The eye has been called
the window to the soul. For
cat owners and veterinarians, however, the eye is often a window to the
rest of the cat as well. Cats
are fortunate to have relatively few primary eye disorders, however, they
often experience eye problems associated with other systemic illnesses,
such as viral and fungal infections, and high blood pressure.
humans as well as animals, the role of the visual system is to collect
light and focuses it onto the retina, where specialized cells convert this
light energy into nerve impulses. The
amount of light that passes through the cornea is controlled by the pupil. Similar to the aperture of a camera, the pupil adjusts to
different light levels by dilating to let in more light during dim
conditions, or by constricting to limit the amount of light in bright
conditions. When light
strikes the retina, light-sensitive cells called photoreceptors are
stimulated, causing them to produce impulses that travel to the brain.
nocturnal creatures, cats are more sensitive to light than humans.
While they can’t see in total darkness, cats require only
one-sixth the amount of light as that of a person to see.
Their pupils can dilate three times larger than a human’s, and
the cornea is bigger, allowing more light in. The feline retina also
contains reflective cells that amplify the light coming into the eye.
These reflective cells (the “tapetum”) are responsible for the
“glowing appearance of the eyes when light strikes them.
in cats is geared to assist in hunting . Being predators, their eyes are
placed on the front of their head. This
results in a larger area of binocular vision, allowing for more accurate
depth perception and coordination of body movements with visual events.
Cats, however, cannot see detail very well.
acuity is the ability to see the detail of an object separately and
clearly. A person with “20/20” vision can discern the details of
an image (such as letters on a chart) perfectly from 20 feet away.
Applied to animals, dogs are said to have a visual acuity of 20/75.
The average cat is believed to have a visual acuity between 20/100
and 20/200. Simply put, cats
can see color, but they do not have as many color-sensitive photoreceptors
as humans. Colors that would appear to be very rich to us are more
pastel-like to the cat. Cats
respond to the blue and yellow wavelengths best, but have trouble with
green and red. What appears
to us as “red” is simply “dark” to cats.
A fraction of the green spectrum in cats is indistinguishable from
white. Cats would see a green, grassy lawn as a whitish lawn, and a
green rosebush with red roses would appear as a whitish bush with dark
flowers. Cats, however, are
very good at distinguishing many different shades of gray.
in the appearance of cats’ eyes are usually apparent to cat owners.
If left untreated, many eye conditions can lead to visual
impairment or even blindness, so any abnormality should be reported to a
veterinarian. A thorough history is important before any eye exam. For
example, outdoor cats are more likely to be exposed to trauma or foreign
bodies than indoor cats. The
general medical history is also very important, because many systemic
diseases can manifest through the eye.
Once at the veterinarian, the eyes can be examined in a systematic
fashion. Ideally, the eye
exam takes place in a quiet area to minimize excessive eye movement.
The lights should be dimmed to eliminate distracting reflections
from the surface of the eye. A
bright, focused light source is necessary for a proper eye exam
of the conjunctiva are the most common eye conditions affecting cats. The conjunctiva is the membrane that lines the lids and
covers the eyeball. Inflammation
of this structure is called conjunctivitis and is often a symptom of a
viral infection, especially the upper respiratory viruses (herpes and
calici), or agents such as chlamydia (Picture: severe bilat conjunc.jpg).
Herpes conjunctivitis is usually bilateral, whereas chlamydia often
begins in one eye and progresses to bilateral involvement in about a week.
Kittens are particularly susceptible.
Dr. Anne Sinclair is the owner of Cat Sense, a feline-exclusive
veterinary hospital in Bel Air, Maryland.
“I encounter a lot of conjunctivitis, especially in orphaned
kittens”, notes Dr. Sinclair. “Fortunately,
most recover with no problems, although I’ve had a few kittens lose
sight in one eye from severe herpes infections”.
“third eyelid” is a membrane located in the inside corner of the eye. It produces a portion of the tear film, helps distribute this
tear film over the surface of the cornea, and protects the cornea from
damage. Elevation of
the third eyelid on one side only is often a sign of local irritation or
trauma to that eye. General
malaise from a variety of well-defined or vague systemic illnesses can
cause bilateral elevation of the third eyelid (often described by cat
owners as a “film over the eyes”). Whenever this symptom appears, a
thorough ocular and systemic examination is warranted.
Therapy is directed at the basic illness, as elevated third eyelids
do not interfere with vision.
diseases, most notably corneal abrasions or ulcers, are common in the cat. Superficial ulcers can occur as a result of trauma, such as a
fight with another cat, or as a result of herpes virus infection.
Regardless of the cause, ulcers must be treated promptly.
The feline cornea is only 0.5 millimeters thick.
Delaying therapy for corneal ulcers could result in rupture,
leading to blindness. Special
fluorescent staining techniques can readily identify corneal ulcers.
Treatment generally involves topical antibiotics.
Herpes ulcerations require antiviral drugs.
Occasionally, topical analgesics are necessary, as corneal ulcers
can be quite painful. Dr. Eric Bregman treats many corneal ulcers at his
feline-exclusive veterinary hospital in Williston Park, New York.
“This is a suburban practice, and many of our clients let their
cats go outdoors”, he says. “Cat
fights probably account for the majority of the corneal ulcers I see in
these outdoor cats. Fortunately,
most cases end favorably, although I’ve seen a few eyes rupture after
the ulcer became infected and the cat wasn’t brought here quickly
(opacities of the lens) are relatively uncommon in the cat.
Unlike dogs, cats with diabetes do not develop cataracts.
Congenital cataracts are occasionally seen in kittens, but they
tend to be non-progressive and improvement in vision may eventually
result. Inherited congenital cataracts usually occur in Persian cats or
breeds with Persian ancestry.
with any organ, cancer can occasionally strike the eye.
Ocular cancer can be categorized as primary (originating from the
eye) or secondary (originating from nearby structures or distant
structures). Recently, my own
16-year old blind cat, Ethel, developed an inflammatory condition of her
left eye that got progressively worse and didn’t respond to
anti-inflammatory medication. I
eventually had to remove the eye surgically.
The pathology report confirmed the eye to be cancerous.
Unfortunately, the cancer had spread to nearby lymph nodes.
This type of cancer (lymphosarcoma) often responds to chemotherapy,
however, I’ve elected not to go this route due to her other medical
problems. Fortunately, my
Ethel seems to be made of steel. She
is doing remarkably well despite her unfavorable biopsy report.
eyes are important and delicate organs.
Any minor eye problem that doesn’t clear up quickly (within 24
hours) should be brought to the attention of a veterinarian.
for most eye disorders entails administering drops or ointments.
Drops are often easier to administer, although many drops require
frequent administration. Ointments have the advantage of providing
lubrication and allowing for increased contact time for the medication,
and are especially useful given at bedtime.
Application involves using the thumb or forefinger to gently roll
the lower eyelid downward. Ointment
is then squeezed into the exposed space (called the “conjunctival
sac”), and the eye is opened and closed by hand several times to evenly
distribute the ointment over the eye. Approaching the eye from the outside
corner can prevent the cat from seeing the tip of the tube, making
administration a bit easier. Eye
drops are instilled with the cat’s nose tilted slightly upward.
To prevent contamination, the tip of the dropper bottle or ointment
tube should not be touched by fingers or any other surface, and should not
come into direct contact with the eye.