Pathophysiology of Dry Eye Disease Using Human Models





Introduction


Dry eye disease (DED) is an umbrella term that represents a wide spectrum of disorders with different clinical presentations and variable risk factors. In 2017, in light of studies performed up to that date, the Tear Film and Ocular Surface Dry Eye Workshop II (TFOS DEWS II) defined DED as, “a multifactorial disease of the ocular surface characterized by a loss of homeostasis of the tear film, and accompanied by ocular symptoms, in which tear film instability and hyperosmolarity, ocular surface inflammation and damage, and neurosensory abnormalities play etiological roles.” This definition clearly aims to embrace the diverse pathophysiological changes underlying the disease, which may be key to early detection and new treatment development.


Studies on DED in humans have led to the classification of the disease as aqueous deficient or evaporative, however, though usually both conditions coexist as part of a continuum called “hybrid” DED. A plethora of risk factors from systemic diseases to environmental pollution can lead to tear film instability or hyperosmolarity, which in turn initiate a vicious circle involving inflammation and apoptosis of ocular surface epithelium, and neural dysfunction. The elucidation of distinct pathways underlying various clinical phenotypes by means of meticulous ophthalmic examination and appropriate testing is critical in planning treatment of individual patients with DED. However, there is currently no “pathognomonic” or “gold standard” test to diagnose DED. Further complicating the diagnosis is the discordance between signs and symptoms of the disease. Therefore, for the clinician, DED mostly remains as a diagnosis made for a variety of ocular surface problems, typically associated with symptoms of eye discomfort or eye pain, after eliminating other potential pathologies.


In this review, we cover some of the major findings related to pathophysiology of DED; tear film/ocular surface alterations, corneal morphological changes observed in neuropathic pain, aqueous versus evaporative DED and some systemic diseases, effect of age, sex, environmental factors on DED, preservative toxicity and DED, and effect of microbiome on DED. As expected, the human data is less coherent than data from animal and/or tissue culture research, but also demonstrates the heterogeneity of DED. Furthermore, outcome measures and measurement techniques may be less than reproducible from site to site, making comparison between studies difficult to analyze. Despite these shortcomings, we aim to highlight some of the major areas of research in humans with and without DED, mainly looking for tests that can be performed in humans to better diagnose DED, determine severity, and/or evaluate response to treatment.


Clinical Findings in Human Studies of Dry Eye Disease


Inflammatory Markers in Tears-Overview


Tear hyperosmolarity and tear film instability are recognized as important drivers of DED, with tear hyperosmolarity a likely trigger of the acute immune response. Stress induced by desiccation provokes epithelial activation, involving pathways by which the production of inflammatory mediators at the ocular surface is stimulated. The subsequent activation of an adaptive immune response drives the inflammatory cascade, causing further damage to the ocular surface, and this vicious cycle continues through the dysregulation of the immune system. To more fully understand the inflammatory cascade in DED, tear analysis has been used to explore key mediators associated with DED. A number of molecular mediators, such as proinflammatory cytokines, chemokines, endopeptidases, and cell adhesion molecules have been studied extensively in DED patients as potential biomarkers of the disease. Sjo some mediators considered important in DED are interleukin-1b (IL-1b), IL-6, tumor necrosis factor-α (TNF-α), interferon-γ (IFN-γ), caspase 3, transglutaminase-2 (TG-2), and matrix metalloproteinases (MMPs), such as MMP-3 and MMP-9. It is hoped that inflammatory tear will better determine candidates for antiinflammatory therapy, more accurately determine disease activity, and provide objective therapeutic endpoints for clinical trials testing efficacy of new treatments for DED.


Matrix metalloproteinase-9


Among other cytokines, MMP-9 has a crucial role in initiation and progression of ocular surface disease. MMP-9 is a zinc and calcium ion–dependent enzyme important for tissue remodeling in normal physiological processes such as wound healing.


MMP-9 activity is regulated by epigenetic processes, cell–cell interactions, and cytokine-mediated pathways. The hyperosmolarity of the tear fluid seen in DED has been shown to trigger the stress-activated protein kinase ( SAPK ) signaling cascade, leading to the release of MMP-9 from corneal epithelial cells themselves, thus initiating a cycle of progressive inflammation. , MMP-9 cleaves tight junction proteins occludin and zonula occludens-1 ( ZO-1 ), thereby disrupting the ocular surface epithelium. MMP-9 also activates other inflammatory factors such as pro-IL-1 beta, pro-tumor necrosis factor ( TNF )-alpha, and substance P. MMP-9 may also play role in processing molecules that initiate a positive feedback loop to increase EGFR signals to increase cell migration and upregulate MMP-9 expression. Overall, T-cell recruitment, the proteolytic activity of the MMP-9 molecule itself, and activation of secretion of additional cytokines initiate a self-perpetuating cycle of inflammation, secretory dysfunction, corneal surface irregularity, and worsening eye dryness. ,


The normal levels of MMP-9 (ng/mL) in human tears range from 3 to 41 ng/mL with a level less than 30 ng/mL in 90% of people. The finding of elevated MMP-9 in experimental models of DED and human DED patients has led to the development of an easy-to-use point-of-care MMP-9 assay in unstimulated tear samples (InflammaDry, Rapid Pathogen Screening, Inc, Sarasota, FL, USA), to aid in the diagnosis of DED. The assay is considered positive when MMP-9 levels over 40 ng/mL are detected.


In various studies, MMP-9 positivity as measured by InflammaDry ranged from 11% to 85% among patients with DED, and 5.6–7.5% in healthy individuals in various studies. , Different diagnostic criteria for DED used in each study were suggested as a possible reason underlying the considerable variation in reported positivity. Furthermore, recently InflammaDry test results were shown to be influenced by tear volume. Low tear volume in aqueous tear-deficient DE may induce false-negative results, and reflex tearing during the test may induce false-positive results. Although not all patients with DED expressed this indicator of cell damage, in some studies, MMP-9 activity was reported to be strongly correlated with symptom scores, tear film break-up time (TBUT), conjunctival staining, corneal staining, Schirmer test, decrease in visual acuity, Schirmer test results, conjunctival staining, corneal staining, as well as the number of obstructed meibomian ducts, pathologic meibomian gland (MG) secretion, and surface area of abnormal superficial corneal epithelia, , , , , whereas, in others, correlation between this test and tear osmolarity was poor in patients with mild DED, and recently, Kook et al. could not demonstrate any correlation between MMP-9 test results and clinical DED indices in Sjögren’s syndrome (SS) patients.


