When discussing aids with the low vision patient, the solution that often first comes to mind is a magnifier – but how do we determine the most suitable one?
Put simply, the magnification required can be determined by:
What the patient can see
What the patient wants to see
However, other factors need to be considered. Research conducted in 19931 determined that visual parameters such as the acuity reserve, contrast reserve, field of view, and the size of any scotoma, affect what and how a person will be able to read, as detailed in the table below.
It is useful to consider acuity reserve when calculating a starting point for the required magnification. If a person wants to read text for a short time, such as instructions on a packet, then the near acuity needed is equal to the near acuity threshold.
For example, a patient with a near acuity of N12 who needs to read instructions on a packet with a font size of N6 would require a magnifier of:
12/6 = 2x N12 What the patient can see
N6 What the patent wants to see
If a person wants to read a longer text that requires more sustained, fluent reading, such as a novel, the acuity reserve needs to be three times greater, so you need to divide the size of the target text by three to get a more realistic target acuity.
So to read a book with the same size font fluently, the patient would need to read a font 3x smaller, i.e. N2, so they would need a magnifier of 12/2 = 6x, i.e.:
N12 What the patient can see
N6/3 What the patent wants to see
So now we have a starting point for the magnifier selection, but we also have to consider how it will be used. For example, a handheld magnifier could be tiring to hold while reading a book, and impossible to hold while knitting. Many patients will also require and benefit from a device with a built-in light, especially if there is some loss of contrast.
The ability to handle a device must also be considered. Think about the weight and size of the device, availability of a power source, the patient’s ability to change batteries and operate controls, and their ability to grip and hold the device.
For some patients, a traditional optical magnifier will not provide the best solution. For example, a 10x magnifier will not have a large enough field of view for fluent reading and, if the patient has a significant loss of contrast, their vision may not improve adequately with an optical magnifier. Both of these difficulties can be overcome by use of a CCTV magnifier.
A wide range of CCTV magnifiers is available – ranging from small portable handheld devices to desktop devices that can be connected to a TV or PC. As well as allowing much higher magnification and improving the contrast of the image, unlike optical systems there is no restriction of the field of view or working distance and no aberration.
The range of magnifiers and other low vision devices is increasing all the time, and it is a good idea to familiarise yourself with what’s available so that you can give your patients the best advice. Many suppliers of high-tech solutions are happy to visit patients in their own homes to demonstrate their range.
Distance vision
Although most visually impaired people struggle more with near tasks, they may also find distance tasks, such as looking at the TV or seeing bus numbers, difficult. As with near vision, the solution will vary from person to person. It may be possible for the patient to get closer to whatever they are struggling with – for example, sitting closer to the TV or using a wristwatch instead of a wall clock.
Another strategy is to increase the physical size of the TV or clock. If a low vision aid is required, the principle for prescribing is the same as for near. For example, a patient whose distance VA is 6/60 wants to see a street sign requiring a visual acuity of 6/12:
Magnification = 60/12 = X5
So, you would start by trying X5 telescopes. Again, once you have a starting point you need to consider the nature of the task the person wants to do and the person doing it.
Mastering distance aids is more challenging than near vision aids; finding, focusing on and following objects is extremely difficult so the patient’s ability to cope with it needs to be carefully assessed. If the person is sitting down for the task, then a spectacle-mounted device should be considered. For outdoor tasks, such as seeing street names or bus numbers, a hand-held telescope or binoculars may be more appropriate.
Reference
1. Whittaker SG and Lovie-Kitchin J. Visual requirements for reading. Optom. Vis. Sci. 1993;70:54-65.
Abi Crutcher is an extended services contact lens optician with a professional certificate in low vision. In addition to working in practice, Abi represents dispensing opticians on her regional optical committee and, through that, on the Welsh Optometric Committee. She represents optometry on her local primary care cluster committee – and is ABDO’s regional lead in Wales.