There have been several posts or comments from visitors throughout the website asking questions about their cat’s eyesight. The most common is that the pupils of their cat’s eyes do not change, constrict or become smaller, when the light becomes brighter. The pupil is the hole located in the center of the eye that lets light into the eye to fall on the retina at the back of the eyeball. The iris is the part of the eye that controls the size and shape of the iris. When the pupil is fixed despite changes in light levels, what is happening?
For me the obvious reason why the pupil does not contract when light falls onto the eye is because the light is not registering with the cat’s nervous system and therefore the natural reflex to narrow the pupil is not instigated.
The obvious reason why the nervous system has not registered light is because the retina is not working properly due to retinal disease such as retinal atrophy. Another reason is that something is wrong with the optic nerves that run from the retina to the brain. If the retina is working but the optic nerve is not functioning for whatever reason the brain will not receive the signals that is the image formed by the retina and lens.
The layperson’s test for checking for blindness is to shine a bright light into a cat’s eyes to test for pupillary constriction. If the pupils do not constrict I think you can make an initial diagnosis of some sort of blindness. This though is not an exact test for blindness. Apparently the pupil may contract due to a simple reflex but that does not mean the cat is seeing. However, I would have thought that if the pupil does not contract the first conclusion is blindness to some degree.
That initial assessment could be backed up by other observations such as:
- In low light a cat with impaired vision might bump into furniture and
- hold their head close to the ground to employ their sensitive whiskers to feel what is before and around them or
- may hesitate to go out if she went out before or
- be unwilling to jump on or off furniture.
It would seem that retinal degeneration is perhaps one of the most common causes. In some purebred cats it is a genetically inherited disease. Bengal cats and Abyssinians can have the problem (see Progressive retinal atrophy (PRA) in Bengal cats). Some authors also say it can affect Siamese and Persian cats too.
Another disease of the retina is retinitis. This is inflammation of the retina which leads to its degeneration. There are a number of causes including feline infectious peritonitis (FIP) and toxoplasmosis. Pulmonary hypertension can cause the retina to detach. If a cat has a deficiency in his diet of taurine it can result in retinal degeneration of the central part of the retina. The cat will have some peripheral vision. The same sort of partial blindness can be caused by an antibiotic called enrofloxacin.
Cataracts are obvious. Anyone can tell if a cat has cataracts. This is because the lens of the eye becomes opaque (cloudy rather than clear). Cats have very clear bright eyes usually. Cataracts will impair vision in varying amounts.