Visual impairment

Visual impairment can manifest itself in essentially three directions (Fire, 2000):

a) by reducing visual acuity limiting or

b) damaging the visual field of vision or

c) color vision disorder.

Decreasing visual acuity requires diminishing the distance or enlarging the object in comparison with the standard. However, visual acuity is not the only criterion of visual impairment.

Visual field impairments occur in most partially sighted and partially visually impaired people. The greatest number of visual field defects occur as a result of atrophy of the optic nerve and retinal damage. Pathological changes in the field of vision lead to disturbance of the visual reflection of space, to its reduction and/or deformation.

When combined with weak sightedness or visual residua, color vision impairments can also be an obstacle to the perception of space.

The quality of color perception influences the differentiation of the following color properties:

a) color (color shade)

b) saturation

c) clarity.

In rare cases of full color blindness, the affected person will perceive all colors as shades of gray, white and black. In most cases, however, only partial color blindness occurs, where the affected person does not distinguish some of the colors, most commonly red and green. Colorblindness represents visual impairment, but is generally not considered to be a visual defect as such.

Types and degrees of visual impairment in children:

Binocular vision disorders

Binocular vision (seeing with both eyes) allows us to see the two images viewed by the eyes at the same time as a single image. This allows us to perceive depth. If binocular vision is impaired, this leads to changes in visual perception due to reduced visual acuity, problems with spatial perception and in the case of significant loss of function or complete loss of one eye, also a restriction of the visual field.  

These disorders include:

• strabismus (“squinting” – causes the visual center in the brain to perceive two images instead of one, the brain eliminates this by ignoring the less clear image from the squinting eye, this state later results in amblyopia),

• amblyopia (“lazy eye” – reduced visual acuity caused by dulling from inactivity of the eye in cases where the brain no longer accepts information from the given eye for a long period of time, instead only from the other eye),

• monocular vision (complete absence of the organ).

Binocular vision disorders are among the most visual disorders in preschool children.