In all canine breeds PRA has certain common features. Early in the disease, affected dogs are nightblind, lacking the ability to adjust their vision to dim light; later their daytime vision also fails. As their vision deteriorates, affected dogs will adapt to their handicap as long as their environment remains constant, and they are not faced with situations requiring excellent vision. At the same time the pupils of their eyes become increasingly dilated, causing a noticeable "shine" to their eyes; and the lens of their eyes may become cloudy, or opaque, resulting in a cataract.

      The big difference in PRA among breeds is in the age of onset and the rate of progression of the disease. Certain breeds, notably including the Collie, the Irish Setter, the Norwegian Elkhound and the Miniature Schnauzer, have early onset forms. In these breeds the disease results from abnormal or arrested development of the photoreceptors -- the visual cells in their retina, and affects pups very early in life. In other breeds, including the Miniature Poodle, the English and American Cocker Spaniel, and the Labrador Retriever, PRA is much later in onset. Affected dogs in these breeds appear normal when young, but develop PRA as adults.

      Diagnosis of PRA is normally made by ophthalmoscopic examination. This is undertaken using an instrument called an indirect ophthalmoscope, and requires dilatation of the dog's pupil by application of eyedrops. Broadly speaking all forms of PRA have the same sequence of ophthalmoscopic changes: increased reflectivity (shininess) of the fundus (the inside of the back of the eye, overlain by the retina); reduction in the diameter and branching pattern of the retina's blood vessels; and shrinking of the optic nerve head (the nerve connecting the retina to the brain). These changes occur in all forms of PRA, but at different times in the different breed-specific forms. Usually by the time the affected dog has these changes there is already significant evidence of loss of vision.

      Confirmation of the diagnosis can be undertaken by electroretinography. This is an electrical measurement of retinal function somewhat similar to an electrocardiographic test of heart function, but with two differences: the electroretinogram (ERG) can only be recorded as a response to a flash of light (ie: it is not a free running signal like the EKG); and accurate recording of the ERG requires that the dog be anesthetized. In all dogs showing clinical evidence of PRA, the ERG is severely diminished or extinguished.