Interocular Comparison by In Vivo Confocal Microscopy of the 2-Dimensional Architecture of the Normal Human Corneal Subbasal Nerve Plexus

The inferocentral whorl region and its directional patterns in the corneal sub-basal nerve plexus: A review

There has been a growing application of in vivo confocal microscopy (IVCM) in the examination of corneal microstructure, including different corneal layers and corneal nerve fibers in health and in pathological conditions. Corneal nerves forming the sub-basal nerve plexus (SBNP) beneath the corneal basal epithelial cell layer in particular have been intensively researched in health and disease as a marker for corneal neurophysioanatomical and degenerative changes. One intriguing feature in the SBNP that is found inferior to the corneal apex, is a whorl-like pattern (or vortex) of nerves, which represents an anatomical landmark. Evidence has indicated that the architecture of this 'whorl region' is dynamic, changing with time in healthy individuals but also in disease conditions such as in diabetic neuropathy and keratoconus. This review summarizes the known information regarding the characteristics and significance of the whorl region of nerves in the corneal SBNP, as a potential area of high relevance for future disease monitoring and diagnostics.

Chronic Kidney Disease Has No Impact on Tear Film Substance P Concentration in Type 2 Diabetes

PURPOSE

The study aimed to ascertain the potential effects of chronic kidney disease (CKD) on substance P concentration in the tear film of people with type 2 diabetes.

METHODS

Participants were classified into two groups: type 2 diabetes with concurrent chronic kidney disease (T2DM-CKD (n = 25)) and type 2 diabetes without chronic kidney disease (T2DM-no CKD (n = 25)). Ocular surface discomfort assessment, flush tear collection, in-vivo corneal confocal microscopy, and peripheral neuropathy assessment were conducted. Enzyme-linked immunosorbent assays were utilized to ascertain the levels of tear film substance P in collected flush tears. Correlation analysis, hierarchical multiple linear regression analysis, and t-tests or Mann-Whitney U tests were used in the analysis of data for two-group comparisons.

RESULTS

There was no substantial difference between the T2DM-CKD and T2DM-no CKD groups for tear film substance P concentration (4.4 (0.2-50.4) and 5.9 (0.2-47.2) ng/mL, respectively; p = 0.54). No difference was observed in tear film substance P concentration between the low-severity peripheral neuropathy and high-severity peripheral neuropathy groups (4.4 (0.2-50.4) and 3.3 (0.3-40.7) ng/mL, respectively; p = 0.80). Corneal nerve fiber length (9.8 ± 4.6 and 12.4 ± 3.8 mm/mm2, respectively; p = 0.04) and corneal nerve fiber density (14.7 ± 8.5 and 21.1 ± 7.0 no/mm2, respectively; p < 0.01) were reduced significantly in the T2DM-CKD group compared to the T2DM-no CKD group. There were significant differences in corneal nerve fiber density (21.0 ± 8.1 and 15.8 ± 7.7 no/mm2, respectively; p = 0.04) and corneal nerve fiber length (12.9 ± 4.2 and 9.7 ± 3.8 mm/mm2, respectively; p = 0.03) between the low- and high-severity peripheral neuropathy groups.

CONCLUSION

In conclusion, no significant difference in tear film substance P concentration was observed between type 2 diabetes with and without CKD. Corneal nerve loss, however, was more significant in type 2 diabetes with chronic kidney disease compared to type 2 diabetes alone, indicating that corneal nerve morphological measures could serve greater utility as a tool to detect neuropathy and nephropathy-related corneal nerve changes.

Impact of Chronic Kidney Disease on Corneal Neuroimmune Features in Type 2 Diabetes

