As the disease progresses, the owner may notice the dog's pupils remaining dilated, as the eye attempts to take in more light. This is due to the retina reflecting light back out through the pupil. As it loses its layers of rods and cones, the underlying tissue allows light to be reflected, instead of absorbed. This is called hyperreflectivity. The eyes may resemble those of a cat, which normally has a very reflective retina.
Diagnosis is made by doing an ophthalmic examination of the eye.
The veterinarian looks for the increased reflectivity of the retina, and a decrease in the numbers and size of blood vessels supplying the retina. An ERG can be used to diagnose PRA before the characteristic changes are seen on ophthalmic exam. While useful, ERG is usually performed under general anesthesia, and is only available at large veterinary teaching hospitals. The cause of the retinal degeneration is not yet understood. However, the genetic nature of the condition is well described.