The meibomian glands in the lids (see diagram below) are responsible for maintaining a strong tear film by providing a protective oil layer to stop your eyes drying out. If the glands aren’t working properly, the lid margin is typically red, the eyes get irritated and dry (gritty, burny), there is watering, vision can be variable (due to butter-like grease in the tears rather than smooth thin oil) and occasionally they will block completely and form a cyst.
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Meibomian cysts are small (2 to 8 mm) and filled with fluid. They're more common on the upper eyelid and are more likely to affect adults than children.
Are they the same as a stye?
It can be hard to tell the difference between a meibomian cyst and a stye. However, while they are both due to blocked oil glands, a stye is a bacterial infection. Styes tend to occur at the very edge of your eyelid and can be very painful. Meibomian cysts (chalazia) tend to be further up (or down) from the edge of your eyelid and might be a bit tender but are not usually sore.
The image below shows a meibomian cyst on an upper eyelid.
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