Visual Impairment Fact Sheet
Contents
Where ever you go you'll hear a different definition of what visual impairment is.
Two popular definitions are:
- According to the British and Irish Orthoptic Society, the term visual impairment
is "any loss or abnormality of visual function" which can lead to either being registered
blind or partially sighted.
- The World Health Organization divides visual impairment into two categories:
- "A partially sighted person is someone who has some remaining vision, but whose
impairment is such that it cannot be corrected by glasses or lenses. He would be
unable to see how many fingers were being held up at a distance of 6 metres or less
(even if wearing glasses or lenses)."
- "A blind person would be unable to see how many fingers were being held up at a
distance of 3 meters or less, note; that this does mean that they may have some
remaining vision." [NHS website, 2007]
In other words, a person who is registered as visually impaired is not necessarily
blind and may still have some aspects of useful vision.
The term 'Low Vision' is described commonly as a:
- Vision of less than 6/18 to light perception and/or
- Visual Field loss of less than 10 degrees from central point of viewing.
A person, who has 'Low Vision' as above, will still have the ability to use his/her
vision for planning and/or the execution of a task.
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How does the eye see?
When light enters the eyes an impulse is sent along the optic nerve to the brain.
The brain receives the impulses and gives them meaning, thus we're able to see the
object before us. Damage anywhere along the pathway whether that is at the front
of the eye or towards the back of the brain, can cause visual loss. Most visual
loss is caused by damage to the nerve layer inside the eyes.
How is eyesight impaired?
The cause of an eye condition can be either cerebral or ocular.
- Cerebral impairments result from brain damage affecting the areas of the
brain responsible for producing images/vision.
- Ocular impairments result from problems with the optic nerves or the eye
itself.
There are several different kinds of ocular impairments, and in addition to the
differences between the conditions themselves, the visual impairment can be either
congenital (occurs early or from birth) or acquired (occurs later on in life). It
might be also be partial or total, stable or deteriorating. Within each eye condition,
there are also variations in severity, effects and causes.
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A person can present with visual impairment at any age and due various reasons.
Ideally, they will require an ocular examination before a diagnosis is made. Some
of the professionals that may be involved are:
- The Ophthalmologist: A medical doctor specialising in the diagnosis and medical/surgical
treatment of visual disorders.
- The Orthoptist: Concerned with the diagnosis and treatment of binocular vision
disorders (such as strabismus/squints and double vision) and vision problems in
children and adults.
- The Optometrist: An eye care practitioner who performs eye examinations and
prescribes glasses or contact lenses if required.
- The Visual Field Technician: Plots central and peripheral visual fields using
an automated visual field analyser (often for patients suffering from glaucoma or
neurological conditions such as strokes).
Children are usually referred to the Ophthalmology team from their GP after a problem
has been noted either in the Health Visitor checks, Vision Screening centres, School
Nurse checks, or if there are any parental concerns. They could also be referred
to the Ophthalmology team from another consultant within the hospital (like a Paediatrician)
who may have noticed an ocular abnormality.
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The figures for England and Scotland are taken from the most recent registration
data, which are for year ending 31 March 2006. They are presented below and are
courtesy of the RNiB and The NHS Information Centre:
Actual numbers in England in 2005/2006 of registered blind and partially sighted
people aged 18 – 64:
- Registered blind = 33,425
- Registered partially sighted = 29,305
- Total = 62,730
Actual numbers in Scotland in 2005/2006 of registered blind and partially sighted
people aged 16 – 64:
- Registered blind = 4,679
- Registered partially sighted = 3,231
- Total = 7,910
Vision disorders are said to be the second most common problem affecting young children,
apart from dental decay. It's important that you look out for your child's vision
from an early stage, especially if there's a strong family history of ocular conditions,
as the visual system is said to have matured by the age of 7. Thus if an ocular
condition is picked up well before this age then a lot may be done to manipulate
the visual system to help it work better.
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There are many ocular conditions but we thought it best to write down some of the
common ocular conditions that affect the visual system:
Myopia: Refractive error where parallel rays of light come to a focus in
front of the retina when the eye is at rest. In simple terms: short sightedness
where the eye is large and thus near vision is better than distance.
Hypermetropia: Refractive error where the principal focus of the eye is calculated
behind the eye. May be axial (caused by abnormal length between the anterior and
posterior poles of the eye), curvature or index (caused by an increase/decrease
in the refractive index of the structures of the eye i.e. cornea and lens). In simple
terms: long sightedness where the eye is small and thus ones distance vision is
better than the near vision.
