Epithelial Innervation of Human Cornea

Pathophysiology of Corneal Endothelial Cell Loss in Dry Eye Disease and Other Inflammatory Ocular Disorders

PURPOSE

Dry eye disease (DED) and other inflammatory ocular disorders have been reported to be associated with decreased corneal endothelial cell density (CECD), however the mechanism of underlying endothelial cell loss remains unknown.

METHODS

We conducted a comprehensive literature search of English-written publications on dry eye disease, corneal endothelial cell loss, Sjögren's syndrome, and Graft Vs Host Disease (GVHD), to review the effects of DED and other inflammatory ocular surface conditions on CECD.

RESULTS

A total of 78 studies were included in our study. Loss of corneal neurotrophic support, cytotoxic stress, and a heightened immune response, all of which may occur secondarily to a common causative agent such as inflammation, are major contributors to reduced CECD.

CONCLUSION

More studies are needed to determine how the interrelated pathways of altered corneal nerve function and upregulated expression of inflammatory activity influence corneal endothelial cell loss.

Corneal Parameters in Patients with Multiple Sclerosis: A Pilot Study

OBJECTIVE

To evaluate the corneal topographic parameter values measured with the Pentacam Scheimpflug system in patients with multiple sclerosis (MS).

METHODS

A total of 108 eyes of 62 MS patients were studied. In addition to a complete examination of anterior and posterior segments, all patients were scanned using the Pentacam Scheimpflug camera. The diagnosis of MS was made according to the McDonald criteria. All MS patients were clinically assessed using the Multiple Sclerosis Severity Score (MSSS).

RESULTS

The mean age was 38.89 ± 10.18 years (36.16 - 41.30) for MS patients and 40.94 ± 9.44 years (38.49 - 43.11) for the controls (p = 0.26). Only central corneal thickness (CCT) and corneal volume (CV) values were significantly lower in MS patients (p < 0.001). The other corneal parameters were not significantly different between the study eyes and control eyes (p > 0.05 for all). Pachymetric measurements at the corneal apex were 525.69 ± 29.35 (518.29 - 533.67) µm for the study eyes versus 563.13 ± 23.70 (562.13 - 576.36) µm for the control eyes. CV were 59.22 ± 4.11(58.18 - 60.20) mm³ for the study eyes versus 62.78 ± 3.09 (62.38 - 64.00) mm³ for the control eyes.

CONCLUSION

This is the first study that has reported lower CCT and CV measurements in MS patients than healthy subjects of a similar age. These results should be supported by further studies.

Evaluation of Corneal Alterations After Short-Term Silicone Hydrogel Contact Lens Use by Confocal Microscopy

Objectives

To evaluate the corneal subbasal nerve morphology, corneal sensitivity, and anterior segment alterations in short-term silicone hydrogel contact lens (SiHCL) users by confocal microscopy.

Materials and Methods

The study included 25 right eyes of 25 male volunteers aged 25-30 years who had never used SiHCLs before. ocular surface disease index (OSDI), tear break-up time, Schirmer test, tear meniscus area, strip meniscometry tube, corneal sensitivity, and corneal subbasal nerve morphology were evaluated before and after 1 month of CL use.

Results

OSDI was 10.6±1.1 before CL use and 17.2±1.2 after 1 month of CL use (p<0.01). Schirmer test distance was 16.3±2.3 mm before and 14.3±1.9 mm after 1 month of CL use (p>0.05). Tear film break-up time was 7.1±0.4 s before and 6.2±0.3 s after CL use (p>0.05). The tear meniscus area was 0.026±0.002 mm² before and 0.024±0.001 mm² after 1 month of CL use (p>0.05). Strip meniscometry tube results were 5.4±0.9 mm before and 4.9±0.8 mm after 1 month of CL use (p>0.05). Corneal sensitivity values were 3.2±0.4 mm before and 2.95±0.3 mm after 1 month of CL use (p>0.05). Dendritic cell density evaluated by confocal microscopy was 14.84±3.1 cells/mm² before and 32.57±4.2 cells/mm² after 1 month of CL use (p<0.01). Subbasal nerve tortuosity was 0.92±0.2 before and 1.03±0.2 after 1 month of CL use (p>0.05). Subbasal nerve density was measured as 4726±310 pixels/frame before and 4570±272 pixels/frame after 1 month of CL use (p>0.05).

Conclusion

After a month of SiHCL use, no significant changes were observed in tear secretion, corneal sensitivity, tear meniscus volume, subbasal corneal nerve density, reflectivity, or tortuosity, while a significant increase was found in OSDI and dendritic cell density.

Corneal toxicity associated with belantamab mafodotin is not restricted to the epithelium: neuropathy studied with confocal microscopy

PURPOSE

The purpose of this study was to investigate epithelial and neuronal changes in patients with refractory/relapsed multiple myeloma (RRMM) before/during belantamab mafodotin (belamaf) treatment using confocal microscopy.

DESIGN

Retrospective case series.

METHODS

RRMM patients underwent best corrected visual acuity (BCVA) testing and slit-lamp examination/photography, followed by corneal confocal microscopy (CCM) to evaluate the epithelium and subbasal nerve plexus (SNP) to measure corneal nerve fiber density (CNFD), -branch density (CNBD), and -fiber length (CNFL) before and during belamaf treatment.

RESULTS

In 14 eyes of 7 patients (4 female, 68±10 years) with complete follow-up (4±2 months), the median BCVA dropped from 20/25 (20/25-20/20) to 20/40 (20/200-20/32) in the worse eye at the end of follow-up. Microcystic epithelial changes and ocular surface disease were demonstrated biomicroscopically. CCM showed "grape-like" hyperreflective spots in the central basal epithelium that changed to polymorphous-structured cysts in the superficial epithelium, with no pathology detected at the(peri-)limbal structures. The baseline, normal SNP morphology with a mean CNFD, CNBD, and CNFL of 20.25±7.06/mm², 19.49±12.34/mm², and 11.8±3.74mm/mm² respectively, showed severe fiber fragmentation during follow-up, and an observed complete loss of the SNP at the end of follow-up in all eyes.

CONCLUSIONS

This study is the first to illustrate neurotoxic effects of belamaf on the human cornea.

