Autofluorescence distribution along the corneal axis in diabetic and healthy humans

Two-Photon Imaging for Non-Invasive Corneal Examination

Two-photon imaging (TPI) microscopy, namely, two-photon excited fluorescence (TPEF), fluorescence lifetime imaging (FLIM), and second-harmonic generation (SHG) modalities, has emerged in the past years as a powerful tool for the examination of biological tissues. These modalities rely on different contrast mechanisms and are often used simultaneously to provide complementary information on morphology, metabolism, and structural properties of the imaged tissue. The cornea, being a transparent tissue, rich in collagen and with several cellular layers, is well-suited to be imaged by TPI microscopy. In this review, we discuss the physical principles behind TPI as well as its instrumentation. We also provide an overview of the current advances in TPI instrumentation and image analysis. We describe how TPI can be leveraged to retrieve unique information on the cornea and to complement the information provided by current clinical devices. The present state of corneal TPI is outlined. Finally, we discuss the obstacles that must be overcome and offer perspectives and outlooks to make clinical TPI of the human cornea a reality.

Eye care providers' emerging roles in early detection of diabetes and management of diabetic changes to the ocular surface: a review

US adults visit eye care providers more often than primary healthcare providers, placing these doctors in a prime position to help identify and manage patients with prediabetes and diabetes. Currently, diabetes is identified in eye clinics in an advanced stage, only after visible signs of diabetic retinopathy. Recent ophthalmic research has identified multiple subclinical and clinical changes that occur in the anterior segment of the eye with metabolic disease. The corneal epithelium exhibits increased defects and poor healing, including an increased risk of neurotrophic keratitis. Increased thickness and stiffness of the cornea artificially alters intraocular pressure. There is damage to the endothelial cells and changes to the bacterial species on the ocular surface, both of which can increase risk of complications with surgery. Decreased corneal sensitivity due to a loss of nerve density predispose patients with metabolic disease to further neurotrophic complications. Patients with diabetes have increased Meibomian gland dysfunction, blepharitis and reduced tear production, resulting in increased rates of dry eye disease and discomfort. Early detection of metabolic disease may allow eye care providers to be more proactive in recommending referral and intervention in order to reduce the risk of blindness and other diabetes-related morbidity. Continued research is needed to better understand the time course of changes to the anterior segment and what can be done to better detect and diagnose patients with prediabetes or undiagnosed diabetes and provide improved care for these patients.

Endogenous fluorescence can differentiate the keratoconic cornea

The purpose of the study was to investigate the endogenous fluorescence of the keratoconic cornea in order to analyze changes in the spectra due to the keratoconic stroma abnormalities. Twenty-two corneal buttons obtained from patients with keratoconus (KC, N = 22) at the time of penetrating keratoplasty were used. As a reference, twelve normal corneas (N = 12): ten from the Eye Bank and two from enucleated eyes due to choroidal melanoma were used. The fluorescence excitation/emission matrices (EEM) in the ranges of 250-400/260-600 nm were recorded. Healthy cornea, keratoconic cornea and sclera showed three main EEM bands, which correspond to the following fluorophores: tryptophan residues in the proteoglycan fraction of corneal/scleral stromas, naturally occurring collagen cross-links and the NAD(P)H fraction present in the metabolically active cells. Relative intensity factors S₁, S₂ and S₃ describing the contribution of each kind of fluorophore to the total fluorescence of the tissue were calculated. Normal and keratoconic corneas show qualitatively similar fluorescence matrices, but the statistically significant differences in the mean values of the S₁, S₂ and S₃ parameters for the KC and normal corneas were observed indicating changes in contribution of different fluorophores to the whole fluorescence of the tissue. Moreover, differences between multidimensional distribution of the relative intensity factors S₁, S₂ and S₃ between these groups were demonstrated (p < 0.001). In conclusions: Differences in the relative intensity factors calculated on a basis of the fluorescence spectra can correspond to the changes found in the KC stroma regarding natural collagen cross-links and the proteoglycan fraction. These parameters well differentiate the KC and normal corneas that could serve as an additional tool for the keratoconus characterization.

Objective Evaluation of Corneal and Lens Clarity in Children With Type 1 Diabetes Mellitus

PURPOSE

To investigate whether abnormal glucose metabolism and duration of diabetes mellitus (DM) affected the corneal and lens clarity in children with well-controlled type 1 DM and to compare the results obtained with those in healthy children.

DESIGN

Cross-sectional prospective study.

METHODS

This multicenter study enrolled 56 patients with DM and 51 control subjects. The duration of DM and the glycosylated hemoglobin (HbA1c) levels of the patients in the DM group were recorded. The Pentacam HR imaging system was used for corneal densitometry (12-mm corneal diameter) measurements. Furthermore, the lens densitometry and lens thickness (LT) measurements were performed after dilation of the pupils, using the same Pentacam HR device.

