Visual Impairment Fact Sheet

What is Visual Impairment?

Where ever you go you’ll hear a different definition of what visual impairment is. Two popular definitions are:

  1. 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.
  2. 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 is vision lost?

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.

  1. Cerebral impairments result from brain damage affecting the areas of the brain responsible for producing images/vision.
  2. 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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How is visual impairment diagnosed?

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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How many people are visually impaired?

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:

  1. Registered Blind = 33,425
  2. Registered partially sighted = 29,305
  3. Total = 62,730

Actual numbers in Scotland in 2005/2006 of registered Blind and partially sighted people aged 16 – 64:

  1. Registered Blind = 4,679
  2. Registered partially sighted = 3,231
  3. 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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Common ocular conditions

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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Can visual impairment be treated?

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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Other Links

General Tips for acting as a Sight Guide

Date Posted: 31/10/2008

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