Corneal epithelial oxygen uptake rate in diabetes mellitus

Diabetic Ocular Surface Has Defects in Oxygen Uptake Revealed by Optic Fiber Microsensor

Purpose

Diabetes mellitus causes diabetic keratopathy (DK). This and other ocular surface disorders are underdiagnosed and problematic for affected patients as well as recipients of diabetic donor corneas. Thus, it is important to find noninvasive means to facilitate determination of the potentially vision-threatening DK. It has been reported that diabetic corneas uptake significantly less oxygen (O2) than healthy controls. However, an integral assessment of the ocular surface is missing.

Methods

Using an optic-fiber O2 micro-sensor (optrode) we demonstrated recently that the healthy ocular surface displays a unique spatiotemporal map of O2 consumption. We hypothesize that diabetes impairs the spatiotemporal profile of O2 uptake at the ocular surface.

Results

Using streptozotocin (STZ)-induced diabetic mice, we found diminished O2 uptake and loss of the unique pattern across the ocular surface. A diabetic cornea consumes significantly less O2 at the bulbar conjunctiva and limbus, but not the central and peripheral cornea, compared to controls. Further, we show that, contrary to the healthy cornea, the diabetic cornea does not increase the O2 consumption at the limbus in the evening as the normal control.

Conclusions

Altogether, our measurements reveal a previously unknown impairment in O2 uptake at the diabetic cornea, making it a potential tool to diagnose ocular surface abnormalities and suggesting a new etiology mechanism.

Immune Fingerprint in Diabetes: Ocular Surface and Retinal Inflammation

Diabetes is a prevalent global health issue associated with significant morbidity and mortality. Diabetic retinopathy (DR) is a well-known inflammatory, neurovascular complication of diabetes and a leading cause of preventable blindness in developed countries among working-age adults. However, the ocular surface components of diabetic eyes are also at risk of damage due to uncontrolled diabetes, which is often overlooked. Inflammatory changes in the corneas of diabetic patients indicate that inflammation plays a significant role in diabetic complications, much like in DR. The eye's immune privilege restricts immune and inflammatory responses, and the cornea and retina have a complex network of innate immune cells that maintain immune homeostasis. Nevertheless, low-grade inflammation in diabetes contributes to immune dysregulation. This article aims to provide an overview and discussion of how diabetes affects the ocular immune system's main components, immune-competent cells, and inflammatory mediators. By understanding these effects, potential interventions and treatments may be developed to improve the ocular health of diabetic patients.

Corneal epithelial defect after pars plana vitrectomy

AIM:

This study aims to study the incidence of corneal epithelial defect (CED) after pars plana vitrectomy (PPV) and associated patient-related risk factors. The incidence of other immediate postoperative complications was also in the scope of this study.

DESIGN:

Retrospective descriptive case series study.

METHODS:

Review of electronic medical records of all patients who underwent PPV alone or combined with cataract surgery and/or scleral buckle in a tertiary hospital by one retinal surgeon. All demographic data, surgery notes, and immediate postoperative findings were obtained for all patients.

RESULTS:

The cohort of the included cases was composed of a total of 168 procedures that were performed in 121 eyes of 106 patients over 5 years. CEDs occurred in 19 eyes (15.7%) of 19 patients (17.9%). Males were affected more than females (90% vs. 10%). Patients with postoperative CED were found to have longer duration of surgery when compared to patients without postoperative CED (P = 0.0038). All cases of CED had a complete resolution of the defects after supportive therapy. Immediate intraocular hypertension (IOH) was found in 30 eyes (24.8%). IOH was controlled in all cases with medical therapy only. Post-PPV immediate complications, other than CED and IOH, occurred in 10.7% of the eyes. These included vitreous hemorrhage, choroidal detachment, corneal edema, anterior chamber fibrin, and hyphema.

CONCLUSION:

Development of CED is not uncommon after PPV. This complication is more common in males and may be related to prolonged duration of surgery. It also develops more in older patients. CED after PPV can be managed with conservative treatment with good outcome.

Diabetes and contact lens wear

The literature suggests that diabetic patients may have altered tear chemistry and tear secretion as well as structural and functional changes to the corneal epithelium, endothelium and nerves. These factors, together with a reported increased incidence of corneal infection, suggest that diabetic patients may be particularly susceptible to developing ocular complications during contact lens wear. Reports of contact lens-induced complications in diabetic patients do exist, although a number of these reports concern patients with advanced diabetic eye disease using lenses on an extended wear basis. Over the past decade or so, there have been published studies documenting the response of the diabetic eye to more modern contact lens modalities. The results of these studies suggest that contact lenses can be a viable mode of refractive correction for diabetic patients. Furthermore, new research suggests that the measurement of tear glucose concentration could, in future, be used to monitor metabolic control non-invasively in diabetic patients. This could be carried out using contact lenses manufactured from hydrogel polymers embedded with glucose-sensing agents or nanoscale digital electronic technology. The purpose of this paper is to review the literature on the anterior ocular manifestations of diabetes, particularly that pertaining to contact lens wear.

