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Chapter 4
Understanding Vision Impairment
What happens when things go wrong with our eyes? If we’re going to understand how to make things right, we need to understand how things go wrong.
When things go wrong with the structure of your eye, those changes will have an effect on your visual function. That’s the relationship we’ll consider now, starting with the front of your eye and then moving backwards.
Very broadly speaking there’s an extremely important dividing line at about the level of your eye’s lens.
The front of your eye is like a camera. Its job is to get a nice focused image on to the retina at the back of your eye.
Problems with the front part of your eye tend to affect your entire visual field reasonably evenly.
The back of your eye is like a camera’s film or sensor. Its job is to ‘see’ the focused image, and convert it into nerve signals to send to the brain.
Problems with the back part of your eye tend to affect your visual field unevenly. Some areas might be badly affected or even blind, while other areas might be completely normal.
Problems With the Front of the Eye
As mentioned above, the job of the front of your eye is to get a nice focused image on to the retina at the back of your eye.
Things right in the front of your eye can’t be focused on the back of the eye though, so any problems up the front tend to have a fairly even effect over most or all of your visual field. (Sure, there are exceptions).
Media haze — the clear bits aren’t clear
The word media here means the clear parts that the light passes through on its journey to the back of the eye. So hazy media means some part of the eye that’s supposed to be perfectly clear is hazy or has opacities. Examples can include dry eye, corneal dystrophies, corneal scarring, cataracts, and vitreous haze.
If you have mild to moderate media haze you may have trouble seeing finer detail, but your bigger issues often revolve around glare (all types) and trouble seeing things that are lower contrast. You may have very significant functional problems, but still have good visual acuity.
If the haziness is denser, the loss of vision can be a lot more severe. The extreme is when the clear part isn’t just hazy, it’s opaque, in which case no light can get through.
The optics are messed up
The usual culprit here is the cornea, but it can occasionally be the lens. Your cornea might be lovely and clear, but if it’s badly shaped then the image on the back of your eye won’t be focused. This is often the case if you have keratoconus or have had a corneal transplant.
On the whole, when optics are the problem the visual acuity (VA) is still a pretty good reflection of how good or bad your vision is. Your functional problem is likely to be about not being able to see fine detail, rather than any problem with low contrast or low light.
In some cases this kind of problem is fixable by using a contact lens to bridge the distorted surface so that a clear image can be formed, so technically they wouldn’t properly count as ‘low vision’. Those types of lens can be exceedingly complicated to fit, so you really need to find someone with real skill in that area.
Magic Glasses — legit?
Interestingly, having distorted optics is one of the few situations where those ‘magic’ pinhole glasses really can be a good answer. You will have seen them advertised, ‘miracle glasses’ that will give you ‘natural vision improvement’ or some such tosh.
They look like sunglasses, but the ‘lenses’ aren’t really lenses at all, just opaque material with a whole heap of tiny holes in it (hence ‘pinholes’). The reason they ‘improve’ vision is the same reason a pinhole camera or a camera obscura work. Check out those pages for a detailed explanation of the optics, but the short summary is that the line of light getting through the hole is so narrow that even if it goes through rubbish optics, it’s still quite constrained by the time it reaches the back of the eye, so it forms an image with decent clarity.
The downside is that about 99% of the light never makes it through the ‘lens’ because it’s simply blocked. So it’s good to make sure you only use them in really good light. Also you need to keep perfect alignment between your eye, the pinhole and what you’re looking at, so you’ll need to keep making tiny movements of your head to shift the pinhole around.
The eyes don’t move properly
If you’re going to see something well, you need to be able to look at it steadily. Any condition that interferes with the accurate pointing of the eye at the thing you’re looking at will make it hard to see detail properly, but it probably won’t have much effect on how you see low-contrast or low light. Examples can include nystagmus (‘wobbly eyes’) and parkinsons disease (which can interfere with eye movements).
Many conditions have multiple effects. For instance, ocular albinism often causes glare problems because the iris doesn’t have enough pigment, so the light leaking through has a similar effect to media opacities — but it frequently also causes nystagmus, affecting detail vision.
Problems With the Back of the Eye
Most people I see in my low vision work have a condition in which the back of their eye is the problem. And in almost all cases the problem area is the retina (or the choroid, which supports the retina, so the retina becomes affected). The most common back-of-eye problem (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, and the same condition can affect different people in many different ways.
As mentioned above, the job of the back of your eye is to receive the nice clear focused image that the front of your eye has given it, convert the light of that image into nerve signals, and transmit those signals out of the eye and back to your brain.
