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.
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.
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.