Anisocoria is defined as unequal pupil size, so your vet was correct. The treatment depends on the cause of this symptom, and it might be best to consult with a veterinary ophthalmologist.
I answered a similar question in May, 2007 and I have posted it below:
Cornell College of Veterinary Medicine lists 54 causes of feline anisocoria.
Your cat's only symptom right now may be anisocoria, but other symptoms could develop over time,
The list of causes can be summarized into the following broad categories: infection (bacteria, fungal, or, viral), toxin ingestion, brain neoplasm (cancer), congenital or hereditary defects, insect infestation (bot, or cutebra), traumatic injury to the head, vitamin deficiency, autoimmune, and others.
The following is the complete list of 54 causes of feline anisocoria pasted from Cornell College of Veterinary Medicine Consultant list:
The 54 Possible Causes for Anisocoria:
o Babesiosis, babesia, in dogs and cats
o Bacterial, vegetative, infective, endocarditis in dogs and cats
o Blastomycosis, blastomyces dermatitidis, in dogs and cats- zoonosis
o Blood transfusion reaction in dogs and cats
o Bromethalin toxicity in dogs and cats
o Cerebellar or pericerebellar abscess
o Cerebellar or pericerebellar brain neoplasia, tumor, cyst
o Cerebellar trauma, hematoma, or ischemia
o Cerebral or pericerebral brain neoplasia, tumor, cyst
o Chronic corneal epithelial defects, indolent ulcer
o Coccidioidomycosis, coccidioides immitis in dogs and cats, valley fever- zoonosis
o Congenital and/or hereditary deafness in dogs and cats
o Congenital retinal detachment in puppies, kittens, and foals
o Cryptococcosis in dogs and cats- zoonosis
o Cuterebra, dermatobia, infection in dogs and cats
o Dysautonomia or dilated pupil syndrome in dogs and cats
o Feline immunodeficiency virus, fiv
o Feline infectious peritonitis, fip, enteritis, coronavirus, fcov
o Fungal, mycotic keratitis, keratomycosis
o Glaucoma in the cat
o Hemifacial spasm in dogs and cats
o Hydrocephalus, hydranencephaly
o Idiopathic tonic pupil, pupillotonia, in dogs and cats
o Injury, trauma, to optic disk (disc) or nerve
o Intraocular parasite, ophthalmomyiasis interna, in dogs and cats
o Ischemic encephalopathy in cats
o Keratoconjunctivitis sicca in dogs, cats, and horses
o Lens induced, phacoclastic, phacolytic, uveitis in dogs and cats
o Lens luxation, subluxation, displacement, dislocation in cats
o Lymphosarcoma, lymphoma in cats, feline leukemia virus, felv
o Meningoencephalitis, meningitis, meningoventriculitis
o Metastatic ocular neoplasia in dogs and cats
o Muscular dystrophy in cats
o Neurotrophic, neuroparalytic, keratitis
o Ocular, corneal, trauma, injury, proptosis of the eye, orbit
o Optic nerve hypoplasia
o Orbital, retrobulbar neoplasia, cyst in dogs and cats
o Orbital, retrobulbar, cellulitis, abscess, foreign body in dogs and cats
o Otitis externa, media, interna, para-aural abscesses, in dogs and cats
o Peripheral vestibular neoplasia in dogs and cats
o Ponto-medullary, brain stem trauma or thrombosis, hematoma
o Ponto-medullary, brain stem, pituitary, inflammation, abscess
o Ponto-medullary, brain stem, pituitary, neoplasia, cyst
o Portosystemic shunts, hepatic microvascular dysplasia, in dogs and cats
o Prenatal or perinatal asphyxia
o Progressive retinal atrophy, degeneration, in dogs and cats
o Pseudorabies, aujeszky's disease, herpesvirus 1 in species other than pigs
o Pupillary dilation resembling pourfour du petit syndrome in cats
o Snake bites, snakebites
o Thiamine, vitamin b1 deficiency in dogs and cats
o Toxoplasmosis, toxoplasma gondii, in cats- zoonosis
o Trigeminal neuropathy, idiopathic neuritis, in dogs and cats
o Ulcerative canine or feline keratitis, corneal ulcer
o Urinary incontinence, feline genitourinary dysplasia, urethral hypoplasia