Self-Report of Severity of Ocular Pain Due to Light as a Predictor of Altered Central Nociceptive System Processing in Individuals With Symptoms of Dry Eye Disease

Dry eye disease (DED) is a diagnosis given to individuals with a heterogeneous combination of symptoms and/or signs, including spontaneous and evoked ocular pain. Our current study evaluated whether and which ocular pain assessments could serve as screening tools for central sensitization in individuals with DED. A cohort of individuals with DED symptoms (n = 235) were evaluated for ocular pain, DED signs (tear production, evaporation), evoked sensitivity to mechanical stimulation at the cornea, and evidence of central sensitization. Central sensitization was defined for this study as the presence of pain 30 seconds after termination of a thermal noxious temporal summation protocol (ie, aftersensations) presented at a site remote from the eye (ventral forearm). We found that combining ratings of average intensity of ocular pain, ratings of average intensity of pain due to light, response to topical anesthetic eye drops, and corneal mechanical pain thresholds produced the best predictive model for central sensitization (area under the curve of .73). When examining ratings of intensity of ocular pain due to light alone (0-10 numerical rating), a cutoff score of 2 maximized sensitivity (85%) and specificity (48%) for the presence of painful aftersensations at the forearm. Self-reported rating of pain sensitivity to light may serve as a quick screening tool indicating the involvement of central nociceptive system dysfunction in individuals with DED. PERSPECTIVE: This study reveals that clinically-relevant variables, including a simple 0 to 10 rating of ocular pain due to light, can be used to predict the contribution of central sensitization mechanisms in a subgroup of individuals with DED symptoms. These findings can potentially improve patient stratification and management for this complex and painful disease.

Regenerative therapy for the Cornea

The cornea is the outmost layer of the eye, unique in its transparency and strength. The cornea not only transmits the light essential for vision, also refracts light, giving focus to images. Each of the three layers of the cornea has properties essential for the function of vision. Although the epithelium can often recover from injury quickly by cell division, loss of limbal stem cells can cause severe corneal surface abnormalities leading to corneal blindness. Disruption of the stromal extracellular matrix and loss of cells determining this structure, the keratocytes, leads to corneal opacity. Corneal endothelium is the inner part of the cornea without self-renewal capacity. It is very important to maintain corneal dehydration and transparency. Permanent damage to the corneal stroma or endothelium can be effectively treated by corneal transplantation; however, there are drawbacks to this procedure, including a shortage of donors, the need for continuing treatment to prevent rejection, and limits to the survival of the graft, averaging 10-20 years. There exists a need for new strategies to promote regeneration of the stromal structure and restore vision. This review highlights critical contributions in regenerative medicine with the aim of corneal reconstruction after injury or disease. These approaches include corneal stromal stem cells, corneal limbal stem cells, embryonic stem cells, and other adult stem cells, as well as induced pluripotent stem cells. Stem cell-derived trophic factors in the forms of secretomes or exosomes for corneal regeneration are also discussed. Corneal sensory nerve regeneration promoting corneal transparency is discussed. This article provides description of the up-to-date options for corneal regeneration and presents exciting possible avenues for future studies toward clinical applications for corneal regeneration.

Corneal Subbasal Plexus in Eyes with Fuchs' Endothelial Corneal Dystrophy after Two Different Endothelial Surgeries

Purpose

To evaluate the morphological features and density of corneal subbasal plexus (SBP) using in vivo corneal confocal microscopy (IVCCM) in patients affected by Fuchs' endothelial corneal dystrophy (FECD) six months after Descemet membrane endothelial keratoplasty (DMEK) and Descemet-stripping automated endothelial keratoplasty (DSAEK).

Methods

We included patients affected by FECD, requiring corneal endothelial surgery due to corneal oedema occurred from 3 to 6 months. 7 eyes underwent DMEK and 7 eyes DSAEK. All patients performed IVCCM preoperative and in six months postoperative. We analyzed SBP parameters, using CS4 Nerves Tracking Tool, and we studied the differences between the two endothelial keratoplasties.

Results

Comparing the eyes treated with DMEK with those treated with DSAEK, preoperative corneal thickness, corrected distance visual acuity (CDVA), and age were similar in both groups. SBP was not detectable at preoperative IVCCM in any eye. Postoperatively, the nerve fibers length, the nerve fibers density, the tortuosity, and the number of fibers and of branching did not differ in the eyes that underwent DMEK compared to DSAEK. The corneal beadings density was higher after DMEK than DSAEK, and this difference was statistically significant (P = 0.004). The type of endothelial keratoplasty was not associated with the presence or absence of postoperative corneal SBP (Pearson' chi-square, 0.755).

Conclusions

Postoperative corneal reinnervation should be easily and noninvasively studied using IVCCM. Morphological postoperative features of SBP did not differ between two different types of endothelial keratoplasty, DMEK and DSAEK, despite the different sizes of the corneal incision. The lower beading density in the DSAEK group should be the consequence of a different distribution of mitochondria along the nerve fibers, as expression of a supposed higher metabolic distress in the DSAEK group.

Comparing the results of manual and automated quantitative corneal neuroanalysing modules for beginners

This study aimed to evaluate the reliability of in vivo confocal microscopic neuroanalysis by beginners using manual and automated modules. Images of sub-basal corneal nerve plexus (SCNP) from 108 images of 18 healthy participants were analyzed by 7 beginner observers using manual (CCMetrics, [CCM]) and automated (ACCMetrics, [ACCM]) module. SCNP parameters analyzed included corneal nerve fiber density (NFD), corneal nerve branch density (NBD), corneal nerve fiber length (NFL), and tortuosity coefficient (TC). The intra-observer repeatability, inter-observer reliability, inter-module agreement, and left–right eye symmetry level of SCNP parameters were examined. All observers showed good intra-observer repeatability using CCM (intraclass correlation coefficient [ICC] > 0.60 for all), except when measuring TC. Two observers demonstrated especially excellent repeatability in analyzing NFD, NBD, and NFL using manual mode, indicating the quality of interpretation may still be observer-dependent. Among all SCNP parameters, NFL had the best inter-observer reliability (Spearman’s rank-sum correlation coefficient [SpCC] and ICC > 0.85 for the 3 original observers) and left–right symmetry level (SpCC and ICC > 0.60). In the additional analysis of inter-observer reliability using results by all 7 observers, only NFL showed good inter-observer reliability (ICC = 0.79). Compared with CCM measurements, values of ACCM measurements were significantly lower, implying a poor inter-module agreement. Our result suggested that performance of quantitative corneal neuroanalysis by beginners maybe acceptable, with NFL being the most reliable parameter, and automated method cannot fully replace manual work.

Oral Aminoacids Supplementation Improves Corneal Reinnervation After Photorefractive Keratectomy: A Confocal-Based Investigation

Aim of this retrospective study was to estimate the effect of oral supplementation with amino acids (AA) on corneal nerves regrowth after excimer laser refractive surgery with photorefractive keratectomy (PRK). Based on the pre and post-surgical treatment received, 40 patients with 12 months of follow-up were distributed in two groups: 20 patients had received oral AA supplementation 7 days before and 30 days after PRK, and 20 patients without AA supplementation, as untreated reference control. All patients followed the same standard post-operative topical therapy consisting of an association of antibiotic and steroid plus sodium hyaluronate during the first week, then steroid alone progressively decreasing during 30 days and sodium hyaluronate for the following 3 months. In vivo corneal confocal microscopy was used to evaluate the presence of sub-basal corneal nerve fibers during 12 months after PRK. Results have shown that sub-basal nerves regenerated significantly faster (p <0.05), and nerve fibers density was significantly higher (p <0.05) with a more regular pattern in the eyes of AA treated patients with respect to the untreated control group. Therefore, our data indicate that oral supplementation with AA improved significantly corneal nerve restoration after PRK and could thus be considered as an additional treatment during corneal surgical procedures.