Aim: To determine the impact of chronic kidney disease on corneal nerve measures and dendritic cell counts in type 2 diabetes. Methods: In vivo corneal confocal microscopy images were used to estimate corneal nerve parameters and compared in people with type 2 diabetes with chronic kidney disease (T2DM-CKD) (n = 29) and those with type 2 diabetes without chronic kidney disease (T2DM-no CKD) (n = 29), along with 30 healthy controls. Corneal dendritic cell densities were compared between people with T2DM-CKD and those with T2DM-no CKD. The groups were matched for neuropathy status. Results: There was a significant difference in corneal nerve fiber density (p < 0.01) and corneal nerve fiber length (p = 0.04) between T2DM-CKD and T2DM-no CKD groups. The two diabetes groups had reduced corneal nerve parameters compared to healthy controls (all parameters: p < 0.01). Immature central dendritic cell density was significantly higher in the T2DM-CKD group compared to the T2DM-no CKD group ((7.0 (3.8−12.8) and 3.5 (1.4−13.4) cells/mm2, respectively, p < 0.05). Likewise, central mature dendritic cell density was significantly higher in the T2DM-CKD group compared to the T2DM-no CKD group (0.8 (0.4−2.2) and 0.4 (0.6−1.1) cells/mm2, respectively, p = 0.02). Additionally, total central dendritic cell density was increased in the T2DM-CKD group compared to T2DM-no CKD group (10.4 (4.3−16.1) and 3.9 (2.1−21.0) cells/mm2, respectively, p = 0.03). Conclusion: The study showed that central corneal dendritic cell density is increased in T2DM-CKD compared to T2DM-no CKD, with groups matched for peripheral neuropathy severity. This is accompanied by a loss of central corneal nerve fibers. The findings raise the possibility of additional local factors exacerbating central corneal nerve injury in people with diabetic chronic kidney disease.

Reproducibility and Reliability of Subbasal Corneal Nerve Parameters of the Inferior Whorl in the Neurotoxic and Healthy Cornea

PURPOSE

The aim of this study was to investigate the reliability of subbasal corneal nerve plexus parameters of the inferior whorl compared with the central cornea with in vivo corneal confocal microscopy and to investigate the impact of inferior whorl pattern complexity on reproducibility.

METHODS

Subbasal corneal nerves of healthy controls (n = 10) and patients with chemotherapy-induced peripheral neuropathy (n = 10) were imaged with a laser scanning confocal microscope. Two masked, experienced observers and the original image taker were tasked with selecting representative images of the central cornea and inferior whorl for each participant. This was conducted on 2 occasions 1 week apart. Corneal nerve fiber length (CNFL) and fractal dimension (CNFrD) [central cornea: CNFL and CNFrD; inferior whorl region: inferior whorl length (IWL) and inferior whorl fractal dimension (IWFrD)] were analyzed. Intraclass correlation coefficient (ICC) was analyzed for interobserver and intraobserver reliability. Inferior whorl complexity was classified according to the ease of identification of the center point of convergence.

RESULTS

Interobserver ICC was 0.992 for CNFL, 0.994 for CNFrD, 0.980 for IWL, and 0.954 for IWFrD. When analyzed by inferior whorl complexity, the interobserver reliability was similar for simple (0.987 for IWL; 0.960 for IWFrD) and complex patterns (0.967 for IWL; 0.949 for IWFrD). However, intraobserver ICC were reduced for complex (IWL 0.841-0.970; IWFrD 0.830-0.955) compared with simple patterns (IWL 0.931-0.970; IWFrD 0.921-0.969).

CONCLUSIONS

Although the overall interobserver reliability was excellent for the central corneal and inferior whorl parameters, there was lower intraobserver reliability for the inferior whorl parameters for complex morphological patterns. To improve reliability, more sophisticated wide-field imaging of the inferior whorl may be needed.

Defining an Optimal Sample Size for Corneal Epithelial Immune Cell Analysis Using in vivo Confocal Microscopy Images

Purpose

In vivo confocal microscopy (IVCM) images are frequently used to quantify corneal epithelial immune cell (IC) density in clinical studies. There is currently limited evidence to inform the selection of a representative image sample size to yield a reliable IC density estimate, and arbitrary numbers of images are often used. The primary aim of this study was to determine the number of randomly selected, unique IVCM images required to achieve an acceptable level of accuracy when quantifying epithelial IC density, in both the central and peripheral cornea. The secondary aim was to evaluate the consistency and precision of an image selection approach where corneal epithelial IC density was quantified from "three representative images" selected independently by three experienced observers.

