Veterinary technicians commonly perform or assist with ophthalmic examinations. This blog is a review on common eye tests and normal values.
A good eye exam begins with a thorough history and appreciating a patient’s signalment (age, male/female/spay/neuter status, breed). Some particular breeds may be predisposed to certain ocular diseases. Taking a good history may help the veterinarian make a proper diagnosis and treatment plan.
Eye tests should be performed in a certain order. The proper order for the most common ocular tests are Schirmer Tear Test (STT), Fluoroscein stain, and Intraocular pressures (IOP).
1) Schirmer Tear Test- The STT tests for dry eye or kerratoconjunctivitis sicca (KCS). This is a decrease in tear production. A small piece of paper made for this test is inserted under the lower eyelid and held in place for 60 seconds. Normal tear production should be greater than 15 mm in 60 seconds. Most normal dogs will produce more tears than this but this is a guideline to follow.
2) Fluoroscein stain- This is a yellow dye which is applied to each eye to look for a corneal abrasion or ulcer. The abrasion or ulcer is seen when highlighting the stained eye with a fluorescent blue light.
3) Intraocular pressure (IOP) – This is the test for glaucoma. Glaucoma is increased pressures in the eyes. The pressure in the eyes is tested using a device called a tonometer (the most commonly used one is the TonoPen). A drop of proparacaine is applied to each eye to numb the surface of the eye. The tonometer is gently touched to the surface of the eye and a proper or good reading should be with less than 5% error. Normal values are less than 20 mmHg (typical normal range 10-20 mmHg). Again, this is only a basic guideline and should be interpreted based on the rest of the exam and clinical signs. A low pressure may be due to uveitis (inflammation). It is important to note: DO NOT put any pressure around the neck region while holding the pet for IOP. This can falsely elevate the readings.