Diffusion and Monod kinetics to determine in vivo human corneal oxygen-consumption rate during soft contact-lens wear

The rate of oxygen consumption is an important parameter to assess the physiology of the human cornea. Metabolism of oxygen in the cornea is influenced by contact-lens-induced hypoxia, diseases such as diabetes, surgery, and drug treatment. Therefore, estimation of in vivo corneal oxygen-consumption rate is essential for gauging adequate oxygen supply to the cornea. Phosphorescence quenching of a dye coated on the posterior of a soft contact lens provides a powerful technique to measure tear-film oxygen tension (Harvitt and Bonanno, Invest Ophthalmol Vis Sci 1996;37:1026-1036; Bonanno et al., Invest Ophthalmol Vis Sci 2002;43:371-376). Unfortunately, previous work in establishing oxygen-consumption kinetics from transient postlens tear-film oxygen tensions relies on the simplistic assumption of a constant corneal-consumption rate. A more realistic model of corneal metabolism is needed to obtain reliable oxygen-consumption kinetics. Here, physiologically relevant nonlinear Monod kinetics is adopted for describing the local oxygen-consumption rate, thus avoiding aphysical negative oxygen tensions in the cornea. We incorporate Monod kinetics in an unsteady-state reactive-diffusion model for the cornea contact-lens system to determine tear-film oxygen tension as a function of time when changing from closed-eye to open-eye condition. The model was fit to available experimental data of in vivo human postlens tear-film oxygen tension to determine the corneal oxygen-consumption rate. Reliance on corneal oxygen diffusivity and solubility data obtained from rabbits is no longer requisite. Excellent agreement is obtained between the proposed model and experiment. We calculate the spatial-averaged in vivo human maximum corneal oxygen-consumption rate as Q(c)(max) = 1.05 x 10(-4) mL/(cm(3) s). The calculated Monod constant is K(m) = 2.2 mmHg.

Corneal Hydration Control in Contact Lens Wearers With Diabetes Mellitus

PURPOSE

The purpose of this study was to assess the acute swelling and recovery response to contact lens-induced corneal hypoxia in diabetic patients who wear contact lenses.

METHODS

A thick, low water content, soft contact lens was fitted under a light patch to one eye of 23 diabetic patients and one eye of 23 nondiabetic control subjects in a single-masked, controlled clinical study. After 2.5 hours an ultrasonic pachymeter was used to measure the induced corneal edema and the rate at which the cornea recovered to baseline thickness.

RESULTS

The induced corneal edema was significantly less for the diabetic patients compared with the nondiabetic control subjects (p = 0.004). The corneas of the diabetic patients were found to recover from the induced edema at a similar rate to the corneas of the nondiabetic control subjects (p = 0.28).

CONCLUSIONS

Diabetes mellitus alters aspects of corneal hydration control.

Safety of contact lenses in patients with diabetes

The purpose of the present study was to determine if increased complications exist among patients with diabetes as compared with individuals without diabetes who wear soft disposable contact lenses for daily wear. Complications among daily-wear contact lens wearers who have worn soft contact lenses for 1 year or more were reported from a retrospective review of records. Eyes from 254 patients with diabetes and 254 individuals without diabetes (control), one eye for each subject, from six clinical practices were compared using chi2 and exact Wilcoxon analyses as appropriate. The overall incidence of complications was 49.8% among eyes of patients with diabetes and 48.0% among controls. The respective values for occurrence of corneal abrasions were 7.9% and 5.9%, those for corneal staining were 3.2% and 5.5%, and those for pain at some time while wearing a contact lens were 17% and 19.7%. No significant difference in complications, corneal abrasions, corneal staining, or experience of pain was found between daily-wear contact lens wearers with diabetes and controls. Thus patients with diabetes showed no increased complications over patients without diabetes for daily-wear soft contact lens wear.

Assessment of induced corneal hypoxia in diabetic patients

PURPOSE

To assess the control of corneal hydration in patients with diabetes during a contact lens-induced hypoxia.

METHODS

Corneal stress was induced in 15 patients with diabetes and 23 healthy patients by having them wear contact lenses for 2 hours. Pachymetries were measured and corneal parameters (percentage recovery per hour [PRPH], time for deswelling [T99%] and induced swelling [IS]) were calculated. In the mean time, tears were collected to assess the activity of lactate dehydrogenase (LDH), and a specular microscopy (SM) was performed.

RESULTS

In patients with diabetes, PRPH, T99%, and LDH activity were statistically significantly modified (p < 0.05), whereas IS and SM were not.

CONCLUSION

This decreased ability to recover the initial corneal thickness after a transient edema caused by hypoxia confirms the enzymatic dysfunction of the endothelial pumps that are partly caused by a shift toward anaerobic metabolism.

Effects of hypoxemia on the electroretinogram in diabetics

PURPOSE

To identify any relationship between retinal dependence on oxygen and the presence or absence of retinopathy in a group of patients with the same duration of diabetes, based on the proposal that diabetic retinopathy results from long-term adaptation to the Crabtree effect, i.e., reduction of oxidative phosphorylation caused by increased intracellular glucose concentrations.

METHODS

Electroretinograms were recorded on 17 patients with insulin-dependent diabetes (of 15 to 17 years duration) and 10 healthy controls. Recordings were made under normal conditions and then after 4 min of hypoxemia (oxygen saturation, 80%). Retinopathy status was assessed from fundus photographs.

RESULTS

Electroretinogram b-wave amplitudes of seven patients without retinopathy were reduced to 69% by the hypoxemia compared to a reduction to 88% for 10 patients with retinopathy (p = 0.039).

CONCLUSIONS

The relative sensitivity to hypoxemia of patients without retinopathy suggests greater dependence on oxidative phosphorylation than in patients with retinopathy. The results appear to be consistent with the Crabtree effect hypothesis. Downregulation of tissue oxygen consumption by the Crabtree effect would lead to a reduction of retinal blood flow by autoregulation and to a reduced concentration of adenosine triphosphate in the retina.