Key concept: ‘field defects’
There are a few conditions that affect the entire back of your eye equally, so they can affect your entire visual field. But in most cases there will be some parts of your visual field that are more affected than others. Many conditions only ever affect one part of your visual field, and leave other parts completely normal (macular degeneration, for instance).
This is where we first encounter the tricky concept of field defects, blind patches or scotomas.
Whenever there is an eye condition that causes impairment to one part of the back of your eye more than other parts, we talk about its effect in terms of effect on your visual field. Where the field defect is severe, that area may be totally blind. Where it’s less severe, the other three aspects of vision will be affected in that area — that is, that area will likely have worse ability to detect low contrast, perform worse in lower light, and have reduced sensitivity to detail.
What is a ‘scotoma’?
‘Scotoma’ means a visual field defect, an area in your visual field that is impaired. It can be either an absolute scotoma (totally blind area) or a relative scotoma (not blind, but significantly less sensitive than normal).
Technically the plural is scotomata, but nowadays we more often say scotomas.
This is where the complexity comes in. In practical terms, there’s so much variation in how visual field loss affects people, depending on things like:
- How much of your visual field is affected?
- Whereabouts in your field are the defects? Are they just on one side? Are they close to where you’re looking?
- How severe are those field defects? Are they just reduced sensitivity, or are those areas blind?
- Is the macular field affected?
- Is the foveal field affected?
Is the fovea involved?
Since the fovea gives us our ultra-fine detail vision, problems with the fovea will always cause reduced fine detail vision. Macular holes are always centred on the fovea, so even a very small one can cause a very significant loss of visual acuity (trouble reading down the eye chart), but every other aspect of your vision might be quite normal.
If the reverse is true — that is, whatever condition you has is affecting the back part of the eye but is not affecting the fovea — you might have close to normal visual acuity, and read down the eye chart really well. But that doesn’t mean your vision is normal. Having non-foveal scotomas can have a huge impact on your visual function.
If your condition doesn’t affect the fovea, then you might have almost normal visual acuity. But that doesn’t necessarily mean your vision is normal.
There are two really frustrating things about visual field impairment.
One is that, there’s not much we can do about it. Remember on the previous page when we talked about something getting in the way of part of our field of vision, we’d deal with that by either moving the thing that was in the way or moving ourselves. Your can’t do that with visual field impairment. The problem is inside your eye, so when you move, it comes with you.
The other is that we usually don’t see it. That is, it’s usually not a positive scotoma (which means one in which we see something blocking out part of our vision, like we do with seeing an afterimage from a bright object). Instead, our brain tends to ‘fill in the gap’ with its best guess based on the vision around the blind area. Because we don’t see anything blocking our vision out, it’s known as a negative scotoma.
Unsurprisingly, that best-guess-filling-in by your brain is not very reliable. When the scotomas are in the macular field, people tend to just have a sense that their vision is ‘blurry’ (even when it’s in perfect focus), or that words ‘jump around’ when they are reading (since the blind areas make individual letters disappear and then reappear again when your eye moves).
You might say that it’s a blessing that it’s a scotoma is negative — who would want to be constantly seeing big black areas in your vision, being constantly reminded that you have vision problems? But there can be serious personal safety implications. When it’s your general field, you might still have a strong sense that you are seeing everything around you, but in fact completely miss seeing some things (even quite big things), which can mean bumping into or tripping over or falling into them. For safety reasons, it would be better to have a positive scotoma so that you always know you can’t rely on seeing in those areas.
Defects in the general visual field, but not the macula
Remember, the macula is only about 2% of your total area of visual field, and your fovea is even smaller that that. The remaining 98% can be thought of as your general field, with roughly the outer half of that being considered peripheral vision.
If you’ve lost a lot of peripheral visual field, you’ll probably have difficulty getting through crowded areas, since you’ll tend to just not see things or people that you normally would have. Or you might have a harder time finding things, because as you are using your eyes to search around your surroundings you have only a smaller general visual field to search with.
Impaired peripheral field — even if it’s only mildly impaired, rather than complete loss — can also mean an increased risk of falls. Let’s say you’re walking along a footpath (pavement). If the surface you’re walking on isn’t completely flat, the only way we see that is by detecting subtle changes in shadow on the ground, and that’s a low-contrast detection task, the sort of thing that even mild field loss makes harder. The same is true when walking on lawn, or dirt roads.
A high risk situation is walking down stairs, especially in dimmer light. The edges of some stairs are clearly marked, but others (especially carpeted stairs) are not. With impaired visual fields, you might find it impossible to see where one step ends and the other begins. Beware of rooms where the level steps down as you move from one room to the other.
