At the outset, glaucoma must be differentiated from other causes of non-specific eye redness and irritation, including conjunctivitis, anterior uveitis, ectropion and entropion, among many others. This is accomplished by a thorough ophthalmic examination, during which the veterinarian looks closely at the external and internal structures of both eyes, even if only one appears affected. Simple vision tests also may be conducted to determine whether a dog’s sight is impaired. These tests usually involve hand movements to assess peripheral vision, and tossing cotton balls around the room to assess the dog’s ability to follow subtle movements.
Assessment of internal eye pressure (intraocular pressure) using a tonometer is essential for the diagnosis and clinical management of glaucoma and is the single most important diagnostic tool in a veterinarian’s arsenal. At present, there are three available types of tonometry: digital, indentation and applanation. Of these, most experts find applanation tonometry to be the most reliable, although personal preferences among veterinarians will vary. Intraocular pressure tends to be higher in early morning and lower in early evening; several reproducible tonometer readings should be taken for a conclusive diagnosis of glaucoma.
Advanced testing, normally conducted upon referral to a veterinary eye specialist, can include radiographic (X-ray) or ultrasonic imaging to look for damage inside the eye. Blood tests can be performed to identify possible underlying infectious or inflammatory disease. Gonioscopy (evaluation of the angle at which the intraocular fluid drains from the eyes), and possibly an electroretinogram (ERG), are also available to help the veterinary ophthalmologist determine whether a dog’s affected eyes are capable of medical or surgical vision restoration.
Most of these diagnostic tests are noninvasive and fairly quick. They are best conducted with only light restraint. They are painless, because topical anesthetic drops are applied to numb the dog’s eyes before the examination begins.