Visiting Veterinarian : Sussing out canine vision problems
It was shaping up to be a complicated day. Morning surgery had been more difficult than anticipated. My faithful assistant was sick — working, but sick — and her sister was about to have a baby. We had just had an emotional loss, a cat passing away suddenly in the car on its way here, and we needed to leave soon on a house call to see seven cats in Chilmark.
But first, Jamyang was here, a ten-year-old Labrador mix scheduled for a physical and vaccinations. "This won't take long," I thought, rushing into the room disheveled, but looking forward to a cheerful, healthy pet visit.
"Jamyang doesn't seem to be seeing well," her owner commented as I began my exam. I wasn't overly concerned. Clients frequently complain of less acute senses in middle-aged dogs. People think the greyish haze in those older eyes means cataracts and assume their pups are going blind. Not so. That cloudy appearance usually indicates a normal aging process called nuclear sclerosis that does not affect vision. The reported behavioral changes are more often the result of deafness, arthritis, even senility, but not blindness.
"Let's take a look," I said, pulling my light closer. Jamyang's pupils were a bit dilated, but that could be anxiety, a common response at the veterinarian. I shone the bright light directly in her face. Jamyang blinked. "See that?" I asked. "That's a dazzle reflex. Doesn't happen if you're blind."
I moved the light away, then back, watching her pupils. The responses seemed a little slow, but both eyes dilated and constricted as expected. However, this reflex doesn't rule out visual impairment. Next I did a menace response test. In other words, I poked my finger at her eye. This takes a certain amount of finesse. The goal is to suddenly threaten the eye, without making any sound, moving air, or touching the dog even slightly. You want to be sure any reaction is the result of vision, not other senses. Normal animals blink. I poked at one eye, then the other. No blink. I did it again. No blink. A third time. She blinked, I think, maybe, a little.
Occasionally an animal is so distracted by their plight of being on the exam table that you literally have to stick your finger in their eye before they blink, but Jamyang was not that nervous. Her menace response was definitely reduced. Time for the cotton ball test.
Standing beside Jamyang, I pulled a small piece of cotton off a roll, then dropped it so that it fell silently past one eye. One side. Then the other. Repeat. It's instinctive to glance at something you see moving out of the corner of your eye, and most animals with normal vision will track the cotton's path by moving their head down, or at least their eyes. Jamyang continued to stare straight ahead.
"I think you're right," I concluded. "She's not completely blind, but something is definitely wrong."
There is a long list of things that can cause partial or complete vision loss in dogs. Progressive retinal atrophy, sudden acquired retinal degeneration, tumors, or infectious diseases affecting the optic nerve or brain, glaucoma, retinal detachment, cataracts, toxins, hereditary conditions, auto-immune diseases. Time to examine the internal structures of her eyes.
An ophthalmoscope is essentially a bright light with a magnifying glass that can be set to focus at different distances. Imagine the eyeball as a hollow globe. The iris is a membrane across the center of the globe, dividing it into front and back hemispheres. The pupil is an adjustable hole in the middle of the iris. The front hemisphere is called the anterior chamber. Jamyang's looked normal, as did her iris.
Change the ophthalmoscope setting. Now look through the pupil to the back hemisphere. Imagine the lens as a small flattened marble right behind the iris. Cataracts are opacifications in the lenses. Jamyang's lenses were clear.
Change the ophthalmoscope setting. Imagine a layer of paint on the inside of the back half of the globe. This is the fundus. A highly reflective area there, called the tapetum, acts like a mirror, which is why dogs' eyes seem to glow in the dark when caught by headlights or other illumination. The retina is a collection of rods and cones lining the back of the globe that collect visual information. The nerve fibers then bundle together in the center of the retina forming a flat white circle. This is the optic disc, the beginning of the optic nerve that transports the information to the brain. The retinal blood supply comes from one large artery that radiates into smaller vessels around the optic disc. Jamyang's retinal blood vessels looked fine — not too big, not too small. Her tapetum had normal reflectivity.
There were no signs of retinal hemorrhage or detachment. But look at that optic disc. Not the usual neat, flat circle. It was subtle but the disc looked irregular and slightly puffy. "I'm not an ophthalmologist," I demurred, "but I think her optic discs are abnormal."
Our working diagnosis is optic neuritis, an inflammation of the optic nerve that causes varying degrees of visual impairment. It can be spontaneous (known as idiopathic optic neuritis) or secondary to other diseases such as infections or cancer. It may resolve, be recurrent, or cause permanent blindness. The prognosis depends largely on the underlying cause and whether it can be successfully treated.
Tests are pending on Jamyang to rule out infectious diseases, and we have started anti-inflammatory medications. If she does not improve, a trip to the ophthalmologist and neurologist may be needed to get a definitive diagnosis. At home, it may be hard to assess her progress. Even totally blind dogs often navigate familiar environments with ease. But her mom will observe her and in a few weeks they will return for a recheck. I will make it rain cotton balls and see if she tracks them. I will poke my finger at her eyes — and hope she blinks.