Speaking of going from room to room, you might also find that if the two rooms have different colour floors, you might find it hard to tell if it’s a change in colour or if in fact the floor level has changed. If in doubt, stop and check before stepping forwards!
The world’s most dangerous carpet?
This carpet is a nightmare! Even people with perfectly normal vision would find it difficult to see the edges of the stairs. It’s not a low contrast problem, but the fact that the pattern interferes with our ability to detect the edges means it’s kind of like the experience of someone with impaired low contrast visual fields trying to find their way down normal stairs.
Defects in the macular visual field
As with the general field, if you have impaired macular fields your eyes won’t work nearly so well when looking at paler shades (greys, pastels) as they do with high contrast (black & white, bold colours), and they won’t see nearly as well in lower light situations (at night, but also just in the evening (when there’s no daylight coming in the windows), or when looking into cupboards, pantries and drawers where the light isn’t great.
Most of us find we see things better when we take things to a sunny window, but that is especially true if your macula is impaired.
Most of us find we see things better when we take things to a sunny window, but that is especially true if your macula is impaired.
Some conditions will affect the macular field more or less evenly, but it’s actually more common for some areas to be more affected than others, so there are areas of better vision mixed in with areas of worse vision or blind areas.
I think it’s useful to think of your macular field as if it’s a circular jigsaw puzzle, one made up of a thousand tiny pieces. Many conditions remove pieces one by one (the geographic atrophy subtype of macular degeneration is notorious for doing this). You can lose quite a few pieces but still be able to figure out what you’re seeing fairly well. But eventually the small gaps start joining up to form bigger gaps, and there comes a point where we’re really starting to struggle to understand what we’re seeing.
When that happens, there are two things that people tend to notice in particular. One is that their reading comfort and fluency starts to get worse, and they other is that they may start having difficulty recognising people’s faces until they are very close, or have difficulty interpreting people’s facial expressions properly. Both fluent reading and face recognition are vision tasks that need you to not just see individual details, but also to see how those details relate to each other in space, or in a pattern.
Signature symptoms of macular field degradation include difficulty reading fluently and comfortably, and difficulty recognising faces.
We’ll talk a lot more about macular field loss later, with special reference to reading fluency and comfort.
Having no rod photoreceptors
You might remember that one of our two types of photoreceptor is the rods, which are designed to let us see in light that would be otherwise too dark to see in. They are exquisitely sensitive to very low levels of light, so once they’ve had time to adapt (they adapt pretty quickly to be begin with when you go from light to dark, but reaching full sensitivity takes 20-30 minutes), they give us enough vision to walk around reasonably safely even by starlight.
Some people have a condition that affects only their rods. That will mean they see perfectly normally during the day (we call that photopic conditions), and also in dimmer conditions that aren’t too dim (we call that mesopic conditions), but when it comes to light levels below that (scotopic conditions) they won’t be able to see at all. This is often referred to as night blindness, and it often goes undetected, since you’ll see perfectly well in the optometrist’s rooms, and if you’ve been born with it you might not realise how much better other people see in the dark, since even people with normal night vision still tend to use a torch or something else to help.
Having no cone photoreceptors
Most of our photoreceptors are cones. They give us colour vision (rods don’t see different colours, just shades of grey), and they are also responsible for giving us detail vision, becoming densely packed in the macula and extremely densely packed in the fovea. They operate over a wide range of levels of brightness, overlapping with the range of rods in the mesopic range (in which it’s on the dim side for the cones, but a bit bright for the rods).
Some people have a condition that affects only their cones. If you have no cones at all, then all your vision comes from rods. That means:
- No colour vision.
- Reduced detail vision.
- The everyday world is way too bright. It’s as if those of us with cones were put in an intensely bright room, so bright it’s like a camera flash hitting our eyes over and over again without a chance to recover.
The single best event in history for a person with no cones was the invention of sunglasses. Generally the darker the better. Other people will probably find it hard to understand why you keep wearing sunglasses even when they are finding the light quite dim, but the point is they keep your vision in the light range that works best for you, not them.
Problems Behind the Eye
When we’re considering problems behind the eye, we’re mostly considering some condition that affects the neural pathways (neural means the nerve fibres) carrying the picture back to the brain.
Remember, the part of your brain that processes vision is right at the back of your head (the area called the occipital lobe), so those nerves have to travel a long way, so there are a lot of places they can be damaged along the way.
