The iris is the colored tissue visible in the eye, and its appearance can vary between species and from animal to animal.  It is rich in blood supply and is made up of tissue called stroma, as well as muscles that control the size of the pupil.  Normally, in bright light, the constrictor (sphincter) muscle causes contraction of the pupil, and in low light conditions the pupil dilates.

Causes of Iris Atrophy

Iris atrophy is a normal aging change and a common diagnosis in animals.  The condition can begin to develop in middle-aged to older pets.  Though both can result in iris atrophy, generally it is sphincter muscle atrophy, versus stromal thinning, that develops.  

Thinning of the iris stroma is commonly seen in certain dog breeds such as the Yorkshire Terrier, Toy Poodle, and Chihuahua, as well as the Siamese cat breed.  In advanced stages, this may cause the iris to have a "lacelike" appearance.  Affected animals may appear as if they have more than one pupil (pseudopolycoria).

Signs of Iris Atrophy

Atrophy of the pupillary sphincter muscle causes a decreased ability for the pupil to constrict in response to light.  This results in varying degrees of pupillary dilation - depending on the severity of atrophy.  Since the degree of atrophy may not be symmetric between both eyes at any given time, anisocoria (uneven pupil size) is commonly diagnosed.  This naturally occurring, age-related iris atrophy needs to be differentiated from other more serious disorders including:  glaucoma, Horner's syndrome, uveitis, cranial nerve abnormalities, among other possible causes, all of which that can possibly lead to loss of vision or imply illness in the animal.

In severe iris atrophy, the decreased ability for the pupil to respond to bright light results in an inability for the eye to control how much light is entering into it.  This can cause affected animals to squint in bright light.
 

Treatment

There is no treatment for iris atrophy, as this is generally regarded as a benign condition.

In some instances, avoiding exposure to bright light or sunlight might be recommended (i.e. walking the dog early in the morning and at (or after) dusk instead of times of peak sunshine).