By Dr Ananya Mandal, MD
The World Health Organization (WHO) International Classification of Impairment, Disabilities, and Handicaps (ICIDH) system is used to classify the types of visual impairment.
This system, as the name suggests, is used to classify disorders, impairments, disabilities, and handicaps.
Definition of impairment
Impairment is defined as “any loss or abnormality in an anatomical structure or a physiological or psychological function.”
Similarly a disability is “any restriction or lack (resulting from an impairment) of ability to perform an activity in the manner or within the range considered normal for a human being.”
This places an individual in a handicap that is a person’s disadvantaged position in society due to an impairment or disability.
Definition of visual impairment
Visual impairment is defined as the limitation of actions and functions of the visual system.
The National Eye Institute defines low vision as a visual impairment not correctable by standard glasses, contact lenses, medication or surgery that interferes with the ability to perform activities of daily living.
Types of visual impairment
According to the CDC and the World Health Organization the classification of visual acuity and impairment includes (1, 2) –
- Low visual acuity means vision between 20/70 and 20/400 with the best possible correction, or a visual field of 20 degrees or less
- Blindness is defined as a visual acuity worse than 20/400 with the best possible correction, or a visual field of 10 degrees or less
- Legal blindness in the US means visual acuity of 20/200 or worse with the best possible correction, or a visual field of 20 degrees or less.
- Visual acuity of 20/70 to 20/400 (inclusive) is considered moderate visual impairment or low vision.
Causes of visual impairment
Types of impairment are different for different causes of visual impairment. In total vision loss for example there may be total darkness of the visual fields. Other types include visual impairment in glaucoma, age-related macular degeneration and so forth. (1-5)
Visual impairment in glaucoma
This condition is due to the rise of normal fluid pressure inside the eyes. The type of vision is usually like a tunnel.
The intact vision remains in the center while progressively the peripheries start decreasing. The center of the tunnel reduces in size progressively till total vision is lost if left uncorrected.
Age-Related Macular Degeneration
A central area of woolly or cottony opacity obscures the central part of the vision.
The peripheries may be normally seen. AMD usually blurs the sharp, central vision that is needed for closely viewed activities like reading, sewing, and driving. This is a painless condition.
There is general clouding of the vision. As the whole eye lens is affected the blurring of vision may be diffuse until it is totally lost.
There may be other symptoms like photophobia – inability to see the light; diplopia – double vision etc. Cataracts are very common in older people.
Diabetes leads to damage of the smaller arteries and blood vessels at the back of the eyes over the retina.
Diabetic retinopathy is the most common diabetic eye disease and a leading cause of blindness in adults.
Usually vision impairment in diabetics begins as black spots or floating shapes that appear in the field of vision. Slowly complete vision may be lost if left unchecked.
Myopia or near sightedness or short sightedness means a person can see nearby objects clearly but distant objects appear blurred. High myopia may lead to vision impairment.
This is a genetic or inherited condition. Initially it manifests as night blindness.
As the disease progresses there may be a tunnelling of vision with loss of peripheral vision followed by complete blindness.
Edited by April Cashin-Garbutt, BA Hons (Cantab)
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