In a retrospective cohort of DED patients, Soifer et al. reported that eyes with detectable MMP-9 had significantly decreased tear production over time compared to those without detectable MMP-9, suggesting a role for MMP-9 as a prognostic biomarker to predict long-term deterioration in DED. Recent evidence also suggests that InflammaDry test may have predictive value for the success of antiinflammatory treatment and used to monitor treatment response. Ryu et al. reported that, following topical steroid treatment in patients with refractory DED, improvements in symptom and ocular surface staining scores were better in the MMP-9–positive subgroup of patients than in the MMP-9–negative group. In another study, MMP-9–positive DED patients showed more favorable responses to topical cyclosporine A, compared to MMP-9–negative patients, with decreased MMP-9 levels, improved OSDI scores, and increased TBUT, and Schirmer test results after a month of treatment. Aragona et al. reported significantly higher expression of MMP-9 in SS compared to MG dysfunction patients, and SS patients with higher MMP-9 levels in the tear film showed a more favorable response to topical corticosteroid treatment. Lifitegrast 5% treatment was also reported to normalize MMP-9 levels in 38.9% of a retrospective cohort of 54 DED patient eyes with initial positive InflammaDry test. Choi et al. reported statistically significantly more MMP-9 conversion rate in tears of patients who underwent botulinum toxin type A (BTX-A) injection to the medial part of the upper and lower eyelids (76.92%) compared to those who received saline injection (38.46%).


Further studies regarding the reproducibility of the InflammaDry test and development of other such point-of-care diagnostics for markers of DED disease, “lab-on-a-stick” may help both diagnosis and evaluation of response to treatment would be valuable for the management of DED.


Inflammatory cytokines in tears of subpopulations of DED


Tear levels of IL-1, IL-6, and IL-8 were noted to be increased, and epidermal growth factor (EGF) decreased in SS patients; these increased concentrations were associated with the severity of DED clinical parameters, such as greater corneal staining and lower tear secretion.


Tear inflammatory cytokine levels in dry eye with rheumatoid arthritis (RA) also correlated with corneal dendritic cell (DC) density at in vivo confocal microscopy (IVCM), and IL-1 and IL-6 concentrations decreased after the systemic treatment of RA. Jackson et al. found significant correlations between tear IFN-γ concentrations, tear osmolarity, total ocular surface staining, and Schirmer’s test score, all key clinical diagnostic parameters for DED, suggesting IFN-γ as a potential biomarker of tear hyperosmolarity associated with evaporative DED.


Regarding changes to tears, previous studies have shown elevated IFN-γ levels in tears in patients with meibomian gland disease (MGD), elevated IL-8 levels in SJS, elevated levels of TNF-γ and TGF-β1 with decreased levels of EGF in trachoma, elevated IL-8 and MMP-9 levels in OCP, and IL-17, IL-1β, IL-6, and IL-8 levels in Graves’ ophthalmopathy with exposure keratitis, compared to normal controls.


Studies also reported reduced tear cytokine levels as to assess the effectiveness of topical steroid treatment in DED , or intense pulsed light therapy in treating dry eye due to MGD.


Among several cytokines, interleukin-8 (IL-8) is consistently found in the tear film and conjunctiva of patients with dry eye. , It can be produced by any cell with toll-like receptors, such as epithelial cells and macrophages, and acts as a chemoattractant for neutrophils, as part of the innate immune response. A recent metaanalysis of 13 articles investigating 342 DED patients and 205 healthy controls reported significantly higher concentrations of the tear inflammatory mediators IL-1, IL-6, IL-8, IL-10, IFN-γ, TNF-α, in the tears of DED patients as compared to age-matched non-DED control subjects. Conversely, the evidence of difference was less strong for IL-17A and IL-2. However, the authors cautioned against determining clinical significance, since they noted large standard deviations reported in these studies, suggesting substantial interindividual variation in cytokine concentrations that was not explained.


Tear analysis and DED


In summary, studies have pointed the association associateion of tear cytokines  with DED however, no consensus has been reached  in regards to tear collection, methods of analysis, cut-off values, and which cytokines are best associated with  ocular surface disease and/or dry eye stymptoms. Overall, although DED seems to be accompanied by release of cytokines in tears, the analysis of tear cytokines as a biomarker for DED is still an unmet need, since the variability in tear collection, sampling, storage, and analytical methods seem to have generated inconsistent data, not allowing the estimation of either a reference interval for control subjects and a cut-off value between the DED and controls.


Cell Surface Markers


Major histocompatibility complex


Human leukocyte antigen D-related (HLA-DR) is a type of major histocompatibility complex class II cell surface receptor involved with antigen presentation. HLA-DR is often used as a marker of loss of immunosilencing on the ocular surface, and is one of the most commonly studied inflammatory markers in DED. Although HLA-DR is a surface receptor that is constitutively expressed on antigen-presenting cells and immune cells, under pathological conditions, it can be conditionally induced in CD45-negative conjunctival epithelial cells to regulate ocular surface immune responses. , In DED, hyperosmolarity was shown to induce HLA-DR overexpression in human conjunctival epithelial cells, , , and this upregulation was suggested to be driven by IFN-γ as shown in Sjögren’s patients.


Studies investigating HLA-DR expression (HLA-DR%) in DED patients using impression cytology (IC) have generally reported elevated HLA-DR%, suggesting that antigen presentation occurs efficiently in DED. However, in a recent study evaluating expression pattern of HLA-DR in conjunctival cell populations from 1049 samples collected from 527 patients with moderate to severe DED, almost 42% of the study patients had less than 5% of conjunctival cells expressing HLA-DR at baseline. Therefore, the authors proposed that conjunctival HLA-DR% did not appear to be a sensitive marker of DED. However, since the mean conjunctival and corneal staining scores increased with increasing levels of HLA-DR%, it was suggested that HLA-DR% levels might prove useful in defining subtypes of DED patients prone to epithelial disease, such as those with higher levels of ocular surface staining. Although the differences in average HLA-DR% between various studies of DED can be attributed to variabilities in inclusion criteria, one major contributor is the lack of standardized methods for flow cytometry instrumentation, data acquisition, and analysis.


There is limited and inconsistent correlation of HLA-DR% with commonly used clinical assessments of signs and symptoms. , , Few studies have reported on statistically significant correlation of HLA-DR% with symptoms, osmolarity, TBUT, corneal fluorescein staining, , and Schirmer’s test.


Statistically significantly decreased levels of HLA-DR% were also reported following different treatment regimens such as following treatment with low-concentration clobetasone butyrate, topical CsA, , , and oral supplementation of omega-3 and omega-6 fatty acids.


