The Glenn A. Fry award lecture 2010: Ophthalmic markers of diabetic neuropathy

Mosaic vs. Single Image Analysis with Confocal Microscopy of the Corneal Nerve Plexus for Diagnosis of Early Diabetic Peripheral Neuropathy

INTRODUCTION

The assessment of the corneal nerve fibre plexus with corneal confocal microscopy (CCM) is an upcoming but still experimental method in the diagnosis of early stage diabetic peripheral neuropathy (DPN). Using an innovative imaging technique-Heidelberg Retina Tomograph equipped with the Rostock Cornea Module (HRT-RCM) and EyeGuidance module (EG)-we were able to look at greater areas of subbasal nerve plexus (SNP) in order to increase the diagnostic accuracy. The aim of our study was to evaluate the usefulness of EG instead of single image analysis in diagnosis of early stage DPN.

METHODS

This prospective study was performed on 60 patients with type 2 diabetes mellitus, classified equally into two subgroups based on neuropathy deficient score (NDS): patients without DPN (group 1) or with mild DPN (group 2). The following parameters were analysed in the two subgroups: corneal nerve fibre length (CNFL; mm/mm²), corneal nerve fibre density (CNFD; no./mm²), corneal nerve branch density (CNBD; no./mm²). Furthermore, we compared the data calculated with the novel mosaic, EG-based method with those received from single image analysis using different quantification tools.

RESULTS

Using EG we did not find a significant difference between group 1 and group 2: CNFL (16.81 ± 5.87 mm/mm² vs. 17.19 ± 7.19 mm/mm², p = 0.895), CNFD (254.05 ± 115.36 no./mm² vs. 265.91 ± 161.63 no./mm², p = 0.732) and CNBD (102.68 ± 62.28 no./mm² vs. 115.38 ± 96.91 no./mm², p = 0.541). No significant difference between the EG method of analysing the SNP and the single image analysis of 10 images per patient was detected.

CONCLUSION

On the basis of our results it was not possible to differentiate between early stages of large nerve fibre DPN in patients with type 2 diabetes mellitus via SNP analysis. To improve sensitivity and specificity of this method newer technologies are under current evaluation.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier NCT05326958.

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.

Corneal nerves in diabetes-The role of the in vivo corneal confocal microscopy of the subbasal nerve plexus in the assessment of peripheral small fiber neuropathy

The cornea's intense innervation is responsible for corneal trophism and ocular surface hemostasis maintenance. Corneal diabetic neuropathy affects subbasal nerve plexus, with progressive alteration of nerves' morphology and density. The quantitative analysis of nerve fibers can be performed with in vivo corneal confocal microscopy considering the main parameters such as corneal nerve fibers length, corneal nerve fibers density, corneal nerve branching density, tortuosity coefficient, and beadings frequency. As the nerve examination permits the detection of early changes occurring in diabetes, the invivo corneal confocal microscopy becomes, over time, an important tool for diabetic polyneuropathy assessment and follow-up. In this review, we summarize the actual evidence about corneal nerve changes in diabetes and the relationship between the grade of alterations and the duration and severity of the disease. We aim at understanding how diabetes impacts corneal nerves and how it correlates with sensorimotor peripheral polyneuropathy and retinal complications. We also attempt to analyze the safety of the most common surgical procedures such as cataract and refractive surgery in diabetic patients and to highlight the specific risk factors. We believe that information about the corneal nerve fibers' condition obtained from the in vivo subbasal nerve plexus investigation may be crucial in monitoring peripheral small fiber polyneuropathy and that it will help with decision-making in ophthalmic surgery in diabetic patients.

Reduced corneal nerve fibre length in prediabetes and type 2 diabetes: The Maastricht Study

PURPOSE

In individuals with diabetes, injury to the corneal nerve fibres predisposes to delayed corneal epithelial healing, reduced corneal sensitivity and corneal erosion. We investigated to what extent a reduction in corneal nerve fibre length (CNFL) is present in individuals with prediabetes or type 2 diabetes (DM2) compared with individuals with normal glucose metabolism (NGM).