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.

Is gabapentin effective in dry eye disease and neuropathic ocular pain?

This study aims to evaluate the efficacy of gabapentin treatment in dry eye disease (DED) and neuropathic ocular pain. Our study was performed with 72 patients. The painDETECT questionnaire was used for neuropathic pain screening. Patients who were thought to have severe DED according to ocular surface disease index (OSDI) questionnaire, Schirmer's test type 1 and tear break up time test results were treated with artificial tear and cyclosporine drops. Gabapentin treatment was also initiated in addition to artificial tear and cyclosporine drops treatments to the patients with neuropathic component and DED findings. We divided the patients into two groups: group 1 (n: 36), patients treated with artificial tear and cyclosporine drops and group 2 (n: 36), patients treated with artificial tear, cyclosporine drops and gabapentin. In the first evaluation, no significant differences were found between groups in terms of OSDI score, Schirmer's test result and TBUT. After the 6 weeks of treatment, in both groups OSDI score, Schirmer's test result and TBUT statistically significantly improved. OSDI score, Schirmer's test result and TBUT significantly improved after the 6 weeks of gabapentin treatment than artificial tear and cyclosporine treatment group (p < 0.001). Dry eye patients should be screened for neuropathic ocular pain symptoms and individualized treatment has to be applied. Our study showed that the use of gabapentin is effective in severe dry eye patients with neuropathic ocular pain.

A Review of Imaging Biomarkers of the Ocular Surface

A biomarker is a "characteristic that is measured as an indicator of normal biological processes, pathogenic processes, or responses to an exposure or intervention, including therapeutic interventions." Recently, calls for biomarkers for ocular surface diseases have increased, and advancements in imaging technologies have aided in allowing imaging biomarkers to serve as a potential solution for this need. This review focuses on the state of imaging biomarkers for ocular surface diseases, specifically non-invasive tear break-up time (NIBUT), tear meniscus measurement and corneal epithelial thickness with anterior segment optical coherence tomography (OCT), meibomian gland morphology with infrared meibography and in vivo confocal microscopy (IVCM), ocular redness with grading scales, and cellular corneal immune cells and nerve assessment by IVCM. Extensive literature review was performed for analytical and clinical validation that currently exists for potential imaging biomarkers. Our summary suggests that the reported analytical and clinical validation state for potential imaging biomarkers is broad, with some having good to excellent intra- and intergrader agreement to date. Examples of these include NIBUT for dry eye disease, ocular redness grading scales, and detection of corneal immune cells by IVCM for grading and monitoring inflammation. Further examples are nerve assessment by IVCM for monitoring severity of diabetes mellitus and neurotrophic keratitis, and corneal epithelial thickness assessment with anterior segment OCT for the diagnosis of early keratoconus. However, additional analytical validation for these biomarkers is required before clinical application as a biomarker.

Atypical Cellular Elements of Unknown Origin in the Subbasal Nerve Plexus of a Diabetic Cornea Diagnosed by Large-Area Confocal Laser Scanning Microscopy

In vivo large-area confocal laser scanning microscopy (CLSM) of the human eye using EyeGuidance technology allows a large-scale morphometric assessment of the corneal subbasal nerve plexus (SNP). Here, the SNP of a patient suffering from diabetes and associated late complications was analyzed. The SNP contained multiple clusters of large hyperintense, stellate-shaped, cellular-like structures. Comparable structures were not observed in control corneas from healthy volunteers. Two hypotheses regarding the origin of these atypical structures are proposed. First, these structures might be keratocyte-derived myofibroblasts that entered the epithelium from the underlying stroma through breaks in Bowman’s layer. Second, these structures could be proliferating Schwann cells that entered the epithelium in association with subbasal nerves. The nature and pathophysiological significance of these atypical cellular structures, and whether they are a direct consequence of the patient’s diabetic neuropathy/or a non-specific secondary effect of associated inflammatory processes, are unknown.

How scars shape the neural landscape: Key molecular mediators of TGF-β1's anti-neuritogenic effects

Following injury to the peripheral and central nervous systems, tissue levels of transforming growth factor (TGF)-β1 often increase, which is key for wound healing and scarring. However, active wound regions and scars appear to inhibit process outgrowth by regenerating neurons. We recently showed that corneal wound myofibroblasts block corneal nerve regeneration in vivo, and sensory neurite outgrowth in vitro in a manner that relies critically on TGF-β1. In turn, delayed, abnormal re-innervation contributes to long-term sensory dysfunctions of the ocular surface. Here, we exposed morphologically and biochemically-differentiated sensory neurons from the ND7/23 cell line to TGF-β1 to identify the intracellular signals regulating these anti-neuritogenic effects, contrasting them with those of Semaphorin(Sema)3A, a known inhibitor of neurite outgrowth. Neuronal morphology was quantified using phase-contrast imaging. Western blotting and specific inhibitors were then used to identify key molecular mediators. Differentiated ND7/23 cells expressed neuron-specific markers, including those involved in neurite extension and polarization. TGF-β1 increased phosphorylation of collapsin response mediator protein-2 (CRMP2), a molecule that is key for neurite extension. We now show that both glycogen synthase kinase (GSK)-3β and Smad3 modulate phosphorylation of CRMP2 after treatment with TGF-β1. GSK-3β appeared to exert a particularly strong effect, which could be explained by its ability to phosphorylate not only CRMP2, but also Smad3. In conclusion, TGF-β1's inhibition of neurite outgrowth in sensory neurons appears to be regulated through a highly-conserved signaling pathway, which involves the GSK-3β/CRMP-2 loop via both canonical and non-canonical mechanisms. It is hoped that by defining the signaling pathways that control neurite outgrowth in wound environments, it will become possible to identify optimal molecular targets to promote re-innervation following injury.

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.

