Palpebral fissure
Palpebral fissure is the anatomic name for the separation between the upper and lower eyelid
An eyelid is a thin fold of skin that covers and protects an eye. With the exception of the prepuce and the labia minora, it has the thinnest skin of the whole body. The levator palpebrae superioris muscle retracts the eyelid to "open" the eye. This can be either voluntarily or involuntarily...

s. In adults, this measures about 10mm vertically and 30mm horizontally.

It can be reduced in horizontal size by fetal alcohol syndrome
Fetal alcohol syndrome
Fetal alcohol syndrome is a pattern of mental and physical defects that can develop in a fetus in association with high levels of alcohol consumption during pregnancy. Current research also implicates other lifestyle choices made by the prospective mother...

 and in Williams Syndrome. The chromosomal conditions Trisomy 9
Trisomy 9
Trisomy 9 is a chromosomal disorder caused by having three copies of chromosome number 9. It can appear with or without mosaicism.-Characteristics:...

 and Trisomy 21 (Down Syndrome) can cause the palpebral fissures to be upslanted, while Marfan Syndrome can cause a downslant.

The fissure may be increased in vertical height in Graves' disease
Graves' disease
Graves' disease is an autoimmune disease where the thyroid is overactive, producing an excessive amount of thyroid hormones...

, which is manifested as Dalrymple's sign
Dalrymple's sign
Dalrymple's sign is a widened palpebral opening, or eyelid spasm, seen in thyrotoxicosis , causing abnormal wideness of the palpebral fissure. As a result of the retraction of the upper eyelid, the white of the sclera is visible at the upper margin of the cornea in direct outward stare...

This is one of the numerous symptoms of genetic disorders like Cri-du-chat.

In animal studies, using four times the therapeutic concentration of the ophthalmic solution Latanoprost (brand name Xalatan), the size of the palpebral fissure can be increased. The condition is reversible. Latanoprost is a prostaglandin F receptor agonist.

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