METHODS

Using composite images acquired by corneal confocal microscopy, we assessed total CNFL per mm² in the subbasal nerve plexus of the cornea in 134 participants (mean age 59 ± 8 years, 49% men, 87 NGM, 20 prediabetes, 27 DM2). Multivariable linear regression was used to assess the association between CNFL and glucose metabolism status, adjusted for age and sex.

RESULTS

In individuals with type 2 diabetes, the mean CNFL was significantly reduced [β = -1.86 mm/mm² (95% CI -3.64 to -0.08), p = 0.04], as compared with individuals with normal glucose metabolism after adjustment for age and sex. Part of the reduction was present in individuals with prediabetes [β = -0.96 mm/mm² (95% CI -2.91 to 0.99), p = 0.34], with a linear trend of corneal nerve fibre reduction with severity of glucose metabolism status (p trend = 0.04).

CONCLUSIONS

A significant reduction in CNFL was found in individuals with DM2 compared with individuals with NGM. A trend of reduction in CNFL was observed between individuals with NGM and prediabetes. The reduction in corneal nerve fibre length could contribute to a delayed corneal healing and an increased risk for corneal complications after surgery.

Effects of Vitamin D Receptor Knockout and Vitamin D Deficiency on Corneal Epithelial Wound Healing and Nerve Density in Diabetic Mice

Diabetic keratopathy occurs in ∼70% of all people with diabetes. This study was designed to examine the effects of vitamin D receptor knockout (VDR-/-) and vitamin D deficiency (VDD) on corneal epithelial wound healing and nerve density in diabetic mice. Diabetes was induced using the low-dose streptozotocin method. Corneal epithelial wounds were created using an Algerbrush, and wound healing was monitored over time. Corneal nerve density was measured in unwounded mice. VDR-/- and VDD diabetic mice (diabetic for 8 and 20 weeks, respectively) had slower healing ratios than wild-type diabetic mice. VDR-/- and VDD diabetic mice also showed significantly decreased nerve density. Reduced wound healing ratios and nerve densities were not fully rescued by a supplemental diet rich in calcium, lactose, and phosphate. We conclude that VDR-/- and VDD significantly reduce both corneal epithelial wound healing and nerve density in diabetic mice. Because the supplemental diet did not rescue wound healing or nerve density, these effects are likely not specifically related to hypocalcemia. This work supports the hypothesis that low vitamin D levels can exacerbate preexisting ophthalmic conditions, such as diabetes.

ASSOCIATIONS BETWEEN INDIVIDUAL RETINAL LAYER THICKNESSES AND DIABETIC PERIPHERAL NEUROPATHY USING RETINAL LAYER SEGMENTATION ANALYSIS

PURPOSE

To evaluate clinical correlations between the thicknesses of individual retinal layers in the foveal area of diabetic patients and the presence of diabetic peripheral neuropathy (DPN).

METHODS

This retrospective, observational, cross-sectional study enrolled a total of 120 eyes from 120 patients. The eyes were divided into 3 groups: normal controls (n = 42 eyes), patients with diabetes mellitus (n = 42 eyes) but no DPN, and patients with diabetes mellitus and DPN (n = 36 eyes). The primary outcome measures were the thickness of all retinal layers in the central 1-mm zone measured using the segmentation analysis of spectral-domain optical coherence tomography. Correlations between the thicknesses of the individual retinal layers and the presence of DPN were also analyzed. Logistic regression analyses were used to determine which change in layer thickness had the most significant association with the presence of DPN.

RESULTS

The mean thicknesses and the ratios of retinal nerve fiber layers to total retina thicknesses in the DPN group were 10.77 ± 1.79 μm and 4.10 ± 0.55%, which was significantly lower than those in normal controls and the diabetes mellitus with no DPN group (P = 0.014 and P = 0.001, respectively). Logistic regression analyses also showed that the decrease in thicknesses of the retinal nerve fiber layers and the inner nuclear layer are significant factors for predicting a higher risk for DPN development (odds ratio = 7.407 and 1.757; P < 0.001 and P = 0.001, respectively).

CONCLUSION

A decrease in the retinal nerve fiber layer and the inner nuclear layer thickness was significantly associated with the presence of DPN.