Validation of the Use of Automated and Manual Quantitative Analysis of Corneal Nerve Plexus Following Refractive Surgery

Following refractive surgery, the cornea is denervated and re-innervated, hence a reproducible tool to objectively quantify this change is warranted. This study aimed to determine the repeatability and reproducibility of corneal nerve quantification between automated (ACCMetrics) and manual software (CCMetrics) following refractive surgery. A total of 1007 in vivo confocal microscopy images from 20 post-small incision lenticule extraction (SMILE) or post-laser-assisted in situ keratomileusis (LASIK) patients were evaluated by two independent observers using CCMetrics for corneal nerve fibre density (CNFD), corneal nerve branch density (CNBD), and corneal nerve fibre length (CNFL). Intra-observer and inter-observer reproducibility and repeatability, as well as agreement and correlation between the measurements obtained by ACCMetrics and CCMetrics, were assessed. We found that CNFL demonstrated the best intra- and inter-observer agreement followed by CNFD (intra-class correlation coefficient (ICC) = 0.799 and 0.740, respectively for CNFL; 0.757 and 0.728 for CNFD). CNBD demonstrated poorest intra- and inter-observer ICC. There was an underestimation in ACCMetrics measurements compared to CCMetrics measurements, although the differences were not significant. Our data suggested that both automated and manual methods can be used as reliable tools for the evaluation of corneal nerve status following refractive surgery. However, the measurements obtained with different methods are not interchangeable.

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.

Impact of Attrition, Intercellular Shear in Dry Eye Disease: When Cells are Challenged and Neurons are Triggered

The mechanical component in the pathophysiology of dry eye disease (DED) deserves attention as an important factor. The lubrication deficit induced impaired mechano-transduction of lid pressure to the ocular surfaces may lead to the dysregulation of homeostasis in the epithelium, with sensations of pain and secondary inflammation. Ocular pain is possibly the first sign of attrition and may occur in the absence of visible epithelial damage. Attrition is a process which involves the constant or repeated challenge of ocular surface tissues by mechanical shear forces; it is enhanced by the thinning of corneal epithelium in severe DED. As a highly dynamic process leading to pain and neurogenic inflammation, the identification of the impact of attrition and its potential pathogenic role could add a new perspective to the current more tear film-oriented models of ocular surface disease. Treatment of DED addressing lubrication deficiencies and inflammation should also consider the decrease of attrition in order to stimulate epithelial recovery and neural regeneration. The importance of hyaluronic acid, its molecular characteristics, the extracellular matrix and autoregulative mechanisms in this process is outlined. The identification of the attrition and recognition of its impact in dry eye pathophysiology could contribute to a better understanding of the disease and optimized treatment regimens.

Use of the Inferior Whorl for Detecting Age-Related Changes in Human Corneal Subbasal Nerve Plexus with Laser Scanning Confocal Microscopy

PURPOSE

The purpose of this study was to determine the effect of aging on the corneal subbasal nerve plexus (SNP) by employing a wide-field mapping technique of composite images, scanned at the location of a distinctive spiraled subbasal nerve pattern located 1-2 mm inferior to the corneal apex (the inferior whorl) for SNP structural quantification.

MATERIALS AND METHODS

Central corneal tactile sensitivity (CCTS) and inferior whorl length (IWL) were compared among individuals in 3 age-groups (20-39 years, 40-59 years, and 60-79 years). Statistical analyses constituted the Kruskal-Wallis test, one-way analysis of variance (with the post hoc least significant difference test), Spearman correlation coefficient, and linear regression analysis.

RESULTS

CCTS remained stable until the age of 50 years, when it began to decrease; the mean CCTS was 58.15 ± 2.46 mm in the group aged 20-39 years, 55.74 ± 3.85 mm in the group aged 40-59 years, and 50.23 ± 3.27 mm in the group aged 60-79 years. IWL decreased with increasing age, with a corresponding linear decline of 0.2088 mm/mm2 per year, and the mean IWL was 25.43 ± 4.50 mm/mm2 in the group aged 20-39 years, 22.71 ± 6.19 mm/mm2 in the group aged 40-59 years, and 18.60 ± 4.21 mm/mm2 in the group aged 60-79 years.

CONCLUSION

Our work provided a more accurate and repeatable method for corneal nerve analysis using laser scanning confocal microscopy. By using this technique, we confirmed that aging is associated with progressive reduction in subbasal nerve length.

Ocular Surface Disease Index and Ocular Thermography in Keratoconus Patients

Purpose

Keratoconus (KC) has been defined as a “noninflammatory” corneal disease, but recent studies have noted a potential inflammatory origin. We analysed the Ocular Surface Disease Index (OSDI) and ocular surface temperature (OST) in KC patients compared to controls. Patients and Methods. A total of 179 eyes in 90 patients with KC (topographic keratoconus classification 0-1 to 4, age 36.1 ± 12.5 years, 65.9% males) and 82 eyes in 41 controls (age 36.4 ± 12.8 years, 47.6% males) were examined. The participants completed the OSDI questionnaire and underwent corneal topography, tomography, and thermography. Additional outcome measures were vision- and discomfort-related OSDI subscores and mean OST  at the corneal centre during 10 seconds of sustained eye opening after blinking.

Results

The OSDI score (31.4 ± 22.4 vs. 17.5 ± 17.9) and vision- (17.7 ± 14.6 vs. 10.5 ± 13.2) and discomfort-related (14.3 ± 10.7 vs. 9.4 ± 10.5) OSDI subscores were significantly higher in KC patients than in controls (p < 0.001). We found no significant difference in the central corneal OST (34.2 ± 0.6°C vs. 34.2 ± 0.7°C; p < 0.001). We found no significant difference in the central corneal OST (34.2 ± 0.6°C vs. 34.2 ± 0.7°C; p < 0.001). We found no significant difference in the central corneal OST (34.2 ± 0.6°C vs. 34.2 ± 0.7°C; r > 0.174, p < 0.001). We found no significant difference in the central corneal OST (34.2 ± 0.6°C vs. 34.2 ± 0.7°C; r > 0.174, r > 0.174,

Conclusion

KC patients had increased OSDI scores and vision- and discomfort-related OSDI subscores without an increase in the OST compared to a normal population. OSDI score/subscores weakly correlate with SAI and SRI but do not correlate with OST in KC patients or controls. Vision- and discomfort-related symptoms of KC have to be managed in parallel in ophthalmological practice, but the necessity of anti-inflammatory treatment cannot be verified through ocular thermography.

Corneal Sub-Basal Nerve Changes in Patients with Herpetic Keratitis During Acute Phase and after 6 Months

Background and objectives: The purpose of this study was to describe corneal sensitivity and the morphological changes of sub-basal corneal nerves using in vivo laser scanning confocal microscopy (LSCM) in herpes simplex virus (HSV) keratitis-affected eyes, and to compare with both contralateral eyes and with the eyes of patients with a previous history of herpes labialis but no history of herpetic eye disease, and with healthy patients with no history of any HSV diseases, during the acute phase of the disease and after six months. Materials and Methods: A prospective clinical study included 269 patients. All of them underwent a complete ophthalmological examination, Cochet-Bonnet aesthesiometry and LSCM within the central 5 mm of the cornea. After six months, all the patients with herpetic eye disease underwent the same examination. Serology tests of the serum to detect HSV 1/2 IgG and IgM were performed. Results: HSV-affected eyes compared with contralateral eyes, herpes labialis and healthy control group eyes demonstrated a significant decrease in corneal sensitivity, corneal nerve fibre density, corneal nerve branch density, corneal nerve fibre length and corneal nerve total branch density (p < 0.05). During follow up after six months, corneal sensitivity and sub-basal nerve parameters had increased but did not reach the parameters of contralateral eyes (p < 0.05). Previous herpes labialis did not influence corneal sensitivity and was not a risk factor for herpetic eye disease. Conclusions: Corneal sensitivity and sub-basal nerve changes in HSV-affected eyes revealed a significant decrease compared with contralateral eyes, and with the eyes of patients with a previous history of herpes labialis, and of healthy controls. Following six months, corneal sensitivity and sub-basal nerve parameters increased; however, they did not reach the parameters of contralateral eyes and the eyes of healthy controls. The best recovery of corneal sensitivity was seen in patients with epithelial keratitis. Herpes labialis was not a risk factor for herpetic eye disease.

