For low vision specialists and those who consult them
Back to basics: What is low vision?
September 8, 2009Posted by on
I went for dinner with some non-optometrist, non-medical friends at the weekend and I realised that whilst within the community we use the phrase “low vision” very widely, it isn’t a term which is particularly well understood by the wider population. There also isn’t a unified definition of low vision: if I use the phrase “low vision” it may be interpreted differently by an ophthalmologist, a rehab worker and someone with (for example) retinitis pigmentosa.
There are many definitions of low vision, but lets start with the one by the World Health Organisation (WHO). They define someone with low vision in a fairly wordy, legalese way:
“a person with low vision is one who has impairment of visual functioning even after treatment and/or standard refractive correction;
* and has a visual acuity of less than 6/18 to light perception, or a visual field less than 10 degrees from the point of fixation,
* but who uses, or is potentially able to use, vision for the planning and/or execution of a task for which vision is essential”
In simpler language, this is saying that someone with low vision has poor vision, even with the best specs or contacts, but has enough vision to perform certain visual tasks.
There is a visual acuity criterion – to be defined as having low vision, the best visual acuity with the optimal glasses or contact lenses must be worse than 6/18 (20/60, 0.50logMAR). This is about half way down a conventional sight chart and slightly poorer than the driving standard in most countries. I wonder what people with visual acuity of 20/50 think of this, who may not be able to receive a driving licence because of their visual acuity but are not classified as having low vision?
There is also a visual field criterion which relates to how far you can see around the central point of your vision without moving your eyes. The WHO low vision definition is again fairly strict, especially when you consider a normal visual field extends around 170º horizontally.
There is an increasing awareness that visual acuity and visual field are not the only determinants of how well you see – the WHO definition has no mention of contrast sensitivity (how well you can see faint objects) and no mention of vision in different lighting conditions (people with some retinal diseases have very poor vision at night but relatively good sight in bright conditions).
My biggest concern with the WHO defintion is that it makes no mention of function. If your visual acuity and visual field fall outside the levels prescribed by the WHO but your greatest passion is sewing, or reading old manuscripts, or fly fishing, and you can’t see to perform these tasks, shouldn’t you be defined as having low vision, and be entitled to a low vision assessment to prescribe magnifiers for this task? Conversely, if your vision is poor enough to meet the WHO definition but you are able to perform every activity you want to without any difficulty is it appropriate to be labelled as having a visual problem?
I think a better definition of low vision is the one advocated by Dr Gordon Legge in Minnesota: “The inability to read regular newsprint with optimal refractive correction”. This is better in that it includes a functional statement, but may exclude people who have good reading acuity but are unable to perform other tasks using vision – such as to safely cross a street or to watch television.
A consensus paper by Morimoto Noriko in Japan suggests this definition, which I quite like even if it does sound slightly strong in translation:
“visual function or vision that interferes with growth and development of children or interferes with any individual’s activities of daily living and functioning in society”. They deliberately do not exclude people with no sight at all from their analysis, who many of us would feel have no vision rather than low vision.
A final question is whether “low vision” is an inappropriate term. We wouldn’t talk about someone with “low intelligence” or “low hearing” – do people take offense from being told they have “low vision”? I am glad we have moved away from using “visually disabled” (or worse, “subnormal vision”) – but should a new phrase be used? Some organisations refer to VIPs (visually impaired persons) – maybe our low vision clinics should be called VIP centres?!
I would be really interested to hear readers’ thoughts on this – should the WHO definition be changed? Should I rename lowvisionnews? Am I inappropriately confusing low vision and visual impairment? The comments section is open…