Corneal pain and experimental model development

The cornea is a valuable tissue for studying peripheral sensory nerve structure and regeneration due to its avascularity, transparency, and dense innervation. Somatosensory innervation of the cornea serves to identify changes in environmental stimuli at the ocular surface, thereby promoting barrier function to protect the eye against injury or infection. Due to regulatory demands to screen ocular safety of potential chemical exposure, a need remains to develop functional human tissue models to predict ocular damage and pain using in vitro-based systems to increase throughput and minimize animal use. In this review, we summarize the anatomical and functional roles of corneal innervation in propagation of sensory input, corneal neuropathies associated with pain, and the status of current in vivo and in vitro models. Emphasis is placed on tissue engineering approaches to study the human corneal pain response in vitro with integration of proper cell types, controlled microenvironment, and high-throughput readouts to predict pain induction. Further developments in this field will aid in defining molecular signatures to distinguish acute and chronic pain triggers based on the immune response and epithelial, stromal, and neuronal interactions that occur at the ocular surface that lead to functional outcomes in the brain depending on severity and persistence of the stimulus.

Opposing Effects of Neuropilin-1 and -2 on Sensory Nerve Regeneration in Wounded Corneas: Role of Sema3C in Ameliorating Diabetic Neurotrophic Keratopathy

The diabetic cornea exhibits pathological alterations, such as delayed epithelial wound healing and nerve regeneration. We investigated the role of semaphorin (SEMA) 3C in corneal wound healing and reinnervation in normal and diabetic B6 mice. Wounding induced the expression of SEMA3A, SEMA3C, and their receptor neuropilin-2 (NRP2), but not NRP1, in normal corneal epithelial cells; this upregulation was suppressed for SEMA3C and NRP2 in diabetic corneas. Injections of Sema3C-specific small interfering RNA and NRP2-neutralizing antibodies in wounded mice resulted in a decrease in the rate of wound healing and regenerating nerve fibers, whereas exogenous SEMA3C had opposing effects in diabetic corneas. NRP1 neutralization, on the other hand, decreased epithelial wound closure but increased sensory nerve regeneration in diabetic corneas, suggesting a detrimental role in nerve regeneration. Taken together, epithelium-expressed SEMA3C plays a role in corneal epithelial wound closure and sensory nerve regeneration. The hyperglycemia-suppressed SEMA3C/NRP2 signaling may contribute to the pathogenesis of diabetic neurotrophic keratopathy, and SEMA3C might be used as an adjunctive therapeutic for treating the disease.

Cornea and anterior eye assessment with slit lamp biomicroscopy, specular microscopy, confocal microscopy, and ultrasound biomicroscopy

Current corneal assessment technologies make the process of corneal evaluation extremely fast and simple, and several devices and technologies show signs that help in identification of different diseases thereby, helping in diagnosis, management, and follow-up of patients. The purpose of this review is to present and update readers on the evaluation of cornea and ocular surface. This first part reviews a description of slit lamp biomicroscopy (SLB), endothelial specular microscopy, confocal microscopy, and ultrasound biomicroscopy examination techniques and the second part describes the corneal topography and tomography, providing up-to-date information on the clinical recommendations of these techniques in eye care practice. Although the SLB is a traditional technique, it is of paramount importance in clinical diagnosis and compulsory when an eye test is conducted in primary or specialist eye care practice. Different techniques allow the early diagnosis of many diseases, especially when clinical signs have not yet become apparent and visible with SLB. These techniques also allow for patient follow-up in several clinical conditions or diseases, facilitating clinical decisions and improving knowledge regarding the corneal anatomy.

Automatic analysis of corneal nerves imaged using in vivo confocal microscopy

Interest has grown over the past decade in using in vivo confocal microscopy to analyse the morphology of corneal nerves and their changes over time. Advances in computational modelling techniques have been applied to automate the estimation of sub-basal nerve structure. These objective methods have the potential to quantify nerve density (and length), tortuosity, variations in nerve thickness, as well as temporal changes in nerve fibres such as migration patterns. Different approaches to automated nerve analysis, methods proposed and how they were validated in previous literature are reviewed. Improved understanding of these approaches and their limitations will help improve the diagnostic leverage of emerging developments for monitoring the onset and progression of a broad class of systemic diseases, including diabetes.