Impact of topical anti-fibrotics on corneal nerve regeneration in vivo

Recent work in vitro has shown that fibroblasts and myofibroblasts have opposing effects on neurite outgrowth by peripheral sensory neurons. Here, we tested a prediction from this work that dampening the fibrotic response in the early phases of corneal wound healing in vivo could enhance reinnervation after a large, deep corneal injury such as that induced by photorefractive keratectomy (PRK). Since topical steroids and Mitomycin C (MMC) are often used clinically for mitigating corneal inflammation and scarring after PRK, they were ideal to test this prediction. Twenty adult cats underwent bilateral, myopic PRK over a 6 mm optical zone followed by either: (1) intraoperative MMC (n = 12 eyes), (2) intraoperative prednisolone acetate (PA) followed by twice daily topical application for 14 days (n = 12 eyes), or (3) no post-operative treatment (n = 16 eyes). Anti-fibrotic effects of MMC and PA were verified optically and histologically. First, optical coherence tomography (OCT) performed pre-operatively and 2, 4 and 12 weeks post-PRK was used to assess changes in corneal backscatter reflectivity. Post-mortem immunohistochemistry was then performed at 2, 4 and 12 weeks post-PRK, using antibodies against α-smooth muscle actin (α-SMA). Finally, immunohistochemistry with antibodies against βIII-tubulin (Tuj-1) was performed in the same corneas to quantify changes in nerve distribution relative to unoperated, control cat corneas. Two weeks after PRK, untreated corneas exhibited the greatest amount of staining for α-SMA, followed by PA-treated and MMC-treated eyes. This was matched by higher OCT-based stromal reflectivity values in untreated, than PA- and MMC-treated eyes. PA treatment appeared to slow epithelial healing and although normal epithelial thickness was restored by 12 weeks-post-PRK, intra-epithelial nerve length only reached ∼1/6 normal values in PA-treated eyes. Even peripheral cornea (outside the ablation zone) exhibited depressed intra-epithelial nerve densities after PA treatment. Stromal nerves were abundant under the α-SMA zone, but appeared to largely avoid it, creating an area of sub-epithelial stroma devoid of nerve trunks. In turn, this may have led to the lack of sub-basal and intra-epithelial nerves in the ablation zone of PA-treated eyes 4 weeks after PRK, and their continuing paucity 12 weeks after PRK. Intra-operative MMC, which sharply decreased α-SMA staining, was followed by rapid restoration of nerve densities in all corneal layers post-PRK compared to untreated corneas. Curiously, stromal nerves appeared unaffected by the development of large, stromal, acellular zones in MMC-treated corneas. Overall, it appears that post-PRK treatments that were most effective at reducing α-SMA-positive cells in the early post-operative period benefited nerve regeneration the most, resulting in more rapid restoration of nerve densities in all corneal layers of the ablation zone and of the corneal periphery.

Corneal esthesiometry and sub-basal nerves morphological changes in herpes simplex virus keratitis/uveitis patients

AIM

To describe and compare corneal sensation and morphological changes of sub-basal corneal nerves by in vivo laser scanning confocal microscopy (LSCM) in herpes simplex virus (HSV) keratitis/uveitis and contralateral, clinically unaffected eyes.

METHODS

A prospective clinical study included 30 HSV eyes and 30 contralateral eyes of 30 patients, diagnosed with unilateral HSV keratitis/uveitis. Both eyes underwent a complete ophthalmological examination, Cochet-Bonnet aesthesiometry and LSCM of the central cornea, using the Heidelberg Retina Tomograph III Rostock Cornea Module. After 6mo, the same examination of the HSV affected and contralateral, clinically unaffected eyes was performed.

RESULTS

HSV eyes, as compared to contralateral eyes, demonstrated a significant decrease in mean corneal sensation (3.1±1.6 vs 5.3±0.8 cm), total nerve fibres number (5.7±4.4 vs 15.1±5.4), nerve branches (3.4±3.0 vs 8.4±4.7), main nerve trunks (2.3±1.6 vs 5.8±2.2), and nerve fibres density (7.5±5.6 vs 18.1±5.3 mm/mm², P<0.05). There was no significant difference between keratitis and uveitis eyes in mean corneal sensation and nerve fibres parameters. After 6mo, corneal sensation and sub-basal nerve fibres parameters were increased significantly, but did not reach the parameters of contralateral, clinically unaffected eyes.

CONCLUSION

Corneal aesthesiometry and LSCM in HSV affected eyes reveals a significant decrease of corneal sensation and sub-basal nerve fibres which recovers at 6mo but does not reach the normal level.

Corneal re-innervation following refractive surgery treatments

Laser refractive surgery is one of the most performed surgical procedures in the world. Although regarded safe and efficient, it has side effects. All of the laser based refractive surgical procedures invoke corneal nerve injury to some degree. The impact of this denervation can range from mild discomfort to neurotrophic corneas. Currently, three techniques are widely used for laser vision correction: small incision lenticule extraction, laser-assisted keratomileusis in situ and photorefractive keratotomy. Each of these techniques affects corneal innervation differently and has a different pattern of nerve regeneration. The purpose of this review is to summarize the different underlying mechanisms for corneal nerve injury and compare the different patterns of corneal reinnervation.

Potential for endocannabinoid system modulation in ocular pain and inflammation: filling the gaps in current pharmacological options

Challenges in the management of ocular pain are an underappreciated topic. Currently available therapeutics lack both efficacy and clear guidelines for their use, with many also possessing unacceptable side effects. Promising novel agents would offer analgesic, anti-inflammatory, and possibly neuroprotective actions; have favorable ocular safety profiles; and show potential in managing neuropathic pain. Growing evidence supports a link between the endocannabinoid system (ECS) and a range of physiological and disease processes, notably those involving inflammation and pain. Both preclinical and clinical data suggest analgesic and anti-inflammatory actions of cannabinoids and ECS-modifying drugs in chronic pain conditions, including those of neuropathic origin. This review will examine existing evidence for the anatomical and physiological basis of ocular pain, specifically, ocular surface disease and the development of chronic ocular pain. The mechanism of action, efficacy, and limitations of currently available treatments will be discussed, and current knowledge related to ECS-modulation of ocular pain and inflammatory disease will be summarized. A perspective will be provided on the future directions of ECS research in terms of developing cannabinoid therapeutics for ocular pain.