In summary, the validity of HLA-DR as an objectively measurable biomarker in DED seems to be limited at this time due to a lack of universal standardization of the methodology for HLA-DR detection and measurement; and due to the limited and inconsistent correlation of HLA-DR% with commonly used clinical assessments of signs and symptoms.


Fas and Fas ligand are two other human leukocyte antigen subtypes that, upon interaction, induce apoptosis. These immunomodulatory molecules have also been demonstrated in the conjunctiva and lacrimal glands of patients with DED.


Cell adhesion molecules


Cell adhesion molecules are surface molecules that enhance cellular migration by binding components within the extracellular matrix and promoting immune cell infiltration onto the ocular surface. Elevated levels of intercellular adhesion molecule-1 (ICAM-1) and vascular cellular adhesion molecule-1 have been identified in the conjunctiva and lacrimal glands in DED. ICAM-1 is also upregulated on the conjunctival epithelium in ocular surface inflammation and could represent a potential biomarker. In one study, conjunctival inflammation was confirmed in DED patients by observing lymphocytic infiltration and immunoreactivity for HLA-DR and ICAM-1 in conjunctival biopsy specimen.


Hyperosmolarity—Measurement of Tear Osmolarity


Animal studies have shown tear film hyperosmolarity to be a fundamental feature in DED irrespective of the dry eye subtype. , Tissue culture studies with human corneal epithelial cells (HCECs) exposed to hyperosmolar stress, with osmolarities ranging between 330 and 512 mOsm/kg, have reported activation of MAPK signaling pathway, leading to expression of cytokines, and initiating the inflammatory circle of DED. , , There is also evidence of a direct cytotoxic effect of hyperosmolarity on HCECs in culture. Given this research in animals and tissue culture, the measurement of tear osmolarity in DED patients is of great interest, particularly considering the limited number of minimally objective metrics available for the diagnosis of DED.


In the past, various measurement techniques have been used to measure tear osmolarity, including the Clifton and vapor pressure osmometers. , Although these methods reportedly had high accuracy, sensitivity, and specificity, they were not practical, requiring significant time and specialized equipment. In 2008, the TearLab osmometer (TearLab, San Diego, CA, USA) became available as a point-of-care test device using a microchip, microelectrode technology to measure the number of charged particles in only a 0.2-μL tear sample to provide an estimate of the tear osmolarity. Today, this test is the most widely used test to measure tear osmolarity at the clinic. Normal value is considered 302 mOsm/L, with minimal intereye difference. A value of 308 mOsm/L in either eye has been used as the threshold in differentiating normal and early stages of DED, with 316 mOsm/L used a cut-off for more advanced DED. Both intereye and repeat tear osmolarity measurements in the same eye have been reported to show variability in DED patients. The worse the severity of dry eyes, the more variable tear osmolarity has been found to be (6.9 ± 5.9 mOsm/L in mild, 11.7 ± 10.9 mOsm/L in moderate, and 26.5 ± 22.7 mOsm/L in severe DES, respectively). Hence, a difference of 8 mOsm/L between two eyes is also considered to be significant and compatible with an unstable tear film.


Several studies have reported that DED patients had significantly elevated tear osmolarity compared to healthy controls and the hyperosmolarity increased with dry eye severity, and some have even concluded that the TearLab osmometer is the best marker for diagnosing and classifying DED levels of severity. , Other studies have proposed that higher cut-off values (316–317 mOsm/L) demonstrated superior accuracy to other single tests for diagnosing DED. , Conversely, many recent clinical studies have raised questions on the diagnostic ability of the test demonstrating high variability between controls and DED patients. Despite a general shift in osmolarity with DED, there was a large overlap in osmolarity values between 293 and 320 mOsm/L between normal individuals and patients with dry eye. Sensitivity and specificity measurements of osmolarity values for DED diagnosis using a threshold of 294 mOsm/L were 67% and 46%, respectively, 40% and 100% using a threshold of >310 mOsm/L in patients with SS.


Significant variability in tear osmolarity has been noted and multiple causes sited including different patient populations, variability in technique, humidity, etc. , Concerns exist particularly on what part of the tear film should be measured—tear meniscus, as measured by TearLab, or the precorneal tear film. , In fact, the precorneal tear film in DED was reported to have higher osmolarity levels, even spiking up to 800–900 mOsm/L in areas of tear film break-up, Moreover, the TearLab system might have led to reflex tear production resulting in varying values, while earlier studies have suggested that pathological changes would be best obtained in basal tears rather than in reflex tears.


In summary, although earlier studies have concluded that tear osmolarity is the best single metric for diagnosing and classifying DED, and suggested 308 mOsm/L as measured by the TearLab osmometer as the most sensitive threshold between normal and mild DED, considering the high variability and overlap of values between healthy eyes and dry eyes, the tear osmolarity measurements using the TearLab osmometer still need to be interpreted cautiously as a stand-alone diagnostic tool. Further research in this area may improve our understanding as to how measurements taken by the TearLab system can be standardized or interpreted, or development of new point-of-care tests measuring tear osmolarity at the clinic may help resolve the variability issues encountered with the current system.


Anterior Segment Imaging: In Vivo Confocal Microscopy (IVCM)


IVCM is a currently evolving imaging and diagnostic tool, that enables real-time visualization of ocular structures in cellular detail. Optimum resolution images 500- to 800-fold magnification are acquired by focusing light onto a certain depth within a sample. There are two basic types of IVCM used in ophthalmology clinics; the slit-scanning microscope (SSCM) that uses white light source, and the laser-scanning microscope (LSCM) that uses 670 nm red wavelength diode laser as its light source. Whereas transverse and axial resolution and contrast are lower with the SSCM, the more widely used LSCM has high resolution and better contrast, but is less accurate in determining depth within the corneal tissue, rendering measurements hard to standardize.


Neuropathic Pain


The cornea is the most densely innervated tissue in the body with a high density of free nerve endings, 200–300 times that of the skin . The majority of nerves in the cornea have a sensory function. Sensory nerve endings termed “nociceptors” function in maintaining homeostasis, in wound healing and in sensing the environment to regulate tear secretion and distribution. IVCM allows the description of the morphology, density, and disease-induced or surgically induced alterations of corneal nerves, particularly of the sub- basal nerves, SBN.


