Dahlia is a good-looking Siamese cross. Her body is pale silver and tan tabby with patches of creamy white around the chest, but you can see her Siamese side in the dark grey and brown “points” on her ears, feet, and tail. Her eyes are a lovely blue. Gaze into them for a moment and you will notice that her eyes flick back and forth constantly in a rhythmic fashion, like she’s watching a tiny tennis match. That’s called nystagmus. And she’s cross-eyed, to boot. That’s called strabismus.
Recently adopted by a new owner, Dahlia’s mom was curious. “What’s the deal with her eyes?” she asked. “Is that normal? Does it affect what she sees?”
Good questions. Let’s start with the basics. Vision requires a complex series of events utilizing a complex set of anatomical structures. Here’s a simplified version. Starting with the eyeball, light passes through the cornea and lens to reach the back of the eye, called the retina. The retina contains light receptors called rods and cones. Cones facilitate color perception and visual acuity. Rods facilitate night vision and motion detection. Light reaching the retina causes a photochemical reaction in the rods and cones, which send nerve impulses to ganglion cells and then the optic nerve, and eventually arrive at the visual cortex in the brain.
Different species have different retinas to suit their specific needs. Most bird retinas, for example, are primarily made of cones, and they have a high number of ganglion cells. This gives good visual acuity and ability to see colors well. That bright red male cardinal wants that female cardinal to fully appreciate his vivid plumage. That chicken pecking for food needs a keen eye to discern small seeds and bugs in the yard, as does that osprey soaring high over the great pond scanning for fish far below her in the water.
Dogs and cats have fewer cones than most birds or people. Cones come in several varieties that respond to different colors. Dog and cat cones are predominantly those that perceive blue and yellow. This means our pets are partially “color blind” when it comes to red and green, seeing these colors for the most part as various shades of grey.
On the other hand, they have far more rods than people, giving them superior night vision. A reflective area on the retina called the tapetum enhances this even further by reflecting what little light is available. This is what makes cats’ eyes appear to glow in the dark, the tapetum acting like a tiny mirror reflecting back any ambient light. One source estimates that the tapetum in a cat’s eye may reflect more than one hundred times as much light as a human eye and that humans need six times as much light as a cat in order to see. Dogs also do better than people in terms of night vision, but cats far exceed both canines and humans in their ability to navigate in the dark.
What about being nearsighted or farsighted? These conditions occur when the eye focuses incoming images either in front of or behind the retina. It’s hard to get Fido to do the eye chart test, but apparently dogs do experience these conditions. Breeds reportedly prone to being farsighted include Australian shepherds, Alaskan malamutes, and English springer spaniels. Breeds reportedly at risk for nearsightedness include German shepherds, rottweilers. miniature schnauzers, toy poodles, Labrador retrievers, English springer spaniels, and collies. Honestly, I don’t know how they test for these, you’d have to ask a veterinary ophthalmologist, but I suspect they rarely have any significant impact on a dog’s well-being. Which brings us back to Dahlia. Do her crossed eyes and nystagmus affect her vision in any major way?
Dahlia has a condition called convergent strabismus and congenital nystagmus. There are many illnesses that can cause nystagmus — feline vestibular syndrome, inner or middle ear infections, and many central nervous system diseases. In these diseases, nystagmus is usually associated with a balance disorder. Animals may have a head tilt, walk in circles, stagger, or fall over. Not so with congenital nystagmus, which is commonly diagnosed in certain breeds or individuals whose coloration is called “imperfect albinos” or subalbinotic — Siamese, Siamese crosses, Himalayans, flame-point Persian, and Burmese as well as white domestic cats and white tigers with blue eyes.
These non-pigmented or poorly pigmented animals have a genetic mutation linked with their subalbinism that results in a relatively benign form of congenital nystagmus (as well as strabismus.) Symptoms are usually evident by six months of age, but most cats appear to acclimate quickly. Affected cats appear to have good functional vision, and owners rarely notice any visual deficits. Congenital nystagmus in animals with “normal” pigmentation is far less common, but, when it does occur, it may be associated with severe vision problems.
We knew that Dahlia was part Siamese. This, and her baby blue eyes, told us she was an “imperfect albino.” Thus the likelihood of significant visual deficits was small, but her owner had noticed that though an efficient hunter, Dahlia appeared not to see a mouse when it wasn’t moving.
“Is that because of her weird eyes?” her mom wondered.
Possibly, but it could also just be normal. The large number of rods in the retinas of dogs and cats make them very good motion detectors. This ability is often the last thing to go when animals are losing their sight, which explains why old Rover can be virtually blind but still chase the neighbor’s cat. It makes sense, right? If you need to hunt for a living, you want to be good at noticing when prey goes dashing past. Conversely, this is why many small prey animals have the defense mechanism of freezing motionless in order to avoid being spotted by predators. (It’s one reason why you shouldn’t run if you ever encounter a mountain lion. Remember, you are the mouse in that situation.)
Dahlia should do just fine, and her new family needn’t worry about her vision.