RESULTS

The corneal densitometry values were similar in all concentric zones and layers in both groups (P > .05, for all). The mean values of the average and maximum lens densitometry measurements of the 2 groups, as well as the mean LT values, were statistically significantly different (P = .021, P = .011, and P < .001, respectively). There were statistically significant correlations between the lens densitometry values and the duration of DM (P < .05, for all). Conversely, no statistically significant relationship was found between the lens densitometry values and HbA1c levels (r = 0.743; P = .084).

CONCLUSIONS

The children with type 1 DM had decreased lens clarity and increased LT, even in cases of well-controlled DM, without DR. It is reasonable to think that these changes might have been caused by the type 1 DM.

Ocular autofluorescence in diabetes mellitus. A review

Diabetes mellitus is a metabolic disease with a considerable impact on healthcare owing to its increased prevalence and high mortality rate. Structural, morphological, and physiological changes in each of the ocular components have been described in detail. Autofluorescence has been described as a good indicator of metabolic activity. The aim of the present review is to provide an overview of ocular endogenous fluorophores in the cornea, the crystalline lens, and the retinal pigment epithelium, the effects of diabetes mellitus and therefore the potential of autofluorescence assessment for screening and monitoring changes in diabetic patients.

Optical quality of the diabetic eye: a review

Diabetes mellitus is a metabolic disorder characterized by the presence of chronic hyperglycaemia. Several structural, morphological, and physiological changes in each of ocular component have been described in detail during the past decades. Due to these abnormalities, the diabetic patient undergoes a degradation of the retinal image by an increase of higher ocular aberrations and ocular scattering coming from mainly tear film, cornea, and crystalline lens. This review aims to provide an overview of current knowledge about the effects of diabetes mellitus in these optical phenomena and its consequence on the visual quality of the diabetic patient.

Diabetes mellitus effect on rat corneal dielectric properties

In the course of the study, we carried out a dielectric examination to determine the effect of diabetes mellitus on the rat corneal function. Measurements were performed over the frequency range of 500 Hz-100 kHz in air and at the temperatures from 25 to 150°C. The frequency dependencies of the loss tangent for the healthy and the diabetic cornea exhibit two peaks at 2 kHz and 16 kHz in the α-dispersion region. The amplitude of these both peaks is smaller for the diabetic cornea than that for the healthy one. The temperature dependencies of the loss tangent for the healthy and the diabetic cornea reveal β-relaxation in the range of 30-70°C and 50-90°C, respectively. The present study exhibits that the dielectric spectroscopy is useful in detection of the effect of diabetes mellitus on the corneal molecular behavior.

Multiphoton spectral microscopy for imaging and quantification of tissue glycation

Tissue glycation from diabetes and aging can result in complications such as renal failure, blindness, nerve damage and vascular diseases. In this work, we applied multiphoton microscopy for imaging and characterizing the extent of tissue glycation. The characteristic features of multiphoton autofluorescence (MPAF) and second harmonic generation (SHG) images as well as MPAF spectra of glycated bovine skin, cornea and aorta were acquired. The analysis of MPAF intensity change accompanying the glycation process shows that collagen is more responsive to the formation of autofluorescent advanced glycation endproducts (AGE) than elastic fibers. Changes in spectral features were also used to estimate the rate of glycation in tissues with intrinsic AF. Our study shows that multiphton imaging may be used for the in vitro investigation of the effects of tissue glycation and that this approach may be used for monitoring AGE formation in the clinical setting.

Diabetes and corneal cell densities in humans by in vivo confocal microscopy

PURPOSE

Diabetes is accompanied by an increased autofluorescence of the cornea, probably because of accumulation of advanced glycation end products (AGEs). The pathogenic mechanism is still unknown. This study aimed to quantify differences in corneal cell densities between diabetic patients and healthy controls.

METHODS

The left cornea of 15 patients with non-insulin-dependent diabetes mellitus (NIDDM) with level of retinopathy 20 according to the Early Treatment of Diabetic Retinopathy Study (ETDRS) and of 15 healthy controls were examined by noninvasive in vivo confocal microscopy in an observational prospective study. The cell densities in 6 corneal layers were determined along the optical axis of the cornea by using stereologic methods.

