CONTACT LENSES TRUSTED AROUND THE WORLD
Physiology of the organ of sight
The physiology of the organ of sight considers the functional processes occurring in the eye. This section contains information on the interaction of water and oxygen for the normal functioning of the cornea and tear film, the role of glucose (a nutrient for cells) and salts in the metabolism of the eye.
Water balance
The norm of water content in a healthy cornea is 78%. There are biophysical systems in the cornea that constantly maintain this level.
The water/ion equilibrium (dissolved salts) between the tear fluid and the cornea is called osmolarity or tonicity. With an isotonic tear film, the cornea maintains normal thickness, since the amount of water penetrating and leaving the cornea is the same.
From the point of view of physiology, the process of maintaining equilibrium is the exchange of salts by water molecules between the cornea, lacrimal and intraocular fluids. The optimal equilibrium is achieved when the tear film contains 0.9% salts (tonicity). This is why most contact lens solutions have the same tonicity: lenses do not change this equilibrium when they are in the eye.
A hypertonic state develops when the concentration of salts in the tear film exceeds 0.9%. In this case, water flows from the cornea into the lacrimal fluid and dehydration or corneal wrinkling occurs.
The epithelium acts mainly as a semipermeable membrane and does not affect the passive diffusion of water.
When the salt concentration in the tear film is below 0.9%, a hypotonic state occurs and water enters the cornea, which swells.
The main function of the endothelial layer is to maintain a clean cornea. The ion “pump” controls the flow of water and ions from and to the cornea so that the water content remains at 78%. Glucose is the main source of energy for this pump.
Glucose is found in our food. It is mainly delivered to the endothelium by intraocular fluid, but first it is necessary that glucose is metabolized or broken down to units of energy (ATP – adenosine triphosphate).
Glucose metabolism is carried out in one of two biochemical ways: anaerobic or aerobic. With anaerobic glycolysis (which means “without oxygen”), two units of energy (ATP) are obtained from one glucose molecule. This amount of energy is not enough for the effective operation of the “pump”, and therefore water accumulates in the cornea, causing it to swell (edema).
Aerobic glycolysis (glucose metabolism “with oxygen”) is much more effective. With aerobic metabolism, 36 units of energy (ATP) are obtained from one glucose molecule. This is a sufficient amount of energy to maintain the necessary balance of water in the cornea with an endothelial “pump”
The tear film is the main supplier of nutrients – oxygen, glucose, salts and minerals – to the cornea. Oxygen from the air is contained in the tear film in a dissolved state. Without contact lenses, up to 21% of all air oxygen can enter an open eye. With eyes closed and without lenses (during sleep), the amount of oxygen is reduced to 7%, with oxygen mainly supplied by diffusion from the conjunctival blood vessels.
Contact lenses significantly impede the penetration of oxygen into the eye. This is not significant when the eye is open. However, with eyes closed, for example during sleep with contact lenses for prolonged wearing, the vital processes in the cornea may decrease.
Cellular glucose metabolism begins to flow along a less efficient anaerobic pathway. The ion “pump” responsible for the water content in the cornea starts to work under pressure. As a result, corneal edema may develop. When choosing lenses, doctors usually take into account and fully trust the oxygen consumption indicator. In general, the permeability of lenses to oxygen is higher, the more water they contain and the thinner they are. Solid lenses let through only 1-3% of all oxygen available to the cornea. In fact, these lenses are impermeable to oxygen and the indicated 1-3% come with a tear fluid that penetrates under the lenses and from the blood vessels of the limb.
Summary
Cellular glucose metabolism begins to flow along a less efficient anaerobic pathway.
The ion “pump” responsible for the water content in the cornea starts to work under pressure. As a result, corneal edema may develop.
When choosing lenses, doctors usually take into account and fully trust the oxygen consumption indicator. In general, the permeability of lenses to oxygen is higher, the more water they contain and the thinner they are. Solid lenses let through only 1-3% of all oxygen available to the cornea. In fact, these lenses are impermeable to oxygen and the indicated 1-3% come with a tear fluid that penetrates under the lenses and from the blood vessels of the limb.