[Confocal microscope examination of the corneal nerve plexus as biomarker for systemic diseases : View from the corneal nerve plexus on diabetes mellitus disease]

It is estimated that approximately 50% of patients with diabetes mellitus suffer from polyneuropathy, which is frequently diagnosed too late. Consequently, the question arises whether imaging procedures of the eye, namely optical coherence tomography of the retina and confocal microscopy of the cornea are suitable for the diagnostics and follow-up control of neurodegenerative changes in patients with diabetes mellitus. De Clerck and co-workers could demonstrate this by a systematic review of studies. Of these studies 11 were further evaluated with respect to corneal confocal microscopy. Approximately 15 years after juvenile type 1 diabetes a reduction of corneal nerve fiber length and density was observed, although clinical signs of neuropathy were absent. At this stage an examination seems reasonable. Type 2 diabetes mellitus in the elderly is often associated with a metabolic syndrome and its time of manifestation remains unknown; therefore, corneal confocal microscopy should be implemented at the time of diagnosis of type 2 diabetes. Patients with long disease duration and significant changes in the corneal nerve plexus already showed clinical signs of polyneuropathy and often suffered from proliferative retinopathy. The accessibility of the eye for non-invasive optical modalities should be used more often in the treatment of patients with diabetes mellitus for early identification of patients at risk. Further longitudinal studies are highly necessary.

Corneal myofibroblasts inhibit regenerating nerves during wound healing

Abnormal nerve regeneration often follows corneal injury, predisposing patients to pain, dry eye and vision loss. Yet, we lack a mechanistic understanding of this process. A key event in corneal wounds is the differentiation of keratocytes into fibroblasts and scar-forming myofibroblasts. Here, we show for the first time that regenerating nerves avoid corneal regions populated by myofibroblasts in vivo. Recreating this interaction in vitro, we find neurite outgrowth delayed when myofibroblasts but not fibroblasts, are co-cultured with sensory neurons. After neurites elongated sufficiently, contact inhibition was observed with myofibroblasts, but not fibroblasts. Reduced neurite outgrowth in vitro appeared mediated by transforming growth factor beta 1 (TGF-β1) secreted by myofibroblasts, which increased phosphorylation of collapsin response mediating protein 2 (CRMP2) in neurons. The significance of this mechanism was further tested by applying Mitomycin C after photorefractive keratectomy to decrease myofibroblast differentiation. This generated earlier repopulation of the ablation zone by intra-epithelial and sub-basal nerves. Our findings suggest that attaining proper, rapid corneal nerve regeneration after injury may require blocking myofibroblast differentiation and/or TGF-β during wound healing. They also highlight hitherto undefined myofibroblast-neuron signaling processes capable of restricting neurite outgrowth in the cornea and other tissues where scars and nerves co-exist.

What is the best strategy on detection of cornea neuropathy in people with diabetes? Recent advances in potential measurements

There are well-acknowledged clinical or pre-clinical measurements concerning diabetic peripheral neuropathy (DPN). The current gold standard for diagnosis of diabetic peripheral neuropathy is nerve conduction suitable for detecting large nerve fiber function and intraepidermal nerve fiber density assessment for small fiber damage evaluation [2]. The lack of a sensitive, non-invasive, and repeatable endpoint to measure changes in small nerve fibers is a major factor holding back clinical trials for the treatment of diabetic peripheral neuropathy. As cornea is the most densely innerved tissue, assessing corneal nerves' structure and function will be promising to predict and assess the degree of DPN. In the diabetic micro-environment, damaged corneal nerves lead to decreased corneal sensitivity, both of which resulting in abnormal tear function. According to this theory, the measurements of nerve structure, corneal sensitivity, tear secretion and tear components, to some extent, can reveal and assess the state of corneal neuropathy. This review focuses on summarizing the knowledge of the latest detective methods of diabetic corneal neuropathy, popular in use or possible to further in study and be applied into clinical practice.

Cellular in vivo 3D imaging of the cornea by confocal laser scanning microscopy

We present an in vivo confocal laser scanning microscopy based method for large 3D reconstruction of the cornea on a cellular level with cropped volume sizes up to 266 x 286 x 396 µm3. The microscope objective used is equipped with a piezo actuator for automated, fast and precise closed-loop focal plane control. Furthermore, we present a novel concave surface contact cap, which significantly reduces eye movements by up to 87%, hence increasing the overlapping image area of the whole stack. This increases the cuboid volume of the generated 3D reconstruction significantly. The possibility to generate oblique sections using isotropic volume stacks opens the window to slit lamp microscopy on a cellular level.

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.

Alterations in Corneal Sensory Nerves During Homeostasis, Aging, and After Injury in Mice Lacking the Heparan Sulfate Proteoglycan Syndecan-1

Purpose

To determine the impact of the loss of syndecan 1 (SDC1) on intraepithelial corneal nerves (ICNs) during homeostasis, aging, and in response to 1.5-mm trephine and debridement injury.

Methods

Whole-mount corneas are used to quantify ICN density and thickness over time after birth and in response to injury in SDC1-null and wild-type (WT) mice. High-resolution three-dimensional imaging is used to visualize intraepithelial nerve terminals (INTs), axon fragments, and lysosomes in corneal epithelial cells using antibodies against growth associated protein 43 (GAP43), βIII tubulin, and LAMP1. Quantitative PCR was performed to quantify expression of SDC1, SDC2, SDC3, and SDC4 in corneal epithelial mRNA. Phagocytosis was assessed by quantifying internalization of fluorescently labeled 1-μm latex beads.

Results

Intraepithelial corneal nerves innervate the corneas of SDC1-null mice more slowly. At 8 weeks, ICN density is less but thickness is greater. Apically projecting intraepithelial nerve terminals and lysosome-associated membrane glycoprotein 1 (LAMP1) are also reduced in unwounded SDC1-null corneas. Quantitative PCR and immunofluorescence studies show that SDC3 expression and localization are increased in SDC1-null ICNs. Wild-type and SDC1-null corneas lose ICN density and thickness as they age. Recovery of axon density and thickness after trephine but not debridement wounds is slower in SDC1-null corneas compared with WT. Experiments assessing phagocytosis show reduced bead internalization by SDC1-null epithelial cells.

Conclusions

Syndecan-1 deficiency alters ICN morphology and homeostasis during aging, reduces epithelial phagocytosis, and impairs reinnervation after trephine but not debridement injury. These data provide insight into the mechanisms used by sensory nerves to reinnervate after injury.

