The length of the fissure is generally defined as the straight line distance from the point of the inner canthus to the point of the outer canthus, which can be measured. The eye fissure, also known as the eyelid fissure, is what is commonly referred to as the eye slit. The front of the eye is covered by the eyelid, which is divided into upper and lower parts, and the gap formed between the upper and lower eyelids is the eye fissure. The length of the fissure is the straight line distance between the medial and lateral canthus points, which are the inner and outer points of the upper and lower eyelids. The length of the fissure varies among individuals and is related to a number of factors such as age, gender, race, lifestyle, and genetics. In adults, the length of the fissure is about 20 to 30 mm, and the width at the widest point is about 10 to 15 mm. If the size of the fissure does not fall within this range, it may be abnormal, and the fissure can usually be observed with the naked eye. Diseases that may lead to an abnormal eye fissure include true microphthalmia, extraocular muscle trauma, upper oblique muscle paralysis in both eyes, chemical eye trauma, and congenital paronychia. It is recommended that patients with abnormal ocular fissures should seek timely hospitalization to evaluate their condition and follow the doctor’s instructions for treatment.