Tear film osmolality

Osmotic Pressure The osmolality of the lacrimal fluid is mainly dependent on the number of ions dissolved in the aqueous layer of the tear film and normally ranges between 310 and 350mOsm/kg [44], When an ophthalmic solution is instilled into the eye, it mixes with the tear fluid, resulting in an osmotic pressure that is dependent on the osmolality of the tears as well as that of the formulation and the amount of the formulation instilled. In general, hypotonic solutions are better tolerated by... 

The exposed part of the eye is covered by a thin fluid layer, the so-called precorneal tear film. The film thickness is reported to be about 3-10 pm depending on the measuranent method used. The resident volume amounts to about 10 pL. According to the three layers theory, the precorneal tear film consists of a superficial Upid layer, a central aqueous layer, and an inner mncus layer. The lipids play an important role in reducing the evaporation rate in a way that normal tear osmolality can be maintained, even when the tear flow is quite low. - ... 

The osmolality of the tear film equals 310-350 mOsm/kg in normal eyes and is adjusted by the principal inorganic ions Na K+, CD, HC03 , and proteins. The mean pH value of normal tears is about 7.4. Depending on age and diseases, values between 5.2 and 9.3 have been measured. 

The lipid composition is kept within physiological limits by androgens [17]. The decrease of their secretion in elderly people is one of the reasons for development of dry eye syndrome. Patients with Meibomian gland dysfunction show a high tear film evaporation rate and a high tear osmolality [18, 19]. 

If the decision is taken to administer very small volumes (1-10 microlitres) of concentrated active substance solutions in order to compensate for dilution in the tear film, irritation becomes a problem. Consequently excessive lachrymation, drainage and wash out will occur, resulting in low active substance availability. To avoid irritation the surface tension, osmolality, pH and buffer capacity should be within certain limits (see Sects. 10.4.5 and 10.4.6). 

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