The influence of age on the fragility of the cornea

The impact of sensory neuropathy and inflammation on epithelial wound healing in diabetic corneas

Diabetic peripheral neuropathy (DPN) is the most common complication of diabetes, with several underlying pathophysiological mechanisms, some of which are still uncertain. The cornea is an avascular tissue and sensitive to hyperglycemia, resulting in several diabetic corneal complications including delayed epithelial wound healing, recurrent erosions, neuropathy, loss of sensitivity, and tear film changes. The manifestation of DPN in the cornea is referred to as diabetic neurotrophic keratopathy (DNK). Recent studies have revealed that disturbed epithelial-neural-immune cell interactions are a major cause of DNK. The epithelium is supplied by a dense network of sensory nerve endings and dendritic cell processes, and it secretes growth/neurotrophic factors and cytokines to nourish these neighboring cells. In turn, sensory nerve endings release neuropeptides to suppress inflammation and promote epithelial wound healing, while resident immune cells provide neurotrophic and growth factors to support neuronal and epithelial cells, respectively. Diabetes greatly perturbs these interdependencies, resulting in suppressed epithelial proliferation, sensory neuropathy, and a decreased density of dendritic cells. Clinically, this results in a markedly delayed wound healing and impaired sensory nerve regeneration in response to insult and injury. Current treatments for DPN and DNK largely focus on managing the severe complications of the disease. Cell-based therapies hold promise for providing more effective treatment for diabetic keratopathy and corneal ulcers.

Diabetic keratopathy: Insights and challenges

Ocular complications from diabetes mellitus are common. Diabetic keratopathy, the most frequent clinical condition affecting the human cornea, is a potentially sight-threatening condition caused mostly by epithelial disturbances that are of clinical and research attention because of their severity. Diabetic keratopathy exhibits several clinical manifestations, including persistent corneal epithelial erosion, superficial punctate keratopathy, delayed epithelial regeneration, and decreased corneal sensitivity, that may lead to compromised visual acuity or permanent vision loss. The limited amount of clinical studies makes it difficult to fully understand the pathobiology of diabetic keratopathy. Effective therapeutic approaches are elusive. We summarize the clinical manifestations of diabetic keratopathy and discuss available treatments and up-to-date research studies in an attempt to provide a thorough overview of the disorder.

Pupil response to noxious corneal stimulation

Purpose

Ocular somatosensory-autonomic reflexes play critical roles in maintaining homeostasis of the eye. The purpose of this study was to investigate the pupil response to nociceptive corneal stimuli.

Methods

A Waterloo-Belmonte pneumatic esthesiometer was used to determine detection thresholds and randomly deliver mechanical and chemical stimuli from levels of detection threshold to twice the threshold in 50% steps to the central cornea of 15 healthy subjects. For each stimulus, imaging of the stimulated/unstimulated eye was performed using two modified/calibrated Logitech c920 digital cameras for 4 seconds each, pre/post stimulus capture. The data were processed with a custom segmentation algorithm to help identify the pupils and pupil diameter was measured using ImageJ software. Pupil dilation response differences between the ipsi- and contralateral eye was analyzed using dependent t-tests. The effect of stimulus intensity, modality and sex of subjects were analyzed using repeated measures.

Results

In mechanical and chemical stimulation experiments, there was no difference in pupil responses between the stimulated eye and the unstimulated eye, (all dependent T-test p > 0.05). On average, pupil diameter increased from baseline as the corneal stimulus intensity increased. This happened regardless of whether mechanical or chemical stimulation occurred (ANOVA p < 0.05). At 200% threshold, pupil diameter was greater than at all stimulus intensities (Tukey HSD, all p < 0.05). Based on stimulus intensity, females had greater pupil diameters than males at levels of 150% threshold and 200% threshold (ANOVA p < 0.05, all Tukey HSD p < 0.05).

