Our house faces East and in spring and autumn the sun is perfectly placed in space and time to shine straight 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 man and animals, resulting in different but equally challenging injuries.  Puppies have to learn the “menace response” which is a reflex closure of the eyelids to protect the eye from a fast-moving object.  As it takes time for this lesson to be learnt, puppies up to 4 or 5 months’ old are vulnerable to cat-scratch corneal lacerations if the playful attentions of the new addition to the household are not appreciated by the feline incumbent.  Thorn injuries in older dogs running through undergrowth can be mistaken for simple conjunctivitis, as 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 (especially the iris and retina) and infection. Both cause inflammation that can result in fibrin deposition (scar tissue) in the eye, resulting 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 a bit theoretical but in fact it’s quite common in breeds with lots of spare facial skin, like the Cocker spaniel.  Surprisingly difficult for owners to spot, as when our dogs look up at us, the facial “mask” slips backwards under gravity 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, covering the eyes and sometimes causing 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 that stops light entering the eye and passing through to the retina to be converted 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, hence the long list of possibilities.  In the clinic, cloudiness or pigmentation of the cornea and cataract (opacity of the lens) are commonly seen (mostly due to trauma, infection and diabetes) 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.  Crossbreeds like the Cockapoo and the Labradoodle can also be affected 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 as 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 down to the back of the eye and 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!