Understanding Vision Impairment
When things to wrong with the back of the eye
We’d count the back of the eye as being everything behind the lens. Most people I see in my low vision work have a condition in which the back of the eye is the problem. The most common by far is macular degeneration (in its various forms), followed by glaucoma and diabetic retinopathy, but there are lots of other conditions, each with their own quirks.
Key Point: When we're dealing with a condition affecting the back of the eye, the main difference is that the problem is almost never even across the visual field. Some areas might be badly affected, while other areas might be completely normal.
Retina and Optic Nerve
The job of the retina in general, and the photoreceptors in particular, is to detect an image (hopefully it’s a focused image) of the world and convert it into a nerve signal. The job of the optic nerve is to carry those nerve signals back to the brain.
Remember that the retina is what sees our visual field, and different fibres in the optic nerve carry information from different parts of the retina. So damage to the optic nerve or the retina causes problems in just certain parts of the visual field, depending on which exact parts have been damaged. That might mean some parts of the visual field are completely blind, or they might just have reduced sensitivity. Areas with reduced sensitivity will almost always have particular difficulty with low light and low contrast.
When we were considering problems with the visual field in normal vision, it was all about things being in the way — either something being completely blocking your view, or partial obstructions such as tree branches interfering with your view. To get a better view we needed to move either the obstruction or ourselves. But it’s not so simple when the problem is inside your eye. When you move your eye, the problem moves with it.
It can be hard to understand problems affecting just one part of our visual field, so here are a couple of examples.
- When someone takes a flash photo or we get a reflection of the sun off a car ahead of us, we get an afterimage — a part of our retina which has been temporarily blinded by far too much light. If we were looking straight at it, it will have affected our fovea, so we’ll have a lot of difficulty with tasks that require fine detail, such as reading or looking at faces. If it happened just to the side of where we were looking we’ll still be able to see fine detail with our fovea, but the blind patch is annoyingly still there, just to the side of whatever we look at.
- If we are wearing a baseball cap or sunglasses with big wide arms or side-shields, they will block out some of our peripheral visual field. We might accidentally walk into a low-hanging branch because the cap blocked our view, or have to turn our head more when we’re driving because some of our side vision is blocked by the sunglass arms.
Thankfully, we can take off the sunnies or the cap, and afterimages go away by themselves after a few minutes. If only it were so simple with eye conditions.
General retina
We generally talk about or peripheral or side vision, as opposed to our straight-ahead vision. But for our purposes it’s more useful to think of our macular field and then everything else. The macular field is so small that it seems to really stretch the definition of the word ‘peripheral’ or even ‘side’, so let’s just call it ‘general’ field. Note though that there are some conditions that really do affect the peripheral retina first, such as retinitis pigmentosa (RP).
You’ll remember the general visual field has the job of telling us what’s around us and keeping us safe. Impairment to the general field makes it harder to get around, and more likely that you will bump into things or trip over. That’s a real worry, and a serious problem — so much so that most definitions of ‘legal blindness’ include being classified on the basis of losing general visual field alone, even if you can still read to the bottom of the VA chart.
Many general field conditions cause problems in low light conditions first, especially if they affect the rod photoreceptors more than the cone photoreceptors. So night-blindness is often an early symptom. Many also have trouble with seeing low contrast. Some have trouble with fine detail, but others see fine detail just fine.
Other conditions may physically damage the general retina, such as retinal detachments or diabetic retinopathy. In retinal detachments, the areas that have come away are left completely blind, although the vision in your remaining visual field should be pretty normal. What problems you have depends on where the blind patches are. For instance, if you’ve lost vision on your right side, you’ll be more likely to bump into or trip over things on your right. People with diabetic retinopathy might have laser surgery to their entire peripheral retina, which can leave them with reduced sensitivity in their peripheral field and more difficulty in lower light. (That consequence of the laser is frustrating, but generally the alternative is letting the diabetes cause complete blindness).
The Fovea
Many macular diseases have the potential to cause damage to the fovea, because the fovea is a part of the macula. Common examples include macular degeneration (wet or dry) and diabetic retinopathy.
The fovea gives us our ultra-fine detail vision, so problems with the fovea will always cause reduced visual acuity. In fact, you only need a very small area affected to reduce the VA by a lot, even to what we’d term ‘legal blindness.’ For instance, if you have a macular hole, you might have lost only your foveal field, which can cause a severe loss of VA, but every other aspect of your vision is quite normal. Usually the other eye makes up for it, but if it has something wrong too it can be very frustrating.
The Macula
Macular diseases such as macular degeneration and diabetic retinopathy can damage the fovea. However, it’s quite common for a condition to damage the macula but leave the fovea untouched — and this is where things get complicated.
