Page 1178 - Problem-Based Feline Medicine

1170 PART 15 CAT WITH EYE PROBLEMS

epithelium from the rich vascular supply of the be blind. Animals with central blindness generally
choroid, to cause retinitis and retinal detachment. have normal pupil reflexes, although they may be
● Systemic conditions that affect the vascular slow.
system can cause changes in the retina. Systemic
Conditions that affect the visual cortex include:
hypertension causes inner retinal hemorrhage and
● Ischemia from oxygen starvation or trauma.
detachment. Anemia, polycythemia vera and
● Vascular accidents causing ischemia.
hyperviscocity syndromes can cause vision
● Inflammation of brain or meninges.
problems.
● Neoplasia of cerebral or meningeal tissue, causing
● Glaucoma (increased intra-ocular pressure) causes
pressure-related cell degeneration.
retinal degeneration by complex mechanisms that
are not well understood, but include retinal vascular Usually other neurological signs are present suggest-
ischemia, and pressure-related changes in the axo- ing there is a central lesion such as seizures, proprio-
plasmic flow of optic nerve fibers. ceptive deficits, ataxia and weakness.
The optic nerve connects the eye to the midbrain.
Inflammation of the optic nerve and conditions that WHERE?
affect the anterior brain and/or midbrain will cause
Structures that may be involved in blindness include:
vision loss, by preventing stimuli from the ganglion
● The ocular media (cornea, anterior chamber, lens
cells from reaching the central processing area of the
and vitreous).
visual cortex. Efferent nerve fibers controlling pupil
● Retina.
function will usually be affected causing changes in
● Optic nerve.
pupillary light reflexes. See the main references on
● Mid-brain.
page 1279 “The Cat With Abnormal Pupil Size, Shape
● Visual cerebral cortex.
or Response” and optic neuritis is a rare condition in
the cat. Causes of optic nerve disease in cats are usually
confined to: WHAT?
● Inflammation or neoplasia affecting the optic
Changes to the ocular media appear as a cloudy eye.
chiasm.
See the main reference on pages 1254, 1233 “The Cat
● Inflammation or neoplasia of the mid-brain.
With a Cloudy Eye” and “The Cat With Abnormalities
Midbrain and visual cortex lesions can present with Confined to the Cornea”.
blindness and normal-appearing eyes.
Most cats with sudden blindness have a retinal
Sometimes ocular lesions are present, but do not detachment caused by infectious agents or hyper-
account for the blindness. tension.
Pupil size and pupillary light reflexes help localize the Cats that have blindness associated with abnormal
site of the lesion causing blindness. pupil reflexes and normal retinas generally have optic
● If the lesion is at the cortical level, there will usu- nerve or mid-brain disease.
ally be a normal to slow pupil light reflex.
Blind cats with normal retinas and normal pupil
● If the lesion is anterior and involves the optic chi-
reflexes usually have central blindness, with lesions
asm, there will be a loss of pupillary light reflex.
involving the visual cortex.
● If the lesion is in the mid-brain, there will be
anisocoria (uneven-sized pupils). Pupillary light reflexes form an important part of eye
examinations, and help determine the part of the visual
The visual cortex occupies approximately one third of
system that is causing blindness.
the cerebrum. Vision is an extremely complex process
that is not fully understood. The ocular structures may The summation of retinal electrical responses to light
be perfectly normal, but if the brain cannot process the can be measured electrophysiologically using a proce-
information sent from the eye, the animal will appear to dure known as electroretinography (ERG).