Methods

All combinations of two to 15 non-overlapping IVCM images were used for deriving IC density estimates, for both the central and peripheral cornea, in 20 healthy participants; the density value from averaging quantifications in the 16 images was defined as the "true mean". IC density estimates were compared with the true mean in each corneal region using a mean ratio. Intraclass correlation coefficients (ICCs) were used to evaluate the consistency of the mean ratios of IC density estimates derived from the method involving the manual selection of "three representative images" by the observers. The precision of the IC density estimates was compared to a scenario involving three randomly selected images.

Results

A total of 12 randomly selected, non-overlapping IVCM images were found to be required to produce a corneal epithelial IC density estimate that was within 30% of the true mean, 95% of the time, for the central cornea; seven such images produced an equivalent level of precision in the peripheral cornea. Mean ratios of corneal IC density estimates derived from "three representative images" methods had poor consistency between observers (ICC estimates <0.5) and similar levels of precision when compared with using three randomly selected images (p > 0.05 for all comparisons), in both the central and peripheral cornea.

Conclusions

Data presented in this study can inform image selection methods, and the sample size required for a preferred level of accuracy, when quantifying IC densities in the central and peripheral corneal epithelium using IVCM images.

Corneal Confocal Microscopy in Type 1 Diabetes Mellitus: A Six-Year Longitudinal Study

Purpose

The current study describes corneal nerve morphology using in vivo confocal microscopy (IVCM) in patients with type 1 diabetes (T1D) who were followed up for 6 years, and it examines the relationship between corneal parameters and metabolic control of glucose and peripheral neuropathy.

Methods

Sixty-two participants (37 with T1D and 25 control participants) were assessed in 2011 and 2017. Participants with bilateral cataract surgery or controls who developed diabetes were excluded. All underwent HbA1c, IVCM, and central corneal sensitivity measurements at both time points in the eye previously examined. A modified total neuropathy score was obtained.

Results

Participants were age and sex matched. The mean duration of diabetes was 32.1 ± 12.0 years at the follow-up visit. The sub-basal nerve density in participants with T1D was lower than that of the controls and did not change (mean ± SD, 11.07 ± 4.0 to 11.41 ± 4.1 mm/mm²; P = 0.71), but it showed a marginal change in controls (19.5 ± 3.7 to 21.63 ± 4.03 mm/mm²; P = 0.06). The corneal sensitivity in T1D did not change (1.3 ± 1.5 to 1.4 ± 1.0 mbar; P = 0.8), and it declined in the controls (0.2 ± 0.3 to 0.6 ± 0.3 mbar; P < 0.001). There were no significant changes in HbA_1c (60.5 ± 12.5 to 61.6 ± 13.7 mmol/mol) or in modified total neuropathy scores (2.4 ± 3.2 to 3.4 ± 3.8; P = 0.2).

Conclusions

The corneal nerve damage and poorer corneal sensitivity reported in the patients with T1D did not change and displayed improvement with good glycemic control.

Translational Relevance

The corneal nerve changes may be of more value in those with a shorter duration of diabetes for the timely prediction of at-risk individuals likely to develop peripheral neuropathy, particularly in type 1 diabetes.

The pattern of the inferocentral whorl region of the corneal subbasal nerve plexus is altered with age

PURPOSE

To describe the pattern of the nerves in the inferocentral whorl region of the human corneal subbasal nerve plexus (SBNP) in health and diseases known to affect the subbasal nerves.

METHODS

Laser-scanning in vivo confocal microscopy (IVCM) was used to image the SBNP bilaterally in 91 healthy subjects, 39 subjects with type 2 diabetes mellitus (T2DM), and 43 subjects with Parkinson's disease (PD). Whorl regions were classified according to nerve orientation relative to age and health/disease status.

RESULTS

Of 346 examined eyes, 300 (86.7%) had an identifiable whorl pattern. In healthy subjects, a clockwise nerve orientation of the whorl was most common (67.9%), followed by non-rotatory or 'seam' morphology (21.4%), and counterclockwise (10.7%). The clockwise orientation was more prevalent in healthy subjects than in T2DM or PD (P < 0.001). Healthy individuals below 50 years of age had a predominantly clockwise orientation (93.8%) which was reduced to 51.9% in those over 50 years (P < 0.001). Age but not disease status explained whorl orientation in T2DM and PD groups. Moreover, whorl orientation is bilaterally clockwise in the young, but adopts other orientations and becomes asymmetric across eyes with age. Finally, we report reflective 'dot-like' features confined to the whorl region of the subbasal plexus, sometimes appearing in close association with subbasal nerves and present in 84-93% of examined eyes regardless of disease status, eye or sex.

