The pupil refers to the round hole in the center of the iris. The diameter of the normal pupil varies with the intensity of light, generally about 2 mm in diameter in newborns and older adults, and 4 to 5 mm in adults. pupillary abnormalities refer to abnormalities in pupil size, position, shape, and reflex to light. The pupil is regulated by parasympathetic fibers (the sphincter that governs the pupil and can constrict the pupil) and sympathetic fibers (the muscle tissue that governs the dilated pupil) and can change at any time with narrowing or dilation. When the sympathetic nerves are excited during panic or pain, the pupil can dilate, while when the parasympathetic nerves are excited, the pupil narrows. Pupil narrowing is common in various diseases, especially brain diseases and neurological diseases. 1. Cerebral vascular sclerosis Pupillary constriction is usually unilateral, depending on the location and extent of vascular damage. Other symptoms include blurred vision, slurred or aphasic speech, loss of muscle tension, vertigo and headache. 2. Cluster headache Severe cluster headache is often accompanied by ipsilateral pupillary constriction, tearing, conjunctival congestion and ptosis. It is also associated with facial redness and sweating, bradycardia, irritability, nasal congestion or rhinorrhea. 3. Corneal foreign body The affected eye shows pupil narrowing with pain, foreign body sensation, mild vision loss, conjunctival congestion, photophobia and tearing. 4. Corneal ulcer The affected eye has a narrow pupil with moderate pain, blurred vision, possible partial loss of vision, and diffuse conjunctival congestion. 5. Horner’s syndrome This neurological condition in which moderate pupillary constriction is common and occurs ipsilateral to the spinal cord lesion. Associated ipsilateral manifestations include dull pupillary reflexes, mild entropion, moderate ptosis, facial anhidrosis, temporary conjunctival congestion, and vascular headache. If this syndrome is congenital, the iris on the damaged side will become shallow. 6. Anterior chamber hemorrhage The anterior chamber hemorrhage is usually the result of a blunt injury and can cause pupillary constriction with moderate pain, blurred vision, diffuse conjunctival congestion, and mild eyelid swelling. The eyeball becomes harder than normal. 7. Iriditis (acute) The affected eye shows typical pupillary constriction with reduced pupillary reflex, severe ocular pain, photophobia, blurred vision, conjunctival hemorrhage and possible pus accumulation in the anterior chamber. The eye becomes cloudy and the iris is bulging. Pupillary constriction is seen during eye examinations.