Degenerative iris atrophy is usually an age-related thinning of the iris sphincter muscle that affects the ability to constrict the pupil. Decreased pupillary light reflex/response occurs despite normal vision. Both direct light response and indirect response from the opposite eye are abnormal (decreased constriction). The iris stroma may also be affected, causing dyscoria, thinning of the iris stroma, occasional color changes within the iris (dark areas may be seen if the underlying pigment is exposed), as well as areas of iris translucency or actual holes in the iris. It most often occurs in small-breed dog such as the miniature poodle or Yorkshire terrier
- Incomplete pupillary light reflex, accompanied by a normal menace response (the reflex to close the eyes)
- Unequal pupil sizes (anisocoria)
- Irregular, scalloped edge to the pupillary margin
- Thin or absent areas of the iris on transillumination
- Strands of iris occasionally remain, spanning across portions of the pupil
- Holes within the iris stroma - black spots that may resemble additional pupils
- Swelling (edema) of the cornea
The condition may be unilateral or bilateral, and is often asymmetrical. As the condition advances, the owner may notice dilation of one or both pupils. Sensitivity to light, particularly when the dog is outside in bright sunlight, can also occur.
Examination of the iris under magnification is often required to confirm the diagnosis. Slit-lamp examination is also helpful to rule out secondary causes such as uveitis or chronic glaucoma. Measurement of intraocular pressure should also be performed. Retroillumination may identify areas of iris thinning. Demonstration of normal vision and the absence of retinal abnormalities that could cause mydriasis (pupil dilation) are also important.
No specific therapy is available.
Prognosis and Monitoring:
Prognosis is good because vision is unaffected. Periodic monitoring can be performed to evaluate progression of the condition and/or development of iris atrophy in the opposite eye.