A challenge in DED is that symptoms of the disease are often discordant with ocular surface findings. In certain cases, DED symptoms can present without measurable abnormalities in tear film or ocular surface, thus bringing up the question of nerve damage as an underlying cause of symptoms. , Classical severe DED often occurs with appropriate nociceptive pain, that is, these nerves can appropriately transmit information from the diseased ocular surface environment (i.e., nociceptive pain). In 2017, the Tear Film and Ocular Surface Dry Eye Workshop II (TFOS DEWS II) Committee, for the first time, suggested “neurosensory abnormalities” to play role in the pathophysiology of DED. Yet, a more treatment-resistant form of DED arises when they can become dysfunctional and submit signals inappropriately. In these cases, there is marked disparity between the severity of symptoms and magnitude of signs and symptoms exceed the clinical signs. This latter scenario is named “neuropathic pain,” which is defined as “pain caused by a lesion or disease of the somatosensory nervous system, occurring in either the peripheral or central nervous system (PNS or CNS)”. Such patients are troubled with profound ocular surface discomfort, burning sensation, pain, hyperalgesia (increased pain sensitivity), or photoallodynia (painful sensitivity to light) despite having a clinically unremarkable ocular surface examination, also described by the phrase “pain without stain”. ,


Another challenge in DED management is that individuals with “neuropathic pain” may present with comorbid ocular surface abnormalities. As such, symptoms of neuropathic pain may overlap with symptoms of DED arising from ocular surface abnormalities. Individuals with both entities usually report sensations of “dryness,” “discomfort,” and “irritation.” However, some symptoms were reported to be more suggestive of a neuropathic origin, including “burning” and evoked pain to wind, light, or extreme temperatures. Therefore, identification of the instances when nerve dysfunction, including NCP, underlies patient symptoms is crucial.


Unfortunately, there are no reliable metrics to confirm the presence of NCP and diagnosis is based on assessing symptoms and clinical features of ocular surface. Absence of clinical signs despite the presence of symptoms or failure of conventional treatment for DED to relieve symptoms suggest the diagnosis of NCP. Positive medical history of a lesion or disease of the corneal nociceptive pathway such as history of corneal refractive surgery, presence of positive findings with corneal confocal microscopy (e.g., anomalies in corneal nerve morphology), presence of local or systemic disorders that may affect nociceptive processing such as fibromyalgia or migraine, and abnormal responses to topical instillation of hyperosmolar drops have been reported as additional diagnostic criteria. , ,


IVCM is the most widely studied clinical test in NCP with or without dry eye. In few studies, subbasal nerve number and density have been reported to be significantly decreased in NOP patients compared to controls, with increased tortuosity and reflectivity. , , , , Yet, the most interesting IVCM findings in patients with NOP were “microneuromas,” which are sites of axonal injury with attempted nerve regeneration, viewed as “abrupt endings,” “stumps,” “terminal enlargements,” or “relatively large, diffuse, poorly demarcated but round appearing bright areas on the nerve itself”. , , , Such microneuroma formations along with activated keratocytes were also reported in the stromal nerves as spindle, lateral, or stump microneuromas.


In two studies from the same group, corneal MNs were reported to occur in 62.5%–100% of individuals with clinically diagnosed NCP , and were proposed as a biomarker of NCP with 100% sensitivity and specificity. Whereas in the latter study no microneuromas were observed in individuals with DED and no NCP, a more recent study reported MN in 11.1% of individuals with NCP versus 21.8% in patients with DED, and 6.3% of postrefractive surgery patients with DED symptoms. In another study, NCP patients due to refractive surgery were reported to show similar clinical characteristics, pain levels, quality of life impact, and IVCM findings as patients with NCP due to herpetic eye disease. Therefore, the significance and extent of MN presence in NCP remains poorly understood. Recently, Aggarwal et al. reported significant improvements in subbasal nerve number, length, tortuosity, and reflectivity of subbasal nerves in NOP patients, in correlation with improvements in symptoms, following treatment with autologous serum drops. In a recent metaanalysis of subbasal nerve metrics in IVCM studies, Hwang et al. concluded that, at this time, no subbasal nerve metrics were consistent in differentiating between specific DED etiologies, reflecting both the heterogeneity within each etiology and the overlapping clinical features between them. Prospective studies with more number of NOP patients are required to better understand IVCM findings, along with their diagnostic potential or their potential as objective metrics of response to treatment.


IVCM in SS vs non- SS DED and Inflammatory cells


IVCM has been used in a number of studies to analyze corneal, conjunctival, or MG morphology in patients with DED. Although IVCM studies have reported decreased superficial epithelial cell density, , increased or decreased epithelial basal cell density and presence of abnormal hyper-reflective keratocytes, it has mostly been used to evaluate the morphology of subbasal nerve plexus of the cornea, and presence and morphology of immune or inflammatory cells in DED.


Subbasal nerve density is decreased in both SS and non-SS DED compared to normal subjects. Nerves were also reported to have increased tortuosity and beaded appearance in DED patients compared to controls, and Tepelus et al. reported a significant correlation between NF density and OSDI. On the contrary, Tuisku et al. and Zhang et al. described an increase in the number of nerve fibers in SS. A recent metaanalysis of IVCM findings in DED populations aiming to determine the relationships between IVCM parameters and specific DED subtypes reported that comprehensive examination of number, density, and beading from all IVCM studies performed on aqueous deficiency DED showed that nerve density values overlapped considerably between SS, non-SS, and healthy control populations when extrapolated. , , , New standardized image capturing techniques and new software are being developed that were reported to detect subbasal nerve alterations in DED patients with better efficacy and reproducibility.


DC infiltration is another common observation in IVCM studies in DED patients. Density of DC, which are interpreted as antigen-presenting cells, was reported to be increased in central and peripheral corneas of SS and non-SS patients compared to controls, , and in one study DC density correlated with nerve fiber density in DED patients. Another recent study aiming to evaluate correlations between DC infiltration and clinical findings in DED reported differential changes in different levels of DED severity, with increased size of DC body, increased number and length of dendrites, and thus a larger DC field as the severity of DED increases.


A more recent observation in DED patients was significantly high light backscattering in corneal layers in SS patients with DED compared to controls. Such observation was proposed as a parameter that can be used in diagnosis and management of this disease. In few studies, the use of IVCM was also evaluated to monitor treatment response in DED. Villani et al. reported that the subbasal DC density and activated keratocyte density significantly decreased after 4 weeks of treatment with topical 0.5% loteprednol etabonate. Six months of treatment with topical 0.05% cyclosporine eye drops also increased the cell density of the corneal intermediate epithelium, decreased hyperreflective keratocytes and density, tortuosity, and reflectivity of corneal nerve fibers, increased corneal subbasal nerve density, and decreased DC density in two studies. Autologous serum eye drops were also reported to decrease corneal basal epithelial cell density and decrease the number of nerve branches and beadings in patients with DED. In patients with SS-DED, NF density and morphology improved, and DC density decreased following treatment with 3% diquafosol sodium for 3 months.


