ANIRIDIA

CONGENTIAL ANIRIDIA:
         Aniridia means the absence of the iris (the coloured part of the eye that causes us to call people "blue-eyed" or "brown-eyed"). Although the iris appears to be absent when examined, it has been shown what remains  in cases of aniridia is merely a thick collar of tissue around the outer edge. The muscles which open and close the pupil are absent.
        Aniridia is a hereditary, developmental defect often associated with additional anomalies. In some cases, there may be a cataract (cloudiness of the lens), under-development of the fovea (the minute area in the centre of the macula which contains the most closely packed cells for our finest sight), persistent hyaloid artery (a vessel which normally disappears before birth, but which can interfere with good vision when it persists after birth), and/or glaucoma (increased pressure within the eye).
        Decreased visual acuity (poor resolution of detail), photophobia (extreme sensitivity to light) and field loss (decreased angle of vision) often result from aniridia.   Other losses are more likely consequences of the other defects that may accompany aniridia.
        Vision may fluctuate depending on lighting conditions and glare. As a result, lighting conditions in the class will need to be controlled. Teachers should not stand in front of a window or other light source when speaking to the student, and extra time to adapt to changes in lighting should be allowed. Paper copies of overhead transparencies or blackboard work can be helpful.

Adapted from "Aniridia" by G.H. Marshall and Low Vision by Nancy Levack

acquired aniridia:

This  condition arises either from a latent congenital defect such as the Axenfeld Rieger (essential iris atrophy syndrome) or from injury to the eye in which all or part of the iris is damaged.