Causes of dry eyes:

Let us consider an example of a wash basin, where we want an optimum level of water to be always present so we can float a paper

boat. The tap produces water at a predetermined rate and a drain stopper regulates the exit of water from the basin. Under these conditions our paper boat can float around. The level of water will decrease if the tap does not produce enough water. If the room is dry, hot or windy the water will vaporize. Finally, if the drain stopper leaks water, the boat will sink. Decreased tear production due to ageing, infection or inflammation is akin to an improperly functioning tap. Increased evaporation may account for decreased tear lake leading to irritated eyes. Rarely the outflow channels, the punctum, may be too big.

Decreased Production:

1. 1. Atrophy of tear producing cells.
2. 2. Inflammation
3. 3. Suppression by drugs
4. 4. Congenital absence
5. 5. Tumors
6. 6. Surgical excision of tear producing glands

Increased Evaporation

1. 1. Air Conditioning
2. 2. Heat in cold climate
3. 3. Outdoor exposure to sun
4. 4. Infrequent blinking

Increased drainage

If the tear punctum is too big or duplicated.

Poor Blinking

1. 1. Habitual
2. 2. Lid tumors
3. 3. Nerve damage

Incorrect Composition

The volume is god but there is a mismatch between the oily and water layers.