Our house faces East. In spring and autumn, the sun is perfectly placed to shine through the bedroom window as Tracey and I try to wake up.  The effect this has on a snoozy brain is profound, a flood of sensory input as light hits the retina for the first time in hours, sending a tidal wave of signals through the optic nerve, the mid-brain and on to the visual cortex.  A connected central activation centre gets a hit and boom! I’m awake. No wonder it takes longer to get going on dark winter mornings.

A network of neurological connections link what we see with the motor cortex and parts of the brain that control our emotions and behaviour.  That’s why we can evade a fast-moving threat without thinking and get pleasure or pain from what we see.   How those of us with eyesight take this amazing sense for granted, only really appreciating it when it’s lost.

So what causes poor eyesight or blindness in animals? Dogs and cats differ in some respects although, of course, trauma is common to both. Deeply penetrating wounds and blunt trauma to an eye are quite common in both people and animals.  Puppies learn the “menace response”, a reflex closure of the eyelids to protect the eye from a fast-moving object.  As it takes time for puppies to learn this lesson, they’re vulnerable to cat-scratch corneal lacerations up to 4 or 5 months’ old if their playful actions aren’t well-met from the resident cat. 

Then there are thorn injuries, sometimes mistaken for conjunctivitis, seen in older dogs running through undergrowth. Both cause a painful, red eye in the early stages.  The prognosis for these two conditions is very different, as I am sure most people are aware.  There are two aspects to eye trauma, mechanical injury to the sensitive structures of the eye and infection. Both cause inflammation that can result in fibrin deposition (scar tissue) in the eye. This can result in adhesions that stop the iris moving and possibly glaucoma, which can cause blindness.

Trauma aside, cats and dogs differ in the commonest causes of blindness that affect them although there is considerable overlap.  As vision loss can result from disorders of the eyelids and cornea right the way through the optic pathway to the back of the brain, the list of causes is very long!  If you can’t open your eyelids, you can’t see even though everything else behind is working fine. 

This might sound theoretical but it’s quite common in breeds with lots of spare facial skin, like the Cocker spaniel. Difficult for owners to spot, when our dogs look up, the facial “mask” slips backwards, so all looks normal. But, these dogs spend most of their time with their noses on the ground, so the skin falls forward. Like a curtain, it covers the eyes and sometimes causes hair to rub on the corneal surface.  A facelift is the treatment of choice, once again proving dogs and humans are not so different.

So, anything stopping light from entering the eye, passing through to the retina and converting into nerve impulses can cause blindness or poor eyesight.  Also, anything that prevents transmission of these impulses through the optic nerve and brain to the visual cortex.  In the clinic, we commonly see cloudiness or pigmentation of the cornea and cataracts (opacity of the lens).

But, two retinal disorders deserve a special mention.  Mostly seen in dogs, retinal degeneration comes in various forms but one, progressive retinal atrophy (PRA) is an important cause of inherited blindness in the Toy and Miniature Poodle, the Labrador Retriever and the English Cocker Spaniel.  It can also affect crossbreeds like the Cockapoo and the Labradoodle, along with other breeds.  This is why screening of young dogs’ eyesight intended for breeding, with genetic tests and an examination by an ophthalmologist is so important.  Only selective breeding will control this disease. PRA is incurable and annoyingly, causes blindness in middle age, after litters have already been born.

Retinal detachment is particularly common in older cats with high blood pressure.  Think of the retina as wallpaper, loosely stuck to the back of the eye, only firmly attached in two places. Any fluid that gathers between the wall and the paper will cause detachment. In the living room and in the eye, this is “unsightly” (pun intended!).  Caught early, treatment can lower blood pressure and restore the retina to its rightful place and amazingly, vision. We do need to be aware that hypertension in cats can be caused by kidney and thyroid dysfunction and so a blood test is needed along with blood pressure (BP) measurement.  In fact, all older cats should have their BP checked regularly, once again proving cats and humans are not so different!