Page 1266 - Problem-Based Feline Medicine

1258 PART 15 CAT WITH EYE PROBLEMS

The fluid is called aqueous humor and is produced
INTRODUCTION
by a process of ultrafiltration and active secretion in
the ciliary body. Aqueous exits the anterior chamber
MECHANISM? through the trabecular meshwork in the iridocorneal
angle to reach the vascular system via the scleral ves-
The eye is an organ with various media that normally
sels. The dynamics of aqueous humor production and
maintain clarity for transmission of light by very spe-
drainage is very complex, but simply, it keeps the eye
cialized means.
inflated at a predetermined pressure known as the
● The corneal stoma maintains optical clarity by the
intra-ocular pressure (IOP). Interference of aqueous
highly organized structure of the collagen fibrils,
drainage causes glaucoma. The increased intra-
lack of blood vessels and lack of pigment.
ocular pressure disturbs the corneal physiology result-
● The lens fibers (cells) are also laid down in a
ing in corneal edema and a cloudy eye. See “The Cat
very orderly fashion and contain soluble lens pro-
with Abnormal Globe Position or Size” (page 1312).
teins known as crystallins.
● The aqueous humor is a clear fluid secreted by the Aqueous is a clear fluid that also supplies nutrients to
ciliary body to maintain the anterior chamber. the inner structures of the eye. When the anterior uveal
● The vitreous is a complex, transparent living tis- tract (ciliary body and iris) becomes inflamed this is
sue made mainly from complex carbohydrates. called anterior uveitis. Inflammation of the vascular-
rich uveal tract causes the blood vessels to leak protein
Cloudiness of the eye results from changes in struc-
and white blood cells into the aqueous causing “aque-
ture and composition of the cornea, aqueous (ante-
ous flare”. Accumulations of protein and white blood
rior chamber), lens or vitreous (posterior chamber).
cells may form opaque clots known as “hypopyon”.
This causes a cloudy anterior chamber. See “The Cat
Cloudy cornea With Abnormal Iris Appearance” (page 1294).
For details on the structure and function of the cornea At the same time, anterior uveitis disturbs the
see “The Cat With Abnormalities Confined to the endothelial pump mechanism, allowing aqueous to
Cornea” (page 1236). enter the stroma. This causes edema of the cornea
seen as a cloudy eye.
The cornea becomes cloudy from edema when the
surface develops an ulcer (ulcerative keratitis). Loss
of epithelial cells allows the stroma to absorb fluid. Cloudy lens
This causes the intercellular matrix to swell, which in
A cloudy lens is seen when the lens develops a
turn, disrupts the normal pattern of the keratocytes and
cataract.
the cornea becomes cloudy.
A cataract is defined as an opacity of the lens. Lens
Corneal edema is also seen in cases of glaucoma and
fibers become opaque when they are no longer viable.
uveitis where the endothelial pump mechanism is dis-
A cataract is classified according to the position in the
turbed, so that aqueous enters the stroma from the ante-
lens, and according to whether it is immature or mature.
rior chamber.
The lens consists of an anterior and posterior capsule
Inflammatory conditions in the cornea cause cloudi-
on the outer surface of the cortex. The central part of
ness by disrupting the normal cellular matrix, either with
the lens is known as the nucleus. The lens is suspended
cellular infiltrates or by changing the normal cellular
by fibers known as zonules, which extend from the
structure, such as scar tissue formation.
equator (outer circumference) of the lens to the ciliary
processes. Lens cells are known as lens fibers and are
Cloudy anterior chamber continually produced from the lens epithelium, which
is most active at the equator.
The anterior chamber is a fluid-filled compartment
bounded by the cornea anteriorly and the lens and The lens has no blood supply, and therefore relies on
ciliary body posteriorly. the nutrients in the aqueous humor and the vitreous for