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Corneal Endothelial Disease


CORNEAL ENDOTHELIAL DISEASE IN DOGS

Endothelial disease causes oedema or excess fluid in the cornea. To understand what is
going on a microscopic level it is important to understand the normal microscopic anatomy
and function of the cornea.


The cornea consists of three layers:


* EPITHELIUM: the outer layer of the cornea represents approximately 10% of the total
thicknesss. It consists of three layers of cells and is the outer protecting coat for the cornea.

* STROMA: the middle and thickest of the three layers forms around 90% of total thickness.
It is composed of hundreds of layers of collagen laid down in very regular sheets. Between
the collagen fibres is a ground substance (water and chemical compounds called GAGS)
which is osmotically active, ie water from inside the eye can be"sucked" into the corneal
stroma where concentration of these chemicals is higher.

* ENDOTHELIUM: is a single layer of cells on the inside of the cornea. The cells are laid
down in a very regular array like a mosaic of hexagonal shaped tiles. The endothelial cells
have little pumps in their membranes which normally move water out of the corneal stroma
back into the front chamber of the eye. Endothelial cells cannot regenerate - the cells present
at birth remain throughout life but may degenerate with age.

The endothelium will continue to function normally until the density of the cells on the
endothelial surface gets below a certain critical level. The overall effect of this is a net
movement of water into the cornea. The water gets between the collagen layers in the stroma
causing abnormal bending of light and a whitish blue opacity will appears as the stromal
oedema develops. When the problem gets advanced, water can break out through the
corneal epithelium, the outer layer causing painful chronic superficial ulcers.

The endothelium can lose its normal function in a number of ways


* natural loss of endothelial cells and decline in density with age.


* some breeds of dogs eg Boston terriers, Chihuahuas, Springer Spaniels seem to have a
natural breed-related dystrophy which results in abnormal function at an earlier age than
other dogs.


* damage by inflammation: when the interior of the eye becomes inflamed, inflammatory
cells and proteins deposit on the endothelial surface and damage it causing loss of normal
water pumping function and oedema


* glaucoma: very high pressure inside the eye can damage the endothelium


Endothelial disease is a very difficult disease to treat in dogs as there are no simple solutions
to replace the abnormally functioning endothelial surface. Complete full thicknesss grafts of
normal cornea from other donor dogs do not work well like they do in people. There are few
drugs which can help to reduce the oedema: at best these drugs will stabilise the problem. If
the disease becomes end stage then two alternative procedures can be tried, but these never
return the cornea to normal clarity:


* thermal keratoplasty: applying tiny multiple burns to the outer layers of the stroma to
create scars or adhesions between the collagen layers and reduce their water holding
capacity


* Gunderson's flap: a very thin layer of conjunctiva is dissected from the edge of the eye and
pulled across the corneal surface. The blood vessels in the conjunctiva help to improve the
egress of water from the water from the cornea. The veterinary ophthalmologist will discuss
the appropriateness of each of these procedures in your dog's case.

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