Diagnostic capability of retinal thickness measures in diabetic peripheral neuropathy

PURPOSE

To examine the diagnostic capability of the full retinal and inner retinal thickness measures in differentiating individuals with diabetic peripheral neuropathy (DPN) from those without neuropathy and non-diabetic controls.

METHODS

Individuals with (n=44) and without (n=107) diabetic neuropathy and non-diabetic control (n=42) participants underwent spectral domain optical coherence tomography (SDOCT). Retinal thickness in the central 1mm zone (including the fovea), parafovea and perifovea was assessed in addition to ganglion cell complex (GCC) global loss volume (GCC GLV) and focal loss volume (GCC FLV), and retinal nerve fiber layer (RNFL) thickness. Diabetic neuropathy was defined using a modified neuropathy disability score (NDS) recorded on a 0-10 scale, wherein, NDS ≥3 indicated neuropathy and NDS indicated <3 no neuropathy. Diagnostic performance was assessed by areas under the receiver operating characteristic curves (AUCs), 95 per cent confidence intervals (CI), sensitivities at fixed specificities, positive likelihood ratio (+LR), negative likelihood ratio (-LR) and the cut-off points for the best AUCs obtained.

RESULTS

The AUC for GCC FLV was 0.732 (95% CI: 0.624-0.840, p<0.001) with a sensitivity of 53% and specificity of 80% for differentiating DPN from controls. Evaluation of the LRs showed that GCC FLV was associated with only small effects on the post-test probability of the disease. The cut-off point calculated using the Youden index was 0.48% (67% sensitivity and 73% specificity) for GCC FLV. For distinguishing those with neuropathy from those without neuropathy, the AUCs of retinal parameters ranged from 0.508 for the central zone to 0.690 for the inferior RNFL thickness. For distinguishing those with moderate or advanced neuropathy from those with mild or no neuropathy, the inferior RNFL thickness demonstrated the highest AUC of 0.820, (95% CI: 0.731-0.909, p<0.001) with a sensitivity of 69% and 80% specificity. The cut-off-point for the inferior RNFL thickness was 97μm, with 81% sensitivity and 72% specificity.

CONCLUSIONS

The GCC FLV can differentiate individuals with diabetic neuropathy from healthy controls, while the inferior RNFL thickness is able to differentiate those with greater degrees of neuropathy from those with mild or no neuropathy, both with an acceptable level of accuracy. Optical coherence tomography represents a non-invasive technology that aids in detection of retinal structural changes in patients with established diabetic neuropathy. Further refinement of the technique and the analytical approaches may be required to identify patients with minimal neuropathy.

Recovery of Corneal Sensitivity and Increase in Nerve Density and Wound Healing in Diabetic Mice After PEDF Plus DHA Treatment

Diabetic keratopathy decreases corneal sensation and tear secretion and delays wound healing after injury. In the current study, we tested the effect of treatment with pigment epithelium-derived factor (PEDF) in combination with docosahexaenoic acid (DHA) on corneal nerve regeneration in a mouse model of diabetes with or without corneal injury. The study was performed in streptozotocin-induced diabetic mice (C57BL/6). Ten weeks after streptozotocin injection, diabetic mice showed significant decreases of corneal sensitivity, tear production, and epithelial subbasal nerve density when compared with age-matched normal mice. After diabetic mice were wounded in the right eye and treated in both eyes with PEDF+DHA for 2 weeks, there was a significant increase in corneal epithelial nerve regeneration and substance P-positive nerve density in both wounded and unwounded eyes compared with vehicle-treated corneas. There also was elevated corneal sensitivity and tear production in the treated corneas compared with vehicle. In addition, PEDF+DHA accelerated corneal wound healing, selectively recruited type 2 macrophages, and prevented neutrophil infiltration in diabetic wounded corneas. These results suggest that topical treatment with PEDF+DHA promotes corneal nerve regeneration and wound healing in diabetic mice and could potentially be exploited as a therapeutic option for the treatment of diabetic keratopathy.

