Acute loss of vision can occur from three broad categories that include:
opaque ocular media (eg. diffuse corneal edema or cataracts)
optic pathway disease
Many ocular diseases (eg, anterior uveitis Anterior Uveitis in Small Animals Aqueous flare and cell are observed with fibrin admixed with blood and white cells in the ventral anterior chamber in the right eye of a cat. Anterior uveitis or iridocyclitis is commonly diagnosed... read more and keratitis Cornea The size of the nearly round to oval cornea (vertical/horizontal) varies by animal species: dog (8.5 × 9.5 mm), cat (8.4 × 8.9 mm), horse (16.6 × 17.9 mm), and cow (15.2 × 16.4 mm). The animal... read more ) may cause vision loss to the affected eye because of loss of transparency to the cornea, anterior chamber, and/or lens. If the opposite eye is blind, then the patient may present with vision loss, although the owner may also notice the change in the appearance of the eye. Thus, this discussion focuses on acute, bilateral vision loss when the ocular media is clear.
A complete history is critical because ivermectin can cause retinal toxicity as well as central blindness in dogs and cats and enrofloxacin can cause acute retinal degeneration in cats. For acute vision loss, a thorough neuro-ophthalmic examination that assesses the menace response, dazzle and palpebral reflexes, and pupillary light reflexes (PLRs) is critical. Because large amounts of the retina must be involved, a fundic examination is also important, to rule out diffuse retinal detachment or progressive retinal degeneration. Lesions of the optic nerve can also cause blindness, and the optic nerve head may appear normal if retrobulbar lesions are present.
If the retina appears normal and the patient is blind, then referral to an ophthalmologist is recommended. The ophthalmologist should perform an electroretinogram to diagnose sudden acquired retinal degeneration syndrome (SARDS) if retinal function is absent or should strongly suspect optic pathway disease if retinal function is present. If retinal function is normal, then a neurologist may need to perform a thorough neurologic examination, and additional testing may be necessary. For a review of the neural pathway for the menace response and PLR, see the UGA CVM Clinical Neurology and Functional Neuroanatomy website.