February is low vision awareness month, so I thought I would do this blog post explaining some of the terms used when discussing vision loss and blindness. Whenever I tell people I have a problem with my eyes and am losing my central vision, a lot of the time I am asked how that is possible and they would never have known, because I have lovely eyes and they don’t look like there is anything wrong. When we think of blind or visually impaired, or when someone is losing their vision, we expect there to be something visible to give it away. Some people think your eyes should be cloudy, or look different in some way, but this is often not the case at all. A lot of people with vision loss can also get around quite well without the use of a cane or a guide dog, because they keep a useful portion of their vision. When my central vision depletes with Stargardts, I should still retain my side vision. In some rare cases some people can lose more vision than just the central, but I can expect to keep my peripheral, so even though I may not be able to see someone’s face who is walking up to me, I may still see the rest of their body up close and surrounding environment. The further you look out into the distance with central vision loss, the blind spots for me get bigger than when you are looking at something up close like a book, so when central vision is lost, you may not even see someone at all further away.
I thought I would run through what some of the terms mean when people talk about vision loss and blindness, so that others can understand a little bit more on what someone can or can’t see.
What is visual acuity and field of vision?
6/6 vision is a term used to express normal visual acuity (the clarity or sharpness of vision) measured at a distance of 6 metres. If you have 6/6 vision, you can see clearly at 6 metres what should normally be seen at that distance. 6/60 vision is 10 times poorer than ‘normal’, i.e., the patient sees at 6m what a person with normal vision sees at 60m.
When you go for an eye test, an optician will use a Snellen chart to determine what your best corrected visual acuity is. This will be which line you can read from on the chart. With Stargardts, the damage to the retina usually means that vision becomes uncorrectable the further the disease progresses.
Sometimes a person can have a visual acuity of 6/6 but have blind spots in their peripheral or lack of depth perception etc which can all affect your overall visual ability.
Your field of vision is measured by a “visual field test”. you will usually have had to have lost a large part of your visual field to be certified as severely sight impaired (blind) or visually impaired.
Visually impaired covers a whole spectrum of vision loss which can range from low vision through to total blindness. Some also include those who have a decreased ability to see because they do not have access to glasses or contact lenses or have something like severe light sensitivity affecting their vision.
Low vision is uncorrectable vision loss that interferes with daily activities. For someone with Stargardts they may have lost all of their central vision but still have use of peripheral vision (side vision) but are unable to read, recognise faces or drive. A person classed as having moderate low vision may have a visual acuity in the range of 6/36 to 6/60 and may need visual aids in their daily life.
In the U.K, legally blind individuals are those whose visual acuity or sharpness (with glasses, if needed) is 6/60 or worse (the top line of the Snellen chart) and/or a visual field of 20 degrees or less.
Light perception is the ability to interpret the surrounding environment using light in the visible spectrum reflected by the objects in the environment. This is different from visual acuity which refers to how clearly a person sees.
Complete blindness is full lack of light perception. About 15% of the total population of the world who is blind have absolutely no light perception at all while 85% of blind people have some light perception.
I am so lucky to have foveal sparing at the moment. So, for me I still see at 6/9 in both eyes but have damage around the very outside of my very centre of vision that includes blind spots. This I believe, is why I never noticed I was losing my vision before my diagnosis. I am currently not classed as low vision but will find out in April if it has deteriorated and I can’t deny, I am extremely anxious about this appointment and it seems to have come around too fast. If it has deteriorated, there is nothing I can do about it anyway and I am trying my hardest to preserve what vision I have left, so I can’t do anymore. I hope this has helped people understand a bit better about the terms used when describing vision loss.