CONCLUSION

Subbasal nerves in the inferocentral whorl region are predominantly clockwise in young, healthy corneas. With aging and conditions of T2DM and PD, counterclockwise and non-rotatory configurations increase in prevalence, and bilateral symmetry is lost. Mechanisms regulating these changes warrant further investigation.

Corneal nerve changes following treatment with neurotoxic anticancer drugs

Survival rates of cancer has improved with the development of anticancer drugs including systemic chemotherapeutic agents. However, long-lasting side effects could impact treated patients. Neurotoxic anticancer drugs are specific agents which cause chemotherapy-induced peripheral neuropathy (CIPN), a debilitating condition that severely deteriorates quality of life of cancer patients and survivors. The ocular surface is also prone to neurotoxicity but investigation into the effects of neurotoxic chemotherapy on the ocular surface has been more limited compared to other systemic etiologies such as diabetes. There is also no standardized protocol for CIPN diagnosis with an absence of a reliable, objective method of observing nerve damage structurally. As the cornea is the most densely innervated region of the body, researchers have started to focus on corneal neuropathic changes that are associated with neurotoxic chemotherapy treatment. In-vivo corneal confocal microscopy enables rapid and objective structural imaging of ocular surface microscopic structures such as corneal nerves, while esthesiometers provide means of functional assessment by examining corneal sensitivity. The current article explores the current guidelines and gaps in our knowledge of CIPN diagnosis and the potential role of in-vivo corneal confocal microscopy as a diagnostic or prognostic tool. Corneal neuropathic changes with neurotoxic anticancer drugs from animal research progressing through to human clinical studies are also discussed, with a focus on how these data inform our understanding of CIPN.

A Cross-Sectional Study of Sub-Basal Corneal Nerve Reduction Following Neurotoxic Chemotherapy

Purpose

Sub-basal corneal nerves have been shown to change during neurotoxic chemotherapy treatment. This cross-sectional study investigated corneal nerve morphology in patients who have completed neurotoxic chemotherapy well after treatment cessation and its association with peripheral nerve function.

Methods

Central corneal nerve fiber length (CNFL) and inferior whorl length (IWL), average nerve fiber length (ANFL), corneal nerve fiber density (CNFD) and corneal nerve branch density (CNBD), and nerve fiber area (CNFA) were examined using in vivo corneal confocal microscopy in patients with cancer who had completed treatment with either paclitaxel or oxaliplatin between 3 and 24 months prior to assessment in comparison with 2 separate groups of healthy controls. Neurological assessments were conducted including clinician- and patient-reported outcomes, and neurological grading scales.

Results

Both paclitaxel- (n = 40) and oxaliplatin-treated (n = 30) groups had reduced IWL and ANFL compared to the respective healthy control groups (n = 15 in each group) (paclitaxel: IWL = P = 0.02, ANFL = P = 0.009; and oxaliplatin: IWL = P = 0.008, ANFL P = 0.02). CNFL and CNFD reduction were observed only in the paclitaxel-treated group compared with healthy controls (P = 0.008 and P = 0.02, respectively), whereas CNFA was reduced in the oxaliplatin-treated group (P = 0.04). IWL reduction correlated with worse fine hand dexterity in chemotherapy-treated patients (r = −0.33, P = 0.007).

Conclusions

There is evidence of corneal nerve loss in patients with cancer who have been treated with paclitaxel and oxaliplatin well after treatment cessation associated with worse upper limb function.

Translational Relevance

Sub-basal corneal nerve reduction is evident even after cessation of neurotoxic treatment. In vivo corneal confocal microscopy may be useful in the monitoring of nerve function in patients receiving chemotherapy.

