Eyelid discharge is usually harmless and is often part of the body's natural immune system for targeting infections. However, eyelid discharge can be the result of a more serious underlying condition that may require treatment.

Conjunctivitis: A thick green or grey discharge often represents an eye infection such as bacterial conjunctivitis. You may find that your eyelids are glued shut in the mornings and this is accompanied by redness, swelling and irritation.

Allergies can also cause conjunctivitis and result in some discharge, although this is usually white and stringy from allergic reaction deposits in the eye that clump together.

Blepharitis: Thick, crusty eye mucous can be caused by a condition known as blepharitis. When blepharitis is the result of an infection, there may be yellow or green discharge that glues the lids together.

Stye: A stye could be the problem if discharge is accompanied by a lump on the eyelid. When eyelid glands become blocked, they may fill with pus and form styes. These can leak mucous but will often clear up of their own accord.

Dacryocystitis: White or yellow balls of eye mucous present in tears may be a sign of dacryocystitis. This infection of the tear drainage system can cause redness and swelling around the nasal part of the eyelid, as well as facial pain. There may also be discharge from the puncta, which is the small drainage hole in the eyelid. Dacryocystitis should be treated quickly with antibiotics, otherwise it can become more serious.

Dry eye: When the eyes don't produce enough tears or good-quality tears it can lead to dry eye.Most tears are made up of water, mucous and oil. If the water level decreases, the oil and mucous may clump together. This can result in a stringy discharge that often accumulates in the corners of the eyes.