The pupil is the opening in the centre of the coloured part of the eye (iris) that constricts and dilates to regulate the amount of light entering the eye.
     Not infrequently, children are born missing the bottom section of their pupil (iris coloboma). Iris coloboma may be  associated with a defect in the back part of the eye, and less frequently defects in other body systems.  However, most children with iris coloboma are healthy.
     Adies pupil is common.  Patients with Adies pupil are frequently young healthy women who initially develop a large pupil on one side in the absence of a droopy eyelid or double vision.  The pupil constricts slowly with light, but better when the patient fixates on a near object.  In general, no treatment is required for Adies pupil, except some patients require a temporary adjustment in their glasses to help them read.  With time Adies pupils will become smaller, and the other pupil may become involved.
Red top eye drops, scopolamine patches, plants (e.g. jimsonweed), and some asthma sprays can dilate the pupil  Some migraine patients develop an enlarged pupil  If someone is hit in the eye very hard an irregular or large pupil may result (traumatic mydriasis).
Patients with a dilated pupil associated with a droopy eyelid, and decreased eyeball movement may have a 3rd nerve palsy. (see above section)  A dilated pupil with a painful eye and misty cornea occurs with angle closure glaucoma.  After eye surgery (e.g. cataract surgery), the pupil may appear a different size or shape.
A new onset droopy lid associated with a small pupil on the same side may indicate Horners syndrome.  (If children are born with Horners syndrome, with time the iris is usually lighter colored on the side with the small pupil.)    Intraocular inflammation (iritis) may cause a small pupil due to a “sticky” iris.  Patients with iritis usually have a red painful eye and aversion to light.