Translational Immunoimaging and Neuroimaging Demonstrate Corneal Neuroimmune Crosstalk

Corneal immunoimaging and neuroimaging approaches facilitate in vivo analyses of the cornea, including high-resolution imaging of corneal immune cells and nerves. This approach facilitates the analyses of underlying immune and nerve alterations not detected by clinical slit-lamp examination alone. In this review, we describe recent work performed in our translational ocular immunology center with a focus on "bench-to-bedside" and "bedside-to-bench" research. The ability to visualize dendritiform immune cells (DCs) in patients with laser in vivo confocal microscopy (IVCM), recently discovered in the central murine cornea, has allowed us to demonstrate their utility as a potential surrogate biomarker for inflammatory ocular surface diseases. This biomarker for inflammation allows the measurement of therapeutic efficacy of anti-inflammatory drugs and its utility as an endpoint in clinical trials with high interobserver agreement. IVCM image analyses from our studies has demonstrated a significant increase in DC density and size in ocular disease, a positive correlation between DC density and clinical signs and symptoms of disease and pro-inflammatory tear cytokines, and a strong negative correlation between DC density and subbasal nerve density. In conjunction with preclinical research investigating the inflammatory state in a partial or fully denervated cornea, our results indicated that corneal nerves are directly involved in the regulation of homeostasis and immune privilege in the cornea.

A systematic review on the impact of diabetes mellitus on the ocular surface

Diabetes mellitus is associated with extensive morbidity and mortality in any human community. It is well understood that the burden of diabetes is attributed to chronic progressive damage in major end-organs, but it is underappreciated that the most superficial and transparent organ affected by diabetes is the cornea. Different corneal components (epithelium, nerves, immune cells and endothelium) underpin specific systemic complications of diabetes. Just as diabetic retinopathy is a marker of more generalized microvascular disease, corneal nerve changes can predict peripheral and autonomic neuropathy, providing a window of opportunity for early treatment. In addition, alterations of immune cells in corneas suggest an inflammatory component in diabetic complications. Furthermore, impaired corneal epithelial wound healing may also imply more widespread disease. The non-invasiveness and improvement in imaging technology facilitates the emergence of new screening tools. Systemic control of diabetes can improve ocular surface health, possibly aided by anti-inflammatory and vasoprotective agents.

Dendritic cell dysfunction and diabetic sensory neuropathy in the cornea

Diabetic peripheral neuropathy (DPN) often leads to neurotrophic ulcerations in the cornea and skin; however, the underlying cellular mechanisms of this complication are poorly understood. Here, we used post-wound corneal sensory degeneration and regeneration as a model and tested the hypothesis that diabetes adversely affects DC populations and infiltration, resulting in disrupted DC-nerve communication and DPN. In streptozotocin-induced type 1 diabetic mice, there was a substantial reduction in sensory nerve density and the number of intraepithelial DCs in unwounded (UW) corneas. In wounded corneas, diabetes markedly delayed sensory nerve regeneration and reduced the number of infiltrating DCs, which were a major source of ciliary neurotrophic factor (CNTF) in the cornea. While CNTF neutralization retarded reinnervation in normal corneas, exogenous CNTF accelerated nerve regeneration in the wounded corneas of diabetic mice and healthy animals, in which DCs had been locally depleted. Moreover, blockade of the CNTF-specific receptor CNTFRα induced sensory nerve degeneration and retarded regeneration in normal corneas. Soluble CNTFRα also partially restored the branching of diabetes-suppressed sensory nerve endings and regeneration in the diabetic corneas. Collectively, our data show that DCs mediate sensory nerve innervation and regeneration through CNTF and that diabetes reduces DC populations in UW and wounded corneas, resulting in decreased CNTF and impaired sensory nerve innervation and regeneration.

Comparison of Standard Versus Wide-Field Composite Images of the Corneal Subbasal Layer by In Vivo Confocal Microscopy

PURPOSE

To evaluate whether the densities of corneal subbasal nerves and epithelial immune dendritiform cells (DCs) are comparable between a set of three representative standard images of in vivo confocal microscopy (IVCM) and the wide-field mapped composite IVCM images.

