the stroma and brightly reflective particles among the endothelial cells were the 2 main corneal findings observed by using in vivo corneal confocal microscopy in patients with Marfan syndrome compared with a control group. Further studies need to be made to confirm these findings and eventually find new criteria for Marfan syndrome from the examination of in vivo corneal confocal microscopy.

Contact lens-induced changes in the anterior eye as observed in vivo with the confocal microscope

The availability of the confocal microscope over the past decade has allowed clinicians and researchers to refine their understanding of the physiological and pathological basis of the ocular response to contact lens wear, and to discover previously unknown phenomena. Mucin balls, which form in the tear layer in patients wearing silicone hydrogel lenses, can penetrate the full thickness of the epithelium, leading to activation of keratocytes in the underlying anterior stroma. Epithelial cell size increases in response to all forms of lens wear, with lenses of higher oxygen transmissibility (Dk/t) interfering least with the normal process of epithelial desquamation. A higher density of Langerhans' cells is observed in the layer of the sub-basal nerve plexus among contact lens wearers, suggesting that contact lens wear may be altering the immune status of the cornea. Dark lines and folds are observed in the oedematous cornea in response to contact lens wear. Mechanical stimulation of the corneal surface, due to the physical presence of a contact lens, and the consequent release of inflammatory mediators, is the likely cause of reduced keratocyte density associated with lens wear. Highly reflective stromal 'microdot deposits' are observed throughout the entire stroma in higher numbers in lens wearers. 'Blebs' in the endothelium have a bright centre surrounded by a dark annular shadow; this appearance is explained with the aid of an optical model. The confocal microscope has considerable clinical utility in diagnosing Acanthamoeba and fungal keratitis. At the limbus, contact lenses can induce structural changes such as increases in basal epithelial cell size. An increased number of rolling leucocytes is observed in limbal vessels in response to low Dk/t lenses. It is concluded that the confocal microscope has considerable utility in contact lens research and practice.

[Assessment of the nerve fiber layer thickness after LASIK using scanning laser polarimetry with variable corneal compensation]

AIM

To evaluate the reliability of scanning laser polarimetry with variable corneal compensation after laser-assisted in-situ keratomileusis (LASIK).

METHODS

Thirty-six eyes of 18 consecutive patients who had LASIK where included in the study. For each eye, one scanning laser polarimetry with variable corneal compensation (GDxVCC) was performed before LASIK (GDx no 1). One month after surgery, two other scanning laser polarimetry procedures were performed: one used the corneal compensation measured preoperatively (GDx no 2) and one used a new corneal compensation, measured postoperatively (GDx no 3). The values measured preoperatively were compared first to the values obtained postoperatively with GDx no 2, then to the values obtained postoperatively with GDx no 3 (Wilcoxon test).

RESULTS

When corneal compensation was re-measured postoperatively (GDx no 3), the measurements were reliable for 35 eyes (97%). We found no significant differences between the preoperative and postoperative values except for the NFI (p=0.032). When the preoperative corneal compensation value was used in the postoperative measurements (GDx no 2), the exam was not reliable for 13 eyes, and it could not be performed at all for two eyes. For the remaining eyes, the values of four parameters were significantly modified: superior thickness (p=0.03), superior ratio (p=0.0005), inferior ratio (p=0.009), and ellipse modulation (p=0.39). The values of the other parameters (average thickness and inferior average) remained unchanged.

CONCLUSION

The use of a preoperative customized corneal compensation for the realization of post-LASIK measurements does not provide reliable results. The GDxTMVCC can be considered a reliable tool after LASIK only when the corneal compensation is re-evaluated postoperatively.

Comparison of the corneal response to laser in situ keratomileusis with flap creation using the FS15 and FS30 femtosecond lasers: clinical and confocal microscopy findings

PURPOSE

To compare the response of the cornea to laser in situ keratomileusis (LASIK) with flap creation using the IntraLase FS15 or FS30 femtosecond laser (IntraLase Corp.).

SETTING

Department of Ophthalmology, University of Texas Southwestern Medical Center, Dallas, Texas, USA.

METHODS

Twenty-three patients (31 eyes) who had LASIK with flap creation using the FS15 or FS30 laser were assessed by clinical examination and confocal microscopy in a nonrandomized parallel treatment group comparative trial. Eight FS15 patients (15 eyes) were examined preoperatively and 3 months postoperatively, and 14 FS30 patients (15 eyes) were examined 3 months postoperatively.