Comparison of parameter agreement for characterization of corneal subbasal nerve plexus in the whorl-like region and central cornea using in vivo confocal microscopy

PURPOSE

To compare the reliability of the whorl-like region with that of the central cornea for accurate assessment of corneal subbasal nerve plexus (SNP) by analyzing the parameter variability of these two anatomical regions in repeated measurements.

METHODS

Participants were scanned in the central cornea and whorl-like region with in vivo confocal microscopy on three occasions by two examiners within a time span of one week. Coefficients of repeatability (CoR), intra-class correlation coefficient (ICC), and Bland-Altman scatter plots with 95% limits of agreement (LOA) in the central cornea and whorl-like region were calculated, respectively, based on the nerve fiber length, then the inter-observer and intra-observer agreement were compared between these two anatomical regions.

RESULTS

The inter-observer ICC was 0.945, the inter-observer CoR was 0.052, the intra-observer ICC was 0.936, and the inter-observer CoR was 0.046, with narrow 95% LOA within 1 standard deviation in the whorl-like region, whereas the inter-observer ICC was 0.600, the inter-observer CoR was 0.207, the intra-observer ICC was 0.206, and the intra-observer CoR was 0.253, with 95% LOA nearly threefold wider than the standard deviation in the central cornea.

CONCLUSIONS

Nerve parameter in the whorl-like region showed higher inter-observer and intra-observer agreement than that of the central cornea. The whorl-like region is a more reliable site for accurate assessment of SNP.

Alteration of the pattern of regenerative corneal subbasal nerves after laser in-situ keratomileusis surgery

PURPOSE

Corneal nerves exhibit high plasticity, which allows successful reinnervation after nerve damage caused by laser in-situ keratomileusis (LASIK) surgery. This study aimed to examine corneal subbasal nerve orientation during regeneration after LASIK.

METHODS

This study involved 20 healthy, myopic subjects who had undergone bilateral Femto-LASIK 12-16 months prior with a superior hinge position. The corneal subbasal nerve plexus at the central, mid-temporal and mid-superior cornea on the right eye were imaged using in vivo confocal microscopy. Global nerve fibre orientation (indicating the overall pattern) and variation of nerve fibre orientation (indicating the consistency of the orientation) was determined using customised MATLAB™ software (www.mathworks.com/products/matlab.html). Differences in nerve orientation variables between groups were examined using the Mann-Whitney U test. Linear mixed models with Bonferroni adjustment were conducted to examine differences between corneal regions, and over time, after LASIK.

RESULTS

There were no differences between post-LASIK and control groups in global nerve orientation at any of the examined corneal regions. The post-LASIK subjects had a greater variation of nerve orientation at the central (p = 0.007) and temporal (p = 0.049) cornea than the controls. There was a difference in global nerve fibre orientation between corneal regions (p < 0.001) in the controls but not in the post-LASIK group. The variation of nerve fibre orientation was higher at the central, compared to the temporal and superior cornea after LASIK (p < 0.001), although there were no differences between corneal regions in controls.

CONCLUSIONS

Our results demonstrate that there was an increased variability in the corneal subbasal innervation patterns following LASIK when compared to controls.

Region of interest and directional analysis of subbasal nerves in wide-area corneal nerve plexus mosaics in type 2 diabetes mellitus

In vivo confocal microscopy (IVCM) imaging of the corneal subbasal nerve plexus (SBNP) is a clinical imaging modality gaining popularity for the diagnosis and follow-up of corneal neuropathy in various conditions such as diabetes mellitus. There remain, however, major limitations to the method, hindering its widespread clinical use. Finding the same exact area of the central cornea to standardize image acquisition is difficult without a reference point. Alternatively, creating wide-area mosaics of the SBNP is resource-intensive and has not yet been developed for routine clinical use. Here, we investigated whether IVCM analysis of the corneal SBNP in a predetermined, anatomically standardized region of interest (ROI) could be applied as an equivalent substitution for wide-area SBNP mosaic generation and analysis. Furthermore, we investigated nerve patterns outside the central corneal region for a possible relationship to type 2 diabetes mellitus status using a publicly available dataset. We found that corneal nerve fibre length density (CNFL) based on the ROI underestimated the mosaic-based CNFL by an average of 34% in 90% of cases (150 eyes), and did not exhibit a significant reduction with diabetes, as seen in the full SBNP. Outside the central cornea, nerve orientation differed depending on the anatomic region (left, central or right superior plexus, P < 0.001). Moreover, in long-term type 2 diabetes mellitus (≥ 10 years, 28 subjects), nerve density in the left superior sector of the SBNP was decreased (P < 0.001) while that in the central superior SBNP increased (P = 0.01) relative to 35 age-matched healthy subjects with normal glucose tolerance. These results indicate that subbasal nerve degeneration in type 2 diabetes mellitus can vary according to anatomic location, and regions with potential diagnostic value outside the central SBNP may warrant further investigation.