TFOS DEWS II pain and sensation report

Pain associated with mechanical, chemical, and thermal heat stimulation of the ocular surface is mediated by trigeminal ganglion neurons, while cold thermoreceptors detect wetness and reflexly maintain basal tear production and blinking rate. These neurons project into two regions of the trigeminal brain stem nuclear complex: ViVc, activated by changes in the moisture of the ocular surface and VcC1, mediating sensory-discriminative aspects of ocular pain and reflex blinking. ViVc ocular neurons project to brain regions that control lacrimation and spontaneous blinking and to the sensory thalamus. Secretion of the main lacrimal gland is regulated dominantly by autonomic parasympathetic nerves, reflexly activated by eye surface sensory nerves. These also evoke goblet cell secretion through unidentified efferent fibers. Neural pathways involved in the regulation of meibomian gland secretion or mucin release have not been identified. In dry eye disease, reduced tear secretion leads to inflammation and peripheral nerve damage. Inflammation causes sensitization of polymodal and mechano-nociceptor nerve endings and an abnormal increase in cold thermoreceptor activity, altogether evoking dryness sensations and pain. Long-term inflammation and nerve injury alter gene expression of ion channels and receptors at terminals and cell bodies of trigeminal ganglion and brainstem neurons, changing their excitability, connectivity and impulse firing. Perpetuation of molecular, structural and functional disturbances in ocular sensory pathways ultimately leads to dysestesias and neuropathic pain referred to the eye surface. Pain can be assessed with a variety of questionaires while the status of corneal nerves is evaluated with esthesiometry and with in vivo confocal microscopy.

Laser stripe extraction method in industrial environments utilizing self-adaptive convolution technique

A line-structured laser scanner is widely applied for 3D reconstruction in industrial environments with ubiquitous various luminance, complicated background, diverse objects, and instable lasers. These elements will show up as noise in the obtained laser stripe images. Therefore, the basic and key point for a line-structured laser scanner is to accurately extract the laser stripe from noise. This paper proposes an effective laser stripe extraction procedure with two steps. First, a novel laser stripe center extraction method based on the geometry information and correlation in the laser stripe, is designed to significantly eliminate noise and accurately extract the laser stripe centers. In addition, the robustness, speed, and accuracy of this method are respectively analyzed in detail. Second, piecewise fitting is adopted to acquire a smooth and continuous laser stripe centerline. In order to select the optimal fitting method, the characteristics of two spline methods, Akima spline and cubic Hermite spline, are deeply analyzed and compared. Finally, an experiment is carried out by using a rough metal step and a line-structured laser scanning system. The experiment results demonstrate that the proposed self-adaptive convolution-mass method can significantly eliminate noise in industrial environments. In addition, the cubic Hermite spline is a better choice for 3D reconstruction, rather than the Akima spline.

[Morphometric characterization of the subbasal nerve plexus : Detection and analysis of networks of nerve fibers]

Confocal laser scanning microscopy is a versatile tool in medical research and enables noninvasive in vivo imaging of the corneal subbasal nerve plexus. The aim of this work is to provide a structured overview about the detection and quantification of nerve fibers of the subbasal nerve plexus from images acquired by confocal laser scanning microscopy. Relevant steps are explained and potential factors influencing the quality of the results are pointed out. Information obtained from the quantification of subbasal nerve fiber structure can be potentially used as clinical parameters in the context of diagnostics and therapy control of diabetic neuropathy.

In Vivo Confocal Microscopy of Corneal Nerves in Health and Disease

In vivo confocal microscopy (IVCM) is becoming an indispensable tool for studying corneal physiology and disease. Enabling the dissection of corneal architecture at a cellular level, this technique offers fast and noninvasive in vivo imaging of the cornea with images comparable to those of ex vivo histochemical techniques. Corneal nerves bear substantial relevance to clinicians and scientists alike, given their pivotal roles in regulation of corneal sensation, maintenance of epithelial integrity, as well as proliferation and promotion of wound healing. Thus, IVCM offers a unique method to study corneal nerve alterations in a myriad of conditions, such as ocular and systemic diseases and following corneal surgery, without altering the tissue microenvironment. Of particular interest has been the correlation of corneal subbasal nerves to their function, which has been studied in normal eyes, contact lens wearers, and patients with keratoconus, infectious keratitis, corneal dystrophies, and neurotrophic keratopathy. Longitudinal studies have applied IVCM to investigate the effects of corneal surgery on nerves, demonstrating their regenerative capacity. IVCM is increasingly important in the diagnosis and management of systemic conditions such as peripheral diabetic neuropathy and, more recently, in ocular diseases. In this review, we outline the principles and applications of IVCM in the study of corneal nerves in various ocular and systemic diseases.

Invisible Shield: Review of the Corneal Epithelium as a Barrier to UV Radiation, Pathogens, and Other Environmental Stimuli

The ocular surface is comprised of the cornea and conjunctiva, which are structures that not only protect the eye but also enable vision. The corneal epithelium is the most superficial layer of the cornea, and therefore first line of defense against external assaults. Damage to this highly specialized structure could lead to vision loss, making it an important structure to investigate and understand. Here, we conducted a search of the current literature on the mechanisms the corneal epithelium has adapted against three frequent insults: UV-radiation, pathogens, and environmental assaults. This review systematically examines the corneal epithelium's response to each assault in order to maintain its role as an invisible shield. The goal of this review is to provide insight into some of the critical functions the corneal epithelium performs that may be valuable to current regenerative studies.

Corneal Reinnervation and Sensation Recovery in Patients With Herpes Zoster Ophthalmicus: An In Vivo and Ex Vivo Study of Corneal Nerves

PURPOSE

To study corneal reinnervation and sensation recovery in Herpes zoster ophthalmicus (HZO).

METHODS

Two patients with HZO were studied over time with serial corneal esthesiometry and laser in vivo confocal microscopy (IVCM). A Boston keratoprosthesis type 1 was implanted, and the explanted corneal tissues were examined by immunofluorescence histochemistry for βIII-tubulin to stain for corneal nerves.

RESULTS

The initial central corneal IVCM performed in each patient showed a complete lack of the subbasal nerve plexus, which was in accordance with severe loss of sensation (0 of 6 cm) measured by esthesiometry. When IVCM was repeated 2 years later before undergoing surgery, case 1 showed a persistent lack of central subbasal nerves and sensation (0 of 6). In contrast, case 2 showed regeneration of the central subbasal nerves (4786 μm/mm) with partial recovery of corneal sensation (2.5 of 6 cm). Immunostaining of the explanted corneal button in case 1 showed no corneal nerves, whereas case 2 showed central and peripheral corneal nerves. Eight months after surgery, IVCM was again repeated in the donor tissue around the Boston keratoprosthesis in both patients to study innervation of the corneal transplant. Case 1 showed no nerves, whereas case 2 showed new nerves growing from the periphery into the corneal graft.