METHODS

This prospective, cross-sectional, and masked study included 110 eyes of 58 patients seen in a neurology clinic who underwent laser-scanning IVCM (Heidelberg Retina Tomograph 3) of the central cornea. Densities of subbasal corneal nerves and DCs were compared between the average of three representative standard images and the wide-field mapped composite images, which were reconstructed by automated mapping.

RESULTS

There were no statistically significant differences between the average of three representative standard images (0.16 mm2 each) and the wide-field composite images (1.29 ± 0.64 mm2) in terms of mean subbasal nerve density (17.10 ± 6.10 vs. 17.17 ± 5.60 mm/mm2, respectively, P = 0.87) and mean subbasal DC density (53.2 ± 67.8 vs. 49.0 ± 54.3 cells/mm2, respectively, P = 0.43). However, there were notable differences in subbasal nerve and DC densities between these two methods in eyes with very low nerve density or very high DC density.

CONCLUSIONS

There are no significant differences in the mean subbasal nerve and DC densities between the average values of three representative standard IVCM images and wide-field mapped composite images. Therefore, these standard images can be used in clinical studies to accurately measure cellular structures in the subbasal layer.

New ophthalmologic imaging techniques for detection and monitoring of neurodegenerative changes in diabetes: a systematic review

Optical coherence tomography (OCT) of the retina and around the optic nerve head and corneal confocal microscopy (CCM) are non-invasive and repeatable techniques that can quantify ocular neurodegenerative changes in individuals with diabetes. We systematically reviewed studies of ocular neurodegenerative changes in adults with type 1 or type 2 diabetes and noted changes in the retina, the optic nerve head, and the cornea. Of the 30 studies that met our inclusion criteria, 14 used OCT and 16 used CCM to assess ocular neurodegenerative changes. Even in the absence of diabetic retinopathy, several layers in the retina and the mean retinal nerve fibre layer around the optic nerve head were significantly thinner (-5·36 μm [95% CI -7·13 to -3·58]) in individuals with type 2 diabetes compared with individuals without diabetes. In individuals with type 1 diabetes without retinopathy none of the intraretinal layer thicknesses were significantly reduced compared with individuals without diabetes. In the absence of diabetic polyneuropathy, individuals with type 2 diabetes had a lower nerve density (nerve branch density: -1·10/mm(2) [95% CI -4·22 to 2·02]), nerve fibre density: -5·80/mm(2) [-8·06 to -3·54], and nerve fibre length: -4·00 mm/mm(2) [-5·93 to -2·07]) in the subbasal nerve plexus of the cornea than individuals without diabetes. Individuals with type 1 diabetes without polyneuropathy also had a lower nerve density (nerve branch density: -7·74/mm(2) [95% CI -14·13 to -1·34], nerve fibre density: -2·68/mm(2) [-5·56 to 0·20]), and nerve fibre length: -2·58 mm/mm(2) [-3·94 to -1·21]). Ocular neurodegenerative changes are more evident when diabetic retinopathy or polyneuropathy is present. OCT and CCM are potentially useful, in addition to conventional clinical methods, to assess diabetic neurodegenerative changes. Additional research is needed to determine their incremental benefit and to standardise procedures before the application of OCT and CCM in daily practice.

In Vivo Confocal Microscopy of the Cornea: New Developments in Image Acquisition, Reconstruction, and Analysis Using the HRT-Rostock Corneal Module

The optical sectioning ability of confocal microscopy allows high magnification images to be obtained from different depths within a thick tissue specimen and is thus ideally suited to the study of intact tissue in living subjects. In vivo confocal microscopy has been used in a variety of corneal research and clinical applications since its development over 25 years ago. In this article we review the latest developments in quantitative corneal imaging with the Heidelberg Retinal Tomograph with Rostock Corneal Module (HRT-RCM). We provide an overview of the unique strengths and weaknesses of the HRT-RCM. We discuss techniques for performing 3-D imaging with the HRT-RCM, including hardware and software modifications that allow full-thickness confocal microscopy through-focusing (CMTF) of the cornea, which can provide quantitative measurements of corneal sublayer thicknesses, stromal cell and extracellular matrix backscatter, and depth-dependent changes in corneal keratocyte density. We also review current approaches for quantitative imaging of the subbasal nerve plexus, which require a combination of advanced image acquisition and analysis procedures, including wide-field mapping and 3-D reconstruction of nerve structures. The development of new hardware, software, and acquisition techniques continues to expand the number of applications of the HRT-RCM for quantitative in vivo corneal imaging at the cellular level. Knowledge of these rapidly evolving strategies should benefit corneal clinicians and basic scientists alike.

