The palpebral fissure is the elliptic space between the medial and lateral canthi of the two open eyelids.

The chromosomal conditions trisomy 9 and trisomy 21 (Down syndrome) can cause the palpebral fissures to be upslanted,[2] whereas Marfan syndrome can cause a downslant.

[3] An increase in vertical height can be seen in genetic disorders such as cri-du-chat syndrome.

The fissure may be increased in vertical height in Graves' disease, which is manifested as Dalrymple's sign.

In animal studies using four times the therapeutic concentration of the ophthalmic solution latanoprost, the size of the palpebral fissure can be increased.