Palpebral fissure
The palpebral fissure is the elliptic space between the medial and lateral canthi of the two open eyelids.  In simple terms, it is the opening between the eyelids. In adult humans, this measures about 10 mm vertically and 30 mm horizontally.

Variations

Congenital dysmorphisms 
It can be reduced (short, "narrow") in horizontal size by fetal alcohol syndrome and in Williams syndrome. The chromosomal conditions trisomy 9 and trisomy 21 (Down syndrome) can cause the palpebral fissures to be upslanted, whereas Marfan syndrome can cause a downslant. An increase in vertical height can be seen in genetic disorders such as cri-du-chat syndrome.

Acquired 
The fissure may be increased in vertical height in Graves' disease, which is manifested as Dalrymple's sign. It is seen in disorders such as cri-du-chat syndrome.

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

See also
 Blepharophimosis

References

External links
 Facial Neurological Examination from University of Toronto

Human eye anatomy