Surgery for the Dry Eye-The Normal Tear Film
Purpose: To survey briefly current knowledge on the normal preocular tear film. Methods: Sections deal with: Formation of the film, its physical dimensions and rates of inflow and outflow of tears, and briefly the pathways of nervous control of tear production in the main lacrimal gland and accessory lacrimal tissue. The protein and electrolyte com¬position of the aqueous part of the tears derived from the lacrimal gland, as well as the accessory secretions of the meibomian glands and conjunctiva; the 'soluble' or gel-forming mucins from the conjunctival goblet cells are described as well as the membrane-spanning epithelial mucins of the glycocalyx which take part in wettability and mucus binding. The functions of the film, including acting as a nutritional route for the anterior epithelium of the cornea, and its protective roles in ocular lubrication and in scavenging and eliminating invad¬ing debris and microorganisms, plus specific antibacterial and immune functions. Problems of structure and stability of the film are discussed, and the wide variety of tests of tear function and quality, with discussion of which tests are suitable for the clinical envi¬ronment, and which laboratory-based tests can be useful in assessing the individual patient. Results: The preocular tear film plays a vital role in nourishing, lubricating and protecting the ocular surface. Many tests can be applied in either the clinical or the laboratory setting, to determine whether the tears of the individual patient exert their physiological and antimicro¬bial functions at the normal level. Conclusions: Knowledge of the normal functions of the film provides a basis for later consideration of clinical and surgical treatment of the dry eye.
Importance of Film
In the open eye, the exposed surfaces of cornea and sclera are covered with a very thin film of tear fluid. This has both protective and nutritional properties; its thickness changes due to evaporation while the eye is held open, and during prolonged eye-opening the film may break up to expose surface epithelial cells directly to the air. The tear break-up time (TBUT) is an important clinical
parameter in defining the normality of function of the eye. During normal life, break-up will perhaps occur only comparatively rarely, as blinking is very rapid and is a nearly automatic response to symptoms of drying. However, normal life for many people now includes prolonged periods of computer or other visual screen use, and it is known that the blink rate falls when paying close attention. Hence the tear film may break up between blinks. The Corneal Protection Index (CPI), defined as the ratio of TBUT to the length of the interblink, can be used to indicate the boundary between normal and dry eye.
Position and Extent of the Tear Film
The term 'tear film' is normally used to describe the film of fluid covering the corneal surface and contained between the lid margins . In fact we should also include the film overlying the exposed bulbar sclera; however, because this surface is rough and irregular it is much harder to obtain information about its nature by the usual reflectance-based methods used for the cornea, so this area is often ignored. At present we cannot even say with certainty that a uniform and continuous tear film is present over all the exposed sclera. However this area is not negligible - in upward gaze it may contribute 60% or more of the total exposed area. It is also neglected because it is of much lower importance than the cornea in the visual process, and because, having its own blood supply, it recovers more readily from injuries and infections.
Purpose: To survey briefly current knowledge on the normal preocular tear film. Methods: Sections deal with: Formation of the film, its physical dimensions and rates of inflow and outflow of tears, and briefly the pathways of nervous control of tear production in the main lacrimal gland and accessory lacrimal tissue. The protein and electrolyte com¬position of the aqueous part of the tears derived from the lacrimal gland, as well as the accessory secretions of the meibomian glands and conjunctiva; the 'soluble' or gel-forming mucins from the conjunctival goblet cells are described as well as the membrane-spanning epithelial mucins of the glycocalyx which take part in wettability and mucus binding. The functions of the film, including acting as a nutritional route for the anterior epithelium of the cornea, and its protective roles in ocular lubrication and in scavenging and eliminating invad¬ing debris and microorganisms, plus specific antibacterial and immune functions. Problems of structure and stability of the film are discussed, and the wide variety of tests of tear function and quality, with discussion of which tests are suitable for the clinical envi¬ronment, and which laboratory-based tests can be useful in assessing the individual patient. Results: The preocular tear film plays a vital role in nourishing, lubricating and protecting the ocular surface. Many tests can be applied in either the clinical or the laboratory setting, to determine whether the tears of the individual patient exert their physiological and antimicro¬bial functions at the normal level. Conclusions: Knowledge of the normal functions of the film provides a basis for later consideration of clinical and surgical treatment of the dry eye.
Importance of Film
In the open eye, the exposed surfaces of cornea and sclera are covered with a very thin film of tear fluid. This has both protective and nutritional properties; its thickness changes due to evaporation while the eye is held open, and during prolonged eye-opening the film may break up to expose surface epithelial cells directly to the air. The tear break-up time (TBUT) is an important clinical
parameter in defining the normality of function of the eye. During normal life, break-up will perhaps occur only comparatively rarely, as blinking is very rapid and is a nearly automatic response to symptoms of drying. However, normal life for many people now includes prolonged periods of computer or other visual screen use, and it is known that the blink rate falls when paying close attention. Hence the tear film may break up between blinks. The Corneal Protection Index (CPI), defined as the ratio of TBUT to the length of the interblink, can be used to indicate the boundary between normal and dry eye.
Position and Extent of the Tear Film
The term 'tear film' is normally used to describe the film of fluid covering the corneal surface and contained between the lid margins . In fact we should also include the film overlying the exposed bulbar sclera; however, because this surface is rough and irregular it is much harder to obtain information about its nature by the usual reflectance-based methods used for the cornea, so this area is often ignored. At present we cannot even say with certainty that a uniform and continuous tear film is present over all the exposed sclera. However this area is not negligible - in upward gaze it may contribute 60% or more of the total exposed area. It is also neglected because it is of much lower importance than the cornea in the visual process, and because, having its own blood supply, it recovers more readily from injuries and infections.
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