Astigmatism: Refractive Error in which a point of focus cannot be formed
on the retina owing to unequal refraction in different meridians. The front of the
eye can be shaped a little like a rugby ball rather than being perfectly rounded.
Vision is therefore blurred at all distances.
Anisometropia: A condition in which there is a difference of refractive error
between the two eyes, for example one eye may be normal and the other eye may be
myopic or hypermetropic. This can lead to amblyopia and strabismus.
Amblyopia: A condition where the eye is not used and the sight deteriorates
if not treated. Early referral is therefore essential as treatment involves occluding
the good eye in order to force the poor eye to work better and become visually stimulated.
Strabismus: A manifest strabismus occurs when the two eyes are not aligned,
i.e. one eye may deviate inwards, outwards, upwards or downwards. A strabismus can
be constant or intermittent; and is commonly known as a squint, wall eye, cross-eyed,
lazy eyed, boss-eyed. Although a strabismus can be present from birth, more appear
in early childhood. At first it may only be present when a child is unwell, tired
or looking at a certain distance but then over time can become constantly noticeable.
In most children there is a common cause for a strabismus, i.e. it could be due
to uncorrected or unequal refractive error, heredity reasons, a developmental anomaly,
trauma to the eye or head, and some squints do have a more serious cause. It is
therefore important that all children with a strabismus, even if it’s suspected,
be examined fully. No child is too young to have an eye examination and a true strabismus
does not disappear. It is impossible to tell accurately from a photograph or video
if a patient has a strabismus and thus checks with the local Orthoptist are really
important.
Any strabismus, whether large or small, can result in the development of amblyopia
and the permanent loss of binocular single vision (how well the eyes work together
as a pair). Thus early detection and referral is therefore vital so that treatment
can commence. Treatment of strabismus can be one or a combination of the following
methods: 1) glasses; 2) occlusion of the good eye to make the amblyopic eye work;
3) surgery; and then 4) orthoptic exercises.
Nystagmus: involuntary oscillations of the eyeball.
Anophthalmos: absent eyes therefore due to abnormalities the child is born
without fully formed eyes.
Delayed Visual Maturation: this is apparent blindness in a normal child until
this child is approximately 12 weeks of age. This is thought to be due to a delay
in the development of the visual fibres.
Cortical Blindness: a term applied to a severe defect of vision of both eyes
as a result of a lesion of the visual cortex. Optic nerve and pupil reflexes may
be normal.
Colour blindness: inability to distinguish certain colours. This is more
common in males than in females and is passed via a mutated X gene from the mother.
Cataracts: Hardening of the crystalline lens due to ageing or acquired from
trauma, metabolic disorders, or a congenital defect.
Retinopathy of Prematurity (ROP): a retinal pathology associated with premature
birth where the child can have a very high degree of myopia and delayed visual maturation
which results in reduction of vision.
Neuropathy: a disorder affecting the structure and function of the nervous
system.
Optic Nerve Atrophy: the Optic Nerve is positioned at the back of the eye
and collates all the signals of light to send to the brain for visual processing.
If the Optic Nerve is abnormal in size or damaged then this can lead to Optic Nerve
Atrophy.
Macular Degeneration: burst blood vessels in the retina obstruct the light
impulses to the back of the eye and thus giving a field defect. The person with
Macular Degeneration will be able to see their peripheral vision but will have a
lack of central vision.
Glaucoma: increased pressure within the eye leads to deformities on the optic
nerve that can reduce the vision and/or cause visual field loss; depending on the
extremity.
Visual Field Defect: a statistical and clinically significant departure from
the normal shape of the hill of vision which may be localised or a general depression
of the whole field. This is when central or peripheral visual function loss is detected
without movement of the eye.
More Ocular Conditions can be found in the British and Irish Society of Orthoptics
Dictionary
of Common Terms in Orthoptic Practise.
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The Low Vision Service is a tool by which a person with low vision is given access
to a range of services which will enable them to use and/or enhance their remaining
vision to its greatest potential. The service provider gives individual tailored
tuition in the use of vision or low vision aids.
Low vision aids can be offered to enhance or magnify their vision, and these aids
can be optical or non-optical. In addition, people with visual impairment can ask
for reading material to be transcribed into Braille or large print, and also allow
the material to be read out to them via various technological programs on a computer.
Low vision training can also be offered in some localities where a social worker
will come to assess the home environment and do their best to adjust your home life
to help you to adapt to your surroundings.
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