Meibomian Gland Disease


In few studies that investigated IVCM features of MGD, reduction of acinar unit density, increases of acinar and orifice diameters, and increased secretion reflectivity were demonstrated compared with those of SS patients and healthy controls. , These alterations were associated with an increase in the meibum viscosity. Ibrahim et al. reported that MG acinar unit density showed a strong and significant correlation with tear function, ocular surface vital staining, MG expressibility, and MG dropout grades. All parameters showed high sensitivity and specificity for MGD diagnosis, suggesting the potential of IVCM for diagnosis and determining severity of MGD and associated DED.


Fu et al. analyzed nerve density, width, tortuosity, and reflectivity of subbasal nerves in mild, moderate, and severe MGD, and reported significant differences between grades of MGD in nerve density and reflectivity. Few studies reported increased inflammatory cell density in MG interstitium , and since inflammatory cell density was decreased in response to a combination of topical antibiotic, topical steroid, and oral tetracycline therapy, Matsumoto et al. suggested that inflammatory cell density can be used as a new parameter for monitorization of antiinflammatory treatment response in MGD. Furthermore, Randon et al. suggested an IVCM-based scoring of MGD (on the basis of meibum reflectivity, inflammation, and fibrosis) that correlated strongly with meibography scores, to better understand pathophysiology of MGD and help develop a treatment plan.


Villani et al. compared corneal IVCM findings of MGD, SS, non-SS, and control patients. SS patients had the lowest nerve fiber number and highest amount of beading, followed by non-SS and MGD, than controls. SS and MGD had higher grades of nerve tortuosity compared with non-SS and controls. SS and MGD patients also had significantly increased subbasal DC density compared to normals. Following 4 weeks of treatment with topical loteprednol etabonate 0.5%, Kheirkhah et al. reported no change in subbasal nerve fiber length in patients with MGD.


In a metaanalysis evaluating the value of IVCM in differential diagnosis of various DED subgroups, Hwang et al. suggested that subbasal nerve tortuosity could be used to differentiate MGD-associated DED from healthy controls, whereas neither nerve density nor morphology could be used to distinguish MGD-associated DED from other etiologies of DED.


IVCM was also used effectively to disclose the demodex mites in the terminal bulbs of the eyelashes, , which were not observed after treatment. , Marked inflammatory infiltrates were observed around the MGs and conjunctiva of eyelids with demodicosis infestation, which also cleared with tea tree oil treatment. Furthermore, Demodex-positive seborrheic blepharitis patients were reported to have significantly reduced subbasal nerve density and increased DC density compared go Demodex-negative blepharitis patients. The risk of false -negative results exists in the diagnosis of Demodex infestation with both eyelash depilation and IVCM since total examination of the eyelid is not possible with either method. However, IVCM technique which allows frequent exams, and easy follow-up over time, seems to be more advantageous in the management of Demodex infestation of lids over depilation technique, which is painful and cannot be repeatedly proposed.


In summary, significant decreases in the acinar unit density and increases in the acinar unit diameter and inflammatory cell density were reported in IVCM of MGs in patients with MGD. Although it can be hypothetized that the activation of inflammatory cells may induce the morphologic changes of MG, the mechanism of enlargement of glandular acinar units in MGD patients is still not yet fully understood. IVCM seems to be useful as a supplementary diagnostic tool for the in vivo assessment of MG in patients with MGD.


Systemic Diseases Associated with DED


Graft-versus-host disease


Tepelus et al. reported decreased superficial, wing, and basal epithelial cell densities in patients with ocular graft-versus-host disease (oGVHD) compared to normals. In few studies a decrease in subbasal nerve fiber density was reported in these patient eyes; however, no significant differences were reported in nerve densities between oGVHD, non-oGVHD, or healthy controls , , , suggesting that nerve density alone cannot be used to differentiate GVHD-associated DED from non–GVHD-associated DED or controls.


In oGVHD-associated DED patients, nerve fiber densities were not reported to be different from healthy controls. , , , However, oGVHD-associated DED demonstrated increased tortuosity of subbasal nerves , , , and reduced nerve reflectivity , compared with healthy controls and HSCT patients without DED.


Interestingly, the DC density did not significantly differ from healthy controls in oGVHD. ,


Sjögren’s syndrome


Superficial epithelial cell density was found to be decreased, , , , , whereas basal epithelial cell density was found to be decreased , , or increased , in SS patients compared to controls.


Number and density of subbasal nerve fibers were reported to be decreased in most studies on SS patients, , , with increased tortuosity and beading , , , , compared to controls. Whereas Tepelus et al. and Gabriellini et al. reported significantly lower nerve fiber density in SS patients compared to non-SS DED patients, Tuisku et al. observed no significant difference. DC density was also reported to be significantly high in SS patients compared to controls. , , , , Nerve fiber density and morphology improved, and DC density decreased following treatment with 3% diquafosol sodium for 3 months. In a metaanalysis, Hwang et al. concluded that DC presence was reported to be higher in individuals with SS compared with non-SS and healthy controls, and, in general, DC presence was the highest in immune-mediated diseases, lower in MGD and non–immune-mediated DED, and the lowest in individuals without DED, suggesting that elevated DC presence can be an indication of a systemic inflammatory condition. In patients with SS, MG morphology was also reported to be altered with an increase in the acinar unit density and decrease in acinar unit diameter compared to controls.


IVCM was also used to study the conjunctiva in dry eye patients, focusing on confocal correlations with IC, inflammatory cells, and goblet cell evaluation. While data are conflicting about the evaluation of goblet cells (repeatability, interobserver agreement, and chances to discriminate these cells). , Inflammatory conjunctival cell density and response to treatment could be monitored with IVCM in Sjögren’s patients. , Finally, IVCM was successfully applied to lacrimal gland examination in some Sjögren’s patients, revealing acinar unit density, acinar unit diameter, and inflammatory cell densities of lacrimal glands were significantly worse in SS patients inflammatory cell infiltration and perilobular fibrosis.


In conclusion, IVCM is a technology that has the potential to help us understand the pathophysiology of DED, improve differential diagnosis, and develop a tailored approach to treatment and also evaluate the response to treatment. Yet, there are still many issues that limit the application of IVCM findings to an individual patient with DED; the field of view is very small, for instance, HRT-II/RCM (Heidelberg Engineering GmbH, Heidelberg, Germany) that has been used in many of the published studies is only capable if imaging a 400 × 400 μm area, making it difficult to locate and serially reimage the same region of the cornea on multiple sessions, images require tiling in order to cover the entire corneal or MG area, acquisition times can be long, and the analysis is based only on reflectivity and morphology.


