WebMD Medical Reference
Reviewed By Charlotte Grayson, MD
on Tuesday, January 01, 2002
What Is a Sty?
A sty is a pimple or abscess that forms in either the upper or lower eyelid. The medical term for sty is hordeolum(say HOR-dee-oh-lumm) and it is an infection caused by bacteria that normally live peacefully on the eyelid skin surface. Some of thee germs get trapped along with dead skin cells inside crypts along the eyelid margin. Stys are usually superficial and plainly visible. Occasionally they can reside deeper within the eyelid.
An external sty starts as a pimple next to an eyelash. It turns into a red, painful swelling that usually lasts several days before it bursts and then heals. Most external stys are short-lived and self-limiting.
An internal sty (on the underside of the lid) also causes a red, painful swelling, but its location prevents the familiar whitehead from appearing on the eyelid. The internal sty may disappear completely once the infection is past, or it may leave a small fluid-filled cyst or nodule that can persist and may have to be opened and drained.
Folks tend to confuse a sty with another common lid lump - the chalazion (say cha-LAY-zee-yon). A chalazion is very different from a sty and is not an infection. It is instead a firm, round, smooth, painless bump usually some distance from the edge of the lid. A chalazion is a local tissue reaction to oily glandular secretions that were unable to reach the lid surface because the duct was blocked by debris.
Stys and chalazia are usually harmless and rarely affect your eyeball or your eyesight. They can occur at any age and tend to periodically recur.
What Causes It?
Stys are usually caused by staphylococcal bacteria, which often live right on the skin surface. Truth be told, our bodies are coated with billions of friendly bacteria that coexist with us. When the conditions are just right the bacteria feast on dead cells and other debris, resulting in the tender pimple.
For the sake of comparison, a chalazion is caused by the blockage of tiny eyelid gland ducts that normally transports an oily substance called meibom. This oily material enters the tear film to prevent tear evaporation. Trapped or misplaced oil stimulates the immune system to cleanup the mess. Chalazia develop over weeks-to-months.
Medically Reviewed by William C. Lloyd, MD, July 2005.
SOURCES: Bradford, C (Editor) Basic Ophthalmology, American Academy of Ophthalmology, 2004. pp 88-91. The Mayo Clinic.
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