Tonopens: How & Why Veterinarians Use Them

Veterinarians use Tonopens primarily to measure eye pressure in dogs, cats, and other animals. There is fluid produced in the front of the eye, and normally it leaves the front of the eye at the same rate that it is produced. This is what creates normal ocular pressure. If the rate of fluid production is faster than the rate of fluid leaving then the pressure is increased. If the rate of fluid leaving is faster than the rate of fluid production then the pressure is decreased. Tonopens can measure these fluctuations in pressure, which are important markers for detecting ocular disease as explained below.

Normal IOP in dogs and cats ranges from 10-25. Each dog or cat will have a slightly different normal value within this range so it is a good idea to measure IOP during a regular exam. Then, if a problem occurs in that animal’s eye the pressure can be checked and compared to the normal value in that animal. It can be measured in other animals as well; guinea pigs and other small mammals have a normal range of 15-25 with 15-20 being ideal.

The main reason for increased IOP is glaucoma. By definition glaucoma is an increase in pressure due to changes the rate of production and/or the rate of fluid leaving the eye. It can be caused by other diseases or illnesses such as high blood pressure or it can be an isolated problem. No matter the initial problem, it will cause changes in the eye that will eventually lead to other problems including pain and blindness. Some clinical signs that can indicate an animal may be dealing with glaucoma are painful eyes, bulging eyes, redness around the edge of the eyes, dilated pupils, and a decrease or loss in vision.

The main reason for decreased IOP is uveitis. Uveitis is inflammation of the uvea which is the middle part of the eye. When this area is inflamed or swollen it does not produce as much fluid for the front of the eye. Clinical signs include painful eyes, sensitivity to light and squinting.

There are other eye problems that can have similar clinical signs of painful eyes, redness, squinting and sensitivity. Some of these include conjunctivitis (pink eye), KCS (dry eye) and corneal ulcers. As your animal’s veterinarian we can take a closer look at their eyes and look for changes deeper in the eye that help us determine what your animal is dealing with.

One of the ways we can further asses your animal’s eyes is by measuring IOP with the Tono-pen. The Tono-pen reads the IOP by tapping the pen on an eye. The resistance from the eye is read and a pressure is calculated. It is done multiple times and an average reading is obtained.

If you have any concerns about your pet’s eyes, don’t hesitate to make an appointment for us to examine them. Many eye problems can be addressed and fixed if caught early enough, and River Road has Tonopens and other diagnostic equipment ready to assist your pet.

Dr. Annie Whitford

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