Persistent Pupillary Membrane
A dog's eyes are vulnerable to literally dozens of diseases. One of the more unusual of these is known as Persistent Pupillary Membrane, or PPM.
With this condition, strands of membrane are present in the eye, often crisscrossing it and connecting to various structures within the eye. These strands are leftover remnants of blood vessels that supplied nutrients to the developing eye before the puppy was born. In most cases these vessels deteriorate and disappear before or shortly after birth. When they do not, PPM is the result.
PPM may or may not impair your dog's vision, depending upon the location of the membrane strands. Often these cross from the iris to the cornea, where they can cause cloudiness (opacity) in the cornea. [...]
In normal circumstances, dogs develop in the womb with a thin film called the pupillary membrane covering each eye. The idea is that since the animal is not yet sighted anyway, this won't interfere with vision and is the most efficient way to supply blood to the developing organ. It usually can still be seen once a dog is born but dissolves on its own within the first few days. In some cases, however, this membrane refuses to dissolve and can cause problems; this is known as a persistent pupillary membrane.
The membrane is made up of thin but tough strands of a silk-like film. Depending on how much of the membrane persists after birth, these strands can be located in any number of places and their location determines to a large extent how much of a problem the persistent pupillary membrane will cause. The strands can stretch across the entire pupil, from the pupil to the lens, from the iris to the cornea, or they can be free-floating, attached at only one eye. [...]