When presented with a dog suspected of having vision impairment or loss, most veterinarians will perform a thorough physical examination and take a complete history. They also typically will draw blood samples for a complete blood count and a serum biochemistry profile, and will take a urine sample for a urinalysis, to assess the overall health of the dog and the status of the function of its vital organs. The dog’s blood pressure may also be assessed, because high blood pressure (hypertension) can contribute to retinal detachment, which is one of the potential causes of blindness.
The veterinarian will also conduct specific tests to assess the dog’s visual capacity. Examination of the function of the cranial nerves is essential to a diagnosis of the possible causes of blindness. This includes assessing: 1) the “menace response” (how the dog reacts to a hand or other object being moved rapidly towards its face); 2) the size and symmetry of the pupils; 3) the pupillary light reflexes (PLRs); 4) the dazzle, palpebral and corneal reflexes; 5) the vestibulo-ocular reflex; 6) facial nerve function/facial sensation; and 7) motor function of the muscles of the eyes and face. Your veterinarian can provide you with the details of these various vision tests.
Another common test of vision is to observe the dog in a dark, familiar room in which the furniture has been rearranged. The same test can then be done with the lights on. A blind dog will react the same way in both situations, while a dog with some vision will react more normally when the lights are on. The attending veterinarian is in the best position to describe the details of these examinations and their results to the owners of affected dogs.
Most veterinarians will also do a thorough neurological examination on dogs with vision loss, and of course will conduct a complete ophthalmic (eye) examination, as well. The ophthalmic exam may include: 1) a fluorescein stain test; 2) assessment of intraocular pressure; 3) evaluation of any opacity of eye structures; and 4) evaluation of all aspects and structures of the eyes after pharmacologically dilating the pupils with special eye drops.
Thoracic radiographs (chest X-rays) may be taken to look for evidence of cancer or systemic infectious disease. Specific blood tests are available to identify specific infectious disease that might contribute to or cause vision loss, such as Lyme disease, Toxoplasmosis, Bartonellosis or systemic fungal infections. Ocular ultrasonography (ultrasound of the eyes) can also be done.
When the local veterinarian cannot determine the cause of blindness, she typically will refer the owner and patient to a specialized veterinary ophthalmologist for advanced testing. This might include: 1) electroretinography, to assess retinal function; 2) a tap of the cerebrospinal fluid; 3) vitreous centesis, with culture, titers and cytologic (cellular) assessment of the sample; 4) evaluation of visual-evoked potentials; 5) magnetic resonance imaging (MRI); and/or 6) computed tomography (CT scan). These are very advanced diagnostic procedures.
A diagnosis of vision impairment or loss should not be taken as a catastrophe or a death sentence. Most dogs normally rely much more on their senses of smell and hearing than they do on their sense of sight. As vision fails, a dog’s senses of hearing and smell become even more acute, which usually helps the dog navigate familiar rooms and areas.