To better evaluate and understand the role of IVCM in DED, longitudinal studies with well-defined DED subtypes, taking sex and age into account, utilizing one type of IVCM model (white light or scanning laser) with uniform software to evaluate various parameters will be required. As such, IVCM-based imaging markers’ utility can validated, confirming IVCM as a useful assessment tool supplementary to other clinical diagnostic modalities. An image-guided approach to diagnosis and treatment would provide objective parameters for evaluating disease severity and potentially monitoring treatment efficacy.


Aging and DED


Age and female sex have been found to be the greatest risk factors for DED. Although prevalence estimates for DED vary with the definition of dry eye used and the characteristics of the population studied, in 2017, TFOS DEWS II reported that symptoms and a clinical diagnosis of dry eye show a modest change below the age of 49, with a gradual increase from age 50 and a more marked increase beyond the age of 80. Large epidemiological studies have shown that after the age of 50, the prevalence of DED in women and men increases every 5 years, and the prevalence rate of women is higher than that of men. Generally, the increase in prevalence for signs of dry eye show a greater increase than for a diagnosis based on symptoms. Also reported was that, in general, females showed a higher prevalence with increased age than males, despite considerable variability among studies.


In age-related DED, a reduction in lacrimal secretion dominates the clinical picture and is the basis of tear hyperosmolarity. This results chiefly from loss of secretory lacrimal tissue, but a fall in corneal sensitivity to all sensory modalities, reported in both SS and non-SS DED may contribute to the reduced secretion based on a lack of sensory drive. With biological aging, the decrease of tear secretion and the increase of evaporation may be the core explanation of DED. , The function and structure of lacrimal gland is impaired with aging. The histopathologic changes such as acinar atrophy, periacinar fibrosis, periductal fibrosis can be observed in the human main lacrimal gland. Diffuse fibrosis and diffuse atrophy in orbital lobes were more frequently observed in women than in men. , As expected, secretion of the lacrimal-derived proteins, lysozyme, lactoferrin, and peroxidase fall with age. From about the age of 40 years, the glands are increasingly infiltrated by CD4 and CD8 T cells, which are considered to be the basis of a gradual destruction of lacrimal acinar and ductal tissue, associated with interacinar and periductal fibrosis, paraductal blood vessel loss, and acinar cell atrophy. Corneal sensitivity to mechanical , and chemical stimuli , falls with age, which could reduce the sensory drive to lacrimal secretion, which would be in keeping with a loss of lacrimal gland function the decrease in corneal sensitivity may explain the lack of correlation of signs and symptoms.


One of the proposed mechanisms of glandular damage over the lifespan was reported to be oxidative stress, resulting from the production of reactive oxygen species such as superoxide and hydrogen peroxide, in the process of aerobic metabolism. Free radical production occurs in the course of mitochondrial electron transfer as part of the process of energy production. In a comparison of human lacrimal tissue from young (17–48 year) versus old (76–87 year) cadavers, evidence of lipid peroxidation and of oxidative DNA damage was also found in the older group.


In MGD, inspissated secretions over lifespan is thought to contribute to stasis, obstruction, and inflammation of the ductal system, leading to gland atrophy and dropout. , These pathophysiological processes result in diminished delivery of MG secretions to the tear film, compromising the integrity of the surface lipid layer and contributing to excessive aqueous tear evaporation and tear film instability. Age-related eyelid alterations like lid laxity and orifice metaplasia may also relate to dry eye. It has been reported that Marx line, the MG orifices located anterior to the mucocutaneous junction in young healthy eyelid, migrates anteriorly with aging.


Few studies have reported that aging is an important risk factor for the development of MGD. However, since MG dropout was also reported in adolescents, hormonal and environmental factors were also hypothesized to influence the structure of MGs. ,


In a recent prospective registry-based cross-sectional study aiming to understand the natural history of DED, signs of MG dysfunction emerged earlier in the natural history of disease progression, with the optimal prognostic cut-off ages for gland dropout, diminished meibum expressibility, and reduced tear film lipid layer quality occurring in the third decade of life, between 24 and 29 years of age. In the aforementioned study, decreased tear meniscus height occurred relatively later in life, with the optimal predictive cut-off age being 46 years suggesting that aqueous tear deficiency predominantly affects the older population, consistent with the trends reported following the metaanalysis conducted by the TFOS DEWS II Epidemiology Subcommittee. Conjunctival and corneal staining were among the final clinical markers to emerge in the natural history of dry eye progression, with the optimal prognostic thresholds being 46 and 52 years of age.


In conclusion, advancing age was identified to be a significant risk factor for DED, and the global burden of this condition is projected to increase with the aging population. Signs of MG dysfunction emerged early in the natural history of disease progression during the third decade of life, and the brief delay prior to the development of other clinical signs of DED might indicate a window of opportunity for exploring preventative interventions in this age group.


Sex and DED


Androgens are extremely important in the regulation of the ocular surface and adnexa. , They also appear to mediate many of the sex-related differences in these tissues. Sex-related differences are evident in the anatomy, physiology, and pathophysiology of the lacrimal gland, as well as the morphological appearance, gene expression, neutral and polar lipid profiles, and secretory output of the MG. Sex also effects density of goblet cells and susceptibility of the ocular surface to inflammation. Although how these variations may relate to the sex-associated prevalence of DED is unclear, androgen deficiency is associated with both aqueous-deficient and evaporative DED. , , ,


Androgens influence structure and function of the lacrimal gland, including its cellular architecture, gene expression, protein synthesis, immune activity, and fluid and protein secretion , , . Androgen deficiency, in turn, has been linked to lacrimal gland dysfunction and a corresponding aqueous tear deficiency. , , , Women with SS are androgen-deficient. , Researchers have also suggested that the decrease in serum androgen levels that occurs during menopause, pregnancy, lactation, or the use of estrogen-containing oral contraceptives may trigger the development of a nonimmune type of DED, termed primary lacrimal gland deficiency.


Meibomian function is strongly influenced by the sex hormones, particularly androgens. In brief, androgens stimulate the synthesis and secretion of lipids by the MG and suppress the expression of genes related to keratinization. ,


Conversely a deficiency of androgen action, such as occurs in aging, SS, antiandrogen treatment, and complete androgen insensitivity syndrome, is associated with MGD, altered meibum lipid profiles, and evidence of decreased tear film stability. ,


The clinical evidence regarding the influence of estrogen and progesterone on dry eye is inconsistent. Although a common assumption exists in the literature that menopause is associated with increased occurrence of dry eye, a closer inspection shows that definitive evidence is lacking. The prevalence of dry eye is higher in females than males across the lifecycle and dry eye prevalence increases gradually with age in both men and women. , Such a gradual increase in both sexes is perhaps more in keeping with the gradual decrease in serum androgen levels which occurs with age rather than the abrupt decline in ovarian estrogen at menopause.