Conclusion

This study serves as a basis for the characterization of the local stimulus-response neural circuitry relating nociceptive stimuli to autonomic responses and in combination with our work on completely separate autonomic circuits of bulbar conjunctival vessel dilation and reflex tearing suggests that the monotonic measurements of redness, tearing and pupils provide accurate, separable responses that reflect painful stimulus intensity.

Analysis of age, refractive error and gender related changes of the cornea and the anterior segment of the eye with Scheimpflug imaging

PURPOSE

To assess age, refractive error and gender related changes occurring in the cornea and the anterior segment of the eye using a Scheimpflug system.

METHODS

The study included 666 healthy eyed subjects with a mean age of 39.3±19.7 years (range: 3-85 years). All analyses were based on the right eyes of the patients as all measured parameters correlated well between the right and left eyes. Each parameter was correlated with age and the right eye's spherical equivalent (SE) using Pearson correlations. Univariate linear regression models were constructed for analyses of parameters.

RESULTS

The anterior corneal surface asphericity showed significant positive correlations whereas posterior corneal surface asphericity showed significant negative correlations with age. Anterior chamber depth (ACD), volume (ACV) and angle (ACA) showed significant negative correlations with age and SE. Age explained 25% of the variance in anterior corneal surface asphericity, 22% of variance in posterior corneal surface asphericity, 26% of variance in ACV, 27% of variance in ACD, and 19% of variance in ACA. In the SE model SE was identified to account for 25% of variance in ACV, 22% of variance in ACD, each, and 17% of variance in ACA. Significant differences were detected in anterior and posterior keratometry values, ACV, ACD and ACA among gender groups (p<0.01).

CONCLUSIONS

The cornea shows a tendency for a decrease in anterior corneal surface asphericity and an increase in posterior corneal surface asphericity with advancing age. Men have flatter corneas and women have shallower anterior chambers and narrower anterior chamber angles.

Assessing corneal nerve structure and function in diabetic neuropathy

The accurate detection and quantification of human diabetic peripheral neuropathy are important to define at-risk patients, anticipate deterioration and assess new therapies. Two emerging ophthalmic techniques, namely, corneal confocal microscopy and corneal aesthesiometry, demonstrate the ability to diagnose, quantify severity and assess therapeutic benefit in diabetic peripheral neuropathy. Corneal confocal microscopy allows quantification of corneal nerve morphology and non-contact corneal aesthesiometry assesses corneal sensitivity. The present review provides a detailed critique of the rationale, practical application in terms of the instruments used to capture images and the basis on which images are interpreted and analysed. We also critically evaluate how these two new non-invasive ophthalmic tests can be deployed to diagnose diabetic and other peripheral neuropathies.

Dysfunctional neural regulation of lacrimal gland secretion and its role in the pathogenesis of dry eye syndromes

Tears are a complex fluid consisting of three layers, each of which is secreted by a different set of tissues or glands. The aqueous portion of the tear film is produced predominantly by the lacrimal gland. Dry eye syndromes are diseases in which the amount and composition of tears are altered, which can lead to ocular surface damage. There are many causes for dry eye syndromes. One such cause is the alteration in the functions of nerves innervating the lacrimal gland and the ocular surface. The autoimmune disease Sjogren syndrome can deleteriously affect the innervation of the lacrimal gland. Damage to the sensory nerves in the ocular surface, specifically the cornea, as a result of refractive surgery and normal aging, prevents the normal reflex arc to the lacrimal gland. Both defects can result in decreased tear secretion and dry eye syndromes. This review will discuss the current information regarding neurally-stimulated protein, water, and electrolyte secretion from the lacrimal gland and delineate how nerve dysfunction resulting from a variety of causes decreases secretion from this gland.

Corneal sensitivity and aqueous tear production in dogs undergoing evisceration with intraocular prosthesis placement

OBJECTIVE

To determine whether there is an association between evisceration with intraocular prosthesis placement via a dorsal scleral approach and decreased corneal sensitivity or aqueous tear production in dogs.

