Dear reader: This page is part of a series of articles written for vision professionals. If that’s not you, it might not make much sense. If you’d like to learn all about eyes, vision impairment and what you can do about it, I strongly recommend you start by reading the article series I wrote for everyone, which starts here (click).
This site is in the process of being updated, with extra content designed for the layperson as well as vision professionals. I’m afraid the formatting of existing pages has been affected — sorry about that. It’s still readable, but hopefully it will all be fixed up soon, better than before.
Pathology always reduces the size of the visual volume. If a patient has impairment of low contrast, they’ll probably have impaired low luminance vision as well.
So, if this is the normal Visual Volume (VV)…
…what does a low vision VV look like?
I’m going to repeat my favourite Tolstoy quote:
“Happy families are all alike; every unhappy family is unhappy in its own way.”
Leo Tolstoy: Anna Karenina (1877)
We know that the VV will be impaired, but different patients have different impairments, so each patient’s VV will be unique. Still, just as we’re used to ‘typical’ field plots for glaucoma patients and stroke patients, etc, there will be a range of ‘typical’ Visual Volumes for each condition.
Aside: What I’d love to do is map out a whole lot of these Visual Volumes for a wide range of individual patients. I’ve got some ideas on how it could be done, but I lack the time and the funds. If anyone wants to support me in that, by all means get in touch! Or if you’re intrigued by this yourself, I’m sure there is a PhD thesis in this area just waiting to be done.
I’m going to introduce you to two archetypal patients, who I’ll use to illustrate these concepts from here.
Alice: Early-to-Moderate AMD
The first is Alice. Alice has early-to-moderate AMD. Her VV looks like this (I’ve left in the outline of the normal VV for comparison, but coloured it magenta):
Points to note:
- Alice’s overall visual volume is much smaller. You could perhaps think of the total volume of the VV as a proxy of overall vision loss.
- The VA is the point furthest to the right on the VV. Alice’s VA is still close to normal, at least in good illumination. This fits very much with clinical observation, that AMD patients often retain good VA even though they struggle with their vision.
- Alice’s contrast is much more depressed though. AMD patients tend to struggle with documents like newspaper, and may also start losing confidence with mobility as they have difficulty reading the subtle shading that shows up the contours of uneven ground.
- There is much more of a difference between medium illumination and good illumination, compared to the normal VV in which they are pretty similar. Most early-AMD patients have already discovered that they can still read pretty well as long as they have good light. My go-to history question is to ask “Do you find that you can see better if you go over near the window?” Alice certainly finds that she’s often needing to take objects over near the window to see things properly.
- Notice that the impairment of contrast vision brings with it an impairment in low luminance vision. (It has to, you couldn’t depress the contrast sensitivity without having an effect on low luminance vision — it’s just not how the 3D shape works). The flip-side of patients’ noticing their vision improving in good light is that their macular vision tends to drop out badly when the light is poor. Patients with early AMD often report struggling with even many basic tasks after sundown, when there is no light coming in the windows, or in nooks & crannies where there is poor lighting. My other go-to history question is “Do you find that you have trouble seeing things when you look into pantries and cupboards?” Alice has already noticed this — she often picks out cans that turn out to not be what she wanted. She’s already started to rely more on the cans being in the right position of the pantry rather than reading the labels, and sometimes she uses a torch to help her find the right thing in cupboards.
Tip: A torch (flashlight) is often a very useful tool for looking into darker spots around the house. LV patients should also consider carrying one in purse or pocket when out & about.
Bob: Advanced AMD
Our second archetype is Bob. Bob has more advanced AMD. Here’s his VV (again with normal in magenta for comparison):
Points to note:
- Bob’s VA is definitely impaired (the VV doesn’t reach over to the right nearly as far).
- The overall volume of the VV is much smaller.
- Bob’s contrast vision is severely impaired, with anything below medium-high contrast just gone.
- Bob’s low luminance vision has gone too. He really struggles in the evenings, when there’s no light coming in the window and he’s reliant on artificial lighting only. Even on dull rainy days he struggles, compared to a nice sunny day.
Important Note: Remember, all these diagrams are for sharp-edge-contrast objects (a ‘square grating’), which means larger object boundaries are always easier to see. If we were to plot VVs for gradual contrast transitions (ie- ‘sine gratings’, which is actually what the image shows in the background — sorry) then contrast transitions on very large objects would actually be harder to see, so the above VV would be more like this:
This is a better reflection of many real-world objects, especially with regard to mobility and navigating in unfamiliar environments. The visual volume is even smaller, and would imply a lot more difficulty with real-world visual tasks involving large objects at low contrast, such as seeing the contours on uneven ground (which is why impairment of low contrast tends to be a falls risk). But again, for the purposes of this exercise I’m primarily interested in helping people read text, so I’ll keep using VVs that relate to sharp contrast boundaries.