Ptosis, also known as blepharoptosis, is a drooping or falling of the upper eyelid. This condition is sometimes called "lazy eye", but that term normally refers to the condition amblyopia. If severe enough and left untreated, the drooping eyelid can cause other conditions, such as amblyopia or astigmatism, so it is especially important to treat the disorder in children before it can interfere with vision development.

Ptosis can be unilateral or bilateral, and may vary in severity. Common signs and symptoms include:

Ptosis occurs as the result of dysfunction of the muscles that raise the eyelid or their nerve supply (oculomotor nerve for levator palpebrae superioris and sympathetic nerves for superior tarsal muscle). It can affect one eye or both eyes and is more common in the elderly, as muscles in the eyelids may begin to deteriorate. Babies may also exhibit ptosis at birth as the result of abnormal development of the levator muscle while the child is in the mother's womb. Congenital ptosis is hereditary in three main forms.

Causes of congenital ptosis remain unknown. Ptosis may be caused by damage to the muscle that raises the eyelid, damage to the superior cervical sympathetic ganglion or damage to the oculomotor nerve, which controls the muscle. Such damage could be a sign of an underlying disease such as diabetes mellitus, a brain tumor, a pancoast tumor (apex of the lung) and diseases that cause weakness in muscles or nerve damage, such as myasthenia gravis or oculopharyngeal muscular dystrophy. Exposure to the toxins in some snake venoms, such as that of the black mamba, may also cause this effect.

Ptosis can be caused by the aponeurosis of the levator muscle, nerve abnormalities, trauma, inflammation or lesions of the lid or orbit. Dysfunctions of the levators may occur as a result of autoimmune antibodies attacking and eliminating the neurotransmitter.

Ptosis may be attributable to a myogenic, neurogenic, aponeurotic, mechanical or traumatic cause, and it usually occurs in an isolated manner. However, it may be associated with various other conditions, such as immunological, degenerative or hereditary disorders as well as tumors or infections.

Acquired ptosis is most commonly caused by aponeurotic ptosis. This can occur because of senescence, dehiscence or disinsertion of the levator aponeurosis. Moreover, chronic inflammation or intraocular surgery can lead to the same effect. Also, wearing contact lenses for long periods is thought to have a certain impact on the development of the condition.

Congenital neurogenic ptosis is believed to be caused by Horner's syndrome, in which a mild ptosis may be associated with ipsilateral ptosis, iris and areola hypopigmentation and anhidrosis caused by paresis of the superior tarsal muscle. Acquired Horner syndrome may result after trauma, neoplastic insult or even vascular disease.