Dropout refers to a loss of meibomian glands, whereas atrophy refers to glands that exist, but no longer function.
Both dropout and atrophy of meibomian glands leads to reduced meibum secretion and evaporative dry eye. They are signs of meibomian gland dysfunction (MGD).
It is not uncommon to have some atrophy or dropout. Both can be seen with meibography. Dropout is sometimes indicated by notches on the lid margin. Dropout or atrophy can occur along the entire length of the gland or intermittently along sections of the gland.
When the section of the gland closest to the opening drops out, but the rest of the gland is still functioning, meibum produced by the functioning section of the gland does not reach the orifice and does not reach the tear film.
Because meibomian glands contain stem cells, atrophy and dropout can sometimes be reversed with intraductal meibomian gland probing, breaking the cycle of inflammation and scarring, by creating an opening through which meibum can flow.