[In vivo imaging of the corneal nerve plexus : From single image to large scale map]

The sub-basal nerve plexus (SNP) of the cornea provides the possibility of in vivo and non-invasive examination of peripheral nerve structures by corneal confocal microscopy (CCM). Thus morphological alterations of the SNP can be directly detected and quantified. A single CCM image is insufficient for a well-founded diagnosis because of the inhomogeneous distribution of the nerve fibers; therefore, there is a demand for techniques for large area imaging of the SNP. This article provides an overview of published approaches to the problem. Current developmental work at the Karlsruhe Institute of Technology and the University of Rostock Eye Clinic is expected to lead to a simplified handling of the technology and a further improvement in the image quality.

Short-Term Effects of Overnight Orthokeratology on Corneal Sub-basal Nerve Plexus Morphology and Corneal Sensitivity

OBJECTIVE

To assess the effects of a short period of orthokeratology (OK) on corneal sub-basal nerve plexus (SBNP) morphology and corneal sensitivity.

METHODS

Measurements were made in 56 right eyes of 56 subjects with low-to-moderate myopia who wore 2 OK lens designs (Group CRT: HDS 100 Paragon CRT, n=35; Group SF: Seefree; n=21) for a period of 1 month and in 15 right eyes of noncontact lens wearers as controls. The variables determined in each participant were corneal sensitivity using a Cochet-Bonnet esthesiometer and 12 SBNP variables determined on laser scanning confocal microscopy images using 3 different software packages. Correlation between SBNP architecture and corneal sensitivity was also examined.

RESULTS

Few changes were observed over the 1-month period in the variables examined in the OK treatment and control groups. However, significant reductions were detected over time in the number of nerves in the central cornea in the groups CRT (P=0.029) and SF (P=0.043) and in central corneal sensitivity in CRT (P=0.047) along with significant increases in central and midperipheral corneal Langerhans cell counts in SF (P=0.001 and 0.048, respectively).

CONCLUSIONS

This study provides useful data to better understand the anatomical changes induced by OK in corneal SBNP. The different response observed to the 2 OK lens designs requires further investigation.

Corneal confocal sub-basal nerve plexus evaluation: a review

The aim of this study was to review the most recent data about corneal sub-basal nerve plexus (SNP) evaluated with the use of corneal confocal microscopy (CCM). For this purpose, an electronic search was conducted based on PubMed and Google Scholar and Web of Science databases from 2008 up to the end of 2016. Ninety-eight articles in English were cited, as well as abstracts in other languages, concerning the morphology and function of corneal SNP in various diseases. Changes in corneal SNP as a result of local treatment were also introduced. Figures with scans from confocal microscopy from our Department were included. The main conclusion of this review was that both corneal SNP diminishment and high tortuosity as well as low sensitivity are in principle related to the presence or level of pathology. In addition, increased nerve tortuosity may represent a morphological determinant of nerve regeneration. However, the presented literature shows that SNP changes are not characteristic for one unified corneal pathology; rather, they reflect the non-specific pathological process present in many diseases. Future studies should use automatized biometric software and also examine the effects of new treatments on SNP.

