Discussion:
This ICIDH-document does not cover visual impairments of persons with intellectual disabilities. In many areas assessment of impairments can be similar to that used in assessment of adult persons or children with normal intellectual functions. When the person cannot perform in a test situation specific tests are needed and should be standardised.

Addition to the chapter:

Chapter 4 (0400-0499) Functions of the eye and of adjoining structures in persons with intellectual disabilities

Oculomotor functions and accommodation

incl. diplopia, seeing double

Functions of the external musculature of the eye

: voluntary eye movements of the eyes; tracking movements of the eyes; saccadic movements of the eyes

Assessment of oculomotor functions is equal to that of adult persons with normal intellectual functions. Measurement of accommodation is particularly important in individuals with Down Syndrome. If the person does not have the ability to fixate on a target, it needs to be clearly stated because it may modify other visual sensory test situations.

Visual-sensory functions

Visual acuity at distance

Visual acuity is measured using line tests when possible.
If single symbol tests are the only optotype tests that can be used, then the line acuity value can be estimated to be three to four lines lower than the acuity measured with single symbols. It is also possible that the central island of vision is so small that only one symbol can be seen.

Normal vision, visual acuity values are the same as in persons with normal intellectual functions

Low vision, visual acuity values are the same as in persons with normal intellectual functions

severe low vision, visual acuity values are the same as in persons with normal intellectual functions

profound low vision
light perception with projection.
light perception without projection.

Total blindness, equal to no light perception

Visual acuity at near
In persons with intellectual disabilities, visual acuity at near is more important a measurement than that of distance visual acuity because learning occurs mostly at close distances and in them also communication is often limited to a near vision function. Visual acuity is measured using line tests, single symbol tests and crowded tests when possible.
Visual acuity measured as grating acuity.
Since grating acuity measures brain functions different from those measured with optotype acuity it cannot be used as a basis of classification of visual impairment. Grating acuity may well be within the range of age normal values, yet central vision is not normal. However, it gives some information about the functions of the visual pathways. Classification of individuals in whom only grating acuity can be measured is based on their overall use of vision.
Visual acuity of persons who cannot respond in test situations using optotype tests needs to be assessed using objects of subjective value. Visual acuity is then defined as recognition, not detection acuity:

ability to recognise small objects at near
(the size to be agreed upon)
ability to recognise medium size objects at near
(the size to be agreed upon)
ability to recognise only large objects at near
(the size to be agreed upon)
ability to recognise small objects at a distance of several meters
ability to recognise medium size objects at a distance of several meters
ability to recognise large objects at a distance of several meters
The person needs in these test situations to be able to look at the object and recognise it when several objects are present in front of him/her.

ability to be aware of an object and reach for it. Note that this may be related to parietal visual functions without form perception, i.e. the person may be profoundly visually impaired although (s)he can reach for and grasp small objects.

Contrast sensitivity is measured using appropriate tests. In persons who can respond in optotype tests, low vision is defined similar to adult persons with normal intellectual functions. In assessment of persons who cannot respond in optotype tests standardised fixation targets at 2.5% contrast are used (tests to be defined) and the greatest distance where the person responds to the stimulus is measured.

Normal vision is defined as responses at a distance greater than one third of the mean distance of responding of normally developing infants/children at the same age or of adult persons with normal intellectual functions,

Low vision is defined as responses at a distance equal or less than one third of the mean distance of responding of normally developing infants/children at the same age, or of adult persons with normal intellectual functions,

: severe low vision is defined as responses at a distance equal or less than one tenth of the mean distance of responding of normally developing infants/children or of adult persons with normal intellectual functions,
In case contrast sensitivity is more affected than visual acuity, classification is based on contrast sensitivity loss.

Visual field, colour vision and visual adaptation are assessed similar to the assessment of children or adult persons with normal intellectual functions using appropriate tests.