Macular conditions tend to cause a lot of difficulty with seeing pale or low contrast things, and also struggle in dimmer light (or even normal light levels).
If your fovea is one of the parts of the macula that’s damaged, you will have a lot of trouble with seeing detail.
But even if your fovea is fine, you’ll probably have a lot of trouble with reading and reading fluency, along with difficulty recognising people’s faces and reading their facial expressions. Why is that?
It’s complicated. Remember — just as one of the general field’s jobs is to guide the macula to points of interest, one of the macular field’s jobs is to guide the fovea to points of finer-detail interest. Two major examples of that type of task are reading text and examining faces.
Think of your macular field as being a round jigsaw puzzle, made of a hundred pieces. Conditions affecting the macula don’t tend to affect it evenly. They will tend to create blind or impaired areas in your macular field, just like removing pieces from the jigsaw. You can lose quite a few of those pieces, and still tell what the jigsaw picture is — especially since your brain gets input from both eyes, so a piece that’s missing in one eye might not be missing in the other.
But eventually there are enough pieces missing that you start to struggle to see what the jigsaw picture is, even if you can see each individual jigsaw piece just fine.
Eccentric Fixation
This is a term meaning ‘looking off centre,’ not that you’re eccentric (although who knows — maybe you are?). It’s a very useful skill if you have problems with your macula and/or fovea.
If there’s a problem with your fovea, your very best bit of vision will be off-centre, so if you look directly at something your vision might be worse. Try looking at a thing just a little to the side. Imagine the thing is in the middle of a clock — look at one o’clock, two o’clock, all the way around the clock. Perhaps you see best if you look just down and to the right a little, or up and to the left? Remember that, and you can use it in daily life.
If your macular field has blind spots but your fovea is fine, your very best bit of vision will still be exactly straight ahead. But for some tasks, such as looking at people’s faces, you might still be better to look a bit off centre — each individual part of their face won’t be as detailed to see, but you’ll be more able to see the person’s face and expression as a whole. Experiment with looking at their neck, or either ear, or imagine they are wearing a hat and look at that.
Eccentric viewing is not easy — we are wired so strongly to look directly at whatever we are paying attention to. But with practise you can get good at it, and it can be very helpful.
Faces become hard to see
Recognising a face doesn’t mean just being able to see an eye, another eye, a mouth and a nose. We recognise faces by being able to put all those things together in relation to each other very precisely, but if our macula field has too many blind spots we struggle to see how all those pieces fit together. Seeing facial expressions is similar, mainly relying on seeing how the eyebrows relate to the eyes and the mouth. Even if you know who you are talking with, it can get hard to read their expression, which can make socialising difficult and communication mix-ups more likely. Was that person speaking in a gruff tone just joking, which they signalled by a twinkle in their eye? If you can’t see the twinkle, you might think they were serious. (Or they might have been seriously cross with you, but you assume they are joking).
We don't see blind spots as blind
You might have seen pictures that claim to show what eye conditions are like, which have a photo with a big black blob in the middle (for macular degeneration) or the middle okay but the rest of the vision gone (for glaucoma). But whenever I’ve asked people with vision impairments how they see, they almost never say they see like that.
Remember, we all have a natural blind spot, which corresponds to the point where the optic nerve fibres leave the eye. We’re not aware of it being blind, because our brain just fills in the gap with its best guess. For most people with blind areas, the brain does the same thing — filling the gap in as best it can. People can lose surprisingly large areas of vision and not be aware of it, although they will often be aware that something’s just not right.
The other thing is that our perception of a view is always a composite picture, with the detail filled in by the macular field scooting around the points of interest. When we see a photo with the centre blocked out, the rest of the photo is still perfectly clear. In reality, a person with a degraded macula won’t see fine detail anywhere in the photo no matter where they look, so it would be more accurate to just degrade the quality of the entire photo.
Reading fluency gets worse
Reading is also a task that requires more field than the foveal field alone. Or rather, it’s more accurate to say that fluent and comfortable reading need more. When we look with the fovea, we see about 2-3 letters in high detail. It’s certainly possible to read by moving this 2-3 letter window along a line of text, making tiny jumps all the way. In fact, that’s what we do when we are proof-reading, looking for any small typos. But it’s slow, and it’s fatiguing.
Normal reading also intensively uses the macular field to the right of the foveal field. That area has detail vision, but not ultra-fine detail. But it’s sharp enough that it tells us things like how long the rest of the word is, and its shape (where there are tall letters and the short letters, and where there are letters that come down below the line like p, q, g and y), which our brains quickly match against words we know that match the context of what we are reading.
I’ll have a lot more to say about this in later sections. For now, it’s enough to say that many people with macular field problems find their reading comfort and fluency become a lot worse, even when they supposedly still have good vision.