DESIGN

Prospective study.

PROCEDURE

Twenty-one dogs scheduled to undergo unilateral evisceration with intraocular prosthesis, and with a normal normotensive contralateral eye, based on slit-lamp biomicroscopy, indirect ophthalmoscopy and applanation tonometry, were included in the study. Central corneal sensitivity was measured with a Cochet-Bonnet esthesiometer prior to (week 0) and 2, 7, 14 and 28 weeks following surgery. Other pre- and postoperative data collected were Schirmer's tear test (STT I), Schirmer's tear test with topical anesthesia (STT II), corneal fluorescein retention, and intraocular pressure (IOP; postoperative control eye only). Axial globe length was determined for both eyes. Scleral incision length in the surgery eye was also recorded.

RESULTS

Corneal sensitivity was significantly lower in eyes undergoing evisceration with intraocular prosthesis than in control eyes both preoperatively and postoperatively at all time points. There was no significant difference between preoperative and postoperative corneal sensitivity within the two groups. There was no significant difference in STT I or STT II-values between control eyes and surgery eyes preoperatively. There was a significant difference between preoperative and 2-week postoperative STT I in the surgery eye. There was a significant difference between the preoperative and 28-week postoperative STT II in the surgery eye. The 28-week postoperative STT I-values differed significantly between control and surgery eyes. Corneal sensitivity tended to decrease with increased degree of buphthalmos or with increased scleral incision length, although neither relationship was statistically significant.

CONCLUSIONS

Eyes that have sustained chronically elevated intraocular pressure had decreased axial corneal sensitivity compared to normal eyes, which persisted even after intraocular pressure was reduced. The presence of buphthalmos appeared to be a factor in globes developing decreased corneal sensitivity. Evisceration via a dorsal scleral incision did not result in a significant further reduction in axial corneal sensitivity. A mild reduction in aqueous tear production occurred over time in globes following evisceration with intraocular prosthesis.

Influence of age, gender and iris color on mechanical and chemical sensitivity of the cornea and conjunctiva

The purpose of this study was to establish the influence of age, gender and iris color on the mechanical and chemical sensitivity of the cornea and the conjunctiva. In 57 healthy subjects (27 males, 30 females; ages between 23 and 71 years), sensory thresholds to mechanical and chemical stimulation were measured in the central cornea and the temporal conjunctiva using a Belmonte's gas esthesiometer. Mechanical stimulation consisted of warmed air pulses of 3s duration at different flow rates (40-200 ml/min). For chemical (acidic) stimulation, 3-s warmed gas pulses containing 10% to 80% CO(2) in air were applied, at a flow below mechanical threshold flow. Corneal and conjunctival thresholds to mechanical and chemical stimuli increased with age. Premenopausal women were more sensitive to corneal stimulation than men of similar ages but overall differences in mechanical and chemical threshold between men and women were not significant. Individuals with blue eyes had significantly lower corneal chemical thresholds than those with brown or green eyes. Multiple linear regression analysis evidenced that corneal mechanical threshold depends on age and iris color according to the equation 58.2 ml/min+1.3.(Age-23 years)-9.7 if blue eyes. For chemical threshold, the equation was 28.6%CO(2)+05.(Age-23 years)-12.5 if blue eyes. In the case of the conjunctiva, only age determined mechanical and chemical thresholds according to the equations: 78.2 ml/min+1.4.(Age-23 years) and 41.6%CO(2)+0.8.(Age-23 years), respectively. Therefore, normal values of mechanical and chemical thresholds of the cornea and conjunctiva measured with the Belmonte gas esthesiometer can be predicted according to age and iris color. Variations of sensitivity with age, iris color and gender may reflect differences in innervation density and neural responsiveness associated with the hormonal status.

The Association Between Eye Color and Corneal Sensitivity Measured Using a Belmonte Esthesiometer

PURPOSE

The purpose of this study is to determine the association between corneal sensitivity measured using a pneumatic esthesiometer and eye color quantified objectively.

