On the previous page I showed how the OVV isn’t something you can simply calculate for a patient with ocular pathology. Patchiness of macular fields is common, and depending on each individual’s fields the OVV can be eroded away to much smaller than the theory would predict.
The next question is, how can we find each patient’s OVV?
One helpful bit of data would be to have an accurate measure of exactly how patchy the patient’s macular fields are. Ideally I would have a 10-2 fields and or microperimetry to give me a clear idea of what’s going on, but even if I had the time and the equipment (we don’t have a microperimeter), many of our patients are elderly and frail, and lack the concentration and physical stamina to give us a good reliable macular fields result.
Using CCTVs to Determine OVV
There is a technique I’ve found quite useful to empirically measure the boundary of a patient’s OVV, which uses a CCTV as a diagnostic tool rather than as a low vision aid. When I get a patient who wants to achieve fluent reading but really struggles with the VA chart (either has a reasonable VA but struggles with much larger lines, or simply has a bad VA), then before anything else I’ll go through this process:
- Sit them in front of the CCTV (in the LLVC it’s a nice large 24″ Topaz HD).
- Put some newspaper column text under the camera. I tend to use the Births column from the weekend newspaper, as we can all have a good laugh at the strange names, but it also includes details such as date, birth weight, etc so I can check the patient is reading accurately.
- Switch the contrast settings to maximum contrast, on reversed contrast to make sure it’s not giving too much glare.
- Ramp up the magnification to maximum, which with our CCTV means that it fits about 5 letters across the screen, about 15x. At this point I check that the patient can see those letters. If they can’t, you can tell straight away that their vision loss is really severe, and reading isn’t going to be possible at all. (Either that or they have some form of dyslexia or alexia, perhaps from a stroke — that’s a useful finding in itself). But the vast majority of patients laugh and confirm they can see those letters easily.
- Then I get the patient to turn the magnification dial to make the magnification smaller and smaller, and ask them to tell me when the print first starts to become difficult. When it does, I turn it back up one step and confirm that the print is quite comfortable for them, and when they’re sure I get them to read out the Births notice, so I can get an idea of their fluency and accuracy.
Assuming they are both fluent and accurate, this represents one boundary of their OVV, like this:
We’re starting way over to the left, with print that should be right in the middle of most patients’ OVVs, (if they have one), and then moving out towards the Unseen. The green/yellow boundary on this diagram indicates the point where the patient tells us they stop reading easily, which is the boundary of their OVV.