Glaucoma is defined as an increase in interlobular pressure (IOP) caused by impairment of normal drainage of fluid inside the eye. Dogs get it. Even cats get it. “Primary” glaucoma is caused by a genetic predisposition to a narrowed or malformed drainage angle. “Secondary” glaucoma is caused by some other disease process like lens dislocation or infection occluding normal fluid outflow. There are many breeds prone to glaucoma including northern dogs such as Samoyeds (like my old Sadie), Siberian huskies, malamutes, and akitas, and many toy breeds like poodles and Shih Tzus, to name a few.
The increased pressure causes pain and redness. If Popeye, the pug, has increased IOP, he may squint from discomfort and have a watery ocular discharge. His pupil may be dilated, though internal structures of the eye are not visible because the pressure inside forces fluid into the normally clear surface, turning the whole eye a cloudy white. The eye may bulge visibly but owners often miss this, especially in the early stages. If only one eye is affected, an owner may also not notice any change in Popeye’s vision. Chronic glaucoma signs may come on more gradually and subtly. Miss the diagnosis early on, and the increased pressure may cause irreparable retinal damage leading to irreversible blindness. In other words, time is of the essence if we want to have the best chance of saving Popeye’s sight.
When I was fresh out of veterinary school many moons ago, the instrument used by private practice vets to measure IOP was a Schiotz tonometer. Unlike the large, high-tech contraption doctors use on people, the Schiotz is a small, hand-held instrument, notoriously difficult to use. It looks like something you’d find on Galileo’s desk — a cross between an old-fashioned scale and a compass used for drawing circles. So when Popeye arrived on my table with a red eye, I was supposed to apply a drop of local anesthetic in the eye, then balance this apparatus on his cornea. That’s right. Directly on the eye. Of a wide-awake dog. Then watch the needle swing and read the pressure.
Yeah, right. I could never get it to work. My patients wouldn’t sit still, and I’d get bizarre, inaccurate readings. I could see clients looking quizzical and thinking “What is this crazy woman doing to my dog?” Sometimes I was confident Popeye did not have glaucoma. “He has a scratch on his cornea,” I would pronounce. Or ” It’s keratoconjunctivitis sicca, a.k.a. dry eye.” Or “Probably just a reaction to an environmental allergen or irritant.” But there was often a nagging worry in the back of my mind. What if I was wrong? What if it was glaucoma?
About ten years ago, I had a sudden run of pets presenting with suspicious eyes. I had long since given up on the Schiotz tonometer, instead referring all possible glaucoma cases to a veterinary ophthalmology off-Island. Some clients went willingly. Some wouldn’t or couldn’t. I fuddled through those that declined referral, dog after cat after dog, the best I could, always with some degree of uncertainty and anxiety. Then late one night, after the last ferry had sailed, I got one more emergency dog with an acutely painful, red eye. That was it. “I’ve had it,” I decided. “I don’t care how much it costs. I’m buying a Tono-pen.”
The Tono-pen is a handy, albeit expensive, little electronic item that was developed in the 1990s. Technically called an applanation tonometer (Tono-pen is a brand name), it looks like a big penlight wearing a condom. Give Popeye those topical anesthetic drops. Calibrate the Tono-pen. Get a good assistant, like Elise. Tap the pen lightly against Popeye’s cornea. Beep, beep, beep. Digital readout. Done. It’s beautiful. Okay, it actually isn’t quite that simple. You’re supposed to take three readings each side. Some dogs are still too squirmy, and if Popeye gets stressed (or if Elsie holds too tight, which she would never do), the IOP can be artificially elevated, But it’s light years better than a Schiotz. Magic. I stopped worrying about missing glaucoma. “Worth every penny in peace of mind,” I told my husband, “even if it never pays for itself.”
So, recently, I got a call from another Island veterinarian. Did I have a Tono-pen? Yes. Could I check the IOP on a cat? Sure. Happy to oblige. When Kitty arrived, at first glance I was puzzled. Her eyes weren’t red or cloudy. Her pupils weren’t dilated. She did not appear to be in pain. There was no ocular discharge.
“Do you know why your vet wanted Kitty’s pressure checked?” I asked, calibrating the pen. “She said the eyes were bulging,” the owner replied. Hhmm. I thought silently. Bulging? Really? I pulled over my bright light. Hhmm again. That’s odd. There were a few abnormally prominent blood vessels on the lower iris, and yes, the iris was bulging. It was subtle but clear. It wasn’t the whole eye that bulged, like in typical glaucoma, just the iris, inside the eye, causing the anterior chamber to be unusually shallow. Weird. I checked the intraocular pressure. It was borderline high. Having never seen anything exactly like this before, I retreated to my desk to do some research.
“Looks like Aqueous Misdirection Syndrome,” I reported to the owner later, as though I knew what I was talking about. “Also known as Vitreous Displacement, Aqueous Diversion, or Humor Misdirection Syndrome.” In this condition, fluid flows in the wrong direction, moving to the back of the eye instead of the front. This misdirected fluid pushes interior structures like the lens and iris forward. The cause is not known, but it occurs most frequently in middle-aged female cats. It can lead to glaucoma and blindness, although affected animals may maintain vision for years. Treatment to decrease the IOP is often recommended. Thanks to the other veterinarian’s astute observation, Kitty’s unusual condition was noticed. Thanks to my Tono-pen, I got a chance to see it and learn something new.