RESULTS

No patient in either group had clinically significant flap interface haze. One FS15 eye and 1 FS30 eye had significant keratocyte activation at the flap interface. The mean difference between the actual flap thickness and intended flap thickness was 16.8 microm +/- 11.1 (SD) and 13.9 +/- 7.1 microm in the FS15 group and FS30 group, respectively (P = .49). The mean measured interface reflectivity was 156.4 +/- 88.6 confocal backscatter units (CBU) and 104.8 +/- 91.2 CBU, respectively (P = .15). The mean density of interface particles was 21.4 +/- 14.8 particles/mm(2) in the FS15 group and 11.0 +/- 7.1 particles/mm(2) in the FS30 group (P<.05).

CONCLUSIONS

Both the FS15 and FS30 lasers provided more reproducible flap thickness and fewer interface particles than previously observed using microkeratomes. The response of corneal keratocytes to intra-LASIK was reduced compared with previous results in which higher raster energies were used. Compared with the FS15, there was an apparent reduction in overall interface reflectivity and fewer interface particles with the FS30 laser.

[In vivo confocal microscopy and ocular surface diseases: anatomical-clinical correlations]

PURPOSE

To describe corneal and conjunctival epithelial changes in various ocular surface disorders with a new in vivo confocal microscope (Corneal HRT II module) and to compare these results with those obtained by immunohistology on impression cytology specimens.

METHODS

In this study, we investigated 36 eyes of 18 patients with ocular surface diseases such as corneal epitheliopathy induced by ocular rosacea (five patients), keratoconjunctivitis sicca related to tear evaporative condition in Meibomian gland dysfunction (eight patients), or tear-deficient dry eye in Sjögren's syndrome (five patients). In vivo confocal microscopy imaging (Heidelberg Retina Tomograph II Rostock Cornea Module) and impression cytology of the cornea and/or conjunctiva were performed. The images were analyzed for epithelial cell morphology at nuclear and cytoplasmic levels, in the cornea and the conjunctiva, number of goblet cells and their possible presence within the corneal epithelium, corneoconjunctival junction at the limbus, and density and presence of inflammatory cells within both epithelia.

RESULTS

Images obtained consisted of two-dimensional 400x400 microm optical sections oriented parallel to the surface of the eye and compared with impression cytology. Conjunctival and corneal epithelia showed dramatic changes in ocular surface disease. In keratoconjunctivitis sicca, we found squamous metaplasia, inflammatory cell infiltration, goblet cell depletion, as well as a nuclear snake-like chromatin pattern. In rosacea associated with corneal epitheliopathy, a corneal conjunctivalization (goblet cells together with conjunctival epithelial cells within the corneal epithelium layer) was found. The images provided by this technique are in excellent correlation with the findings using impression cytology.

CONCLUSION

The HRT II in vivo confocal microscope facilitates the study of both central and peripheral ocular surface epithelia compared to first-generation confocal microscopy devices. In vivo confocal microscopy, showing corneal and conjunctival epithelial structures in the live eye at the cellular level, is likely to open up a new promising way to investigate ocular surface involvement in complex diseases, providing a new insight on corneal and conjunctival disorders in the future.

Confocal Microscopy Comparison of IntraLase Femtosecond Laser and Moria M2 Microkeratome in LASIK

PURPOSE

To analyze the quality of LASIK flaps created using a femtosecond laser or a mechanical microkeratome.

METHODS

A prospective study of consecutive bilateral LASIK flaps (100 patients) created with a 15 kHz femtosecond laser (IntraLase) or with a mechanical microkeratome (Moria M2) was performed. Outcome measures included visual and refractive outcomes, flap dimensions, and intraoperative complications. The first consecutive ten patients (20 eyes) were examined using confocal microscopy 1 and 3 months following surgery.

RESULTS

No significant differences were noted in the visual outcomes between groups (t test, P > .05 for all comparisons). Analysis of superficial measurements revealed more round flaps and greater predictability in surface dimensions for the femtosecond laser group (t test, P = .016). Mean deviation of flap thickness from target was the same for both instruments, 10 microm, with greater variability for the mechanical microkeratome. The incidence of diffuse lamellar keratitis was significantly greater in the femtosecond laser group (17%) than in the mechanical group (0%) (chi-square test, P < .001). Confocal microscopy revealed a higher wound healing opacity index in the femtosecond laser group.

CONCLUSIONS

Although the clinical results are similar, the femtosecond laser produces LASIK flaps with more accurate dimensions compared to the mechanical microkeratome.

Prospective evaluation of factors associated with post-LASIK corneal birefringence with scanning laser polarimetry

PURPOSE

To identify factors that affect corneal birefringence, such as the corneal polarization axis (CPA) and corneal polarization magnitude (CPM), after laser in situ keratomileusis (LASIK) using scanning laser polarimetry with a variable corneal polarization compensator.

