The eye fissure is the gap between the upper and lower eyelids, which is commonly referred to as the eye slit. The most important factor affecting the size of the eye fissure is the inner canthus, followed by the outer canthus. The inner canthus is an additional half-moon skin fold that connects the upper and lower eyelids, covering the intersection of the upper and lower lines of the inner canthus, the inner lake, and the meatus, making the inner corner of the eye at an obtuse angle, giving the impression of a wider orbital distance and a shorter eye fissure. The eye fissure is the crack formed between the upper and lower eyelids. A small eye fissure is a narrow eye slit, and there are several general causes of a small eye fissure that need attention when diagnosing. 1. Genuine microphthalmia. 2, trauma to the extraocular muscles. 3.Superior oblique muscle paralysis of both eyes. 4, Chemical ocular trauma. 5, Congenital parametrial ankylosis.