For low vision specialists and those who consult them
Peripheral prisms for hemianopia
November 25, 2009Posted by on
Nearly 1% of people over 50 years of age have visual field problems following a stroke or brain injury. The classic pattern of this visual field loss is to lose half of the visual field from each eye, which is known as homonymous (meaning the same side) hemianopia (meaning a loss of half of the visual field). The effect of this is to not be able to see anything to the left or right side of the central point of the vision. As you can imagine this has a significant impact on reading, navigating, and walking in crowded environments.
One strategy to overcome this is to make more eye movements into the blind side of the vision – for example, if the right side of the visual field is missing, the person makes repeated head and eye movements to their right in order to move the region of good vision to the right hand side. However this is a difficult strategy to learn. An obvious alternative approach is to use prism lenses to move the image of objects which fall on the non-seeing side into the healthy part of the visual field. A problem with this is that they create central double vision – which can be more disabling than having missing visual field.
For the past 10 years or so, Dr Eli Peli from Harvard Medical School has been using an alternative approach – a peripheral prism system which is fitted on the top and bottom of a spectacle lens, leaving the central region clear. The idea of this system is that things which fall into the top or bottom of the nonseeing visual field are moved into the seeing region, alerting the person to make a head or eye movement to examine what is there.
A report on this system appeared in Archives of Ophthalmology last year, reporting that 74% of people who were fitted with the prisms still used them six weeks after they were dispensed, and that about half of the people who were fitted with the prisms still used them one year later. People with hemianopia report these glasses as being particularly useful for avoiding obstacles at home, in shops and shopping centres, and when walking in unfamiliar environments.
More information on this system can be found at www.hemianopia.org.
It is good news that peripheral prism spectacles seem to be useful for walking and navigation for a reasonable proportion of people with hemianopia. Unfortunately they are less useful for reading and computer work, which in my experience is a very frequent complaint of people with this type of visual field loss. However given that 1% of the older population has heminaopia, and that the population are ageing, I would not be surprised if adaptive devices to help reading with visual field loss are developed in the future.