Sensory nerve regeneration after epithelium wounding in normal and diabetic cornea

The cornea is the most densely innervated mammalian tissue. The sensory nerves are responsible for sensations of dryness, temperature, touch, and pain, and play important roles in the blink reflex, wound healing, and tear production. Many ocular and systemic diseases can adversely affect corneal sensory nerve and consequently impair their function. One of such systemic diseases is diabetes mellitus (DM) which causes sensory degeneration, neurotrophic keratopathy (DNK), and delayed wound healing. In this review, we summarize recent discoveries revealing mechanisms underlying the pathogenesis of DNK and the impairment of sensory nerve regeneration in post wound diabetic corneas in using animal model of human diabetes. Because it is generally believed that common mechanisms are operative in the pathogenesis of diabetic peripheral neuropathy in different tissues, the findings in the corneas have implications in in other tissues such as the skin, which often leads to foot ulceration and amputation in diabetic patients.

Corneal confocal microscopy predicts 4-year incident peripheral neuropathy in type 1 diabetes

OBJECTIVE

This study determined if deficits in corneal nerve fiber length (CNFL) assessed using corneal confocal microscopy (CCM) can predict future onset of diabetic peripheral neuropathy (DPN).

RESEARCH DESIGN AND METHODS

CNFL and a range of other baseline measures were compared between 90 nonneuropathic patients with type 1 diabetes who did or did not develop DPN after 4 years. The receiver operator characteristic (ROC) curve was used to determine the capability of single and combined measures of neuropathy to predict DPN.

RESULTS

DPN developed in 16 participants (18%) after 4 years. Factors predictive of 4-year incident DPN were lower CNFL (P = 0.041); longer duration of diabetes (P = 0.002); higher triglycerides (P = 0.023); retinopathy (higher on the Early Treatment of Diabetic Retinopathy Study scale) (P = 0.008); nephropathy (higher albumin-to-creatinine ratio) (P = 0.001); higher neuropathy disability score (P = 0.037); lower cold sensation (P = 0.001) and cold pain (P = 0.027) thresholds; higher warm sensation (P = 0.008), warm pain (P = 0.024), and vibration (P = 0.003) thresholds; impaired monofilament response (P = 0.003); and slower peroneal (P = 0.013) and sural (P = 0.002) nerve conduction velocity. CCM could predict the 4-year incident DPN with 63% sensitivity and 74% specificity for a CNFL threshold cutoff of 14.1 mm/mm(2) (area under ROC curve = 0.66, P = 0.041). Combining neuropathy measures did not improve predictive capability.

CONCLUSIONS

DPN can be predicted by various demographic, metabolic, and conventional neuropathy measures. The ability of CCM to predict DPN broadens the already impressive diagnostic capabilities of this novel ophthalmic marker.

Differential reduction in corneal nerve fiber length in patients with type 1 or type 2 diabetes mellitus

AIM

To examine the relationship between corneal nerve fiber length (CNFL) and diabetic neuropathy (DN) status in patients with type 1 or type 2 diabetes mellitus (DM).

METHODS

In this cross-sectional study, we examined 25 diabetic patients without DN, 10 patients with mild DN, 8 patients with severe DN, and 9 controls without diabetes. DN status was assigned based on a combination of clinical symptoms, signs, and electrophysiological testing. Patients underwent corneal confocal microscopy (CCM) of the sub-basal nerve plexus. Post-hoc analysis of the CCM images was performed to quantify the average CNFL, and ANOVA was used to assess for differences in CNFL.

