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Ptosis is an abnormally low position (drooping) of the upper eyelid. The drooping may be worse after being awake longer, when the individual's muscles are tired. This condition is sometimes incorrectly referred to as a "lazy eye", a separate condition known as Amblyopia. If severe enough and left untreated, the drooping eyelid can cause other conditions, like Amblyopia or Astigmatism. This is why it is especially important for this disorder to be treated in children at a young age, before it can interfere with vision development.
A drooping of the upper eyelid when the eyes are open; may occur in one or both eyes; may be constant or intermittent. If the condition is congenital, it is usually a failure of the levator muscle to develop; it may be hereditary (dominant). If the condition is acquired, it is usually the result of a mechanical factor (the lid is simply too heavy 
for the levator muscle to lift it), associated with disease (commonly muscular dystrophy or myasthenia gravis), or paralytic (related to the malfunctioning of the 3rd cranial nerve). If the lid droops enough to partially cover the pupil, the person attempts to compensate by raising the eyebrow and/or by tilting the head back.