Normal Intraocular Pressure in Dogs
A balance between production and draining of fluid within the eyeball, called "aqueous humor," is what determines the level of intraocular pressure (IOP). Normal pressure readings may vary depending on the age of the dog and the breed. Most dogs will live a healthy, full life without the owner worrying about intraocular pressure, but if the dog is a breed prone to eye problems, this measurement could be crucial to watch for the dog's healthy eyesight.
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How IOP is Measured
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Intraocular pressure in a dog is measured with a special tool called a tonometer, which exerts gentle pressure on the cornea and measures the speed of corneal rebound. People who have experienced an ophthalmic exam may remember a puff of air or a blue light--this was the tonometer mearuring intraocular pressure. Many types of tonometers exist on the market and each can give different readings from the others. Veterinarians have found that an applanation tonometer gives the most accurate readings. Readings are given in pressure millimeters of mercury, written "mmHg." Often an addition of "± SD" will be listed with this, giving a standard deviation from the mean pressure listed.
The Facts
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Normal, average intraocular pressure in dogs is 19 mmHg. Depending on the brand of tonometer used and the age of the dog, normal IOP can measure as little as 10 or as high as 29 mmHg. Aqueous humor fluid is responsible for nourishing tissues inside the eye and is regularly created fresh by the cilary epithelium cells near the iris. When draining of the fluid does not keep pace with creation, abnormal IOP problems will result.
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Mean Intraocular Pressure
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The Department of Small Animal Clinical Sciences and the Gwathmey-Adams Laboratory for Vision Sciences conducted a study published in 1998 that concluded normal intraocular pressure was consistent across 53 breeds of dog studied. Among 1,027 dogs studied, their findings also revealed that sex of the dog made no difference. Age was a deciding factor in IOP, showing a decline of 2 to 4 mmHg as dogs aged from under two years old to more than six years old.
Function and Abmormality
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Normal intraocular pressure in a dog's eye maintains the shape of the cornea. Abnormal pressure can distort not only the cornea but also blood vessels in the eye and the muscles holding the lens, leading to blindness. Abnormally high IOP and its resultant symptoms is called glaucoma. Abnormally low IOP is called hypontony; this sometimes results from inflammation and injury.
Breed-Specific Problems
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Northern breeds are more prone to developing glaucoma--Arctic Circle breed dogs such as huskies, Norwegian elkhounds and samoyeds. Chow chows, Labrador retrievers, shar peis, American cocker spaniels and basset hounds are also known to be prone to glaucoma. Scientists are unsure why these breeds develop glaucoma at statistically higher rates, but glaucoma is a broad term applied to three distinct high-IOP issues. Breeds prone to this problem should get regular measurements with an applanation tonometer to monitor intraocular pressure.
Indirect Therapy
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Glaucoma can be painful for a dog due to intraocular pressure three times the normal measurement. Researchers at Seoul National University College of Veterinary Medicine and Department of Veterinary Surgery showed acupuncture reduces the rate of aqueous humor production by reducing the dopamine and norepinephrine levels. Along with reducing blood pressure and increasing endorphins to deal with pain, acupuncture may be a beneficial therapy to maintain normal intraocular pressure for dogs.
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