Corneal Nerve Fiber Structure, Its Role in Corneal Function, and Its Changes in Corneal Diseases

Recently, in vivo confocal microscopy is used to examine the human corneal nerve fibers morphology. Corneal nerve fiber architecture and its role are studied in healthy and pathological conditions. Corneal nerves of rats were studied by nonspecific acetylcholinesterase (NsAchE) staining. NsAchE-positive subepithelial (stromal) nerve fiber has been found to be insensitive to capsaicin. Besides, NsAchE-negative but capsaicin-sensitive subbasal nerve (leash) fibers formed thick mesh-like structure showing close interconnections and exhibit both isolectin B4- and transient receptor potential vanilloid channel 1- (TRPV1-) positive. TRPV1, TRPV3, TRPA (ankyrin) 1, and TRPM (melastatin) 8 are expressed in corneal nerve fibers. Besides the corneal nerve fibers, the expressions of TRPV (1, 3, and 4), TRPC (canonical) 4, and TRPM8 are demonstrated in the corneal epithelial cell membrane. The realization of the importance of TRP channels acting as polymodal sensors of environmental stresses has identified potential drug targets for corneal disease. The pathophysiological conditions of corneal diseases are associated with disruption of normal tissue innervation, especially capsaicin-sensitive small sensory nerve fibers. The relationships between subbasal corneal nerve fiber morphology and neurotrophic keratopathy in corneal diseases are well studied. The recommended treatment for neurotrophic keratopathy is administration of preservative free eye drops.

Unravelling the stromal-nerve interactions in the human diabetic cornea

Corneal defects due to diabetes mellitus (DM) may cause severe vision impairments. Current studies focus on the corneal epithelium and nerve defects neglecting the corneal stroma. The aim of this study was to develop a 3D in vitro model to examine the interactions between corneal stroma and nerves in the context of DM. Primary human corneal stromal fibroblasts isolated from healthy (HCFs), Type 1 (T1DM) and Type 2 (T2DM) patients were stimulated with stable ascorbic acid to secrete and assemble an extracellular matrix (ECM). Human neuronal cells were then seeded on top and differentiated to create the 3D co-cultures. Our data revealed successful co-culture of stromal fibroblasts and neuronal cells with large elongated neuron extensions. T2DM showed significant upregulation of Collagen III and IGF1 when compared to T1DM. Interestingly, upon nerve addition, those markers returned to HCF levels. Neuronal markers were also differentially modulated with T2DM co-cultures expressing high levels of βIII tubulin where T1DM co-cultures expressed Substance P. . Overall, our unique 3D co-culture model provides us with a tool that can be utilized for both molecular and therapeutic studies for diabetic keratopathy.

Corneal nerve microstructure in Parkinson's disease

Ocular surface changes and blink abnormalities are well-established in Parkinson's disease. Blink rate may be influenced by corneal sub-basal nerve density, however, this relationship has not yet been investigated in Parkinson's disease. This case-control study examined the ocular surface in patients with moderately severe Parkinson's disease, including confocal microscopy of the cornea. Fifteen patients with moderately severe Parkinson's disease (modified Hoehn and Yahr grade 3 or 4) and fifteen control participants were recruited. Ophthalmic assessment included slit-lamp examination, blink rate assessment, central corneal aesthesiometry and in vivo corneal confocal microscopy. The effect of disease laterality was also investigated. Of the 15 patients with Parkinson's disease, ten were male and the mean age was 65.5±8.6years. The corneal sub-basal nerve plexus density was markedly reduced in patients with Parkinson's disease (7.56±2.4mm/mm²) compared with controls (15.91±2.6mm/mm²) (p<0.0001). Corneal sensitivity did not differ significantly between the patients with Parkinson's disease (0.79±1.2mBAR) and the control group (0.26±0.35mBAR), p=0.12. Sub-basal nerve density was not significantly different between the eye ipsilateral to the side of the body with most-severe motor symptoms, and the contralateral eye. There was a significant positive correlation between ACE-R scores and sub-basal corneal nerve density (R²=0.66, p=0.02). This is the first study to report a significant reduction in corneal sub-basal nerve density in Parkinson's disease and demonstrate an association with cognitive dysfunction. These results provide further evidence to support the involvement of the peripheral nervous system in Parkinson's disease, previously thought to be a central nervous system disorder.

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.

