Many different conditions can cause glaucoma. Glaucoma is classified as either primary or secondary in animals.
Primary Glaucoma is an inherited condition. Primary glaucoma occurs in many breeds of dogs, including the American Cocker Spaniel, Basset Hound, Chow Chow, Shar Pei, Jack Russell Terrier, Shih Tzu, and Arctic Circle breeds (including the Siberian Husky and Elkhound). Primary glaucoma is rare in cats.
Primary Glaucoma usually begins in one eye, but in most patients it eventually affects both eyes, leading to complete blindness if not controlled.
Secondary Glaucoma occurs when other eye diseases cause decreased drainage of fluid from the inside of the eye. Common causes of secondary glaucoma include: inflammation inside the eye (uveitis); advanced cataracts; cancer in the eye; lens subluxation or luxation (i.e. displacement of the lens from its normal position; a completely luxated lens is free of all attachments and can “float around” inside the eye, causing both damage and pain) and chronic retinal detachment. Glaucoma in cats is almost always secondary to chronic uveitis. Treatment for secondary glaucoma is too broad to be presented here; it is critical to treat the cause of the glaucoma whenever possible.
Diagnosing whether your dog has primary or secondary glaucoma is important because the treatment needed and the prognosis for vision is different for each type of glaucoma. Veterinary ophthalmologists measure the intraocular pressure (IOP) and use slit lamp biomicroscopy, indirect ophthalmoscopy, and gonioscopy to determine the type and cause of glaucoma in your pet. Tonometry is measurement of IOP, and there are three basic types of instruments (tonometers) that can be used to measure IOP. The best tonometers are the TonoPen™ and the TonoVet™; these are costly computerized handheld devices. Another device is a Schiotz tonometer; this is an inexpensive stainless steel device, but is more difficult to use in animals. Gonioscopy helps determine how predisposed the remaining visual eye is to develop glaucoma when primary glaucoma is present in the other eye–i.e. what is the risk level of the remaining eye to develop glaucoma? Gonioscopy involves placing a special contact lens (“goniolens”) on the eye, which allows examination of the drainage angle. Gonioscopy is usually performed under sedation.