RESULTS

All 25 subjects without DN had type 1 DM, and subjects with DN had type 2 DM. Participants with severe DN had significantly lower CNFL (12.5±6.1mm/mm(2)) compared to controls (20.7±2.2mm/mm(2)) (p=0.009). However, lower CNFL was also found in participants with type 1 DM who did not have DN (15.1±4.7mm/mm(2)) relative to controls (p=0.033).

CONCLUSIONS

CCM of the sub-basal nerve plexus may be an indicator of early peripheral nerve degeneration in type 1 DM. Type of diabetes, in addition to degree of neuropathy, may influence the extent of corneal nerve damage.

Reduced innervation and delayed re-innervation after epithelial wounding in type 2 diabetic Goto-Kakizaki rats

Patients with diabetes are at an increased risk for developing corneal complications including delayed wound healing and potential vision loss. To understand the cause of diabetic keratopathy, we investigated innervation and its correlation with delayed corneal epithelial wound healing in type 2 diabetic Goto-Kakizaki (GK) rats. GK rats are smaller than the age-matched control Wistar rats from which the GK rats were derived. The blood sugar levels of GK rats are significantly higher than those of Wistar rats. GK rats had increased rose bengal staining and cornea fragility. Fewer nerve fibers were detected compared with Wistar rats. Although nerve fiber densities detected by whole-mount immunohistochemistry were similar near the limbal region, in the central cornea the subbasal nerve plexuses were thinner, less abundant, and showed less branching in GK rats. Corneal epithelial wound closure was delayed and re-innervation was slow and incomplete in GK rats. These abnormalities were more apparent in older GK rats (12 months). Our data suggest that diabetic neuropathy occurs in the cornea of type 2 diabetic GK rats, and defects in the sensory nerve and/or tear film may contribute to diabetic keratopathy and delayed epithelial wound healing in diabetic corneas.

Retinal neuropathy precedes vasculopathy in diabetes: a function-based opportunity for early treatment intervention?

Diabetes, now at epidemic levels, can have devastating effects on the eye and vision. Treatments of the ocular complications are currently focused on relatively advanced stages and are limited to the slowing down of the progressive sight-threatening retinal vasculopathy (diabetic retinopathy). Tiny signals from the neural retina have been shown to reveal early diabetic neuropathy prior to vascular retinopathy. These signals, in a clinical test format, are predictive, by precise retinal location, of impending vasculopathy in the retina within a year, including sight-threatening oedema. The discovery opens possibilities for the future development of treatments to prevent the onset of retinopathy and the more sight-threatening retinal oedema and changes patient management strategies.

Changes in corneal innervation and sensitivity and acetylcholine-mediated vascular relaxation of the posterior ciliary artery in a type 2 diabetic rat

PURPOSE

Corneal confocal microscopy is emerging as a clinical tool to evaluate the development and progression of diabetic neuropathy. The purpose of these studies was to characterize the changes in corneal sensitivity and innervation in a rat model of type 2 diabetes in relation to standard peripheral neuropathy endpoints. Assessment of diabetes-induced changes in corneal innervation and sensitivity in animal models will be important for determining the usefulness of corneal markers for preclinical studies to test potential new treatments for diabetic neuropathy.

METHODS

High-fat/low-dose streptozotocin diabetic rats were used to examine diabetes-induced changes in standard diabetic neuropathy endpoints and innervation of the cornea using confocal microscopy, corneal sensitivity using a Cochet-Bonnet esthesiometer, and vascular reactivity of the posterior ciliary artery.

RESULTS

Compared with age-matched control rats, the induction of hyperglycemia in rats fed high-fat diets caused a decrease in nerve conduction velocity, thermal hypoalgesia, and intraepidermal nerve fiber profiles. In the cornea there was a decrease in corneal nerve fiber length and sensitivity. In addition, vascular relaxation in response to acetylcholine was decreased in the posterior ciliary artery.

CONCLUSIONS

These studies suggest that in a type 2 diabetic rat model, changes in corneal nerve innervation and sensitivity occur that are consistent with changes seen in diabetic patients. Corneal sensitivity and innervation may be valuable endpoints for examining the potential treatments of diabetic neuropathy in preclinical studies.