Aqueous deficiency is a contributor to evaporation-related dry eye disease

Dry eye disease caused by viral infection: Past, present and future

Following viral infection, the innate immune system senses viral products, such as viral nucleic acids, to activate innate defence pathways, leading to inflammation and apoptosis, control of cell proliferation, and consequently, threat to the whole body. The ocular surface is exposed to the external environment and extremely vulnerable to viral infection. Several studies have revealed that viral infection can induce inflammation of the ocular surface and reduce tear secretion of the lacrimal gland (LG), consequently triggering ocular morphological and functional changes and resulting in dry eye disease (DED). Understanding the mechanisms of DED caused by viral infection and its potential therapeutic strategies are crucial for clinical interventional advances in DED. This review summarizes the roles of viral infection in the pathogenesis of DED, applicable diagnostic and therapeutic strategies, and potential regions of future studies.

Review of Hydroxypropyl Methylcellulose in Artificial Tears for the Treatment of Dry eye disease

Dry eye disease (DED) is a highly prevalent condition, resulting in reduced quality of life, lower participation in social life and impaired work efficiency. Hydroxypropyl methylcellulose (HPMC) is a cellulose-based viscosity-enhancing agent and is one of the most popular therapeutic ingredients in artificial tears. This review aims to evaluate the literature on the efficacy and safety of HPMC used in the treatment of DED. Literature searches were conducted in PubMed and Cochrane CENTRAL. A total of 28 clinical trials from 26 publications are included in this review, including 21 clinical intervention studies evaluating the effect of HPMC treatment over time and seven single instillation studies evaluating the short-term physical and symptomatic effects of HPMC after drop-instillation. The duration of clinical intervention studies ranged from 2 weeks to 5.5 months. DED severity ranged from mild to severe. Drop frequency ranged from two to up to 16 drops per day. HPMC concentration in artificial tears ranged from 0.2% to 0.5%. No major complications or adverse events were reported. Artificial tears containing HPMC were effective at improving symptoms and some signs of DED. However, combination drops with HPMC plus other therapeutic ingredients seem more effective than HPMC alone. HPMC appears to be equally effective or inferior to hyaluronic acid (HA). There is no evidence of superiority or inferiority to either carboxymethylcellulose (CMC) or polyethylene glycol 400/propylene glycol (PEG/PG). No single study explained the choice of drop frequency or HPMC concentration. More well-designed studies are needed to determine an evidence-based standard for HPMC treatment, including drop frequency, concentration and molecular weight for different DED severity and subgroups.

Expression of Androgen and Estrogen Receptors in the Human Lacrimal Gland

Lacrimal gland dysfunction causes dry eye disease (DED) due to decreased tear production. Aqueous-deficient DED is more prevalent in women, suggesting that sexual dimorphism of the human lacrimal gland could be a potential cause. Sex steroid hormones are a key factor in the development of sexual dimorphism. This study aimed to quantify estrogen receptor (ER) and androgen receptor (AR) expression in the human lacrimal gland and compare it between sexes. RNA was isolated from 35 human lacrimal gland tissue samples collected from 19 cornea donors. AR, ERα, and ERβ mRNA was identified in all samples, and their expression was quantified using qPCR. Immunohistochemical staining was performed on selected samples to evaluate protein expression of the receptors. ERα mRNA expression was significantly higher than the expression of AR and ERβ. No difference in sex steroid hormone (SSH) receptor mRNA expression was observed between sexes, and no correlation was observed with age. If ERα protein expression is found to be concordant with mRNA expression, it should be investigated further as a potential target for hormone therapy of DED. Further research is needed to elucidate the role of sex steroid hormone receptors in sex-related differences of lacrimal gland structure and disease.

Lacrimal Gland Epithelial Cells Shape Immune Responses through the Modulation of Inflammasomes and Lipid Metabolism