CONCLUSIONS

We demonstrate that regaining corneal innervation and corneal function are possible in patients with HZO as shown by corneal sensation, IVCM, and ex vivo immunostaining, indicating zoster neural damage is not always permanent and it may recover over an extended period of time.

Corneal epithelial cells function as surrogate Schwann cells for their sensory nerves

The eye is innervated by neurons derived from both the central nervous system and peripheral nervous system (PNS). While much is known about retinal neurobiology and phototransduction, less attention has been paid to the innervation of the eye by the PNS and the roles it plays in maintaining a functioning visual system. The ophthalmic branch of the trigeminal ganglion contains somas of neurons that innervate the cornea. These nerves provide sensory functions for the cornea and are referred to as intraepithelial corneal nerves (ICNs) consisting of subbasal nerves and their associated intraepithelial nerve terminals. ICNs project for several millimeters within the corneal epithelium without Schwann cell support. Here, we present evidence for the hypothesis that corneal epithelial cells function as glial cells to support the ICNs. Much of the data supporting this hypothesis is derived from studies of corneal development and the reinnervation of the ICNs in the rodent and rabbit cornea after superficial wounds. Corneal epithelial cells activate in response to injury via mechanisms similar to those induced in Schwann cells during Wallerian Degeneration. Corneal epithelial cells phagocytize distal axon fragments within hours of ICN crush wounds. During aging, the proteins, lipids, and mitochondria within the ICNs become damaged in a process exacerbated by UV light. We propose that ICNs shed their aged and damaged termini and continuously elongate to maintain their density. Available evidence points to new unexpected roles for corneal epithelial cells functioning as surrogate Schwann cells for the ICNs during homeostasis and in response to injury. GLIA 2017;65:851-863.

Corneal Nerve and Epithelial Cell Alterations in Corneal Allodynia: An In Vivo Confocal Microscopy Case Series

PURPOSE

To investigate morphological changes of the corneal epithelium and subbasal nerves in patients with corneal allodynia using in vivo confocal microscopy (IVCM).

DESIGN

Case-control study of patients with corneal allodynia and healthy controls.

METHODS

Ten eyes of six patients were diagnosed with corneal allodynia at a single center and compared to fifteen healthy eyes. IVCM of the central cornea was performed on all subjects and controls. Images were retrospectively analyzed numbers of total corneal subbasal nerves, main trunks and branches, total nerve length and density, nerve branching, and tortuosity, superficial and basal epithelial cell densities, and superficial epithelial cell size.

RESULTS

Corneal allodynia was seen in patients with dry eye disease, recurrent corneal erosion syndrome, exposure to ultraviolet radiation, and Accutane use. Compared to controls, patients with corneal allodynia had a significant decrease in the total numbers of subbasal nerves (P=.014), nerve branches (P=.006), total nerve length (P=.0029), total nerve density (P=.0029) and superficial and basal epithelial cell densities (P=.0004, P=.0036) with an increase in superficial epithelial cell size (P=.016). There were no statistically significant differences in the number of subbasal nerve main trunks (P=.09), nerve branching (P=.21), and nerve tortuosity (P=.05).

CONCLUSIONS

Corneal IVCM enables near-histological visualization and quantification of the cellular and neural changes in corneal allodynia. Regardless of etiology, corneal allodynia is associated with decreased corneal epithelial cell densities, increased epithelial cell size, and decreased numbers and lengths of subbasal nerves despite an unremarkable slit-lamp examination. Therefore, IVCM may be useful in the management of patients with corneal allodynia.

Rare human nerve growth factor-β mutation reveals relationship between C-afferent density and acute pain evaluation

The rare nerve growth factor-β (NGFB) mutation R221W causes a selective loss of thinly myelinated fibers and especially unmyelinated C-fibers. Carriers of this mutation show altered pain sensation. A subset presents with arthropathic symptoms, with the homozygous most severely affected. The aim of the present study was to investigate the relationship between peripheral afferent loss and pain evaluation by performing a quantification of small-fiber density in the cornea of the carriers, relating density to pain evaluation measures. In vivo corneal confocal microscopy (CCM) was used to quantify C-fiber loss in the cornea of 19 R221W mutation carriers (3 homozygous) and 19 age-matched healthy control subjects. Pain evaluation data via the Situational Pain Questionnaire (SPQ) and the severity of neuropathy based on the Neuropathy Disability Score (NDS) were assessed. Homozygotes, heterozygotes, and control groups differed significantly in corneal C-nerve fiber density, with the homozygotes showing a significant afferent reduction. Importantly, peripheral C-fiber loss correlated negatively with pain evaluation, as revealed by SPQ scores. This study is the first to investigate the contribution of small-fiber density to the perceptual evaluation of pain. It demonstrates that the lower the peripheral small-fiber density, the lower the degree of reported pain intensity, indicating a functional relationship between small-fiber density and higher level pain experience.

Small-Fiber Neuropathy Is Associated With Corneal Nerve and Dendritic Cell Alterations: An In Vivo Confocal Microscopy Study

PURPOSE

In patients with small-fiber neuropathy (SFN), noninvasive diagnostic tests that allow accurate monitoring of disease progression are urgently needed. The aim of this study was to assess corneal trigeminal small sensory nerves and immune cells by in vivo corneal confocal microscopy (CCM) in SFN.

METHODS

In this prospective single-center study, 14 patients with histologically confirmed SFN were analyzed. CCM parameters [corneal nerve fiber density (NFD); the total number of nerves, main trunks, and branches; nerve tortuosity; and dendritic cell density] were compared with 14 age-matched healthy controls and correlated with clinical symptoms, disease course, and histopathological findings.

RESULTS

Corneal NFD (15,489.3 ± 5927.6 μm/mm² vs. 22,687.1 ± 4328.7 μm/mm²; P = 0.001) and the total number of nerves (10.4 ± 4.6/frame vs. 18.5 ± 4.8/frame; P < 0.0001) were significantly reduced in patients with SFN. In contrast, nerve tortuosity was significantly increased (2.2 ± 0.3 vs. 1.7 ± 0.5; P = 0.02). Corneal NFD did not correlate with intraepidermal NFD (ρ = -0.158; P = 0.5) or clinical symptoms (cold P = 0.1; prickling P = 0.2; burning P = 0.8; formication P = 0.7; stabbing P = 0.4; rubbing 0.1; pressure P = 0.1). The average dendritic cell density was increased in SFN (33.5 ± 57.5 cells/mm² vs. 16.1 ± 13.7 cells/mm²) but did not reach significance (P = 0.7).

CONCLUSIONS

CCM provides parameters that reliably indicate injury to sensory afferents of the trigeminal nerve in patients with SFN. Our data suggest that CCM may serve both as a noninvasive diagnostic test and as a surrogate marker in SFN.