Inadequate tear production is the cause. This may be due to injuries to the tear glands, such as infections or trauma. The nerves of these glands may also become damaged.
Some cases are also the result of the gland of the third eyelid being surgically removed by mistake.
The eyes typically develop a thick, yellowish discharge. Infections are common as the lack of the bactericidal tears allows bacterial organisms to overgrow on the eye. Additionally, inadequate lubrication allows dust, pollen, etc., to accumulate. As a result the eyes lose their ability to flush away foreign particles and protect themselves from bacteria. To confirm a case of dry eye, a measurement of tear production is performed.