Rheum is a medical term for the natural watery discharge from the eyes, which forms a crust on the eyelids during sleep (contrast to Mucopurulent discharge). It is known to most people as "sleep." It is formed by a combination of mucus consisting of mucin discharged from the cornea or conjunctiva, tears, and dust from the eyelid. This substance is normally washed away from the eyes by tears or blinking, but the absence of this function during sleep results in a small amount forming around the eyes even among healthy individuals, especially children. However, the formation of a large amount of crust or presence of pus within the crust may indicate dry eye or other more serious eye infections. Removal of the crust can be done easily by washing the eye with water or simply brushing them away with clean fingers. If left alone, the crust may cause the eyelids to stick together and make it difficult for the eyes to open.
