By Dr. Laura Hady
We often don’t think about the complexity of vision in our pets until we see them bump into objects, have decreased maneuvering in dim light, or when we see redness, cloudiness or discharge in their eyes.
Vision begins when light enters the eye through the cornea, the clear outer layer. The light then passes through the lens, which focuses it onto the retina at the back of the eye, creating a sharp image. The retina has specialized cells that convert the light into electrical signals that travel to the brain via the optic nerve. It’s there that these signals are processed into a picture.
Humans tend to have sharper vision (20/20 versus 20/75 in pets) and better color vision than pets, but pets have more peripheral vision, night vision, and a greater ability to see motion. The following are some vision abnormalities in pets and how they are diagnosed and treated.
- Lenticular sclerosis is a normal vision change associated with age. It tends to start around ages 6 to 8 in pets. It is caused by a hardening of the lens and may appear as bluish-grey haze to the eyes. It does not significantly affect vision in pets but should be monitored at yearly veterinary exams. To help differentiate vision issues, your veterinarian may use a series of vision tests. These include having your pet track an object, such as a falling cotton ball; the vet may check how a pet’s pupils respond to light and check the blink response as a finger comes closer to the eye; they will examine the structures in the back of the eye with an ophthalmoscope.
- Cataracts appear as a distinct cloudiness in the lens of pets. Causes include genetics (think poodles, cocker spaniels, and miniature schnauzers), uncontrolled diabetes mellitus, and age. If diagnosed early, cataracts can be treated with eye drops. Mature cataracts can be treated with a surgery that involves removal and replacement of the lens. Left untreated, cataracts can cause glaucoma (increased ocular pressure), pain, or permanent vision loss.
- Uveitis involves inflammation in the uvea of the eye, which is made up of the iris (colored portion around the pupil), the ciliary body (the ring-shaped tissue controlling the lens shape), and the choroid, which is the layer of blood vessels providing nutrition to the retina. Causes of uveitis in pets include infections, immune-mediated diseases, trauma, high blood pressure, cancer and structural changes to the eye like cataracts. The most common cause of uveitis in dogs is tick-borne disease, such as Lyme disease or Ehrlichia. The most common causes of uveitis in cats include feline infectious peritonitis and lymphoma. Your veterinarian may need to conduct lab work to find an underlying cause. Pets with uveitis often have red and cloudy eyes, pain, and light sensitivity. Anti-inflammatory medication is often prescribed to control pain while treating the underlying cause with antibiotics or anti-viral medication in the case of infections.
- Hypertension (increased blood pressure) is often the silent disease that can affect vision. Cats are more likely to go blind from hypertension than dogs. Conditions such as hyperthyroidism and kidney disease can damage the blood vessels in the eyes causing bleeding or retinal detachment. Cats with hypertension will often have enlarged (dilated) pupils that don’t become smaller in response to light and have tortuous retinal blood vessels on opthalmic exam. Blood pressure can be monitored at the clinic so the appropriate blood pressure medication can be given.
- Keratoconjunctivitis sicca (KCS – dry eye) is a condition where the eyes do not make enough tears to lubricate the eye. The dryness results in inflammation, thick discharge, redness, squinting, and can lead to blindness. Causes for KCS include genetics, infections, and immune-mediated issues. Cocker spaniels, pugs, poodles, bulldogs, and schnauzers are the most common breeds affected. These patients are diagnosed and monitored with the Schrimer tear test, which is a small paper filterstrip placed in the lower eyelid that measures how far tears are wicked up on the paper. KCS requires lifelong treatment with eye lubricants, anti-inflammatories, and tear stimulants such as cyclosporine. Surgery for KCS that is non-responsive to medications consists of repositioning the parotid salivary gland to deliver moisture-rich saliva to the eye.
- Glaucoma involves increased fluid pressure inside the eye caused by a drainage problem in the eye. It is typically an inherited condition, or there may be an underlying issue such as trauma or inflammation. The eye may appear bloody or cloudy with a dilated pupil, and the pet may often squint and paw at the painful eye. Diagnosis is made by measuring the pressure in the eye using a tonometer. Treatment often involves eye drops to help lower the pressure in the eye. Surgical options use lasers to decrease fluid production, implanting a shunt to help with fluid drainage, or, in severe case, removal of the eye.
- Corneal ulcers are the most common vision emergency that we see in the clinic. These ulcers can be caused by scratches, infections, infolded eyelids, and dry eyes. Smaller snout dogs (think pugs) and cats (think Persians) are more prone to corneal ulcers. Pets may present with squinting, pawing at the eye, cloudiness, and sensitivity to light. Testing involves placing a fluorescein dye strip in the lower lid and inspecting the cornea for uptake of the green dye. Treatment involves pain medication and eye drops containing antibiotics, atropine (to minimize eye muscle spasm), and solutions to keep enzymes from breaking down the cornea. Most corneal ulcers heal within two weeks, but some deeper and infected ulcers may require surgery to place a third eyelid flap for protection.
Let your family veterinarian know as soon as possible if you notice your pet stumbling or having large pupils. The annual veterinary visit is a good place to monitor structural changes with the ophthalmoscope. In some cases, your pet may be referred to a veterinary ophthalmologist for confirmation of a diagnosis via additional testing and specialized care such as surgery. Keeping a consistent layout of furniture, blocking off stairs, and using verbal and scented clues can help your pet with decreased vision navigate more safely in your home.
