Visual Impairment

What does  Visual Impairment mean?

     Visual impairment is when a person has sight loss that cannot be fully corrected using glasses or contact lenses. It's important to visit an optometrist (optician) for an eye test at least every two years so they can check for signs of vision loss. 

How vision is tested and measured

visual acuity – central vision used to look at objects in detail, such as reading a book or watching television visual field – ability to see around the edge of your vision while looking straight ahead

 Visual acuity testing:

           A test called the Snellen test is often used to measure your visual acuity. It involves reading letters off a chart on which the letters become progressively smaller. This chart is used during a routine eye test.

          After the test you are given a score for your visual acuity. A Snellen score consists of two numbers. The first number represents how far away from the chart you were able to successfully read the letters on the chart. The second number represents how far away a person with healthy vision should be able to read the chart.So if you were given a visual acuity score of 6/60, it means you can only read at 6 metres away what a person with healthy eyesight can read at 60 metres away.

 Visual field testing:

        There are a number of different tests that can be used to assess your visual field.One test involves looking straight ahead at a device while lights are flashed on and off at the edges of your vision. You will be asked to press a button every time you see a light. This shows any gaps in your field of vision.

       Alternatively, you may be asked to follow an object (or the tester's hand) with your eyes as it is moved across your field of vision. You will be asked to say when you first see the object and when you can no longer see it.

 Types of visual impairment

Visual impairment is usually classified as either ’sight impaired’ or ‘severely sight impaired’. These classifications are based on the results of the tests described above.

Sight impaired

Sight impairment, previously called ‘partial sight’, is usually defined as: having poor visual acuity (3/60 to 6/60) but having a full field of vision, or having a combination of slightly reduced visual acuity (up to 6/24) and a reduced field of vision or having blurriness or cloudiness in your central vision, or having relatively good visual acuity (up to 6/18) but a significantly reduced field of vision.

Severely sight impaired

 The legal definition of severe sight impairment (which was previously called ‘blindness’) is when ‘a person is so blind that they cannot do any work for which eyesight is essential’.

This usually falls into one of three categories: having very poor visual acuity (less than 3/60), but having a full field of vision having poor visual acuity (between 3/60 and 6/60) and a severe reduction in your field of vision having slightly reduced visual acuity (6/60 or better) and a significantly reduced field of vision.

Causes of visual impairment

Some of the most common causes of visual impairment include: age-related macular degeneration (AMD) – where the central part of the back of the eye (the macula, which plays an important role in central vision) stops working properly cataracts – where cloudy patches can form within the lenses of the eyes glaucoma – where fluid builds up inside the eye, damaging the optic nerve (which relays information from the eye to the brain) diabetic retinopathy – where blood vessels that supply the eye become damaged from a build-up of glucose

taken from: www.nhs.uk/conditions/Visual-impairment/Pages/Introduction.aspx