However, more dry eye symptoms were demonstrated in a study of 17- to 43-year-old women with premature ovarian failure, than in an age-matched control group, alluding to a positive role for ovarian estrogen. Three reports of increased risk of dry eye in postmenopausal women undergoing aromatase inhibitor treatment for breast cancer highlight the importance of peripheral estrogen synthesis in ocular surface homeostasis.


Assessment of pain, which is a hallmark of DED, may also be affected by gender difference. Although many studies show that men have higher pain thresholds than women, reliable evaluation of a possible sex difference in subjective reporting of pain in DED remains lacking.


Further studies are required to clarify the precise nature, extent, and mechanisms of these sex, endocrine, and gender effects on the eye in health and disease.


Environmental Factors and DED


Established risk factors of DED include environmental factors such as extreme temperature or reduced relative humidity, , contact lens (CL) wear, use of video display terminals, and smoking.


Digital Display Use


Digital display (DD) use is ubiquitous and various forms of DDs, such as laptops, smartphones, tablets, or even e-readers, are being used widely in addition to desktop computers. Studies indicate a significantly higher dry eye symptom score in DD users as compared to controls, as well as significantly lower fluorescein break-up time (FBUT), noninvasive break-up time, and tear meniscus height. Additionally, oxidative stress markers in the tear film, inflammatory mediators, and tear osmolarity have shown to be altered in DD users. Consequently, DD use has been implicated as a contributing factor to DED. The overall prevalence of DED in computer users is thought to be around 49.5%, and ranges from 9.5% to 87.5%.


Abnormalities of blinking, including reduced blink rate and incomplete eyelid closure during DD use, are considered as the main mechanisms of DD-related dry eye. Incomplete blinking, resulting from increased cognitive and task demand, was suggested to be a more pertinent issue in DD users with a significant positive correlation with total symptom scores. Incomplete blinking alters the distribution of mucin over the ocular surface, causes poor maintenance of lipid layer integrity, and reduces tear film thickness in the inferior cornea, rendering an instable tear film prone to break-up problems. Since small aliquots of oil are delivered from the MGs with each blink, abnormal blinking may alter MG secretion, leading in the long run to chronic changes in the gland, which may eventually cause inflammation, gland obstruction, and a further reduction of the outflow of meibum. Incomplete blinking was also reported to be associated with greater levels of MG dropout, decreased tear film lipid layer thickness, tear film stability, and expressed meibum quality. ,


Additionally, DD use was reported to lead to reduced tear volume and tear stability. , , , , Whereas tear stability can be affected even after a few minutes of computer visualization, it was also shown to decrease with the duration of computer use , , Significantly increased osmolarity was measured in computer users at the end of a 9-h working day, and osmolarity was negatively correlated with the duration of computer use and with FBUT and Schirmer scores. ,


Contact Lens Wear


CL wear is considered a risk factor for DED. According to TFOS DEWS II report, CL wear increases the risk of developing dry eye from between 2.01 and 2.96 times. , , Since CLs compartmentalize the tear film into two layers, namely the outer prelens and the inner postlens tear film, they lead to biophysical and biochemical alterations in the tear film. The use of CLs leads to a thinner and irregular lipid layer with poor wettability, tear film instability, increased tear evaporation and osmolarity, lower basal tear turnover rate, decreased tear volume, , and reduced levels of the mucin MUC5AC. Furthermore, MGs were also reported be adversely affected by CL use, with morphological changes and increased gland dropout in time.


Temperature, Humidity, Altitude, Pollution


Studies have reported strong association between low relative humidity environments and prevalence of DED. Tear evaporation rate, lipid layer thickness, ocular comfort, and tear film stability and production have shown to be adversely affected by low relative humidity. , Low relative air humidity in office buildings and air-conditioned rooms negatively impact the tear film, causing symptoms of DED and leading to conjunctival and limbal hyperemia as well as a reduction in tear meniscus height. Exposure of the ocular surface to low relative humidity was reported to cause conjunctival goblet cell cornification, with alterations in the delivery of mucins to the tear film. High horizontal or downward air velocity can also increase tear film evaporation leading to exposure keratitis and epithelial damage.


Tear film quality is also adversely affected from high temperature. Lowering room temperature by 1°C (within 22–26°C) was reported to decrease dry eye symptoms by 19%. Cold thermoreceptors in the cornea regulate the basal flow of tears. Blinking and basal tear secretion are suppressed in warm environments, resulting in a less stable tear film lipid layer. , Dry air and cold temperatures at high altitudes are associated with the development of dry eye symptoms, hyperosmolarity, and decreased tear film stability.


Another factor that may impact the tear film is air pollution. Finally, glare and reflections due to improper lighting can cause discomfort and disability glare.


Preservatives (Benzalkonium Chloride, BAK) and DED


Topical drugs may cause allergic, toxic, and/or immunoinflammatory effects on the ocular surface or may undergo chemical interaction with the tear film, either by disrupting the lipid layer through detergent effects, by reducing aqueous secretion, or by damaging goblet cells, surface epithelia, corneal nerves, or even eyelids at the skin or MG level. Delayed allergic reactions can also occur, often mimicking blepharitis with low-grade inflammation.


The most studied detergent compounds are quaternary ammoniums, particularly benzalkonium chloride (BAK), which is commonly used in eye drops at concentrations ranging from 0.004% to 0.02%. In vitro or animal models have suggested that BAK has cytotoxic effects on several structures of the eye, with a threshold of toxicity found at about 0.005%, i.e., below the concentration used in most eyedrops. BAK may cause or aggravate DED through various mechanisms such as toxic and proinflammatory effects; however, BAK has strong detergent properties, dissolving lipids and destroying the bacterial walls and cell membranes.