RESULTS

The average cell density per unit area in the superficial and basal epithelium and the endothelial layer was 725 +/- 171, 5950 +/- 653, and 2690 +/- 302 cells/mm in controls and 815 +/- 260, 5060 +/- 301, and 2660 +/- 364 cells/mm in diabetic patients. The cell density per unit volume in the anterior, mid-, and posterior stroma was 26,300 +/- 4090, 19,390 +/- 3120, and 25,700 +/- 3260 cells/mm in controls and 27,560 +/- 3880, 21,930 +/- 2110, and 25,790 +/- 3090 cells/mm in patients with diabetes. In both groups, the density in the midstroma was significantly lower than in both the anterior stroma and the posterior stroma (P < 0.02). The cell density in the basal layer of diabetic patients was significantly lower than in healthy controls (-15.0%, P < 0.0004). In the other layers, no significant differences between both groups (P > 0.07) were observed.

CONCLUSIONS

The lower basal cell density found in patients with diabetes may result from a combination of different mechanisms including decreased innervation at the subbasal nerve plexus, basement membrane alterations, and higher turnover rate in basal epithelial cells. The lower cell density in the midstroma of diabetic patients and healthy controls may be attributed in part to differences in oxygen concentration in the stromal layers (depths). Changes in cellular density did not seem to be responsible for the increased autofluorescence in diabetes.

Diabetic nephropathy and retinopathy

Diabetic nephropathy and retinopathy are arguably the two most dreaded complications of diabetes. Together they contribute to serious morbidity and mortality. As they progress to end-stage renal disease and blindness, they impose enormous medical, economic,and social costs on both the patient and the health care system. Because nephropathy and retinopathy are frequently linked in patients,this article reviews their common and individual aspects of pathophysiology, clinical features, and management.

Influence of viscoelastic substances used in cataract surgery on corneal metabolism and endothelial morphology: comparison of Healon and Viscoat

PURPOSE

To evaluate the influence of cohesive and dispersive ophthalmic viscosurgical devices (OVDs) on endothelial morphology and corneal metabolism during cataract surgery.

SETTING

Department of Ophthalmology, University of Vienna Medical School, Vienna, Austria.

METHODS

In this prospective randomized blind study, 50 eyes of 43 patients were randomized into 2 groups before surgery. Phacoemulsification with implantation of a posterior chamber intraocular lens was performed in all patients. In half the patients, sodium hyaluronate 1% (Healon) was used as the OVD and in the other half, sodium hyaluronate 3%-chondroitin sulfate 4% (Viscoat). Corneal metabolism was evaluated by fluorophotometric measurement of corneal autofluorescence. The corneal fluorescence values were corrected for interference by fluorescence of the ocular lens. Specular microscopy (Noncon Robo SP800, Canon) was used to evaluate the endothelial cell density, coefficient of variation, and percentage of hexagonal cells. Examinations were performed preoperatively and 3 days, 1 and 4 weeks, and 3 months postoperatively.

RESULTS

There were no significant changes between preoperative and postoperative endothelial cell density measurements in either group (P =.1717). The percentage of hexagonal cells was similar (P =.3489); however, there was a slightly increasing tendency toward polymorphism in both groups. Corneal autofluorescence decreased 3 days after surgery, increased after 1 week, and decreased again subsequently in both groups. There was no significant difference in the influence on corneal metabolism between the 2 OVDs (P =.9899).

CONCLUSIONS

There was no significant difference between Healon and Viscoat. Thus, this study did not confirm an advantage of either for endothelial protection of healthy corneas.

Corneal autofluorescence in choroidal melanoma or in choroidal naevus

AIMS

To investigate whether corneal autofluorescence is different in patients with choroidal melanoma or choroidal naevus.

METHODS

Corneal autofluorescence was determined by fluorophotometry in both eyes of 32 patients with a unilateral choroidal melanoma, 32 patients with a unilateral choroidal naevus, and 32 age matched healthy controls. The corneal autofluorescence ratio between affected and contralateral eyes of patients or between randomly selected eyes of healthy controls was calculated.

RESULTS

Mean corneal autofluorescence ratio of patients with a choroidal melanoma was significantly higher than that of healthy controls (mean ratio: 1.09 (SD 0.15) and 1.00 (0.09), respectively, ANOVA p=0.014), and than that of patients with choroidal naevus (mean ratio 0.96 (0.09), p<0.001). Mean ratios of patients with choroidal naevus and healthy controls were not significantly different (p=0.27).

CONCLUSIONS

Corneal autofluorescence ratio of patients with a unilateral choroidal melanoma is increased. This is probably due to an increased flow of glucose through the impaired blood-aqueous barrier in the affected eye, resulting in additional glycation of corneal proteins and hence in increased autofluorescence. The corneal autofluorescence is not increased in patients with a choroidal naevus, because the blood-aqueous barrier is not impaired in the affected eye in these patients. Measurement of corneal autofluorescence is simple, fast, and non-invasive, and might be helpful to distinguish between patients with choroidal melanoma and those with choroidal naevus.