What is a sty (stye)?
A sty is a bump that forms on or in the eyelid as the result of a blocked gland. The word sty can also be spelled stye. There are two distinct types of styes: hordeolum and chalazion. Each has different causes and treatments.
A hordeolum is a blockage of one of the sweat glands found in the skin of the lid and base of the eyelashes, or one of the small sebaceous glands found at the base of the eyelashes. Sebaceous glands secrete sebum, a waxy, oily material.
A chalazion is a blockage of a meibomian gland, which is a special sebaceous gland unique to the eyelids. These glands form a single row in each lid, with the body of the gland located inside the eyelid, and the opening located at the rim of the lid, posterior to the lashes. They secrete an oily material onto the surface of the eye, preventing the water layer of tears from evaporating too rapidly from the eye's surface between blinks. Therefore, poorly functioning meibomian glands can lead to dry eye symptoms.
What causes a sty?
Styes occur when a gland in or on the eyelid becomes plugged or blocked. This can occur if the gland's opening is obstructed by scar tissue or a foreign substance (makeup, dust), or if there is thickening of the substance produced by the gland, causing the material to flow sluggishly or not at all.