Goblet cells, which are particularly sensitive to toxic and inflammatory stress, are decreased in density in humans after short exposure to BAK or BAK-containing timolol. BAK also causes disruption of the tight junctions of the corneal epithelium, and increase epithelial permeability, an effect that has led to BAK being considered an enhancer of drug penetration into the anterior chamber. Hence, BAK may cause some level of toxicity in normal eyes as well as eyes that need chronic treatment such as glaucomatous eyes. Cumulative amounts of benzalkonium chloride (BAK) were shown to disrupt the tear film stability and increase tear film osmolarity. Tear film alterations may stimulate a series of biological changes in the ocular surface, leading to subsequent neurogenic inflammation and further impairment of the tear film, creating a vicious cycle. Glaucomatous eyes undergoing treatment with BAK-containing antiglaucoma medications exhibit more lid margin abnormalities and worse MG morphology compared to those that are not treated. Last, but not the least, BAK has shown neurotoxic effects to the trigeminal nerve endings, density of superficial epithelial cells and the number of subbasal nerves were reduced in the preservative-containing groups, and stromal keratocyte activation and bead-like nerve shaping were higher in the glaucoma preservative therapy groups than in the control and preservative-free groups, together with a decrease in corneal sensitivity. Several studies have shown that switching from a preserved to nonpreserved formulation significantly improved the ocular surface and reduced symptoms from 54% to 65%. Interestingly, in those studies, reversibility of inflammatory lesions can be obtained rapidly, as also shown with DC numbers returning to normal levels in less than 1 month.


In one study, unpreserved ketotifen 0.025% eye drops were more effective and better tolerated than BAK-preserved 0.05% drops in patients with seasonal allergic conjunctivitis, a fact that was attributed to the toxic effect of BAK. As allergic patients often exhibit impaired and inflammatory tear film and ocular surface, BAK-free compounds should be the first choice when treating allergic conjunctivitis or DED.


In conclusion, there is abundance of evidence that strongly supports the use of BAK-free solutions in the treatment of patients with chronic diseases, such as ocular allergy or glaucoma, to avoid ocular surface disease and findings typical of DED.


As BAK toxicity is dose-dependent, at least reducing the number of preserved eyedrops may be helpful and may reduce the burden of side effects to acceptable levels. , Low toxicity preservatives have been developed and have shown little if no adverse effects on the ocular surface. , However, their possible effects on the tear film and tolerance in dry eye patients need to be investigated.


Microbiome and DED


Mucosal surfaces of the human body are associated with commensal microorganisms with a mutualistic/symbiotic relationship with the human host. These microorganisms were referred to as the “normal flora”; now, commensal microorganisms are often referred to as the “microbiota” (the microbial cells), and the genetic information of the microorganisms is referred to as the “microbiome”.


Microbial commensal organisms can alter Th17 populations in the host organism, both in homeostasis and when perturbed. Each individual encompasses a unique gut microbiota profile that changes over time, depending on certain variables such as lifestyle, physical exercise, body mass index (BMI), and cultural and dietary habits. The gut microbiota has been reported to be interactive in maintaining balance in immune responses between regulatory T cells (Tregs) and T helper 17 (Th17) cells at mucosal surface and to act as a trigger of inducing autoimmunity. , Emerging evidences indicate that gut dysbiosis contributes to the pathophysiology or exacerbation of autoimmune diseases, including RA, SLE, systemic sclerosis, ankylosing spondylitis, and SS through the imbalance of the immune system. Emerging findings also suggest the existence of a gut–eye axis, wherein gut dysbiosis may be a crucial factor influencing the onset and progression of multiple ocular diseases, including uveitis, dry eye, macular degeneration, and glaucoma.


Studies have demonstrated that the gut microbiome is altered in dry eye, and there are specific bacterial classes associated with dry eye signs and symptoms. Gut dysbiosis is prevalent in SS and gut dysbiosis is associated with ocular disease severity. ,


Overall, studies agree on the presence of a significantly different gut microbiota of SS subjects compared to healthy controls through diversity analysis. , , , In most studies, in SS, greater relative abundances of Pseudobutyrivibrio , Escherichia/Shigella , and Streptococcus and reduced relative abundances of Bacteroides , Parabacteroides, Faecalibacterium, and Prevotella were noted compared to controls. Given that gut microbiota is easily influenced by diet, ethnicity, and gender, search for a specific causal bacteria is difficult; however, some degree of gut dysbiosis seems to be present in SS subjects compared to healthy individuals. Furthermore, subjects with severe gut dysbiosis exhibited higher disease activity with hypocomplementemia and higher F-calprotectin . Reduced gut microbiome diversity was also found to correlate with overall disease severity. , , Furthermore, individuals that did not meet full Sjögren’s criteria (SDE) were also found to have gut microbiome alterations compared to controls.


Gut dysbiosis leads to an aberrant diversification of the B-cell repertoire and an imbalance between Treg and TH17 cell responses in adaptive immunity, subsequently triggering ocular autoimmune diseases. Both non-Sjögren and SS-related dry eyes as well as uveitis share key pathogenic features, such as an imbalance in Treg/TH17 cells, or reduced SCFAs-producing bacteria.


Recently, ocular microbiota of DED patients have also been examined. In general, these studies mostly found CNS , Staphylococcus epidermidis , Corynebacterium , and Propionibacterium acnes , as have been described in other disease states. Reduced microbiome diversity has been identified in OSD patients compared to healthy controls, , which also have been shown to have greater fungal diversity. Further studies are warranted to uncover the role of the OSM in ocular diseases.


In summary, the beneficial or harmful effects of the gut microbiome in the pathogenesis of dry eye and other autoimmune ocular diseases are now just beginning to be understood. However, functional studies of gut microbiota are still preliminary to fully understand the pathogenesis of dry eye associated with gut dysbiosis.


Future innovation in this field may lead to a new target in ophthalmology to understand and manage ophthalmic diseases, providing alternative or adjunctive local or systemic treatments to modulate the ocular surface and gut microbiota. It remains unclear whether the OSM plays a role in the etiology of these diseases, or whether these diseases alter the OSM.


Summary


In summary, DED represents an unmet medical need—a common, multifactorial, chronic disease with significant impact on quality of life. The challenge remains in the multifactorial etiology and lack of concordance between signs and symptoms of the disease limited availability of adequate questionnaires and minimallly invasive objective tests make identification of various patient subgroups difficult and therefore it is hard to determine specific mechanisms of disease and best targets for treatment. Research on DED has historically suffered from the lack of validated biomarkers and less than objective and reproducible endpoints. Identification of biomarkers using minimally invasive methods that will lead to the development of objective metrics are hoped to improve our understanding of DED, and hence establish effective treatment strategies. Studies on various constituents of the tear film and ocular surface in health and disease, in vivo evaluation of morphological features, investigation of effects of aging, sex and environmental factors, microbiome and other risk factors on physiology and function of the ocular surface will improve our understanding of DED. Specific markers that can become validated biomarkers for DED will propel our understanding of DED and lead to better diganosis, classification and ultimately imporved treatment efficaty.



References


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Nov 10, 2024 | Posted by in OPHTHALMOLOGY | Comments Off on Pathophysiology of Dry Eye Disease Using Human Models

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