Chronic migraine is associated with reduced corneal nerve fiber density and symptoms of dry eye

BACKGROUND

We used in vivo corneal confocal microscopy to investigate structural differences in the sub-basal corneal nerve plexus in chronic migraine patients and a normal population. We used a validated questionnaire and tests of lacrimal function to determine the prevalence of dry eye in the same group of chronic migraine patients. Activation of the trigeminal system is involved in migraine. Corneal nociceptive sensation is mediated by trigeminal axons that synapse in the gasserian ganglion and the brainstem, and serve nociceptive, protective, and trophic functions. Noninvasive imaging of the corneal sub-basal nerve plexus is possible with in vivo corneal confocal microscopy.

METHODS

For this case-control study, we recruited chronic migraine patients and compared them with a sex- and age-similar group of control subjects. Patients with peripheral neuropathy, a disease known to be associated with a peripheral neuropathy, or prior corneal or intraocular surgery were excluded. Participants underwent in vivo corneal confocal microscopy using a Heidelberg Retinal Tomography III confocal microscope with a Rostock Cornea Module. Nerve fiber length, nerve branch density, nerve fiber density, and tortuosity coefficient were measured using established methodologies. Migraine participants underwent testing of basal tear production with proparacaine, corneal sensitivity assessment with a cotton-tip applicator, measurement of tear break-up time, and completion of a validated dry eye questionnaire.

RESULTS

A total of 19 chronic migraine patients and 30 control participants completed the study. There were no significant differences in age or sex. Nerve fiber density was significantly lower in migraine patients compared with controls (48.4 ± 23.5 vs. 71.0 ± 15.0 fibers/mm2 , P < .001). Nerve fiber length was decreased in the chronic migraine group compared with the control group, but this difference was not statistically significant (21.5 ± 11.8 vs. 26.8 ± 5.9 mm/mm2, P < .084). Nerve branch density was similar in the two groups (114.0 ± 92.4 vs. 118.1 ± 55.9 branches/mm2 , P < .864). Tortuosity coefficient and log tortuosity coefficient also were similar in the chronic migraine and control groups. All migraine subjects had symptoms consistent with a diagnosis of dry eye syndrome.

CONCLUSIONS

We found that in the sample used in this study, the presence of structural changes in nociceptive corneal axons lends further support to the hypothesis that the trigeminal system plays a critical role in the pathogenesis of migraine. In vivo corneal confocal microscopy holds promise as a biomarker for future migraine research as well as for studies examining alterations of corneal innervation. Dry eye symptoms appear to be extremely prevalent in this population. The interrelationships between migraine, corneal nerve architecture, and dry eye will be the subject of future investigations.

Differences and similarities in development of corneal nerve damage and peripheral neuropathy and in diet-induced obesity and type 2 diabetic rats

PURPOSE

Peripheral neuropathy has been shown to exist in prediabetic and diabetic patients and animal models. However, the development of peripheral neuropathy in prediabetes and posthyperglycemia is likely different. The purpose of this study was to examine the progression of peripheral neuropathy in diet-induced obese rats and high-fat-fed rats treated with a low dose of streptozotocin, a model for type 2 diabetes, using standard endpoints as well as corneal sensitivity and innervation.

METHODS

Diet-induced obese rats and high-fat/low-dose streptozotocin diabetic rats were used to examine standard peripheral neuropathy endpoints and innervation of the cornea and corneal epithelium using corneal and standard confocal microscopy, respectively, and corneal sensitivity using a Cochet-Bonnet esthesiometer at three different time points.

RESULTS

Obese rats and to a greater extent diabetic rats were insulin resistant. Obese and diabetic rats had developed sensory nerve deficits, but only diabetic rats had motor nerve dysfunction as determined by measuring nerve conduction velocity, thermal nociception, and intraepidermal nerve fiber density. In the cornea there was a decrease in corneal nerve fiber length, innervation of the corneal epithelium, and corneal sensitivity in both diet-induced obese and diabetic rats.

CONCLUSIONS

These studies demonstrate that changes in corneal nerve innervation and sensitivity occur in both obese and type 2 diabetic rat models that are consistent with development of peripheral neuropathy. Examination of corneal nerve changes may be valuable endpoints for exploring potential treatments for peripheral neuropathy in both prediabetes with insulin resistance and diabetes.