METHODS

The CPA and CPM from 42 patients (42 eyes) who underwent LASIK and 24 normal untreated subjects (24 eyes) were measured 1 week before and 1 week after LASIK. Changes in the CPA and CPM after LASIK were studied using Student t test and Pearson correlation coefficient.

RESULTS

In the LASIK group, the post-LASIK CPA and CPM significantly changed, whereas both values did not change in normal subjects. Among 5 candidate factors [preoperative CPA (pre-CPA), preoperative CPM (pre-CPM), age, change in corneal curvature, and ablation depth], the change in CPA was significantly correlated with the pre-CPA (r=-0.793) and pre-CPM (r=0.339). The change in CPM was correlated with the pre-CPA (r=-0.455) and pre-CPM (r=-0.411). Over 83% case of postoperative corneal birefringence can be explained by only 2 parameters with regression analysis.

CONCLUSIONS

The pre-CPA and pre-CPM are strongly associated with LASIK-induced changes in CPA and CPM. Variations in age, changes in corneal curvature, and ablation depth do not contribute to changes in either parameter.

Contemporary in vivo confocal microscopy of the living human cornea using white light and laser scanning techniques: a major review

In vivo confocal imaging of the cornea has evolved exponentially over the last few decades and it has increasingly emerged from the laboratory to be used in the clinical setting in relation to inherited corneal diseases, corneal infections, contact lens wear and the effects of corneal surgery. This evolution has led to significant enhancement of our knowledge of the living cornea in both its physiological and pathological states. A number of in vivo confocal microscope devices using white, and more recently coherent, light sources have been developed to provide non-invasive assessment of the corneal microstructure at a lateral resolution of 1-2 microm. The fundamental principles of in vivo confocal microscopy and the key differences between these devices are highlighted in this review. By providing a systematic review of the extensive literature on the human cornea, this perspective paper aims to provide an overview of how in vivo confocal microscopy has contributed to our greater understanding of the human cornea in health, in disease, and following surgery, with a particular emphasis on quantitative data. The utility and limitations of available data are highlighted as are possibilities for the future development of this innovative technology.

Clinical corneal confocal microscopy

Confocal microscopy allows non-invasive in vivo imaging of the ocular surface. Its unique physical properties enable microscopic examination of all layers of the cornea and have been used to investigate numerous corneal diseases: epithelial changes, numerous stromal degenerative or dystrophic diseases, endothelial pathologies, corneal deposits, infections, and traumatic lesions. It offers a new approach to study the physiological reactions of the cornea to different stimuli and the pathophysiologic events leading to corneal dysfunction in certain diseases. Confocal microscopy proves to be a powerful diagnostic tool and is especially of value in certain corneal diseases by allowing straightforward and non-invasive recognition of the pathologic conditions.

Comparative anatomy of laboratory animal corneas with a new-generation high-resolution in vivo confocal microscope

PURPOSE

The aim of the current study was to compare the corneas of three commonly used laboratory animals with a new in vivo confocal microscope.

METHODS

Six eyes of three adult male New Zealand albino rabbits, six eyes of three adult male Lewis rats, and six eyes of three adult male Swiss mice were used in this study. Corneas were analyzed in vivo using the Rostock Cornea Module of the Heidelberg Retina Tomograph (HRT)-II. For all eyes, 20 confocal microscopic images of each layer, that is, the superficial and basal corneal epithelia, the Bowman layer, the anterior and posterior stroma, and the endothelium, were recorded. The images were then analyzed qualitatively and compared among animals. Cellular densities of anterior and posterior stroma keratocytes of rabbits and endothelium density of the three different animals were also measured and compared.

RESULTS

The Rostock Cornea Module of the HRT II was successfully used to analyze all corneal layers of these three commonly used laboratory animals. Although the cellular patterns of the corneal layers of these three animals, as observed with in vivo confocal microscopy, were quite similar, some differences were seen in terms of endothelial cell density and stroma appearance. Superficial cells were seen as hyper- and hyporeflective polygonal cells. Basal cells had dark cytoplasm without visible nuclei and were closely organized. A Bowman layer was observed in all three animals as an amorphous tissue containing fine subepithelial nerve plexus. In rabbits, the stroma consisted of an amorphous ground substance with hyper-reflective structures corresponding with keratocyte nuclei. In rats and mice, numerous reflective stellate structures with no clearly visible nuclei were observed within the stroma. Besides endothelial cell density, the endothelium was similar among the three animals and was seen as hyper-reflective cells with dark limits organized in a honeycomb pattern.

CONCLUSIONS

The Rostock Cornea Module of the HRT II can provide high-resolution images of all corneal layers of rabbits, rats, and mice without sacrificing animals or preparing tissue. This new device may be useful for evaluating the cornea during experimental animal studies.