How do blepharitis, a sty, and a chalazion differ?

These three conditions are distinct inflammatory disorders of the eyelid:

• Blepharitis is a chronic inflammation of the eyelid margins characterized by redness, scaling, crusting, and a burning or itching sensation. It often involves the entire lid margin and can be caused by bacterial colonization, skin disorders like seborrhea, or allergic reactions.12
• A sty is an acute, painful, localized infection of an eyelid gland or lash follicle. It typically presents as a swollen, red, pimple-like lesion. External sties affect glands in the lid margin, while internal sties involve the meibomian glands.32
• A chalazion is a painless, chronic swelling of a meibomian gland. Unlike a sty, which is an acute infection, a chalazion is a firm lump that develops slowly in the deeper tissue of the lid. While it may arise as an aftereffect of an internal sty, it is distinct due to its chronic, non-infectious, and painless nature.32

Treating underlying blepharitis is often necessary to prevent the recurrence of sties.3