Persistent pupillary membranes, or PPMs as they are often called are common findings on CERF examinations. They may or may not be a problem in a breed and/or individual dogs. PPMs are remnants of a fetal structure called the pupillary membrane. This membrane covers the pupil before an animal is born. It is part of the blood supply to the developing lens (the structure in the eye that focuses light on the retina). Normally the pupillary membrane completely absorbs before birth in foals and calves but is partially present and continues to disappear in neonatal dogs. Absorption may not be complete in puppies when the eyes first open and small strands or a web-like structure may be seen across the pupil. These strands normally disappear by four to five weeks of age. In some dogs these strands do not disappear and become PPMs.
PPMs may be found in several configurations in the anterior chamber (see figure). They may span across the pupil (iris to iris), from the iris to the lens, from the iris to the cornea, or they may float free on one end, only connected to the iris. In general, iris to iris PPMs cause no problems. They may be single strands or a forked structure. These PPMs may break and become less prominent as the puppy gets older, but they usually do not disappear completely. Iris to lens PPMs are more problematical. These PPMs cause opacities (cataracts) at the point where they are attached to the lens capsule. The cataracts do not usually progress and cause only minor visual deficits. Iris to cornea PPMs cause opacities on the cornea due to their ability to damage the corneal endothelium (the inner lining of the cornea). These opacities may be small or may be severe due to the development of corneal edema (fluid in the cornea). Severely affected puppies (with numerous strands) may be blind (they may improve as they get older). The strands may regress but do not disappear.
PPMs are found in many breeds of dog. In most of these breeds, iris to iris PPMs are classified by CERF as a "breeder option" problem. This means that most of the PPMs which have been reported in these breeds have been small and are probably sporadically occurring and not hereditary defects. Dogs with these small iris to iris PPMs who have been bred have not been reported to have puppies with vision problems. This does not mean that problems will never occur in these breeds. Owners with dogs diagnosed with PPMs should be aware of the situation and should probably either not breed affected dogs or should breed the affected dogs only to unaffected dogs.
In some breeds, PPMs are known to be hereditary and puppies who have any type of PPM will not receive a certification number. The Basenji is the most well known but CERF will also not certify Chow Chows, Mastiffs, Pembroke Welsh Corgis, or Yorkshire Terriers with PPMs. Members of these breeds have been shown to produce offspring with blindness directly associated with their PPMs. In these breeds, the mechanism of inheritance is not known but breeding any of these dogs with PPMs is highly discouraged.