ABSTRACT
When an ophthalmic formulation is instilled into the eye, it is mixed with the precorneal tear film. The osmotic pressure of the mixture depends upon the osmolarity of the tears and of the ophthalmic formulation. If the osmotic pressure obtained is within defined limits, no discomfort is experienced, but if the osmotic pressure is outside these limits the patient experiences irritation eliciting reflex tears and blinking. The original osmolarity of the precorneal tear film is regained two minutes after the non-isotonic solution is administered, mainly due to a rapid flow of water across the cornea. The instillation of a hypotonic drug solution creates an osmotic gradient between the tear film and the surrounding tissues. This induces a flow of water from the eye surface to the cornea, hence the drug concentration on the eye surface is temporarily increased.
