The vicious circle in dry eye syndrome: a mechanistic approach

The mechanistic view of dry eye disease aims at completing the classic etiological approach that classifies the disease as parallel ocular surface disorders leading to lacrimal film impairment and dry eye. This approach proposes two levels of ocular surface impairment (with standard etiologies, previously validated in the NEI/Industry workshop), which may not be independent diseases but rather risk factors and/or ways to enter a self-stimulated biological process involving the ocular surface. All external disorders proposed in this model, although unlikely to be fully exhaustive, are classical mechanisms considered to be causes of tear film impairment and ocular surface damage, by tear instability and evaporation, tear hyposecretion, or both. These mechanisms, sometimes alone-when severe or becoming chronic or repeatedly present on the ocular surface and when two or more are present-may cause the patient to enter the self-stimulated loop. Tear film instability/imbalance can be considered as the key point of dry eye disease. It will cause local or diffuse hyperosmolarity of the tear film and therefore of superficial epithelial cells of the cornea and/or conjunctiva, stimulating epithelial cells and resident inflammatory cells. Cell damage in the cornea and conjunctiva, by means of apoptosis and direct mechanical and/or osmotic stress, will stimulate the reflex neurosensory arc, in turn stimulating acninal gland and neurogenic inflammation, with inflammatory cytokine release, MMP activation, and inflammatory involvement of the conjunctival epithelium. Goblet cell loss is thus directly related to chronic inflammation and surface cell apoptosis subsequent to cell hyperosmolarity and chronic damage, resulting in further tear film instability/imbalance. On the other hand, bacterial changes and an imbalance resulting from specific diseases or from tear film abnormalities may trigger release of endotoxins, lipopolysaccharides, and/or lipase activation, causing eyelid inflammation, meibomian gland dysfunction, and lipidic changes, directly influencing tear film stability and favoring tear evaporation. The lipidic hypothesis therefore participates in the vicious circle as a parallel, independent, or complementary loop. This mechanistic approach proposes a synthetic combination of mechanisms previously validated independently, with two levels of ocular surface impairment, a first level including many possible acute or chronic causes that favor or trigger the imbalance and can be reversible if correctly and specifically managed when possible, and the further involvement of a series of biological cascades centered by tear film imbalance and inflammatory stimulation, finally acting as an independent vicious circle, however the patient entered the loop. Clinically, this approach may explain examples of dry eye syndrome occuring after ocular surgery, contact lens wear, chronic allergy or systemic or topical drugs, and the long-lasting effect even though all causal factors have been removed or have disappeared. This model should be considered as a basis for further reflection on biological mechanisms that could be even more complex but individually constitute potential leads for targeting therapeutic strategies to allow patients to leave the loop even though the triggering factors are still present or can only be attenuated, such as in Sjogren syndrome or ocular rosacea. It also should be considered a complement to more classic etiological and severity classifications aimed at understanding and classifying the large number of diseases that may cause dy eye disease and better assessing the major impairment it causes on the patient's quality of life.