The production and turnover of an efficient tear film is an essential condition for ocular surface health.
The eye is covered externally by a thin layer of tears, called the tear film, which represents the complex end product of the activity of structures belonging to a functional unit known as the ocular surface. This includes the lacrimal apparatus, responsible for the production and excretion of tears, the eyelids, the conjunctiva, the limbus, and the cornea. The lachrymal film therefore represents the interface between the eye and the external environment and performs protective, nourishing and optical functions. Its structure consists of a mixture of water, mucins, lipids, products of epithelial metabolism, cell debris and cells of the immune system.
Different insults can alter the tear film: external factors such as wind, air conditioning, low humidity, smoke, foreign bodies (including contact lenses), micro-organisms (such as viruses, bacteria and mycetes), visual activities such as prolonged use of video-terminals, systemic or topical medications. All these factors can cause suffering of the ocular surface, the severity of which varies from the simple onset of discomfort symptoms to the presence of lesions that can interfere even considerably with the quality of vision.
The production and turnover of an efficient tear film is an essential condition for a healthy ocular surface. Tears nourish, remove products of cell metabolism, provide immunological protection and lubricate the ocular surface through their physico-chemical properties. To fulfil its task, the tear film must remain stable. The stability of the tear film depends on its volume and composition. Normally, the film remains on the ocular surface for at least 7-10 seconds. When qualitative changes in the tears occur, the tear film becomes unstable, leading to suffering of the corneo-conjunctival epithelium and subsequent inflammation and onset of symptoms, such as burning and foreign body sensation, which can significantly interfere with the quality of life of affected patients.
The malfunctioning of the tear film leads to a state of suffering of the ocular surface known as dry eye or, more commonly, dry eye. The causes of dry eye can be traced to two main pathogenic forms: that of reduced tear production and that of increased evaporation. Both of these forms have in common that they induce a strong instability of the tear film. Mucins play a key role in maintaining the volume of the film and ensuring its anchorage with the epithelial cells, contributing decisively to its stability.
An effective therapeutic approach to dry eyes requires a restoration of tear film stability. This can be achieved with eye drops based on mucomimetic substances. Of these, the molecule most studied and whose beneficial effects are best known is hyaluronic acid (HA), normally present in ocular fluids and the extracellular matrix. HA is endowed with non-Newtonian mucomimetic, lubricating and viscoelastic activity (low viscosity during blinking, high viscosity between blinks), so that it offers significantly less resistance to the movement of the eyelids on the eyeball during blinking, while remaining very dense when the eyelids are open between blinks. Moreover, due to its richness in negative charges, it retains large volumes of water, thus ensuring the maintenance of an efficient tear film volume. Its biological characteristics mean that it can bind to epithelial cells via a specific receptor, CD44, so that it has been shown to have healing properties and to promote corneal and conjunctival re-epithelialisation. The continued use of HA-based eye drops, especially if they are preservative-free, is to be considered essential in the treatment of a chronic condition such as dry eye, which afflicts about 20% of the adult population, but can also be beneficial in postmenopausal women and younger individuals who use video-terminals or are exposed to the increasing pollution of our cities.
Prof. Pasquale Aragona Regional Reference Centre for Diseases of the Ocular Surface, University of Messina
'Eye health' - Corriere della sera (Sette) - 21 November 2014
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