Lacrimal gland inflammation triggers dry eye disease through impaired tear secretion by the epithelium. As aberrant inflammasome activation occurs in autoimmune disorders including Sjögren's syndrome, we analyzed the inflammasome pathway during acute and chronic inflammation and investigated its potential regulators. Bacterial infection was mimicked by the intraglandular injection of lipopolysaccharide (LPS) and nigericin, known to activate the NLRP3 inflammasome. Acute injury of the lacrimal gland was induced by interleukin (IL)-1α injection. Chronic inflammation was studied using two Sjögren's syndrome models: diseased NOD.H2^b compared to healthy BALBc mice and Thrombospondin-1-null (TSP-1^-/-) compared to TSP-1^WTC57BL/6J mice. Inflammasome activation was investigated by immunostaining using the R26^ASC-citrine reporter mouse, by Western blotting, and by RNAseq. LPS/Nigericin, IL-1α and chronic inflammation induced inflammasomes in lacrimal gland epithelial cells. Acute and chronic inflammation of the lacrimal gland upregulated multiple inflammasome sensors, caspases 1/4, and interleukins Il1b and Il18. We also found increased IL-1β maturation in Sjögren's syndrome models compared with healthy control lacrimal glands. Using RNA-seq data of regenerating lacrimal glands, we found that lipogenic genes were upregulated during the resolution of inflammation following acute injury. In chronically inflamed NOD.H2^b lacrimal glands, an altered lipid metabolism was associated with disease progression: genes for cholesterol metabolism were upregulated, while genes involved in mitochondrial metabolism and fatty acid synthesis were downregulated, including peroxisome proliferator-activated receptor alpha (PPARα)/sterol regulatory element-binding 1 (SREBP-1)-dependent signaling. We conclude that epithelial cells can promote immune responses by forming inflammasomes, and that sustained inflammasome activation, together with an altered lipid metabolism, are key players of Sjögren's syndrome-like pathogenesis in the NOD.H2^b mouse lacrimal gland by promoting epithelial dysfunction and inflammation.

Hyaluronic acid in the treatment of dry eye disease

Dry eye disease (DED) is a highly prevalent and debilitating condition affecting several hundred million people worldwide. Hyaluronic acid (HA) is a naturally occurring glycosaminoglycan commonly used in the treatment of DED. This review aims to critically evaluate the literature on the safety and efficacy of artificial tears containing HA used in DED treatment. Literature searches were conducted in PubMed, including MEDLINE, and in Embase via Ovid with the search term: "(hyaluronic acid OR hyaluronan OR hyaluronate) AND (dry eye OR sicca)". A total of 53 clinical trials are included in this review, including eight placebo-controlled trials. Hyaluronic acid concentrations ranged from 0.1% to 0.4%. Studies lasted up to 3 months. A broad spectrum of DED types and severities was represented in the reviewed literature. No major complications or adverse events were reported. Artificial tears containing 0.1% to 0.4% HA were effective at improving both signs and symptoms of DED. Two major gaps in the literature have been identified: 1. no study investigated the ideal drop frequency for HA-containing eyedrops, and 2. insufficient evidence was presented to recommend any specific HA formulation over another. Future investigations assessing the optimal drop frequency for different concentrations and molecular weights of HA, different drop formulations, including tonicity, and accounting for DED severity and aetiology are essential for an evidence-based, individualized approach to DED treatment.

Hyaluronic acid in the treatment of dry eye disease

Dry eye disease (DED) is a highly prevalent and debilitating condition affecting several hundred million people worldwide. Hyaluronic acid (HA) is a naturally occurring glycosaminoglycan commonly used in the treatment of DED. This review aims to critically evaluate the literature on the safety and efficacy of artificial tears containing HA used in DED treatment. Literature searches were conducted in PubMed, including MEDLINE, and in Embase via Ovid with the search term: "(hyaluronic acid OR hyaluronan OR hyaluronate) AND (dry eye OR sicca)". A total of 53 clinical trials are included in this review, including eight placebo-controlled trials. Hyaluronic acid concentrations ranged from 0.1% to 0.4%. Studies lasted up to 3 months. A broad spectrum of DED types and severities was represented in the reviewed literature. No major complications or adverse events were reported. Artificial tears containing 0.1% to 0.4% HA were effective at improving both signs and symptoms of DED. Two major gaps in the literature have been identified: 1. no study investigated the ideal drop frequency for HA-containing eyedrops, and 2. insufficient evidence was presented to recommend any specific HA formulation over another. Future investigations assessing the optimal drop frequency for different concentrations and molecular weights of HA, different drop formulations, including tonicity, and accounting for DED severity and aetiology are essential for an evidence-based, individualized approach to DED treatment.