METHODS

Twenty subjects had ocular surface sensitivity measured using a Belmonte esthesiometer. An ascending method of limits followed by the method of constant stimuli were used to estimate 1) cold detection thresholds, 2) discomfort detection thresholds (both using pneumatic stimuli at 20 degrees C, 3) mechanical detection thresholds using pneumatic stimuli at 50 degrees C (ocular surface temperature approximately 33 degrees C), and 4) percent CO2 chemical detection thresholds using 50 degrees C pneumatic stimuli at flow rates set at half of each subject's pneumatic detection threshold (therefore detected by the chemical content and not the mechanical content). Eye color was estimated 1) clinically by two observers ranking the color (light to dark) of digital images of each subject's iris, 2) photometrically by measuring iris luminance, and 3) using chromaticity obtained from a Photo Research 650 spectroradiometer with controlled illumination. Correlation and linear and nonlinear regression analyses were used to examine relationships between variables.

RESULTS

There were no associations between eye color (determined clinically or objectively) for mechanical and chemical detection thresholds (best r = 0.15, all p > 0.05). There was a significant linear association between 20 degrees detection thresholds and eye color (r = 0.39), which was substantially improved with a two-line function (part level and part increasing linearly, r = 0.65).

CONCLUSIONS

We were generally unable to demonstrate the relationship between eye color and sensitivity reported previously using a Cochet-Bonnet esthesiometer. However, for a subset of subjects with palest irises, there appears to be a linear association between eye color and sensitivity to cooling stimuli.

A population study of the normal cornea using an in vivo, slit-scanning confocal microscope

PURPOSE

To document qualitative and quantitative changes in the normal, healthy human cornea with age using the confocal microscope.

METHODS

The central corneas of 120 subjects (mean age, 41 years; range, 11 to 80 years) were examined using an in vivo slit-scanning real-time confocal microscope. Images of the corneal stroma and endothelium from both eyes of each subject were semiautomatically analyzed in an observer-masked, randomized manner.

RESULTS

Anterior keratocyte density, posterior keratocyte density, and endothelial cell density were shown to be unaffected by the sex of the subject with p values of 0.46, 0.55, 0.50, respectively (multivariate analysis of variance). No statistically significant difference was detected between right and left eyes for all corneal layers examined. The anterior keratocyte density, posterior keratocyte density, and endothelial cell density decreased at a rate of 0.48, 0.22, and 0.33% per year, respectively. A positive correlation was found between the coefficient of cell variation and age.

CONCLUSIONS

This data constitutes essential normative data that can be used as a control in further research into abnormal corneal conditions.

The effect of scleral search coil lens wear on the eye

BACKGROUND/AIM

Scleral search coils are used to measure eye movements. A recent abstract suggests that the coil can affect the eye by decreasing visual acuity, increasing intraocular pressure, and damaging the corneal and conjunctival surface. Such findings, if repeated in all subjects, would cast doubt on the credibility of the search coil as a reliable investigative technique. The aim of this study was to reassess the effect of the scleral search coil on visual function.

METHODS

Six volunteer subjects were selected to undergo coil wear and baseline measurements were taken of logMAR visual acuity, non-contact tonometry, keratometry, and slit lamp examination. Four drops of 0.4% benoxinate hydrochloride were instilled before insertion of the lens by an experienced clinician. The lens then remained on the eye for 30 minutes. Measurements of the four ocular health parameters were repeated after 15 and 30 minutes of lens wear. The lens was then removed and the health of the eye reassessed.

RESULTS

No obvious pattern of change was found in logMAR visual acuity, keratometry, or intraocular pressure. The lens did produce changes to the conjunctival and corneal surfaces, but this was not considered clinically significant.

CONCLUSION

Search coils do not appear to cause any significant effects on visual function. However, thorough prescreening of subjects and post-wear checks should be carried out on all coil wearers to ensure no adverse effects have been caused.