The characteristic effect of damage to those pathways is again visual field defects. Just like damage to the retina, they can be totally blind areas, or they can be less severe, just causing areas that have reduced sensitivity to light, contrast and detail.
Optic nerve is damaged
The Optic Nerve is Both Back and Behind
You might notice I’ve referred to glaucoma (which is an optic nerve disease) already, in the back-of-the-eye section, but now I’m talking about it in the behind-the-eye section. Why?
It’s because the glaucoma damages the nerve fibres that make up the optic nerve. Those nerve fibres actually start before the optic nerve, (just above the photoreceptors in the retina), ‘flow’ across the surface of the retina to the optic nerve itself (they are completely clear, so they don’t block the light getting to the photoreceptors), and then head on down the optic nerve all the way through to the optic chiasm and even onwards deep into the brain. So glaucoma and other optic nerve diseases are kind of both in-the-eye and behind-the-eye problems.
Strictly speaking, the ‘optic nerve’ covers just that section between where the nerve fibres leave the eyeball and them reaching the crossing-over point at the optic chiasm. But when any of those nerve fibres are killed off, we can detect their absence in the retina itself (using scanners such as an OCT) and also back in some parts of the brain (although not so easy to scan).
Damage to the optic nerves themselves will cause visual field loss in only one eye. Conditions include glaucoma, along with many different forms of optic neuritis and optic neuropathies. The field loss affects only that eye, and can affect any part of the visual field. Totally cutting the optic nerve would make that eye totally blind, but it wouldn’t affect the other eye at all.
Other things can also damage the optic nerve, and they have variable effects, but again they tend to be visual field defects affecting just the one eye. Examples include:
- NAAION (Non-Arteritic Anterior Ischaemic Optic Neuropathy), which is effectively a stroke affecting just the eyeball (NOT a stroke affecting the vision part of the brain — see below).
- Toxic or nutritional neuropathies, which is damage to the optic nerve fibres from some sort of toxin/poison or drug/medication, or nerve fibres dying off because of lack of certain important micronutrients.
- Multiple sclerosis, in which the nerve becomes inflamed (can happen as an episode that later gets better).
Damage to the vision parts of the brain
As I alluded to at the end of Chapter 2, damage to the vision part of the brain is very different to everything we’ve considered above. Remember, the nerve fibres are re-sorted into two new groups at the optic chiasm — one group that is all the nerve fibres from both eyes that carry information about things in the left half of your visual field, which then proceed off to the right side of the brain, and the other group being all those (again, from both eyes) that carry information about your right hemifield (which go off to the left brain).
That means that damage to just one side of your brain will cause loss of vision (field loss) in just one side of your vision, but it will happen in both eyes. Depending on how extensive the damage is, and exactly what part of the brain it’s in, the result can be partial or total loss of vision in one hemifield. The most common cause of this sort of vision loss is a stroke damaging part of the brain, or a traumatic brain injury.
Blind Side, not Blind Eye
People with loss of one side of their vision (one hemifield) often have a profound sense that they are blind in one eye, rather than blind on one side.
Any damage to the nerve pathways at the optic chiasm — the point where the nerve fibres are dividing themselves into the two new groups — can cause quite complicated patterns of visual field impairment that can affect either side (or both sides) of vision in one or both eyes.
In practical terms, the effects are similar to the visual field impairments that we talked about in the ‘back of the eye’ section (above). But the tendency for the field defects to be mostly or totally on one side means people may have the potential to learn to use the relatively normal other side of their vision better to compensate.
Real World: Everything, Everywhere, All At Once
In talking about vision impairment, I’ve been mostly dealing with each aspect — detail, contrast, brightness, visual field — separately, considering them in turn. But the reality for many people is their vision has a mixture of those impairments. Many conditions affect more than one aspect of your vision, and even if you have a condition that affects just one of those aspects, you might also have more than one condition — for instance, having a macular hole but also a cataract.
The image above is a scan I made from a roll of negatives that unfortunately had been water damaged. Even a master of Photoshop isn’t going to be able to make much of it! But I’ve kept the image, because I think it’s a wonderful illustration of why we can’t use visual acuity (the eye chart) as the be-all and end-all measure of how good or bad our vision is.
What I find most interesting is that the actual sharpness of the image isn’t that bad: I can recognise the face of my friend; I can see the pattern on his shirt; and if there was an eye chart on the wall behind him I reckon I’d be able to read it pretty well. But despite that, the overall picture is terrible, with poor contrast, large absolute field defects, smaller relative field defects, and multiple tiny field defects starting to interfere with the view of his face.