Why are a cat's eyes always dilated?

The first important point to note is that the question asks why a cat's eyes are ALWAYS dilated. In other words, this is a permanent state of affairs. In this state of affairs, the irises do not contract to a narrow pupil but there are always wide open with a big circular black orb for the eye and almost no visible iris.

Dilated pupils of a cat.

The second point to note is that the muscles of the iris contract to allow much less light through the pupil on a bright day. And they relax to let in more light when the conditions are dark. There is a 135 times difference in the amount of light that the iris allows to pass into the eye between wide open and closed to a slit.

The nerves of the retina send a signal to the brain which sends another signal to the irises to relax or contract depending upon the light conditions.

If the retina is malfunctioning and not able to convert light into an image the brain receives a signal that the light conditions are dim and then directs the iris to open up to allow more light in to the eye.

And if the retina no longer works at all due to a disease such as progressive retinal atrophy or retinitis, then there will be a permanent signal to the brain to say that no light is being received and therefore the brain tells the iris to remain permanently wide open; the muscles of the iris are completely relaxed. Like a camera lens the iris is not going to be 'stopped down'. This, as stated, is a permanent state of affairs.

And behind this situation is a retina which is either malfunctioning or is no longer functioning. And therefore, if they cat's eyes are always dilated, in my view, it means that the cat's retina is malfunctioning and the cat is functionally either going blind or blind.

Progressive retinal atrophy is a gradual degeneration of the retinal cells. This is a disease that is inherited in certain breeds such as Persians, Abyssinians and possibly Siamese. It is caused by an autosomal recessive mutated gene as I understand it. There is no treatment.

Retinitis is a disease in which inflammation of the retina leads to degeneration and destruction of the light respecting cells in the retina. It may occur because of hypertension or perhaps an eye injury or for unknown reasons. High blood pressure or hypertension is one of the most common causes of retinitis. And hypertension is usually associated with hyperthyroidism and/or renal failure.

RELATED: Different pupil size in a cat's eyes - time to visit a vet.

Retinitis is also associated with feline infectious peritonitis, lymphoma, cryptococcosis, feline infectious peritonitis and toxoplasmosis. These are the underlying diseases which can lead to damage to the retina which in turn these to the eyes always being dilated.

It is normal for the eyes of a person or a cat to dilate from time to time when conditions are dark but in my view, they should not be dilated all the time. Something is wrong and a veterinarian needs to be consulted.