Transdermal anti-inflammatory therapy for aqueous deficiency

PURPOSE

Lacrimal gland inflammation has been identified as an important limitation on aqueous production and associated dry eye disease. Ocular surface inflammation in dry eye disease can be a downstream response to reduced quantities of warmer hyperosmotic aqueous being delivered from an inflamed lacrimal gland with high concentrations of inflammatory mediators. This review examines evidence which shows how topical applications of anti-inflammatory drugs have very limited access to the lacrimal gland and an associated limited capacity to increase aqueous flow by reducing inflammation in the main lacrimal gland.

RECENT FINDINGS

Using cyclosporine as an exemplar immunomodulatory drug, this review examines problems associated with the topical administration of all anti-inflammatory drugs in the treatment of dry eye disease.

SUMMARY

Limited access to the lacrimal gland for topical applications and their very short on-eye residence times are compared with the therapeutic potential of prolonged therapeutic episodes that could be achieved with transdermal applications of a drug to the skin at the site of the lacrimal gland. Poor access to the lacrimal gland for topically administered drugs is a major barrier to the treatment of aqueous deficiency. While topical inflammatory drug access to the ocular surface is direct, poor access to the lacrimal gland is partly due to drop placement being downstream to the flow of aqueous (Eye Vis 2020;7:1; Eye Vis 2019;6:1). This barrier is much greater according to the degree that reflex tear flow is stimulated by irritation associated with adverse drop temperature, and/or pH and/or tonicity for example.

Protection against corneal hyperosmolarity with soft-contact-lens wear

Hyperosmotic tear stimulates human corneal nerve endings, activates ocular immune response, and elicits dry-eye symptoms. A soft contact lens (SCL) covers the cornea preventing it from experiencing direct tear evaporation and the resulting blink-periodic salinity increases. For the cornea to experience hyperosmolarity due to tear evaporation, salt must transport across the SCL to the post-lens tear film (PoLTF) bathing the cornea. Consequently, limited salt transport across a SCL potentially protects the ocular surface from hyperosmotic tear. In addition, despite lens-wear discomfort sharing common sensations to dry eye, no correlation is available between measured tear hyperosmolarity and SCL-wear discomfort. Lack of documentation is likely because clinical measurements of tear osmolarity during lens wear do not interrogate the tear osmolarity of the PoLTF that actually overlays the cornea. Rather, tear osmolarity is clinically measured in the tear meniscus. For the first time, we mathematically quantify tear osmolarity in the PoLTF and show that it differs significantly from the clinically measured tear-meniscus osmolarity. We show further that aqueous-deficient dry eye and evaporative dry eye both exacerbate the hyperosmolarity of the PoLTF. Nevertheless, depending on lens salt-transport properties (i.e., diffusivity, partition coefficient, and thickness), a SCL can indeed protect against corneal hyperosmolarity by reducing PoLTF salinity to below that of the ocular surface during no-lens wear. Importantly, PoLTF osmolarity for dry-eye patients can be reduced to that of normal eyes with no-lens wear provided that the lens exhibits a low lens-salt diffusivity. Infrequent blinking increases PoLTF osmolarity consistent with lens-wear discomfort. Judicious design of SCL material salt-transport properties can ameliorate corneal hyperosmolarity. Our results confirm the importance of PoLTF osmolarity during SCL wear and indicate a possible relation between PoLTF osmolarity and contact-lens discomfort.

Why the symptoms and objective signs of dry eye disease may not correlate

Cases of dry eye disease involving a neuropathic basis for symptoms and a poor correlation between symptoms and objective signs of dry eye disease can be associated with unsatisfactory responses to treatments which are limited to attempts to restore lacrimal function unit deficiencies. This review examines a wider range of circumstances under which the same kind of poor correlation between signs, symptoms and treatment results can be found. Some cases of computer vision syndrome can present for examination at times when objective signs related to reported symptoms have dissipated. A thorough history should explain this type of presentation for which symptoms might otherwise appear to be unexplained. However, mental health disorders can also be the basis for apparently unexplained levels of symptoms of dry eye disease. Anxiety, depression, hypochondriasis, stress, sleep and mood disorders as well as neuroticism for example, may be associated with exacerbation of symptoms to degrees that are not consistent with the levels of tear homeostasis anomalies that are assessed. The conclusion is drawn that failure to consider mental health comorbidities may result in symptomatic patients being exposed to less successful attempts to remediate tear dysfunctions when, for example, the symptoms have a somatic basis. Appropriate screening and referral to a psychologist or psychiatrist may be the key to managing some patients whose symptoms do not correlate with objective evidence of dry eye disease.