Persistent Pupillary Membranes

or PPMs

PPMs are remnants of a structure called the pupillary membrane which covers the pupil before a puppy is born. The membrane carries part of the blood supply for the developing lens which focuses light on the retina in the back of the eye. The pupillary membrane continues to disappear in puppies before they are born and soon after but it may not be completely gone when the eyes first open. Sometimes small strands or web-like structures may be seen across the pupil. These strands usually disappear by four to five weeks of age. In some dogs the strands persist and are called PPMs.

PPMs may be found in several configurations in the anterior chamber. They may be attached on the iris and float free on the other end, go across the pupil from the inner side of the iris to other inner side of iris as either a simple string or a forked string, go from the iris to the lens, or go from the iris to the cornea.

In general, iris to iris PPMs cause no problems. They may be simple single strands or a forked structure or “Y” shaped structure. These PPMs may break and become less prominent as the puppy gets older, but they usually do not completely disappear.

Iris to lens PPMs are of more concern. These PPMs cause cataracts (cloudy areas) at the point where they are attached to the lens capsule. The cataracts do not usually grow and cause only minor visual deficits throughout the life of a dog.

Iris to cornea PPMs cause opacities on the cornea, the whites of the eye, due to their ability to damage the inner lining of the cornea. These opacities may be small or severe due to the development of the fluid in the cornea. Some affected puppies may have many strands and be blind. Their eyesight may improve as they get older because the strands may regress although they do not disappear completely.

PPMs are found in many breeds of dogs and are believed to be hereditary in Basenjis, Chow Chows, Mastiffs, Pembroke Welsh Corgis, and Yorkshire Terriers. Members of the aforementioned breeds have been shown to produce offspring with blindness associated with their PPMs. The exact mechanism of inheritance is not known but breeding dogs of those breeds with iris to lens or iris to cornea PPMs is discouraged.

Types of PPMs: -
a. One end of PPM floating in anterior chamber - no clinical significance
b. Single iris to iris band - no clinical significance
c. "Y" shaped iris to iris band - no clinical significance
d. Iris to lens - results in capsular cataract
e. Iris to cornea - results in adherent leukoma of the cornea