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Visual Impairment :
Special Educational Needs
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VISUAL ACUITY

The designation of visual impairment into severely visually impaired (blind) and partially sighted takes into account visual acuity and visual field. Their medical reports and EHCPs include such measurements which help to describe some of their vision problems. The following explanation is taken from the RNIB web-site, (http://www.rnib.org.uk).

A consultant has rules to follow when completing the Certificate of Vision Impairment (CVI), the form used in England. Other certification forms use similar rules.

These rules take into account:

  • visual acuity : Central vision, the vision used to see detail

  • visual field : how much can be seen around the edge of vision, while looking straight ahead.

Visual acuity is measured by reading down an eye chart while wearing any glasses that may be needed. This is known as a Snellen test. Field of vision is measured by a 'field of vision test'. Measuring visual acuity and visual field helps the ophthalmologist to decide whether a person is eligible to be registered as severely sight impaired (blind) or sight impaired (partially sighted).

The Snellen scale

Visual acuity is measured using the 'Snellen scale'. A Snellen test usually consists of a number of rows of letters which get smaller as you read down the chart.

On the Snellen scale, normal visual acuity is called 6/6, which corresponds to the bottom or second bottom line of the chart. If a person can only read the top line of the chart then this would be written as 6/60. This means he/she can see at 6 metres what someone with standard vision could see from 60 metres away. (NB. 6 metres is almost 20 feet, so 6/6 is the equivalent to the American 20/20 vision)

The figures 6/60 or 3/60 are how the result of a Snellen test are written. The first number given is the distance in metres from the chart someone sits when reading it. Usually this is a 6 (for 6 metres) but would be 3 if he/she were to sit closer to the chart, ie 3 metres away. The second number corresponds to the number of lines that he/she is able to read on the chart. The biggest letters, on the top line, correspond to 60. As a person reads down the chart the numbers that correspond to the lines get smaller, ie 36, 18, 12, 9 and 6. The bottom line of the chart corresponds to the number 6. Someone with standard vision who can read to the bottom of the chart would have vision of 6/6.

For example, a person with standard vision would be able to read the second line on the chart when 36 metres away. However, if he/she had a Snellen score of 6/36, he/she would only be able to read the same line at 6 metres away. In other words he/she need to be much closer to the chart to be able to read it. Generally the larger the second number is, the worse his/her sight is.

The ophthalmologist uses a combination of both visual acuity and field of vision to judge whether someone is eligible to be registered, and at which level. If a person has good visual acuity, he/she will usually have had to have lost a large part of his/her visual field to be registered as severely sight impaired (blind) or sight impaired (partially sighted).

Alternatively, if he/she have all his/her visual field, he/she will usually have to have a very poor visual acuity to be registered as severely sight impaired (blind) or sight impaired (partially sighted).

Snellen chart

Generally, to be registered as severely sight impaired (blind), sight has to fall into one of the following categories :

visual acuity of less than 3/60 with a full visual field

visual acuity between 3/60 and 6/60 with a severe reduction of field of vision, such as tunnel vision

visual acuity of 6/60 or above but with a very reduced field of vision, especially if a lot of sight is missing in the lower part of the field.

To be registered as sight impaired (partially sighted) sight has to fall into one of the following categories :

visual acuity of 3/60 to 6/60 with a full field of vision

visual acuity of up to 6/24 with a moderate reduction of field of vision or with a central part of vision that is cloudy or blurry

visual acuity of up to 6/18 if a large part of your field of vision, for example a whole half of your vision, is missing or a lot of your peripheral vision is missing.

Visual acuity is often expressed using a measurement called LogMAR (Log of the minimum angle of resolution). The table shows the LogMAR scale derived from the distance measurements.

LogMar table

LogMAR scores greater than 1 represent visual acuity of less than 6/60, i.e. severely visually impaired. LogMAR 1.3 is 3/60.

VISUAL FIELD ANALYSIS

Visual field is analysed by measuring absorbance of light by the cells in the retina. It is based on the reflection of light from a pure white reflective surface and measured in units of luminance called apostilbs. No reflection gives a score of 0 while complete reflection gives a score of 10,000. As the range of measurements is so great it is converted to a log scale and then into decibel units. A score of 40 or more represents excellent light absorption while 0 represents no light detection, i.e. the retinal cells are not functioning.

The diagrams below show the visual field analysis and visual field of a 16 year old girl. Her central vision was relatively good. A normal visual field would be 90o from the centre but hers was only 5o so she sees the world as if she was looking through a straw. The photographs show a scene as seen by a normally sighted person compared with the same scene as she would see it. Although she has good central vision the limitations of her sight means that she is registered as severely visually impaired/blind
Visual field analysis