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.

This common infection or inflammation results from blocked glands within the eyelid. When the gland is blocked, the oil produced by the gland occasionally backs up and extrudes through the wall of the gland, forming a lump, which can be red, painful, and nodular. Frequently, bacteria can infect the blocked gland, causing increased inflammation, pain, and redness of the eye and even redness of the surrounding eyelid and cheek tissue.

The lump can point externally (outward) or internally (inward). Frequently, the lump appears with a visible whitish or yellowish spot that looks much like a large pimple.

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.

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.

Good hand and facial washing may prevent styes from forming or coming back.