Why is my senior cat's one eye dilated and unresponsive to light?

There is anisocoria (the two pupils are unequal in size) and possibly ptosis of the upper eyelid (the eyelid cannot open normally) on the left side (where the pupil is bigger).
There are many causes that can provoke this presentation:
-Corneal injury such as an ulcer.
-Disease or injury to the brain or to the nerves running to the affected eye, such as Horner’s syndrome.
-Glaucoma, a disease in which there is increased pressure within the eye (the pupil in the affected eye will be dilated).
-Uveitis, or inflammation of the interior of the eye (the pupil in the affected eye will usually be constricted).
-Retinal disease.
-Scar tissue formation between the iris and the lens (called posterior synechia), a condition that may develop following uveitis.
-Iris atrophy (a decrease in the amount of tissue within the iris) usually a degenerative change associated with aging.
-Congenital defect of the iris, in which the iris tissue does not develop properly.
-Cancer within the affected eye.
-Spastic pupil syndrome - a syndrome that may be associated with feline leukemia virus infection.
-Other infectious diseases such as feline immunodeficiency virus or toxoplasmosis.
Unfortunately is difficult to be more specific than that and a clinical examination at your local veterinarian is essential to confirm these clinical suspicions and treat appropriately. Antibiotics may be needed.
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