Chloe is a 7 year old, female spayed cockapoo. She has always been generally healthy and is not on any chronic medications. She is obsessed with tennis balls and loves to play fetch with them. They were taken away at home because she started to eat the fuzz off the outside. A couple of months ago she was staying with family friends for a few days. They also have a dog obsessed with tennis balls. The friends noted that she had trouble finding the balls in the evening when the lighting was decreased. This had not been noted at home. On physical examination Chloe is in good body condition. There is moderate tartar. Her pupils are widely dilated and more light than normal is reflected from the retinas. There are small opacities in the lenses of both eyes. The retinal vessels are smaller than normal in both eyes with the left side smaller than the right. The eyes are otherwise quiet. The physical exam is otherwise normal.
What is Chloe's problem and what can be done about it?
Chloe has Progressive Retinal Atrophy (PRA) and small cataracts.
The retina is the thin layer of cells in the back of the eye that is stimulated by light. There are 2 types of retinal cells, rods (providing black and white vision) and cones (3 colors to provide color vision). These are really nerve cells that create a signal that is transmitted to the brain for interpretation. Therefore, attached to each rod/cone there is a nerve fiber that travels to and through what is called the optic disk where the optic nerve attaches to the eyeball.
The eye has many structures that help enhance vision. These include the lens that focuses light on the retina, the muscular iris that opens and closes the pupil to allow more or less light to enter the eye depending on conditions, and the tapetum lucidum. This later structure is like a mirror in the back of the eye. It reflects the light that has passed through the retina so that it has a second chance to stimulate the rods and cones. This greatly increases an animal's ability to see in low light situations.
Supplying the retina with nutrition are the retinal blood vessels. These enter the eye through the optic disk and then fan out across the retina.
PRA is a condition in which the rods and cones gradually die and disappear. When this happens the retina becomes physically thinned. Light can more easily pass through the retina both initially and after reflection by the tapetum. As there are fewer cells to become stimulated by the light, vision is gradually decreased. As there are fewer cells present in the retina, the retinal vessels do not need to carry as much to and from the retina and the vessels become smaller.
The rods, which are used more for black and white vision at night, are usually affected first. Therefore, night blindness is usually seen first, even when vision during the day still appears normal based on activity. In actuality, vision in good lighting is also decreasing but it is not noticeable to us and the dogs cannot verbalize what is happening to their vision.
What we see with the ophthalmoscope is smaller retinal vessels than normal along with an increased tapetal reflection. We also see the pupils become more dilated than normal in a given light intensity. Behaviorally, night blindness is the first symptom. Sometimes this is difficult to notice as the dog has memorized its surroundings and uses its nose to help navigate. In pictures, the eyes often glow dramatically.
Chloe was actually diagnosed just over a year ago with PRA and small cataracts. She is now almost 9 years old and has done well. The owners do not really notice a decrease in her vision. However, when at the friend's house, symptoms are more obvious. She is reluctant to go outside at night or into darker areas and coaxing is required. Her ability to chase a tennis ball is decreased. The ball can be bounced over her head and she cannot see which direction it has gone. The amount of light does not seem to matter. She now uses her nose more to find the ball (and it still has to be the ball of her choosing as there are at least 6 out at any given time.)
Chloe will eventually become completely blind. There is no treatment for progressive retinal atrophy. It is a genetic condition affecting a gene that helps maintain the rods and cones. Most dogs completely lose vision over about 2-3 years. However, most dogs learn to compensate extremely well and go on to live nearly normal lives.